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Impact of Access Cavity Design and Root Canal Taper on Fracture Resistance of Endodontically Treated Teeth: An Ex Vivo Investigation
Basic Research—Technology
Impact of Access Cavity Design and Root Canal Taper on Fracture Resistance of Endodontically Treated Teeth: An Ex Vivo Investigation Mohammad Sabeti, DDS, MS,* Majid Kazem, DDS, MS,†‡ Omid Dianat, DDS, MS,‡§ Nazanin Bahrololumi, DDS,k Amirreza Beglou, DDS,k Kasra Rahimipour, DDS,k and Farshad Dehnavi, DDSk Abstract Introduction: The susceptibility of endodontically treated teeth (ETT) to fracture is mainly associated with the loss of tooth structure. This study evaluated the effect of the access cavity design and taper preparation of root canals on ETT fracture resistance of maxillary molars. Methods: For tapering assessment, 30 sound distobuccal roots of maxillary molars were randomly assigned to 1 of 3 groups (n = 10): a .04 taper, a .06 taper, or a .08 taper. Endodontic canal preparations were performed using the Twisted Files rotary system (Kerr Co, Glendora, CA). In addition, 48 intact maxillary first and second molars were randomly assigned to 1 of 3 groups (n = 16) for cavity preparation approaches: intact teeth, traditional access cavity (TAC), or conservative access cavity (CAC). Fracture resistance was tested using a universal testing machine. For statistical analysis, the level of significance was P # .05. Results: The .04 taper instrumentation had the highest fracture resistance (259.61 52.06), and the .08 taper had the lowest (168.43 59.63). The .04 and .06 groups did not differ significantly (P > .05); however, these groups differed significantly from the .08 group (P # .05). Regarding the cavity preparation approaches, the 3 groups of intact teeth, CAC, and TAC showed fracture resistance mean values of 2118.85 336.97, 1705.69 591.51, and 1471.11 435.34, respectively, with no significant difference between the CAC and TAC groups (P > .05). Conclusions: Increasing the taper of the root canal preparation can reduce fracture resistance. Moreover, access cavity preparation can reduce resistance; however, CAC in comparison with TAC had no significant impact. (J Endod 2018;:1–5)
Key Words Access cavity, endodontically treated teeth, fracture resistance, maxillary molars, minimally invasive, root taper
T
ooth fracture is 1 of the Significance most undesirable pheThe effect of the root canal taper and the influence nomena in endodontically of the access cavity design on tooth fracture treated teeth (ETT) and resistance remains limited and controversial. We usually leads to tooth provide information regarding the effect of the extraction (1). Research root canal taper and access cavity design on has reported that the fracture resistance of root canal–treated maxillary susceptibility of ETT to molars. Our results showed that increasing the fracture is mainly associroot canal taper can predispose them to fracture, ated with the loss of but CAC designs did not show benefits compared tooth structure because with TACs. of dental carries or therapeutic endodontic procedures such as access cavity and root canal preparation (2, 3). Hence, the amount of remaining structure appears to be a major factor determining the prognosis of ETT. The endodontic access cavity is considered an important step in endodontic treatments (4). Recently, a new concept of a conservative access cavity (CAC), inspired by concepts of minimally invasive dentistry, has been designed and developed in order to minimize the removal of the chamber roof and pericervical dentin (5). The rationale of this approach is to avoid excessive dentin removal from tooth structures (6, 7). With advances in the field of imaging, endodontic instruments, visual enhancers, and clinical microscopes, the traditional requirements of the endodontic access cavity start to diminish. For instance, newly developed ultraflexible canal preparation instruments make straight-line access to the canals less important; also, the progress in visual magnification makes it easier to find canal orifices without the need for excessive expansion of access cavity walls (4, 8). However, this relatively new cavity design may confine cleaning, shaping, and obturation of root canals (9). An inadequate access cavity also increases the prevalence of iatrogenic complications during endodontic procedures (4).
From the *Endodontic Department, School of Dentistry, University of California, San Francisco, California; †Endodontic Department, School of Dentistry, Shahid Beheshti University of Medical Sciences, Tehran, Iran; ‡Iranian Center For Endodontic Research, Research Institute of Dental Sciences, Dental School, Shahid Beheshti University of Medical Sciences, Tehran, Iran; §Endodontic Division, School of Dentistry, University of Maryland, Baltimore, Maryland; and kResearch Institute of Dental Sciences, Dental School, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Address requests for reprints to Dr Omid Dianat, Endodontic Division, School of Dentistry, 650 West Baltimore Street, Baltimore, MD 21201. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2018 American Association of Endodontists. https://doi.org/10.1016/j.joen.2018.05.006
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Impact of Access Cavity Design and Root Canal Taper
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Basic Research—Technology Rotary systems facilitate debridement of canals, and the higher instrument tapers lead to superior canal and canal wall cleanliness and decrease the concerns regarding microbial removal of canal walls (1). However, there are some concerns regarding the excessive removal of radicular dentin because of increased instrumentation taper (10). Fundamentally, any removal of hard tissue from the canal walls increases the chance of root fracture (3). On the contrary, some authors claim that increased canal preparation taper allows forces to be better distributed in the apical third of the canal; this better distribution increases the tooth’s fracture resistance (11). Accordingly, the aim of the present study was to evaluate and compare the effect of access cavity preparation and 3 different root taper preparations on ETT fracture resistance of maxillary molars. The 2 null hypotheses tested were there would be no difference in the fracture resistance of teeth with different access cavity designs, and there would be no difference in the fracture resistance of roots with different root canal tapers.
