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Effect of bleaching agent on interfacial fracture toughness of resin composite-dentin interfaces Far, Cyrus 2002

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EFFECT OF BLEACHING AGENT ON INTERFACIAL FRACTURE TOUGHNESS OF RESIN COMPOSITE-DENTIN INTERFACES by CYRUS FAR B . S c , The University of British Columbia, 1999 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE D E G R E E OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Oral Biological and Medical Sciences) We accept this thesis as conforming to the/required standard THE UNIVERSITY OF BRITISH COLUMBIA April 2002 © Cyrus A. Far, 2002 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of the department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Oral Biological and Medical Sciences The University of British Columbia Vancouver, Canada Date Of. v^rr?1:?. i^??*^. Abstract The effect of bleaching on fracture toughness (Kic) of existing composite-dentin interfaces was assessed using the notchless triangular prism (NTP) specimen fracture toughness test. Human molars and premolars (<6 months old) were wet ground on 600 SiC to obtain 4x4x4x4mm triangular prisms with buccal or lingual exposed for bonding. Buccal or lingual dentin surfaces, ground on 600 grit sandpaper immediately before bonding, were bonded using a resin composite (Z-250, 3M) and a dentin bonding agent (Single Bond, 3M) to obtain 4x4x4x8mm dentin-composite NTP specimens. Samples were randomly divided into 18 groups (n=9) and exposed to four concentrations of bleaching agent (11,13,16, and 21% carbamide peroxide). Controls were exposed to the same conditions as experimental groups, but were treated with tap water instead of carbamide peroxide. Exposure to the bleaching agent took place in 30-minute increments for 2 hours per day during the first week and 4 hours per day in the following two weeks. Samples were stored in tap water for 1 hour in between treatments, and 7 hours overnight. Testing was conducted after a cumulative exposure of 14, 42, and 70 hours. The maximum force required to fracture specimens, using an Instron 4311 universal testing machine at a cross head speed of 0.1 mm/min, was recorded and used to calculate Kic in order to investigate the effect of concentration and length of exposure to the bleaching agent on interfacial fracture toughness. Results were analyzed using two-factor ANOVA and Bonferroni multiple means comparisons. Increasing length of exposure to the bleaching agent resulted in a significant decrease in fracture toughness (p<.01). No statistical differences in bleaching agent concentration were detected (p=0.78). SEM observations of fractured surfaces revealed differences in fracture path of control and experimental groups. ii Table of Contents Abstract Table of Contents List of Tables List of Figures Acknowledgements Introduction Bleaching Industrial Bleaching Oxidizing Bleaching Agents Reducing Bleaching Agents 5 Dental Bleaching 6 Historical Perspective 6 Dental Discolouration 8 Classification 8 Mechanism 10 Dental Bleaching Procedures and Chemicals 14 Mechanism of Bleaching 16 Effect on Restorative Materials 19 Dental Amalgam and Alloys ...19 Resin Composites 20 Luting Agents, Temporary Restorative Materials, and Porcelain 22 Effect on Bonding 23 Enamel 23 Dentin. 25 Effect on Microleakage and Sealing Ability 26 Effects on Enamel 27 i i i Effects on Dentin and Cementum 28 Dentin Structure 28 Effects 30 Fracture Toughness 32 Fundamentals 32 Chevron Notch Fracture Toughness Test 37 Notchless Triangular Prism Fracture Toughness Test 39 Validity 40 Accuracy 43 Precision 44 Thesis Aims 47 Materials and Methods 48 Specimen Preparation 48 Exposure to Carbamide Peroxide 52 Fabrication of Bleaching Trays 53 Procedures for Exposure 54 Exposure to Carbopol and Urea 56 Fracture Toughness Testing 57 Scanning Electron Microscopy Observations 59 Statistics 59 Results 61 Discussion • 69 Conclusion 76 Reference List 78 iv List of Tables TABLE 1. COMMON CAUSES AND COLOURS OF STAINS 1 TABLE 2. Kic OF PMMA OBTAINED BY NTP AND CNSR TESTS 4 TABLE 3. Kic OF LUTING CEMENTS OBTAINED BY NTP AND CNSR TESTS TABLE 4. EXPERIMENTAL DESIGN I.. TABLE 5. EXPERIMENTAL DESIGN II. TABLE 6. RESULTS TABLE 7. TWO FACTOR ANOVA TABLE 8. ONE-WAY ANOVA List of Figures FIGURE 1. CHEMICAL STRUCTURE AND ELECTRONIC SPECTRA 13 FIGURE 2. EPOXIDATION AND CARBONYL COMPOUNDS 17 FIGURE 3. MECHANISMS FOR PRODUCTION OF FREE RADICALS 18 FIGURE 4. STRESS CONCENTRATION IN AN ELLIPTOID CRACK 33 FIGURE 5. QUANTIFICATION OF STRESSES AT THE CRACK TIP 37 FIGURE 6. BARKER'S CHEVRON NOTCH SHORT ROD SPECIMEN TEST... 38 FIGURE 7. CHEVRON NOTCHED SHORT ROD DESIGN FOR COMPOSITE-TOOTH INTERFACIAL STUDIES 39 FIGURE 8. NTP TESTING JIGS 40 FIGURE 9. SCHEMATIC PRESENTATION OF FABRICATED RESIN COMPOSITE-DENTIN NTP SPECIMENS 48 FIGURE 10. MANUFACTURER'S RECOMMENDED WEAR SCHEDULE 53 FIGURE 11. NTP SPECIMEN TESTING USING INSTRON 4311 58 FIGURE 12. 95% CONFIDENCE INTERVALS FOR MEAN FRACTURE TOUGHNESS 63 FIGURE 13. ADHESIVE FAILURE 66 FIGURE 14. FRACTURED SURFACES OF SAMPLES EXPOSED TO 42 HOURS OF BLEACHING 67 FIGURE 15. OBSERVED EFFECTS AFTER 70 HOURS OF BLEACHING 68 FIGURE 16. THE EFFECT OF TIME ON FRACTURE TOUGHNESS OF DIFFERENT CONCENTRATION LEVELS 75 vi Acknowledgements This thesis is more than a collection of words put together to satisfy the requirements of a Master of Science degree. It represents a thought process and many hours of gaining familiarity with the field of research, acquiring the necessary background knowledge and finally executing a research plan. I am greatly indebted to my academic superiors for their participation and input throughout the above process. I first and foremost wish to express my gratitude to my supervisor, Dr. Dorin Ruse, who despite his many other commitments, has guided and supported me every step of the way. I also wish to thank Dr. Brunette for his wisdom and vast knowledge, a fraction of which I hope to have absorbed over these last two years, and hope to carry with me through the rest of my academic journey, as well as my life. I would like to express my thankfulness to Dr. John Gosline for first sparking my interest in biomaterials and for providing extremely thoughtful input during this thesis. My sincere thanks to our graduate secretary, Vicky Koulouris, our graduate advisor Dr. Douglas Waterfield, Dr. Ravi Shah, Mr. Andre Wong, Dr. Nanako Iwamoto, and all others "upstairs" who have had a positive impact on my graduate education and have made me feel welcome in my new academic home. Thank you to Drs Braverman, McDonald, Seth, Slemko, Walter and staff, for their commitment to providing our laboratory with teeth for the past two years. This thesis is also dedicated to my parents Firooz and Simin, my Sister Susan, and my brother Sassan, who have enriched my life in more ways than words could ever express, and have provided me with genuine support through all stages of life. vii Introduction Over the past two to three decades, the dental profession has witnessed a shift from disease-oriented restorative procedures to elective cosmetic procedures.1 Billions of discretionary dollars are spent on cosmetic procedures each year as patients seek perfect "Hollywood smiles".2 Realization of the perfect smile has been made possible in part due to recently made improvements to dental materials such as bonding agents and composite resins (white fillings). The increase in bond strength to enamel and dentin using recent bonding agents,3 as well as improvements in wear characteristics and aesthetics of composite materials,4 have enabled practitioners to successfully restore carious teeth, as well as unsightly stains, such as fluorosis, with dental composites. Placement of composite or porcelain veneers on anterior teeth, which involves reducing tooth tissue(s) before bonding an ultra-thin shell, is another cosmetic procedure which has been made possible as a result of improvements in bonding. 3 , 5 , 6 In addition to these aesthetic procedures, tooth whitening has played a significant role in improving the smiles of aesthetically inclined patients. This procedure, also known as bleaching, refers to a process whereby stained or discoloured teeth are whitened using in-office procedures or at-home dentist-supervised bleaching systems, both of which contain hydrogen peroxide in some form or other as the active ingredient. Bleaching is considered a highly effective, inexpensive, and relatively painless procedure.1 For this reason, it has gained enormous popularity among dental patients in the last decade and is the most common aesthetic treatment for adults.7 According to a recent member poll (2000) conducted by the American Academy of Cosmetic Dentistry (AACD)(, demand for bleaching has increased 300 percent over the last five years, and its demand as 1 an aesthetic procedure exceeds the demand for orthodontic braces or composite bonding among patients under 20. Bleaching is also the most popular cosmetic procedure among patients 20 to 50 years of age, exceeding the demand for veneers and composites. Bleaching is also highly sought after by dentists. According to a survey on bleaching conducted by Clinical Research Associates (CRA) in 1997, of members and guests of the American Academy of Esthetic Dentistry, 90% of respondents prescribed bleaching to 19% of their incoming patients. In addition, the majority of those surveyed (65%) favoured and prescribed hydrogen peroxide-containing home bleaching kits over in-office bleaching procedures.8 While these statistics demonstrate the popularity of tooth whitening with dentists and patients, as well as the rise in demand for this aesthetic procedure, they fail to account for many bleaching systems available over the counter (OTC) or through infomercials neither of which require dentist supervision. Apparently more than 50% of all people indicate a desire for brighter teeth.2 Therefore, these figures are likely to be an underestimation of how widespread tooth whitening really is. The effectiveness of bleaching agents in whitening teeth has been validated through millions of applications8 while a large body of knowledge regarding possible consequences is still missing. One question of importance to dental materials, specifically bonding, is the effect of these bleaching systems on bonded tooth-composite interfaces. Fracture toughness, which measures the ability of a material or interface to resist crack propagation, is one way of assessing bond integrity. The use of fracture mechanics methodology for assessment of bond integrity has been advocated by experts in light of questions raised about the validity of bond strength tests. 9 , 1 0 2 Therefore, the aim of this thesis is to explore the effect of a carbamide peroxide-containing at-home bleaching system on fracture toughness of dentin-composite interfaces. Bleaching Industrial Bleaching Bleaching refers to the process whereby a coloured substrate is whitened, or at least lightened through chemical reaction with a bleaching agent. Natural tissues and fibres owe at least a part of their colour to complex organic molecules they contain. The whitening or decolourisation action of most bleaching agents is due to their ability to either modify, or break down these molecules via oxidative or reductive reactions such that they no longer exhibitl their characteristic colour. The uses of bleaching agents extend beyond the field of dentistry. In fact, most industrial bleaching agents were used long before tooth whitening came along. As early as 300 B.C., a simple bleaching agent (sea weed ash) in conjunction with soured milk and sunlight was used to bleach cloth.11 Today, a multitude of bleaching agents are commercially available, each of which are used for a large number of industrial applications. Industrial bleaching agents can be divided into two categories: 1) Oxidizing bleaching agents containing either chlorinated or peroxygen-containing compounds, and 2) reducing bleaching agents such as those containing elemental sulphur. Oxidizing Bleaching Agents Chlorinated Agents Chlorine-containing bleaching agents are one group of oxidizing agents. These are one of the most cost-effective bleaching agents known to the industry and 3 are divided into four classes: elemental chlorine, hypochlorites, N-chloro compounds, and chlorine dioxide. Except to bleach wood pulp and flour, elemental chlorine itself has relatively few applications as a bleaching agent. It is however, commonly used to produce alkali and alkali earth hypochlorite salts, such as sodium hypochlorite (NaOCI) and calcium hypochlorite Ca(OCI)2. Hypochlorites are one of the strongest classes of bleaching agents known and are widely used in laundering, sanitizing and for pulp and textile bleaching. The active ingredient in hypochlorite bleaching agents is hypochlorous acid and is produced in solution according to the following equation: XOCI + H 2 0 HOCI + X + + OH -N-chloro compounds, in other words compounds in which chlorine is complexed with nitrogen, make up another class of chlorinated bleaching agents. Examples of N-chloro compounds include chloramines, chloramides, chlorimides, and chlorosulfinamides, and are used to make dishwashing products. Similar to hypochlorites, the active ingredient of N-chloro compounds is hypochlorous acid, and is released upon hydrolysis in accordance with the following equation: RR'NCI + H 2 0 RR'NH + HOCI Chlorine dioxide constitutes the last class of chlorine-containing bleaching agents. Unlike the other two classes that yield hypochlorous acid in solution, chlorine dioxide is itself the reactive species. Chlorine dioxide has recently gained popularity in paper pulp bleaching due to its ability to yield far less undesirable effluent during the bleaching process than sodium hypochlorite and chlorine gas. However, the extent of its use as a bleaching agent for other applications is limited due to its physical properties: CI0 2 is gaseous at room temperature, and is explosive in high concentrations.12 4 Hydrogen Peroxide and Derivatives The second group of oxidative bleaching agents is composed of hydrogen peroxide itself and a number of hydrogen peroxide-based compounds. Chemistry texts often refer to this group as peroxygen-containing bleaching agents due to the existence of peroxygen (O—O) in its chemistry. This moiety is considered to be active during bleaching processes. A more detailed treatment of the mechanism of hydrogen peroxide-based bleaching agents will be given in a later section under "mechanism of bleaching". The fact that hydrogen peroxide is a liquid at room temperature and is completely miscible with water,13 makes this agent a popular choice for a number of industrial applications. The most important uses for hydrogen peroxide are the bleaching of pulp for paper production, and the bleaching of textiles. Hydrogen peroxide and derivatives have a number of applications like deinking of wastepaper during the recycling process, as well as bleaching of natural fibres such as cotton, wool, and hair.13 While hydrogen peroxide itself is a liquid, a number of hydrogen peroxide based solids are available which are generally stable and allow for ease of transportation. A number of these peroxygen-based compounds are used in the industry, all of which hydrolyse in the presence of water, and yield hydrogen peroxide. The two most prominent of these solid compounds are sodium perborate, and sodium carbonate peroxyhydrate,12 both of which are used in making denture cleaners, tooth powders, detergents, and dry bleaches. Reducing Bleaching Agents Reducing bleaching agents are, in general, sulphur-containing compounds. Of these agents, zinc and sodium dithionite (Na2S204 and Zn S2O4) have a wide range of applications from bleaching silk, wool, hair, to household products and foods like soap, glue, sugar, and molasses. 