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Biomechanical Effects of Bonding Pericervical Dentin in Maxillary Premolars
Basic Research—Technology
Biomechanical Effects of Bonding Pericervical Dentin in Maxillary Premolars Nghia Huynh, HBSc, DDS, Fang-Chi Li, DDS, Shimon Friedman, DMD, and Anil Kishen, BDS, MDS, PhD Abstract Introduction: Pericervical dentin (PCD) loss may increase root fracture propensity in root-filled teeth. This study evaluated the impacts of bonding PCD with composite resin (CR) on radicular microstrain distribution and load at failure of root-filled maxillary premolars. Methods: Ten single-canal maxillary premolars decoronated 2 mm coronal to the cementoenamel junction (CEJ) had canals enlarged with ProTaper Universal instruments (Dentsply Tulsa Dental Specialties, Tulsa, OK) to F3. They were root filled with gutta-percha (GP) to the CEJ and restored with Cavit (3M Deutschland GmbH, Neuss, Germany) (GP group, n = 5) or 6 mm apical to the CEJ and restored with bonded CR to simulate bonding of PCD (bonded PCD group, n = 5). Digital moire interferometry was used to evaluate pre- and postoperative whole-field microstrain distribution in the root dentin under physiologically relevant loads (10–50 N). Another 30 premolars, similarly treated as groups 1 and 2 or left untreated as controls (n = 10/group), were subjected to cyclic loads (1.2 million cycles, 45 N, 4 Hz) followed by uniaxial compressive load to failure. Mechanical data were analyzed with 1-way analysis of variance and the post hoc Tukey test at a 5% level of significance. Results: Microstrain distribution showed bending and compressive patterns at the coronal and apical root dentin, respectively. In the GP group, microstrain distribution was unaltered. In the bonded-PCD group, different microstrain distribution suggested stiffening at the PCD. The load at failure did not differ significantly for the GP, bonded PCD, and control groups (P > .05). Conclusions: CR bonding of PCD might impact the biomechanical responses in maxillary premolar roots at low-level continuous loads. The effect of this impact on root fracture loads when subjected to cyclic load warrants further investigation. (J Endod 2018;-:1–6)
Key Words Bonding, fracture resistance, pericervical dentin
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ith approximately 15 Significance million endodontic Bonding pericervical dentin with composite resin treatments performed annureduced pericervical dentin bending and apical mially in the United States (1), crostrain distribution for continuous loads at physthe reported 5%–10% (2, iological ranges, without providing a significant 3) prevalence of vertical increase in the load to failure subsequent to cyclic root fracture (VRF) leading loading. The long-term advantage of bonding perito tooth loss posttreatment cervical dentin with composite resin was not estabrepresents a considerable lished. societal burden (2). Of the multifactorial causes of VRF (4), iatrogenic and noniatrogenic loss of dentin predisposes teeth to mechanical failure under functional stresses (4). Typically, when a root-filled tooth is exposed to chewing forces, under certain conditions, a cumulative process of crack initiation and propagation may occur with time, leading to fatigue failure (4). Although crack initiation and propagation induced by engine-driven canal instrumentation and root filling are currently the focus of research (5), the biomechanical impact of bonded restoration on instrumented root canals has not been thoroughly explored. Biomechanics is the study of structure and function of biological systems using the principles of engineering mechanics (6). The biomechanical response of bulk dentin tissue to functional forces determines its mechanical integrity (7). Microcrack events leading to catastrophic fracture are locally strain controlled (8); thus, assessing the mechanical stress/strain distribution in root dentin may explain some of the causes of VRF in root-filled teeth (9). Greater dentin loss generates higher stress concentrations, which significantly compromises the fracture strength of teeth (10). Different degrees of dentin removal may occur during canal instrumentation, which may alter the biomechanical response of the remaining root dentin and resistance to VRF (4). Of particular interest in this regard is pericervical dentin (PCD), extending 6 mm apical and 4 mm coronal to the crestal bone (11), which distributes occlusal stresses through the long axis of the root (11). Loss of PCD is implicated in the weakening of the root structure and decreased resistance to VRF (11). Digital moire interferometry (DMI) uses the principles of optical interferometry to determine the microstrain distribution in dento-osseous structures with high resolution (12), offering substantial advantages over conventional microstrain assessments (13). Contrary to conventional mechanical testing, DMI provides whole-field strain distribution patterns on specimens for low-level loads within physiological limits. Our group has recently used DMI to assess microstrain distribution in root dentin in response to root canal instrumentation (5). The aims of this study were to assess the impacts of restoring PCD with bonded composite resin in root-filled maxillary premolars on inplane microstrain distribution in root dentin using DMI and load at failure using cyclic
From the University of Toronto, Toronto, Canada. Address requests for reprints to Dr Anil Kishen, Faculty of Dentistry, University of Toronto, 124 Edward Street, Toronto, ON, Canada M5G1G6. E-mail address: anil. [email protected] 0099-2399/$ - see front matter Copyright ª 2018 American Association of Endodontists. https://doi.org/10.1016/j.joen.2018.01.002
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Bonding Pericervical Dentin in Maxillary Premolars
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Basic Research—Technology and subsequent continuous compressive loading. It was hypothesized that bonding PCD with composite resin would have no effect on the strain distribution patterns and subsequent loads to failure in endodontically treated roots.
(Dentsply Tulsa Dental Specialties) to size F3. During instrumentation, canals were intermittently irrigated with 10 mL 2.5% sodium hypochlorite using a ProRinse side-vented 30-G needle (Dentsply Tulsa Dental Specialties).