Materials and Methods Sample Preparation This study used a total of 78 sound maxillary first and second molars. All of these teeth were extracted for periodontal reasons after written informed consent was obtained. The inclusion criteria were noncarious teeth with mature apices, absence of cracks, and free of any defects. The study was approved by the Ethics Committee of Shahid Beheshti University of Medical Sciences, Tehran, Iran. For infection control, the teeth were stored in 0.5% chloramine-T trihydrate for 2 weeks before the experiment. At no stage in the procedure were the teeth allowed to dehydrate. After comprehensive visual and stereomicroscopic assessment of all intact teeth, 30 maxillary first molars were similarly decoronized by a calibrated endodontist. Three roots of each tooth were divided by burs (ISO no. 806 104 199 544 016; NTI, Kahla, Germany) under sufficient water cooling. The distobuccal roots were randomly assigned to 1 of 3 groups (n = 10) for tapering evaluation (ie, a .04 taper, a .06 taper, or a .08 taper). The buccolingual and mesiodistal dimensions of the roots were measured using a digital caliper, and roots with more than 20% deviation were replaced. All specimens of the tapering groups were prepared for endodontic treatment. Canals were negotiated with size 10 K-type files (Flexofile; Dentsply Maillefer, Ballaigues, Switzerland) to the apical foramen, and the working length was established 0.5 mm shorter. After the initial preparation, canals were instrumented up to an apical size of 25; instrumentation was performed with the Twisted Files rotary system (taper .04, .06, and .08; Kerr Co, Glendora, CA) according to the manufacturer’s instructions. Twisted Files are a progressive tapered instrument used for shaping and finishing root canals. In our study, a new set of instruments was used for each tooth. Intermittent irrigation with 5 mL 2.5% sodium hypochlorite was applied with 30-G needles. In order to omit other covariates, the specimens were not obturated. Another 48 teeth that were free of carious lesions, previous restorations, and any enamel or dentin defects and had a similar crown anatomy and size with 3 separate mature roots were selected. The teeth, which were similar in buccolingual and mesiodistal size, were randomly assigned to 1 of 3 groups (n = 16): intact teeth, no treatment (negative control); traditional access cavity (TAC, positive control); and conservative access cavity (CAC, experimental). Access cavities were prepared using coarse, flat-end diamond burs (ISO no. 806 104 199 544 016, NTI) in a high-speed handpiece with sufficient water cooling. For each 10-access cavity preparation, a separate diamond bur was used. 2
Sabeti et al.
In the intact group, no treatment was performed on teeth, and they remained intact until the fracture resistance test. In the TAC group, traditional endodontic access cavities were prepared following conventional guidelines (12). In order to obtain straight-line access to all canal orifices, outlines of the cavity and pericervical dentin were removed or modified where necessary (Fig. 1A). In the CAC group, in order to determine the outlines of the access cavity and locate the pulp chamber and canals, we used 2 periapical radiographs made from buccal and mesial aspects as a guide. Then, starting from the central fossa, cavities were extended only as necessary to visualize and locate canal orifices while taking care to preserve pericervical dentin and the root chamber vault where possible as described previously (Fig. 1B) (2, 13). Using the analyzing rod of the dental surveyor, each tooth or root was positioned vertically in a brass ring of self-cured acrylic resin, and teeth were embedded up to 2 mm below the cementoenamel junction. To simulate the periodontal ligament space, roots were covered before acrylic embedment with a uniformly thin 0.2-mm layer of light body silicone impression material.
Fracture Testing For both experimental groups (ie, the tapering and cavity preparation groups), a fracture resistance test was conducted at the laboratory of the Dental Research Center at Shahid Beheshti University of Medical Sciences. The testing device used was the Universal Testing Machine (Model 55144; Zwick/Roell, Ulm, Germany). The testing machine allowed an error of 0.04% for a maximal load of 10,000 kg, 0.01% for a repetitive maximal load of 10,000 kg, resolution of displacement of 0.01 mm (10 mm), and an accurate speed of 0.01% of full scale. Statistical Analysis To analyze the results, statistical software (SPSS 17; SPSS Inc, Chicago, IL) was used. Averages and standard deviations were established. Statistical tests of normality (the Shapiro-Wilk test) were used to check whether data matched a particular distribution such as the normal distribution or the exponential distribution. For all comparisons, the level of importance was P # .05, and the Tukey test was used as the post hoc test.
Results Root Canal Tapers The normality of the obtained data was confirmed using the Shapiro-Wilk test (P > .05). The .04 taper showed the highest (259.61 52.06) and the .08 taper showed the lowest (168.43 59.63) fracture resistance values (Fig. 1). Comparison of data using the 1-way analysis of variance test revealed a significant difference between groups (Fig. 2). Pair-wise comparison of the groups using the Tukey test showed the .04 taper and .06 taper groups did not significantly differ (P > .05), but both groups differed significantly from the .08 taper group (P # .05). Effect of Access Cavity Design Three groups of intact teeth, CAC, and TAC showed fracture resistance mean values of 2118.85 336.97, 1705.69 591.51, and 1471.11 435.34 N, respectively (Table 1). The 1-way analysis of variance test showed no significant difference between the groups (P > .05). Pair-wise comparison of the groups using the Tukey test revealed no significant difference between CAC and TAC (P > .05), but they both showed a significant difference with intact teeth (P # .05). JOE — Volume -, Number -, - 2018
Basic Research—Technology
Figure 1. (A) Conventional access cavity design. (B) CAC design.
