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The relationship between specific occlusal contacts and jaw closing muscle activity during parafunctional… MacDonald, James W. C. 1982

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THE RELATIONSHIP BETWEEN SPECIFIC OCCLUSAL CONTACTS AND JAW CLOSING MUSCLE "ACTIVITY DURING PARAFUNCTIONAL CLENCHING TASKS IN MAN BY JAMES W.C. MacDONALD B.Sc, D.M.D., THE UNIVERSITY OF BRITISH COLUMBIA, 1982 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF ORAL BIOLOGY We accept this thesis as conforming to the required standard The University of British Columbia September, 1982 ©JAMES W.C. MacDONALD, 1982 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Oral Biology The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date May 10, 1983 DE-6 (3/81) i i ABSTRACT Parafunctional clenching and grinding habits have been associated with masticatory muscle hyperactivity (18,39,167,176). Muscle tenderness to palpation, muscle fatigue, and stresses to the teeth and joints are the most common signs and symptoms of parafunction and are thought to result from prolonged periods of muscle contraction (158). The object of this work was to examine and describe the relationships between the electromyographic (muscle) activity in the jaw closing muscles and the location, size, and direction of applied effort to specific bite points, some chosen to simulate clincal conditions. The experiment included two preliminary studies which utilized multiple small acrylic occlusal bite stops. The stops caused an approximate 1mm vertical opening of the occlusion from the intercuspal position as measured at the incisors. They were employed to control the location of the bite point and the variables caused by cuspal inclines and lateral displacement as their design incorporated both upper and lower teeth. The stops were used in both vertical and eccentric clenching tasks. A third study was performed to examine similar clenching tasks on specific tooth contacts on natural cuspal inclines. Bipolar surface electrodes recorded electromyographic activity from the anterior temporal, posterior temporal, and superficial masseter muscles bilaterally, while paired, insulated fine wire electrodes recorded activity in the left medial pterygoid muscle. Data were collected for i i i each muscle and task and were stored in digitized form for analysis by a disc based computer system that calculated means and standard deviations over a 400 msec period about the centre of the response. Data were then normalized to each subject's peak task for each muscle which allowed comparisons for the group as a whole. Muscle patterns for the majority of subjects were consistently dependent upon the position and number of occlusal contacts and the direction of effort applied to the contacts. Vertical clenching efforts in the natural intercuspal position generally exhibited the highest activity for all the muscles recorded of all the tasks. As isolated bite points moved posteriorly along the arch from incisors to molars an increase in activity was observed in the ipsilateral temporal muscles while the ipsilateral medial pterygoid and the masseter muscles bilaterally were seen to decrease. The ipsilateral temporal and contralateral pterygoid muscles comprised the majority of activity during subjective vertical clenches on natural cuspal inclines as well as in lateral efforts on specific bite points. The temporal muscles, especially the posterior fibers, exhibited the most activity during retrusive efforts and the least during protrusive and i nei sal clenches which were primarily handled by the masseter and pterygoid muscles. When the size and number of contacts were increased anteriorly a generalized increase in muscle activity was witnessed while the same trends were not as consistent or significant posteriorly. Cross-arch contacts, when present, were associated with a slight significant increase in activity of the masseter muscle ipsilateral to the cross-arch contact but the majority of the i v activity was s t i l l provided by the ipsilateral temporal and contralateral pterygoid muscles. Proportionately, muscle activity between subjective maximal and half maximal clenches was similar and the same relationships of muscle and task could in general be described for both efforts. The findings have shown that patterns of muscle activity during parafunctional clenching behaviour vary predictably according to the location, size, and direction of effort applied to specific tooth contacts and are relevant to both local muscle soreness and joint biomechanics in mandibular dysfunction. V TABLE OF CONTENTS PAGE Abstract i i List of Tables ix List of Figures xi Acknowledgement xi i i I. Introduction 1 A. Bruxism 2 1. Definition 2 2. Eti ology 3 3. Signs and Symptons 6 4. Summary 9 B. Neuromuscular control of Interocclusal Forces 10 1. Gingiva, Periosteum and Periodontal Ligament 10 2. Muscle 12 3. Temperomandibular Joint 15 4. Central Control 16 C. Anatomy and Function of the Major Masticatory Muscles 19 1. Temporali s 19 2. Masseter 20 3. Medial Pterygoid 21 4. Lateral Pterygoid 21 5. Digastric 22 6. Summary 22 VI PAGE D. Interocclusal Force 23 1. Measurement 23 2. Interocclusal Force and Electromyographic Activity 24 3. Joint Forces 26 II. Statement of Problem 30 III. Methods 32 A. Study #1 - Vertical Effort Study 32 1. Occlusal Stops 32 2. Electromyography 34 3. Tasks 36 4. Data Handling 36 a) Normalization 42 B. Study #2 - Eccentric Effort Study 42 1. Occlusal Stops 42 2. El ectromyography 43 3. Tasks 43 4. Data Handling 43 C. Study #3 - Natural Tooth Study 45 1. El ectromyography 45 2. Tasks 45 3. Data Handling 46 vii PAGE IV. Results 48 A. Study #1 - Vertical Effort Study 48 1. Intercuspal and Simulated Intercuspal Position 48 2. Anterior Bite Block 53 3. Antero-Posteri or Relati onshi ps 53 4. Cross-Arch Relationships 53 5. Ipsilateral-Contralateral Contact Relationships 55 6. Empty Side Relationships 55 7. Half Maximal Clenching Relationships 59 B. Study #2 - Eccentric Effort Study 59 1. Vertical -Lateral Relationshi ps 59 2. Eccentric Efforts on Molar Contacts 61 a) Vertical-Lateral Relationships 61 b) Lateral Relationships 61 c) Vertical-Protrusive Relationships 64 d) Vertical-Retrusive Relationships 64 e) Protrusive-Retrusive Relationships 64 C. Study #3 - Natural Tooth Study 64 1. Antero-Posterior Relationships 65 2. Cross-Arch Relationships 7 7 3. Ipsilateral-Contralateral Contact Relationships 67 4. Intercuspal Relationships 7 9 a) Vertical-Eccentric Relationships 69 b) Lateral Relationships 69 c) Protrusive-Retrusive Relationships 71 vi i i PAGE V. Discussion 72 A. Discussion of Methods 72 1. Occlusal Stop Design 72 2. Jaw Position 73 3. Random Subject Group 74 4. Fatigue 74 5. Practice 75 B. Discussion of Data 75 1. Intercuspal and Simulated Intercuspal Position 75 2. Antero-Posterior Location of Bite Point 76 3. Cross-Arch Contacts 78 4. Eccentric Contacts and Direction of Effort 78 5. The Number and Site of Tooth Contacts 80 6. Empty Side Efforts 82 7. Joint Loading 82 C. Parafunctional Considerations 83 D. Future Directions 84 VI. Summary 87 VII. Bibliography 89 Appendix A Statisical Analysis of Specific Task Comparisons 120 Appendix B (n) Values for the Tasks and Muscles Illustrated in the Figures 137 i x L I ST OF TABLES TABLE PAGE I O c c l u s a l S t op C o m b i n a t i o n s U t i l i z e d i n S t u d y #1 37 I I T y p i c a l Computer Data F o r a S i n g l e C l e n c h 40 I I I T y p i c a l Computer Data W i t h Means and S t a n d a r d D e v i a t i o n s f o r One Task 41 IV O c c l u s a l S t op C o m b i n a t i o n s and Tasks Employed i n S t u d y #2 44 V O c c l u s a l C o n t a c t C o m b i n a t i o n s and Tasks Employed i n S t udy #3 47 VI Compar i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between C l e n c h e s i n a N a t u r a l I n t e r c u s p a l P o s i t i o n and a S i m u l a t e d I n t e r c u s p a l P o s i t i o n 120 VI I Compar i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between V e r t i c a l C l e n c h e s on an A n t e r i o r B i t e B l o c k and O t h e r C o n t a c t P o s i t i o n s 121 V I I I Compar i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between V e r t i c a l C l e n c h e s on A n t e r i o r and P o s t e r i o r C o n t a c t P o s i t i o n s 122 IX Compa r i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between V e r t i c a l C l e n c h e s on U n i l a t e r a l C o n t a c t s and t h e C o r r e s p o n d i n g C r o s s - A r c h C o n t a c t C o m b i n a t i o n 123 X Compar i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between V e r t i c a l C l e n c h e s on I p s i l a t e r a l and C o n t r a l a t e r a l C o n t a c t s 124 XI Compar i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between U n i l a t e r a l Group C o n t a c t and E f f o r t on the Empty S i de 125 X TABLE PAGE XII Comparison of Maximum and Half Maximum Vertical Clenching Relationships 126 XIII Comparison of Normalized Muscle Activity Between Vertical and Lateral Clenching Efforts 129 XIV Comparison of Normalized Muscle Activity Between Vertical and Eccentric Efforts on Molar Contacts 130 XV Comparison of Normalized Muscle Activity Between Lateral Efforts to Opposite Sides on Molar Contacts 131 XVI Comparison of Normalized Muscle Activity Between Protrusive and Retrusive Efforts on Molar Contacts 132 XVII Comparison of Normalized Muscle Activity Between Clenches on Anterior and Posterior Contacts on Natural Teeth 133 XVIII Comparison of Normalized Muscle Activity Between Clenches on Unilateral Contact and the Corresponding Cross-Arch Contact Combination on Natural Teeth 134 XIX Comparison of Normalized Muscle Activity Between Clenches on Ipsilateral and Contralateral Contact Combinations on Natural Teeth 135 XX Comparison of Normalized Muscle Activity Between Intercuspal and Other Clenching Efforts on Natural Teeth 136 XXI (n) Values for Each Task and Muscle Used in Study #1 137 XXII (n) Values for Each Task and Muscle Used in Study #2 138 XXIII (n) Values for Each Task and Muscle Used in Study #3 139 xi LIST OF FIGURES FIGURE PAGE 1 Occlusal Stop Design 33 2 Experimental Design 39 3 Raw Electromyographic Response for a Series of Clenches 39 4 Effects of Seven Different Tooth Contact Combinations on the Response of One Individual's Left Superficial Masseter Muscle 50 5 Effects of Seven Different Tooth Contact Combinations on the Normalized Muscle Activity of the Left Superficial Masseter Muscle for the Group 50 * Histogram Key 51 6 Comparison of Normalized Muscle Activity Between Clenches in a Natural Intercuspal Position and a Simulated Intercuspal Position 52 7 Comparison of Normalized Muscle Activity Between Vertical Clenches on an Anterior Bite Block and an Incisal Stop 52 8 Comparison of Normalized Muscle Activity Between Vertical Clenches on Anterior and Posterior Contact Positions 54 9 Comparison of Normalized Muscle Activity Between Vertical Clenches on Unilateral Contacts and the Corresponding Cross-Arch Contact Combination 56 10 Comparison of Normalized Muscle Activity Between Vertical Clenches on Ipsilateral and Contralateral Contacts 57 xii FIGURE 11 Comparison of Normalized Muscle Activity Between Unilateral Group Contact and Effort on the Empty Side 12 Comparison of Normalized Muscle Activity Between Vertical and Lateral Clenching Efforts 13 Comparison of Normalized Muscle Activity Between Vertical and Eccentric Efforts on Unilateral Molar Contacts 14 Comparison of Normalized Muscle Activity Between Vertical and Eccentric Efforts on Bilateral Molar Contacts 15 Comparison of Normalized Muscle Activity Between Clenches on Anterior and Posterior Contacts on Natural Teeth 16 Comparison of Normalized Muscle Activity Between Clenches on Unilateral Contact and the Corresponding Cross-Arch Contact Combination on Natural Teeth 17 Comparison of Normalized Muscle Activity Between Intercuspal Clenching Efforts on Natural Teeth xi i i ACKKNOWLEGEMENTS I would like to extend my deepest gratitude to my supervisor, Dr. Alan G. Hannam, whose instruction, guidance and inspiration have made this project a most pleasant and rewarding learning experience. I would like to thank Dr. Alan Lowe and Dr. Bill Wood for their helpful suggestions and critiques concerning the thesis content. I am most grateful to Mr. Richard DeCou for his expert technical advice and professional artwork and to Mrs. Joy Scott for her expertise in solving the various computer problems that arose. Special thanks also to Ms. Linda Skibo for her professional collection and organization of the material for typing of the final draft. Finally, a thank you to my wife, Cyndi, for her inspiration and good nature during the length of the project. - 1 -I. INTRODUCTION Bruxism i s thought to be a habitual c lenching or gr ind ing of the teeth in response to various psychologica l (114,176,181,184,216) and dental f a c to rs (88,109,162,176,207). It has been reported to cause s t ress in the tee th , muscles and j o i n t s that may progress to pathosis in those areas (161,176). Nocturnal electromyographic studies (32,153,154,182) have demonstrated that subjects may indeed clench or gr ind t he i r teeth fo r extended periods of time at high force l eve l s (6,176). Numerous reports (29,30,150,163) have demonstrated that long periods of c lenching in acute experiments can resu l t in muscle pain and fat igue s im i l a r to that witnessed in bruxism pa t i en ts . It i s well known that the s i t e and number of tooth contacts vary considerably in natural and restored dent i t ions (212). A l t e r i ng the height of even a s ing le tooth has been reported to cause a marked asymmetry in the a c t i v i t y of the masticatory muscles. If l e f t c h r o n i c a l l y , a prematurity may cause problems in the tee th , muscle and j o i n t s and even i n i t i a t e bruxism i t s e l f (162). An apparent assoc ia t ion ex i s t s between parafunct ional habits and the signs and symptoms of muscle dysfunct ion and stresses to the teeth and j o i n t s . However no formal work has attempted to study the s p e c i f i c muscle groups involved and t h e i r electromyographic a c t i v i t i e s during various parafunct ional tasks on s p e c i f i c tooth contacts . It would be useful to know i f s p e c i f i c tooth contac ts , hab i tua l l y used to generate high fo r ces , were associated with a c t i v i t y in s p e c i f i c muscle groups. If t h i s could be demonstrated i t would a s s i s t in the understanding of some of the symptomology of parafunct ional hab i t s . It - 2 -could also be of help to the biomechanical approach, seeking an explanation for joint pathosis. A. Bruxism 1. Definition Bruxism is considered a parafunction (45) or functional disturbance of the masticatory system (85). It is believed to involve long lasting tooth contacts during clenching or grinding, when an individual is neither chewing nor swallowing (25,121,160,176). Bruxism has been subdivided into . eccentric bruxism, a gnashing or grinding of the teeth in eccentric excursions, and centric bruxism which is a clenching of the teeth in or around centric occlusion (161). It has been difficult to assess the incidence of bruxism in the population as i t is performed on a subconscious level, (2,24,78) to estimates as high as 88%, based on dental wear facet analysis (24,85,116). A reasonable estimate of bruxism in the population has been suggested to be 30%, with a smaller percentage actually presenting with symptoms (170). Bruxism may be a daytime habit, a nighttime habit, or both (61,170). Sleep studies (32,50,182,189) have shown that many tooth contacts can occur, for long periods of time, at reasonably high force levels (6,176). Those studies have led to a description of two basic patterns of nocturnal bruxism (170). Rhythmic, chewing-like movements have been observed which involve intense vertical and lateral pressures to the teeth. Also, intense, prolonged muscle contractions have been described with the jaw in either centric or eccentric positions for periods up to - 3 -300 seconds. Nocturnal bruxism has been demonstrated to occur generally during stages of light sleep, (50,154,165,189) especially when the subject moved from deeper to lighter stages of sleep. It has also been associated with rapid eye movement (REM) (108,153,165) sleep. Bruxism has often been associated with body mobility, an increased pulse rate and asynchronous respiration (6,170,189). This led one author to conclude an association between bruxism and the autonomic nervous system (189). Other studies reported an inconsistency in bruxism, as events varied from night to night, sometimes disappearing altogether (154,165). Recently, a small patient group of bruxers has been found active during deep sleep as opposed to earlier findings (119). This group also displayed generalized body muscle hyperactivity when asleep, and would awaken unrefreshed. 