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Fracture Strength of Endodontically Treated Teeth with Different Access Cavity Designs
Basic Research—Technology
Fracture Strength of Endodontically Treated Teeth with Different Access Cavity Designs Gianluca Plotino, DDS, PhD,* Nicola Maria Grande, DDS, PhD,† Almira Isufi, DDS, PhD, MSc,* a, DDS, PhD,§ Rossella Bedini, DSc, PhD,‡ Pietro Ioppolo, DpHS,‡ Eugenio Pedull Gianluca Gambarini, MD, DDS,* and Luca Testarelli, DDS, PhD* Abstract Introduction: The purpose of this study was to compare in vitro the fracture strength of root-filled and restored teeth with traditional endodontic cavity (TEC), conservative endodontic cavity (CEC), or ultraconservative ‘‘ninja’’ endodontic cavity (NEC) access. Methods: Extracted human intact maxillary and mandibular premolars and molars were selected and assigned to control (intact teeth), TEC, CEC, or NEC groups (n = 10/group/type). Teeth in the TEC group were prepared following the principles of traditional endodontic cavities. Minimal CECs and NECs were plotted on conebeam computed tomographic images. Then, teeth were endodontically treated and restored. The 160 specimens were then loaded to fracture in a mechanical material testing machine (LR30 K; Lloyd Instruments Ltd, Fareham, UK). The maximum load at fracture and fracture pattern (restorable or unrestorable) were recorded. Fracture loads were compared statistically, and the data were examined with analysis of variance and the Student-Newman-Keuls test for multiple comparisons. Results: The mean load at fracture for TEC was significantly lower than the one for the CEC, NEC, and control groups for all types of teeth (P < .05), whereas no difference was observed among CEC, NEC, and intact teeth (P > .05). Unrestorable fractures were significantly more frequent in the TEC, CEC, and NEC groups than in the control group in each tooth type (P < .05). Conclusions: Teeth with TEC access showed lower fracture strength than the ones prepared with CEC or NEC. Ultraconservative ‘‘ninja’’ endodontic cavity access did not increase the fracture strength of teeth compared with the ones prepared with CEC. Intact teeth showed more restorable fractures than all the prepared ones. (J Endod 2017;43:995–1000)
Key Words Conservative access cavity, endodontic access cavity, fracture resistance, ‘‘ninja’’ cavity, traditional endodontic cavity
O
ne of the most imporSignificance tant steps for successCEC and NEC access was proposed to reduce ful endodontic treatment is fracture risk of endodontically treated teeth. Teeth access cavity preparation. with CEC and NEC showed similar fracture The traditional endodonstrength, which was higher than that of teeth with tic cavity (TEC) design for traditional endodontic access. different tooth types has remained unchanged for decades, and only minor modifications have been done (1). However, the removal of tooth structure needed for access cavity preparation may undermine the strength of the tooth to fracture under functional loads (2, 3). Extraction is the most frequent consequence of fracture of endodontically treated teeth (4–6). Extended preparation of endodontic access cavities critically reduces the amount of sound dentin (7–10) and increases the deformability of the tooth (8), compromising the strength to fracture of endodontically treated teeth (7). Recently, conservative endodontic cavity (CEC) preparation (11, 12) to minimize tooth structure removal and preserve some of the chamber roof and pericervical dentin was reported in the literature. This sound dentin preservation could be achieved with the help of cone-beam computed tomographic (CBCT) imaging to identify all the canals (13, 14). This could improve the fracture strength of endodontically treated teeth (11). Following this concept, an extreme conservative approach has recently been proposed, which is conventionally known as ‘‘ninja.’’ This technique may improve the fracture strength of endodontically treated teeth (15). To date, there are no studies comparing CEC access cavity preparation with ultraconservative ‘‘ninja’’ endodontic cavity (NEC) access. Therefore, the purpose of this study was to investigate the fracture strength of endodontically treated teeth with a TEC, CEC, or NEC access cavity.
Materials and Methods Specimen Selection and Preparation After ethics approval, 160 recently extracted intact human maxillary and mandibular molars and premolars from a white population with completely formed apices were used. The exclusion criteria for the tested teeth were the presence of caries, previous restoration, and visible fracture lines or cracks. After a debridement with hand scaling instruments and cleansing with a rubber cup and pumice, the teeth were stored in individually numbered containers with 0.1% thymol solution at 4 C until used and during all the time between the different phases of the experiment in order to prevent their dehydration.