Materials and Methods
Microstrain Distribution After canal enlargement, specimens were subjected to microstrain assessment with DMI. The experimental setup for the highresolution DMI and the process of grating replication were based on our previous experiments (12). Mesial and distal surfaces of 10 specimens were ground down equally on wet emery paper of grit sizes 800 and 1200 under constant running water to prepare 3-mm-thick, parallel-sided longitudinal sections. High-frequency cross grating (f = 1200 lines/mm, diffraction efficiency of 10% and intensity variation of <15%) was replicated on the longitudinal root surface with a thin layer of epoxy-based adhesive (PC-10; Measurements Group, Raleigh, NC) and was allowed to set for 5 hours in ambient conditions (22 C, 55% relative humidity) before being subjected to DMI. Specimens acted as their own control in this experiment. Mounted on a specially fabricated loading jig (5), the grating-replicated specimen was compressively loaded from 0 N to 50 N with 10-N increments. At each loading interval, whole-field digitized fringe patterns in root dentin were acquired by a high-resolution charge-coupled device camera and recorded as the control microstrain distribution (after canals were enlarged to F3). Subsequently, the grating material was gently removed with wet emery paper as described. Specimens were root filled with thermoplasticized gutta-percha (GP) and Pulp Canal Sealer (SybronEndo Endodontics, Orange, CA) by vertical compaction and stored in 100% humidity for 24 hours to allow the sealer to set. In the GP group (n = 5), canals were root filled to the CEJ level and the remaining coronal 2 mm restored with Cavit (3M Deutschland GmbH, Neuss, Germany). In the bonded PCD group (n = 5), canals were root filled to 6 mm below the CEJ level and the remaining coronal 8 mm restored with dentin-bonded composite resin as follows: a 20-second application and light cure of Clearfil DC Bond (Kuraray America, New York, NY) followed by Clearfil DC Core Plus Resin (Kuraray America). To minimize voids in the restoration, the dual cure resin was syringed in
Specimens The university ethics board approved the study protocol. Sample size calculation considered previous studies (14, 15) on load at failure under compressive loads in which differences in the range of 22% reached statistical significance with samples of 10 teeth per group. Accordingly, a sample size of 10 teeth per group was used in the present study to analyze data with 80% power and 5% significance. Ten human extracted noncarious maxillary premolars with single canals, closed apices, and no cracks/fractures were selected. Teeth were stored in a phosphate-buffered saline solution at 4 C before testing. Conventional radiographic images of the teeth were captured from 2 perpendicular exposures, and the teeth were characterized for length and overall dimensions. Three sets of teeth matched for length and canal dimensions were assembled to minimize variation among groups. Teeth were decoronated under a dental operating microscope (OPMI Pico; Zeiss, Oberkochen, Germany) at 10 magnification with a low-speed saw (Isomet; Buehler, Lake Bluff, IL) under water cooling. They were sectioned at 2 mm coronal to the average level of the buccal, lingual, mesial, and distal cementoenamel junction (CEJ) levels. In the absence of crestal bone, the level of CEJ, located approximately 2 mm supracrestal in normal periodontal architecture (16), was used as the reference for measuring PCD in this in vitro model (Fig. 1). The single root canal was negotiated with a size 10 K-type file (Dentsply Tulsa Dental Specialties, Tulsa, OK) to the major apical foramen as observed at 4 magnification, and the working length was established 1 mm short of this point. The glide path was established with 3 PathFile instruments (Dentsply Tulsa Dental Specialties). The canal was enlarged to the working length with ProTaper Universal instruments
GROUP GP :
GROUP Bonded-PCD:
CEJ Simulated crestal bone
2mm 2mm 6 mm
Cavit Gua-percha + Pulp Canal Sealer Bonded composite-resin – Clearfil DC Core Plus
Diagram not to scale
Figure 1. A schematic diagram showing specimens used in the GP and bonded PCD groups.
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Basic Research—Technology place with 2-mm increments and cured. The root-filled specimens were again grating replicated and subjected to microstrain distribution as described earlier, which was recorded for the GP and bonded PCD groups.
Mechanical Testing Thirty premolars were collected, prepared, and assigned to the following groups: control, GP, and bonded PCD (n = 10/group). In the untreated controls (size F3, without root filling), the pulp chambers were restored with Cavit. All 30 specimens were mounted in custom devices and imaged with a micro–computed tomographic (micro-CT) scanner (SkyScan 1172; Bruker MicroCT, Toronto, Ontario, Canada) at an 8-mm voxel size (pretreatment scan) to standardize root canal geometry and volume and to rule out any dentinal defects. After instrumentation, specimens were imaged again (posttreatment scan) to capture the enlarged canal geometry and posttreatment defects. Amira 3D software (FEI, Hillsboro, OR) was used for micro-CT image analysis including comparison of pre- and posttreatment canal volumes (15, 17). Specimens were mounted on brass rings with the roots embedded in self-curing resin (Justis Quick Resin; Ivoclar Vivadent, Schaan, Lichtenstein) up to a level 2 mm apical to the CEJ. A 0.5-mmthick silicone rubber barrier (Aquasil LV; Dentsply DeTrey GmbH, Konstanz, Germany) was applied to the root surfaces to simulate the periodontal ligament. The embedded specimens were stored in deionized water before mechanical testing. Specimens were then mounted in the Instron Universal Testing Machine (Instron, Canton, MA). In a custom-made water bath, a force of 45 N was cyclically applied at a frequency of 4 Hz, with a 5-mm spherical crosshead at the center of the occlusal access cavity aligned with the longitudinal axis of the tooth (18). If the sample survived 1.2 million loading cycles without fracture, simulating approximately 5 years of function (19), they were subsequently loaded with the same spherical crosshead applying a continuous compressive force at 1 mm/min until fracture occurred (25% drop in applied force). The load at failure (N) was recorded as a measure of fracture strength. Upon completion, radiographic images of all specimens were again captured from 2 perpendicular exposures and characterized for location (cervical, middle, or apical) and pattern (comminuted, buccopalatal, or mesiodistal) of fracture. Data Analysis The acquired whole-field moire fringe patterns were used to evaluate qualitatively the microstrain distribution pattern at different regions of interest at the cervical and the apical third of the root dentin (Fig. 2A). The in-plane microstrain values in regions of interest were calculated using the following relationship. Strain (εxx) in the long axis of the tooth is given as DU/Dx = P/Dx, where DU is the relative displacement in the x direction between 2 points, P is the pitch of the reference grating, and Dx is the fringe spacing in the x direction. The microstrain values were used to examine the nature of microstrain distribution at regions of interest in the cervical and apical regions of the root for the unrestored and bonded PCD specimens (12). For the mechanical testing under cyclic and continuous loading, the mean load at failure values of the 3 groups were analyzed with 1-way analysis of variance and the post hoc Tukey test at a 5% level of significance.