2. Etiology The etiology of the muscle hyperactivity, thought responsible for the pain and other effects of bruxism, is thought to be primarily due to occlusal interferences (11,70,174,204,205) and emotional stress (170,176,181). Early reports thought bruxism was simply an unconscious attempt to establish greater tooth contact (143) by relieving irritating occlusal discrepancies (170). The literature concerning occlusal interferences as etiologic agents in bruxism is contradictory. Some electromyographic studies ciaimed resolution of bruxism following occlusal adjustment, thereby associating occlusal interferences with bruxism (158,161). Introduction of a centric interference in the form of a gold occlusal inlay was seen to cause tooth pain, mobility, muscle pain, and - 4 -even to initiate bruxism in some subjects (162). Another study suggested that minimal alteration of occlusal height of even a single tooth could cause elevator muscle asynchrony (11). Increased forces on teeth, analagous to interferences, allegedly cause an increased activity in the periodontal mechanoreceptor afferent nerves (177). This may result in increased or decreased motor unit activity in the jaw elevator muscles which may contribute to both muscle asynchrony and hyperactivity associated with a change in jaw position. Reflex changes in the muscles can occur with changes in articular capsular tension due to positional changes at the temporomandibular joints (36,103). Altered neuromuscular control (138) resulting from interferences altering the path of jaw movement may be the basis upon which muscle hyperactivity develops and the associated symptoms occur (6,60,158,161,176). However there is no scientific evidence to support these assertions. Balancing-side interferences and those located between intercuspal position and centric relation have been claimed to initiate bruxism and temporomandibular joint problems in susceptible individuals (136,144,159). There is no conclusive information directly relating interferences to the initiation of bruxism (130). Other researchers have found no difference in the electromyograms after occlusal adjustment (92). They have concluded that interferences had l i t t le or nothing to do with triggering bruxism (10,13). Further studies in this area would be helpful. Historically, a psychological etiology (143,181,184) has been associated with bruxism. This has been exhibited by some of the diverse - 5 -terminology employed such as bruxomania (131) and occlusal habit neurosis (198). Subjects with portable electromyographic recorders demonstrated a marked increase in masseter muscle activity during stressful situations, such as freeway driving, work, and children returning from school (169). Sleep studies utilizing portable recorders have confirmed the association and have claimed nocturnal bruxism to be a stress-related sleep disorder closely related to emotional stress and physical exhaustion during waking hours (33,56,168,173). Apparently each individual has his or her own unique response to stress and it has been suggested that some may even have a genetic predisposition to nocturnal bruxism (1). In addition to emotional stress and occlusal factors, reports of brain damage and drug side effects appear as etiologies of bruxism in the literature (46,143,161,170,176). Amphetamine (8) and dopamine (126) precursors have been shown to give rise to bruxism. Patients with tardive dyskinesia, (91) thought to be a complication of long term phenothiazine useage, also exhibit bruxism. As opposed to most jaw movements elicited by cortical stimulation, animal and human experimentation (96) have described rhythmic bruxing-like movements to be stimulated from the limbic system and some lower brain areas (175). The limbic system is thought to be a connecting brain structure between the conscious, voluntary cortex and the hypothalamic vegetative centers (5). Some evidence for this association is that tranquillizers used in bruxists are effective in the limbic system (151) as well. It has been suggested that emotional tension in man manifested - 6 -through bruxism must originate, i f only partially, within the limbic system. Daytime bruxing has been connected with abnormal peripheral stimuli from the oral cavity creating disturbances in the reticular system (161). Altered activity from the reticular substance leads to increased reflex activity within the jaw-closing muscles. However, l i t t le evidence exists that peripheral afferents from the oral cavity can exert a disturbing effect on the reticular system (42). Recent evidence indicates supraspinal and supramedullary structures and not peripheral structures greatly influence reflex control. Most mechanisms contributing to increased stretch reflexes are due to altered supraspinal control. The reticular formation itself is under feedback control from the cerebral cortex and experimental evidence supports the concept that bruxism is primarily a central nervous system phenomenon (176). 3. Signs and Symptoms The most common symptoms associated with bruxism are related to masticatory muscle hyperactivity (60,104,161,167,176). Prolonged periods of muscle contraction may result in tenderness to palpation, muscle fatigue, and occasionally general tension (161,167,169,170). Patients with dysfunction have been shown to exhibit elevated muscle activities in masseter and temporal muscles (120). The muscle soreness has been related to myalgic pain, (125,176) which has been produced in rigorous clenching experiments (29,30,150,163). The pain mechanisms remain unknown but some theories have been suggested. An increase in the internal pressure of the - 7 -masseter muscle has been observed, leading some authors to believe tissue edema may be present and exert pressure on pain receptors (30). Inflammatory chemical changes due to trauma may irritate pain afferent nerve fiber plexes, resulting in pain (9). Muscle fatigue may irritate pain receptors due to inadequate blood flow being unable to remove metabolite build-up (104,140,146,150,163). Inadequate blood flow may be due to the increase of intramuscular pressure from the contraction itself (44,139,167) or due to compression of the arteries and veins entering the muscle (63). Finally, fatigue from prolonged postural jaw positioning may cause pain from mechanical stimulation of the articular ligaments and capsules, and their respective receptors (104,138). Tooth damage from abrasion during bruxism can result in characteristic nonfunctional bruxo-facets (46,161,170,176). Facets are a common sign and may lead to tooth sensitivity from dentin and even pulpal exposure (56) and possibly pulp death (86). A direct relation has been described experimentally in 50% of the cases studied, between the degree of abrasion and nocturnal jaw muscle activity (108). Observations, including nocturnal recordings, have estimated functional tooth contact during chewing and swallowing to occur approximately 17.5 minutes a day (60). As a result, most abrasion has been attributed to bruxism (59,115). Analysis of wear patterns has revealed that bruxism often involves eccentric, unstable positioning of the jaw (170). Horizontal and vertical force application over long periods of time may be responsible for the development of primary occlusal traumatism, (50,58,60,158) with its - 8 -deleterious effects on the teeth and supporting structures (95,165,185). Severe periodontal injury has occurred in cases where the occlusion contained steep cusps. Lateral stresses from bruxism were applied near the cusp tips, resulting in a longer lever arm than i f the cusp was in its corresponding fossa (195). Animal studies have described an increase in the width of the periodontal ligament space, tooth mobility, and tissue necrosis from occlusal trauma (16,57). However, no loss of tissue attachment occurred in the absence of inflammation (152). A relationship between occlusal trauma and mobility has been described (141) and the suggestion made, that although occlusal trauma does not cause gingivitis i t may be an essential factor in its spread (54). Temperomandibular joint pain and dysfunction may also result from bruxism (105,161,170). The symptoms resemble that of bruxism, with muscles tender to palpation due to muscle hyperactivity (176). Other common symptoms (161) include clicking, jaw deviation, limitation of movement and chronic headache. A summary of the signs and symptoms of bruxism has been fashioned from a report by Ramfjord (158). 1. Facets indicating a nonmasticatory pattern of occlusal wear. 2. Excessive, often uneven occlusal wear with minimal cupping of the exposed dentin. 3. Increased mobility of the teeth (80). 4. Pulpal sensitivity to cold. 5. Soreness of the teeth to. biting stress. 6. Dull percussion sound from the teeth. - 9 -7. Tenderness of the masticatory muscles to palpation. 8. Increased muscle tonus and uncontrolled resistance to manipulation of the mandible. 9. Tired feeling in the muscles upon waking in the morning. 10. Compensatory hypertrophy of the masticatory muscles, especially the masseter. 11. Temperomandibular joint discomfort or pain. 12. Locking of the jaw and a tendency to bite the cheeks, l ips, or tongue. 13. Audible sounds from bruxism. 4. Summary In summary, the signs and symptoms of bruxism are restricted primarily to the teeth, muscles and joints, and pathosis may develop in all three of these sites either separately or together. Although the mechanisms are not totally understood, i t is known that the bruxing act is associated with excessive muscle activity in the muscles of mastication. Clenching tasks have been included in other studies but none of these have attempted to systematically evaluate the muscle responses to clenching in different positions and in different directions. It would be pertinent to review here the current knowledge on those particular aspects of the masticatory system which have a direct bearing upon the production of interocclusal forces. This discussion will include the neuromuscular mechanisms controlling the generation of interocclusal force, a - 10 -description of the actions of the muscles of mastication, and the forces produced at the teeth and joints as a consequence of muscular activity. B. Neuromuscular Control of Interocclusal Forces Generation of interocclusal force during function requires neural control via afferent sensory feedback. Although the knowledge is incomplete, the innervation of the gingiva, periosteum, periodontal ligament, muscle and the temporomandibular joint, by an assorted description of receptors are assumed to supply some of this information. 1. Gingiva, Periosteum and Periodontal Ligament For simplicity, the receptors of the gingiva, periosteum and periodontal ligament can be referred to collectively as either periodontal or dental mechanoreceptors. A greater number of mechanoreceptors have been located in the maxillary incisor region than in the molar area while in the mandible the molar area was found to possess more receptors than the incisor region (26,41). Mechanoreceptors have also been found in greater density in the more apical regions of the periodontal ligament (26,46,111). Periodontal mechanoreceptors have been classified by qualities and characteristics as slowly adapting, rapidly adapting, and nociceptive units (46). Slowly adapting units have been the easiest to locate and have been found to respond to minor tooth movements and low increments of - 11 -force (89). They then relay information concerning the amplitude and changes in amplitude of the stimulus (73). Slowly adapting units can be direction sensitive, (7,72,74,172) responding to several directions but only one or two maximally. Whether this imparts a marked direction sense to the central nervous system is unclear (74). Rapidly adapting units are less understood but have been found to respond to higher thresholds (71) of mechanical stimulation. Their reaction is limited to rates of change of stimulation and not sustained displacement. The high thresholds are thought consistent with periosteal involvement during transient tooth stimulation (171). Nociceptor units have also been described and are found to be slowly conducting and react only to strong, noxious mechanical stimulation of the teeth, to warming and chemical stimulation with bradykinin, but not to normal stimulation. Periodontal projections to the mesencephalic nucleus of the trigeminal nerve suggest a possible interaction of muscle spindle and periodontal afferents. This could affect the functional elevator muscle activity as well as the brainstem pattern generator, responsible for rhythmic jaw movements, by reflexes that tend to be monosynaptic and can be excitatory (123), inhibitory (177,193,200), or both. Assuming that periodontal innervation via its thalamocortical projection contributes to the conscious perception of interocclusal force, and grading of force during a voluntary effort, then periodontal feedback is probably employed at a conscious level during masticatory performance (74,123). - 12 -2. Muscle Muscle spindles have been reported to be numerous in the jaw elevator muscles (53,81). They are found localized in specific areas such as the deep belly of the masseter muscle and in the horizontal and vertical fibers of the temporalis muscle (93,110). These areas are associated with type I, small cross sectional, highly oxidative muscle fibers which are smaller and slower units, active in the tonic stretch reflex. Taylor (194) suggests that these spindles are well situated to react to jaw opening. Cody (38) found the mesencephalic nucleus in the cat to contain the cell bodies of the f i rst order spindle afferents, both primary and secondary, from the masseter, temporals, and pterygoids. A constant tonic stretch reflex mediated via muscle spindles aids in maintaining mandibular posture. The stretch reflex is basic for maintenance of muscle tone and is capable of increasing the tension of selected muscle groups. This provides a background muscle tone upon which voluntary movements can be superimposed. The reflex can be tested by a menton tap which produces a stretch in the elevator muscles. This stretching of extrafusal fibers results in a concurrent stretch of intrafusal fibers which stimulates the afferent endings. Historically, i t was thought that a monosynaptic reflex stimulating alpha motor neurons mediated by Ia afferents resulted in a jaw jerk or jaw muscle contraction (178,188,192). However, recent findings by Kirkwood and Sears (101) suggested that secondary afferents or type II fibers could also provide monosynoptic excitatory input to the motor neurons. - 13 -Intrafusal fiber function informs the central nervous system of the length and rate of change in the length of the extrafusal fibers. Clark (34) reported that primary Ia afferents were associated with length and rate of change in length while secondary type II afferents were primarily involved as length receptors. Much spindle information is utilized by supraspinal centers such as the cerebral cortex and cerebellum. These can influence the descending pathways that facilitate and inhibit alpha and gamma motorneurons (22,34,37). Gamma motorneurons may also influence the sensitivity of the spindles by initiating intrafusal fiber contraction. Taylor (194) suggested that the stretch reflex exhibited more strength than that required for tonic posture alone, and he questioned whether spindle capabilities might extend to include a level of control over active movement. He discussed the following methods by which the stretch reflex could be modified. Static fusimotor (gamma) drive may enhance spindle excitation to the motor neurons which could fire with l i t t le evidence of stretch reflex modulation. Dynamic fusimotor drive could influence the reflex to act at high gain, provided there was sufficient motor neuron excitation, a third mode of reflex control. Finally, pre-synaptic spindle afferent terminal inhibition is also possible but lacks substantial evidence at this time. Movement control may involve co-activation of alpha and gamma fusimotor systems but evidence is currently lacking about the extent to which static and dynamic fusimotors are independent in action. Taylor (194) commented that postural regulation is controlled by spindles and small, slow motor units, strategically located in muscle - 14 -fibers most sensitive to stretch. These are linked via spindle primary and secondary afferents to the motorneurons monosynaptically. He also contends that some level of motorneuron excitation from other sources is necessary for the act to occur while the actual level of jaw support may be initiated by tonic firing of the fusimotor system. The strength at which alterations of jaw posture are