1 4 Both of these bleaching 5 compounds exhibit increased reducing strength with increasing temperature up to about 75°C, above which both decompose rapidly. The use of sodium dithionite is generally favoured over zinc due to restrictions prohibiting the presence of excessive zinc in water.14 Sulfinic acid derivatives, are another group and are used at higher temperatures where sodium and zinc dithionite decompose rapidly.12 Commonly used derivatives are zinc and sodium salts of hydroxymethanesulfinates as well as formamedine sulfinic acid, which, in addition to being used for bleaching soaps, glue, and gelatin, are used to brighten textiles and remove dyes from fabrics. Formamedine sulfinic acid is also known as thiourea peroxide due the fact that it is synthesized from thiourea and hydrogen peroxide.14 In light of peroxide bleaching agents previously discussed, which are oxidizing agent, it seems counterintuitive that thiourea peroxide is a reducing agent; however, the active group in this case is the thiol, rather than hydrogen peroxide, and has reductive potential. Other sulphur-containing agents include sulphur dioxide derivates. However, sulphur dioxide is no longer widely used due to its relatively poor bleaching effect.14 Dental Bleaching Historical Perspective Tooth whitening, also commonly referred to as bleaching, refers to the process of brightening or whitening stained or discoloured teeth using either in-office procedures or dentist supervised at-home bleaching systems. While the demand for tooth whitening has increased dramatically over the past decade or so, the origins of whitening stained and/or discoloured teeth dates back to before the turn of the century. 6 One of the first to attempt whitening was Dwinelle,15 who in 1850, attempted to bleach non-vital or "dead" teeth using a chloride of lime and soda. The general approach of most of these early attempts was the application of an acid such as nitric, sulphurous, or hydrochloric acid. 1 5 Around 1916, fluorosis stains—caused by ingestion of excess fluoride during the developmental stage of enamel—had high prevalence in South-western United States, where fluoride concentration in drinking water exceeded 4ppm. 1 6 While Dr. Kane was not aware of the cause of fluorosis, he apparently was the first to successfully remove these stains by applying 18% hydrochloric acid. 1 5 The main drawback to these approaches, despite their relative efficacy, was the lack of selectivity as the bleaching agent abraded away not only the stain, it also removed most of the superficial enamel. In addition, treated teeth were found to have a higher incidence of decay. 1 5 While the use of a hydrogen peroxide containing agents for the removal of fluorosis stains had been suggested as early as 1882, its use was only encouraged after Prinz published an article, in 1924, demonstrating the efficacy of sodium perborate, in removing fluorosis stains.1 5 Then in 1937, Ames reported on the use of hydrogen peroxide in its pure form in combination with ether. His solution containing five parts 100% hydrogen peroxide, one part ether, was found to be effective in removing fluorosis stains. However, the technique required multiple visits and extended chair time.1 7 Since 1937, a number of techniques using variations of hydrogen peroxide and/or hydrochloric acid have been applied to teeth affected by fluorosis as well as other stain types, details of which will be discussed in the next section. 1 5 , 1 8 All of these procedures were or are currently being carried out at the office, with little, if any, patient involvement. 7 In 1989, dental bleaching was revolutionized with the advent of a new technique19 that moved bleaching from dental offices to patient homes. This procedure, which is known as "at-home bleaching", is performed by the patients. Progress and the effect of bleaching on the oral cavity is—with the exception of OTC products—monitored by the dentist during relatively infrequent, say weekly or biweekly, visits to the dentist.19 However, the ease of application has led to the production of over-the-counter systems as well which do not involve the dentist. Compared to in-office procedures previously described, this bleaching technique is unique in that it is reasonably inexpensive and efficacious. In addition, dentist-supervised at-home systems require significantly less time input from the dentist. These reasons, combined with the increased obsession with physical appearance, have transformed bleaching teeth from a procedure reserved for patients with severe conditions into an industry, to the extent that I recently discovered tooth whitening floss along with numerous other whitening products during a recent visit to the drug store. Provided the trend continues, bleaching is likely to become as commonplace as dental check-ups in the near future. Dental Discolouration Classification Tooth discoloration is generally classified as extrinsic or intrinsic depending on the location of the stain. Stains on the surface of the tooth are classified as extrinsic, whereas stains caused by the penetration of a staining agent into the bulk of the tooth are defined as intrinsic stains. Intrinsic stains can be further classified according to the time during which the stain developed. Pre-eruptive stains occur during the developmental stage of dentin and enamel, while post-eruptive stains occur after the teeth have erupted and are fully developed.20 8 Fluorosis stains, described by hypomaturation of enamel, are categorized as pre-emptive intrinsic stains caused by excessive ingestion of fluoride during the developmental stage of enamel. As previously mentioned, patients living in regions where fluoride levels in natural water are above 1ppm, or those exposed to excessive amounts of fluoride through fluoride mouth rinses and toothpastes are susceptible to this condition.21 Another example of this type of intrinsic discolouration is tetracycline staining, which is caused by ingestion of the antibiotic tetracycline, from the fourth month of pregnancy to seventh year of life. Unlike fluorosis, tetracycline primarily affects dentin and is caused by the deposition of its ingredients within dentin. The severity of stains is governed by the dose and length of time the fetus or child is exposed to the antibiotic.22 Similarly, haematological disorders such as sickle cell anaemia and erythroblastosis fetalis, which affect the blood's ability to coagulate, can lead to an accumulation of blood pigments within the structure of the tooth within pulp and dentin, and subsequent discolouration.23 The second type of intrinsic discolouration is known as post-eruptive. The most common causes of this type of discoloration are pulp necrosis, and iatrogenic dental procedures. The former can be caused by 1) severe trauma resulting in rupturing of blood vessels and deposition of blood pigments, or 2) pulp degeneration itself, without rupturing of any blood vessels. 1 8 The latter can be caused by the gradual break down of restorative materials such as amalgams, composite resins, and cements. Alternatively, failure to fully extract the pulp during endodontic procedures, or inflicting trauma during endodontic procedures can lead to discolouration due to break down of organic material and/or deposition of blood pigments.16 9 Classically extrinsic stains have been defined as those which form on the surface of the tooth, and are caused by extrinsic rather than intrinsic agents. Therefore, in contrast to intrinsic stains which generally result from systemic administration of a stain-causing agent, extrinsic stains act directly on the substrate.24 Extrinsic stains are classically divided into two categories, metallic or non-metallic, based on the chemistry of the staining agent. 2 2 , 2 3 According to this system, agents of metallic origin such as potassium permanganate and iodine or iron solutions defined as metallic stains, whereas tobacco stains, being organic in nature, are considered non-metallic. Extrinsic stains are most commonly caused by intake of foods and beverages, all of which contain naturally occurring pigments; coffee, tea, spices, cause stains ranging from yellow to black. Other causes of extrinsic staining include smoking cigarettes, pipes, cigars, marijuana, as well as chewing tobacco, all of which are exacerbated in the absence of poor oral hygiene.23 Table 1 shows causes and colours associated with commonly observed dental stains.2 5 Mechanism Wavelengths of light to which the human eye is sensitive are those belonging to the visible spectrum which ranges in colour from violet to red and has wavelengths, from 400 to 700nm, respectively. Our ability to detect different colours is due to the presence of colour detecting cell types within the retina. These cells, known as cones, are divided into three groups: those which are stimulated upon receiving light in the blue end of the spectrum, those which are stimulated upon detecting light having a wavelength corresponding to the green region, and lastly, cones receptive to the yellow and red region of the visible spectrum.2 6 The mixture of all wavelengths in the visible region is known as white light. If the light striking the eye contains all components of white light, and all these components have the same intensity, the eye fails to detect colour. 10 Cause Extrinsic Discoloration Colour(s) Cigarettes, pipes, cigars, tobacco Marijuana Coffee, tea, foods Poor oral hygiene Extrinsic and Intrinsic Discoloration Fluorosis Aging Intrinsic Discoloration Yellow-brown to black Dark brown to black rings Brown to black Yellow or brown White, yellow, brown, grey, Black Yellow Genetic conditions, e.g., amelogenesis imperfecta Systemic conditions e.g., jaundice Porphyria Medications during tooth development, e.g., Tetracycline Fluoride Body by-products, e.g., Bilirubin Haemoglobin Pulp changes, e.g., pulp canal obliteration Pulp canal obliteration Pulp necrosis -with haemorrhage -without haemorrhage Iatrogenic causes, e.g., Trauma during pulp extirpation Tissue remnants in pulp chamber Restorative dental materials Endodontic materials Brown, black Blue-green or brown Purple-brown Brown, grey, or black Brown, grey, or black Grey, black Yellow Grey, black Yellow, grey-brown Grey, black Brown, grey, black Brown, grey, black grey, black Table 1. Common causes and colours of stains. 11 This particular scenario occurs if the substrate from which the light originates does not absorb any spectral portions of white light. However, if the substrate absorbs any spectral portions, such that the light reflected no longer contains all components of white light or contains different intensities of these spectral components, the sensation of colour will result.27 The perceived colour is therefore that which it is not absorbed by the substrate and is referred to as the complementary colour 2 6 The observed colour of stains is due to the presence of coloured chemical compounds known as chromophores or pigments which absorb visible light.28 The main distinction between these compounds and those which absorb little of the incident beam, is mainly the complexity of their chemical structure. Chromophores have many unsaturated or conjugated double bonds and consequently, mobile electrons which absorb light in the UV or visible range of the spectrum. As a general rule, the more conjugated double bonds a compound contains in its structure, the higher its wavelength of absorption. Other factors contributing to the ability of these compounds to absorb light include the presence of substituent groups on the conjugated double bonds such as aromatic groups, as well as position of conjugated bonds with respect to each other (i.e. Planar versus co-planar) 2 9 Examples of pigments found in foods include lycopene, which is a naturally occurring pigment in tomatoes, and 3-carotene, the principle pigment in carrots and sweet potatoes. These pigments contain more than ten conjugated bonds in addition to possessing ring structures,28 and absorb wavelengths in the blue to blue-green range of the visible spectrum (450-500nm) (figure 1). As previously said the perceived colour is that which is not absorbed by the substrate. Therefore, these pigments exhibit the complementary colours, red and orange.2 8 12 B 270 330 390 450 510 540 c Figure 1. Chemical structure and electronic spectra. A) Chemical structure of lycopene, B) Chemical structure of R-carotene C) Electronic spectra of lycopene and B-carotene. 13 Substituents on conjugated double bonds, such as aromatic rings were previously described as additional factors contributing to the ability of a compound to absorb components of visible light. Tea is one such example, containing a ring structure known as quercitin.30 The tea chromophore, similar to B-carotene and lycopene, absorbs in the blue and green region of the visible spectrum as well, hence the characteristic colour associated with tea. 2 9 Similarly, the pigment responsible for the brown colour exhibited by tetracycline stains is a quinone, which is deposited deep within dentin as a result of systemic administration of this antibiotic.30 Dental Bleaching Procedures and Chemicals All currently used dental bleaching agents are hydrogen peroxide-based oxidizing agents; however, acid etchants are sometimes used in conjunction with the bleaching agent to render the substrate more porous and allow for subsequent penetration of bleaching agent. Four general procedures are used for tooth whitening. The first of these is an in-office procedure known as power bleaching and refers to the use of high concentrations of the bleaching agent, in conjunction with heat or light. Cotton gauzes saturated with high concentrations (ca. 30%) of hydrogen peroxide are applied to external surfaces of teeth prior to the application of heat and/or light. This in-office procedure can be applied to external surfaces of vital as well as non-vital teeth. 1 6 , 3 1 A variation of this technique known as laser bleaching involves the application of the same active ingredient, hydrogen peroxide, followed by CO2 or Argon laser beam activation. In some cases the two lasers are applied sequentially, where the bleaching agent is applied and activated using argon laser for about five minutes, following which the bleaching agent is suctioned off, replaced with fresh hydrogen peroxide, and activated by C 0 2 laser 14 for an additional 5 minutes. The process is repeated for about 20-25 minutes unless patient sensitivity occurs. 