Results Microstrain Distribution Moire fringes are formed from the constructive and destructive interference between the virtual and specimen gratings. They provide JOE — Volume -, Number -, - 2018
Figure 2. (A) Typical moire fringe patterns obtained from the representative, unfilled specimen in the control group showing microstrain distribution patterns in the cervical and apical regions of interest (denoted by the red boxes) where microstrain values were measured. The microstrain distribution pattern at the coronal regions was characteristic of bending. Compressive strain characteristics were observed at the apical region. The red stars indicate the dentin at the center of the root section corresponding to the root canal dentin.(B) Typical moire fringe patterns obtained from GP group specimens (unrestored PCD). The microstrain distribution characteristic of bending at the coronal region of the control specimens was reduced in this group. (C) Typical moire fringe patterns obtained from the bonded PCD group. Distinct microstrain distribution at the coronal region, which decreased toward the apical region, was observed.
information on the nature of deformation on the test specimen. In the control specimens, the microstrain distribution exhibited patterns consistent with bending at the cervical third of the root and patterns consistent with compression at the apical third (Fig. 2A). Both coronal and apical microstrain distribution patterns increased with higher applied loads (Fig. 3). In the GP group (Fig. 2B), bending microstrain distribution patterns in the cervical dentin and microstrain distribution patterns at the apical regions were lower compared with controls (Fig. 2A). In the bonded-PCD group (Fig. 2C), the degree of bending
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Figure 3. A graphic representation of the (A) cervical and (B) apical microstrain response to increasing applied loads. In the bonded PCD group, an increased shift of microstrain away from the apical region toward the cervical region was evident. In contrast, the GP group and the unfilled controls exhibited increasing microstrains with an increase in the applied loads at both the cervical and apical regions.
microstrain distribution at the cervical region was obviously lower compared with controls (Fig. 2A); the microstrain distribution at the apical third was also much lower. All groups showed an increase in microstrain with applied loads at the cervical region (Fig. 3A). The control and GP groups showed an increase in microstrain with applied loads at the apical region, whereas the bonded PCD group samples had very little change in apical microstrain (Fig. 3B).
Load at Failure and Fracture Patterns The mean load at failure values were highest in the bonded PCD group (1703.74 N) and lowest in the GP group (1467.65 N) (Fig. 4A). Differences among the groups were not statistically significant (P = .46 > .05). When comparing the fracture patterns, all specimens exhibited fractures contained in the cervical region (Fig. 4B). In the GP and control groups, the pattern of fracture was consistent with comminuted fractures (Fig. 4B). In the bonded PCD group, fractures in the buccopalatal direction and interfacial failures between the composite resin and the dentin surface were noted (Fig. 4B).
Discussion Cumulative loss of tooth structure because of caries, trauma, and treatment procedures increases the risk of tooth fracture (4, 20). To restore fracture resistance in teeth, modern dentistry has shifted toward bonded restorative procedures (21). Bonded restorations have been suggested to improve the long-term survivability of root-filled teeth 4
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(10). Because the modulus of elasticity of composite resin is close to that of dentin (10), restoring endodontic cavities with composite resin showed a minimal stress jump at the resin-dentin interface, reduced cuspal flexure, and lowered probability of coronal fractures (10, 22, 23). Although previous studies have examined the effects of bonded restorations at the coronal tooth level (23, 24), the feasibility of using bonded composite resin to restore the biomechanical integrity in instrumented radicular dentin was not well studied. This study focused on the restoration of PCD with dentin-bonded composite resin and its impacts on biomechanical responses of root dentin, including microstrain distribution and load to failure. Tooth selection was based on the reported higher incidence of root fractures in endodontically treated maxillary second premolars (3). The application of pre– and post–micro-CT scans allowed a nondestructive means to evaluate specimens for intraradicular cracks as well as to match root canal volume and canal geometry for specimens used in this study (25, 26). DMI has been previously applied in dental biomechanics (13). It enables examination of dental hard tissues under physiologically relevant loads to evaluate high-resolution microstrain distributions over the whole sample in real time (13). One of the inherent challenges in the application of DMI to biological structures is the difficulty of comparing high-resolution microstrain quantitatively between specimens (6, 17). Nevertheless, the whole-field microstrain distribution patterns generated in real time allow for qualitative comparison within samples subjected to physiological range of forces, which cannot be achieved with conventional strain gauge measurements. The Instron Universal Testing machine is commonly used to assess resistance to fracture of root-filled teeth (27), with cyclic loading considered to represent clinical function (19). In this study, root specimens were subjected to a hybrid analysis comprised of cyclic loading to simulate physiological aging followed by compressive loading to failure (28). Cyclic fatigue is imperative when assessing bonded composite resin restorations because it may undermine the bond strength because of increased flexure of the supporting tooth structure (29), leading to bond failures (30). Because incomplete bonding, interfacial voids, and polymerization shrinkage may all undermine bond strength (31), we used a self-polymerizing dual-cured resin, which was applied incrementally within the canal space. In vivo and in vitro stress/strain analyses of teeth have shown that stress was concentrated toward the cervical region when restored access cavities were subjected to a compressive load (10). Our results indicated that both the untreated control GP group experienced increased cervical and apical microstrain with increasing applied loads. Restoring PCD with bonded composite resin (bonded PCD group) contributed to a shift in microstrain distribution away from the apical root dentin toward the cervical dentin, resulting in the greatest increase in cervical microstrain and the least apical microstrain increase. This finding suggested that bonding of PCD reduced flexing or caused stiffening of cervical root dentin, resulting in redistribution of functional loads away from the apical region. The definition of a true VRF according to the American Association of Endodontists is a complete or incomplete fracture initiated from the root at any level, usually directed buccolingually (32). The reduction in apical microstrain achieved by bonding of PCD might be expected to reduce the propensity to VRF. Although bonded PCD withstood physiological level loads and distributed radicular microstrain away from the apical region, under cyclic fatigue conditions the value of bonding PCD appeared to be limited. It did not show a significant improvement in fracture strength but possibly a modified fracture pattern characteristic of interfacial failure. These observations suggested that composite resin bonding to PCD has its limitations when cyclic mechanical loads are JOE — Volume -, Number -, - 2018
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Figure 4. (A) The average load at failure for specimens in the tested groups. The load at failure was as follows: the bonded PCD group > the control group > the GP group (P > .05). (B) Typical examples of fracture patterns observed in the 3 groups tested in the study (blue arrows). All fracture planes were contained within the cervical region.