Discussion ETT are prone to root fracture, which may lead to extraction and undermine the long-term benefits of endodontic treatment (7,14–16). A major factor endangering the survival of root-filled teeth is the loss of dentin (17, 18). Endodontic procedures encompass various steps such as access cavity preparation and root canal preparation, which may result in the loss of excessive tooth structure, weakening of the tooth, and a subsequent reduction in the tooth’s capability to resist forces (3, 17, 19). Therefore, recently, the conventional methods in the aforesaid procedures have given rise to some criticism because of the possibility of excessive tooth removal, and conservative approaches
have gained attention for the restoration of tooth stability (3, 20). However, this relatively new concept has been slow to impact the endodontists’ mainstream practice and had not been well supported by research data. In this regard, this study sought to evaluate and compare the fracture resistance of ETT in both conventional and conservative approaches of access cavity preparation and canal shaping. Because the available data on the fracture resistance of maxillary molars are lacking (unlike respective data on mandibular molars) and it was shown in a previous study that the impact of CAC varied in different unrestored tooth types (5), this study assessed the influence of different endodontic procedures on fracture resistance in maxillary molars.
Figure 2. Fracture resistance values of roots after different root canal tapers.
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Basic Research—Technology TABLE 1. Fracture Resistance Values of Teeth with Different Access Cavity Designs Access cavity type
Break force (N)
Conservative access cavity Traditional access cavity Intact teeth
1705.691250 (591.51)a 1471.113125 (435.34)a 2118.851250 (336.92)b
Values are expressed in newtons (N). Values in the parenthesis represent standard deviation. Mean values with the same superscript letters are not significantly different (P > .05).
In this study, periapical radiographs of teeth from 2 different planes were used for planning conservative access outlines (13). In addition, the method of root embedment may affect the fracture resistance significantly. Thus, in the present study, simulation of the periodontal ligament around the tooth was done with polyvinyl siloxane, an elastomeric material that is able to prevent concentration of stresses in the cervical region of the tooth because of imitating the accommodation of the tooth in the alveolus (21). Also, an Instron (Norwood, MA) Universal Testing machine was used to measure tooth fracture resistance because the use of this machine is the simplest and most frequently used method to evaluate tooth strength (17, 19). However, this in vitro test provides a static load until failure occurs, whereas in the oral cavity loads are dynamic and, thus, it may not simulate in vivo conditions (22). Furthermore, to avoid confounding by covariates, we assessed the net impacts of access cavity and root tapering preparations by not restoring access cavities and obturating root canals. In the conventional approach, access cavity designs are determined according to some main principles. According to straight-line access, convenience form, and extension for prevention, the outline of the access cavity must be extended beyond gaining access to the canal orifices in order to enable thorough debridement of the canals and prevent procedural complications such as instrument fracture (9, 13). As shown in a study by Krishan et al (13), minimal access cavities were associated with compromised canal instrumentation in the distal canals of mandibular molars. Moreover, the lack of straight-line access because of inadequate tooth tissue removal may compromise the delivery of irrigants to the apical portion of the roots because it avoids the needle reaching further into the canal (23). However, some researchers believe that the superfluous sound tooth structure is sacrificed in this way, which is unnecessary and can decrease resistance to tooth fracture (6, 7, 14). According to Clark and Khademi (6), in TAC preparation, the dentist’s needs for facile access to the canal systems are placed above the tooth needs, whereas in CAC preparation the emphasis is on banking of pericervical dentin by maintaining soffit dentin, skewing the access toward fillings/carries, and altering the traditional reference points. Taken together, it seems that the benefits of thorough canal instrumentation and diminished complications must be weighed against the risk of the tooth’s resistance to fracture being reduced. The results of the present study showed no significant difference between conservative and conventional endodontic cavity preparations, whereas both showed significant differences compared with intact teeth. In agreement with our results, Moore et al (5) reported that the fracture strength in maxillary molars with CACs and TACs was remarkably lower than that of intact molars. Also, although the CAC’s fracture strength was 23% greater than that of TAC, this difference was not statistically significant; this finding suggests that, in comparison with TAC preparation, CAC preparation provides insignificant advantages with regard to fracture strength. In our study, the removal of the pulp chamber roof as the reinforcing structure (24) and the disruption of tooth integrity (25) may have been responsible for the substantial 4
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reduction in fracture resistance of the ETT of both groups in comparison with the intact teeth, which is supported by the findings of many other studies (3, 26–28). In agreement with our study, Rover et al (29) showed no significant difference in the means of the fracture resistance test between conservative and traditional endodontic cavities in maxillary first molars. Their study suggested a reduced possibility of canal orifice detection in the CAC group in comparison with the TAC group. They also observed an increased possibility of canal transportation associated with CACs. Considering the drawbacks of CACs, their study did not support the notion of CAC preparation in maxillary first molars. On the contrary, Plotino et al (30) observed a significant difference in the fracture resistance amounts of CACs and TACs in maxillary and mandibular premolars and molars, whereas CACs did not differ significantly from intact teeth. Furthermore, Cobankara et al (19) showed that none of the tested restoration techniques in their