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Micro-computed tomography analysis of post space preparation in teeth obturated with carrier-based thermoplasticized… Schroeder, Agmar Anthony 2014

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MICRO-COMPUTED TOMOGRAPHY ANALYSIS OF POST SPACE PREPARATION IN TEETH OBTURATED WITH CARRIER-BASED THERMOPLASTICIZED GUTTA PERCHA TECHNIQUES  by Agmar Anthony Schroeder B.Sc. (Hons), University of Western Ontario, 1995 D.D.S., University of Western Ontario, 2000  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Craniofacial Science)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  December 2014  © Agmar Anthony Schroeder, 2014  ii Abstract  Many dentists use the obturation technique of carrier-based thermoplasticized gutta percha. Placement of a post may be indicated for teeth in which there has been extensive loss of coronal tooth structure. This study aimed to determine if post space preparation deviated from the endodontic preparation in teeth obturated with the Thermafil, GuttaCore, or gutta percha material. Forty-two extracted human permanent maxillary lateral incisors were decoronated and their root canals instrumented with nickel-titanium rotary files using a standardized protocol. Samples were divided into three groups and filled with Thermafil, GuttaCore, or gutta percha, before post space preparation was performed. Teeth were scanned using micro-computed tomography after obturation, and again after post space preparation. Scans were examined for post space deviation, volume and linear deviation of post space preparation, and minimum root thickness before and after post space preparation. Data were analyzed with parametric and non-parametric statistical methods. Deviation occurred in eight teeth; seven from the Thermafil group (50%), one from the GuttaCore group (7%), and none from the gutta percha group. Deviation occurred statistically more often in the Thermafil group than in each of the other two groups (p < 0.05). The volume difference before and after post space preparation was statistically greater in the Thermafil group than in each of the other two groups (p < 0.05). Linear deviation of post space preparation was greater in the Thermafil group than in both of the other groups, and was statistically greater than that of the gutta percha group (p < 0.05). The minimum root thickness before post space preparation was significantly greater than it was after post space preparation for all groups (p < 0.01). There was no difference between the groups with  iii respect to the difference in minimum root thickness before and after post space preparation. The differences in the number of samples showing post space deviation, volume, and linear deviation of post space preparation amongst the Thermafil, GuttaCore, and gutta percha groups were due to the presence or absence of a carrier as well as the different carrier materials.   iv Preface  The research question and study design were identified by Dr. Agmar Schroeder and were subsequently revised with contributions from Dr. Jeffrey Coil and Dr. Markus Haapasalo. Collection and preparation of the samples and performance of the research were carried out by Dr. Agmar Schroeder. Micro-CT scans were done by John Schipilow at the UBC Faculty of Dentistry Centre for High Throughput Phenogenomics. Micro-CT data collection and analysis were performed by Dr. Agmar Schroeder with assistance from Dr. Nancy Ford and John Schipilow. Ethics approval was required and granted from the University of British Columbia Clinical Research Ethics Board (certificate number H13-00109).  v Table of Contents  Abstract .............................................................................................................................. ii Preface ............................................................................................................................... iv Table of Contents ................................................................................................................v List of Tables .................................................................................................................... vii List of Figures .................................................................................................................. viii List of Abbreviations ......................................................................................................... xi Acknowledgements ........................................................................................................... xii Dedication ........................................................................................................................ xiii Chapter  1: Introduction .....................................................................................................1 1.1 Root Filling ............................................................................................................1 1.2 Root Filling Techniques .........................................................................................2 1.3 Posts and Post Space Preparation ...........................................................................9 1.4 Micro-computed Tomography .............................................................................. 15 1.5 Hardness of Root Filling Materials ....................................................................... 17 Chapter  2: Rationale and Hypothesis .............................................................................. 18 Chapter  3: Materials and Methods .................................................................................. 19 3.1 Sample Selection and Preparation ........................................................................ 19 3.2 Experimental Apparatus ....................................................................................... 20 3.3 Root Canal Instrumentation .................................................................................. 23 3.4 Root Canal Filling ................................................................................................ 25 3.5 Micro-CT Scans: First Scan ................................................................................. 27  vi 3.6 Post Space Preparation ......................................................................................... 28 3.7 Micro-CT Scans: Second Scan ............................................................................. 32 3.8 Data Analysis and Parameters of Interest ............................................................. 32 Chapter  4: Results ............................................................................................................ 43 4.1 Root Canal Curvature........................................................................................... 43 4.2 Occurrence of Deviation ...................................................................................... 43 4.3 Images ................................................................................................................. 45 4.4 Volumetric Analysis ............................................................................................ 47 4.5 Linear Deviation .................................................................................................. 51 4.6 Minimum Root Thickness .................................................................................... 54 Chapter  5: Discussion ....................................................................................................... 58 Chapter  6: Conclusion...................................................................................................... 72 References .......................................................................................................................... 74 Appendices ......................................................................................................................... 82 Appendix A - Confirmation of MicroView Volume Measurements ................................. 82 Appendix B - Volume Difference between First and Second Scans .................................. 83   vii List of Tables  Table 3.1 Post drills used and sequence ............................................................................... 28 Table 4.1 Samples with root canal curvature and group assignment ..................................... 43 Table 4.2 Number of samples with deviation and with no deviation of post space preparation ............................................................................................................................................ 44 Table 4.3 Volumes of root filling and air before and after post space preparation ................ 49 Table 4.4 Mean and range of volume difference before and after post space preparation...... 51 Table 4.5 Linear deviation of post space preparation ........................................................... 53 Table 4.6 Mean and range of linear deviation of post space preparation ............................... 54 Table 4.7 Minimum root thickness before and after post space preparation .......................... 56 Table 4.8 Mean and range of minimum root thickness before and after post space preparation ............................................................................................................................................ 57 Table B.1 Volume of tooth plus filling as shown in Figure A.1 before and after post space preparation .......................................................................................................................... 83   viii List of Figures  Figure 3.1 Sample with putty matrix and typodont. .............................................................. 21 Figure 3.2 Mannequin head and clinical simulation. ............................................................ 22 Figure 3.3 Expected canal preparation diameter in relation to sectioned surface of the root. . 24 Figure 3.4 Depth of post space preparation based on expected canal preparation diameter. .. 29 Figure 3.5 Schematic of MicroView’s Synchronized View showing registered images of axial slices from the first scan (left) and second scan (right) of the same sample side-by-side. ............................................................................................................................................ 33 Figure 3.6 Schematic of MicroView’s Fusion View showing registered images of axial slices from the first and second scans of the same sample superimposed. ...................................... 33 Figure 3.7 Schematic of deviation (left) and no deviation (right) of post space preparation. . 34 Figure 3.8 Schematic of volumes to be determined (indicated by dashed red outlines) before post space preparation (left) and after post space preparation (right). ................................... 35 Figure 3.9 MicroView Histogram from the region of interest of a sample after post space preparation. The large peak represents tooth structure. The smaller peaks represent water (left) and air (far left).   The peak representing root filling is not shown. .............................. 37 Figure 3.10 Schematic of MicroView’s Synchronized View showing selection of the axial slice to be used for measurement of linear deviation of post space preparation from the second scan (right). ............................................................................................................. 39 Figure 3.11 Schematic of MicroView’s Fusion View showing measurement of linear deviation of post space preparation (red arrow heads) from the centre of the endodontic  ix preparation (from the first scan) to the centre of the post space preparation (from the second scan). ................................................................................................................................... 39 Figure 3.12 Schematic of MicroView’s Synchronized View showing measurement of minimum root thickness before post space preparation (left)................................................ 41 Figure 3.13 Schematic of MicroView’s Fusion View showing measurement of minimum root thickness after post space preparation. ................................................................................. 41 Figure 4.1 Micro-CT slices showing a Thermafil sample with deviation (left) and a gutta percha sample with no deviation (right). .............................................................................. 44 Figure 4.2 Micro-CT reconstructions: before (left) and after (right) post space preparation - Gutta percha. ....................................................................................................................... 45 Figure 4.3 Micro-CT reconstructions: before (left) and after (right) post space preparation - GuttaCore. ........................................................................................................................... 46 Figure 4.4 Micro-CT reconstructions: before (left) and after (right) post space preparation - Thermafil. ........................................................................................................................... 46 Figure 4.5 Region of interest (yellow box) for volumetric analysis of a sample before (left) and after (right) post space preparation. ............................................................................... 48 Figure 4.6 Volumetric analysis (green) for filling and post space within the region of interest of a sample before (left) and after (right) post space preparation. ......................................... 48 Figure 4.7 MicroView’s Synchronized View showing registered images of axial slices from the first scan (left) and second scan (right) of the same sample side-by-side. ....................... 52 Figure 4.8 MicroView’s Fusion View (left) showing registered images of axial slices from the first and second scans of the same sample superimposed................................................ 52  x Figure 4.9 MicroView’s FusionView showing measurement of linear deviation of post space preparation (dashed red line) from the centre of endodontic preparation (lower green arrowhead) to the centre of the post space preparation (upper green arrowhead). ................. 53 Figure 4.10 MicroView’s Synchronized View showing measurement of minimum root thickness (dashed red line between green arrowheads) before post space preparation........... 55 Figure 4.11 MicroView’s Fusion View showing measurement of minimum root thickness (dashed red line between green arrowheads) after post space preparation. ............................ 55 Figure A.1 Measurement of the volume of tooth structure plus root filling material of a sample before (left) and after (right) post space preparation for comparison of volume measurement accuracy. ........................................................................................................ 