3 2 The second procedure, known as internal or intracoronal bleaching, is used to whiten endodontically treated (non-vital) teeth, and involves the application of a hydrogen peroxide-saturated gauze to an open pulp chamber, following removal of carious or stained tissue. Intracoronal bleaching sessions last 20 to 25 minutes and are repeated until ideal whiteness is achieved.7 Another bleaching procedure is the walking bleach technique for non-vital teeth. This procedure involves placing a solution of sodium perborate and water in the pulp chamber, and sealing the pulp chamber with a temporary restorative material until next visit. A variation of this technique uses a solution of Superoxol (30% hydrogen peroxide) instead of water.33 The advantage offered by this technique over the more conventional procedure for internal bleaching is reduced chair time. The last and most popular technique is at-home bleaching, which was initially referred to as nightguard vital bleaching.19 The procedure involves providing the patient with custom-fitted trays of the arch(s) to be treated. The patient is also provided with a bleaching agent to be placed in the tray before inserting the tray in the mouth. Some of these systems are intended for overnight use, others for 1 to 2 hours daily for a period of 3 to 6 weeks depending on severity of stains, and the recommendations of the dentist and manufacturer. At-home bleaching systems frequently use hydrogen peroxide or a derivative, carbamide peroxide also known as urea peroxide, hydrogen peroxide carbamide, or perhydrolurea.34 While hydrogen peroxide and sodium perborate were discovered to play a role in industrial as well as dental bleaching, carbamide peroxide bleaching is unique to 15 dentistry. Upon contact with water, carbamide peroxide releases urea and the familiar hydrogen peroxide. Urea is then broken down further, releasing ammonia and carbon dioxide. The breakdown of hydrogen peroxide into species involved in the bleaching process will be discussed in the next section. In addition to the above, carbamide peroxide bleaching agents contain Carbopol, a carboxymethylene polymer. 7 This polymer is a thickening agent, also used as a thickening agent in shampoos, 1 2 and allows for sustained release of carbamide peroxide by providing a higher viscosity gel which adheres to external surfaces of the tooth for a longer period of time. Mechanism of Bleaching As the perception of colour is due to the ability of certain chemical moieties such as conjugated double bonds to absorb spectral portions of light, removing or modifying chemical components, such that they no longer absorb in the visible range of the spectrum, results in a whitening effect since the eye no longer perceives the colour normally associated with these compounds. The principle reason why pigments appear colourful is due to the presence of mobile electrons within conjugated double bonds and other substituent groups. Bleaching agents oxidize chromophoric groups such that electrons are mobilized. This process involves the attack of carbon-carbon double bonds either through epoxidation, or through free radical attack on double bonds leading to cleavage 2 9 , 3 5 Despite its widespread use in the industry as well as dentistry, the exact mechanism of hydrogen peroxide is still unclear. Mechanical cleansing due to oxygen release, production of perhydroxyl anion, as well as free radical production, are proposed mechanisms for bleaching. The most widely accepted mechanism involves production of the perhydroxyl anion, according to equation 1. 3 0- 3 6- 3 7 Evidence for implication of perhydroxyl 16 anion as the active species in peroxide bleaching is the observed increase in bleaching activity with increasing alkalinity.38 ,39 H2O2+ OH"->"OOH + H 2 0 (eq. 1) One way in which whitening is achieved is through epoxidation, more simply addition of oxygen across the double bond. 3 5 Reaction mechanism for perhydroxyl attack on a carbonyl group is illustrated in figure 2. C H 2 = C H - C H H — 0 - - O : • • * • nucleophiUc attack at the fi-carbon atom C H 2 - AHIL—• C H 2 \.* \ wtu 6: :6» I :0. H C H displacement of hydroxide ion by the carbanion :0: * it 0 ; + C H , - C H - C H V / .O. Figure 2. Epoxidation and carbonyl compounds. Production of free radicals is another proposed mechanism for the bleaching action of hydrogen peroxide. Under the influence of light or heat, hydrogen peroxide undergoes lysis releasing one of two free radicals implicated as active species involved in bleaching.13 The first of these, the hydroxyl radical (HO) is released by homolysis of the relatively weak O—O bond in accordance with equation 2: H 2 0 2 ^ HO* + HO* (eq. 2) 17 Additionally, under acidic conditions in the presence of reducing metals such as iron II, or under basic conditions, hydrogen peroxide can break down releasing perhydroxyl radicals (HO2), the second free radical thought to be involved in bleaching, as well the hydroxyl radical.40 Metal-induced activation of hydrogen peroxide involves the transfer of one electron from a metal (M) such as iron (II) to hydrogen peroxide. In the case of perhydroxyl-induced activation, perhydroxyl anion acts as a nucleophilic body. Reaction mechanisms of metal- and perhydroxyl-induced activation of hydrogen peroxide are summarized in figure 3. A) M 2 + + H 2 0 2 ^ M 3 + + HO" + HO* HO* + H 2 0 2 ^ H 2 0 + H0 2 * H0 2 * + M 3 + ^ M 2 + + 0 2 + H + H0 2 * + H 2 0 2 ^ HO* + 0 2 + H 2 0 B) H0 2 " + H 2 0 2 ^ H0 2* + HO* + HO-Figure 3. Mechanisms for production of free radicals. A) metal-induced activation under acidic conditions B) activation by perhydroxyanion under basic conditions. According to this model a series of intermediate reactions occur before hydrogen peroxide breaks down into water and oxygen. In the presence of an oxidizable substrate such as stains, these free radicals react with conjugated bonds within stains, resulting in bleaching and termination of the chain reaction.37 Of the two free radicals proposed, the hydroxyl radical would appear to be the more dominant species, since the dissociation energy for homolysis of HO—OH bonds is significantly less than the dissociation energy required to cause lysis of HOO—H bonds 4 3 18 Regardless of reaction intermediates, hydrogen peroxide eventually breaks down into oxygen and water in accordance with equation 3. The last and simplest proposed mechanism for bleaching is mechanical cleansing by oxygen released as a result of hydrogen peroxide decomposition. 2 H 2 0 2 ^ 2H 2 0 + 0 2 (eq. 3) The release of oxygen leads to a bubbling effect thought to mechanically loosen the attachment of stains to tooth structure resulting in whitening.7 , 4 1 '4 2 However, it has long been established, at least in the industry, that bubbling due to release of oxygen does not contribute to the bleaching process. 3 8 Moreover, the ability of bleaching agents to remove intrinsic stains such as tetracycline stains, cannot be explained by this proposed mechanism. Atomic oxygen, released from the perhydroxyl anion (equation 4) has also been suggested as the active species in hydrogen peroxide bleaching. However, experimental evidence for the presence of atomic oxygen in aqueous solution has not been provided.36 H 0 2 H O + O (eq. 4) Effect on Restorative Materials Dental Amalgam and Alloys Only a handful of publications have investigated the effect of bleaching agents on amalgam restorations. However, possible consequences of bleaching amalgam restorations are of primary significance since bleaching agents frequently come in direct contact with this restorative material during at-home as well as in-office bleaching procedures. Exposure of dental amalgam to hydrogen and carbamide peroxide bleaching agents results in increased release of mercury and other alloys.4 4 Moreover, 40% carbamide peroxide releases significantly more mercury than lower carbamide peroxide concentrations, suggesting that mercury release is affected by bleaching agent concentration4 5 One study, reported on 19 differences in the amount of mercury released from 4 dental amalgam brands tested, but was unable to find any correlation between these differences and alloy composition.46 Due the controversy surrounding the use of dental amalgam, 4 7 it may be wise to protect dental amalgam such that mercury release is minimized. The application of a dental varnish (Copalite) to amalgam restorations prior to bleaching is one such approach, and it has been found to significantly reduce mercury release over non-coated controls 4 5 Resin Composites An ideal bleaching agent is one that exerts the same lightening effect on resin composite restorations as it does on natural dentition and does not alter mechanical or surface properties of the resin composite. A significant number of studies have investigated the possible consequences of bleaching agents on resin composite restorative materials. While the use of 35% hydrogen peroxide has been shown to significantly lighten the colour of a number of commercially available composites 4 8 carbamide peroxide does not appear to alter resin composite colour. One group reported no colour changes following 312 exposure hours 4 9 while another reported statistically significant, yet clinically undetectable lightening in 3 out of 12 possible resin composite-bleaching agent combinations.50 For this reason patients are advised that old restorations may require replacement following bleaching. In cases where teeth are lightened to a shade unobtainable by normal resin composites, newer and lighter resin composites known as "ultra light" composites have been developed to meet clinical demands.51 20 While carbamide peroxide is an ineffective agent for lightening non-stained composite resin restorations, one in vitro study has reported its efficacy in decolourising stained resin composites.5 2 Resin composite samples were first stained for 120 hours using cranberry juice/tea, chlorohexidine, following which they were exposed to the bleaching agent with (treatment) or without the active ingredient (control). Similar levels of decolourisation were reported for both groups, leading to the conclusion that ingredients other than carbamide peroxide may play a role in stain removal. However, the efficacy of a simple control such as water in removing the stains was not investigated in this study. Exposure to bleaching agents can potentially alter mechanical and surface properties, although the occurrence appears to depend on chemical formulation of the composite and bleaching agent. For instance, while carbamide peroxide does not appear to significantly affect the tensile strength of posterior composites, exposure to 30% hydrogen peroxide significantly reduces tensile strength of microfill, but not hybrid or posterior composites. 5 3 Effect of bleaching agents on resin composite hardness is controversial, as some investigators have reported increased hardness following bleaching, while others have reported little or no change in hardness. Cooley and Burger5 0 reported a statistically significant increase in hardness of three composites exposed to carbamide peroxide. In contrast, Bailey and Swift54 found that of the two composites—hybrid and microfilled—tested, the microfilled composite became significantly softer in the presence of a carbamide peroxide bleaching agent, and others showed a trend towards decrease in hardness. Nathoo and coworkers were unable to detect any significant changes in Knoop's microhardness tests conducted following 2 hours daily for 14 days treatment with 10% carbamide peroxide; however, tested surfaces were polished prior to microhardness evaluation.55 Further research is needed to determine factors responsible for hardness changes. 21 In addition to affecting mechanical properties, bleaching agents have been shown to affect surface morphology of resin composites. Significant increases in surface roughness following bleaching agents 5 0 , 5 4 or whitening dentifrices,56 as well as increased adherence of S.mutans, and S.sobrinus to bleached surfaces have been reported.57 However, the clinical significance of increased bacterial adhesion is questionable since the salivary proteins onto which bacteria adhere within the oral cavity appear to attach themselves as well to bleached composite surfaces as they attach themselves non-bleached surfaces.5 8 Luting Agents, Temporary Restorative Materials, and Porcelain As far as I am aware, there are no published articles on the effect of bleaching on porcelain, however, abstracts presented at dental conferences have shown minor, clinically insignificant changes in porcelain colour5 9 and surface morphology60 following bleaching. The effect of carbamide on methacrylate-based provisional restorations, resin composite material, and polycarbonate crowns was investigated in one in vitro study showing methacrylate-based restorations are significantly darkened by exposure to carbamide peroxide bleaching agent, while crowns and resin composites are not affected.61 Another intermediate restorative material affected by carbamide peroxide and hydrogen peroxide are zinc eugenol cement, commonly used between visits as a temporary restorative following endodontic therapy. Bleaching agents have been shown to alter the surface morphology of IRM, a well known zinc eugenol cement, resulting in macroscopically visible cracked surface following hydrogen peroxide exposure, and microscopically visible granular surface with crystalline areas following exposure to carbamide peroxide.62 Differences in surface morphology as well as the finding that carbamide peroxide reduces surface zinc oxide levels more drastically than hydrogen peroxide, suggest differences in mechanism of degradation of zinc eugenol cements by these two agents. 22 A number of adverse effects of bleaching agents on cements have been reported. Unlike resin composites which undergo little or no colour change, colour stability of luting agents and cements is significantly affected by exposure to bleaching agents.5 9 Lim et aF investigated the effect of 10% carbamide peroxide on colour change of glass ionomers, polyacid-modified resin-based composites, and a resin composite when exposed to 10% hydrogen peroxide, and found glass ionomers and polyacid-modified resin-based composites to show a high, and clinically significant, level of colour change in comparison to the resin composite which showed excellent colour stability. Colour stability of glass ionomer cements can be improved by glazing. Unglazed surfaces are susceptible to staining as well as bleaching,64 whereas glazed surfaces exhibit high colour stability and resist bleaching.52 In addition to colour changes, hydrogen and carbamide peroxide affect surface morphology. Following carbamide peroxide bleaching, zinc phosphate cement surfaces appear crystalline in texture, while glass ionomer cement surfaces appear washed off.65 Carbamide peroxide exposure has also been reported to cause severe erosion of zinc phosphate,65 and glass ionomer cements. 6 5 ' 6 6 In one recent study, on the colour stability of glass ionomers during bleaching, application of 10% hydrogen peroxide for 7-14 weeks completely dissolved samples such that colour measurements could no longer be taken. 6 3 Effect on Bonding Enamel It is now common knowledge that our ability to bond resin composite materials to enamel is affected by bleaching. Earlier investigations conducted on bovine teeth reported reduced bond strength following as little as a few minutes of exposure to 35% hydrogen peroxide 6 7 ' 6 8 This reduction in bond strength is time 23 dependent and has been confirmed in human teeth . Carbamide peroxide causes a similar but less drastic decrease in bond strength.73 Our ability to bond other dental materials to enamel has also been investigated. Exposure to 10% carbamide peroxide reduces bond strength of ceramic, 7 4 but not metal orthodontic brackets.75 Decreased bond strength to human and bovine teeth is at least partially reversible. While bonding is generally postponed for one week to insure proper bonding, conflicting results regarding the length of time required to insure proper bonding have been published. The variation is presumably due to the interaction of a number of factors such as concentration, type of bleaching agent, carbamide versus hydrogen peroxide, as well as length of exposure to the bleaching agent. Two studies recommend 24 hours of leaching, one following 3 to 6 hours exposure to 10% carbamide peroxide,76 the other after two 10-minute increments of exposure to 25% hydrogen peroxide;69 1 week of water leaching has been demonstrated to restore bond strength to near normal, after bleaching bovine enamel with 35% hydrogen peroxide,77 and human enamel with in-office or at-home bleaching procedures for 30 days. 7 8 Longer recovery times still of two weeks after exposure to different at-home bleaching agents for 60 hours followed by storage in artificial saliva 7 9, and three weeks after to exposure to 25% hydrogen peroxide, for 4 minutes have been reported.80 Two studies reported no significant difference in bond strengths to bleached and unbleached enamel. Despite lack of statistical significance, Josey and co-workers81 suggested that restorations not be placed immediately after bleaching as a trend for decreased bond strength was noticed. However, exposing samples to artificial saliva for 1 to 6 weeks allowed hydrogen peroxide to diffuse out of enamel, and bond strength to return to normal.81 The authors speculated that artificial saliva, which they used in their experiment, was able to remineralize enamel and in doing so inhibit reduction in adhesiveness. The other study failed 24 to bleach for a clinically relevant length of time, exposing enamel to carbamide peroxide bleaching for a total of 5 consecutive days. 8 2 The cause of this transient decrease is speculated to be the existence of residual peroxide on enamel surface. 