resisted may be determined by the spindle sensitivity. This sensitivity may be changed by tonic firing of the dynamic fusimotor system. It has also been suggested (194) that during voluntary movements the intended pattern of movement may be expressed as a temporal template by the discharge of static fusimotor fibers. If the movement proceeds smoothly then spindle influence would not be expected. However, i f the movement is altered, changes in afferent spindle excitation could occur with subsequent muscle compensation, the sensitivity determined by the dynamic fusimotor system. Golgi tendon organs are encapsulated receptors found amongst the large collagenous fibers of tendons, near the muscle insertion (94). They act as tension recorders and respond to active contractions of the muscle via lb afferents. In limb muscle they polysynaptically inhibit agonists and facilitate antagonist muscles. However, Golgi tendon organs have only recently been unequivocally demonstrated in the masticatory muscles (124). - 15 -3. Temperomandibular Joint The temperomandibular joint, like the periodontal ligament, contains a variety of receptors. Free nerve endings are most abundant with far less numbers of specialized Ruffini endings in capsular tissue, the Golgi tendon organs in ligament, and the Paciniform receptors. Receptor densities are greatest in the lateral and posterior portions of the capsule, supplied by the auriculotemporal nerve, while the articular surfaces and meniscus have no innervation (46,186). Rapidly adapting receptor units have been found that fire only when the condylar head is rotated through particular positions (98). This receptor is thought to be important in relaying information on movement of the condylar head and i t has been suggested that i t relays information on acceleration rather than velocity (67). Slowly adapting units have been found that fire continuously as long as the condylar head is in a specific position (98). This unit is thought to be an important conveyor of positional information. Studies on the cat knee joint suggested that joint receptors sensed resistance to movement and angle of rotation (67). Several studies have indicated that temperomandibular joint receptor input can alter the ongoing activity of the jaw elevator muscles (35,66,97). Mandibular rotation in a closing direction has been seen to inhibit masseteric activity i f the mandibular position is maintained (97). Holding the mandible in an open position, however, has been seen to increase the activity in masseteric motorneurons (97). Klineberg (102) has reported both jaw opening and jaw closing reflexes by stimulating - 16 -nerves that supply the j o i n t . It has been suggested that these are protec t i ve re f lexes and are designed to protect the j o i n t (186). Temperomandibular j o i n t receptors have a lso been impl icated in contro l of mandibular and tongue posture , as both are somewhat a f fec ted by j o i n t anesthesia (122,186). It is thought that the receptors could provide a condi t ioned st imulus fo r learned re f lexes a r i s i n g from other s i t e s (186). For ins tance , Scharer (174) has descr ibed balancing-side in ter ferences that he postulates s t imulate re f lexes r e s u l t i n g in mandibular dev ia t ion . Storey (186) comments that th i s could be viewed as a learned r e f l ex in order to avoid fu r ther contact with the of fending tooth . Recordings from temperomandibular j o i n t receptors ind ica te that they probably monitor both pos i t ion and movement of the mandible (46). However, near ly a l l the experimental work in th i s area has been performed on laboratory animals, e spec i a l l y the c a t , which has a d i f f e r en t masticatory pattern than man. A l s o , the i d e n t i f i c a t i o n of the receptors i n i t i a t i n g the s igna ls as well as t he i r funct iona l ro les is l a rge l y specu la t i ve and the i r centra l pro ject ions have not ye t been worked out. 5. Central Control Jaw e leva tor muscles are invo lved i n a var ie ty of motor tasks requ i r ing d i f f e r en t l eve l s of c o n t r o l . It i s the job of the propr ioceptors to adjust centra l commands in accordance with the present status of the oral environment. However, the central e f f e c t s of the - 17 -proprioceptors are subject to modification by pre- and post-synaptic mechanisms which are s t i l l poorly understood. Considerable evidence suggests that limbic forebrain structures such as amygdala and hippocampus are important in goal directed, behavioural responses which may contribute to the initiation of action (133,134). Stimulation of these areas as well as the basal ganglia results in rhythmic chewing-like and bruxing-like movements. However, l i t t le is known of how these processes gain access to the motor system which then translates the cognitive and emotional processes into movements appropriate to the environment. Skilled voluntary movements can be considered to be continuously controlled by sensory input, and the motor commands adjusted by comparison of current to intended status (43). A movement may also be preprogrammed by the motor centers and triggered off as a unit, or ball istic act, which runs its full course without modification (43). The cerebral cortex, cerebellum, or basal ganglia may first select the motor program, then prepare the brainstem and spinal cord for the response, and finally initiate the muscle contraction (135). Neuronal circuits or pattern generators may exist in the brainstem or spinal cord, possibly within the reticular formation, which activate stereotyped coordinated movements (46,183). Limbic structures may have access to the pattern generators and in that fashion have some involvement in the initiation of the motor response (135). A readiness potential has been described (40,52,106,201) which is a cerebral event-related potential preceding voluntary initiated movements - 18 -in man. A slow widespread negative potential was reported to move over the posterior half of the scalp 0.7 seconds prior to the actual movement event and clearly preceded the motor potential, confined to the precentral motor area. The precentral motor cortex could not initiate meaningful movements on its own as i t lacks the sensory input from the periphery, according to Desmedt and Godaux (43). Mogenson (135) suggests that voluntary motor actions are organized and initiated in the association areas of the parietal, temporal, and occipital lobes where the various afferent data can be pooled. Patients with lesions of the association areas develop apraxia which is an inability to initiate and carry out skilled movements. If the voluntary movements are planned posteriorly in the association areas, they cannot transfer directly to the motor cortex as there are no direct connections. Their projections instead go to the premotor areas of the frontal lobe (90,148). Anatomical studies have led to the suggestion that the nucleus accumbens may be the functional link between the limbic and basal ganglia motor systems (64). It receives direct connections from the amygdala, hippocampus and other forebrain structures as well as indirect input from the ventral tegmental area and is able to send signals direct to the motor system via the globus pallidus. As a result, the nucleus accumbens has been implicated in both limbic and motor functions. At present it would be sufficient to say that the organization and initiation of voluntary movements involves several aspects of central nervous system control and the patterning of the command depends upon sensory input, essential for the updating of the motor command. - 19 -Some of the aspects of sensory feedback and central control and how they can affect the muscles of mastication have been considered above. The discussion will now examine the anatomy and function of the major masticatory muscles where this control is manifested. C. Anatomy and Function of the Major Masticatory Muscles Electromyography has been a valuable tool in exploring some of the complex synergistic and antagonistic activities that occur during specific masticatory actions. Simple mechanics can also be used to deduce important aspects relating to muscle function, with a knowledge of the muscles origins and insertions. For a better understanding of the relationship between form and function in the masticatory system, a description of the masticatory muscles, their origins and insertions, and their major functions will be presented. 1. Temporalis The temporal muscle has a broad fan-shaped origin from the lateral surface of the skull. It inserts into the coronoid process and along the anterior border of the ascending ramus. The muscle is generally described in three components due to fiber orientation, with the anterior fibers running a vertical course from just posterior to the supraorbital ridge, the middle fibers running an oblique course, and the posterior fibers running a horizontal course just superior to the external auditory meatus. - 20 -The temporal muscle innervation is via the mandibular division of the trigeminal nerve through three branches of the temporal nerve. Ramfjord and Ash (161) refer to the temporal muscle as the primary positioner of the mandible during elevation. The anterior fibers tend to be more active during elevation with a vertical component, consistent with its fiber orientation. All three components tend to be equally active upon retraction of the mandible. 2. Masseter The masseter muscle is a thick rectangular shaped muscle comprised of at least two and possibly as many as four muscle bundles (15,23). The muscle originates along the zygomatic0arch and inserts into the ramus and body of the mandible. The superficial masseter muscle is oriented obliquely in a postero-inferior direction from the anterior zygomatic arch to the angle of the mandible. The deep masseter muscle is arranged more vertically and runs from the posterior part of the zygomatic arch to the angle of the mandible. The innervation of the two bellies is via the mandibular division of the trigeminal nerve through its masseteric branches. Ramfjord and Ash (161) describe the primary action of the masseter muscles as elevation and this is demonstrated by both bellies. The superficial masseter is quite active in protraction, especially i f elevation is involved. It is also active in extreme lateral movements. The deep masseter is primarily involved in retraction of the mandible (20,65) similar to the posterior fibers of the temporal muscle. - 21 -3. Medial .Pterygoid The medial pterygoid muscle, like the masseter, is a thick retangular muscle. It originates in the pterygoid fossa and along the medial wall of the lateral pterygoid plate. It then runs inferiorly, posteriorly, and laterally to insert on the medial surface of the angle of the mandible. Its innervation is the nerve to medial pterygoid, a branch of the mandibular division of the trigeminal nerve. The medial pterygoid, with its similar fiber orientation to the masseter (15), is active in elevation and protraction as well. However, due to its laterally directed component i t is also quite active in lateral jaw position (75). 4. Lateral Pterygoid The lateral pterygoid muscle has a broad origin by two bellies. The inferior belly originates from the whole lateral aspect of the lateral pterygoid plate while the superior belly or sphenoidal head (48) takes its origin from the adjacent part of the undersurface of the skull. Both heads pass posteriorly and laterally, inserting by a strong tendon into the neck of the condyle. Some fibers also insert into the capsule while a few superior fibers, approximately 10-20%, attach to the foot of the articular disc (208). Of interest is an actual joining of superior and inferior fibers approximately 5mm from their common insertion at the condylar neck. This common insertion may be behind some of the controversial electromyography from that area. - 22 -The prinicpal function of the lateral pterygoid muscle is protraction of the condyle (62,142). It is also presumed quite active during lateral movements. There is currently a controversy concerning the actual function of the superior belly. Recent anatomical evidence has reported that only a very few fibers of the superior belly actually attach to the articular disc. This would appear to weaken its traditional role of drawing the disc forward (208). It has been suggested that the superior belly may stabilize the disc when the condyle is seated in its fossa, by applying tension to the anterior foot of the disc. 5. Di g a s t r i c As opposed to the mandibular elevator muscles the anterior belly of the digastric muscle is a mandibular depressor. It arises from the inner aspect of the mandible near the mental symphysis, and inserts as an intermediate tendon to the hyoid bone by cervical fascia. Its innervation is via the mylohyoid nerve, a branch of the mandibular division of the trigeminal nerve. The digastric muscle is involved in some mandibular reflexes and is concerned with depression of the mandible along with the lateral pterygoid and suprahyoid muscles. 6. Summary The muscle anatomy and functions have been well documented for mastication and some postural positions. However, no thorough description of the interaction of muscle groups has been compiled during - 23 -parafunctional clenching and grinding tasks. Furthermore, few studies have reported the relationship between the muscle activity observed and the particular location of the occlusal contacts employed. Since the neural controls of interocclusal force and the major muscles of mastication, the means of force production, have been discussed, a review of interocclusal force relationships would now be worthwhile. D. Interocclusal Force 1. Measurement Many studies have dealt with interocclusal (bite) force and these are well reviewed elsewhere (17,27). Carlsson (27) commented that up to 1950, at least fifty different measuring devices had been described. The devices most often used have been strain gauge transducers, either placed between the teeth, or within crowns and dentures. However, the differences in vertical dimension, the number of teeth or surface area covered, and the type of act measured are a few of the variables that have severely limited the comparison of results. The f i rst molar teeth have been found to generate the highest forces while the premolars and anterior teeth have exhibited lower levels (215). Howell and Manly (82), using an electronic strain gauge reported maximal biting force at the f irst molar teeth to vary between 91-198 lb. (41.3-89.8 kg.) and at the incisors 29-51 lb. (13.2-23.1 kg.). Lower values were obtained from children (215), denture (117) cases, and during - 24 -chewing (4) and eccentric clenching (129) in lateral, protrusive, or retrusive positions. Values were seen to increase on the preferred chewing side and with practice (215). Anatomic factors have also been implicated in one study. Half the variation in maximal bite force was explained by anatomic variations of the jaws such as the length of the mandible and the gonial angle (166). 