82   xi List of Abbreviations   ISO…………………………………………….International Organization for Standardization rpm………………………………………………………………………..rotations per minute N∙cm……………………………………………………………………….Newton centimeters NaOCl........................................................................................................Sodium hypochlorite GC...............................................................................................................................GuttaCore GP............................................................................................................................Gutta percha TF.................................................................................................................................Thermafil Micro-CT………………………………………………………..Micro-computed tomography CBCT....................................................................................Cone-beam computed tomography SEM………………………………………………………….....Scanning electron microscopy mm………………………………………………………………………………….millimeters mm3…………………………………………………………………………..cubic millimeters µm………………………………………………………………………………....micrometers kVp……………………………………………………………………………peak kilovoltage µA………………………………………………………………………………...microampere ms…………………………………………………………………………………..millisecond std. dev.…………….………………………………………………………..standard deviation  xii Acknowledgements  I would like to express my gratitude to my supervisor, Dr. Jeffrey M. Coil, whose guidance has been essential not only in this research, but in the Graduate Endodontics program overall. I would also like to thank my committee members Dr. Markus Haapasalo and Dr. Nancy Ford. Without their important contributions this research would not have been possible. I am grateful to Dr. Coil and Dr. Haapasalo for allowing me to conduct research on this particular topic, which arose from observations I made during the clinical practice of dentistry before commencing my graduate studies at UBC. I would like to thank John Schipilow for his assistance with the micro-CT scans and Rick White for his assistance with the statistical analysis. Lastly, I would like to extend my appreciation to the Canadian Academy of Endodontics Endowment Fund, the American Association of Endodontists Foundation, and EndoTech Inc. for their generous financial support of this research.   xiii  Dedication  I dedicate this to my family, whose support and words of encouragement have enabled me to realize this achievement. 1 Chapter  1: Introduction  1.1 Root Filling Root filling has been assigned three primary functions: (i) sealing against ingrowth of bacteria from the oral cavity, (ii) entombment of remaining microorganisms, and (iii) complete obturation at a microscopic level to prevent stagnant fluid from accumulating and serving as nutrients for bacteria from any source [1]. In addition, the materials used for root filling may actively kill microorganisms which remain in, or which later gain entry to, the root canal system after endodontic treatment [2].  Gutta percha has been used as a root filling material for over one hundred and fifty years, and is the most common endodontic obturation material in use today [3]. Gutta percha is derived from dried juices from trees of the family Sapotaceae, which are native to Southeast Asia and South America. Gutta percha is a polymer and is the trans- isomer of isoprene (C5H8) [i.e., trans-(1,4)-polyisoprene]. Gutta percha in the crystalline state may occur in alpha (α) or beta (β) phase. The α-phase occurs in nature; the β-phase is produced during the gutta percha refining process and is the phase most frequently found in endodontic products. Dental gutta percha used for root canal obturation contains approximately 20% gutta percha (β-phase) and 75% zinc oxide; the remaining constituents are various metal sulfates (for radiopacity), resins, waxes and dyes. Gutta percha has been evaluated using cell culture systems, and found to have low or no cytotoxicity. In addition, there have been no clinical reports of systemic reactions to gutta percha [4].  2 1.2 Root Filling Techniques Obturation of the prepared root canal system with gutta percha may be accomplished using one of several methods. The most commonly used techniques include cold lateral compaction, warm vertical compaction, single-cone obturation, and carrier-based obturation [5].  Cold lateral compaction is possibly the most widely practiced and taught  obturation technique [6]. The cold lateral compaction technique begins by coating the walls of the prepared canal with sealer. The sealer may be applied using the master cone, a paper point, or a small file. The master cone is placed to working length, and a pre-measured spreader is inserted with gentle apical pressure to deform the gutta percha apically and laterally (‘compaction’) [7]. The spreader is withdrawn with a watch-winding motion to prevent dislodging of the master cone, and the first accessory cone, lightly coated with sealer, is immediately placed to length. Spreader insertion, compaction and accessory cone placement are repeated; each successive spreader insertion and accessory cone placement penetrate less deeply into the canal than the previous ones. This process continues until the spreader insertion reaches approximately three millimeters into the canal. A heated instrument is then used to sever the gutta percha cones at or below the level of the canal orifice, and the heat-softened gutta percha is compacted apically with a cold plugger. Heating softens the gutta percha several millimeters below the canal orifice, and apical compaction of softened gutta percha improves the seal [8]. Clinical reports have shown that laterally compacted gutta percha root canal fillings are able to prevent the entry of oral bacteria into the root canal system after long term exposure [9]. As well, most epidemiological studies regarding the  3 outcome of endodontic treatment are based on cold lateral compaction root canal filling [10]. Many clinical trials involving cold lateral compaction from around the world have confirmed the efficacy of this technique in clinical practice [11].  Warm vertical compaction was advocated and popularized by Schilder in the 1960’s [12]. In order for this obturation technique to be used, the preceding canal shaping criteria were: (i) continuously tapering preparation, (ii) original anatomy maintained, (iii) position of the apical foramen maintained, and (iv) foramen diameter as small as practicable. Canal preparation is adequate when a non-standardized or taper-matched gutta percha cone can fit to the working length. The cone is then trimmed to fit snugly 0.5-1.0 mm short of this length, and fit is confirmed by slight resistance to withdrawal (‘tug-back’). Compaction of the root filling occurs in multiple steps as the gutta percha is first softened with heat, then directed apically with a series of variously-sized cold pluggers. The first heating step severs the gutta percha cone at the level of the canal orifice. The tip of the largest cold plugger is positioned on the softened gutta percha and controlled pressure is used to compact it apically. The diameter of the selected plugger should be smaller than the diameter of the canal for each compaction step, so that the plugger will not engage the canal walls during the compaction process. The next heating step follows: the heat carrier tip is inserted about 3 mm into the compacted gutta percha and then withdrawn, removing with it the most coronal portion of gutta percha from the canal. Compaction is carried out as before, using the next smaller plugger. Subsequent steps of heating, gutta percha removal and compaction continue until the apical 4-5 mm of the canal is filled with thermally softened and compacted gutta percha and sealer. It is expected that the repeated heating and compaction during this ‘downpack’  4 portion of the warm vertical compaction technique will result in gutta percha and sealer being compacted into the apical 0.5-1.0 mm portion of the canal, as well as into any accessible canal ramifications [13]. For the ‘backfill’ portion of the warm vertical compaction technique, the unfilled coronal portion of the canal can be filled using a heat-softened gutta percha injection system. The needle tip of the system is inserted into the canal in contact with the surface of the gutta percha, and it is held in place for a few seconds before beginning injection. An increment of about 3 mm is injected and then compacted using the cold plugger to offset cooling shrinkage. This process is repeated, adding successive increments which are compacted in turn using the next larger pluggers, until the canal is filled to the level of the canal orifice. Clinical studies have shown that the warm vertical compaction technique can have a success rate comparable to that of lateral compaction [14].  A variation of warm vertical compaction is the ‘continuous wave of condensation’ technique, as described by Buchanan [15]. In contrast to the warm vertical compaction technique in which heating and compaction of the root filling occur in multiple steps, in the continuous wave of condensation technique, heating and compaction of the root filling occur in a single step or ‘wave’. This technique was facilitated by the development of electric heat carrier tips which can be heated rapidly and cooled rapidly. This allows the tip to be used as both a heat carrier and a (‘cold’) plugger. A tip is selected which binds at a point 4-5 mm from the working length when inserted into the canal preparation. After sealer application and placement of the previously selected gutta percha point, the heat source is set to a temperature of 200° C and to its maximum rate of increase. The cold tip is brought close to the canal orifice and the heat source is then activated. In a few seconds, the tip reaches the  5 operating temperature, and it is then directed apically in the canal over a period of two to three seconds, adjacent to or through the gutta percha point, until it is approximately 2 mm short of its binding point. The heat source is deactivated, and the tip cools rapidly. Apical pressure is maintained on the gutta percha with the tip for 5-10 seconds to offset cooling shrinkage. At this point, the tip is held in the canal by the cooling and hardening gutta percha. The heat source is activated again for one second (the ‘separation burst’), which disengages the tip from the gutta percha in the apical portion of the canal. The tip is withdrawn from the canal, removing with it the coronal portion of the gutta percha. The softened gutta percha in the apical portion of the canal is then compacted using cold hand pluggers. The unfilled coronal portion of the canal is then backfilled as described previously.  The single cone obturation technique became common in the 1960’s with the introduction of International Organization for Standardization (ISO) standardization of endodontic instruments and gutta percha cones [16]. Canal instrumentation for this obturation technique involves the creation of a ‘stop’ preparation in the apical 2 mm of the canal. Then, a single gutta percha cone is selected to fit with tug-back at the level of the stop to confirm a close fit between the gutta percha cone and the canal preparation here. The cone is then cemented in the canal with a thin layer of sealer. The use of nickel-titanium rotary endodontic files for root canal instrumentation makes centered canal preparations possible in both straight and curved canals [17], and may make accurate apical cone fit achievable in many cases. Current matched rotary file and gutta percha cone systems may further popularize the single cone obturation technique. Evidence suggests that the cross-sectional area of the canal occupied by  6 gutta percha using the single cone technique is comparable to that using lateral compaction, and can be accomplished more quickly [18].  The carrier-based technique is a method of delivering thermoplasticized gutta percha for obturation of the root canal system. This technique was introduced by Johnson in 1978 [19], and involves the use of a carrier which is coated by heat-softened gutta percha, both of which remain in the canal as root filling materials. After completion of the canal preparation, an instrument which is an analogue of the carrier is tried in the canal to verify that the obturation materials will be able to reach the working length. A small amount of sealer is applied to the canal preparation using a paper point. Additional dry paper points are subsequently inserted to blot out excess sealer to ensure that a thin coating remains on the canal walls. The obturation device is heated to a particular temperature for a specified time in the manufacturer’s recommended oven. The device is then removed from the oven, and immediately inserted into the canal to the working length over a five second interval with an even and continuous motion. The carrier is then severed at the level of the canal orifice with a bur or other manufacturer’s high speed rotary instrument. A small vertical plugger may be used to compact the gutta percha around the carrier to counteract the cooling contraction. The insertion of a carrier-based obturation device with a correctly-sized carrier into a prepared root canal is analogous to inserting the plunger of a syringe into its matched barrel; the insertion pressure on the carrier causes the thermoplasticized gutta percha and sealer to be forced into all accessible anatomy and portals of exit of the root canal system. One clinical trial showed similar success rates for teeth filled with a carrier-based obturation system compared to lateral compaction at 3 year recall [20].  