6 8 , 8 3 The fact that fewer resin tags are visible in bleached compared to non-bleached samples has led to the speculation that polymerization of the resin is inhibited by presence of surface oxygen, released during degradation of hydrogen peroxide (free radicals in the resin preferentially attack oxygen and no longer are involved in polymerization of the composite).84 Pre-treatment of bleached surfaces with alcohol 8 5 or the use of bonding agents containing acetone or ethanol 8 5 , 8 6 reduces this adverse affect, apparently due the ability of these chemicals to displace water and as well as the oxygen contained within it. However, two independent investigations into the presence of oxygen on hydrogen peroxide-87 and carbamide peroxide-treated88 surfaces have detected no differences in elemental composition, giving rise to the speculation that dentinal tubules act as hydrogen peroxide and oxygen reservoirs, facilitating gradual release onto the surface. 7 2 , 7 8 Dentin Our ability to bond restorative materials to dentin is also adversely affected by exposure to bleaching agents. Studies on dentin bonding have primarily focused on the effect of high concentrations of hydrogen peroxide on dentin bonding. Reduced ability to bond a glass ionomer cement to bovine dentin;89 lack of ability90 or reduced ability91 to bond resin composite to bovine dentin; and reduced bond strength of composites to human dentin 9 2 , 9 3 have been reported in the literature. The effect of water leaching on the ability to bond to dentin is less clear. Delaying bonding for 1 week restores bond strength of composite to hydrogen peroxide-treated human and bovine dentin surfaces to near normal. 9 1 , 9 3 On the other hand, water storage of bovine dentin treated with 10% carbamide peroxide for a period 25 of one week does not improve its adhesion to composite, and water leaching hydrogen peroxide treated surfaces for a similar length of time results in decreased adhesion of glass ionomer to bovine dentin.89 It is common practice however to postpone bonding for 1 to 2 weeks in order to reduce adverse affects on bonding. As far as I am aware no studies have investigated the effect of bleaching agents on existing bonds between dentin and composite or other restorative materials. Effect on Microleakage and Sealing Ability In addition to interfering with the ability to bond to enamel and dentin, bleaching agents in general increase microleakage of bonded restorations. Crim and co-workers9 4 were the first to assess the affect of bleaching on microleakage of existing class V resin composite restorations and reported increased microleakage at dentin margins following thermocycling of carbamide peroxide-treated restorations. "Post-operative" microleakage has since been confirmed in class V composite and glass ionomer restorations treated with hydrogen peroxide as well as at-home carbamide peroxide bleaching systems.9 5 Considering our reduced ability to bond to bleached enamel and dentin surfaces, it is surprising that Crim and coworkers96 were unable to detect any differences between microleakage of class V composite restorations bonded to teeth previously treated with carbamide peroxide and non-bleached controls. However, more recent studies suggest that the ability to bond is in fact compromised following internal bleaching, as demonstrated by increased microleakage in composite restorations bonded to the pulp dentin, as well as dentin margins of composite class V restorations following internal bleaching. 9 7 , 9 8 In addition, to effects on microleakage, other studies have reported on reduced sealing ability of a number of intermediate restorative materials when used in 26 combination with sodium perborate and 35% hydrogen peroxide or sodium perborate and water. 9 9 ' 1 0 0 - 1 0 3 Effects on Enamel The effect of bleaching agents on surface morphology of enamel has been investigated extensively. A large majority of these studies suggest that enamel surface is altered following exposure to hydrogen peroxide, carbamide peroxide, or sodium perborate exposure. Titley and coworkers observed a white precipitate on bovine and human enamel surfaces following hydrogen peroxide exposure 104,105 | n a d d j t j 0 r i i increased porosity; 8 1 ' 1 0 4 ' 1 0 6 , 1 0 7 surface degradation and irregular etch pattern108; and demineralization of the substrate 8 1 , 1 0 9 have been reported. In addition, one profilometric study reported a tendency toward smoother surfaces following exposure to home bleaching systems. 1 1 0 A few studies have reported little or no significant changes in surface morphology following bleaching. 1 1 1" 1 1 3 However, these studies either failed to bleach for a sufficient, and clinically relevant, length of time or utilized faulty methodology. In one study, investigators detected little change after exposure to 10 to 30 minutes of exposure to 30% hydrogen peroxide (alone and with sodium perborate) and 6 hours of exposure to 10% carbamide peroxide; both application times, however, are below recommended exposure times for these agents. 1 1 1 Investigating the effect of bleaching agent on surface morphology of tea- and coffee-stained enamel Scherer and co-workers1 1 2 suggested that while bleaching may adversely affect non-stained enamel, stained enamel surfaces are protected against these adverse affects since the bleaching agent preferentially attacks the stain, leaving enamel unharmed. However, the study exposed samples to bleaching agent for 72 consecutive hours of bleaching and failed to refresh the bleaching agent on regular intervals, a necessary requirement since carbamide peroxide activity declines rapidly over a short period of time. 1 1 4 It is therefore, more likely that the lack of findings is due to absence of bleaching activity rather than the presence 27 of stain. Lastly, Haywood and coworkers1 1 3 exposed samples to 10% carbamide peroxide for an appropriate length of time, but designated one half of the tooth as control, covered its surface with varnish, and assumed this approach would provide the control portion with adequate protection from the bleaching agent. However, bleaching agents are known to move freely within enamel and dentin 4 2 and controls in this experiment were most likely exposed to the same bleaching regimen as treatment groups. Bleaching also alters mechanical properties of enamel. The effect of carbamide peroxide on enamel microhardness in the presence and absence of saliva has been assessed. Most of these studies report reduced hardness and calcium loss 108,109,115-118^  w n j | e a f e w r e p 0 r t no change in hardness following carbamide peroxide bleaching. 5 5 , 1 1 9 ' 1 2 0 Studies that were unable to detect any changes in hardness either bleached samples for a shorter periods of time than is clinically relevant, 8 2 , 1 1 9 or measured subsurface rather than surface hardness.55 Despite non-significant findings one of these studies reported significantly more enamel abrasion and reduced fracture toughness in bleached specimens. 1 1 9 A plausible explanation for these mixed findings is perhaps that while subsurface enamel is not adversely affected by bleaching, surface hardness is reduced. This hypothesis is supported by the fact that removing enamel before bonding returns bond strength to normal.71 Similar to carbamide peroxide, hydrogen peroxide severely affects enamel: the use of 30% hydrogen peroxide has been shown to significantly reduce enamel microhardness in as little as 15 minutes.121 Effects on Dentin and Cementum Dentin Structure Dentin is a hard, yellowish white, avascular tissue which underlies enamel and forms the bulk of the tooth. The modulus of elasticity of dentin is about 13-28 17GPa, compared to enamel which has a modulus of elasticity of about 84GPa. 1 2 2 Since dentin is significantly more extensible than enamel, it is tougher and acts as a support for the more brittle overlying enamel. 1 2 3 Chemical composition of dentin is estimated at approximately 30 vol% organic material, largely in the form of type I collagen, and 50 vol% mineral, mainly in the form of a relatively calcium-deficient carbonate-rich apatite with the formula Ca 1 0 (PO 4 )6„ and 20 vol% fluid. 1 2 4" 1 2 6 In its physiological state, dentin is a well-hydrated and porous tissue. The wetness is due to the existence of an intrapulpal pressure, the magnitude of which is approximately 25 mm Hg tor human teeth. However, the use of local anaesthetics, which are also vasoconstrictors, results in a significant decrease in blood pressure and subsequently intrapulpal pressure. 1 2 7 When viewed microscopically, several structural features of dentin can be identified, the most important of which as far as the current discussion is concerned, are dentinal tubules, and regions known as peritubular and intertubular dentin, respectively. The porosity of dentin can be attributed to the existence of dentinal tubules. These tubules are canal-like spaces within the dentin and are filled with tissue fluid in vital teeth. Dentinal tubules follow an S-shaped pattern beginning at the dentinoenamel junction (DEJ), and traversing through the entire thickness of dentin to the pulp. 1 2 8 Moreover, each tubule is an inverted cone, with its smallest diameter at the DEJ and its largest diameter at the pulp, ranging from about 0.8 /nm at the DEJ to approximately 3 /um near the pulp. 1 2 5 The main structural elements of dentin, besides the tubules themselves, are peritubular and intertubular dentin. Surrounding each dentinal tubule a collar of highly mineralized dentin is observed. This cuff of hypermineralized dentin, 29 known as peritubular dentin, is composed primarily of hydroxyapetite (ca. 95 vol%) and contains little, if any, collagen. 1 2 3 Intertubular dentin, on the other hand, covers the remaining areas and is primarily composed of a partially mineralized (30%) collagen matrix , 1 2 9 The hardness of peritubular dentin does not depend on location. Kinney and coworkers1 3 0 reported a value of 2.45 + 0.14 GPa for peritubular dentin irrespective of dentin depth. Hardness of intertubular dentin, on the other hand, was found to decrease almost four-fold from 0.51 ±0.02 GPa near the DEJ, to 0.13 ± 0.01 GPa near the pulp. 1 3 0 The portion of the overall surface area covered by intertubular and peritubular dentin as well the tubule density is a function of dentin depth and varies according to location. This can be attributed to two factors: Firstly, dentinal tubules converge onto the pulp, a natural consequence of which, is an increase in the number of tubules per unit area as compared to superficial dentin where dentinal tubules are relatively spaced out. Secondly, as previously mentioned, the diameter of dentinal tubules increases with increasing dentinal depth giving dentinal tubules their characteristic inverted cone shape. Tubule density increases from about 1.9xlO6 tubules/cm2 at the DEJ to between 4.5xl0 6and 6.5xlO6 tubules/cm2 near the pulp. 1 2 5 As a result of this increase in tubule density, a smaller portion of the overall surface area is covered by intertubular dentin at the pulp as compared to the DEJ; intertubular matrix area covers 12% of the overall area near the pulp, whereas it covers more than 96% of the overall area near the D E J . 1 3 1 Effects Increased incidence of root resorption reported in the literature following application of intracoronal bleaching involving application of 30% hydrogen peroxide and sodium perborate 1 3 2 , 1 3 3 has led to the launch of a number of 30 investigations into possible mechanism, consequently exploring effects on dentin and cementum. The presence of a white precipitate on the surface of dentin, 1 3 4 as well as a rough and "etched-like" appearance following application of 30% hydrogen peroxide or carbamide peroxide has been reported with bleaching agents of different pH exerting similar affects.1 0 7 Additionally, dentin hardness is adversely affected by hydrogen peroxide, carbamide peroxide, and various combinations of sodium perborate and hydrogen peroxide or water 1 2 1 , 1 3 5. One in situ study was able to detect differences in microhardness of enamel but not dentin, presumably due to alteration of dentin as a result of steam sterilization procedures that were utilized;1 1 7 another failed to detect surface hardness changes in specimens stored in human saliva during testing.55 This latter study attributed lack of difference to the use of saliva with its potential ability to remineralize; however, exposed surfaces were flattened with a diamond polishing disc before microhardness testing. Therefore, results of this study are—at the very best— merely a reflection of subsurface hardness. As far as I am aware, the effects of bleaching agent on fracture toughness of dentin has not been addressed. A plausible explanation for the observed reduction in hardness is the ability of the bleaching agent to affect organic and inorganic components of dentin, evidence for which is as follows: firstly, a reduction in Ca/P ratio of dentin occurs following exposure to carbamide and hydrogen peroxide, suggesting alteration of hydroxyapatite crystals within dentin; 1 1 5 and secondly a decrease in the organic-inorganic ratio of crushed dentin is observed following as little as15 minutes of exposure to high concentration 30% hydrogen peroxide.1 3 6 While the latter phenomenon has only been observed with hydrogen peroxide, similar results should be expected with carbamide peroxide. Lastly, exposure to hydrogen peroxide or carbamide peroxide has been reported to alter surface morphology of cementum resulting in a fragmented surface with 31 multiple irregularities,107 and alter the calcium/phosphate ratio of cementum in a similar fashion to that seen in dentin.1 1 5 Fracture Toughness Fundamentals Evaluation of bond strength has captured the interest of dental researchers for many years and has traditionally been assessed using tensile or shear test methods. The evaluation of bond strength in dentistry dates back to 1950's when Buonocore measured the amount of force required to dislodge a dental restorative material (methacrylate) from dentin.1 3 7 In recent years, three-dimensional finite element analyses of tensile and shear test methods have been conducted. The results of these analyses have revealed forces along the interface of resin composite-dentin specimens to be unevenly distributed in tensile as well as shear modes. In addition, the distribution of stresses along the interface has been found to be highly dependent on the test geometry, loading configuration, and mechanical properties of the materials being tested. 