study was able to utterly restore the fracture resistance lost from access cavity preparation. In agreement with our study, it was shown in another study that if at least 1 marginal ridge could be preserved, with any thickness more than 0.5 mm, its thickness did not affect the fracture resistance of the teeth (31). In contrast, mesial, occlusal, and distal cavity preparations weakened the teeth severely (14, 24). Thus, the null hypothesis regarding access cavity design has been approved. The findings of this study do not support the use of minimally invasive access cavity preparation because this approach still has drawbacks (6, 7), and, regarding fracture resistance, the CAC approach was not more advantageous than the TAC approach. Considering that maxillary molars have challenging root canal systems in mesiobuccal roots, where secondary canals are difficult to locate and instrument, the application of CACs in these teeth necessitates careful consideration. The primary aim of endodontic treatment is to eliminate microorganisms (32). Research has established that bacteria can penetrate into and colonize almost half the length of dentinal tubules (23, 33). Accordingly, inadequate removal of infected dentin within the canals can decrease the prognosis and lead to posttreatment failures (23). On the other hand, the thickness of the dentin has a stabilizing influence on the root. Consequently, any dentin removal in the canal can decrease the stability of the root (3, 34). For root canal shaping, the current study showed no significant reduction of the fracture resistance of the root by .06 taper instrumentation compared with .04, whereas preparation with the .08 taper showed a significant decrease in fracture resistance. In agreement with our results, a previous study has shown that during instrumentation, maintaining the natural geometry of the root canals is a paramount stabilizing factor for the tooth, and, therefore, if the root canal outline is not substantially altered, tooth fracture resistance is relatively unaffected (3). Conceivably, in our study, the decrease in fracture resistance that followed .08 taper instrumentation might have been the result of geometric alterations of the root canals because .08 taper files are more rigid and less adaptable. Another factor could be the typically small diameter of distobuccal roots because root preparation with a thicker file leads to a further weakening of it (35). Root fracture occurs as a result of propagation of microcracks created in the root canal shaping process with occlusal forces (36). Thus, we suggest that the increased risk of fracture with the .08 taper in this study might be associated with the greater number of craze lines and the greater degree of imposed stress in root dentin. Moreover, our findings corroborated the results of a previous study that reported that preparation with larger taper instruments significantly weakened the roots. Also, Zandbiglari et al. (35) suggested that this result was probably caused by the greater amount of dentin removed with larger tapering instruments compared with common taper hand files. JOE — Volume -, Number -, - 2018
Basic Research—Technology Therefore, the null hypothesis regarding root taper has been partially rejected because the increase in tapering from .06 to .08 influenced tooth resistance significantly. Given our finding that in molar teeth the .08 taper adversely affected tooth resistance, our results did not support the use of the .08 taper in these teeth. Notably, 1 limitation of the present study is that the variability of the tested teeth may have confounded our results. In addition, preparing the conventional and conservative access cavities with the same degree of accuracy in all teeth is difficult. Given these limitations, we suggest that in order to achieve experimental conditions that are more similar to clinical conditions, subsequent studies should use finite element analysis. Moreover, it should be mentioned that until now no standard method for preparing conservative access cavities has been established. Therefore, conservative access cavities are difficult to standardize, and, thus, this type of research is hard to reproduce. The data achieved in this ex vivo study provide insight into the fracture resistance of different access designs and root canal tapers in the clinical setting. However, for more certain results, clinical trials with long-term follow-ups are required. In conclusion, within the limitations of this study, for endodontic treatment of teeth, the following can be concluded: 1. Increasing the tapering of root canal preparation can detrimentally affect tooth fracture resistance 2. Conservative endodontic access in maxillary molars was not shown to significantly increase fracture resistance over traditional access
Acknowledgments The authors deny any conflicts of interest related to this study.
References 1. Lam PP, Palamara JEA, Messer HH. Fracture strength of tooth roots following canal preparation by hand and rotary instrumentation. J Endod 2005;31:529–32. 2. Al Amri MD, Al-Johany S, Sherfudhin H, et al. Fracture resistance of endodontically treated mandibular first molars with conservative access cavity and different restorative techniques: an in vitro study. Aust Endod J 2016;42:124–31. 3. Lang H, Korkmaz Y, Schneider K, Raab W-M. Impact of endodontic treatments on the rigidity of the root. J Dent Res 2006;85:364–8. 4. Boveda C, Kishen A. Contracted endodontic cavities: the foundation for less invasive alternatives in the management of apical periodontitis. Endod Topics 2015;33:169–86. 5. Moore B, Verdelis K, Kishen A, et al. Impacts of contracted endodontic cavities on instrumentation efficacy and biomechanical responses in maxillary molars. J Endod 2016;42:1779–83. 6. Clark D, Khademi JA. Case studies in modern molar endodontic access and directed dentin conservation. Dent Clin North Am 2010;54:275–89. 7. Clark D, Khademi J. Modern molar endodontic access and directed dentin conservation. Dent Clin North Am 2010;54:249–73. 8. Gluskin AH, Peters CI, Peters OA. Minimally invasive endodontics: challenging prevailing paradigms. Br Dent J 2014;216:347–53. 9. Patel S, Rhodes J. A practical guide to endodontic access cavity preparation in molar teeth. Br Dent J 2007;203:133–40. 10. Capar ID, Altunsoy M, Arslan H, et al. Fracture strength of roots instrumented with self-adjusting file and the ProTaper rotary systems. J Endod 2014;40:551–4. 11. Harvey TE, White JT, Leeb IJ. Lateral condensation stress in root canals. J Endod 1981;7:151–5.