7 The most commonly used carrier-based obturation technique is likely Thermafil (DENTSPLY Tulsa Dental Specialties, Johnson City, TN). The first version of the Thermafil carrier was an endodontic hand file in which the coronal flutes had been removed and the apical flutes left intact. Later versions included specifically designed and manufactured carriers made of stainless steel, and subsequently titanium, and whose surfaces were fluted [21]. Plastic carriers were then introduced. The current generation of carriers is unfluted and made from plastics with tungsten added as a radiopaquer; the smaller carriers (ISO tip size 20-40) are liquid crystal polymer, and the larger carriers (ISO tip size 45 and larger) are polysulfone. Evidence indicates that Thermafil obturation is a low temperature [22] and low pressure [23] technique which is capable of rapid and dense filling of root canal systems [24]. The carrier-based technique is reported to be particularly effective in long, curved canals, where the insertion of spreaders or pluggers necessary for other obturation techniques may be restricted [25]. Potential problems associated with the use of Thermafil include periapical extrusion of root filling materials [26], as well as difficulties in carrier removal during retreatment [27], apical retropreparation [28], and post space preparation [29]. In response to these concerns, the manufacturer of Thermafil introduced a new carrier-based thermoplasticized gutta percha obturation device in 2010, which they called GuttaCore (DENTSPLY Tulsa Dental Specialties). The main difference between the Thermafil obturator and the GuttaCore obturator is the material from which the carrier is manufactured: the Thermafil carriers are made from plastics, and the GuttaCore carrier is made from a proprietary cross-linked gutta percha. The manufacturer has made the claims that GuttaCore: “retains shape when heated”, “removes easily” [30], and “it’s easier to create post space with GuttaCore than with any plastic carrier-based obturator” [31]. In addition, the  8 manufacturer’s directions for severing the GuttaCore carrier are different from those for Thermafil: “Remove the shaft and handle at the orifice by bending to either side of the canal wall. Alternatively, while stabilizing the GuttaCore obturator with your index finger, use a round bur, or an inverted cone bur in a high-speed handpiece, or use a sharp spoon excavator” [32]. To date, there have been five studies published regarding GuttaCore. Beasley et al. [33] evaluated the time required to retreat GuttaCore, Thermafil, and traditional gutta percha obturations with ProTaper files in moderately curved canals. They found that GuttaCore was significantly quicker to remove than either Thermafil or warm vertically compacted gutta percha, Thermafil was less efficiently removed than GuttaCore, and there was a trend for carrier-based obturations to be more difficult to remove from the canals than the warm vertical obturations. Alhashimi et al. [34] compared the push-out bond strength between gutta percha coatings and three types of carrier materials: Thermafil, GuttaCore, and an experimental carrier. They found that the experimental obturators exhibited significantly higher push-out bond strength than those of GuttaCore and Thermafil, and that GuttaCore demonstrated significantly higher bond strength than Thermafil. Li et al. [35] examined the quality of obturation in single-rooted canals obturated by the GuttaCore system by comparing the results with similar canals obturated by the cold lateral compaction technique or the warm vertical compaction technique, using micro-CT and SEM. They found that canals obturated with GuttaCore had the lowest incidence of interfacial gaps and voids, although the results were not significantly different from canals obturated by warm vertical compaction. Both the GuttaCore and the warm vertical compaction groups had significantly lower incidences of gaps and voids than the cold lateral compaction group. Scotti et al. [36] compared the bond  9 strength of fiber posts cemented in root canals filled with gutta percha, Thermafil, or GuttaCore. In addition, they evaluated debris and dentinal tubule-opening using SEM. They found that bond strength was significantly higher in the gutta percha group than in the Thermafil and GuttaCore groups, and that apical debris scores were significantly higher in the Thermafil and GuttaCore groups than in the gutta percha group. They concluded that the thermoplasticized alpha gutta percha of Thermafil or GuttaCore seemed to worsen the cleaning of post space walls and hence reduced fiber post bond strength. Mancini et al. [37] evaluated the accuracy of an electronic apex locator (EAL) during the retreatment of canals filled with ProTaper or GuttaCore obturators, and evaluated whether the two different materials influenced the accuracy of the EAL differently. They concluded that the measurements obtained with the EAL during orthograde retreatment can lead clinicians to overinstrument and consequently overfill the endodontic space, and that the two different materials did not influence the accuracy of the EAL differently.  1.3 Posts and Post Space Preparation Placement of a post may be indicated for teeth in which there has been extensive loss of coronal tooth structure. The primary purpose of a post in such teeth is to retain a core that, in turn, can be used to retain a definitive coronal restoration [38].  Post space preparation has been studied in teeth root filled with Thermafil obturations, with respect to its effects on the coronal seal, efficacy of retreatment, and apical sealing ability.   10 Two in vitro studies examined the effect of post space preparation on the coronal seal. Ravanshad & Torabinejad [39] compared dye leakage after post space preparation in teeth which had been obturated with the lateral, vertical, and Thermafil metal carrier techniques, and concluded that lateral and vertical obturations produce a better seal than Thermafil. Gopikrishna & Parameswaren [40] compared dye leakage after post space preparation in teeth which had been obturated with the cold lateral compaction, SimpliFill (SybronEndo Corporation, Orange, CA), Thermafil plastic carrier and warm vertical compaction techniques, and concluded that the obturation techniques of SimpliFill, Thermafil and warm vertical compaction are superior to lateral compaction.  Zuolo et al. [41] compared the efficacy of root filling removal after post space preparation following obturation with Thermafil metal carrier, Thermafil plastic carrier and lateral compaction. There was no statistically significant difference among the groups with regard to the percentage of remaining root filling materials. However, six of fifteen Thermafil metal carriers could not be removed during the retreatment procedure, and Thermafil metal carrier group took significantly more time to be retreated than the other two groups.  Other studies have examined the effect of post space preparation on the apical sealing ability of teeth obturated with the Thermafil technique. Saunders et al. [42] in a study comparing apical dye leakage after post space preparation in teeth obturated with the Thermafil plastic carrier technique, found no effect of post space preparation, whether immediate or delayed, on apical dye leakage. Similarly, a study by Dalat & Spångberg [43] found no differences in apical linear dye penetration after post space preparation in teeth obturated with lateral  11 compaction and with the Thermafil plastic carrier technique. Rybicki & Zillich [44] studied the apical seal of obturations after post space preparation using volumetric dye leakage and spectrophotometry. They compared Thermafil plastic carrier obturations with immediate post space preparation, delayed post space preparation, without post space preparation, and lateral compaction without post space preparation, and found no significant differences in leakage among the groups. Ricci & Kessler [29] examined the effect of post space preparation on apical dye penetration in teeth obturated with the Thermafil plastic carrier, Thermafil metal carrier, or the lateral compaction technique. In contrast with the results of other studies, these authors found approximately three times more dye penetration in the Thermafil plastic carrier group when compared with the other groups.  A variety of methods were used to prepare the post space in the Thermafil obturations in the above-mentioned studies. As the Thermafil metal carrier cannot be partially removed for the purpose of post space preparation once placed in the canal, the technique used with this type of carrier is sectional obturation, facilitated by notching the metal carrier prior to its insertion into the canal. Per the manufacturer’s instructions: "Use a #1558 fissure or a tapered diamond bur to lightly nick the carrier as the obturator is rotated until the diameter has been reduced to approximately 0.06 mm. Test the notch; at the proper diameter it will appear that the tip is close to severing. The obturator should retain sufficient vertical strength to cause an indentation when pressed gently into the fingertip and should stay bent (will not rebound) when light lateral pressure is applied against the apical segment” [45] .  12 The notched obturator is heated in the manufacturer’s oven for the recommended time and then placed in the canal. While pressing apically, the handle of the metal obturator is rotated counterclockwise until the coronal segment separates, while the apical segment remains seated. Studies examining post space preparation in obturations using the Thermafil metal carrier and the sectional obturation technique include those of Ravanshad & Torabinejad [39], Ricci & Kessler [29], and Zuolo et al. [41]. Interestingly, the study by Gopikrishna & Parameswaren [40] also used the sectional obturation technique, even though they examined obturations using the Thermafil plastic carrier.  In contrast to the Thermafil metal carrier, the Thermafil plastic carrier can be partially removed for the purpose of post space preparation once placed in the canal. Saunders et al. [42] undertook a pilot study to determine the feasibility of sectioning the plastic carrier with the Touch 'n Heat unit (Analytic Technology, Redmond, WA). At the time of their study, the heat removal technique for post space preparation had been recommended by the manufacturers of Thermafil. The authors found that the plastic carrier could not be completely softened and sectioned in a longitudinal manner using this heat removal technique. In addition, the time required to heat the carrier resulted in large temperature increases on the external surface of the root. Therefore, this technique was discontinued.  In order to remove the plastic carrier and prepare a post space in Thermafil obturations, Saunders et al. [42] as well as Zuolo et al. [41] used Peeso reamers in a low-speed contra-angle handpiece. It is unknown whether or not this practice was endorsed by the manufacturers of Thermafil.  13 Another method to remove the Thermafil plastic carrier in order to prepare post space is to use a bur specifically designed by the manufacturer for this purpose. The Prepi bur (DENTSPLY Tulsa Dental Specialties) consists of a spherical smooth head mounted on a bur shaft. It is available in five sizes with varying head sizes of 0.5, 0.7, 0.9, 1.2, and 1.5 mm.  This instrument is operated in a high-speed handpiece and generates frictional heat when placed in contact with the Thermafil plastic carrier. The heat causes the carrier to melt and then be severed. Clinically this device is very efficient, but little is known about the effect on the integrity of the remaining root filling. Studies examining post space preparation in obturations using the Thermafil plastic carrier and the Prepi bur technique include those of Dalat & Spångberg [43], Rybicki & Zillich [44], and Ricci & Kessler [29]. As noted previously, the study of the latter authors found approximately three times more dye penetration in the Thermafil plastic carrier group when compared with the Thermafil metal carrier and the lateral compaction groups. The authors concluded that: “The method of post space preparation probably caused the loss of apical integrity of the Thermafil plastic obturator group. These results may be explained by the possible cooling of the Prepi bur, causing it to stick to the plastic obturator and dislodging it. Also, some of the plastic may remain on the bur and cause it to turn eccentrically and vibrate excessively. This vibration of the Prepi bur may disturb the apical seal of the plastic obturator” [29].  The manufacturer’s current recommended technique for post space preparation in teeth obturated with the Thermafil plastic carrier technique involves the initial use of the Prepi bur, followed by a post drill designed by the manufacturer and specifically intended for this purpose. The GT Post drill (DENTSPLY Tulsa Dental Specialties) is a parallel-sided post  14 drill with an eccentric cutting tip. It is available in three sizes, with diameters of 1.0, 1.25, and 1.5 mm.  This instrument is operated in a low-speed handpiece and is able to cut through and remove the Thermafil plastic carrier. The manufacturer’s directions for use state: “If the canal was obturated with a solid core obturator, such as a Thermafil obturator, initiate post space with a Prepi Bur, fluteless round bur or conventional high speed round bur. Use the selected bur at 150,000-200,000 rpm and allow it to create a 1 mm-2 mm dimple below the orifice. Attach the GT Post drill to your motor and set the motor speed to 2,000 rpm. Use intermittent apical pressure to create your parallel post space. The drill’s eccentric (angled) cutting tip will remove the plastic core and filling material” [46].   The manufacturer’s current recommended technique for preparation of post space in teeth obturated with the GuttaCore technique states: “For creating post space, remove the GuttaCore obturator by selecting an appropriately sized GT Post drill” [32]. “If the canal was obturated with gutta-percha or other filling material, set the motor speed to 2,000 RPM. The drill’s eccentric (angled) cutting tip will remove the filling material. Use intermittent apical pressure to create your parallel post space” [46].  It has been suggested that following post space preparation, a minimum of 4-5 mm of intact root filling should remain apically [47]. This position is supported by an in vitro fluid transport study [48]. If, however, one considers the possible need for future surgical endodontic treatment of a tooth which has previously undergone root canal treatment and post placement, one might consider that a minimum of 6 mm of root filling should be left intact apically. It has been shown that approximately 3 mm of the root tip should be resected  15 during the apicoectomy procedure in order to eliminate the majority of apical ramifications and lateral canals [49]. In addition, in order for the root-end filling to provide an adequate seal, the root-end preparation should be prepared to a depth of 3 mm [50]. Many researchers advocate post space preparation which does not remove tooth structure in addition to that which was previously removed during the endodontic preparation [51].  1.4 Micro-computed Tomography Micro-computed tomography (micro-CT) was first developed in the 1980s [52]. CT is a three-dimensional X-ray imaging method that involves obtaining X-ray projection images at many angles of view around an axis through an object and then applying a tomographic reconstruction algorithm to generate a stack of thin tomographic images of contiguous transaxial slices through the object. A specimen is usually scanned by rotating it around a vertical axis within a system comprised of a stationary X-ray source and X-ray imaging array. The transaxial images are made up of voxels. The term micro-CT is commonly used for CT scanners with sub-millimeter voxel resolution [53].   Micro-CT was introduced to the field of endodontic research by Nielsen et al. [54] in 1995. This was followed by studies from Dowker et al. [55] in 1997, Bjorndal et al. [56] and Rhodes et al. [57] in 1999, and Peters et al. [58] in 2000. Since its introduction, micro-CT has been used to study many areas of experimental endodontics, including tooth morphology, root canal anatomy, the evaluation of endodontic instrumentation, root filling, irrigation and hard tissue debris accumulation, and retreatment procedures [59].   