1 3 8 Another common criticism of these traditional tests is their reliability, as very large variations in test results have been observed. 1 3 9 Despite questions regarding the validity of these tests in measuring material properties, tensile and shear test methods are widely used in dental research. 1 4 0 This is presumably due to factors like familiarity with test methodology, the ability to test specimens of any dimension, and straightforward calculation of results, requiring knowledge of two simple parameters: average applied force and the area to which the force is applied. An alternative to tensile and shear tests is fracture toughness testing. Fracture toughness is a measure of the ability of a material or bonded interface to resist unstable crack propagation. Fracture toughness values are commonly reported 32 as the critical stress intensity coefficient, K|C (MPa.m ) or strain energy release rate Gic (J/m2) which indicate the critical level of force or energy after which a defect of a critical size will advance. 1 4 1 In addition, K|C is a quantitative measure of fracture toughness with a known point of origin (zero), which allows for direct comparison and quantification of individual measurements. The unique advantage offered by fracture toughness methodology is the ability to evenly distribute stresses along interfaces. Principles of fracture toughness only date back to the turn of the century and the discipline has gained popularity in engineering circles in a relatively short period of time. It was less than one hundred years ago when Inglis142 argued for the presence of stress concentrations at the crack tip. Inglis suggested that a solid body with an elliptical hole in the center, which is pulled in tension, does not see stresses evenly. 1 4 2 Moreover, maximum stress is present the two crack tips on either side of the ellipse (Figure 4). t o Figure 4. Stress concentration in an elliptoid crack. 33 The magnitude of this maximum stress (a m a x) , while a function of applied stress (©"applied), is actually larger than the applied stress by a factor of 1+2 a/b, where a is one-half the major axis of the ellipse and b is one-half the minor axis. Therefore maximum stress seen by the object is near the two tips and is given by the equation: 0" m a x =oapPiiedQ + 2alb) Given the nature of this equation, the longer the long axis of the ellipse is with respect to its minor axis, in other words, the longer the crack length is, the higher the stress concentration will be. Similarly, the shorter the minor axis with respect to the long axis of the ellipse, which incidentally is a measure of crack sharpness, the higher the concentration of force at the crack tip. Similar to the case for elliptical holes, other objects, which have defects present in them, experience maximum stress at the crack tip, the magnitude of which is actually larger than the tensile force applied. For this reason, failure initiates from the crack tip where maximum stresses are present. The factor by which this stress increases is known as the stress concentration factor kt and is dependent on the geometry of a given crack or defect.1 4 2 The field of fracture mechanics is based on the very principle that materials have defects and inherent flaws which act as stress concentrators, and dictate the strength of the material.1 4 3 The aim of applying fracture methodology, as said earlier, is to quantify the ability of materials or bonded interfaces to resist failure resulting from these increases in local stress. Griffith142 was the first to formulate an equation describing the fracture of cracked of elastic solids with infinitely sharp cracks and reckoned the existence of equilibrium between energy invested into the specimen to grow a crack, and the 34 surface energy released due to the formation of new surfaces and related the two with the following equation: where U = Potential energy of body with crack U0 = potential energy of body without crack o = applied stress a =one-half crack length f = thickness E = modulus of elasticity Ys = specific surface energy By differentiating the energy difference (U - U0) with respect to crack length, Griffith was able to formulate the following expression describing stress as a function of crack length at equilibrium: While Griffith's reasoning was sound, it was based on experiments conducted on elastic materials and failed to adequately describe the situation for plastically deforming materials like metals and polymers. Since Griffith's expression was intended for very sharp cracks in elastic materials, which yield little, the expression was later modified to include a yp term describing plastic deformation energy. Hence, the following expression was suggested: Alternatively, Irwin suggested the use of dU/da, the amount of elastic energy required to drive the crack by an amount da, which he denoted as G, the elastic energy release rate (J/m 1 / 2). 1 4 2 7 9 no a t + 4atys E 35 V m It can be readily seen that the elastic energy required to grow the crack by an amount da is a function of the stress (a), and crack length (a). Comparing this equation to the previous it can be readily seen, however, that the elastic energy release rate, G , is actually describing the sum of ys and yp: G = 2 (ys + Y P ) At the point of instability, G reaches a critical value G C , and uncontrolled fracture occurs. In the case of a straight-through crack where the cracked area covers the entire thickness of the specimen, the magnitude of G C can be estimated by Q JPmJ2 dC 2 da where P m a x is the maximum load recorded during testing, and dC/da is the rate of change of compliance with respect to crack length. This latter variable depends on specimen geometry and is determined experimentally. The alternative to energy analysis is the stress analysis approach, also known as the K approach 1 4 2 . Using the coordinate system, this approach allows for three-dimensional analysis of the stresses present at the crack tip with the use of two simple parameters: radius (r), and angle (0) according to the following equations: K d„ . 6 . 36/ a Y = . cos — (1 + sin — sm—) V27rr 2 2 2 K e „ . e . 30, (Jy = . cos — (1 - sin — sin—) V 2 ^ 2 2 2 K t . e e 3d, TYy - , (sin — cos — cos—) V 2 ^ 2 2 2 In this way, K serves as a scale factor and defines the extent to which force is concentrated at the crack tip (figure 5). Equations used to calculate critical 36 intensity factor K is dependent on specimen geometry and the type of laboratory test used. Solutions for commonly used configurations and tests have been calculated.1 4 2 Y Figure 5. Quantification of stresses at the crack tip. Chevron Notch Fracture Toughness Test A commonly used method to measure fracture toughness is the standardized chevron notch short rod (CNSR) fracture toughness test developed by Barker in 1977. 1 4 4 This test involves the application of a tensile load to a cylindrical specimen into which a chevron notch has been cut (Figure 6). In addition to 37 cutting a chevron notch, two shallow grooves are placed across top and bottom faces of the sample in order to allow for specimen gripping, and subsequent measurement of tensile force. The unique advantage offered by this test is the ability to determine fracture toughness (K|C) by measuring only one parameter: peak load required to fracture the sample, according to the equation: _ Pmax x Y * min D V W where Pm ax= maximum force recorded during loading D = specimen diameter W = specimen width Barker validated the use of technique by comparing fracture toughness values aluminum alloy, polymethylmethacrylate, fused quartz, and siltstone rock as determined by his test, to fracture toughness values reported in the literature. It was found that the results of his Kic calculations were in excellent agreement with Kic values reported in the literature for the same materials. F F Figure 6. Barker's chevron notch short rod specimen test Soon after the introduction of the technique Koblitz and coworkers1 4 5 used the so-called short-rod fracture toughness test to determine Kic for polymethylmethacrylate and a dental composite (Adaptic). The significance of this work was its ability to demonstrate excellent agreement with Barker's results, 38 while miniaturizing the test specimens to dimensions which were more representative of a typical dental restoration. While this test has been applied to testing numerous dental materials, few studies have investigated fracture toughness of bonded interfaces using this technique. One plausible explanation for this is the complexity in making samples. In order to fabricate a specimen that can be gripped for testing, both ends of the sample are made of composite. The side opposite to the bonding surface of the tooth slice is first embedded into composite. Next a highly polished spacer is placed on top of the tooth surface to create a chevron-shaped bonding surface. This exposed surface is then bonded and built up with composite in order to make the second half of the specimen (Figure 7). 1 4 6 Alternatively, the tooth slice is bonded on both sides without the use of a spacer and the chevron notch is created at the interface using a cutting blade. 1 4 7 Both of these procedures are cumbersome, time-consuming, expensive due to the amount of material required to fabricate a specimen, as well as prone to error. loading hole and direction of FIRST HALF RESIN Figure 7. Chevron notched short rod design for composite-tooth interfacial studies. Notchless Triangular Prism Fracture Toughness Test In 1996, a new method for determining the fracture toughness of materials and adhesive interfaces was developed.1 4 8 The notchless triangular prism (NTP) 39 specimen, is based on the chevron notch short rod fracture toughness test, and attempts to maintain specimen geometry as well as the theoretical principles supporting the short rod fracture toughness test, while overcoming difficulties in sample making. One way in which the new test simplifies sample-making procedures is through the use of two stainless steel holders which grip the specimen and eliminate the need to build resin composite arms for sample gripping (Figure 8). Secondly, the new test eliminates the need to introduce a chevron notch using spacers or blades. This is due to the fact that the two stainless steel halves which grip the sample can be separated during specimen mounting in order to create a gap similar to what would be created in the chevron notch short rod test using spacers or blades. Calculation of fracture toughness results, like the chevron notch short rod test, only requires knowledge of the maximum force at fracture. Figure 8. NTP testing jigs. Validity Tensile and shear tests, which fail to detect high and low stresses present at the interface, do not provide valid measures of bond strength. In contrast, fracture mechanics tests are based on the "real local effort it takes to create a certain amount of new crack surface".9 Hence, these tests are at least theoretically valid in that they measure what they are intended to measure, namely the maximum 40 force a material or interface can withstand before it undergoes unstable crack growth. In order to confirm that forces are evenly distributed along the interface, it is required that so-called "plane strain" conditions be satisfied. Brown and Strawley 1 4 2 experimentally determined the conditions required for a valid plane strain fracture toughness test to be performed. For the test to be valid, it is required that specimen thickness (D) be larger than or at least equal to D > 2.5 Kic where K|C is the stress intensity coefficient determined by the test, and a y s is the .2% offset yield strength of the material in the direction of loading in the test. As K I C approximates the size of the plastic (permanent deformation) zone, valid Kic testing must somehow be influenced by the relationship between overall dimensions of the plastic zone and specimen thickness. Brown and Strawley observed that for as long as the thickness is greater than the size of the plastic zone, the reported Kic adequately represents the sum of all stresses available at the crack tip. However, if specimen thickness is not large enough, then the size of the plastic zone will exceed specimen thickness and Kic will no longer be representative of stresses which are present at the interface. 1 4 9 Fracture toughness of dentin, which has a .2% yield strength value of 200 MPa, was recently studied using the NTP test. 1 5 0 Results revealed dentin to have K|C values ranging from 1-2 M P a m 1 / 2 depending on tubule orientation. 1 5 1 Based on these values, the minimum obligatory dimension for valid fracture toughness testing is 0.25mm. As the NTP specimen thickness (8mm) is well in excess of this minimum obligatory value, the test method is of value for reporting the K|C of dentin. 41 Similarly, the NTP fracture toughness test has been used to measure fracture toughness of various dental composites. Fracture toughness values for various composites range from 1.18 to 1.95 MPa m 1 / 2 . 1 5 2 The minimum obligatory value for valid fracture toughness testing of dental composites based on yield strength values in the range of 35-80 MPa, appears to be 1.48 mm. 1 5 3 Again, as the NTP specimen thickness (8mm) is considerably larger than this value, the test appears to be valid for fracture toughness testing of commercially available dental composites. The validity of chevron notch and NTP tests in measuring fracture toughness of homogenous materials can be determined using previously mentioned criteria. However, validity of these tests in reporting fracture toughness of interfaces remains unknown and warrants further investigation.154 This is due to the fact that Brown and Strawley's plane strain criteria are based on stresses applied to homogeneous materials. However, all available evidence, as revealed by analysis of fractured surfaces, suggests that fracture toughness testing of interfaces is valid. This evidence includes stability of crack growth under testing conditions, as well as planar crack propagation, both of which suggest the satisfaction of plane strain conditions. In addition, the NTP test shows excellent correlational validity,155 as fracture toughness values obtained using the NTP specimen fracture toughness test are in agreement with values previously reported in the literature. Ruse and coworkers compared fracture toughness test results for selected dental materials to values reported in the literature and found the correlation to be very good. 1 4 8 The literature was searched to include any additional dental materials that may have since been tested. Two dental composites, Surefil and Prodigy, which had been tested using the NTP test as well as another accepted fracture toughness 42 test, were included in the calculation of correlation. The coefficient of correlation was found to be .942 (p< .01). A c c u r a c y In the previous section, a high degree of correlation was observed between the results of the NTP and chevron notch tests. However, a high degree of correlation would be observed even if one test consistently scored lower than the other by the same amount each time. 1 5 5 Accuracy is a measure of how close the measurement is to the actual or true value. The formula used to calculate Kic values for the NTP test is the standardized formula used to calculate stress intensity coefficients for chevron notch specimens. K|C is obtained using the formula:1 4 8 Pmax x Y * min K|C= 7 = D V W where P m a x is the maximum load recorded at fracture ; Y * m i n is the minimum dimensionless stress intensity coefficient, estimated at 28; D is specimen holder diameter (8mm); and W is the specimen length (4V3 mm). While the use of this familiar formula allows for calculation of K|C by measuring only the peak force, it raises uncertainty about the closeness of the calculated K|C to the actual or true value. This is due to uncertainty about the exact value of Y*min, which may potentially introduce systematic error into the calculation of fracture toughness values. The dimensionless stress intensity coefficient, Y * m i n , is a constant. The value of this coefficient is not dependent on material properties but changes depending 43 on specimen geometry. The value for Y* m j n has been experimentally determined for a range of chevron notch dimensions through the use of a compliance calibration test. 1 5 6 Since dimensions of the NTP specimen fall outside the range of experimentally determined values for the chevron notch short rod test, the value of Y * m i n for the NTP test has been extrapolated using results from the previously described compliance calibration test. However, as Y * m i n is a constant, its deviation from the actual value will affect all Kic calculations equally. Therefore, the ability of the NTP test to rank materials and adhesives according to their fracture toughness values is not diminished by this extrapolation. Additionally, the uncertainty of this extrapolation is estimated at under 10%, which means the K|C value obtained from the NTP fracture toughness test is likely to be reasonably close to the actual fracture toughness value. 