applied. Earlier studies have shown that the initial bond strength is greatest at the time of bonding, but the durability of the bond is reduced with repeated compressive loads (33, 34). The findings also corroborated the challenge of strengthening root dentin postinstrumentation (35). It is important to note that although in this study comparable models were used in both the experiments, altering the load regimen in fatigue testing, simulating a greater degree of root canal dentin removal, and altering the nature and angulation of loads may have generated different findings. In conclusion, this study suggested that in maxillary premolars with a single canal, restoration of PCD with dentin-bonded composite resin impacted a shift in microstrain distribution away from the apical region toward the pericervical region when subjected to loads in physiologic ranges. However, this effect did not appear to impact the load at failure when subjected to fatigue cycling followed by compressive
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loading. The long-term clinical effect of composite resin bonding of PCD in root-filled teeth requires further investigation.
Acknowledgments Supported in part by a research grant from the American Association of Endodontists Foundation (494223), the University of Toronto (928133) (AK), and the Canadian Academy of Endodontics Endowment Fund. The authors deny any conflicts of interest related to this study.
References 1. American Association of Endodontists. AAE Endodontic treatment statistics. Available at: http://www.aae.org/about-aae/news-room/endodontic-treatment-statistics. aspx. Accessed February 8, 2018. 2. Landys Boren D, Jonasson P, Kvist T. Long-term survival of endodontically treated teeth at a public dental specialist clinic. J Endod 2015;41:176–81.
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Basic Research—Technology 3. Tamse A, Fuss Z, Lustig J, Kaplavi J. An evaluation of endodontically treated vertically fractured teeth. J Endod 1999;25:506–8. 4. Kishen A. Mechanisms and risk factors for fracture predilection in endodontically treated teeth. Endod Topics 2006;13:57–83. 5. Lim H, Li FC, Friedman S, Kishen A. Residual microstrain in root dentin after canal instrumentation measured with digital moire interferometry. J Endod 2016;42:1397–402. 6. Asundi A, Kishen A. A strain gauge and photoelastic analysis of in vivo strain and in vitro stress distribution in human dental supporting structures. Arch Oral Biol 2000;45:543–50. 7. Kishen A, Ramamurty U, Asundi A. Experimental studies on the nature of property gradients in the human dentine. J Biomed Mater Res 2000;51:650–9. 8. Nalla RK, Kinney JH, Ritchie RO. Effect of orientation on the in vitro fracture toughness of dentin: the role of toughening mechanisms. Biomaterials 2003;24:3955–68. 9. Kishen A, Kumar GV, Chen NN. Stress-strain response in human dentine: rethinking fracture predilection in postcore restored teeth. Dent Traumatol 2004;20:90–100. 10. Soares PV, Santos-Filho PC, Queiroz EC, et al. Fracture resistance and stress distribution in endodontically treated maxillary premolars restored with composite resin. J Prosthodont 2008;17:114–9. 11. Clark D, Khademi J, Herbranson E. Fracture resistant endodontic and restorative preparations. Dent Today 2013;32:118–20. 12. Kishen A, Asundi A. Photomechanical investigations on the stress-strain relationship in dentine macrostructure. J Biomed Opt 2005;10:034010. 13. Kishen A, Tan KB, Asundi A. Digital moire interferometric investigations on the deformation gradients of enamel and dentine: an insight into non-carious cervical lesions. J Dent 2006;34:12–8. 14. Assif D, Nissan J, Gafni Y, Gordon M. Assessment of the resistance to fracture of endodontically treated molars with amalgam. J Prosthet Dent 2003;89:462–5. 15. Krishan R, Paque F, Ossareh A, et al. Impacts of conservative endodontic cavity on root canal instrumentation efficacy and resistance to fracture assessed in incisors, premolars, and molars. J Endod 2014;40:1160–6. 16. Albandar JM, Abbas DK. Radiographic quantification of alveolar bone level changes. Comparison of 3 currently used methods. J Clin Periodontol 1986;13:810–3. 17. Peters OA, Laib A, Gohring TN, Barbakow F. Changes in root canal geometry after preparation assessed by high-resolution computed tomography. J Endod 2001;27:1–6. 18. De Boever JA, McCall WD Jr, Holden S, Ash MM Jr. Functional occlusal forces: an investigation by telemetry. J Prosthet Dent 1978;40:326–33. 19. DeLong R, Sakaguchi RL, Douglas WH, Pintado MR. The wear of dental amalgam in an artificial mouth: a clinical correlation. Dent Mater 1985;1:238–42. 20. Reeh ES, Messer HH, Douglas WH. Reduction in tooth stiffness as a result of endodontic and restorative procedures. J Endod 1989;15:512–6.