16 Micro-CT has also been used to evaluate carrier-based thermoplasticized gutta percha obturations. Mirfendereski et al. [60] assessed the carrier-based and continuous wave of condensation root filling methods with respect to the quality of the fill, technique acquisition rate, and the perceived ease of use when undertaken by a group of inexperienced dental students. Endal et al. [61] measured, among other things, the percentage of the volume of the isthmus area of mesial roots of lower molars filled by gutta percha-coated solid core thermoplastic root filling. Somma et al. [62] evaluated and compared the voids in root fillings in extracted teeth using two thermoplasticized gutta percha techniques (Thermafil and System B) versus cold gutta percha (single point) technique. Zogheib et al. measured the volume of voids within 5 mm of the apex in root canals prepared with three different taper sizes in one study using Thermafil [63], in a second study using RealSeal 1 (SybronEndo Corporation, Orange, CA) [64], and measured the same parameter when comparing these two root filling materials in a third study with root canals prepared with one taper [65]. Gandolfi et al. [66] investigated the percentage of voids and marginal gaps in Thermafil obturations with AH Plus sealer or MTA Flow sealer after 7 days and after 6 months.  Three studies have been published which used finite element analysis based on micro-CT data to evaluate the stress distribution on endodontically treated and post-restored teeth under mechanical loading [67], [68], [69]. One study used micro-CT to examine the successive volume of hard tooth tissue lost in carious premolar teeth after each of: caries removal, access cavity preparation, root canal preparation, fibre post space preparation, and cast post space preparation [70]. Micro-CT has not been used to directly investigate post space  17 preparation in root canal treated teeth, whether obturated with carrier-based thermoplasticized techniques or with other root filling techniques.  1.5 Hardness of Root Filling Materials Hardness has been defined as the resistance of a material to permanent deformation by another material. In the area of endodontic research, hardness testing has been used primarily to examine the physical and mechanical properties of sealers [71], and of mineral trioxide aggregate [72]. It has not been used extensively to study other root filling materials. However, empirical clinical evidence suggests that the Thermafil carrier is much harder than gutta percha and that the GuttaCore carrier is also harder than gutta percha, but the hardness of the GuttaCore carrier is closer to that of gutta percha than to that of the Thermafil carrier.  18  Chapter  2: Rationale and Hypothesis  Many dentists use the obturation technique of thermoplasticized gutta percha on a carrier [73]. Clinical experience suggests that the hardness of the carriers is greater than that of the gutta percha. When ranking the hardness, the hardness of the Thermafil carrier is greater than that of the GuttaCore carrier, which in turn is greater than that of the gutta percha. Restoration of the endodontically treated tooth may require placement of a retentive post if a significant amount of coronal tooth structure has been lost or destroyed [38]. No study has yet examined possible deviation of post space preparation in teeth which have been obturated with the Thermafil or GuttaCore techniques, and no study has analyzed post space preparation using micro-computed tomography. Thus, the specific aims were: (i) to determine if post space preparation will be deviated from the endodontic preparation in teeth which have been obturated with the Thermafil or GuttaCore techniques due to the hardness of the carriers, and (ii) to determine the amount of deviation, if it occurs, using linear and volumetric analyses. The null hypothesis (H0) is: There will be no deviation of the post space preparation from the endodontic preparation in teeth obturated with Gutta percha, Thermafil, or GuttaCore techniques.   19  Chapter  3: Materials and Methods   3.1 Sample Selection and Preparation Forty-two extracted human permanent maxillary lateral incisors were used in this study. A sample size calculation with α = 0.05 and power = 0.80 with μ₂ = 1.25 x μ₁ with an assumption of equal standard deviations yielded a sample size estimate of 14. Teeth were collected from the Faculty of Dentistry clinics at the University of British Columbia, and from various private dental offices in the greater metropolitan area of Vancouver, British Columbia, Canada. Teeth were extracted during the course of the normal practice of dentistry and were donated anonymously. Immediately upon extraction and thereafter, teeth were stored in 0.05% sodium hypochlorite (NaOCl) at room temperature. Inclusion criteria for teeth in this study were: permanent maxillary lateral incisors with normal anatomy, a minimum of 12 mm of intact root length as measured from the anatomic root apex, with a single root and single root canal. Exclusion criteria included: abnormal anatomy, inadequate root length, incomplete root development or apical root fracture, root resorption, previous endodontic treatment, complete or incomplete horizontal or vertical root fracture, deep root caries or restoration thereof, extremely large pulp space, extremely calcified pulp space, and large bucco-lingual or mesio-distal curvature (> 30 degrees) in the coronal portion of the root canal. Each candidate tooth was inspected grossly and under magnification using a dental operating microscope (Global Surgical Corporation, St. Louis, MO) to determine if it met the inclusion criteria. Teeth were additionally screened by exposing one bucco-lingual radiograph and one mesio-distal radiograph using digital radiography (Planmeca Intra,  20 Helsinki, Finland) with intraoral photostimulable phosphor storage plates and ScanX Classic Digital Imaging System (Air Techniques, Melville, NY); radiographs were analyzed using digital radiography imaging software (Planmeca Romexis, Helsinki, Finland) to determine if the tooth met the inclusion criteria. Root canal curvature was measured using the technique described by Pruett et al. [74]. Selected teeth were each assigned a unique sample number which was inscribed under magnification into the mesial or distal root surface using a long shank friction grip ¼ round carbide bur in a high-speed handpiece. Teeth were again visualized using the operating microscope and measured against a millimeter ruler. Roots were marked circumferentially with an ultra-fine point permanent marker (Sharpie, Newell Rubbermaid Inc., Freeport, IL) at a point 12 mm from the root apex. Each tooth was then sectioned at the level of the marking and perpendicular to its long axis using an IsoMet 5000 Linear Precision Saw (Buehler, Lake Bluff, IL). Roots were allocated equally to three groups of 14 based on canal size, bucco-lingual canal curvature, and mesio-distal canal curvature.  3.2 Experimental Apparatus A custom matrix was fabricated for each root by inserting the root into freshly mixed vinyl polysiloxane putty (Aquasil Easy Mix Putty, DENTSPLY Caulk, Milford, DE). Every effort was made to place the root in the correct anatomic orientation, such that the buccal surface of the root was aligned with the buccal surface of the putty matrix. The top surface of the putty matrix was trimmed such that the coronal portion of the root protruded approximately 2 mm above the surface of the matrix. The other external surfaces of the putty matrices for all samples were identical and designed to fit in a single endodontic X-ray training typodont (Frasaco USA, Greenville, NC). One surface of each putty matrix contained a registration  21 mark which was used to align the matrix within the typodont repeatedly in the same orientation (Figure 3.1).      This typodont was then placed in a dental mannequin head (Frasaco USA) attached to the headrest of the dental chair (Planmeca, Helsinki, Finland). This arrangement facilitated the easy removal and replacement of the samples between various stages of the study, and allowed for the study to be conducted under simulated clinical conditions (Figure 3.2).   Figure 3.1 Sample with putty matrix and typodont.  22    Experimental procedures were carried out under 5.1X magnification using the dental operating microscope, and were performed by one operator (A.S.). Figure 3.2 Mannequin head and clinical simulation.  23 3.3 Root Canal Instrumentation As was the case with most of the samples, the root canal was exposed and determined to be patent after the sectioning procedure. In cases where the canal was calcified, root canal exposure was established using a ProUltra Piezo Ultrasonic unit and ProUltra Endo Tips (DENTSPLY International, York, PA). Working length was determined clinically by introducing a size 06 hand K-file (Lexicon, DENTSPLY International) into the canal and observing the point at which this file emerged from the apical foramen, and subtracting 1 mm from this length. Each canal was instrumented with size 06, 08, 10, and 15 hand K-files sequentially, ensuring that each file fit loosely in the canal at the working length before proceeding to the next larger size file. Further canal preparation was performed using Profile Vortex nickel-titanium rotary endodontic files (DENTSPLY Tulsa Dental Specialties) incorporating an Aseptico DTC Torque Control Motor (DENTSPLY Tulsa Dental Specialties) with a W&H 8:1 gear reduction electric contra angle endodontic handpiece (DENTSPLY Tulsa Dental Specialties). Root canal instrumentation was carried out in a step-back fashion, beginning with a size 15/.06 taper file taken to working length, and proceeding in the same manner with each successive rotary file: 20-, 25-, 30-, 35-, 40-, 45-, and 50/.06. Final instrumentation of the coronal portion of the canal was performed using a size 50/.12 taper GT rotary file (DENTSPLY Tulsa Dental Specialties). This file was inserted in the canal and advanced until the widest fluted part of the file was level with the sectioned surface of the root (8.75 mm). The use of rotary file systems with a known and constant taper in combination with their use in relation to the sectioned surface of the root allowed for estimation of the canal preparation diameter at various distances from this reference point (Figure 3.3).  24   Figure 3.3 Expected canal preparation diameter in relation to sectioned surface of the root.  The pre-programmed settings on the Aseptico DTC torque control motor for speed (rpm) and torque (N∙cm) were used for each file system, size, and taper, per the manufacturer’s instructions. Canals were irrigated throughout manual and rotary instrumentation with 6% NaOCl using a 10 mL syringe (Becton, Dickinson and Company, Franklin Lakes, NJ) and a 30-gauge side-vented needle (ProRinse Endodontic Irrigation Probes, DENTSPLY Tulsa Dental Specialties) placed as closely to the working length as possible without going beyond it. Between each rotary file and after use of the last rotary file, canals were recapitulated with a size 10 hand K-file and rinsed with approximately 1 mL of irrigating solution. After the final rinse, canals were dried using matching Vortex .06 taper paper points (DENTSPLY Tulsa Dental Specialties).   25 3.4 Root Canal Filling The three groups were each obturated with a different root filling technique. The first group was obturated using Gutta percha and warm vertical compaction, the second group with Thermafil, and the third group with GuttaCore.  For samples in the Gutta percha group, the root filling procedure was as follows: a matching Vortex 50/.06 taper gutta percha point (DENTSPLY Tulsa Dental Specialties) was fit with tugback at the working length of the prepared canal. Sealer (ThermaSeal Plus, DENTSPLY Tulsa Dental Specialties) was applied to the canal on a matching Vortex .06 paper point, and excess sealer was removed by the subsequent insertion of a second dry paper point. The apical 5 mm of the previously selected gutta percha point was coated thinly with sealer, and the point was gently inserted into the canal until the working length was reached. Using the continuous wave of condensation technique described previously, the coronal portion of the gutta percha point was severed and removed with the System B Heat Source and matching heat carrier tip (SybronEndo Corporation) to a depth of about 7 mm from the sectioned surface of the root. This left approximately 4 mm of gutta percha remaining in the apical portion of the canal. This thermoplasticized gutta percha was then compacted using a size 8½ posterior Schilder plugger (DENTSPLY Maillefer, Ballaigues, Switzerland). The canal was then backfilled with thermoplasticized gutta percha using the Calamus Flow Obturation Delivery System with 23-gauge injection tip cartridges (DENTSPLY Tulsa Dental Specialties) and sizes 9, 9½, and 10 posterior Schilder pluggers. The root filling was levelled with the sectioned root surface using a sharp lab knife.   26 For samples in the Thermafil group, the root filling procedure was as follows: canal preparation at the working length was verified using a size 50 Thermafil Plus nickel-titanium size verifier (DENTSPLY Tulsa Dental Specialties). Sealer (ThermaSeal Plus, DENTSPLY Tulsa Dental Specialties) was applied to the canal on a matching Vortex .06 paper point, and excess sealer was removed by the subsequent insertion of a second dry paper point. A size 50 Thermafil Plus endodontic obturator was selected and placed into the ThermaPrep Plus obturator oven (DENTSPLY Tulsa Dental Specialties) at the recommended settings per the manufacturer’s directions for use. On the oven’s signal that the obturator was ready, it was removed from the oven and immediately inserted into the prepared canal to the working length with a smooth and continuous motion over a period of about 5 seconds. The obturator handle was stabilized and the obturator carrier was severed using a long shank friction grip ½ round carbide bur in a high-speed handpiece just below the level of the sectioned root surface. The thermoplasticized gutta percha around the carrier was then compacted using a size 9 posterior Schilder plugger, and the root filling was levelled with the sectioned root surface using a sharp lab knife.  For samples in the GuttaCore group, the root filling procedure was as follows: canal preparation at the working length was verified using a size 50 GuttaCore size verifier (DENTSPLY Tulsa Dental Specialties). Sealer (ThermaSeal Plus, DENTSPLY Tulsa Dental Specialties) was applied to the canal on a matching Vortex .06 paper point, and excess sealer was removed by the subsequent insertion of a second dry paper point. A size 50 GuttaCore endodontic obturator was selected and placed into the GuttaCore obturator oven (DENTSPLY Tulsa Dental Specialties) at the recommended settings per the manufacturer’s  27 directions for use. On the oven’s signal that the obturator was ready, it was removed from the oven and immediately inserted into the prepared canal to the working length with a smooth and continuous motion over a period of about 5 seconds. In the first sample obturated with GuttaCore, the technique used to sever the carrier was bending of the carrier handle in a side-to-side fashion against the canal walls, per the manufacturer’s instructions. This technique proved to be less effective than claimed by the manufacturer, so for the remainder of the GuttaCore samples, the manufacturer’s alternate recommended method was used, as described below. The obturator handle was stabilized and the obturator carrier was severed using a long shank friction grip ½ round carbide bur in a high-speed handpiece just below the level of the sectioned root surface. The thermoplasticized gutta percha around the carrier was then compacted using a size 9 posterior Schilder plugger, and the root filling was levelled with the sectioned root surface using a sharp lab knife.  