1 4 8 In order to be certain that reported Kic values obtained using the NTP test are the true Kic values, it is necessary to perform a compliance calibration test similar to that conducted by Bubsey and others in order to determine the exact value of Y* m i n for the NTP configuration. Precision Ruse and coworkers compared the fracture toughness of polymethylmethacrylate (PMMA) as determined by the NTP test and three different chevron notch geometries (Table 2).148 Results from this comparison suggest that the NTP and chevron notched tests are equally precise in testing fracture toughness of dental materials. These results also provide evidence for the accuracy of the technique. The evidence is particularly strong since all conditions except the test method 44 were kept constant. Test groups were tested using similar sample sizes, the same material and similar testing conditions (same time interval, etc.). Sample W/D a 0 Kic C N S R 1.23 0.27 1.15 ± .15 C N S R 1.63 0.518 1.08 ± . 1 3 C N S R 1.75 0.552 1.15 ± .12 NTP 0.88 0.5 1.03 ± . 1 5 Table 2. K,c of PMMA obtained by NTP and CNSR tests. Further evidence is a comparison of dental luting cements using the two tests. Mitchel and coworkers1 5 7 have determined the fracture toughness of six luting cements, using the chevron notch fracture toughness test, while Feduik and Ruse have determined fracture toughness values for five luting cements using the NTP fracture toughness test. 1 5 8 In order to allow for crude comparison between these dissimilar luting agents, the coefficient of variation was calculated for each reported standard deviation (table 3). NTP (n=8) C V C N S R (n=11) C V Fleck's .13 ± . 0 1 7.70% ScotchBond 1.31 ± . 1 7 12.90% Fuji I .32 ± .06 18.70% Fuji I .34 ±. 04 11.70% Panavia .98 ± . 1 9 19.40% KetacCem .37 ± .05 13.50% Durelon .35 ± .03 8.60% Fuji Cap I .37 ± .04 10.80% Vitremer .79 ± .1 12.60% Vitremer 1.08 ± .1 9.25% Table 3. K|C of luting cements obtained by NTP and CNSR tests. Two of the luting cements shown (Fuji I and Vitremer), have been tested using both fracture toughness techniques. It appears that a coefficient of variation between 10 -20 % of the mean can be expected for both techniques. At first 45 glance it appears as though the CNSR fracture toughness is more precise than the NTP test. However, different laboratories were involved in preparing samples. In addition, CNSR tests had a sample size of 11 per group, while NTP tests had a sample size of 8. As mentioned, the search for better bonding agents, as well more efficacious bond strength measurement techniques continues. Having addressed the precision of the two tests in comparing dental materials, it is now appropriate to address the degree of precision offered by these tests in evaluating interfacial fracture toughness. A difficulty that is encountered in evaluating precision of these techniques in evaluating adhesive interfaces is the lack of data comparing the tests under identical conditions. Tarn and Pilliar evaluated the fracture toughness of the resin composite dentin interface using the CNSR fracture toughness test and reported a Kic of .34 ± .21 MParn 1 7 2 using ScotchBond Multi Purpose (CV=61%, n=10).146 The same adhesive was tested using the NTP test to report a Kic of .50 ± .14 MPam 1 / 2 (CV=28%, n=8).159 To date ScotchBond is the only adhesive that has been tested using both techniques. Further testing should therefore explore fracture toughness of a range of adhesives under identical lab conditions using the two test methods. Fracture toughness values for adhesives tested using the chevron notch technique range from .2 ± .14 to .98 ± .22 MPam 1 / 2 , with coefficient of variation for the data ranging from 22-70%. 1 4 6 , 1 4 7 On the other hand, fracture toughness of adhesives tested using the NTP fracture toughness test range from .31 ± .11 to .93 ± .13 MPam 1 / 2 with coefficient of variation for adhesives tested so far ranging from 14 to 35% of the mean. 1 5 9 , 1 6 0 46 Thesis Aims Since the effect of bleaching on interfacial fracture toughness of resin composite-dentin interfaces has not been previously explored, the specific aims of this work are to evaluate the effect of different concentrations of a carbamide peroxide bleaching agent, and different lengths of exposure to the bleaching agent on interfacial fracture toughness of existing resin composite-dentin interfaces. The following hypotheses will be tested: H 0 : Interfacial fracture toughness is not affected by exposure to a carbamide peroxide bleaching agent. H a : Interfacial fracture toughness is affected by exposure to a carbamide peroxide bleaching agent. Moreover, since the bleaching solution contains carbopol and urea, the following hypothesis will be tested: H 0 : Interfacial fracture toughness is not affected by exposure to carbopol or the urea-carbopol combination. H a : Interfacial fracture toughness is affected by exposure to carbopol or the urea-carbopol combination. 47 Materials and Methods Specimen Preparation This section will describe procedures undertaken to fabricate 8mm long equilateral (4mmx4mmx4mm) triangular prisms. All fabricated specimens were one-half dentin and one-half composite with a thin layer of bonding agent sandwiched between them (figure 9). Figure 9. Schematic presentation of fabricated resin composite-dentin NTP Human molars and premolars, all of which were collected from Dental Surgery practices in the Greater Vancouver Regional District, were used in this experiment. Dental practices were instructed to store extracted, caries-free molars and premolars in tap water at room temperature, in jars which they were previously supplied with. Once jars were at least half full, teeth were collected. Upon collection, contents of jars were exposed to air under the fume hood for five minutes, and then flushed under cold tap water for three to five minutes. Soft tissues still adherent to teeth were removed using a hand instrument. No chemical or abrasive agent was used during the course of cleaning. In addition, pulp tissues were not removed since roots were generally not truncated during surgery. J^r^ composite ^ b o n d i n g agent dentin specimens. Cleaning 48 Storage Jars containing cleaned teeth were marked with date of receipt and stored in tap water in the refrigerator (s4°C) until the start of the experiment. At the time of usage teeth were no more than six months old. Obtaining Dentin NTP Specimen In order to fit the jig designed to be used during the NTP fracture toughness test, final samples needed to have the shape of a triangular prism. It was decided in advance to use buccal or lingual dentin for bonding. Therefore, all dentin prisms were prepared such that the long axes of prisms were oriented in the buccolingual direction, exposing buccal or lingual dentin for bonding. Following identification of buccal, lingual, distal and mesial aspects of each tooth using a dental anatomy textbook,161 buccal and lingual aspects of each tooth were marked with equilateral triangles. If one were to imagine the triangular tip of an arrow, the triangular were drawn such that the tip of this imaginary arrow would point towards the pulp, and the side opposite to this tip would lie parallel to the occlusal surface. 240 grit SiC sandpaper, mounted on a Buehler wet grinding machine was used to remove the roots as well as most of the occlusal enamel. The outline of the triangle was then used to place two cuts at approximately 60° to the occlusal surface. These cuts resulted in a roughly triangular prism oriented in the direction previously described. The triangular faces of the specimen were briefly placed on 240 grit sandpaper in order to align them perpendicular to the long axis of the prism. The long faces of the prism were then constantly rotated on the grinder using a custom made 49 specimen holder to reduce the dimensions and remove as much enamel and pulp dentin as possible. Once dimensions of the triangular face were relatively close to the desired specification (=4.5 x 4.5 x 4.5mm), 600 grit SiC sand paper was used in order to achieve a smooth surface, and grinding was continued until all three sides of the triangular prism were 4mm in length. Specimens were then stored in tap water at 37°C until bonding. Bonding surfaces were not exposed until immediately prior to bonding. Bonding It appears that 600grit sandpaper produces a smear layer similar to that observed in clinical situations.162 Therefore, bonding surfaces were ground on 600 grit sandpaper, and thoroughly dried using oil-free air. An acid etchant (ScotchBond Acid Etchant, 3M Dental), containing 37% phosphoric acid, was applied to dried surfaces for 15 seconds in accordance with recommendations of the manufacturer. The bonding surface was then rinsed with water for 10 seconds. As with most other current bonding agents, it was recommended by the manufacturer to keep the bonding surface moist in order to avoid collapse of collagen fibres. For this reason, while excess water was gently wiped off from the sides of the specimen, the bonding surface was not dried. Two consecutive coats of SingleBond (dentin bonding agent, 3M, USA) were brushed onto the surface and gently air-dried with oil-free air for 5 seconds as suggested by the manufacturer. The bonding agent was then polymerized with a curing light for 10 seconds. Bonding surfaces were then carefully inspected to ensure they appeared glossy. While a glossy surface implied the saturation of the so-called hybrid layer by the bonding agent, a bumpy, and non-glossy surface implied that the hybrid layer had not been fully saturated. In cases where the latter was observed, surfaces were reground with 600 grit SiC and bonding procedures were repeated until a glossy surface was observed. 50 Composite Build-Up In order to pack the dental composite onto the bonding agent, a reverse impression mould o f a 4 x 4 x 4 x 8 mm triangular prism was made. This was achieved by holding a 4 x 4 x 4 x 14mm Plexiglass triangular prism in the centre of a hollow cylinder with a diameter and height of 8 and 16 millimetres, respectively. An impression material (Coltoflax, USA) was then injected into the space between the prism and cylinder walls and the reverse impression of a 4x4x4x14mm triangular prism was obtained. In order to facilitate sample placement and removal from the mould, the hollow cylinder and the set impression material were sectioned longitudinally using a diamond cutting disc (Isomet, Buehler) and a surgical blade. After opening the mould, the dentin-adhesive complex was placed inside the mould. The mould was then closed and placed in a block of Plexiglas that had previously been bored out to match the diameter of the cylinder described above. The block enclosed the cylinder tightly in order to ensure proper marginal adaptation of the composite. Z-250 dental composite (3M, USA) was packed onto the bonding agent using a hand instrument, and subsequently light cured twice for 60 seconds: once from the top surface, and a second time from the side (along the long axis of the prism) after removing the mould, to ensure complete polymerization. For water to adequately penetrate the composite resin and counter the effects of polymerization shrinkage, a period of 1 week is required.1 6 3 For this reason, all bonded dentin/composite specimens were stored in tap water at 37°C for no less than 1 week to allow for adequate water sorption prior to exposure to the bleaching agent. Final trimming of dentin composite specimens, which was performed by wet grinding on 600 grit SiC sandpaper, was left until after storage. 51 Exposure to Carbamide Peroxide Following water storage, samples were assigned numbers and randomly divided into groups using a table of random numbers, 1 6 4 according to the experimental design to be described shortly. Samples were randomized not only with respect to numbers they were assigned, but also with respect to the treatment which they were to receive. The following experimental design was used: Cumulative Hours of Exposure 14 42 70 Control 9(A) 9(B) 9(C) 11% CP 9(D) 9(E) 9(F) 13% CP 9(G) 9(H) 9 (I) 16% CP 9(J) 9(K) 9(L) 21% CP 9 (M) 9(N) 9(0) 21%* CP 9(P) 9(Q) 9(R) Table 4. Experimental design I. CP = carbamide peroxide As previously mentioned, the question of interest was the effect of a carbamide peroxide-containing bleaching agent on interfacial fracture toughness. Perfecta carbamide peroxide bleaching agent (Premier Dental, USA) was chosen for this particular experiment because it offered a wide range of carbamide peroxide concentrations (11, 13, 16, 21% carbamide peroxide), covering the spectrum of carbamide peroxide concentrations offered by dental manufacturers. A different bleaching regimen is recommended for the 21% carbamide peroxide as compared to lower concentrations (11, 13, and 16%). The bleaching regimen suggested by the manufacturer for different concentrations of carbamide 52 peroxide is summarized in figure 10. A detailed description of the bleaching regimen as well procedures undertaken in order to expose teeth to different treatments follows. For 11,13, and 16% carbamide peroxide Week 1:1-2 hours daily (1 hour twice per day maximum) Week 2: 2-4 hours daily, with only 2 hours of continuous wear For 21% carbamide peroxide Week 1: V2 to 1 hour daily Week 2: 1-2 hours daily, with only 1 hour of continuous wear Figure 10. Manufacturer's recommended wear schedule. Fabrication of Bleaching Trays In order to expose teeth to carbamide peroxide, bleaching trays which could accommodate 4x4x4x8 mm triangular specimens were needed. A plexiglass rod with a diameter of 5mm was cut into nine 16mm long pegs. A five mm thick plexiglass slab was bored out with nine holes matching the diameter of the pegs. Pegs were then inserted into the holes, creating a model from which bleaching trays could be made. A vacuum impression apparatus (StaVac, USA) was used to heat up viscoelastic sheets supplied by the manufacturer, and subsequently form the reverse impression of the model previously described. Reverse impressions, obtained in this manner, had nine wells in which dentin-composite samples could be bleached. These wells had a radius of 5mm and a final height of 11mm. A total of 18 bleaching trays were made and distributed among the following six categories: Control, 11%, 13%, 16% CP, 21%, and 21%*. This resulted in three bleaching trays per concentration group capable of holding 27 samples, which could then be used to test the effect of concentration over three time intervals. Trays were then clearly marked in order to avoid confusion during the bleaching 53 procedure. Additionally, the volume of bleaching agent required to fully embed specimens was determined and a line corresponding to this level (5mm from the base) was placed along the side of each well. Procedures for Exposure The manufacturer recommends a different bleaching during the first week as compared to the second and third weeks. In addition, a different bleaching regimen is suggested for the 21% as compared to lower concentration groups. Therefore, two separate treatment plans were used for groups intended for bleaching with 21% carbamide peroxide: one treatment plan followed the manufacturer's recommendations (groups P through R)(table 4), while the other ignored these recommendations exposing samples to the same bleaching regimen as that suggested for the lower concentrations (groups M through O). Exposure took place in 30-minute increments until maximum daily limits were reached. 