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12. Schroeder KP, Walton RE, Rivera EM. Straight line access and coronal flaring: effect on canal length. J Endod 2002;28:474–6. 13. 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. 14. Shahrbaf S, Mirzakouchaki B, Oskoui SS, Kahnamoui MA. The effect of marginal ridge thickness on the fracture resistance of endodontically-treated, composite restored maxillary premolars. Oper Dent 2007;32:285–90. 15. Tzimpoulas NE, Alisafis MG, Tzanetakis GN, Kontakiotis EG. A prospective study of the extraction and retention incidence of endodontically treated teeth with uncertain prognosis after endodontic referral. J Endod 2012;38:1326–9. 16. Toure B, Faye B, Kane AW, et al. Analysis of reasons for extraction of endodontically treated teeth: a prospective study. J Endod 2011;37:1512–5. 17. Assif D, Nissan J, Gafni Y, Gordon M. Assessment of the resistance to fracture of endodontically treated molars restored with amalgam. J Prosthet Dent 2003;89: 462–5. 18. Linn J, Messer HH. Effect of restorative procedures on the strength of endodontically treated molars. J Endod 1994;20:479–85. 19. Cobankara FK, Unlu N, Cetin AR, Ozkan HB. The effect of different restoration techniques on the fracture resistance of endodontically-treated molars. Oper Dent 2008;33:526–33. 20. Bremer BD, Geurtsen W. Molar fracture resistance after adhesive restoration with ceramic inlays or resin-based composites. Am J Dent 2001;14:216–20. 21. Soares CJ, Pizi ECG, Fonseca RB, Martins LRM. Influence of root embedment material and periodontal ligament simulation on fracture resistance tests. Braz Oral Res 2005;19:11–6. 22. Molinaro JD, Diefenderfer KE, Strother JM. The influence of a packable resin composite, conventional resin composite and amalgam on molar cuspal stiffness. Oper Dent 2002;27:516–24. 23. Mannan G, Smallwood ER, Gulabivala K. Effect of access cavity location and design on degree and distribution of instrumented root canal surface in maxillary anterior teeth. Int Endod J 2001;34:176–83. 24. Belli S, Erdemir A, Ozcopur M, Eskitascioglu G. The effect of fibre insertion on fracture resistance of root filled molar teeth with MOD preparations restored with composite. Int Endod J 2005;38:73–80. 25. Reeh ES, Messer HH, Douglas WH. Reduction in tooth stiffness as a result of endodontic and restorative procedures. J Endod 1989;15:512–6. 26. Ree M, Schwartz RS. The endo-restorative interface: current concepts. Dent Clin North Am 2010;54:345–74. 27. Kishen A. Mechanisms and risk factors for fracture predilection in endodontically treated teeth. Endod Topics 2006;13:57–83. 28. Tang W, Wu Y, Smales RJ. Identifying and reducing risks for potential fractures in endodontically treated teeth. J Endod 2010;36:609–17. 29. Rover G, Belladonna FG, Bortoluzzi EA, et al. Influence of access cavity design on root canal detection, instrumentation efficacy, and fracture resistance assessed in maxillary molars. J Endod 2017;43:1657–62. 30. Plotino G, Grande NM, Isufi A, et al. Fracture strength of endodontically treated teeth with different access cavity designs. J Endod 2017;43:995–1000. 31. Kalburge V, Yakub SS, Kalburge J, et al. A comparative evaluation of fracture resistance of endodontically treated teeth, with variable marginal ridge thicknesses, restored with composite resin and composite resin reinforced with Ribbond: an in vitro study. Indian J Dent Res 2013;24:193–8. 32. Zehnder MS, Connert T, Weiger R, et al. Guided endodontics: accuracy of a novel method for guided access cavity preparation and root canal location. Int Endod J 2016;49:966–72. 33. Shovelton DS. The presence and distribution of microorganisms within non-vital teeth. Br Dent J 1964;117:101–7. 34. 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. 35. Zandbiglari T, Davids H, Sch€afer E. Influence of instrument taper on the resistance to fracture of endodontically treated roots. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2006;101:126–31. 36. Adorno CG, Yoshioka T, Jindan P, et al. The effect of endodontic procedures on apical crack initiation and propagation ex vivo. Int Endod J 2013;46:763–8.
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Impact of Access Cavity Design and Root Canal Taper on Fracture Resistance of Endodontically Treated Teeth: An Ex Vivo Investigation Mohammad Sabeti, DDS, MS,* Majid Kazem, DDS, MS,†‡ Omid Dianat, DDS, MS,‡§ Nazanin Bahrololumi, DDS,k Amirreza Beglou, DDS,k Kasra Rahimipour, DDS,k and Farshad Dehnavi, DDSk Abstract Introduction: The susceptibility of endodontically treated teeth (ETT) to fracture is mainly associated with the loss of tooth structure. This study evaluated the effect of the access cavity design and taper preparation of root canals on ETT fracture resistance of maxillary molars. Methods: For tapering assessment, 30 sound distobuccal roots of maxillary molars were randomly assigned to 1 of 3 groups (n = 10): a .04 taper, a .06 taper, or a .08 taper. Endodontic canal preparations were performed using the Twisted Files rotary system (Kerr Co, Glendora, CA). In addition, 48 intact maxillary first and second molars were randomly assigned to 1 of 3 groups (n = 16) for cavity preparation approaches: intact teeth, traditional access cavity (TAC), or conservative access cavity (CAC). Fracture resistance was tested using a universal testing machine. For statistical analysis, the level of significance was P # .05. Results: The .04 taper instrumentation had the highest fracture resistance (259.61 52.06), and the .08 taper had the lowest (168.43 59.63). The .04 and .06 groups did not differ significantly (P > .05); however, these groups differed significantly from the .08 group (P # .05). Regarding the cavity preparation approaches, the 3 groups of intact teeth, CAC, and TAC showed fracture resistance mean values of 2118.85 336.97, 1705.69 591.51, and 1471.11 435.34, respectively, with no significant difference between the CAC and TAC groups (P > .05). Conclusions: Increasing the taper of the root canal preparation can reduce fracture resistance. Moreover, access cavity preparation can reduce resistance; however, CAC in comparison with TAC had no significant impact. (J Endod 2018;:1–5)
Key Words Access cavity, endodontically treated teeth, fracture resistance, maxillary molars, minimally invasive, root taper
T
ooth fracture is 1 of the Significance most undesirable pheThe effect of the root canal taper and the influence nomena in endodontically of the access cavity design on tooth fracture treated teeth (ETT) and resistance remains limited and controversial. We usually leads to tooth provide information regarding the effect of the extraction (1). Research root canal taper and access cavity design on has reported that the fracture resistance of root canal–treated maxillary susceptibility of ETT to molars. Our results showed that increasing the fracture is mainly associroot canal taper can predispose them to fracture, ated with the loss of but CAC designs did not show benefits compared tooth structure because with TACs. of dental carries or therapeutic endodontic procedures such as access cavity and root canal preparation (2, 3). Hence, the amount of remaining structure appears to be a major factor determining the prognosis of ETT. The endodontic access cavity is considered an important step in endodontic treatments (4). Recently, a new concept of a conservative access cavity (CAC), inspired by concepts of minimally invasive dentistry, has been designed and developed in order to minimize the removal of the chamber roof and pericervical dentin (5). The rationale of this approach is to avoid excessive dentin removal from tooth structures (6, 7). With advances in the field of imaging, endodontic instruments, visual enhancers, and clinical microscopes, the traditional requirements of the endodontic access cavity start to diminish. For instance, newly developed ultraflexible canal preparation instruments make straight-line access to the canals less important; also, the progress in visual magnification makes it easier to find canal orifices without the need for excessive expansion of access cavity walls (4, 8). However, this relatively new cavity design may confine cleaning, shaping, and obturation of root canals (9). An inadequate access cavity also increases the prevalence of iatrogenic complications during endodontic procedures (4).