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21. Ericson D. What is minimally invasive dentistry? Oral Health Prev Dent 2004;2(Suppl 1):287–92. 22. Mannocci F, Qualtrough AJ, Worthington HV, et al. Randomized clinical comparison of endodontically treated teeth restored with amalgam or with fiber posts and resin composite: five-year results. Oper Dent 2005;30:9–15. 23. Taha NA, Palamara JE, Messer HH. Fracture strength and fracture patterns of rootfilled teeth restored with direct resin composite restorations under static and fatigue loading. Oper Dent 2014;39:181–8. 24. Panitvisai P, Messer HH. Cuspal deflection in molars in relation to endodontic and restorative procedures. J Endod 1995;21:57–61. 25. Nielsen RB, Alyassin AM, Peters DD, et al. Microcomputed tomography: an advanced system for detailed endodontic research. J Endod 1995;21:561–8. 26. Peters OA, Laib A, Ruegsegger P, Barbakow F. Three-dimensional analysis of root canal geometry by high-resolution computed tomography. J Dent Res 2000;79: 1405–9. 27. Tang W, Wu Y, Smales RJ. Identifying and reducing risks for potential fractures in endodontically treated teeth. J Endod 2010;36:609–17. 28. Stappert CF, Guess PC, Chitmongkolsuk S, et al. Partial coverage restoration systems on molars–comparison of failure load after exposure to a mastication simulator. J Oral Rehabil 2006;33:698–705. 29. Janda R, Roulet JF, Latta M, Ruttermann S. The effects of thermocycling on the flexural strength and flexural modulus of modern resin-based filling materials. Dent Mater 2006;22:1103–8. 30. Jantarat J, Palamara JE, Messer HH. An investigation of cuspal deformation and delayed recovery after occlusal loading. J Dent 2001;29:363–70. 31. Braga RR, Ballester RY, Ferracane JL. Factors involved in the development of polymerization shrinkage stress in resin-composites: a systematic review. Dent Mater 2005;21:962–70. 32. American Association of Endodontists. Cracking the cracked tooth code: detection And treatment of various longitudinal tooth fractures. Available at: http:// www.aae.org/uploadedfiles/publications_and_research/endodontics_colleagues_for_ excellence_newsletter/crackedteethecfe_onlineversion.pdf. Accessed February 8, 2018. 33. Montagner AF, Opdam NJ, Ruben JL, et al. Bonding effectiveness of composite-dentin interfaces after mechanical loading with a new device (Rub&Roll). Dent Mater J 2016;35:855–61. 34. Toledano M, Osorio R, Albaladejo A, et al. Effect of cyclic loading on the microtensile bond strengths of total-etch and self-etch adhesives. Oper Dent 2006;31:25–32. 35. Gesi A, Raffaelli O, Goracci C, et al. Interfacial strength of Resilon and gutta-percha to intraradicular dentin. J Endod 2005;31:809–13.
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Biomechanical Effects of Bonding Pericervical Dentin in Maxillary Premolars Nghia Huynh, HBSc, DDS, Fang-Chi Li, DDS, Shimon Friedman, DMD, and Anil Kishen, BDS, MDS, PhD Abstract Introduction: Pericervical dentin (PCD) loss may increase root fracture propensity in root-filled teeth. This study evaluated the impacts of bonding PCD with composite resin (CR) on radicular microstrain distribution and load at failure of root-filled maxillary premolars. Methods: Ten single-canal maxillary premolars decoronated 2 mm coronal to the cementoenamel junction (CEJ) had canals enlarged with ProTaper Universal instruments (Dentsply Tulsa Dental Specialties, Tulsa, OK) to F3. They were root filled with gutta-percha (GP) to the CEJ and restored with Cavit (3M Deutschland GmbH, Neuss, Germany) (GP group, n = 5) or 6 mm apical to the CEJ and restored with bonded CR to simulate bonding of PCD (bonded PCD group, n = 5). Digital moire interferometry was used to evaluate pre- and postoperative whole-field microstrain distribution in the root dentin under physiologically relevant loads (10–50 N). Another 30 premolars, similarly treated as groups 1 and 2 or left untreated as controls (n = 10/group), were subjected to cyclic loads (1.2 million cycles, 45 N, 4 Hz) followed by uniaxial compressive load to failure. Mechanical data were analyzed with 1-way analysis of variance and the post hoc Tukey test at a 5% level of significance. Results: Microstrain distribution showed bending and compressive patterns at the coronal and apical root dentin, respectively. In the GP group, microstrain distribution was unaltered. In the bonded-PCD group, different microstrain distribution suggested stiffening at the PCD. The load at failure did not differ significantly for the GP, bonded PCD, and control groups (P > .05). Conclusions: CR bonding of PCD might impact the biomechanical responses in maxillary premolar roots at low-level continuous loads. The effect of this impact on root fracture loads when subjected to cyclic load warrants further investigation. (J Endod 2018;-:1–6)
Key Words Bonding, fracture resistance, pericervical dentin
W
ith approximately 15 Significance million endodontic Bonding pericervical dentin with composite resin treatments performed annureduced pericervical dentin bending and apical mially in the United States (1), crostrain distribution for continuous loads at physthe reported 5%–10% (2, iological ranges, without providing a significant 3) prevalence of vertical increase in the load to failure subsequent to cyclic root fracture (VRF) leading loading. The long-term advantage of bonding perito tooth loss posttreatment cervical dentin with composite resin was not estabrepresents a considerable lished. societal burden (2). Of the multifactorial causes of VRF (4), iatrogenic and noniatrogenic loss of dentin predisposes teeth to mechanical failure under functional stresses (4). Typically, when a root-filled tooth is exposed to chewing forces, under certain conditions, a cumulative process of crack initiation and propagation may occur with time, leading to fatigue failure (4). Although crack initiation and propagation induced by engine-driven canal instrumentation and root filling are currently the focus of research (5), the biomechanical impact of bonded restoration on instrumented root canals has not been thoroughly explored. Biomechanics is the study of structure and function of biological systems using the principles of engineering mechanics (6). The biomechanical response of bulk dentin tissue to functional forces determines its mechanical integrity (7). Microcrack events leading to catastrophic fracture are locally strain controlled (8); thus, assessing the mechanical stress/strain distribution in root dentin may explain some of