2. Interocclusal Force and Electromyographic Activity The literature has often been confusing and contradictory concerning the relationship between electromyographic (muscle) activity and interocclusal (bite) force in the masticatory muscles during isometric contractions. The terminology has been confusing with respect to the terms bite force and muscle force which have been used interchangeably by some authors. Bite force is the value indicated on the recording instrument, measured at the teeth, and is thought to be a measure of the contributions of the jaw elevator and depressor muscles. Muscle force, on the other hand, has been used to describe the tension developed in the jaw elevator muscles in order to perform the particular task. There is also a question as to how accurately the electromyographic signal depicts the true amount of muscle activity. As more motor units are recruited in a response, the positive-and negative-going action potentials tend to cancel each other out, resulting in the interference pattern. This is claimed to result in a flattening of the response when high muscle forces are generated. - 25 -A linear relation has been described in the limb muscles between force and electromyographic activity (87,118,157,164), but until recently a similar relationship has been difficult to determine for the muscles of mastication. Numerous reports (199,200) in the dental literature have simply assumed a linear relation. Pruim (155,156) suggested that external (bite) force can only be considered representative of total isometric force when antagonistic action is taken into account. His experiments were performed at a large vertical dimension and he proposed that any alinearity observed between bite force and muscle activity may be due to an alinearity between muscle force and bite force and not necessarily due to an alinearity between muscle activity and muscle force. He concluded that there was no reason to doubt a linear relationship between muscle activity and muscle force. Other authors (3,51,127,128) have described a direct relationship between bite force and muscle activity. Carlsson (27) argues that values measured for bite force should not be directly correlated with masticatory muscle strength as force is seen to increase under the effects of local anesthesia and analgesics (200). He suggests that pain or fear of pain associated with a test is a severe limiting factor in application of muscle force. Despite these contentions, a linear relation between bite force and muscle activity would s t i l l be valid for the conscious subject with all neuromuscular pathways intact. Garrett (51) described a relationship between bite force and muscle activity that would be expected from isometric contractions. He found that increased pressure exerted on a dynamometer resulted in an increased - 26 -bite force. He concluded that this was probably due to an increase in myofiber activity resulting in an increased electromyographic activity. Manns (127,128) thorough investigations into the effects of muscle activity, bite force, and vertical dimension support this view. He described a linear relationship between bite force and muscle activity that held for small discrepancies in vertical dimension from the habitual occlusion or intercuspal position. However at vertical dimensions of 5 mm and greater, at which much of the previous works had been performed, the relationship was not seen to hold. 3. Joint Forces During clenching and chewing the masticatory muscles generate forces which are applied to the teeth and may also be transmitted to the joint. Various articles have reported soft and hard tissue deformities of the temperomandibular joint apparatus which have been correlated with aged subjects with defective occlusions due to tooth loss (76,132,147). The question has been raised whether specific occlusal conditions could generate stress to the joints that could lead to pathological change. Hylander (83) has described an anatomical basis for accepting the view that the temperomandibular joint is stress bearing. The functional stress bearing surfaces of the joint should be the posterior surface of the emminence and the antero-superior surface of the condyle. Both these areas consist of thick cancellous bone with a dense cortical plate, probably capable of bearing large amounts of reactive joint force. The portion of the disc that intervenes between the articulating surfaces in - 27 -function is avascular and lacks both nerves and synovium. The disc consists of fibrocartilage arranged in bundles of strong white fibrous tissue. It is tougher, less cellular, less homogenous, and more flexible than hyaline cartilage rendering i t more capable of bearing stress with better adaptation to tensile stress. Anatomically, i t would appear that the joint structures are well adapted to bearing stress. If this is so, what type of stresses may act at the joint? The literature contains differing analyses (12,55,77,83,145,156,180,203) of the mandible considered to act as a lever, link, beam, or other such system. Each method has been subject to a number of variables such as the tooth contact point, muscle angulation, bite force resultant angulation, cross sectional muscle bulk, and others. Refer to Hylander (83) for a review of this area. The mandible has often been classified as a Class III lever during clenching, with the resultant force being divided between the bite force at the teeth and the compressive or reactive force at'the condyle. That is , three forces are involved in the lever model: an applied force (muscle); a bite force along the tooth row, and a reactive force at the joint, where the condyle is considered to act as a fulcrum. Biting on the molars in this system has been predicted to be more efficient than incisal biting (12,77,83,145,156). Electromyographic and force studies have reported greater muscle activity and force levels to be produced when clenching on molars rather than incisors (137,215). A recent modelling system has predicted resistance forces to be greater at both the teeth and the condyle when clenching on a molar. However, proportionately the - 28 -condylar force to tooth force is greater when functioning on the incisors (145). That is , similar joint forces may be generated with less muscle and bite force when clenching on an incisor rather than a molar. This reinforces previous predictions implicating incisor biting as being more stressful to the system. The link model of the mandible is considered a special lever situation where the applied muscle force and resultant bite forces are aligned, thereby creating no reaction forces at the condyle with muscle force and bite force being equal. Gingerich (55) suggested that middle and posterior temporal may create bite force by retracting the mandible as a link because their muscle angulations allign with the tooth row. However, the other elevator muscles do not allign with the tooth row and therefore rotate the mandible about the condyle functioning as a lever. His conclusion was that due to the different muscle orientations the mandible could be considered to act as both a lever and a link. Smith (180) considered the mandible to work as a lever during elevation because the only potential area of contact was the fulcrum at the condyle. However, in a unilateral bite situation, both joint and tooth are possible areas of contact. The system could then be viewed as a beam supported at both ends. He predicted three possible points of contact during biting to be the bite point, the working-side condyle in its fossa, and the balancing-side condyle on i t emminence. The total reactive force at the condyle was predicted to be 75% of the total bite force. Also 70-80% of the total reactive force was thought to be concentrated at the balancing condyle. Hylander (83) predicted from - 29 -Moller's (137) electromyographic findings of greater muscle activity on the working side, that a resultant would be placed between the bite point and the midsagittal plane. This necessitated a reactive or compressive force to act at the balancing condyle in order for equilibrium to be established in the system (83,145,180,203). This was also thought to be a possible reason for the clinical observation that some patients with diseased joints preferentially function on the involved side as there is less loading. Nelson and Hannam (145) have reinforced this theory with their model which also predicts the balancing condyle to bear more load in a unilateral bite situation. They also commented that the balancing condyle resistance force was directed downward and back whereas the working condyle resistance force was directed downward and forward resulting in a torquing effect at the tooth contact in the horizontal plane. Thus, the literature appears to support the view that the joint is load bearing (12,55,77,83,145,156,180,203). The fact that incisor biting is theoretically more stressful than molar biting is important when considering cases of posterior bite collapse in association with recent cadaver studies (76,147). Although much work has been done on the joint force problem most of i t has assumed bilateral symmetry of muscle action and no specific associations have been revealed between the reactive joint forces and the activities of the specific muscle groups active for particular bite positions. - 30 -II. Statement of Problem The l i t e r a t u r e i s rep le te with a r t i c l e s concerned with bruxism, i t s e t i o l o g y , and treatment. The bruxing ac t , whether c lenching or g r ind ing , has been considered to involve long l a s t i n g tooth contac ts . These have been associated with symptoms in the tee th , muscles, and j o i n t s that may progress to patho log ica l changes in those areas. There i s an apparent assoc ia t ion between muscle a c t i v i t y during c lench ing or g r ind ing , and signs of s p e c i f i c muscle dysfunct ion and s t ress to the teeth and j o i n t s . However, no formal work has been conducted to study the s p e c i f i c muscle groups involved and the i r electromyographic a c t i v i t i e s during c lenching or gr inding in s p e c i f i c d i r ec t ions on pa r t i cu l a r occ lusa l contac ts . If i t could be demonstrated that pa r t i cu l a r tooth contac ts , hab i tua l l y used to generate high fo r ces , were assoc ia ted with a c t i v i t y in s p e c i f i c muscle groups, i t may help in the understanding of some of the symptomology. Moreover, i t may be used to study biomechanics when seeking explanat ions for degenerative j o i n t changes. To gain i n s i gh t in to these problems the fo l low ing questions were addressed. 1. How do s p e c i f i c jaw e levator muscle a c t i v i t i e s react to s p e c i f i c changes in tooth contact pos i t ion during c lenching? 2. How are s p e c i f i c muscle a c t i v i t i e s a f fec ted by s p e c i f i c changes in the d i r e c t i on of the appl ied e f f o r t on a p a r t i c u l a r tooth contact during clenching? - 31 -3. How do specific muscle activities respond to a change in surface area of the tooth contact during clenching? 4. What effect do cross-arch molar contacts have on the muscle activity as compared to a unilateral canine or group function clench alone? 5. Do the muscle activities vary proportionately when comparing maximal and half maximal clenching? Answers to these questions could provide useful information in the understanding of some clinical problems. - 32 -III. Methods The experiments performed were divided into three studies that were designed to examine the effects of particular tooth contacts on specific muscle groups, during various clenching tasks in human subjects. Encountering variables such as vertical dimension, lateral displacement, cuspal inclines, and directional control of a clench was conceivable in the natural situation. Therefore, two preliminary studies were designed to control these variables prior to examining the natural situation, to allow an easier interpretation of the third study. A. Study #1 - Vertical Effort Study Ten male subjects were chosen at random with respect to malocclusion and craniofacial type, the only criterion being that each subject possessed a reasonably intact dentition with good occlusal support around the arch. The ages of the sample ranged from 25 years to 44 years with a mean age of 35.4 years. 1. Occlusal Stops Casts were mounted on a Denar Mk II semi-adjustable articulator and six acrylic occlusal stops were fabricated for each subject (figure 1). The stops were approximately 1 mm in thickness, at the incisors, with the articulator opened vertically from the subjects intercuspal position. They incorporated both upper and lower teeth in the second molar and - 33 -Figure 1 Occlusal Stop Design Legend Figure 1 One molar stop is pictured on a subjects lower cast. This illustrates the size of a typical molar stop and the indentations of the opposing cusps which aid in preventing displacement during the desired clenching task. - 34 -canine areas bilaterally, with two possible anterior stops being an anterior ineisal stop and an anterior bite block which covered from canine to canine. The stop design included both upper and lower teeth to increase the stability of the stops, eliminate the effect of cuspal inclines, and prevent lateral movement, while directing forces to the teeth as axial as possible. Zinc oxide eugenol paste relines were performed prior to each recording session to enhance the stability and f i t of the stops. 2. Electromyography The action of the jaw elevator muscles cannot be measured directly and certainly cannot be appreciated from visual inspection and palpation. However, the electrical activity, produced by the summation of action potentials from numerous motor units, can be recorded electro-myographically. Surface electrodes can record from a sizeable area of muscle and therefore provide information about the whole muscle, while fine wire electrodes record activity from a small localized portion of muscle. Surface electrodes were used when possible in the present study with fine wire electrodes placed only when needed. Six muscle channels were available in the system for electromyographic recordings. In all ten subjects, bipolar surface electrodes recorded muscle activity from the anterior temporal muscle and the superficial masseter muscle bilaterally. In three of the ten subjects the posterior temporal muscle was recorded bilaterally due to patient reluctance to undergo a medial pterygoid penetration. Electrode placement - 35 -was chosen by palpation of the muscles prior to the session. Skin impedance was reduced with an alcohol scrub and the electrodes were placed paralell with the muscle fiber orientation, secured with collodion, and f i l led with Hewlett-Packard Redux-Creme electrode conducting paste. A ground electrode was secured to the right wrist. In seven subjects, electromyographic activity was measured in the left medial pterygoid muscle by means of paired, insulated, fine wire electrodes, .002" diameter. These were introduced intraorally via a 27 gauge hypodermic needle into the middle anterior portion of the muscle. The wires were then attached to the buccal mucosa of the left maxilla with adhesive wax, before being led out the corner of the mouth. Care was taken not to disturb the wire positioning during the subsequent placement of the occlusal stops. The recorded signals were amplified and filtered with 3-dB points at 60 and 350 Hz by means of an optically isolated, battery powered amplifier system. They were then sampled by the A-D converter of a disc-based computer system (Hewlett-Packard 1000 series E and peripherals). The f i l ter bandwidths were chosen carefully to reconcile the sampling rate of the computer (1 msec.) with the dominant frequency component of the interference electromyograms. - 36 -3. Tasks Each subject was instructed to perform ten vertical clenches per task, alternating between a subjective maximum comfortable clench and a subjective half maximal clench. The half maximal clench was introduced not only to allow the subject a rest but also to gather sub-maximal clenching data which may have some behavioural significance. As the occlusal stops were easy to remove and replace, a different combination was utilized after each set of ten clenches. The stop combinations that will be discussed are described in Table I. Each subject clenched for a 1.5 second period following a visual task target initiated by the operator (figure 2). Concurrent with target initiation, sampling by the computer system took place. 4. Data Handling A typical raw response for an intercuspal clench, protrusive clench and retrusive clench is pictured in figure 3. The raw data could be examined and errant clenches edited or removed i f judged to be spurious as a result of the subject not performing the clench, or although rare, i f a stop broke during the clench. Raw data in digitized form were stored for each muscle and task, and means and standard deviations were calculated over a 400 msec period about the centre of the response for each clench (Table II). The mean and standard deviation for the task as a whole could then be derived util izing the mean values of each of the five responses comprising the task (Table III). - 37 -Table I: Occlusal Stop Combinations U t i l i z e d i n Study #1 Task Stop Combination 1 natural intercuspation 2 simulated intercuspation (bilateral molars, canines and incisor stops) 3 incisor stop 4 anterior bite block 5 left canine 6 right canine 7 left molar 8 right molar 9 left group contact (left canine and molar) 10 right group contact (right canine and molar) 11 left group contact (effort on right empty side) 12 right group contact (effort on left empty side) 13 left canine with right cross-arch molar 14 right canine with left cross-arch molar 15 left group contact with right cross-arch molar 16 right group contact with left cross-arch molar - 38 -Figure 2 Experimental Design Figure 3 Raw Electromyographic Response for a Series of Clenches. Legend Figure 2 The operator controls the visual task target (lower left) which initiates the task concurrent with computer (middle) sampling. The subject clenches over a 1.5 sec period when the target bar is at its height. Bipolar surface electrodes (upper left) record muscle activity from the six muscles shown on the right and the raw data is stored as absolute counts in digitized form. An example of the raw data for one clench is shown at the right. Means and standard deviations are calculated for each muscle and clench over the 400 msec period illustrated about the centre of the response. Legend Figure 3 Typical raw electromyographic signals for a sequence of intercuspal clenches viewed on a storage oscilloscope. Muscles are LAT, left anterior temporal; LPT, left posterior temporal; LM, left superficial masseter; RM, right superficial masseter; RPT, right posterior temporal; and RAT, right anterior temporal. Calibration bars are illustrated on the lower right of the figure with the vertical bar representing 300 microvolts and the horizontal bar representing 1 sec. The vertical displacement channel was used only to identify any gross displacements and was not calibrated. - 40 -Table I I : Typical Computer Data f o r a Single Clench Channel Numbers Run 0 1 2 3 4 5 1 1 1 8 ( 4 7 . 