From the *Endodontic Department, School of Dentistry, University of California, San Francisco, California; †Endodontic Department, School of Dentistry, Shahid Beheshti University of Medical Sciences, Tehran, Iran; ‡Iranian Center For Endodontic Research, Research Institute of Dental Sciences, Dental School, Shahid Beheshti University of Medical Sciences, Tehran, Iran; §Endodontic Division, School of Dentistry, University of Maryland, Baltimore, Maryland; and kResearch Institute of Dental Sciences, Dental School, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Address requests for reprints to Dr Omid Dianat, Endodontic Division, School of Dentistry, 650 West Baltimore Street, Baltimore, MD 21201. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2018 American Association of Endodontists. https://doi.org/10.1016/j.joen.2018.05.006
JOE — Volume -, Number -, - 2018
Impact of Access Cavity Design and Root Canal Taper
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Basic Research—Technology Rotary systems facilitate debridement of canals, and the higher instrument tapers lead to superior canal and canal wall cleanliness and decrease the concerns regarding microbial removal of canal walls (1). However, there are some concerns regarding the excessive removal of radicular dentin because of increased instrumentation taper (10). Fundamentally, any removal of hard tissue from the canal walls increases the chance of root fracture (3). On the contrary, some authors claim that increased canal preparation taper allows forces to be better distributed in the apical third of the canal; this better distribution increases the tooth’s fracture resistance (11). Accordingly, the aim of the present study was to evaluate and compare the effect of access cavity preparation and 3 different root taper preparations on ETT fracture resistance of maxillary molars. The 2 null hypotheses tested were there would be no difference in the fracture resistance of teeth with different access cavity designs, and there would be no difference in the fracture resistance of roots with different root canal tapers.
Materials and Methods Sample Preparation This study used a total of 78 sound maxillary first and second molars. All of these teeth were extracted for periodontal reasons after written informed consent was obtained. The inclusion criteria were noncarious teeth with mature apices, absence of cracks, and free of any defects. The study was approved by the Ethics Committee of Shahid Beheshti University of Medical Sciences, Tehran, Iran. For infection control, the teeth were stored in 0.5% chloramine-T trihydrate for 2 weeks before the experiment. At no stage in the procedure were the teeth allowed to dehydrate. After comprehensive visual and stereomicroscopic assessment of all intact teeth, 30 maxillary first molars were similarly decoronized by a calibrated endodontist. Three roots of each tooth were divided by burs (ISO no. 806 104 199 544 016; NTI, Kahla, Germany) under sufficient water cooling. The distobuccal roots were randomly assigned to 1 of 3 groups (n = 10) for tapering evaluation (ie, a .04 taper, a .06 taper, or a .08 taper). The buccolingual and mesiodistal dimensions of the roots were measured using a digital caliper, and roots with more than 20% deviation were replaced. All specimens of the tapering groups were prepared for endodontic treatment. Canals were negotiated with size 10 K-type files (Flexofile; Dentsply Maillefer, Ballaigues, Switzerland) to the apical foramen, and the working length was established 0.5 mm shorter. After the initial preparation, canals were instrumented up to an apical size of 25; instrumentation was performed with the Twisted Files rotary system (taper .04, .06, and .08; Kerr Co, Glendora, CA) according to the manufacturer’s instructions. Twisted Files are a progressive tapered instrument used for shaping and finishing root canals. In our study, a new set of instruments was used for each tooth. Intermittent irrigation with 5 mL 2.5% sodium hypochlorite was applied with 30-G needles. In order to omit other covariates, the specimens were not obturated. Another 48 teeth that were free of carious lesions, previous restorations, and any enamel or dentin defects and had a similar crown anatomy and size with 3 separate mature roots were selected. The teeth, which were similar in buccolingual and mesiodistal size, were randomly assigned to 1 of 3 groups (n = 16): intact teeth, no treatment (negative control); traditional access cavity (TAC, positive control); and conservative access cavity (CAC, experimental). Access cavities were prepared using coarse, flat-end diamond burs (ISO no. 806 104 199 544 016, NTI) in a high-speed handpiece with sufficient water cooling. For each 10-access cavity preparation, a separate diamond bur was used. 2
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In the intact group, no treatment was performed on teeth, and they remained intact until the fracture resistance test. In the TAC group, traditional endodontic access cavities were prepared following conventional guidelines (12). In order to obtain straight-line access to all canal orifices, outlines of the cavity and pericervical dentin were removed or modified where necessary (Fig. 1A). In the CAC group, in order to determine the outlines of the access cavity and locate the pulp chamber and canals, we used 2 periapical radiographs made from buccal and mesial aspects as a guide. Then, starting from the central fossa, cavities were extended only as necessary to visualize and locate canal orifices while taking care to preserve pericervical dentin and the root chamber vault where possible as described previously (Fig. 1B) (2, 13). Using the analyzing rod of the dental surveyor, each tooth or root was positioned vertically in a brass ring of self-cured acrylic resin, and teeth were embedded up to 2 mm below the cementoenamel junction. To simulate the periodontal ligament space, roots were covered before acrylic embedment with a uniformly thin 0.2-mm layer of light body silicone impression material.