The samples from all three groups were stored at 100% humidity and 37°C for one week to allow for setting of the sealer [75] before proceeding to the next phase of the study.   3.5 Micro-CT Scans: First Scan All samples were scanned using a MicroCT 100 (SCANCO Medical AG, Bruettisellen, Switzerland) with the following settings: isotropic voxel size of 20.0 µm, energy of 90 kVp, tube current of 200 µA, integration time of 300 ms, and a 0.5 mm aluminum filter. Scan resolution was determined by examining existing endodontic micro-CT studies, and by performing a number of pilot scans using different voxel sizes. Samples were wrapped in plastic food wrap prior to their placement in the scanner’s sample tube to prevent both desiccation and movement during scanning, and scan time was about 52 minutes in duration.  28 The data was analyzed and images were reconstructed using two micro-CT software packages: MicroView v. 2.2 software (GE Healthcare, London, ON) and Amira v. 5.6 (FEI Visualization Sciences Group, SAS, Burlington, MA).  3.6 Post Space Preparation Post space was prepared in all samples using the small (1.0 mm diameter) GT Post drill (DENTSPLY Tulsa Dental Specialties) in a W&H 8:1 gear reduction electric contra angle endodontic handpiece (DENTSPLY Tulsa Dental Specialties) and Aseptico DTC Torque Control Motor (DENTSPLY Tulsa Dental Specialties) at 2,000 rpm per manufacturer’s instructions. Each post drill was used on six samples in an alternating fashion between the three root filling types, two samples of each type. The fourth, fifth, and sixth uses were in the same order as the first, second, and third uses. In total, seven post drills were used in the study. The sequence of use was different for each of the first six post drills used. The sequence of use for the seventh post drill was determined randomly and was a repetition of the sequence of use for one of the previous post drills, as there are only six possible sequence combinations. The post drill sequence information is shown in Table 3.1.  Table 3.1 Post drills used and sequence Post Drill Sequence 1 TF, GP, GC, TF, GP, GC 2 TF, GC, GP, TF, GC, GP 3 GC, TF, GP, GC, TF, GP 4 GC, GP, TF, GC, GP, TF 5 GP, TF, GC, GP, TF, GC 6 GP, GC, TF, GP, GC, TF 7 GP, GC, TF, GP, GC, TF  29 For the purposes of this study, it was desirable for the path of the post drill to be influenced only by the properties of the root filling materials; it was undesirable for the path of the drill to be guided by the dentin of the canal preparation. It was anticipated that if the post space preparation was centred within the canal preparation and root filling materials, the initial contact between the post drill and the canal preparation would be at a point approximately 4.6 mm from the sectioned surface of the root. This measurement corresponds to the distance from the sectioned surface of the root where the canal preparation was expected to measure 1.0 mm in diameter, which is the same diameter as the post drill (Figure 3.4).                      Figure 3.4 Depth of post space preparation based on expected canal preparation diameter.  30 Prior to use, each post drill was visualized using the dental operating microscope and measured against a millimeter ruler. Post drills were marked circumferentially with an ultra-fine point permanent marker (Sharpie) at a point approximately 4.6 mm from the tip of the drill. The post space was prepared to this depth in all samples. As with the other parts of the study, post space preparation was performed under 5.1X magnification using the dental operating microscope. Prior to commencing the post space preparation for each sample, it was possible to estimate the required angulation of the post drill from the coronal portion of the root protruding from the surface of the putty matrix. The technique of post space preparation was different between the three types of root filling materials.  For samples in the Gutta percha group, post space was prepared as follows: post drill angulation was estimated visually, the centre of the post drill was placed over the centre of the root filling on the sectioned surface of the root, the handpiece was activated, and the post drill was directed apically while maintaining the desired angulation until the mark on the post drill was level with the sectioned surface of the root. In this group, the post space preparation was most often completed in one pass.  For samples in the Thermafil group, post space was prepared as follows: using a long shank friction grip ½ round carbide bur in a high-speed handpiece, the coronal portion of the Thermafil carrier was removed to a depth of about 2 mm below the level of the sectioned surface of the root, post drill angulation was estimated visually, the centre of the post drill was placed over the centre of the Thermafil carrier, the handpiece was activated, and the post drill was directed apically while maintaining the desired angulation until the mark on the post  31 drill was level with the sectioned surface of the root. In this group, the post space preparation was most often completed in more than one pass.  In the first GuttaCore sample, the technique for post space preparation was the same as that for the Gutta percha group, namely, preparation of post space without prior removal of the coronal portion of the carrier. This is the recommended technique per the manufacturer’s directions for use. However, this method proved to be less effective than claimed, so for the remainder of the GuttaCore samples, the technique for post space preparation was the same as that used for the Thermafil group: using a long shank friction grip ½ round carbide bur in a high-speed handpiece, the coronal portion of the GuttaCore carrier was removed to a depth of about 2 mm below the level of the sectioned surface of the root, post drill angulation was estimated visually, the centre of the post drill was placed over the centre of the GuttaCore carrier, the handpiece was activated, and the post drill was directed apically while maintaining the desired angulation until the mark on the post drill was level with the sectioned surface of the root. In this group, the post space preparation was most often completed in one or two passes.  For all samples, the post space was cleared of debris using a Stropko irrigator with a curved Luer-Lok syringe tip and a focused stream of air directed into the post space preparation. Samples were again stored at 100% humidity before proceeding to the next phase of the study.     32 3.7 Micro-CT Scans: Second Scan Samples were scanned a second time using a MicroCT 100 (SCANCO Medical AG) with the same settings as in the first set of scans. The data was analyzed and images were reconstructed using MicroView v. 2.2 (GE Healthcare) and Amira v. 5.6 (FEI Visualization Sciences Group, SAS) software packages. Amira was used to create images and animations, while MicroView was used to create images and for the quantitative analysis of the data.  3.8 Data Analysis and Parameters of Interest Images from the first and second micro-CT scans for each sample were registered using the MicroView software. Registration is a process that co-aligns two images, such that identical landmarks in the two separate images are given the same spatial coordinate. Once two images are registered, they can be visualized side-by-side using the Synchronized View tool (Figure 3.5) or superimposed using the Fusion View tool (Figure 3.6). This allows for visualization, identification, and comparison of corresponding features in the two images. Next, the image volumes of both scans for all samples were reoriented in relation to the axial, sagittal, and coronal planes such that the axial plane was made parallel to the sectioned surface of the root.  33                                           Reconstructed images of all samples from the second scans were analyzed to determine if post space preparation deviated from the endodontic preparation. Deviation or its absence was most easily determined by scrolling through the image volume along the sagittal and/or Figure 3.5 Schematic of MicroView’s Synchronized View showing registered images of axial slices from the first scan (left) and second scan (right) of the same sample side-by-side. Figure 3.6 Schematic of MicroView’s Fusion View showing registered images of axial slices from the first and second scans of the same sample superimposed.  34 coronal plane. Deviation of post space preparation was considered to have occurred when root filling material remained alongside the entire length of the post space preparation from the bottom of the preparation to the sectioned surface of the root, or when post space preparation was seen to have removed root dentin adjacent to the existing canal preparation. Deviation of post space preparation was considered not to have occurred when no root filling material remained alongside the post space preparation in the manner described above, and when post space preparation was seen not to have removed root dentin adjacent to the existing canal preparation (Figure 3.7).                Small isolated remnants of root filling may have remained in fins or irregularities of the obturated root canal system adjacent to the post space preparation without post space Figure 3.7 Schematic of deviation (left) and no deviation (right) of post space preparation.  35 deviation having had occurred. The groups were compared using Fisher’s exact test for binary outcomes with regard to the number of samples showing deviation.  For the two registered and reoriented images of each sample, an identical region of interest was created whose boundaries were described by: the sectioned surface the root superiorly, two voxels below the bottom of the post space preparation inferiorly, and root dentin circumferentially, not encroaching upon the margins of the endodontic preparation nor of the post space preparation. The region of interest in the second scan image was analyzed to determine the volume of the post space preparation plus the volume of the root filling material remaining after post space preparation. The region of interest in the first scan image was analyzed to determine the volume of the root filling material prior to post space preparation (Figure 3.8).            Figure 3.8 Schematic of volumes to be determined (indicated by dashed red outlines) before post space preparation (left) and after post space preparation (right).  36 The volume of the root filling material from the first scan was subtracted from the volume of the post space preparation plus the volume of the remaining root filling material from the second scan. The difference between these values represented the volume of tooth structure removed by the post space preparation. The mean volume difference was calculated for each group and these values were compared using one-way analysis of variance. If the overall results were significant, individual group means were compared post hoc using Tukey’s HSD test. The volumes of the post space preparation and the root filling material were measured separately using different greyscale value thresholds. MicroView is a 16-bit imaging software program, able to distinguish 65,536 different greyscale values. Upper and lower greyscale thresholds were chosen to exclude the tooth structure from the volumetric analysis, based on Histogram data (greyscale value vs. number of voxels) from the region of interest (Figure 3.9).   37  Figure 3.9 MicroView Histogram from the region of interest of a sample after post space preparation. The large peak represents tooth structure. The smaller peaks represent water (left) and air (far left).   The peak representing root filling is not shown.  The post space preparation is a low greyscale value material (i.e., air or water or both). As such, a lower threshold (6,000 greyscale units and below) was chosen to measure the post space preparation. In contrast, root filling is a high greyscale value material (about 33,000 greyscale units). Therefore, an upper threshold (10,500 greyscale units and above) was chosen to measure the root filling. These thresholds were used for the volume analysis on all scans and for every sample. Once a region of interest had been defined and thresholds had been specified, MicroView’s ‘Region Grow’ tool was used to measure the volumes. Region Grow is a method of segmentation where an initial voxel is selected within the region of  38 interest, and the criteria for connectivity is provided (i.e., the threshold). For measuring the volume using the lower threshold, the initial voxel must have a greyscale value which is less than or equal to the lower threshold value. For measuring the volume using the upper threshold, the initial voxel must have a greyscale value which is greater than or equal to the upper threshold value. Then, this voxel’s 26 neighbouring voxels are examined to see if each one meets the criteria for connectivity. If a voxel meets the criteria for connectivity, its neighbouring voxels are examined in turn. The Region Grow process is constrained by the defined region of interest.  Linear deviation of the post space preparation was determined by measuring the length of a line segment whose starting point was the centre of the endodontic preparation from the first scan image, and whose termination point was the centre of the post space preparation from the second scan image. In order to determine where linear deviation of the post space preparation would be measured for each sample, the registered and reoriented images from both scans were visualized side-by-side using MicroView’s Synchronized View. An axial slice was selected from the bottom of the post space preparation from the second scan image. The post drill used to create the post space preparation has an eccentric cutting tip. As such, when it is used in rotary motion, it creates a cone shape in the root filling material remaining in the tooth below the post space preparation. The axial slice selected for the linear deviation measurement was the slice in which the apex of the remaining root filling cone first appeared when scrolling through the image volume using the axial slices in a coronal-to-apical direction (i.e., toward the bottom  39 of the post space preparation). The apex of the remaining root filling cone was considered to be the centre of the post space preparation (Figure 3.10).                         