the causes of VRF in root-filled teeth (9). Greater dentin loss generates higher stress concentrations, which significantly compromises the fracture strength of teeth (10). Different degrees of dentin removal may occur during canal instrumentation, which may alter the biomechanical response of the remaining root dentin and resistance to VRF (4). Of particular interest in this regard is pericervical dentin (PCD), extending 6 mm apical and 4 mm coronal to the crestal bone (11), which distributes occlusal stresses through the long axis of the root (11). Loss of PCD is implicated in the weakening of the root structure and decreased resistance to VRF (11). Digital moire interferometry (DMI) uses the principles of optical interferometry to determine the microstrain distribution in dento-osseous structures with high resolution (12), offering substantial advantages over conventional microstrain assessments (13). Contrary to conventional mechanical testing, DMI provides whole-field strain distribution patterns on specimens for low-level loads within physiological limits. Our group has recently used DMI to assess microstrain distribution in root dentin in response to root canal instrumentation (5). The aims of this study were to assess the impacts of restoring PCD with bonded composite resin in root-filled maxillary premolars on inplane microstrain distribution in root dentin using DMI and load at failure using cyclic
From the University of Toronto, Toronto, Canada. Address requests for reprints to Dr Anil Kishen, Faculty of Dentistry, University of Toronto, 124 Edward Street, Toronto, ON, Canada M5G1G6. E-mail address: anil. [email protected] 0099-2399/$ - see front matter Copyright ª 2018 American Association of Endodontists. https://doi.org/10.1016/j.joen.2018.01.002
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Basic Research—Technology and subsequent continuous compressive loading. It was hypothesized that bonding PCD with composite resin would have no effect on the strain distribution patterns and subsequent loads to failure in endodontically treated roots.
(Dentsply Tulsa Dental Specialties) to size F3. During instrumentation, canals were intermittently irrigated with 10 mL 2.5% sodium hypochlorite using a ProRinse side-vented 30-G needle (Dentsply Tulsa Dental Specialties).
Materials and Methods
Microstrain Distribution After canal enlargement, specimens were subjected to microstrain assessment with DMI. The experimental setup for the highresolution DMI and the process of grating replication were based on our previous experiments (12). Mesial and distal surfaces of 10 specimens were ground down equally on wet emery paper of grit sizes 800 and 1200 under constant running water to prepare 3-mm-thick, parallel-sided longitudinal sections. High-frequency cross grating (f = 1200 lines/mm, diffraction efficiency of 10% and intensity variation of <15%) was replicated on the longitudinal root surface with a thin layer of epoxy-based adhesive (PC-10; Measurements Group, Raleigh, NC) and was allowed to set for 5 hours in ambient conditions (22 C, 55% relative humidity) before being subjected to DMI. Specimens acted as their own control in this experiment. Mounted on a specially fabricated loading jig (5), the grating-replicated specimen was compressively loaded from 0 N to 50 N with 10-N increments. At each loading interval, whole-field digitized fringe patterns in root dentin were acquired by a high-resolution charge-coupled device camera and recorded as the control microstrain distribution (after canals were enlarged to F3). Subsequently, the grating material was gently removed with wet emery paper as described. Specimens were root filled with thermoplasticized gutta-percha (GP) and Pulp Canal Sealer (SybronEndo Endodontics, Orange, CA) by vertical compaction and stored in 100% humidity for 24 hours to allow the sealer to set. In the GP group (n = 5), canals were root filled to the CEJ level and the remaining coronal 2 mm restored with Cavit (3M Deutschland GmbH, Neuss, Germany). In the bonded PCD group (n = 5), canals were root filled to 6 mm below the CEJ level and the remaining coronal 8 mm restored with dentin-bonded composite resin as follows: a 20-second application and light cure of Clearfil DC Bond (Kuraray America, New York, NY) followed by Clearfil DC Core Plus Resin (Kuraray America). To minimize voids in the restoration, the dual cure resin was syringed in
Specimens The university ethics board approved the study protocol. Sample size calculation considered previous studies (14, 15) on load at failure under compressive loads in which differences in the range of 22% reached statistical significance with samples of 10 teeth per group. Accordingly, a sample size of 10 teeth per group was used in the present study to analyze data with 80% power and 5% significance. Ten human extracted noncarious maxillary premolars with single canals, closed apices, and no cracks/fractures were selected. Teeth were stored in a phosphate-buffered saline solution at 4 C before testing. Conventional radiographic images of the teeth were captured from 2 perpendicular exposures, and the teeth were characterized for length and overall dimensions. Three sets of teeth matched for length and canal dimensions were assembled to minimize variation among groups. Teeth were decoronated under a dental operating microscope (OPMI Pico; Zeiss, Oberkochen, Germany) at 10 magnification with a low-speed saw (Isomet; Buehler, Lake Bluff, IL) under water cooling. They were sectioned at 2 mm coronal to the average level of the buccal, lingual, mesial, and distal cementoenamel junction (CEJ) levels. In the absence of crestal bone, the level of CEJ, located approximately 2 mm supracrestal in normal periodontal architecture (16), was used as the reference for measuring PCD in this in vitro model (Fig. 1). The single root canal was negotiated with a size 10 K-type file (Dentsply Tulsa Dental Specialties, Tulsa, OK) to the major apical foramen as observed at 4 magnification, and the working length was established 1 mm short of this point. The glide path was established with 3 PathFile instruments (Dentsply Tulsa Dental Specialties). The canal was enlarged to the working length with ProTaper Universal instruments
GROUP GP :
GROUP Bonded-PCD:
CEJ Simulated crestal bone
2mm 2mm 6 mm
Cavit Gua-percha + Pulp Canal Sealer Bonded composite-resin – Clearfil DC Core Plus
Diagram not to scale
Figure 1. A schematic diagram showing specimens used in the GP and bonded PCD groups.