30-minute increments of bleaching were chosen due to the fact that extending bleaching time beyond 30 minutes has been shown to result in a significant decrease in strength of the bleaching solution.7 The bleaching procedure involved three main stages: treatment, rinsing, and storage. During the first week of bleaching it was recommended that teeth exposed to 11 through 16% carbamide peroxide be treated for a maximum of two hours. Groups D through O were, therefore, exposed to four 30-minute increments of carbamide peroxide. Controls (groups A through C) were exposed to the same daily limits as groups D through O. As it was recommended that 21% carbamide peroxide be used for no more than 1 hour during the first week, groups P through R (21%*) were exposed to two 30-minute increments of carbamide peroxide during the first week. The following procedure was undertaken to treat specimens. Three trays belonging to the control group and representing controls were filled with tap 54 water, while trays belonging to bleaching groups were sequentially filled with the correct concentration of bleaching agent. Next, teeth in each group were placed in their respective solutions in the same sequence as trays were filled. In all cases the dentin side of the sample was inserted first in order to avoid desiccation of dentin during the bleaching treatment. Trays were then stored in the incubator at 37°C for 30 minutes. At the end of the 30-minute period, bleaching trays were removed from the incubator. Dentin-composite samples were then carefully removed from the wells using a water syringe and gentle pressure. Samples were then rinsed with water in a 50ml container. Contents were swirled for 5 seconds and rinsed for a total of five times to ensure full removal of the bleaching agent from the sample. Following this, rinsed specimens were stored in tap water in the incubator for 1 hour. All other trays were removed in the same sequence as they were initially placed in the incubator, rinsed, and stored in the incubator in the same manner and length for the same length of time. Time at which trays were filled with the bleaching agent; teeth were placed in trays; trays were placed in incubator; removed from incubator; as well as time when samples were rinsed and placed back in the incubator in tap water were carefully monitored to expose all groups to the same conditions. While samples were stored in the incubator in tap water, trays were rinsed using pressurized water to ensure complete removal of the bleaching agent from wells, and dried using compressed air. The above procedure was carried out until a cumulative exposure of 7 hours was achieved for 21%* groups, and 14 hours of cumulative exposure was achieved in the case of groups belonging to 11, 13, 16, and 21%. Groups A, D, G, J , M, and P were removed at this time and stored in tap water in the incubator for 12 hours 55 before testing. The testing methodology will be treated separately in the next section. In the second week, exposure was doubled as suggested by the manufacturer. This was achieved by exposing samples to twice as many treatments rather than extending treatment time, which would render the bleaching solution ineffective. Therefore, all groups except those which were removed for testing (A, D, G, J , M, and P) and those which belonged to the 21%* group (Q and R), were exposed to the bleaching agent for eight 30-minute increments, totalling 4 hours/day for 7 days resulting in 42 hours of cumulative exposure. The 21%* groups (Q and R) were also exposed to twice the exposure of first week, totalling 2 hours/day for 7days (21 hours of cumulative exposure). At this point groups B, E, H, K, N, and Q were removed and stored in water for 12 hours prior to testing. Bleaching regimen for the last week remained the same as that for the previous week: samples belonging to control, 11, 13, 16, and 21% (C, F, I, L, O) were exposed to eight 30-minute increments of bleaching treatment, while 21%* (group R) was exposed to 21% carbamide peroxide for four 30-minute increments. Once 21%* (R) had been exposed to 35 cumulative hours, and others (C, F, I, L, O) to 70 hours of cumulative exposure, samples were removed and stored in water for 12 hours before testing. Bleaching procedure (exposure, rinsing, and storage) remained the same as previous weeks. Exposure to Carbopol and Urea The investigation of the effect of urea and carbopol on interfacial fracture toughness of composite-dentin interfaces was undertaken separately. Two additional test solutions were selected to accomplish this. The first of these was a hand-mixed solution of carbopol in water. A number of trial tests were conducted to estimate the amount of carbopol and water required to produce a solution offering similar consistency and viscosity as the bleaching agent. It was 56 determined that the gradual dissolution of 2.64 grams of carbopol in 100 millilitres of water would result in a consistency and viscosity similar to the bleaching agent. The other test solution was a mixture of carbopol and urea. The amount of urea (percent weight) present in the highest concentration of the bleaching agent (21%) was determined and added to carbopol and water in order to produce a solution of urea in carbopol. The amount of urea required was estimated at approximately 4 grams per 29 grams of the carbopol water solution. Experimental design for carbopol and urea testing is summarized in table 5. Samples were exposed to either carbopol (S) or carbopol and urea (T) in bleaching trays similar to those previously described. Hours of Exposure 70 Carbopol 9(S) Carbopol & Urea 9(T) Table 5. Experimental Design II. In order to prevent moisture loss trays were sealed with cellophane. Trays were then stored in the incubator at 37°C for 70 consecutive hours. Specimens were then removed from the incubator, rinsed in water according to procedures described for carbamide peroxide bleaching, allowed to rehydrate in tap water for 12 hours, and later tested for fracture toughness. Fracture Toughness Testing The notchless triangular prism (NTP) fracture toughness test was used to assess interfacial fracture toughness. 57 The dentin-composite samples were mounted in NTP testing jigs, by first placing and securing the dentin half in the holding jig. Dentin samples were mounted such that the crack tip would always be located in deep (pulp) dentin. In securing Figure 11. NTP specimen testing using Instron 4311. dentin, the adhesive interface was placed 100 microns outside the testing jig under 16-fold magnification, the rational for which will become clear shortly. A small initiate, approximately 100 microns in length, was placed at the interface under 16-fold magnification. Following initiation, a 200micron thick spacer was held adjacent to the dentin half before mounting the second jig. This procedure ensured that the two jigs were 200microns apart, and that the initiate was located in the middle of this gap (figure 11). This complex was then mounted onto an Instron 4301 Universal Testing Machine (Instron Canada, Canada) using specialized pins, and the machine was operated under tensile loading conditions and a cross head speed of .1mm/min. 58 Maximum load applied to sample at fracture was used to calculate the critical stress intensity factor (Kic) according to the following equation: Pmax x Y * min K|C = 7= D V W In this equation, P m a x is the maximum load recorded at fracture ; Y * m i n is the minimum dimensionless stress intensity coefficient estimated at 28 for NTP samples; 1 4 8 D is specimen holder diameter; and W is the specimen length. Scanning Electron Microscopy Observations Subsequent to fracture toughness testing, samples were dried for a period of 30 days. Following this, samples thought to be representative of each group, based on slopes of load-displacement curves and fracture toughness means were selected for observation under the scanning electron microscope. Corresponding dentin and composite fractured surfaces were mounted on the same stub using adhesive tape such that both fractured halves could be observed. Surfaces were sputter coated with gold using a sputter coater (Hummer VI, Technics) and observed using a scanning electron microscope (StereoScan 260, Cambridge). Photographs were taken at X25, X50, X500, and in some cases X1500, and compared for surface changes. More than 70 dentin and composite fractured surfaces were observed under the scanning electron microscope and nearly 300 SEM images at various locations and magnifications were recorded. Statistics SPSS statistical software was used to analyze the data. The general linear model (GLM) (Type III Sum of Squares) was employed to carry out a two-factor analysis of variance (ANOVA) and assess the effect of 6 levels of concentration and 3 59 levels of cumulative exposure time on interfacial fracture toughness. The Bonferroni multiple means comparison test was used to identify significant differences (p<0.05). Additionally, one-way ANOVA was employed to test the effect of carbopol and urea on interfacial fracture toughness. 60 Results The results of interfacial fracture toughness tests are summarized in table 6. Cumulative Hours of Exposure 14 42 70 Control 0.94 ±0.12 0.91 ±0.26 0.99 ±0.26 Carbopol 0.93 ±0.15 Carbopol & Urea 0.78 ±0.10 11% CP 0.87 ±0.19 0.76 ± 0.09 0.42 ±0.12 13% CP 1.08 ±0.20 1.00±0.13 0.63 ±0.10 16% CP 1.10 ±0.44 0.73 ±0.15 0.53 ±0.17 21% CP 1.2 ±0.41 0.82 ±0.11 0.42 ±0.13 21%* CP 1.01 ±0.20 0.66 ±0.11 0.62 ±0.11 Table 6. Results. Mean fracture toughness (Kic) ± SD Experimental design used to investigate the effect of carbamide peroxide on fracture toughness is a two-factor ANOVA with six levels of concentration, and three levels of exposure time being independent variables, and fracture toughness being the dependent variable. Two-factor analysis of variance indicated a statistically significant difference in fracture toughness based on concentration (p=0.003), as well as length of exposure (p=0.000). Additionally, the interaction between these two factors was also significant (p=0.000)(Table 7). Confidence intervals plotted in figure 12 help illustrate significant differences identified by two-factor ANOVA and Bonferroni multiple means post hoc comparison tests. 61 Tests of Between-Subjects Effects Dependent Variable: Kic Source Type III Sum of Squares3 df Mean F Sig. Corrected Model 5.995 17 .353 7.997 .000 Intercept 76.588 1 76.588 1736.881 .000 TIME 3.702 2 1.851 41.978 .000 CONCENTRATION .858 5 .172 3.894 .003 TIME*CONCENTRATION 1.709 10 .171 3.875 .000 Error 4.410 100 4.41 E-02 Total 93.733 118 Corrected Total 10.405 117 Table 7. Two Factor ANOVA. With the exception of experimental controls, groups exposed to 70 hours of carbamide peroxide bleaching had significantly lower fracture toughness than counterparts exposed to 14 hours of bleaching; in addition a pattern of decreasing fracture toughness with increasing time was noted from 14 to 42 hours, and from 42 to 70 hours. There were no significant differences between different concentrations exposed to bleaching for the same length of time, with the exception of groups K (16%, 42hr) and Q (21*%, 42 hr), which exhibited decreased fracture toughness compared to group H (13%, 42hr) (p=0.021 and p=0.003, respectively). 62 1.8 CM 1.6' 1.4' 1.2' < E 1.0' CC 0_ 2 .8' O ^ .6 • .2' 0.0 N = [I] 7 5 8 Control 6 7 5 11% 8 8 6 13% 7 6 5 16% 7 8 6 21% • 7 5 7 21%* I time I • 14hrs I O 42hrs T J • 70hrs Concentration Figure 12. 95% confidence intervals for mean fracture toughness. Controls tested after 14, 42, and 70 hours (Groups A, B, and C) were not significantly different from one another (p=0.78). After 70 hours of cumulative exposure, all treatment groups (F, I, L, O, and R) had significantly lower interfacial fracture toughness values than the control group (C) with significance levels ranging from 0.000 to 0.003. Moreover, fracture toughness values of carbopol (group S) and urea in carbopol (group T), tested after 70 consecutive hours of exposure, were not significantly different from pooled controls tested previously, which were stored in tap water (p=0.10) (table 8). 63 ANOVA Dependent Variable: Kic Sum of Squares df Mean Square F Sig. Between Groups Within Groups 1.072 .165 32 2 3.349E-02 8.230E-02 2.45 .102 Total 1.236 34 Table 8. One-Way ANOVA SEM observations of fractured surfaces suggested different patterns of failure for treated and non-treated groups. Examined fracture surfaces belonging to controls tested at the three set intervals, as well as samples representative of groups D (11%, week 1) and G (13%, week 1) suggested failure within the adhesive layer. Signs of adhesive failure are apparent at low and high magnification, and can be seen in figure 13. In contrast, bonded interfaces exposed to higher concentrations (i.e. 16, and 21%), which were selected for SEM observation exhibited partial cohesive failure in dentin along boundaries as early as 14 hours after bleaching. Following 42 hours of cumulative exposure to the bleaching agent, all treatment groups failed at least partially in dentin, and exhibited fracture surfaces similar to that of bonded interfaces exposed to higher bleaching agent concentrations for 14 hours (Figure 14). Following 70 hours of cumulative exposure to the bleaching agent, composite-dentin samples were affected such that fracture took place almost entirely in 64 dentin. In addition dentinal surfaces appeared rough, etched-like, and degraded, as can be seen in figure 15. 65 B 25.4>: l:«.D H0<8NR SI llvl P < 3: 3d I Figure 13. Adhesive failure. A) Fractured composite surface of 42-hour control (X24). B) Fractured 42hr control dentin surface corresponding to composite seen in A (X500). C) Fractured 14hr 13% dentin surface (X24). D) Fractured 14hr 13% composite surface corresponding to dentin in C observed at X500 magnification. 66 Figure 14. Fractured surfaces of samples exposed to 42 hours of bleaching. A) Dentin fractured surface exposed to 13% carbamide peroxide. B) Composite surface corresponding to dentin seen in A. C) Fractured surface of sample exposed to 16% carbamide peroxide D) Higher magnification (X1500) observation of sample seen in C. E) Fractured surface of a sample exposed to 42 hours of 21% carbamide peroxide. F) Higher magnification of sample seen in E, showing intact resin tags pulled out of dentin. 