Next, MicroView’s Fusion View was used to visualize the selected axial slice from the first and second scans superimposed in one image. The centre of the endodontic preparation was designated as the starting point of a line segment, and the centre of the post space preparation was designated as the termination point of the line segment (Figure 3.11).                                               Figure 3.11 Schematic of MicroView’s Fusion View showing measurement of linear deviation of post space preparation (red arrow heads) from the centre of the endodontic preparation (from the first scan) to the centre of the post space preparation (from the second scan). Figure 3.10 Schematic of MicroView’s Synchronized View showing selection of the axial slice to be used for measurement of linear deviation of post space preparation from the second scan (right).  40 MicroView automatically calculated the length of the line segment between the two points. The length of the line segment represented the linear deviation of the post space preparation. The mean linear deviation of the post space preparation was calculated for each group and these values were compared using one-way analysis of variance. If the overall results were significant, individual group means were compared post hoc using Tukey’s HSD test.  Minimum root thickness was determined from the first scan image by measuring the shortest distance between the root surface and the endodontic preparation. Minimum root thickness was determined from the second scan image by measuring the shortest distance between the root surface and the post space preparation or the endodontic preparation, whichever was shorter.  In order to determine where minimum root thickness would be measured for each sample, the registered and reoriented images from both scans were visualized side-by-side using MicroView’s Synchronized View. The axial slice from the bottom of the post space preparation which had previously been used for the linear deviation measurement was also used for the minimum root thickness measurements. Line segments were created with starting points and termination points as described above. The line segment from the first scan image was the minimum root thickness before post space preparation, and the line segment from the second scan image was the minimum root thickness after post space preparation (Figures 3.12 and 3.13).   41                                                                            The mean minimum root thicknesses before post space preparation and after post space preparation were calculated for each group, and these values were compared within groups using one-tailed paired Student’s t-tests. The minimum root thickness after post space preparation was subtracted from the minimum root thickness before post space preparation. The difference between these values represented the component of the linear deviation of the Figure 3.12 Schematic of MicroView’s Synchronized View showing measurement of minimum root thickness before post space preparation (left). Figure 3.13 Schematic of MicroView’s Fusion View showing measurement of minimum root thickness after post space preparation.  42 post space preparation which occurred toward the proximal surfaces of the root. The mean difference was calculated for each group and these values were compared between groups using one-way analysis of variance. If the overall results were significant, individual group means were compared post hoc using Tukey’s HSD test. 43 Chapter  4: Results  4.1 Root Canal Curvature Five of the forty-two teeth included in the study had curvature in the coronal portion of the root canal. The curvature data is shown in Table 4.1.  Table 4.1 Samples with root canal curvature and group assignment Sample Type of Curvature Degree of Curvature Group Assignment 28 Bucco-lingual 13° Thermafil 36 Mesio-distal 16° Thermafil 35 Bucco-lingual 10° GuttaCore 46 Mesio-distal 10° GuttaCore 43 Bucco-lingual 15° Gutta percha   4.2 Occurrence of Deviation Deviation occurred in eight of the forty-two samples in this study: in seven samples obturated with Thermafil, in one sample obturated with GuttaCore, and in none of the samples obturated with gutta percha. Figure 4.1 shows a micro-CT slice of a Thermafil sample showing deviation, and a micro-CT slice of a gutta percha sample showing no deviation.    44    Note that the Thermafil carrier can be distinguished from the surrounding root filling material in the sample showing deviation.  The summarized occurrence of deviation data is shown in Table 4.2.  Table 4.2 Number of samples with deviation and with no deviation of post space preparation Group Deviation No Deviation Total Thermafil                  7 a,b 7 14 GuttaCore 1b 13 14 Gutta percha                  0 a 14 14 Total                  8 34 42 The same superscript indicates a significant difference between groups; a: p = 0.006, b: p = 0.033.  Fisher’s exact test for occurrence of deviation yielded a significant p-value for the overall data (p = 0.003). This test also revealed a significant difference (p < 0.05) between the Gutta Figure 4.1 Micro-CT slices showing a Thermafil sample with deviation (left) and a gutta percha sample with no deviation (right).  45 percha group and the Thermafil group (p = 0.006), and between the GuttaCore group and the Thermafil group (p = 0.033). There was no significant difference between the gutta percha group and the GuttaCore group.  4.3 Images Reconstructed micro-CT images of one representative sample from each group before and after post space preparation are shown in Figures 4.2, 4.3, and 4.4.        Figure 4.2 Micro-CT reconstructions: before (left) and after (right) post space preparation - Gutta percha.  46         Figure 4.3 Micro-CT reconstructions: before (left) and after (right) post space preparation - GuttaCore. Figure 4.4 Micro-CT reconstructions: before (left) and after (right) post space preparation - Thermafil.  47 In a Gutta percha sample with no deviation (Figure 4.2), note the root filling material remaining buccally and lingually to the post space preparation in the absence of post space deviation, due to the anatomy of the root canal system. In a GuttaCore sample with no deviation (Figure 4.3), note the obturation of the large lateral canal with root filling material. It was not possible to discriminate between the GuttaCore carrier and the other root filling materials. In a Thermafil sample with deviation (Figure 4.4), note the root filling material remaining alongside the entire length of the post space preparation. It is possible to distinguish between the carrier and the other root filling materials, due to a difference in greyscale values between these materials; the carrier has a slightly lower greyscale value than the other root filling materials. In addition, the other root filling materials have been made partially transparent in this image such that it is possible to visualize the carrier within the surrounding gutta percha.  4.4 Volumetric Analysis Images of the volumetric analysis from MicroView are shown in Figures 4.5 and 4.6.   48  Figure 4.5 Region of interest (yellow box) for volumetric analysis of a sample before (left) and after (right) post space preparation.    Figure 4.6 Volumetric analysis (green) for filling and post space within the region of interest of a sample before (left) and after (right) post space preparation.   49 The volumetric analysis data is shown in Table 4.3. Samples showing deviation are indicated by bold typeface.  Table 4.3 Volumes of root filling and air before and after post space preparation Sample Scan      # Volume of filling (mm3) Volume of air (mm3) Total Volume Difference (mm3) GC_12 2 2.72 4.37 7.09 0.58 1 6.43 0.08 6.51 GC_13 2 0.27 6.06 6.33 -0.30 1 6.63 0.00 6.63 GC_21 2 0.23 5.94 6.17 -0.44 1 6.54 0.07 6.61 GC_31 2 0.42 6.09 6.51 -0.04 1 6.53 0.02 6.55 GC_35 2 0.19 6.15 6.34 -0.36 1 6.67 0.03 6.70 GC_37 2 0.30 5.85 6.15 -0.37 1 6.52 0.00 6.52 GC_46 2 0.29 6.83 7.12 0.15 1 6.87 0.10 6.97 GC_1211 2 0.25 5.96 6.21 -0.26 1 6.40 0.07 6.47 GC_1413 2 0.19 6.09 6.28 -0.39 1 6.54 0.13 6.67 GC_1817 2 0.22 6.44 6.66 -0.35 1 7.01 0.00 7.01 GC_2131 2 0.19 6.29 6.48 -0.56 1 7.04 0.00 7.04 GC_2223 2 0.27 5.72 5.99 -0.31 1 6.30 0.00 6.30 GC_2728 2 0.22 6.00 6.22 -0.52 1 6.73 0.01 6.74 GC_3433 2 0.24 6.06 6.30 -0.23 1 6.50 0.03 6.53 GP_16 2 0.31 5.58 5.89 -0.60 1 6.49 0.00 6.49 GP_18 2 0.26 5.91 6.17 -0.32 1 6.49 0.00 6.49 GP_25 2 0.28 5.80 6.08 -0.48 1 6.56 0.00 6.56 GP_33 2 0.25 6.02 6.27 -0.46 1 6.73 0.00 6.73 GP_41 2 0.18 5.66 5.84 -0.61 1 6.45 0.00 6.45  50 Sample Scan      # Volume of filling (mm3) Volume of air (mm3) Total Volume Difference (mm3) GP_42 2 0.20 5.57 5.77 -0.46 1 6.23 0.00 6.23 GP_43 2 0.23 6.26 6.49 -0.17 1 6.66 0.00 6.66 GP_1343 2 0.20 6.14 6.34 -0.23 1 6.57 0.00 6.57 GP_1545 2 0.21 6.17 6.38 -0.46 1 6.84 0.00 6.84 GP_1716 2 0.24 5.60 5.84 -0.46 1 6.30 0.00 6.30 GP_2333 2 0.23 5.84 6.07 -0.54 1 6.61 0.00 6.61 GP_2838 2 0.26 6.06 6.32 -0.33 1 6.65 0.00 6.65 GP_4243 2 0.24 6.10 6.34 -0.29 1 6.63 0.00 6.63 GP_4647 2 0.22 6.17 6.39 -0.49 1 6.88 0.00 6.88 TF_11 2 1.72 4.45 6.17 -0.05 1 6.21 0.01 6.22 TF_14 2 2.05 4.63 6.68 -0.07 1 6.75 0.00 6.75 TF_15 2 2.98 4.45 7.43 0.51 1 6.89 0.03 6.92 TF_17 2 0.24 6.12 6.36 -0.43 1 6.78 0.01 6.79 TF_27 2 1.57 5.05 6.62 0.07 1 6.53 0.02 6.55 TF_28 2 0.20 6.47 6.67 -0.03 1 6.69 0.01 6.70 TF_32 2 0.22 5.75 5.97 -0.45 1 6.42 0.00 6.42 TF_36 2 0.23 6.53 6.76 -0.10 1 6.72 0.14 6.86 TF_47 2 0.26 5.75 6.01 -0.25 1 6.18 0.08 6.26 TF_1242 2 0.22 6.30 6.52 -0.02 1 6.50 0.05 6.54 TF_1747 2 1.98 4.66 6.64 0.29 1 6.31 0.04 6.35 TF_2232 2 0.31 6.12 6.43 -0.13 1 6.48 0.08 6.56 TF_2324 2 2.71 4.70 7.41 0.42 1 6.98 0.01 6.99 TF_2425 2 2.48 4.13 6.61 0.39 1 6.22 0.00 6.22  51 The volume data from Table 4.3 is shown summarized in Table 4.4.  Table 4.4 Mean and range of volume difference before and after post space preparation Group Volume Difference [mean ± std. dev.] (mm3) Range Thermafil +0.01 ± 0.30 a,b -0.45 to +0.51 GuttaCore -0.24 ± 0.30 b -0.56 to +0.58 Gutta percha -0.42 ± 0.13 a -0.61 to -0.17 The same superscript indicates a significant difference between groups; a: p < 0.01, b: p < 0.05.  One-way analysis of variance for mean volume difference yielded a significant p-value for the overall data (p = 0.0003). Tukey’s HSD post hoc test revealed a significant difference between the Gutta percha group and the Thermafil group (p < 0.01), and between the GuttaCore group and the Thermafil group (p < 0.05). There was no significant difference between the Gutta percha group and the GuttaCore group.  4.5 Linear Deviation Images of the measurement of linear deviation of post space preparation from MicroView are shown in Figures 4.7, 4.8, and 4.9.   52        Figure 4.7 MicroView’s Synchronized View showing registered images of axial slices from the first scan (left) and second scan (right) of the same sample side-by-side. Figure 4.8 MicroView’s Fusion View (left) showing registered images of axial slices from the first and second scans of the same sample superimposed.  53     The linear deviation of post space preparation data is shown in Table 4.5. Samples showing deviation are indicated by bold typeface.  Table 4.5 Linear deviation of post space preparation Sample Linear deviation (mm) Sample Linear deviation (mm) Sample Linear deviation (mm) GC_12 0.57 GP_16 0.03 TF_11 0.23 GC_13 0.05 GP_18 0.04 TF_14 0.26 GC_21 0.11 GP_25 0.07 TF_15 0.42 GC_31 0.03 GP_33 0.17 TF_17 0.01 GC_35 0.05 GP_41 0.06 TF_27 0.22 GC_37 0.04 GP_42 0.05 TF_28 0.01 GC_46 0.06 GP_43 0.06 TF_32 0.05 GC_1211 0.11 GP_1343 0.08 TF_36 0.11 GC_1413 0.03 GP_1545 0.02 TF_47 0.04 GC_1817 0.04 GP_1716 0.04 TF_1242 0.01 GC_2131 0.02 GP_2333 0.10 TF_1747 0.40 GC_2223 0.15 GP_2838 0.07 TF_2232 0.04 GC_2728 0.07 GP_4243 0.03 TF_2324 0.42 GC_3433 0.08 GP_4647 0.01 TF_2425 0.41 Figure 4.9 MicroView’s FusionView showing measurement of linear deviation of post space preparation (dashed red line) from the centre of endodontic preparation (lower green arrowhead) to the centre of the post space preparation (upper green arrowhead).  54 The linear deviation data from Table 4.5 is shown summarized in Table 4.6.  Table 4.6 Mean and range of linear deviation of post space preparation Group Linear Deviation [mean ± std. dev.] (mm) Range Thermafil 0.19 ± 0.17 a 0.01 to 0.42 GuttaCore                          0.10 ± 0.14 0.02 to 0.57 Gutta percha 0.06 ± 0.04 a 0.01 to 0.17 The same superscript indicates a significant difference between groups; a: p < 0.05.  One-way analysis of variance for mean linear deviation yielded a significant p-value for the overall data (p = 0.036). Tukey’s HSD post hoc test revealed a significant difference between the Gutta percha group and the Thermafil group (p < 0.05). There was no significant difference between the GuttaCore group and the Thermafil group, or between the Gutta percha group and the GuttaCore group.  4.6 Minimum Root Thickness Images of the measurement of minimum root thickness from MicroView are shown in Figures 4.10 and 4.11.  The minimum root thickness data is shown in Table 4.7. Samples showing deviation are indicated by bold typeface.  55         Figure 4.10 MicroView’s Synchronized View showing measurement of minimum root thickness (dashed red line between green arrowheads) before post space preparation. Figure 4.11 MicroView’s Fusion View showing measurement of minimum root thickness (dashed red line between green arrowheads) after post space preparation.  56 Table 4.7 Minimum root thickness before and after post space preparation Sample Before (mm) After (mm) Difference (mm) GC_12 1.06 0.95 0.11 GC_13 1.00 1.00 0.00 GC_21 1.26 1.22 0.04 GC_31 1.43 1.38 0.05 GC_35 1.13 1.13 0.00 GC_37 1.06 1.06 0.00 GC_46 0.92 0.90 0.02 GC_1211 1.35 1.32 0.03 GC_1413 0.95 0.95 0.00 GC_1817 1.25 1.23 0.02 GC_2131 1.12 1.06 0.06 GC_2223 1.51 1.48 0.03 GC_2728 1.39 1.39 0.00 GC_3433 1.47 1.45 0.02 GP_16 1.32 1.29 0.03 GP_18 1.05 1.02 0.03 GP_25 1.23 1.20 0.03 GP_33 1.43 1.40 0.03 GP_41 1.21 1.21 0.00 GP_42 1.33 1.27 0.06 GP_43 1.32 1.32 0.00 GP_1343 1.28 1.24 0.04 GP_1545 1.45 1.41 0.04 GP_1716 1.56 1.56 0.00 GP_2333 0.95 0.94 0.01 GP_2838 0.92 0.89 0.03 GP_4243 1.03 0.99 0.04 GP_4647 1.07 1.05 0.02 TF_11 1.24 1.23 0.01 TF_14 1.09 0.97 0.12 TF_15 1.20 1.06 0.14 TF_17 1.47 1.43 0.04 TF_27 0.98 0.98 0.00 TF_28 1.28 1.27 0.01 TF_32 1.20 1.16 0.04 TF_36 1.19 1.11 0.08 TF_47 0.93 0.92 0.01 TF_1242 0.98 0.97 0.01 TF_1747 1.12 1.07 0.05 TF_2232 1.25 1.21 0.04 TF_2324 1.04 0.82 0.22 TF_2425 1.06 1.06 0.00  57 The minimum root thickness data from Table 4.7 is shown summarized in Table 4.8.  Table 4.8 Mean and range of minimum root thickness before and after post space preparation Group Before [mean ± std. dev.] (mm) Range After [mean ± std. dev.] (mm) Range Difference [mean ± std. dev.] (mm) Range TF 1.15 ± 0.15 a 0.93 - 1.47 1.09 ± 0.16 a 0.82 - 1.43 0.06 ± 0.06 0.00 - 0.22 GC 1.21 ± 0.20 b 0.92 - 1.51 1.18 ± 0.20 b 0.90 - 1.48 0.03 ± 0.03 0.00 - 0.11 GP 1.23 ± 0.20 c 0.92 - 1.56 1.20 ± 0.20 c 0.89 - 1.56 0.03 ± 0.02 0.00 - 0.06 The same superscript indicates a significant difference between means; a: p = 0.004, b: p = 0.003, c: p < 0.0001.  