7 ) 1 0 5 ( 4 6 . 0 ) 1 3 5 ( 6 1 . 0 ) 1 6 5 ( 5 8 . 4 ) 9 4 ( 4 6 . 6 ) 1 1 4 ( 5 7 . 3 ) Legend, Table II T y p i c a l d a t a f o r one i n d i v i d u a l ' s i n t e r c u s p a l c l e n c h c a l c u l a t e d o v e r t h e c e n t r a l 400 msec o f t h e r e s p o n s e . Computer c o u n t s and s t a n d a r d d e v i a t i o n . a r e shown f o r each m u s c l e under i t s channe l number o f 0 - 5 . Channe l ( 0 ) , l e f t . a n t e r i o r t e m p o r a l (1) l e f t p o s t e r i o r t e m p o r a l (2) l e f t s u p e r f i c i a l m a s s e t e r (3 ) r i g h t s u p e r f i c i a l m a s s e t e r (4 ) r i g h t p o s t e r i o r t e m p o r a l and (5 ) r i g h t a n t e r i o r t e m p o r a l m u s c l e . - 41 -Table III: Typical Computer Data for One Task Channel Numbers 0 1 2 3 4 5 means 119.46 107.77 134.30 158.07 90.80 108.20 5.0 7.97 7.14 10.34 17.65 7.34 11.51 Legend, Table III Typical data for the same individual as Table II representing the meaned computer counts and the standard deviation for the five intercuspal clenches comprising the task. Muscle channels are the same as Table II. - 42 -a) Normalization Histograms of the meaned data, for each individual, revealed a strong tendency for the group to respond similarly to the different tooth contact combinations. As a result, the data for each subject were f irst normalized for the series of tasks to their greatest individual task or peak response. That is , each individual's greatest task would be graded 100% normalized activity, and all other tasks performed by that subject would be expressed as a percentage of that peak task. The normalized data were were then grouped and comparisons made by paired t tests to describe the particular trends in muscle activity between specific tasks. Histograms with means and standard deviations of the group normalized data were then compiled for the series of tasks. B. Study #2 - Eccentric Effort Study A second study utilizing occlusal stops was designed to examine the effects of directional efforts on the muscle groups. Ten male subjects again took part, nine of whom had participated in the f i rst study. (The ages of the sample ranged from 25 years to 42 years with a mean age of 33.5 years. The methods were essentially the same as the f i rst study apart from the following. 1. Occlusal Stops The occlusal stop fabrication and design was as described in the f i rst study. However, combinations of only three stops, the left and right molar stops and the left canine stop, were utilized. - 43 -2. Electromyography Six muscle channels were again available for electromyographic recordings. The anterior temporal muscles, both left and right, the left posterior temporal muscle, and the superficial masseter muscles of both sides were recorded on all ten subjects. The remaining channel was reserved for the left medial pterygoid muscle. However, due to patient reluctance in five cases, only five medial pterygoid penetrations were performed and the right posterior temporal muscle was substituted in five cases. 3. Tasks Each subject was instructed to perform five subjective maximum comfortable clenches per task. The subject was also instructed to clench either vertically, left , right, protrusively, or retrusively dependent on the contact combination employed. In combinations involving the canine contact, only vertical and lateral efforts to the side of the canine were considered relevant while on molar contacts vertical, left , right, protrusive, and retrusive efforts were all considered relevant to clinical situations. The stop design stabilized the jaw and essentially prevented any lateral movement during the eccentric efforts from the opened intercuspal position. The tasks are summarized as for stop combination and direction of effort in Table IV. 4. Data Handling The computer sampling, editing, normalization, and other handling of data were consistent with the f i rst study. - 44 -Table IV: Occlusal Stop Combinations in Study #2 Task Stop Combination Direction of Effort 1 natural intercuspation vertical 2 left molar vertical 3 left molar ipsilateral (left) 4 left molar contralateral (right) 5 left molar protrusive 6 left molar retrusive 7 left canine vertical 8 left canine ipsilateral (left) 9 left group contact vertical 10 left group contact ipsilateral (left) 11 left canine, right molar vertical 12 left canine, right molar ipsilateral (left) 13 left group contact, right molar vertical 14 left group contact, right molar ipsilateral (left) 15 bilateral molars vertical 16 bilateral molars left 17 bilateral molars ri ght 18 bilateral molars protrusive 19 bilateral molars retrusi ve - 45 -C. Study #3 - Natural Tooth Study The third study in this series was performed on natural tooth contacts. All ten subjects returned from the second study but as some of them were reluctant to undergo another medial pterygoid penetration an additional ten subjects were selected at random for malocclusion and craniofacial type. This resulted in twenty participants all of whom possessed reasonably intact dentitions with good occlusal support around the arch. The sample consisted of eighteen males and two females with an age range of 23 years to 42 years and a mean of 30.8 years. 1. Electromyography Six muscle channels were again available for electromyography and the anterior temporal muscles, both left and right, the left posterior temporal muscle, and the superficial masseter muscle on both sides, were recorded from all twenty subjects. Nine medial pterygoid penetrations were performed and the right posterior temporal muscle was recorded in the eleven subjects reluctant to undergo the penetration procedure. 2. Tasks Each subject was instructed to perform five subjective maximum comfortable clenches per task. The subject was instructed to position his lower teeth as comfortably as possible on the lingual inclines of upper buccal cusps, for the particular task, and clench with a subjective vertical and presumed lateral component of effort, without slipping on the - 46 -incline. The subjects attempted to clench unilaterally on canine and group function contacts. Group function was considered to be any number of posterior contacts in addition to the canine contact on the working or clenching side. They were,..also?asked to attempt canine and group function contacts with cross arch molar contact. The teeth in contact were checked both visually and with articulating paper to confirm the particular contact combination. Four additional tasks were attempted in comparison to a baseline intercuspal vertical clench. These were intercuspal clenches with left , right, protrusive and retrusive effort, and with as l i t t le eccentric movement of the jaw as subjectively possible. The tasks were limited by each subjects own occlusal constraints. That is , not all twenty subjects were able to perform each task. The tasks are summarized for contact combination and direction of effort in Table V. 3. Data Handling The computer sampling and data analysis were similar to the previous two studies which involved editing of spurious clenches, normalization to each subject's peak response for each muscle for the series of tasks, followed by analysis by the paired t test and calculation of group means and standard deviations for each task. - 47 -Table V: Occlusal Contact Combinations in Study #3 Task Contact Combination Direction of Effort 1 natural intercuspat ion ve r t i ca l 2 natural intercuspat ion prot rus ive 3 natural in tercuspat ion re t rus i ve 4 natural intercuspat ion l e f t 5 natural in tercuspat ion r i g h t 6 i n c i so r s 7 l e f t canine 8 r i gh t canine 9 l e f t group funct ion 10 r i gh t group funct ion 11 l e f t group funct ion with r i g h t cross-arch molar 12 r i g h t group funct ion with l e f t cross-arch molar - 48 -IV. RESULTS A. Study #1 - Vertical Effort Study It was evident in the f irst study, despite marked individual variations, that the subjects performed the tasks uniformly, with respect to muscle function for a given occlusal contact combination. A typical example of the data for one subject's left superficial masseter muscle is displayed in figure 4. The same trends are observed in figure 5 which exhibits the group normalized activity for the left superficial masseter muscle. 1. Intercuspal and Simulated Intercuspal Position The natural intercuspal position was not used for comparisons in the above figures as, (with one exception), its data were found to be insignificantly different in normalized activity from that in the simulated intercuspal position (figure 6).* The posterior temporal muscle was the exception as i t was found to decrease significantly in activity with the simulated task (Table VI, Appendix A). Despite this, the simulated intercuspal task was chosen for comparisons with the other tasks utilizing occlusal stops. *An explanatory key for the histograms to follow should be viewed on page 51 before proceeding to the figures. - 49 -Figure 4 Effects of Seven Different Tooth Contact Combinations on the Response of One Individual's Left Superficial Masseter Muscle. Figure 5 Effects of Seven Different Tooth Contact Combinations on the Normalized Muscle Activity of the Left Superficial Masseter Muscle for the Group. Legend Figure 4 Mean masseteric muscle activity in mV is plotted against the task performed. The tasks are illustrated to show the stops used (solid circles). The simulated intercuspal position (1) approximates the peak activity exhibited by group contact with cross-arch molar (7). Incisal (2) and canine (3) contacts exhibit less activity than any of the other combinations, containing molar contacts. Canine alone (3) and group contact alone (6) show less activity than their corresponding cross-arch combinations, (5) and (7). Bars represent one standard deviation. Legend Figure 3 Percent maximum (normalized) masseter muscle activity plotted against the task performed for all ten subjects. The mean data for each subject is displayed in a separtate histogram according to task. Similar tendencies to Figure 4 are evident for the group. Though the data are consistent in general, marked individual variations can be observed. Missing histograms represent occasions where data was unavailable such as when a stop was fractured or the subject did not perform that specific task. - 51 -LEFT SIDE RIGHT SIDE MUSCLES MUSCLES A MEDIAL PTERYGOID B POSTERIOR TEMPORAL C ANTERIOR TEMPORAL D MASSETER Histogram Key All figures to follow are histograms that conform to the format of this figure. The mean normalized EMG for the grouped data are always shown coded for each muscle according to the key. In the figures to follow, bars representing one standard deviation are included in each histogram. Statistical comparisons of the data for each figure are presented in Appendix A and B and referred to in the text. Descriptions of the tasks and interpretations of the figures are also found in the text. - 52 -Figure 6 Comparison of Normalized Activity Between Clenches in a Natural Intercuspal Position and a Simulted Intercuspal Position 100 UJ Q 50 UJ N _J < CC o 0 ANTERIOR INCISAL BITE BLOCK STOP Figure 7 Comparison of Normalized Muscle Activity Between Vertical Clenches on an Anterior Bite Block and an Incisal Stop - 53 -2. Anterior Bite Block The anterior bite block, with coverage from canine to canine, when compared to the incisal stop, was associated with significantly greater normalized muscle activity in all muscles except the medial pterygoid, which maintained high activity in both tasks (figure 7). The activity produced by the bite block task was seen to approach and even surpass the activity of some of the molar combinations (Table VII, Appendix A). 3. Antero-Posterior Relationships The least muscle activity was observed during the incisal clench, followed by the canine clench, with the greatest activity along the arch exhibited by the molar clench (Table VIII, Appendix A). However, the difference in normalized activity between incisor and canine clenches was significant for the ipsilateral temporal muscles only. In contrast, all the muscles recorded, both ipsilateral and contralateral, showed significant increases during a molar clench as compared to an incisor or canine clench for left side contacts while the contralateral masseters and pterygoids were non-significant on vertical clenching efforts on the right side (figure 8). 4. Cross-Arch Contact Relationships The canine with cross-arch molar contact was seen to produce significantly greater activity in all the muscles recorded, than simply the canine clench alone. Group contact with cross-arch molar contact was seen to cause a significant increase in the activity of both masseter - 54 -Figure 8 Comparison of Normalized Activity Between Anterior and Posterior Contact Positions Vertical Clenches on - 55 -muscles as well as the temporal muscles ipsilateral to the cross-arch contact, when compared to group contact alone (figure 9 and Table IX, Appendix A). 5. Ipsilateral and Contralateral Contact Relationships Comparisons were also made to determine the relationship between ipsilateral and contralateral muscle sensitivity to unilateral stop combinations (Table X, Appendix A). The temporal muscles increased significantly in activity when contact was ipsilateral on a canine or group contact combination while medial pterygoid and masseter muscles did not. In contrast the medial pterygoid and masseter muscles increased significantly in activity on an ipsilateral molar contact while the other muscles did not (figure 10). 6. Empty Side Relationship The tasks performed when the subject attempted to close on the empty side exhibited the greatest variation in the series (figure 11). Left group contact compared to left group contact with attempted effort on the right side displayed a significant decrease in the muscles ipsilateral to the contact with insignificant changes to the contralateral muscles on the empty side. However, when the similar task was attempted with the stops on the right side and the subjects effort on the left , significant decreases in activity were seen for the masseter and anterior temporal muscle ipsilateral to the contact while a nonsignificant decrease was observed in the contralateral medial pterygoid muscle on the empty side. The remaining muscles showed insignificant changes (Table XI, Appendix A). - 56 -gure 9 Comparison of Normalized Muscle Activity Between Vertical Clenches on Unilateral Contacts and the Corresponding Cross-Arch Contact Combination - 57 -Figure 10 Comparison of Normalized Muscle Activity Between Vertical Clenches on ipsilateral and Contralateral Contacts - 58 -100-0 ^ UJ Q LU N cc o 50-^ 0 -I LEFT SIDE VERTICAL FORCE LEFT SIDE RIGHT FORCE 100-0 UJ 3 50-N _ l < CC o RIGHT SIDE VERTICAL FORCE RIGHT SIDE LEFT FORCE F i g u r e 11 Compa r i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between U n i l a t e r a l Group C o n t a c t and E f f o r t on t h e Empty S i d e - 59 -7. Half Maximal Clenching Relationships The comparison of half maximal clenching activity to^that of maximal clenching is summarized in Table XII, Appendix A. Except for some minor differences, i f the maximum tasks differed significantly so did the half maximal tasks. The two varied l i t t le for the number of combinations, and even then only in magnitude. It would be sufficient to say that proportionately the muscle activity was similar between half maximal and maximal clenching data but the maximal clenching data was much more consistent, with less variance. B. Study #2 - Eccentric Effort Study It was evident in the second study, despite some individual variations, that when the subjects changed the direction of applied effort to a particular contact, the group performance was quite homogenous with respect to muscle function for a given directional task. 1. Vertical-Lateral Relationships Vertical clenching efforts were found to be significantly greater in activity than an applied lateral effort towards the ipsilateral side of the contact (figure 12). Most comparisons exhibited a significant decrease in the activity of all the