Fracture Testing For both experimental groups (ie, the tapering and cavity preparation groups), a fracture resistance test was conducted at the laboratory of the Dental Research Center at Shahid Beheshti University of Medical Sciences. The testing device used was the Universal Testing Machine (Model 55144; Zwick/Roell, Ulm, Germany). The testing machine allowed an error of 0.04% for a maximal load of 10,000 kg, 0.01% for a repetitive maximal load of 10,000 kg, resolution of displacement of 0.01 mm (10 mm), and an accurate speed of 0.01% of full scale. Statistical Analysis To analyze the results, statistical software (SPSS 17; SPSS Inc, Chicago, IL) was used. Averages and standard deviations were established. Statistical tests of normality (the Shapiro-Wilk test) were used to check whether data matched a particular distribution such as the normal distribution or the exponential distribution. For all comparisons, the level of importance was P # .05, and the Tukey test was used as the post hoc test.
Results Root Canal Tapers The normality of the obtained data was confirmed using the Shapiro-Wilk test (P > .05). The .04 taper showed the highest (259.61 52.06) and the .08 taper showed the lowest (168.43 59.63) fracture resistance values (Fig. 1). Comparison of data using the 1-way analysis of variance test revealed a significant difference between groups (Fig. 2). Pair-wise comparison of the groups using the Tukey test showed the .04 taper and .06 taper groups did not significantly differ (P > .05), but both groups differed significantly from the .08 taper group (P # .05). Effect of Access Cavity Design Three groups of intact teeth, CAC, and TAC showed fracture resistance mean values of 2118.85 336.97, 1705.69 591.51, and 1471.11 435.34 N, respectively (Table 1). The 1-way analysis of variance test showed no significant difference between the groups (P > .05). Pair-wise comparison of the groups using the Tukey test revealed no significant difference between CAC and TAC (P > .05), but they both showed a significant difference with intact teeth (P # .05). JOE — Volume -, Number -, - 2018
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Figure 1. (A) Conventional access cavity design. (B) CAC design.
Discussion ETT are prone to root fracture, which may lead to extraction and undermine the long-term benefits of endodontic treatment (7,14–16). A major factor endangering the survival of root-filled teeth is the loss of dentin (17, 18). Endodontic procedures encompass various steps such as access cavity preparation and root canal preparation, which may result in the loss of excessive tooth structure, weakening of the tooth, and a subsequent reduction in the tooth’s capability to resist forces (3, 17, 19). Therefore, recently, the conventional methods in the aforesaid procedures have given rise to some criticism because of the possibility of excessive tooth removal, and conservative approaches
have gained attention for the restoration of tooth stability (3, 20). However, this relatively new concept has been slow to impact the endodontists’ mainstream practice and had not been well supported by research data. In this regard, this study sought to evaluate and compare the fracture resistance of ETT in both conventional and conservative approaches of access cavity preparation and canal shaping. Because the available data on the fracture resistance of maxillary molars are lacking (unlike respective data on mandibular molars) and it was shown in a previous study that the impact of CAC varied in different unrestored tooth types (5), this study assessed the influence of different endodontic procedures on fracture resistance in maxillary molars.
Figure 2. Fracture resistance values of roots after different root canal tapers.
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Basic Research—Technology TABLE 1. Fracture Resistance Values of Teeth with Different Access Cavity Designs Access cavity type
Break force (N)
Conservative access cavity Traditional access cavity Intact teeth
1705.691250 (591.51)a 1471.113125 (435.34)a 2118.851250 (336.92)b
Values are expressed in newtons (N). Values in the parenthesis represent standard deviation. Mean values with the same superscript letters are not significantly different (P > .05).