One-tailed paired Student’s t-tests revealed a significant difference between the mean minimum root thicknesses before and after post space preparation for all groups (TF group:  p = 0.004; GC group: p = 0.003; GP group: p < 0.0001). One-way analysis of variance of the mean difference in minimum root thickness between groups yielded a non-significant p-value for the overall data (p = 0.136). Therefore, Tukey’s HSD post hoc test was not performed.  58 Chapter  5: Discussion  This study examined whether post space preparation deviated from the endodontic preparation in teeth obturated with the Thermafil, GuttaCore or Gutta percha materials. Deviation occurred in the Thermafil and GuttaCore groups, but did not occur in the Gutta percha group. Deviation occurred statistically more often in the Thermafil group than in each of the other two groups. Deviation occurred in 50% of the Thermafil samples (7/14), while it occurred in only 7% of the GuttaCore samples (1/14). The volume difference before and after post space preparation was statistically greater in the Thermafil group than in each of the other two groups. Linear deviation of post space preparation was greater in the Thermafil group than in both of the other groups, and was statistically greater than that of the Gutta percha group. The minimum root thickness before post space preparation was significantly larger than it was after post space preparation in all obturation groups. There was no difference amongst the groups with respect to the difference in minimum root thickness before and after post space preparation.   This study represents the first attempt to analyze post space preparation and its deviation in carrier-based obturations using micro-computed tomography. Remaining dentin thickness after post space preparation has been measured by many authors using various methods. Kivanc et al. [76], Mireku et al. [77], and Barcellos et al. [78] used calipers to measure remaining dentin thickness in studies of fracture resistance of endodontically treated and post-restored teeth. Raiden et al. [79], Kuttler et al. [80], and Alomari et al. [81] used sectioning procedures combined with microscopic visualization to measure remaining dentin  59 thickness after post space preparation in maxillary first premolars, distal roots of mandibular molars, and maxillary incisors, respectively. Radiography was used to evaluate post space deviation and remaining dentin thickness in studies of mandibular canines by Gegauff et al. [82], mandibular incisors by Katz and Tamse [83], and single-rooted teeth by Huysmans et al. [84]. The technique of sectioning combined with radiography to analyze root thickness after post space preparation was employed by Raiden et al. [85] in maxillary first premolars, and by Souza et al. [86] in mandibular premolars. Ghoddusi  et al. [87] measured residual dentin thickness after post space preparation in two-rooted maxillary first premolars using cone-beam computed tomography (CBCT). Although CBCT and micro-CT are both nondestructive techniques, micro-CT produces higher resolution images of dental hard tissues and root canal anatomy than does CBCT, and is therefore considered the standard in three-dimensional dental research [88].  Our study is the first to examine deviation of post space preparation and remaining root thickness in teeth which have been obturated with the Thermafil or GuttaCore techniques. A study by Scotti et al. [36] compared the bond strength of fiber posts cemented in root canals filled with gutta percha, Thermafil, or GuttaCore. However, their study did not investigate deviation of post space preparation. Many studies have been carried out with post space preparation following endodontic preparation in the absence of root filling materials [79], [81], [83], [85], [86], [89], [90], [91], [92]. A number of other studies measuring remaining dentin thickness have been carried out with preparation of the post space in teeth with gutta percha root fillings [76], [77], [78], [80], [82], [84], [87]. Ghoddusi et al. [87] compared residual dentin thickness after post space preparation using Gates-Glidden drills and Peeso  60 reamers in bifurcated maxillary first premolars. They concluded that post space preparation should be avoided in buccal roots because of the presence of a furcation groove on the palatal aspect of these roots, and that the use of Gates-Glidden drills is preferred to Peeso reamers when post space preparation is necessary. Kuttler et al. [80] examined the effects of post space preparation with Gates-Glidden drills on residual dentin thickness in distal roots of mandibular molars. They found that both the furcal and distal dentin thicknesses were reduced significantly when instruments larger than a #3 Gates-Glidden drill were used, and recommended that such instruments should not be used for post space preparation in distal roots of mandibular molars. Gegauff et al. [82] compared post preparation deviation using ParaPost drills and Gates-Glidden drills in mandibular canine teeth. The mean deviation from the center of the canal was 0.34 mm for the ParaPost drills and 0.17 mm for the Gates-Glidden drills; this difference was statistically significant. Huysmans et al. [84] studied deviation of post space preparation and remaining dentine thickness using Gates-Glidden drills followed by ParaPost drills in four types of single-rooted teeth: maxillary second premolars, maxillary lateral incisors, mandibular canines, and mandibular first premolars. Overall mean deviations were 0.27 mm to the buccal and 0.07 mm to the mesial. The mean deviations for the maxillary lateral incisor group were 0.51 mm to the buccal and 0.01 mm to the distal. The maxillary lateral incisor group had significantly more buccal deviation than the other groups. In our study, the mean linear deviations in the Thermafil, GuttaCore, and Gutta percha groups were 0.19, 0.10, and 0.06 mm, and the mean differences in minimum root thickness before and after post space preparation were 0.06, 0.03, and 0.03 mm, respectively. Before post space preparation, the minimum root thickness of the samples always occurred toward  61 the proximal surfaces of the root (mesial or distal). After post space preparation, the minimum root thickness again always occurred toward the proximal surfaces of the root. The difference in minimum root thickness represented the component of the linear deviation of the post space preparation which occurred toward the proximal surfaces of the root. The greater component of the linear deviation was not toward the proximal surfaces of the root, but toward the buccal or lingual surfaces. Our results compare favourably with the deviation results reported by Gegauff et al. [82] and by Huysmans et al. [84] noted above.  However, there are important differences between our study and those of Gegauff et al. [82] and Huysmans et al. [84]. In their studies the teeth were root filled with gutta percha using cold lateral compaction, while in our study the teeth in two of the three groups were root filled using carrier-based obturations. The post space was prepared in our study using the GT Post drill, in contrast to their studies in which the post space was prepared using Gates-Glidden drills or ParaPost drills or both. In addition, the endodontic preparations in the other studies were performed using hand files, versus our study which incorporated nickel-titanium rotary endodontic files in a standardized protocol to complete the endodontic preparations. As such, it is probable that, in the other studies, there was more variability between the samples with regard to the endodontic preparations when compared to our study. This may have had an effect on the deviation outcomes in the other studies, as the shape of the endodontic preparation, and therefore the root dentin, may have influenced the path of the post drill during post space preparation. In our study, the final step in the standardized canal instrumentation protocol was the preparation of the coronal portion of the canal using a 0.12 taper rotary endodontic file. The use of such a file for root canal instrumentation in maxillary  62 lateral incisors in a clinical situation would likely not be routinely indicated. However, its use in this study created a canal preparation whose diameter was larger than the diameter of the post drill over the entire length of the post space preparation. This allowed for post space preparation without the influence of the root canal dentin. In addition, this canal preparation simulated clinical situations in which large root canal systems may normally exist, such as in maxillary anterior teeth and in mandibular canines and premolars.  None of the five samples with curvature in the coronal portion of the root canal showed deviation of the post space preparation. It might be expected that deviation of the post space preparation would be more likely to occur in a curved canal when compared to a straight canal. In a scenario where canal curvature was preserved after root canal instrumentation, the post drill would not follow the curvature, resulting in deviation of the post space preparation. As described previously, the coronal portion of the canal was prepared using a size 50/0.12 taper rotary endodontic file. Although this file is manufactured from nickel-titanium wire, its large size and taper make it quite rigid. There is evidence that such files may straighten curved canals [93]. As such, the use of this file to instrument the coronal part of the canal of the samples in our study may have resulted in straightening of the curved canals. It was not possible to determine if canal straightening occurred, as micro-CT scans of the samples were not performed prior to root canal preparation. Even if canal straightening did not occur, deviation of the post space preparation in samples with curved root canals was probably not more likely to occur than in samples with straight root canals, because the diameter of the coronal portion of the canal preparation was greater than the diameter of the post drill, as described above.   63 It is interesting to note that in the samples which did not show deviation, the use of the parallel-sided GT Post drill in the 0.12-tapered canal preparation resulted in the complete removal of the root filling material from the coronal part of the canal corresponding to the depth of the post space preparation. The objective of the post space preparation was to prepare the post space to the specified depth with the post drill centered within the root filling. There was no intent to completely remove the root filling material; such an attempt would likely have necessitated contact between the post drill and the endodontic preparation. In fact, the geometry of the canal preparation was intended to prevent contact between the post drill and the root canal dentin coronal to the point where the diameter of the post drill and the canal preparation was the same. The root filling materials were cohesive. As such, removal of the root filling materials in the path of the post drill caused the root filling materials surrounding the post drill to be pulled away from the canal walls. When the post drill contacted the root dentin at the apical extent of the post space preparation where their diameters were the same, the root filling materials surrounding the post drill were severed from the root filling materials apical to the post space preparation. Thus, the amount of root filling removed from the canal was greater than the size of the post drill and the post space preparation.  Volume differences before and after post space preparation were calculated by subtracting the volume of the root filling before post space preparation from the volume of the root filling plus the post space after post space preparation. It is curious to note that many of the calculated volume differences had negative values. This suggested that the volume after post space preparation was smaller than the volume before post space preparation, which is  64 illogical. The volume before post space preparation is essentially comprised of one measurement, the root filling volume, while the volume after post space preparation is comprised of two measurements, the root filling volume plus the post space volume. Potential sources of error leading to this result could include inaccurate measurement of the root filling volume, or of the post space volume, or of both. The mean volume differences for the Gutta percha, GuttaCore, and Thermafil groups were -0.42, -0.24, and +0.01 mm3, respectively. Examination of this data indicates a trend which is consistent with the results of the other parts of this study: the smallest difference is seen in the Gutta percha group, the largest difference in the Thermafil group, and an intermediate difference in the GuttaCore group. This suggests that the error in measurement may not have been random, but rather may have been systematic.  The MicroView software was investigated after completion of the study as a possible source of the confounding volume results. To test MicroView’s accuracy, additional volumetric analysis was performed on six samples, two from each group: the one with the largest volume difference and the one with the smallest volume difference. For the two registered and reoriented images of each of these samples, an identical region of interest was created whose boundaries were described by: two voxels below the bottom of the post space preparation superiorly, the anatomic apex of the root inferiorly, and air circumferentially while not encroaching upon the root surface. The region of interest in both images was analyzed to determine the volume of the tooth structure plus the volume of the root filling material (Figure A.1, Appendix A). The same software tools and thresholds were used which had been used previously in the study. The volume difference was calculated by subtracting the smaller volume from the larger volume. This value was then divided by the larger volume  65 and multiplied by 100, converting it to a percentage. This data showed that the mean volume difference between the two scans of the same sample for tooth structure plus root filling material was 0.23% (Table B.1, Appendix B). The largest root filling volume measured previously in the study and reported in Table 4.3 was approximately 7.0 mm3. The volume difference of a 0.23% error on a volume of 7.0 mm3 is 0.016 mm3. Thus, it is unlikely that measurement of the root filling volume is the cause of the confounding volume results.  