muscles except for the ipsilateral posterior temporal muscle (Table XIII, Appendix A). The ipsilateral anterior temporal muscle and the contralateral posterior temporal muscle were seen to decrease in activity, but insignificantly when comparing a vertical group contact clench to a lateral effort. - 60 -100-r | 1 l 1 l IT LEFT SIDE LATERAL CLENCH CANINE MOLAR CANINE -(-MOLAR F i g u r e 12 Compa r i son o f N o r m a l i z e d M u s c l e and L a t e r a l C l e n c h i n g E f f o r t s A c t i v i t y Between V e r t i c a l - 61 -2. Eccentric Efforts from Molar Contacts a) Vertical - Lateral Vertical efforts on both a unilateral or bilateral molar contacts exhibited significantly greater activity than lateral efforts to either side for the masseter muscles bilaterally the temporal muscles contralateral to the direction of effort and the medial pterygoid muscle ipsilateral to the direction of effort (figure 13 and 14). The medial pterygoid muscle contralateral to the direction of effort did not change significantly. The temporal muscles ipsilateral to the direction of effort showed some variation with the posterior temporal insignificant on all counts while the anterior temporal significantly decreased for one direction and not the other (Table XIV, Appendix A). b) Lateral Relationships When lateral efforts to opposite sides were compared for the same molar contacts (Table XV, Appendix A), the medial pterygoid muscle contralateral to the effort and the temporal muscles ipsilateral to the effort were seen to increase significantly. The masseter muscle contralateral to the effort did not change significantly. The masseter muscle ipsilateral to the effort and the contralateral temporal muscles showed significant decreases in activity. The only exception was the medial pterygoid muscle which did not change significantly in the unilateral molar combination (figure 13 and 14). - 62 -lOO - r LEFT MOLAR VERTICAL LEFT RIGHT 100-r L E F T MOLAR PROTRUSIVE RETRUSIVE Figure 13 Comparison of and Eccentric Normalized Efforts on Muscle A c t i v i t y Between Ve r t i ca l Un i l a te ra l Molar Contacts - 63 -F i g u r e 14 Compar i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between V e r t i c a l and E c c e n t r i c E f f o r t s on B i l a t e r a l M o l a r C o n t a c t s - 64 -c) Vertical-Protrusive Relationships When a protrusive effort was applied to the same molar contacts all muscles exhibited a significant decrease in activity compared to a vertical effort (figure 13 and 14). Activity was seen to decrease, but insignificantly for the left medial pterygoid muscle with bilateral molar contact (Table XIV, Appendix A). d) Vertical-Retrusive Relationships A retrusive effort on the unilateral or bilateral molar contacts when compared to a vertical clench (Table XIV, Appendix A), resulted in significant decreases in activity for the pterygoid, masseter, and anterior temporal muscles while the posterior temporal muscles did not change significantly (figure 13 and 14). e) Protrusive-Retrusive Relationships A retrusive effort on the molar contacts when compared to a protrusive effort (Table XVI, Appendix A), resulted in a significant increase in activity in the temporal muscles bilaterally while a significant decrease in activity was observed in the medial pterygoid and masseter muscles (figure 13 and 14). C. Study #3 - Natural Tooth Study The third study, performed on natural teeth consisted of tasks similar to those employed in the f irst two studies. Despite some marked - 65 -individual variations, i t was again observed that the group performed very uniformly with respect to muscle function for the given occlusal contact combination. 1. Antero-Posterior Relationships A clench on a canine contact when compared to an incisal clench (Table XVII, Appendix A), resulted in a significant increase in muscle activity in the ipsilateral temporal muscles. A significant decrease in activity occurred in the ipsilateral medial pterygoid and the masseter muscles bilaterally. No signficant change was noted for the contralateral medial pterygoid and temporal muscles (figure 15). Single molar contact, used for antero-posterior comparisons in the f i rst two studies, was unobtainable on the natural dentitions of the subjects. Group function was selected instead to compare to anterior contact (figure 15). As compared to an incisal clench the group function clench was seen to result in a significant increase in muscle activity in the ipsilateral temporal muscles. Significant decreases were observed in the ipsilateral medial pterygoid and masseter muscles. No other significant changes were seen (Table XVII, Appendix A). The results were not as uniform when comparing a canine clench to a group function clench (Table XVII, Appendix A). Again the ipsilateral temporal muscles were seen to increase significantly. The contralateral masseter muscle was also seen to increase but on one side only. The same occurred for the posterior temporal muscle. No other significant changes were seen. - 66 -100-r INCISOR CLENCH LEFT SIDE CANINE LEFT SIDE GROUP FUNCTION 100-r RIGHT SIDE RIGHT SIDE CANINE GROUP FUNCTION Figure 15 Comparison of Normalized Muscle Activity Between Clenches on Anterior and Posterior Contacts on Natural Teeth - 67 -2. Cross-Arch Relationships The group on the whole could not produce a canine with cross arch molar contact so the only cross-arch comparison performed was that of group function with cross arch molar contact as compared to group function alone (Table XVIII, Appendix A). The only significant change observed was an increase in the masseter muscle ipsilateral to the cross arch contact. This occurred on one side only while the other side exhibited an increase that was insignificant (figure 16). 3. Ipsi1ateral-Contra!ateral Relationships When an ipsilateral canine clench and a contralateral canine clench were compared (Table XVIII, Appendix A), the ipsilateral clench resulted in a significant increase in muscle activity in the contralateral medial pterygoid and masseter muscles and the ipsilateral temporal muscles (figure 15). The ipsilateral medial pterygoid and masseter muscles and the contralateral temporal muscles significantly decreased in activity. Similar results were obtained when both group function and group function with cross-arch molar contact were compared ipsilateral to contralateral contact. The only exceptions were the masseter muscles which did not always show significant changes (Table XIX, Appendix A). - 68 -Figure 16 Comparison of Normalized Muscle Activity Between Clenches on Unilateral Contact and the Corresponding Cross-Arch Contact Combination on Natural Teeth - 69 -4. Intercuspal Relationships a) Vertical-Eccentric Relationships The i n t e r c u s p a l p o s i t i o n was compared t o t h e i n t e r c u s p a l p o s i t i o n w i t h e f f o r t e x e r t e d p r o t r u s i v e l y , r e t r u s i v e l y and l e f t and r i g h t ( T a b l e XX , A p p e n d i x A ) . In a l m o s t e v e r y i n s t a n c e t h e i n t e r c u s p a l p o s i t i o n e x h i b i t e d s i g n i f i c a n t l y g r e a t e r musc l e a c t i v i t y than the e c c e n t r i c c l e n c h e s ( f i g u r e 1 7 ) . The o n l y e x c e p t i o n s were t h e m e d i a l p t e r y g o i d m u s c l e i n p r o t r u s i v e and c o n t r a l a t e r a l e f f o r t s wh i ch d i d no t change s i g n i f i c a n t l y and t h e l e f t p o s t e r i o r t e m p o r a l m u s c l e i n r e t r u s i v e and i p s i l a t e r a l e f f o r t s wh i ch were a l s o i n s i g n i f i c a n t l y d i f f e r e n t . The i n t e r c u s p a l p o s i t i o n was a l s o compared t o a l l o t h e r t a s k s . I t was f o u n d t h a t f o r a l l t h e m u s c l e s r e c o r d e d , m u s c l e a c t i v i t y was a lways s i g n i f i c a n t l y g r e a t e r i n t h e i n t e r c u s p a l p o s i t i o n t h a n f o r any o t h e r c o m b i n a t i o n . However t h e med i a l p t e r y g o i d was an e x c e p t i o n as i t d i d no t e x h i b i t s i g n i f i c a n t changes d u r i n g i n c i s a l , p r o t r u s i v e , and c o n t r a l a t e r a l e f f o r t s . b) Lateral Relationships The l a t e r a l e f f o r t s f rom t h e i n t e r c u s p a l p o s i t i o n were compared i n T a b l e XX , A p p e n d i x A . The m e d i a l p t e r y g o i d and m a s s e t e r m u s c l e s c o n t r a l a t e r a l t o t h e e f f o r t i n c r e a s e d s i g n i f i c a n t l y as d i d t h e t e m p o r a l m u s c l e s i p s i l a t e r a l t o the e f f o r t . The i p s i l a t e r a l p t e r y g o i d and m a s s e t e r and t h e t e m p o r a l m u s c l e s c o n t r a l a t e r a l t o t h e e f f o r t d e c r e a s e d s i g n i f i c a n t l y i n m u s c l e a c t i v i t y ( f i g u r e 1 7 ) . - 70 -INTERCUSPAL POSITION VERTICAL PROTRUSIVE RETRUSIVE Figure 17 Comparison of Normalized Muscle Activity Between Intercuspal Clenching Efforts on Natural Teeth - 71 -c) Protrusive-Retrusive Relationships The intercuspal protrusive and retrusive tasks were also compared (Table XX, Appendix A). When the retrusive effort was compared to the protrusive effort a significant increase in muscle activity was observed in the temporal muscles bilaterally while a significant decrease was noted in the medial pterygoid and masseter muscles bilaterally (figure 17) for the retrusive effort. - 72 -V. Discussion A. Discussion of Methods 1. Occlusal Stop Design The design included both upper and lower teeth in each stop to prevent eccentric movements of the jaw. Good stability was achieved and the design allowed the subject support to apply effort in any direction desired. The only control, however, was the operator's command to the subject to clench in a particular direction. Discrepancies in the direction of applied effort may have resulted in some of the individual variations encountered. Nevertheless, the subject response was quite uniform overall. This problem could be managed in future by use of a three dimensional force transducer which could quantify the force and direction of the applied effort and possibly explain some of the individual differences observed in behaviour. Another problem with the stop design, although it occurred quite infrequently, was the possibility of fracture. Although duplicates were available or could be fashioned quite satisfactorily intraorally, the fact that a fracture had occurred could have biased the subjects applied effort from that point on. The use of cast metal stops in future could solve this problem. The occlusal stops, by nature of their design, introduced the variable of vertical dimension into the study. Manns (127,128), from a set of thorough experiments, described electromyographic activity and bite - 73 -force to be l i n e a r l y related at small discrepancies in the v e r t i c a l dimension of occlusion. In the present experiments, some increase in the v e r t i c a l dimension was required to i s o l a t e s p e c i f i c tooth contact combinations. It would appear, according to Manns, that the minimal a l t e r a t i o n caused by the stops should have had no s i g n i f i c a n t e f f e c t on the muscle a c t i v i t y . 2. Jaw Position In the natural tooth study, minor displacement of the jaws was required to create c e r t a i n tooth contacts. This was monitored v i s u a l l y and with a r t i c u l a t i n g paper. Positioning of the teeth on cuspal i n c l i n e s varied between subjects and presumably in the same subject between clenches. In future experiments t h i s could be c o n t r o l l e d by using standardized castings that would allow contact in only c e r t a i n jaw p o s i t i o n s . However, i t was f e l t that such an expensive procedure was unwarranted as the discrepancy at the teeth was small, probably even smaller at the muscles, and therefore not a s i g n i f i c a n t factor in most cases. The s i m i l a r i t i e s demonstrated between the eccentric e f f o r t study and the natural tooth study imply that i t i s the contact, contact area, and d i r e c t i o n of e f f o r t that are the most important variables and not the small displacements of the mandible that occurred in t h i s study. However, further studies would be necessary to determine the e f f e c t of displacement as an independent v a r i a b l e . - 74 -3. Random Subject Group In the natural tooth study some subjects had difficulty performing certain tasks while others were unable to perform some tasks at all due to the physical constraints of their own occlusions. In future this problem could be solved by selecting subgroups for specific task comparisons. However, this would require much greater sample sizes to allow the subsets to be arranged. Despite the random selection of subjects and some of the difficulties encountered the results were surprisingly similar for the group as a whole. In the same sense, craniofacial morphology was another variable not specifically controlled, as the study of a heterogenous group response to particular tasks was desired. However, control of this variable in the future may help in the understanding of some of the individual differences observed. 4. Fati gue Fatigue could certainly be considered an important variable in the f i rst and second studies, with the number of clenching tasks involved. However, i t appears that enough rest was supplied between clenches and tasks as the subjects were found to perform as well or better at the end of the experiment in similar tasks. For that reason the effects of fatigue appear to be negligible. The randomized order of the stop positions may also have eliminated any specific trends due to fatique even i f i t were present. - 75 -5. Practice The effect of practice could also be considered a variable. If this were true, one would expect to see greater electromyographic signals as the experiment unfolded. However, base line intercuspal clenches, generally performed f i rst , were usually found to exhibit the highest activities. Also, the fact that the experimental sequence was randomly altered and the same trends were evident among tasks suggests that the effects of practice could also be considered negligible. B. Discussion of Data 1. Intercuspal and Simulated Intercuspal Position The results have shown that for minimal bite opening from the intercuspal position, in healthy subjects, the muscle activity in most elevator muscles generally remains similar to that during natural intercuspal clenching. The exception was the posterior temporal muscle which was seen to decrease in the simulated task. The findings agree with previous research (28,31,99,197) which utilized bite splints and found similar muscle activities between clenching on splints and on natural teeth. An explanation for the decreased posterior temporal activity could be that the enveloping nature of the occlusal stop design provided an excellent platform for a more anteriorly directed force, otherwise denied by the natural dentition. A slight anterior vector for force production is unsuitable for posterior temporal activity due to its muscle fiber - 76 -orientation that favours jaw retraction. This reduced response in the posterior temporal muscle however, was not considered a problem in accepting the stops as an experimental model of the natural dentition. 