67 S 21wk3 P: 535-Figure 15. Observed Effects after 70 Hours of B leach ing. A) 16% carbamide peroxide-treated dentin. B) Composite surface corresponding to A. C) Peritubular dentin dislodged form collagen matrix on dentin surface of sample treated with 16% carbamide peroxide. D) Intact resin tag within partially broken peritubular dentin. E) Rough and etched-like appearance of dentin. F) Side view of interface showing freestanding resin tags. 68 Discussion The results of this thesis, which set out to test the effect of different concentrations and different lengths of exposure to the bleaching agent on interfacial fracture toughness of existing composite-dentin interfaces, suggest that increasing exposure to carbopol-containing carbamide peroxide bleaching agent decreases interfacial fracture toughness. In other words, existing composite-dentin interfaces that have been bleached are more susceptible to fracture than controls exposed to the same environmental conditions without bleach. Under conditions of this experiment, a step-wise reduction in fracture toughness with increasing length of exposure to the bleaching agent was observed. However, bleaching agent concentration did not appear to play a statistically significant role in the fracture toughness of existing composite-dentin interfaces; all bleaching agents affected fracture toughness similarly. Relatively few studies have reported on the fracture toughness of interfaces between two dissimilar materials, and in particular composite-dentin, and composite-enamel fracture toughness. This is presumably due to complexity involved in making and testing specimens. Control groups, which were bonding agent-mediated composite-dentin specimens tested at 1, 2, and 3 weeks, had statistically similar fracture toughness values of approximately 0.9 ± 0.1. This value is similar to previously reported fracture toughness values for dentin-composite interfaces. Tarn and Pilliar (1993) reported a Kic of 0.67 ± 0.29 MPa.m 1 7 2 using chevron-notch methodology, a different bonding agent, and bovine dentin, which is known to produce lower bond strength values than human dentin. 8 9 , 1 4 6 Similarly, Armstrong and coworkers reported a K|C of 0.88 MPa.m 1 7 2 for human dentin using chevron-notch methodology and a different bonding agent. Samples were stored in water for a period of 30 days, closely matching the storage conditions of the current experiment.147 69 In this experiment, dentin-composite interfaces were directly exposed to the bleaching agent. Direct exposure to the bleaching agent can occur in cases where carious, lesions extend into dentin, or when enamel abrasion occurs. 9 1 , 1 1 5 Additionally, defects in the cementoenamel junction can reveal a strip of exposed dentin in cases where enamel does not overlap cementum.1 6 5 Indirect exposure of composite-dentin interfaces, on the other hand, can occur in a number of ways. Permeability of dental hard tissues to hydrogen and carbamide peroxide is well established. Arwill and coworkers1 6 6 demonstrated increased penetration of radiolabled Na through sound enamel and dentin following exposure to hydrogen peroxide or urea. Interestingly, current at-home bleaching systems use carbamide peroxide, which breaks down into hydrogen peroxide and urea, both of which appear to increase permeability of dental hard tissues. Similarly, exposure to as little as 15 minutes of 30% hydrogen peroxide or 10 to 15% carbamide peroxide has been shown to readily penetrate the walls of sound teeth and enter the pulp chamber. 1 6 7 , 1 6 8 Moreover, permeability of dentin itself to hydrogen and carbamide peroxide has been established.1 6 9 Dentin-composite interfaces may come in further contact with the bleaching agents in teeth restored with resin composites since more hydrogen and carbamide peroxide penetrate into the pulp chamber of restored teeth in comparison to sound teeth, suggesting contact of hydrogen peroxide with composite dentin interfaces through surrounding tooth hard tissues, as well as the material with which the tooth has been restored. 1 7 0 , 1 7 1 Another way in which exposure can occur is through leaky margins. One commonly cited problem with resin composite restorations is leaky margins, which can potentially lead to secondary caries. However, the presence of leaky margins alone is not a criterion for replacement of these restorations.172 Since restorations exhibiting microleakage alone are not replaced, leaky margins can 70 potentially act as micro-sized channels allowing for exposure of dentin-composite margins to hydrogen peroxide. Bleaching existing class V restorations for 18 hours, a below average exposure time, has been shown to cause microleakage along dentin composite margins.94 Increased microleakage can, in turn, lead to further exposure of composite-dentin interfaces to the bleaching agent, and comprised integrity of these interfaces. A significant decrease in interfacial fracture toughness was observed with increasing exposure to the bleaching agent. Scanning electron microscopy of fractured surfaces revealed differences in fracture pattern between controls, exposed to tap water, and experimental groups exposed to different concentrations of the bleaching agent. Observed specimens from the control groups all failed adhesively and showed no sign of alteration along the edges of the specimen. This type of interfacial failure is in agreement with failure mode of dentin-composite restorations in clinical practice.1 7 3 In fracture toughness testing the crack propagates along the path of least resistance. Therefore, adhesive failure, as seen in controls, suggests the adhesive layer to be the least fracture-resistant component of this bonded joint. On the other hand, observed fracture surfaces belonging to groups exposed to the bleaching agent followed a different fracture path than control groups. Following 42 hours, and in some cases as early as 14 hours after bleaching, interfaces exposed to bleaching agent exhibited signs of the bleaching agent penetrating along the edges of the specimen, causing partial cohesive failure in dentin. The change in fracture path suggests a change in fracture resistance of bonded joint components, and possibly a weakening of the dentin or hybrid layer compared to the adhesive. In addition, resin tags were observed to have pulled out of dentin, suggesting one of two possibilities: 1) dissolution of the bonding agent as a result of exposure to the bleaching agent to the extent that resin tags no longer adhere to dentinal 71 tubules; or 2) degradation of dentin by the bleaching agent. Previously reported effects of bleaching agents on dentin support the latter hypothesis. Alteration of the Ca/P ratio of dentin 1 1 5 suggests alteration of hydroxyapatite crystals within dentin. Hardness, generally considered to have high correlation with calcium content, has been shown to decrease following bleaching. 1 2 1 , 1 3 5 Moreover, greater dissolution of the organic component of dentin as a consequence of bleaching has been reported.136 Recent dentin bonding agents, such as the one used to bond specimens in this thesis rely on removal of the smear layer and opening of dentinal tubules by the use of acid etchant, and subsequent mechanical interlocking of the resin within exposed collagen and dentinal tubules through the use of hydrophilic monomers with potential to displace water.3 Since successful bonding depends on interlocking within the collagen network and or dentinal tubules, both of which are adversely affected by the bleaching agent, compromised bonding and lower fracture toughness are plausible consequences of bleaching. The question of extending the conclusions of this study to other bonding agents currently available in the market remains and could potentially warrant further investigation. However, regardless of mechanism of bonding, it is speculated that similar results to those reported in this study can be expected with other bonding agents since all evidence suggests degradation of dentin, the bonding substrate. Earlier bonding agents relied on bonding to smear layer components.1 7 4 Since composition of the smear layer is similar to dentin from which it is prepared, 1 7 5 it is likely that prolonged exposure to hydrogen peroxide will exert similar effects on the smear layer as dentin. Regardless, degradation of the bonding substrate will inevitably result in lower adhesion. Under the experimental conditions, all bleaching agents affected interfacial fracture toughness similarly. This is in agreement with findings on the effect of bleaching agents on the mineral component of dentin: Rotstein and coworkers1 1 5 72 reported similar reductions in the Ca/P ratio of dentin with bleaching agent concentrations ranging from 10 to 15%. However, the organic component of dentin is more significantly affected by 30% hydrogen peroxide than 3% hydrogen peroxide,1 3 6 suggesting that our inability to detect differences in concentration may have been due to narrow range of bleaching agent concentrations tested. Nevertheless, concentrations tested covered the spectrum of bleaching agent concentrations currently available for at-home bleaching. Future research, could potentially investigate the affect of sodium perborate and high concentration (30%) hydrogen peroxide, both of which are commonly used during in-office procedures, on interfacial fracture toughness of dentin-composite interfaces. Such an investigation would report on adverse effects of these agents on composite-dentin interfaces and test the accuracy of this explanation. Specimens were stored in tap water when not exposed to the bleaching agent. The rational behind choosing tap water in place of distilled water was to include electrolytes. Saliva contains a variety of electrolytes including sodium, potassium, calcium, magnesium, bicarbonate, and phosphates.1 7 6 It is composed of more than 99% water but also nitrogenous compounds such as urea and ammonia. 1 7 6 In addition to the above, saliva contains a number of enzymes, one of which is peroxidase with the ability to break down hydrogen peroxide produced by oral bacteria.1 7 6 A recent in situ study on the effect of bleaching agent on hardness noted demineralization of enamel despite the presence of saliva. 1 1 7 The same study was unable to detect differences in dentin hardness, potentially, due to sterilization technique utilized involving exposure to 120°C, which may have altered physical properties of dentin prior to the start of the experiment. The explanation for demineralization of enamel, despite presence of saliva, however, may be two fold: firstly, very little saliva is present during in-office or at-home procedures since the field is isolated in both cases; secondly, the volume and concentration of the bleaching agent is likely many fold higher than levels which oral cavity enzymes would be expected to encounter under normal physiological conditions. 73 Studies on the ability to bond to enamel have shown a transient decrease in bond strength lasting one to three weeks. The effect of long-term water storage on interfacial fracture toughness was not assessed during this experiment. However, one may speculate that the observed reduction in fracture toughness may not to be transient for a number of reasons: 1) unlike the ability to bond where effect of bonding surface conditions are assessed, the present study assesses the effect on existing and already-bonded restorations, removing concerns about the presence of residual peroxide on bonding surface; and 2) in light of the effect of bleaching agents on organic tissues, the component of dentin which current bonding agents rely upon to achieve bonding, it is unlikely that remineralization of the relatively little inorganic component of dentin, at least in relation to enamel would improve bonding to any significant extent. Nevertheless, the effect of long-term water storage on interfacial fracture toughness cannot be ascertained without further investigation. The primary interest of this thesis was to determine the effect of a carbamide peroxide bleaching agents on existing dentin-composite interfaces. However, the effect of carbopol and urea on interfacial fracture toughness, were also investigated. It was determined that fracture toughness of dentin composite interfaces exposed to carbopol or urea in solution with carbopol, were not significantly different from controls stored in tap water (p=0.12). Analysis of fractured surfaces, similar to controls, exhibited adhesive failure suggesting a similar fracture path to that seen in controls. Moreover, neither of these test solutions appeared to penetrate the interface. However, a different exposure regimen was followed for carbopol and urea groups. The effect of incremental exposure to urea and carbopol, could potentially be the subject of further research. A significant interaction between concentration and time was noted. This interaction may be explained by the differential affect which time had on controls as compared to treatment groups: while treatment groups exhibited decreased 74 fracture toughness over time, fracture toughness of controls remained constant (figure 16). Conducting statistical analysis following exclusion of controls confirms this hypothesis. 1.4 T 1 4 w » 14hrs 42hrs 70hrs time Figure 16. The effect of time on fracture toughness of different concentration levels. Lastly, the effect of bleaching agent on fracture toughness of resin-composite enamel interfaces remains the subject of further investigation. A similar investigation to that conducted during this thesis should therefore be conducted on existing composite-enamel interfaces. 75 Conclusion We are the first to report on the effects of a bleaching agent on fracture toughness of resin composite-tooth interfaces. The results of this thesis suggest that fracture toughness of resin-composite dentin interfaces is decreased as a result of carbamide peroxide bleaching. It is further concluded, that of the two factors, namely concentration and cumulative exposure time, that were tested, the latter appears to significantly reduce fracture toughness. Scanning electron microscopy observations of fractured surfaces, confirm these findings, as samples exposed to bleaching agent exhibit a different failure mode from control controls. The apparent shift from failure within the adhesive layer, as observed in controls, to partial failure in dentin observed in treatment groups, suggests the ability of the bleaching agent to affect dentin, and consequently, interfacial fracture toughness. This hypothesis is in agreement with previously reported effects of hydrogen and carbamide peroxide on organic and inorganic components of dentin. Under experimental conditions of this investigation, all concentrations appeared to affect fracture toughness similarly. However, our inability to detect differences between concentrations does not necessarily suggest that concentration does not affect interfacial fracture toughness of existing composite-dentin interfaces. Rather, it suggests that within the range of concentrations tested, the adverse affect on interfacial fracture toughness is predominantly due to increased length of exposure. With the exception of the current investigation, no studies to date have investigated in vitro or in vivo effects of carbamide peroxide on interfacial fracture toughness tests. 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