Further analysis was performed on several samples to determine if measurement of the post space volume was the source of the error. Post space volume was measured again, but indirectly this time, using the same software tools and thresholds which had been used previously in the study. The indirect volume measurement was compared to the direct volume measurement previously reported for the sample in Table 4.3. These two volume measurements were found to be similar but not identical, and the overall results were the same: the total volume after post space preparation (root filling plus post space) was smaller than the volume before post space preparation (root filling only). McNiven et al. [94] imaged four small volume ion chambers (56 mm3) with micro-CT using the same voxel size used in this study (20 µm) and measured their air volumes with MicroView software. For air volume comparisons, experimental ionization measurements were also performed. Differences in the air volumes using micro-CT agreed with differences in the ionization response with a mean of 0.73% for the four chambers. It is inconclusive whether the post space volume measurement contributed to the error in the volume results in our study. Another potential source of error may include the interface between the root filling and the post space. This interface was comprised, in part, of voxels with greyscale values similar to those of tooth structure. As such, the interface voxels were excluded from the volumetric analysis by the  66 thresholds selected to exclude tooth structure. The excluded interface could be visualized between the root filling volume and the post space volume (Figure 4.6), and its width appeared to measure as little as one voxel. However, it was not possible to measure the volume of the excluded interface. Presumably, an error related to the interface would exert its largest influence on samples showing deviation, as the interface between the post space and the remaining root filling would likely be larger in samples showing deviation compared to samples in which deviation did not occur. This does not seem to be the case, as the largest negative values for volume difference before and after post space preparation occurred in the Gutta percha group, in samples in which deviation did not occur (see Tables 4.3 and 4.4).  The literature was searched in an effort to compare our study to other micro-CT endodontic studies which used the MicroView software for data analysis, but no such study was found. The confounding volume measurement results in this study, and our inability to account for them, is disappointing. Many other endodontic micro-CT studies using various software packages for the data analysis have measured volumes, apparently accurately. Perhaps there is an as yet undetected or unreported deficiency in the MicroView software which is responsible for the volume outcome results in our study.  The largest value with respect to both volume difference and linear deviation of post space preparation occurred in the one GuttaCore sample which showed deviation. The reason for this is not fully understood. The reconstructed micro-CT image data of this sample was examined and no irregularities were detected in the anatomy of the root canal system or in the root filling which might have contributed to this outcome. In the first GuttaCore sample in which post space was prepared, the technique used for post space preparation was the  67 same as that used for the gutta percha samples, namely, preparation of post space with the GT Post drill without initial removal of the coronal portion of the carrier. This is the recommended technique per the manufacturer’s directions for use. However, this method proved to be less effective than claimed, so for the remainder of the GuttaCore samples, the technique used for post space preparation was the same as that used for the Thermafil samples (i.e., initial removal of the coronal 2 mm of the carrier with a small high-speed round bur, followed by the GT Post drill). The one GuttaCore sample in which deviation of the post space preparation occurred was not the first GuttaCore sample in which post space preparation was performed. Post space preparation for the samples was performed over the course of two sessions: 15 samples in the first session and the remaining 27 samples in the second session. As described previously, each post drill was used on six samples in an alternating fashion between the three root filling types, on two samples of each type. Thus, the post drill used for the first three samples in the second session had been used for the last three samples in the first session (one GuttaCore, one Thermafil, and one Gutta percha). The previous use of this post drill on the Thermafil sample (#2425) had resulted in deviation. As such, the cutting tip of the post drill had been in contact with root dentin and had therefore likely been dulled. The first post space preparation of the second session was in a GuttaCore sample (#12) and resulted in deviation. Thus, it is possible that the deviation of the post space preparation in this sample was due to decreased cutting efficiency of the tip of the post drill caused by the previous deviation. It is interesting to note that the next use of this post drill on a Thermafil sample (#11) also resulted in deviation. If future studies are carried out using a protocol similar to the one used in this study, it may be advisable to discard the post drill after a  68 deviation occurs, or even to use a new post drill for each post space preparation. The manufacturer’s directions for use of the GT Post drill state that: “drills are reusable”, and that “drills must be examined for wear before each use”, but do not specify how many times the drill may be used, or for which materials, before they should be discarded. Further study is required to determine the effect of post space preparation in Thermafil, GuttaCore, and gutta percha obturations, as well as contact with root dentin, on the sharpness of the post drill. The results of such a study might be used to establish guidelines for the number of times the post drill could safely be used for each type of root filling material.  This study was carried out under simulated clinical conditions. Post space preparation was performed by a human operator using a technique described previously, with the objective of preparing the post space to a specified depth. If the operator perceived resistance to advancement of the post drill through proprioceptive feedback mechanisms, he could respond by altering the angulation of the post drill, or by increasing the apical pressure on the handpiece, or both. An alternative study design could include preparation of the post space using a mechanized apparatus analogous to a drill press. This would allow for control of variables such as force and rate of post drill insertion. Such a design would necessitate that the endodontic preparation be centered in all planes in relation to the post drill. This would likely be very difficult to accomplish using extracted human teeth. A material with similar properties to root dentin could be used as a substitute for natural teeth, and straight artificial root canals created within the samples. The root-filled sample would be fixed in a non-rigid manner below the post drill and allowed some freedom to move in the x- and y-planes as a consequence of the post space preparation. This experimental set-up would provide  69 information about the properties of the various root filling materials, but would be a considerable departure from a clinical situation and would, therefore, be of limited usefulness.  The operator was not blinded with regard to the type of root filling within each sample prior to the post space preparation procedure. This was due to the necessity to use slightly different techniques for post space preparation with the different types of root filling materials; specifically, whether or not a carrier was present. It was assumed that deviation would be less likely to occur in carrier-based obturations if the centre of the post drill was aligned with the centre of the carrier at the beginning of, and throughout, post space preparation. In this way, the carrier would become ‘trapped’ within the inverted cone produced by rotation of the eccentric tip of the post drill. A carrier constrained thusly would be more easily cut and removed by the post drill. At completion of the root filling procedure with the carrier-based techniques, the obturator carrier was most often located eccentrically within the root filling at the coronal extent of the canal. In this scenario, if the operator had been blinded with regard to the type of root filling, the post drill would have been placed in the centre of the root filling, and would then be adjacent to the carrier at the initiation of post space preparation. This would likely have influenced the outcome with regard to both the number of samples in which deviation occurred, as well as the amount of deviation which occurred. Thus, it was necessary for the operator to not only see whether or not there was a carrier present in the sample, but also to see where the carrier was located within the root filling in order to orient the post drill appropriately prior to starting the post space preparation. The absence of  70 blinding also replicated a realistic clinical situation in which the practitioner was aware of the material used to obturate the canal, either by direct observation or by communication.  A pilot project was undertaken to determine the type of teeth suitable for this ex vivo study. Initial experimentation was carried out using extracted human permanent canine teeth. The canine root canal system is generally broader in the bucco-lingual dimension compared to the mesio-distal dimension, even after endodontic instrumentation. When post space preparation was attempted in the pilot teeth obturated with Thermafil and GuttaCore, it was discovered that the post drill did not remove the carrier, but removed only gutta percha adjacent to the carrier, either buccaly or lingually. Thus, teeth and roots with root canal systems broader in one dimension compared to the other were deemed unsuitable for this study. Permanent maxillary lateral incisors were ultimately chosen for three reasons: (i) their root canal cross-sectional dimensions are often similar in the bucco-lingual aspect compared to the mesio-distal aspect [95], (ii) their roots and canals are generally straight in the coronal and middle portions of the root, and (iii) anterior teeth are the tooth types which most frequently require post placement following the loss of tooth structure as a result of trauma or caries. In addition, maxillary lateral incisors are commonly subject to iatrogenic complications related to post space preparation. In a clinical study of 55 cases of root perforation, Kvinnsland et al. [96] found that perforation occurred about three times more often in maxillary teeth than in mandibular teeth. More than half the perforations were due to post space preparation. The most frequently perforated tooth was the maxillary canine, followed by the maxillary lateral incisor. All the perforations on buccal root surfaces were found in maxillary anterior teeth. The authors speculated that these perforations may have resulted from inadequate extension  71 of the access preparation, or from the operator's underestimation of the palatal inclination of the roots of the maxillary teeth. Thus, the teeth in our study were decoronated to permit unrestricted access to the root canal system, and to simulate a clinical scenario in which the clinical crown of the tooth had been lost and post placement was required.  72  Chapter  6: Conclusion This study yielded interesting results. Half the Thermafil samples showed post space deviation, only one GuttaCore sample showed deviation, while none of the gutta percha samples showed post space deviation. Thus, the null hypothesis that there would be no deviation of the post space preparation from the endodontic preparation was rejected. It can be concluded that the differences were due to the presence or absence of a carrier as well as the different carrier materials, as the endodontic preparation was the same size in all samples, and larger than the post drill. The manufacturer’s claim that it is ‘easier’ to create post space with GuttaCore than with plastic carrier-based obturators may be true, at least for the polysulfone plastic carrier-based obturator Thermafil, and under the conditions of this study.  Our study was carried out under ideal conditions in several respects, including tooth selection and decoronation, canal preparation, and microscope magnification. Thus, our results may be conservative, and one might expect that more deviations would occur, in both number and extent, under in vivo clinical conditions.  Deviation of post space preparation from the endodontic preparation causes needless additional removal of tooth structure and may lead to sequelae such as iatrogenic root perforation, ultimately resulting in a poorer prognosis for the endodontically treated tooth. Based on the results of this study, if post space preparation is planned for a root canal which has not yet been obturated, the clinician could confidently fill the canal with gutta percha or GuttaCore prior to post space preparation. In a scenario where post space preparation is  73 necessary for a root canal which was previously obturated with Thermafil, the clinician could consider retreatment of the canal with a different obturation material prior to post space preparation to prevent its possible deviation and sequelae.  Future studies could investigate the successive effects of post space deviation. One such study might consist of bonding posts in roots with and without post space deviation and comparing the roots’ fracture resistance. Another such study might include bonding posts in roots with and without post space deviation and comparing the posts’ pull-out bond strength. Yet another study could use hardness testing to determine the relative hardnesses of the plastic Thermafil carriers, the cross-linked gutta percha GuttaCore carrier, the gutta percha, and the root canal dentin.    74 References  [1] Ørstavik D. Materials used for root canal obturation : technical , biological and clinical testing. Endod Top 2005;12:25–38. [2] Saleh IM, Ruyter IE, Haapasalo M, Ørstavik D. Survival of Enterococcus faecalis in infected dentinal tubules after root canal filling with different root canal sealers in vitro. Int Endod J 2004;37:193–8. [3] Gatewood RS. Endodontic materials. Dent Clin North Am 2007;51:695–712. [4] Dahl JE. Toxicity of Endodontic Filling Materials. 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Int Endod J 1989;22:75–84.    82 Appendices  Appendix A  - Confirmation of MicroView Volume Measurements             Figure A.1 Measurement of the volume of tooth structure plus root filling material of a sample before (left) and after (right) post space preparation for comparison of volume measurement accuracy.  83 Appendix B  - Volume Difference between First and Second Scans   Table B.1 Volume of tooth plus filling as shown in Figure A.1 before and after post space preparation Sample Previous volume difference (mm3) * Scan # Volume of tooth plus filling (mm3) Difference (mm3) Difference (%) GC_12 + 0.58 2 102.28 0.20 0.20 1 102.48 GC_2131 - 0.56 2 59.41 0.15 0.25 1 59.26 GP_43 - 0.17 2 85.64 0.32 0.37 1 85.32 GP_41 - 0.61 2 52.31 0.16 0.30 1 52.47 TF_15 + 0.51 2 81.86 0.20 0.24 1 81.66 TF_32 - 0.45 2 71.24 0.03 0.04 1 71.21 Mean volume difference (%) 0.23 * see Table 4.3 (page 49-50)  

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