2. Antero-Posterior Location of Bite Point The antero-posterior position of tooth contact was seen to have an effect on the specific muscles involved in particular clenching tasks. Greater activity was observed with posterior contacts and this finding is in agreement with previous measurements of bite force (215) under similar conditions. Assuming that the electromyographic activity (muscle force) and bite force are linearly related at small vertical openings in an isometric situation (51,127,128), this result would be expected. It has been suggested that the biomechanics are better suited to create greater muscle activity and bite force posteriorly in the arch (156,187). Periodontal mechanoreceptor feedback may also be involved, as a greater density of mechanoreceptors, with lower stimulus thresholds are located anteriorly (41,74). As mechanoreceptors have been implicated in inhibitory feedback mechanisms to the jaw elevator muscles (100,200), greater inhibition and subsequent lower muscle activity would be expected from anterior bite points given sizeable bite forces and lower thresholds anteriorly. A difference was observed between the muscle activity during vertical clenching in the f irst study and the activity during eccentric and natural tooth clenching of the latter two studies. Although the same antero-posterior trends were evident, the latter two studies exhibited the - 77 -majority of their activity in the ipsilateral temporal and contralateral pterygoid muscles, whereas the vertical effort study demonstrated higher levels of activity in all muscles recorded. A different mechanism of control must be at work in the latter two studies as only the ipsilateral temporal muscles were observed to increase consistently as the bite point moved posteriorly along the arch; the other muscles either decreased in activity or did not differ significantly. Periodontal mechanoreceptor innervation is apparently greater in the buccal periodontal ligament of some species (26) and may be stimualted during lateral clenching efforts, resulting in feedback inhibition to the jaw elevator muscles. However, i f this was to take place, one would expect the ipsilateral temporal muscles to be affected as well, and not exhibit a significant increase in activity. Temperomandibular joint receptors could possibly play a role by either registering changes in condylar position or stresses at the joint, resulting in feedback inhibition of the jaw elevator muscles. Again, this would not explain the increase of activity observed in the ipsilateral temporal muscles. Hellsing's (79) suggestion that in maximal voluntary efforts in trained and prepatterned motor acts the motor activity may function without involvement of the peripheral feedback mechanisms may offer an explanation here. If in fact the clenching task was prepatterned, previous experience based on peripheral feedback may have patterned specific muscles for optimal action with the least potential for damage in a particular position or effort. - 78 -3. Cross-Arch Contacts The results of the vertical effort study with addition of cross-arch contacts indicated that cross-arch combinations were associated with a generalized increase in muscle activity. In fact, the activity produced in the cross-arch task was similar to that produced in the simulated intercuspal position. These observations are consistent with the findings of Wood and Tobias (214) who described an insignificant 2% decrease in total muscle activity when they compared maximum vertical clenching on an equilibrated bite plane to clenching on the bite plane with one side of contact removed except for the second molar. In the latter two studies only the masseter muscle ipsilateral to the cross-arch contact exhibited a slight but significant increase in activity while the other muscles were not seen to differ. It appeared that in the vertical study on. occlusal stops, the cross-arch contacts acted in a supporting or bracing fashion for their unilateral counterparts and allowed muscle activity to increase bilaterally during clenching. In the natural tooth situation the apparent lateral effort involved in cuspal stabilization appeared to severely limit the effort applied. This effort was confined primarily to the ipsilateral temporal and contralateral pterygoid muscle. 4. Eccentric Contacts and Direction of Effort The direction of effort applied to a particular contact is clearly an important factor in determining the muscle groups involved and their particular proportions. The eccentric effort tasks in intercuspation, in - 79 -the natural tooth study, varied significantly with the direction of effort applied though the position remained constant. In order for the subject to stabilize functional cusps on inclines in eccentric positions without slipping, i t was expected that similar muscle activity to that seen in the lateral efforts of the second study would be required. This assumption proved correct as the values of lateral efforts in figure 12 are nearly identical to natural tooth clenching of figure 15 for similar contacts. In the lateral clenching efforts of the second study and the unilateral natural clenching efforts of the third study the most active muscles were the ipsilateral temporal, contralateral medial pterygoid, and presumably the contralateral lateral pterygoid muscles (208,213). Only minor contributions were observed from the masseters and contralateral temporal muscles. These findings are in general agreement with previous works (65,69,191,202,213) that included some eccentric clenching acts. This sidedness effect was masked in the vertical unilateral clenching of the f irst study presumably due to the increased activity observed in all the muscles. This difference may have simply been due to the direction of effort being more vertically oriented. The temporal muscles, especially the posterior temporals, were most sensitive to retrusive clenching which was consistent with their fiber orientation. This would agree with previous findings (21,65). Recently the deep masseter muscle has been described to act in a similar fashion during retrusive acts (20,65). The medial pterygoid and masseter muscles were found quite sensitive to protrusive and incisal clenching. This could be inferred from their - 80 -fiber orientations and previous literature concerned with similar clenching acts (65,191). The masseter muscle appears to function optimally only in the intercuspal position, moderately in the intercuspal protrusive and incisal clenches, and only in a minor capacity in lateral efforts or lateral and retrusive positions. The activity in the protrusive and incisal clenches may have been affected by periodontal feedback from the anterior region. Medial pterygoid activity remains consistently high in both cases. 5. The Number and Site of Tooth Contacts Moller (137) suggested that jaw muscle stimulation was dependent on the number of occlusal contacts while Tallgren (191) concluded that an optimal antero-posterior jaw relationship rather than the number of contacts may be important. Compared to the incisor and canine stops, clenching on the large anterior bite block resulted in a generalized increase in muscle activity. From this i t could be concluded that the number of occlusal contacts and the surface area of contact were important. The results were far from consistent when comparing increases in numbers of contacts posteriorly. When muscle activity was compared between a clench on the anterior bite block, involving twelve anterior teeth, and a clench on a unilateral molar stop, which included two posterior teeth, no significant differences were observed. However, as the number of contacts increased in the posterior combinations, the anterior bite block clenches were seen to decrease significantly in comparison. It appears then, that an increase in the - 81 -number of contacts, and probably therefore, the contact surface area is of importance anteriorly. However, posteriorly the muscles reach near optimal levels earlier with fewer contacts. Therefore, the changes do not appear as great or significant to increased number and area of contacts posteriorly. A particular clinical simulation with relevance to the number and site of tooth contacts is the bilateral molar clench. The normalized muscle activity proportions were similar to the unilateral molar bite in all directions of effort and exhibited higher amplitudes. Although the levels of activity were generally slightly lower than the intercuspal clench for vertical and eccentric efforts, not all the values were significantly different. This suggests in a clinical case with partial posterior tooth loss, that although the site and area of contact may be decreased, similar muscle responses may be produced given the same muscle bulk. Similar forces produced on fewer contacts may thus predispose the patient to localized dental pathosis. The fact that greater joint forces are theoretically produced by molar biting (145) and that anterior open bite patients function only on their posterior teeth therefore may predispose them to joint pathosis. Due to the constant function on posterior teeth with no distribution of load to the anteriors the dental destruction of these teeth would be expected and can be seen in adult open bite cases (179). Morphologic and electromyographic studies have determined a correlation that a subject exhibiting a large gonial angle, such as may be found in open bite cases, cannot produce as great electromyographic signals as a subject exhibiting more rectangular features, with parallel - 82 -jaws, a small gonial angle, and diminished lower face height (84,137). The skeletal features combined with the nature or angle of contact may not be favourable for the development of muscle bulk required to generate heavy bite force. Although the forces may not be as great, they must be at a sufficiently elevated level over a certain length of time to result in pathology. However, such speculation requires further experimentation, including for example control over such variables as craniofacial morphology, bite point, electromyographic responses, and bite force. 6. Empty Side Efforts The empty side task is analagous to a rare clinical situation where the patient presents with a unilateral bridge in hyper-occlusion, with sensitive teeth, and attempting to function on the empty side. The subjects found these tasks difficult to perform in general and that may have been the reason for the larger variation observed in the results. When comparing a unilateral group contact situation with effort on the same side to effort on the empty side, the muscles ipsilateral to the contact were seen to decrease in activity. This may have some significance to joint loading for current biomechanical modelling studies (145) suggest that the balancing condyle assumes greater reactive forces during a normal unilateral clenching situation. 7. Joint Loading When considering joint loading during clenching, i t is difficult at this stage to speculate upon the clincal significance of cross-arch - 83 -contacts. Further information, using the type of data obtained in this study, is needed from biomechanical modelling systems to help solve this problem. Subjects with posterior bite collapse who function only on anterior teeth may be predisposed to specific kinds of joint loading. Incisal clenching has been assumed to create proportionately more reactive force at the joint than molar contact (145). The present experiments have shown that the greater area covered by the anterior bite block was associated with greater muscle activity in general. Posterior bite collapse cases generally function on a greater number of anterior teeth. Here, greater reactive forces are probably produced at the joints as compared to clenches limited to the incisors alone. This may render these patients more susceptible to tissue derangements of the joint area. C. Parafunctional Considerations Clenching is considered parafunctional i f i t involves long lasting tooth contacts when the subject is neither chewing nor swallowing. The most common symptoms of parafunctional behaviour are said to be related to muscle hyperactivity. It can be assumed that the muscles exhibiting the greatest activities during particular clenching tasks would be the most susceptible to developing tenderness from muscle hyperactivity. From the present study i t seems that when a subject clenches maximally on his natural teeth, in a vertical direction in the intercuspal position, all the muscles will respond at or near their optimum levels. - 84 -However if the subject is to change the direction of effort of the clench, the active muscle combinations will be dependent on the specific direction of the effort. If the clench is performed in a protrusive direction from the intercuspal position, the masseters and pterygoids will be most active. If the clench is directed retrusively, the temporal muscles, especially the posterior fibers will be most active. If a lateral clench is attempted from the intercuspal position, or on any combination of cuspal inclines, the ipsilateral temporal and contralateral pterygoid muscles should exhibit the highest levels of activity. Therefore, in eccentric excursions, although the muscle activity may be somewhat lower, the temporal and pterygoid muscles appear to be the most susceptible to developing hyperactive tenderness in parafunctional situations. D. Future Directions Results from this study suggest that in patients with signs and symptoms associated with bruxism that i t might be possible to correlate the site of pathosis with activity in specific muscles. For example, a given population exhibiting symptoms of muscle soreness could be examined for site and severity of bruxo-facets. Prediction of the sore muscles could then be made on the basis of the teeth involved and the directional nature and severity of the actual facet. The prediction could then be tested against a correlation of the actual teeth and muscles involved. A similar study could be performed substituting joint pathosis for bruxo-- 85 -facets. However more knowledge of the biomechanics is required to make more reasonable predictions. In the present study the lateral pterygoid muscle was not studied. However, one would expect this muscle to contribute reasonable levels of activity during some of the clenching tasks and this has been alluded to in the text. It would be useful to repeat some of the tasks of the present experiments in order to describe the contribution of the lateral pterygoid which has been implicated in dysfunction cases (39,161). This information would aid the predictive capabilities of the preceding paragraph. Another variable not tested was the effect of cuspal inclines on the muscle activities produced. It is possible that different angles of cuspal inclines allow more support for the teeth, thereby favouring greater force production in certain tasks. A study with controlled cuspal inclines could lead to further studies on quantitated natural cuspal inclines that may provide information relevant to the interpretation of some of the variation observed in the present study. As mentioned previously, craniofacial morphology is an independent variable that warrants further study per se. The literature on the relation between craniofacial morphology and the electromyographic activity in the jaw elevator muscles during clenching is limited and to a certain extent contradictory. Some papers (84,137,190) have stated that a curved mandibular base, prognathism, anterior inclination of the mandible, and a small gonial angle are correlated with stronger electromyographic activity in the major - 86 -jaw elevator muscles. Garrett et a l . have also described specifically that Class II Div. 1 malocclusions exhibit greater muscle activity than normal occlusions. Retrognathism, inclined incisors, and a large gonial angle are associated with less muscle activity. However, other electromyographic studies (3,14,47,211) as well as bite force studies (112,113,206,209,210) found no evidence of differing absolute muscle activities between malocclusion and normal cases. It has been suggested that coordination of the elevator muscles depends on the sagittal placement of the mandible (68,69,196,197) and this has been observed in the present series of experiments. Control of the variations in craniofacial morphology and malocclusion in the future may help explain some of the individual differences encountered. - 87 -VI. SUMMARY 1. Electromyographic recordings from the anterior temporal muscle b i l a t e r a l l y , the p o s t e r i o r temporal muscle b i l a t e r a l l y , the s u p e r f i c i a l masseter muscle b i l a t e r a l l y , and the l e f t medial pteryoid muscle were used to study the e f f e c t s of changing the l o c a t i o n , s i z e , and d i r e c t i o n of e f f o r t on s p e c i f i c b i t e points during maximal clenching tasks in human subjects. 2. V e r t i c a l clenching e f f o r t s in the natural intercuspal p o s i t i o n generally exhibited the highest muscle a c t i v i t i e s f o r a l l the muscles recorded. 3. When the b i t e point moved p o s t e r i o r l y along the arch from i n c i s o r s to molars, the muscle a c t i v i t y i n the i p s i l a t e r a l temporal muscles was seen to increase while the a c t i v i t y in the i p s i l a t e r a l medial pterygoid and the masseter muscles b i l a t e r a l l y was seen to decrease, during subjective v e r t i c a l clenching tasks on cuspal i n c l i n e s . 4. The i p s i l a t e r a l temporal and c o n t r a l a t e r a l pterygoid muscles exhibited the most a c t i v i t y during subjective v e r t i c a l clenching on cuspal i n c l i n e s . 5. Eccentric e f f o r t s on s p e c i f i c b i t e points generally resulted in lower a c t i v i t i e s than the corresponding v e r t i c a l e f f o r t . This was usually seen i n a l l muscles, but not a l l values were s i g n i f i c a n t . 6. The i p s i l a t e r a l temporal and c o n t r a l a t e r a l pterygoid muscles exhibited the most a c t i v i t y during maximal clenches in a l a t e r a l d i r e c t i o n with l i t t l e contribution from the other muscles. - 88 -7. The temporal muscles showed the most a c t i v i t y i n retrusive clenching with a c t i v i t y i n the other muscles nearly non-existent. 