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Basic Research—Technology place with 2-mm increments and cured. The root-filled specimens were again grating replicated and subjected to microstrain distribution as described earlier, which was recorded for the GP and bonded PCD groups.
Mechanical Testing Thirty premolars were collected, prepared, and assigned to the following groups: control, GP, and bonded PCD (n = 10/group). In the untreated controls (size F3, without root filling), the pulp chambers were restored with Cavit. All 30 specimens were mounted in custom devices and imaged with a micro–computed tomographic (micro-CT) scanner (SkyScan 1172; Bruker MicroCT, Toronto, Ontario, Canada) at an 8-mm voxel size (pretreatment scan) to standardize root canal geometry and volume and to rule out any dentinal defects. After instrumentation, specimens were imaged again (posttreatment scan) to capture the enlarged canal geometry and posttreatment defects. Amira 3D software (FEI, Hillsboro, OR) was used for micro-CT image analysis including comparison of pre- and posttreatment canal volumes (15, 17). Specimens were mounted on brass rings with the roots embedded in self-curing resin (Justis Quick Resin; Ivoclar Vivadent, Schaan, Lichtenstein) up to a level 2 mm apical to the CEJ. A 0.5-mmthick silicone rubber barrier (Aquasil LV; Dentsply DeTrey GmbH, Konstanz, Germany) was applied to the root surfaces to simulate the periodontal ligament. The embedded specimens were stored in deionized water before mechanical testing. Specimens were then mounted in the Instron Universal Testing Machine (Instron, Canton, MA). In a custom-made water bath, a force of 45 N was cyclically applied at a frequency of 4 Hz, with a 5-mm spherical crosshead at the center of the occlusal access cavity aligned with the longitudinal axis of the tooth (18). If the sample survived 1.2 million loading cycles without fracture, simulating approximately 5 years of function (19), they were subsequently loaded with the same spherical crosshead applying a continuous compressive force at 1 mm/min until fracture occurred (25% drop in applied force). The load at failure (N) was recorded as a measure of fracture strength. Upon completion, radiographic images of all specimens were again captured from 2 perpendicular exposures and characterized for location (cervical, middle, or apical) and pattern (comminuted, buccopalatal, or mesiodistal) of fracture. Data Analysis The acquired whole-field moire fringe patterns were used to evaluate qualitatively the microstrain distribution pattern at different regions of interest at the cervical and the apical third of the root dentin (Fig. 2A). The in-plane microstrain values in regions of interest were calculated using the following relationship. Strain (εxx) in the long axis of the tooth is given as DU/Dx = P/Dx, where DU is the relative displacement in the x direction between 2 points, P is the pitch of the reference grating, and Dx is the fringe spacing in the x direction. The microstrain values were used to examine the nature of microstrain distribution at regions of interest in the cervical and apical regions of the root for the unrestored and bonded PCD specimens (12). For the mechanical testing under cyclic and continuous loading, the mean load at failure values of the 3 groups were analyzed with 1-way analysis of variance and the post hoc Tukey test at a 5% level of significance.
Results Microstrain Distribution Moire fringes are formed from the constructive and destructive interference between the virtual and specimen gratings. They provide JOE — Volume -, Number -, - 2018
Figure 2. (A) Typical moire fringe patterns obtained from the representative, unfilled specimen in the control group showing microstrain distribution patterns in the cervical and apical regions of interest (denoted by the red boxes) where microstrain values were measured. The microstrain distribution pattern at the coronal regions was characteristic of bending. Compressive strain characteristics were observed at the apical region. The red stars indicate the dentin at the center of the root section corresponding to the root canal dentin.(B) Typical moire fringe patterns obtained from GP group specimens (unrestored PCD). The microstrain distribution characteristic of bending at the coronal region of the control specimens was reduced in this group. (C) Typical moire fringe patterns obtained from the bonded PCD group. Distinct microstrain distribution at the coronal region, which decreased toward the apical region, was observed.
information on the nature of deformation on the test specimen. In the control specimens, the microstrain distribution exhibited patterns consistent with bending at the cervical third of the root and patterns consistent with compression at the apical third (Fig. 2A). Both coronal and apical microstrain distribution patterns increased with higher applied loads (Fig. 3). In the GP group (Fig. 2B), bending microstrain distribution patterns in the cervical dentin and microstrain distribution patterns at the apical regions were lower compared with controls (Fig. 2A). In the bonded-PCD group (Fig. 2C), the degree of bending
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Figure 3. A graphic representation of the (A) cervical and (B) apical microstrain response to increasing applied loads. In the bonded PCD group, an increased shift of microstrain away from the apical region toward the cervical region was evident. In contrast, the GP group and the unfilled controls exhibited increasing microstrains with an increase in the applied loads at both the cervical and apical regions.
microstrain distribution at the cervical region was obviously lower compared with controls (Fig. 2A); the microstrain distribution at the apical third was also much lower. All groups showed an increase in microstrain with applied loads at the cervical region (Fig. 3A). The control and GP groups showed an increase in microstrain with applied loads at the apical region, whereas the bonded PCD group samples had very little change in apical microstrain (Fig. 3B).