In this study, periapical radiographs of teeth from 2 different planes were used for planning conservative access outlines (13). In addition, the method of root embedment may affect the fracture resistance significantly. Thus, in the present study, simulation of the periodontal ligament around the tooth was done with polyvinyl siloxane, an elastomeric material that is able to prevent concentration of stresses in the cervical region of the tooth because of imitating the accommodation of the tooth in the alveolus (21). Also, an Instron (Norwood, MA) Universal Testing machine was used to measure tooth fracture resistance because the use of this machine is the simplest and most frequently used method to evaluate tooth strength (17, 19). However, this in vitro test provides a static load until failure occurs, whereas in the oral cavity loads are dynamic and, thus, it may not simulate in vivo conditions (22). Furthermore, to avoid confounding by covariates, we assessed the net impacts of access cavity and root tapering preparations by not restoring access cavities and obturating root canals. In the conventional approach, access cavity designs are determined according to some main principles. According to straight-line access, convenience form, and extension for prevention, the outline of the access cavity must be extended beyond gaining access to the canal orifices in order to enable thorough debridement of the canals and prevent procedural complications such as instrument fracture (9, 13). As shown in a study by Krishan et al (13), minimal access cavities were associated with compromised canal instrumentation in the distal canals of mandibular molars. Moreover, the lack of straight-line access because of inadequate tooth tissue removal may compromise the delivery of irrigants to the apical portion of the roots because it avoids the needle reaching further into the canal (23). However, some researchers believe that the superfluous sound tooth structure is sacrificed in this way, which is unnecessary and can decrease resistance to tooth fracture (6, 7, 14). According to Clark and Khademi (6), in TAC preparation, the dentist’s needs for facile access to the canal systems are placed above the tooth needs, whereas in CAC preparation the emphasis is on banking of pericervical dentin by maintaining soffit dentin, skewing the access toward fillings/carries, and altering the traditional reference points. Taken together, it seems that the benefits of thorough canal instrumentation and diminished complications must be weighed against the risk of the tooth’s resistance to fracture being reduced. The results of the present study showed no significant difference between conservative and conventional endodontic cavity preparations, whereas both showed significant differences compared with intact teeth. In agreement with our results, Moore et al (5) reported that the fracture strength in maxillary molars with CACs and TACs was remarkably lower than that of intact molars. Also, although the CAC’s fracture strength was 23% greater than that of TAC, this difference was not statistically significant; this finding suggests that, in comparison with TAC preparation, CAC preparation provides insignificant advantages with regard to fracture strength. In our study, the removal of the pulp chamber roof as the reinforcing structure (24) and the disruption of tooth integrity (25) may have been responsible for the substantial 4
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reduction in fracture resistance of the ETT of both groups in comparison with the intact teeth, which is supported by the findings of many other studies (3, 26–28). In agreement with our study, Rover et al (29) showed no significant difference in the means of the fracture resistance test between conservative and traditional endodontic cavities in maxillary first molars. Their study suggested a reduced possibility of canal orifice detection in the CAC group in comparison with the TAC group. They also observed an increased possibility of canal transportation associated with CACs. Considering the drawbacks of CACs, their study did not support the notion of CAC preparation in maxillary first molars. On the contrary, Plotino et al (30) observed a significant difference in the fracture resistance amounts of CACs and TACs in maxillary and mandibular premolars and molars, whereas CACs did not differ significantly from intact teeth. Furthermore, Cobankara et al (19) showed that none of the tested restoration techniques in their study was able to utterly restore the fracture resistance lost from access cavity preparation. In agreement with our study, it was shown in another study that if at least 1 marginal ridge could be preserved, with any thickness more than 0.5 mm, its thickness did not affect the fracture resistance of the teeth (31). In contrast, mesial, occlusal, and distal cavity preparations weakened the teeth severely (14, 24). Thus, the null hypothesis regarding access cavity design has been approved. The findings of this study do not support the use of minimally invasive access cavity preparation because this approach still has drawbacks (6, 7), and, regarding fracture resistance, the CAC approach was not more advantageous than the TAC approach. Considering that maxillary molars have challenging root canal systems in mesiobuccal roots, where secondary canals are difficult to locate and instrument, the application of CACs in these teeth necessitates careful consideration. The primary aim of endodontic treatment is to eliminate microorganisms (32). Research has established that bacteria can penetrate into and colonize almost half the length of dentinal tubules (23, 33). Accordingly, inadequate removal of infected dentin within the canals can decrease the prognosis and lead to posttreatment failures (23). On the other hand, the thickness of the dentin has a stabilizing influence on the root. Consequently, any dentin removal in the canal can decrease the stability of the root (3, 34). For root canal shaping, the current study showed no significant reduction of the fracture resistance of the root by .06 taper instrumentation compared with .04, whereas preparation with the .08 taper showed a significant decrease in fracture resistance. In agreement with our results, a previous study has shown that during instrumentation, maintaining the natural geometry of the root canals is a paramount stabilizing factor for the tooth, and, therefore, if the root canal outline is not substantially altered, tooth fracture resistance is relatively unaffected (3). Conceivably, in our study, the decrease in fracture resistance that followed .08 taper instrumentation might have been the result of geometric alterations of the root canals because .08 taper files are more rigid and less adaptable. Another factor could be the typically small diameter of distobuccal roots because root preparation with a thicker file leads to a further weakening of it (35). Root fracture occurs as a result of propagation of microcracks created in the root canal shaping process with occlusal forces (36). Thus, we suggest that the increased risk of fracture with the .08 taper in this study might be associated with the greater number of craze lines and the greater degree of imposed stress in root dentin. Moreover, our findings corroborated the results of a previous study that reported that preparation with larger taper instruments significantly weakened the roots. Also, Zandbiglari et al. (35) suggested that this result was probably caused by the greater amount of dentin removed with larger tapering instruments compared with common taper hand files. JOE — Volume -, Number -, - 2018
Basic Research—Technology Therefore, the null hypothesis regarding root taper has been partially rejected because the increase in tapering from .06 to .08 influenced tooth resistance significantly. Given our finding that in molar teeth the .08 taper adversely affected tooth resistance, our results did not support the use of the .08 taper in these teeth. Notably, 1 limitation of the present study is that the variability of the tested teeth may have confounded our results. In addition, preparing the conventional and conservative access cavities with the same degree of accuracy in all teeth is difficult. Given these limitations, we suggest that in order to achieve experimental conditions that are more similar to clinical conditions, subsequent studies should use finite element analysis. Moreover, it should be mentioned that until now no standard method for preparing conservative access cavities has been established. Therefore, conservative access cavities are difficult to standardize, and, thus, this type of research is hard to reproduce. The data achieved in this ex vivo study provide insight into the fracture resistance of different access designs and root canal tapers in the clinical setting. However, for more certain results, clinical trials with long-term follow-ups are required. In conclusion, within the limitations of this study, for endodontic treatment of teeth, the following can be concluded: 1. Increasing the tapering of root canal preparation can detrimentally affect tooth fracture resistance 2. Conservative endodontic access in maxillary molars was not shown to significantly increase fracture resistance over traditional access
Acknowledgments The authors deny any conflicts of interest related to this study.
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