8. The medial pterygoid and masseter muscles were found to be the most act i v e muscles during protrusive and i n c i s a l clenching while temporal muscle a c t i v i t y was low. 9. When the size and number of contacts was increased a n t e r i o r l y , a generalized increase in muscle a c t i v i t y was witnessed. The same trend occurred p o s t e r i o r l y but was not as consistent or s i g n i f i c a n t . 10. Cross-arch contacts were associated with a s l i g h t but s i g n i f i c a n t increase in masseter muscle a c t i v i t y i p s i l a t e r a l to the cross-arch contact. Although t h i s difference was observed in the masseter the clench s t i l l consisted of a c t i v i t y primarily a r i s i n g from the i p s i l a t e r a l temporal and c o n t r a l a t e r a l pterygoid muscles. Their a c t i v i t y remained consistent with that of u n i l a t e r a l contacts. 11. Proportionately, the muscle a c t i v i t y in maximal and h a l f maximal clenching was s i m i l a r and the same relationships could be described in general for tasks of both e f f o r t s . 12. The findings of t h i s electromyographical study on changes of bite point, s i z e of bite point, and the d i r e c t i o n of e f f o r t applied on a b i t e point confirm t h e i r s p e c i f i c associations with the a c t i v i t y of muscle groups. Data of t h e o r e t i c a l s i g n i f i c a n c e has also been made available for a biomechanical approach into the i n v e s t i g a t i o n of degenerative j o i n t changes. - 89 -BIBLIOGRAPHY 1. Abe, K. and Shimakawa, M. 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NIH publication #79-1845, 1979. - 120 -A p p e n d i x A : S t a t i s t i c a l A n a l y s i s o f S p e c i f i c Task C o m p a r i s o n s T a b l e VI Compar i son o f N o r m a l i z e d M u s c l e A c t i v i t y Between C l e n c h e s i n a N a t u r a l I n t e r c u s p a l P o s i t i o n and a S i m u l a t e d I n t e r c u s p a l P o s i t i o n Task M U S C 1 e C o m p a r i s o n LMPT LSM RSM LAT RAT LPT RPT 1. I . C . P . : N . S . N .S . N .S . N .S . N .S . ** N.S . S i m . I . C P . n = 7 10 9 10 9 10 3 Legend T a b l e VI N o r m a l i z e d mean da t a have been s t a t i s t i c a l l y a n a l y z e d as f o r the c o m p a r i s o n d e p i c t e d and numbered a t t he l e f t : I . C P . , n a t u r a l i n t e r c u s p a l p o s i t i o n ; s i m . I . C P . , s i m u l a t e d i n t e r c u s p a l p o s i t i o n w i t h o c c l u s a l s t o p s on se cond m o l a r s , c a n i n e s , and i n c i s o r s . P r o b a b i l i t y (P ) o f a d i f f e r e n c e by chance d e t e r m i n e d by the s t u d e n t ' s p a i r e d t - t e s t : N .S . ( n o n - s i g n i f i c a n t ) , p > 0 . 0 5 ; *p < 0 . 0 5 ; * * p < 0 . 0 1 ; * * * p < 0 . 0 0 1 . The v a l u e d e s i g n a t e d by (n) i n d i c a t e s the number o f c o m p a r i s o n s a n a l y z e d f o r t h a t m u s c l e . A c t u a l (n ) v a l u e s a re r e p r e s e n t e d f o r each t a s k and m u s c l e i n A p p e n d i x B. M u s c l e s a r e r e p r e s e n t e d a s : LMPT, l e f t m e d i a l p t e r y g o i d ; LSM, l e f t s u p e r f i c i a l m a s s e t e r ; RSM, r i g h t s u p e r f i c i a l m a s s e t e r ; LAT, l e f t a n t e r i o r t e m p o r a l ; RAT, r i g h t a n t e r i o r t e m p o r a l ; LPT , L e f t p o s t e r i o r t empora l and RPT, t h e r i g h t p o s t e r i o r t empora l m u s c l e . T h i s same c o n v e n t i o n i s used f o r each t a b l e o f A p p e n d i x A . H e n c e f o r t h , o n l y r e l e v a n t d i f f e r e n c e s a r e r e p o r t e d i n s u b s e q u e n t l e g e n d s . - 121 -Table VII Comparison of Normalized Muscle Activity Between Vertical Clenches on an Anterior Bite Block and Other Contact Positions Task M u s e 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. Ant. N.S. *• ** * * * *** incisor n = 5 9 8 9 8 9 5 2. Ant. : N.S. **• ** N.S. ** N.S. ** 1. canine n = 5 10 9 10 9 10 5 3. Ant. • N.S. N.S. N.S. N.S. N.S. ** N.S. 1. molar n = 5 10 9 10 9 10 5 4. Ant. : N.S. N.S. N.S.' N.S. N.S. N.S. N.S. 1. group n = 5 10 9 10 9 10 5 5. Ant. N.S. N.S. N.S. ** N.S. * N.S. 1. canine(x) n = 4 9 8 9 8 9 5 6. Ant. : N.S. N.S. * * *** ** N.S. 1. group(x) n = 5 9 8 9 8 9 5 7. Ant. N.S. N.S. N.S. ** *** ** N.S. bilateral n = 5 10 9 10 9 10 5 molar Legend Table VII Comparison of normalized muscle activity produced on a bite block clench to various other positions. Abbreviations: Ant., anterior bite block; 1. canine(x), left canine with cross-arch molar contact; 1. group(x), left group contact with cross-arch molar contact. - 122 -Table VIII Comparison of Normalized Muscle Activity Between Vertical Clenches on Anterior and Posterior Contact Positions Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. incisor • N.S. N.S. N.S. ** N.S. *** N.S. 1. canine n = 7 9 8 9 8 9 2 2. incisor • N.S. N.S. N.S. N.S. ** N.S. N.S. r. canine n = 7 9 8 9 8 9 2 3. incisor : * ** • *** *• *** N.S. 1. molar n = 7 9 8 9 8 9 2 4. incisor • N.S. N.S. N.S. ** ** ** N.S. r. molar n = 7 9 8 9 8 9 2 5. 1. canine • * ** * ** ** * N.S. 1. molar n = 7 10 9 10 9 10 3 6. r. canine N.S. N.S. ** ** N.S. ** * r. molar n = 7 10 9 10 9 10 3 Legend Table VIII Significance values for antero-posterior comparisons. Abbreviations: (r.), right and (1.) left. - 123 -Table IX Comparison of Normalized Muscle Activity Between Vertical Clenches on Unilateral Contacts and the Corresponding Cross-Arch Contact Combination Task M u s e 1 e Comparison LMPT LSN RSM LAT RAT LPT RPT 1 . 1 . canine : ** ** *** ** ** * * 1. canine(x) n = 6 9 9 9 9 9 3 2. r. canine '• ** *** ** *** *** *** ' N.S. r. canine(x) n = 6 9 9 9 9 9 3 3. 1. group • N.S. * * N.S. * N.S. * 1. group(x) n = 7 10 9 10 9 10 3 4. r. group * ** N.S. ** * *• N.S. r. group(x) n = 7 10 9 10 9 10 3 Legend Table IX Comparison of muscle activities produced on unilateral and corresponding cross-arch contact combinations. - 124 -Table X Comparison of Normalized Muscle Activity Between Vertical Clenches on Ipsilateral and Contralateral Contacts Task M u s e ! e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. 1. canine N.S. N.S. N.S. ** *** *** N.S. r. canine n = 7 10 9 10 9 10 3 2. 1. molar • * * N.S. N.S. N.S. N.S. N.S. r. molar n = 7 10 9 10 9 10 3 3. 1. group : N.S. N.S. N.S. ** N.S. * * r. group n = 7 10 9 10 9 10 3 Legend Table X Comparison of normalized muscle activities when the side of tooth contact changed. - 125 -Table XI Comparison of Normalized Muscle Activity Between Unilateral Group Contact and Effort on the Empty Side Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1.1. group : N.S. * N.S. * N.S. * N.S. effort r. n = 7 10 9 10 9 10 3 2. r. group : N.S. N.S. * N.S. * N.S. N.S. effort 1. n = 7 9 8 9 8 9 3 Legend Table XI Comparison of normalized activities between a vertical clench on a unilateral group contact and an attempt to clench on the empty side with the same contact. Abbreviations: 1. group: effort r., the subject was 'attempting to clench on the right empty side with a left unilateral group contact present; r. group: effort 1., comparison of a vertical clench on a right group contact to a vertical clench on the left empty side with the same contact present. - 126 -Table XII Comparison of Maximum and Half Maximum Vertical Clenching Relationships a) Intercuspal and Simulated Intercuspal Position Relationships Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. I.CP. : N.S., N.S., N.S., N.S., N.S., ** 9 N.S., sim. I.CP. N.S. N.S. N.S. N.S. N.S. ** N.S. n = 7 10 9 10 9 10 3 b) Antero-Posterior Relationship Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. incisor : N.S., N.S., N.S., ** 9 N.S., *** 9 N.S., 1. canine N.S. N.S. N.S. ** N.S. * N.S. n = 7 9 8 9 8 9 2 2. incisor * > ** > * » *** 9 ** 9 *** 9 N.S., 1. molar N.S. ** N.S. *** * *** N.S. n = 7 9 8 9 8 9 2 3. incisor . ** *** • *** ** *** N.S. 1. group N.S. * N.S. *** N.S. *** N.S. n = 7 9 8 9 8 9 2 - 127 -Table XII Comparison of Maximum and Half Maximum Vertical Clenching Relationships c) Cross-Arch Relationships Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. 1. canine 1. canine(x) . ** N.S! ** N.S.' *** > * ** N.S.' ** > ** * N.S! * N.S! n = 6 9 9 9 9 9 3 2 . 1 . group 1. group(x) : N.S., N.S. * ** * > ** N.S., N.S. * 9 N.S., N.S. * N.S! n = 7 10 9 10 9 10 3 d) Ipsilateral/Contralateral Relationships Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1 . 1 . canine r. canine : N.S., N.S. N.S., N.S. N.S., N.S. ** *** *** *** *** 9 ** N.S., N.S. n = 7 10 9 10 9 10 3 2. 1. molar r. molar * * * 9 ** N.S., N.S. N.S., ** N.S., N.S. N.S., N.S. N.S., N.S. n = 7 10 9 10 9 10 3 3. 1. group r. group : N.S., N.S. N.S., N.S. N.S., * ** 9 •* N.S., N.S. * 9 *** * 9 * n = 7 10 9 10 9 10 3 - 128 -Legend Table XII T h i s t a b l e has been p r e p a r e d s i m p l y t o show t h e s i m i l a r i t i e s between maximum and h a l f maximum c l e n c h i n g t a s k s . The f i r s t s i g n i f i c a n c e v a l u e l i s t e d under each m u s c l e i s f o r the maximum c l e n c h i n g c o m p a r i s o n f o l l o w e d by t h e v a l u e f o r t h e h a l f maximum c o m p a r i s o n . The v a l u e o f (n) r e m a i n s c o n s t a n t f o r bo th s i t u a t i o n s as b o t h maximal and h a l f maximal c l e n c h e s were p e r f o r m e d d u r i n g each t a s k . - 129 -Table XIII Comparison of Normalized Muscle Activity Between Vertical and Lateral Clenching Efforts Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. 1. canine(v) •* *** ** * * N.S. * (1. lat.) n = 5 10 10 10 10 10 5 2. 1. molar(v) * *** *** • ** N.S. * (1. lat.) n = 5 9 9 9 9 9 4 3 .1 . group(v) • *** ** *** N.S. *** N.S. N.S. (1. lat) n = 5 9 9 9 9 9 4 4. 1. group(x) ** *** *** * *** N.S. * (1. lat) n = 4 9 9 9 9 9 5 5. 1. canine(x) • * *** *** * *** N.S. * (1. lat) n = 4 9 9 9 9 9 5 6. bilateral * *** *** * *** N.S. *** molar(v) 4 9 9 9 9 9 5 (1. lat.) Legend Table XIII Comparison of vertical and lateral clenching efforts. Abbreviations: (v), vertical clench; (1. lat . ) , left lateral clench on same contact. - 130 -Table XIV Comparison of Normalized Muscle Activity Between Vertical and Eccentric Efforts on Molar Contacts Task M u S c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. 1. molar(v) N.S. *** *** *** N.S. *** N.S. (r. lat) n = 5 10 10 10 10 10 5 2. b i . molar(v) : N.S. *** *** *** N.S. *** N.S. (r. lat) n = 5 10 10 10 10 10 5 3. 1. molar(v) : * *** *** *** *** *•* * (pro.) n = 5 10 10 10 10 • 10 5 4. b i . molar(v) : N.S. *** •** *** *** *** *** (pro.) n = 5 10 10 10 10 10 5 5. 1. molar(v) : *** *** *** ** * N.S. N.S. (ret.) n = 5 10 10 10 10 10 5 6. b i . molar(v) ** *** *** ** ** N.S. N.S. (ret.) n = 5 10 10 10 10 10 5 Legend Table XIV Comparison of vertical and eccentric efforts on unilateral and bilateral molar contacts. Abbreviation: b i . molar, bilateral molar; r. lat . , right lateral effort; pro., protrusive effort; ret., retrusive effort. - 131 -Table XV Comparison of Normalized Muscle Activity Between Lateral Efforts to Opposite Sides on Molar Contacts Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. 1. molard .) : N.S. N.S. ** *** * *** * 1. molar(r.) n = 5 9 9 9 9 9 4 2. b i . molard .) : * N.S. * *** *** •** ** bi . molar(r.) n = 4 9 9 9 9 9 5 Legend Table XV Comparison of normalized activities when effort is applied to opposite side on molar contacts. - 132 -Table XVI Comparison of Normalized Muscle Activity Between Protrusive and Retrusive Efforts on Molar Contacts Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. 1. molar(pro) : ** * *** •* ** *** *** (ret.) n = 5 10 10 10 10 10 5 2. bi . molar(pro) : ** *** *** *** *** *** * (ret.) n = 5 10 10 10 10 10 5 Legend Table XVI Comparison of protrusive and retrusive efforts on molar contacts. - 133 -Table XVII Comparison of Normalized Muscle Activity Between Clenches on Anterior and Posterior Contacts on Natural Teeth Task M u s e 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. incisor : * *** * **• N.S. *** N.S. 1. canine n = 9 19 19 19 19 19 10 2. incisor : N.S. ** *** N.S. *** N.S. *** r. canine n = 9 20 20 20 20 20 11 3. incisor . ** * N.S. *** N.S. *** N.S. 1. group n = 8 16 16 16 16 16 8 4. incisor : N.S. N.S. ** N.S. *** N.S. * r. group n = 9 17 17 17 17 17 8 5. 1. canine : N.S. N.S. N.S. ** N.S. * N.S. 1. group n = 8 16 16 16 16 16 8 6. r. canine : N.S. * N.S. N.S. *** N.S. N.S. r. group n = 9 17 17 17 17 17 8 Legend Table XVII Comparison of clenching on anterior and posterior contacts on natural teeth. Values of (n) are seen to vary more due to the difficulty some subjects had in producing the desired contact. - 134 -Table XVIII Comparison of Normalized Muscle Activity Between Clenches on Unilateral Contact and the Corresponding Cross-Arch Contact Combination on Natural Teeth Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1.1. group N.S. N.S. * N.S. N.S. N.S. N.S. 1 group(x) n = 8 16 16 16 16 16 8 2. r. group • N.S. N.S. N.S. N.S. N.S. N.S. N.S. r. group(x) n = 6 15 15 15 15 15 8 Legend Table XVIII Comparison of unilateral group function and the corresponding cross-arch combination showing l i t t l e difference. - 135 -Table XIX Comparison of Normalized Muscle Activity Between Clenches on Ipsilateral and Contralateral Contact Combinations on Natural Teeth Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. 1. canine • •** ** ** •** *** *** **• r. canine n = 9 19 19 19 19 19 10 2. 1. group : *** ** N.S. *** *** *** •** r. group n = 8 16 16 16 16 16 8 3. 1. group(x) • *** N.S. N.S. *** ** *** *** r. group(x) n = 6 15 15 15 15 15 8 Legend Table XIX Comparison of ipsilateral and contralateral clenches on natural teeth showing many significant differences. - 136 -Table XX Comparison of Normalized Muscle Activity Between Intercuspal and Other Clenching Efforts on Natural teeth Task M u s c 1 e Comparison LMPT LSM RSM LAT RAT LPT RPT 1. I.CP. N.S. *** *** *** *** *** *** I.CP.(pro.) n = 9 20 20 20 20 20 11 2. I.CP. *** *** *** *** *** N.S. * I.CP.(ret.) n = 9 20 20 20 20 20 11 3. I.CP. ** *** *•* ** **• N.S. *•* I .CP.d .) n = 7 13 13 13 13 13 6 4. I .CP. : N.S. •** *•* **• *** *** * I.C.P.tr.) n = 7 13 13 13 13 13 6 5. I.CP.(pro.) *** *•* *** *** *•* *•* *** I.CP.(ret.) n = 9 20 20 20 20 20 11 6. I .CP.d.) *** •* * *** *** *** *** I.C.P.(r.) n = 7 13 13 13 13 13 6 7. I.CP. : N.S. *** *** *** *** *** *** incisor n = 9 20 20 20 20 20 11 8. I.CP. ** *** *** *** *** *** *** 1. canine n = 9 19 19 19 19 19 10 9. I.CP. N.S. *** *** *** *•* *** *** r. canine n = 9 20 20 20 20 20 11 10. I.CP. ** *** *** *** *** * *** 1. group n = 8 16 16 16 16 16 8 11. I.CP. N.S. *** •** *** *** *** * r. group n = 9 17 17 17 17 17 8 12. I.CP. : *•* *** *** *** *•* *•* *** 1. group(x) n = 8 16 16 16 16 16 8 13. I.CP. : * *** *** *** *** *** *** r. group(x) n = 6 15 15 15 15 15 9 Legend Table XX This table simply shows that for other than the medial pterygoid in selected clenching acts that the intercuspal clench generally exhibits significantly different activity than all other contact positions. - 137 -Appendix B: (n) Values for the Tasks and Muscles Illustrated in the Figures Table XXI Values for Each Task and Muscle in Study #1 Task LMPT LSM RSM LAT RAT LPT RPT 1 7 10 9 10 9 10 3 2 7 10 9 10 9 10 3 3 7 9 8 9 8 9 2 4 5 10 10 10 10 10 5 5 7 10 9 10 9 10 3 6 7 10 9 10 9 10 3 7 7 10 9 10 9 10 3 8 7 10 9 10 9 10 3 9 7 10 9 10 9 10 3 10 7 10 9 10 9 10 3 11 7 10 9 10 9 10 3 12 6 9 8 9 8 9 3 13 6 9 9 9 9 9 3 14 6 9 9 9 9 9 3 15 7 10 9 10 9 10 3 16 7 10 9 10 9 10 3 Legend Table XXI Tasks are numbered on the left as for Table I of the methods section. The (n) values are presented for each muscle and task and indicate the number of subjects that performed the clench successfully. - 138 -Table XXII (n) Values for Each Task and Muscle Used in Study #2 Task LMPT LSM RSM LAT RAT LPT RPT 1 5 10 10 10 10 10 5 2 5 10 10 10 10 10 5 3 5 9 9 9 9 9 4 4 5 10 10 10 10 10 5 5 5 10 10 10 10 10 5 6 5 10 10 10 10 10 5 7 5 10 10 10 10 10 5 8 5 10 10 10 10 10 5 9 5 10 10 10 10 10 5 10 5 9 9 9 9 9 4 11 5 10 10 10 10 10 5 12 4 9 9 9 9 9 5 13 5 10 10 10 10 10 5 14 4 9 9 9 9 9 5 15 5 10 10 10 10 10 5 16 4 9 9 9 9 9 5 17 5 10 10 10 10 10 5 18 5 10 10 10 10 10 5 19 5 10 10 10 10 10 5 Legend Table XXII The tasks numbered on the left are presented in Table IV of the methods section. - 139 -Table XXIII (n) Values for Each Task and Muscle Used in Study #3 Task LMPT LSM RSM LAT RAT LPT RPT 1 9 20 20 20 20 20 11 2 9 20 20 20 20 20 11 3 9 20 20 20 20 20 11 4 7 13 13 13 13 13 6 5 7 13 13 13 13 13 6 6 9 20 20 20 20 20 11 7 9 19 19 19 19 19 10 8 9 20 20 20 20 20 11 9 8 16 16 16 16 16 8 10 9 17 17 17 17 17 8 11 8 16 16 16 16 16 8 12 6 15 15 15 15 15 9 j Legend Table XXIII Tasks numbered on the left are presented in Table V of the methods section concerning clenching on natural teeth. 

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