Load at Failure and Fracture Patterns The mean load at failure values were highest in the bonded PCD group (1703.74 N) and lowest in the GP group (1467.65 N) (Fig. 4A). Differences among the groups were not statistically significant (P = .46 > .05). When comparing the fracture patterns, all specimens exhibited fractures contained in the cervical region (Fig. 4B). In the GP and control groups, the pattern of fracture was consistent with comminuted fractures (Fig. 4B). In the bonded PCD group, fractures in the buccopalatal direction and interfacial failures between the composite resin and the dentin surface were noted (Fig. 4B).
Discussion Cumulative loss of tooth structure because of caries, trauma, and treatment procedures increases the risk of tooth fracture (4, 20). To restore fracture resistance in teeth, modern dentistry has shifted toward bonded restorative procedures (21). Bonded restorations have been suggested to improve the long-term survivability of root-filled teeth 4
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(10). Because the modulus of elasticity of composite resin is close to that of dentin (10), restoring endodontic cavities with composite resin showed a minimal stress jump at the resin-dentin interface, reduced cuspal flexure, and lowered probability of coronal fractures (10, 22, 23). Although previous studies have examined the effects of bonded restorations at the coronal tooth level (23, 24), the feasibility of using bonded composite resin to restore the biomechanical integrity in instrumented radicular dentin was not well studied. This study focused on the restoration of PCD with dentin-bonded composite resin and its impacts on biomechanical responses of root dentin, including microstrain distribution and load to failure. Tooth selection was based on the reported higher incidence of root fractures in endodontically treated maxillary second premolars (3). The application of pre– and post–micro-CT scans allowed a nondestructive means to evaluate specimens for intraradicular cracks as well as to match root canal volume and canal geometry for specimens used in this study (25, 26). DMI has been previously applied in dental biomechanics (13). It enables examination of dental hard tissues under physiologically relevant loads to evaluate high-resolution microstrain distributions over the whole sample in real time (13). One of the inherent challenges in the application of DMI to biological structures is the difficulty of comparing high-resolution microstrain quantitatively between specimens (6, 17). Nevertheless, the whole-field microstrain distribution patterns generated in real time allow for qualitative comparison within samples subjected to physiological range of forces, which cannot be achieved with conventional strain gauge measurements. The Instron Universal Testing machine is commonly used to assess resistance to fracture of root-filled teeth (27), with cyclic loading considered to represent clinical function (19). In this study, root specimens were subjected to a hybrid analysis comprised of cyclic loading to simulate physiological aging followed by compressive loading to failure (28). Cyclic fatigue is imperative when assessing bonded composite resin restorations because it may undermine the bond strength because of increased flexure of the supporting tooth structure (29), leading to bond failures (30). Because incomplete bonding, interfacial voids, and polymerization shrinkage may all undermine bond strength (31), we used a self-polymerizing dual-cured resin, which was applied incrementally within the canal space. In vivo and in vitro stress/strain analyses of teeth have shown that stress was concentrated toward the cervical region when restored access cavities were subjected to a compressive load (10). Our results indicated that both the untreated control GP group experienced increased cervical and apical microstrain with increasing applied loads. Restoring PCD with bonded composite resin (bonded PCD group) contributed to a shift in microstrain distribution away from the apical root dentin toward the cervical dentin, resulting in the greatest increase in cervical microstrain and the least apical microstrain increase. This finding suggested that bonding of PCD reduced flexing or caused stiffening of cervical root dentin, resulting in redistribution of functional loads away from the apical region. The definition of a true VRF according to the American Association of Endodontists is a complete or incomplete fracture initiated from the root at any level, usually directed buccolingually (32). The reduction in apical microstrain achieved by bonding of PCD might be expected to reduce the propensity to VRF. Although bonded PCD withstood physiological level loads and distributed radicular microstrain away from the apical region, under cyclic fatigue conditions the value of bonding PCD appeared to be limited. It did not show a significant improvement in fracture strength but possibly a modified fracture pattern characteristic of interfacial failure. These observations suggested that composite resin bonding to PCD has its limitations when cyclic mechanical loads are JOE — Volume -, Number -, - 2018
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Figure 4. (A) The average load at failure for specimens in the tested groups. The load at failure was as follows: the bonded PCD group > the control group > the GP group (P > .05). (B) Typical examples of fracture patterns observed in the 3 groups tested in the study (blue arrows). All fracture planes were contained within the cervical region.
applied. Earlier studies have shown that the initial bond strength is greatest at the time of bonding, but the durability of the bond is reduced with repeated compressive loads (33, 34). The findings also corroborated the challenge of strengthening root dentin postinstrumentation (35). It is important to note that although in this study comparable models were used in both the experiments, altering the load regimen in fatigue testing, simulating a greater degree of root canal dentin removal, and altering the nature and angulation of loads may have generated different findings. In conclusion, this study suggested that in maxillary premolars with a single canal, restoration of PCD with dentin-bonded composite resin impacted a shift in microstrain distribution away from the apical region toward the pericervical region when subjected to loads in physiologic ranges. However, this effect did not appear to impact the load at failure when subjected to fatigue cycling followed by compressive
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loading. The long-term clinical effect of composite resin bonding of PCD in root-filled teeth requires further investigation.
Acknowledgments Supported in part by a research grant from the American Association of Endodontists Foundation (494223), the University of Toronto (928133) (AK), and the Canadian Academy of Endodontics Endowment Fund. The authors deny any conflicts of interest related to this study.
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