From the *Department of Endodontics, La Sapienza University of Rome, Rome, Italy; †Catholic University of Sacred Heart, Rome, Italy; ‡Technologies and Health Department, Istituto Superiore di Sanita, Rome, Italy; and §Department of General Surgery and Surgical-Medical Specialties, University of Catania, Catania, Italy. Address requests for reprints to Dr Eugenio Pedulla, Via Cervignano, 29, 95129, Catania, Sicily, Italy. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2017 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2017.01.022
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Figure 1. (A–D) Sketches with an (A–C) occlusal view and a (D) sagittal view of access cavity designs of a first mandibular molar. (A–D) A traditional access cavity (black line dashed), (A, C, and D) conservative access cavity (green), and (B–D) ultraconservative ‘‘ninja’’ access cavity (red). Comparison of the 3 access cavity designs in the (C) occlusal and (D) sagittal view, respectively. The sagittal view shown as a conservative access cavity maintains a robust amount of pericervical dentin. B, buccal; D, distal; L, lingual; M, mesial.
Forty maxillary first molars with 3 separate roots, 40 mandibular first molars with 2 separate roots, 40 maxillary first premolars with 2 separate roots, and 40 mandibular first single-rooted premolars were selected based on similar dimensions. The anatomic crown height was measured from the occlusal surface to the cementoenamel junction on all 4 sides of the teeth; buccolingual and mesiodistal (MD) dimensions were measured at the occlusal surface. Tooth measurements were taken with a digital caliper (Digimatic 500; Mitutoyo, Kanagawa, Japan). Specimens were subsequently assigned to 4 groups (n = 10) for each tooth type. Therefore, the following homogenous groups were created based on the averages of tooth dimensions in order to minimize the influence of size and shape variations on the results:
1. 2. 3. 4.
Group A: the control group, which included teeth that were left intact Group B: the TEC group Group C: the CEC group Group D: the NEC group
TEC, CEC, and NEC cavity accesses of all teeth were drilled with size 856 diamond burs (Komet Italia srl, Milan, Italy) mounted on a highspeed handpiece with water cooling (16). Teeth in the TEC, CEC, and NEC groups were mounted in a custom-made device (17) and imaged with a CBCT scanner (Kodak 9000 3D; Carestream Health, Inc, Marnela-Vallee, France) with a spatial resolution of 200 mm; the scans were used to plan TEC, CEC, and NEC outlines. Teeth in the TEC group
Figure 2. (A–F) CBCT 3-dimensional reconstructions and segmentations of lower molars prepared with different access cavity designs in (A–C) the sagittal view and (D–F) the axial view at the occlusal surface. (A and D) A traditional access cavity (purple), (B and E) conservative access cavity (green), and (C and F) ultraconservative ‘‘ninja’’ access cavity (red) are segmented on CBCT reconstructions.
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Basic Research—Technology
Figure 3. (A–D) Representative pictures of fractured molars for different groups. (A) Intact teeth had more restorable fractures than teeth prepared with (B) TEC, (C) CEC, and (D) ultraconservative NEC, which had unrestorable fractures often.
were prepared following the principles of TECs as previously reported (1, 18). In the CEC group, premolars were accessed 1 mm buccal to the central fossa, and cavities extended apically, maintaining part of the chamber roof and lingual shelf. Molars were accessed at the mesial quarter of the central fossa, and cavities extended apically and distally while maintaining part of the chamber roof. Mesiodistal, buccolingual, and circumferential pericervical dentin removal was minimized to ensure the maintenance of the part of the chamber roof compatible with the localization of all root canal orifices from the same visual angulation (11, 12, 19). This was caused by the shape of preparation; the occlusal enamel was only beveled at 45 (12). The extension was not balanced equally between the buccal and palatal orifices but rather slightly favored the buccal orifice (11). In the NEC group, premolar and molar teeth were accessed in the same way as the teeth in the CEC group, but the chamber roof was maintained as much as possible. The access ‘‘ninja’’ outline was derived from the oblique projection toward the central fossa of the root canal orifices on the occlusal plane. By doing this, localization of all the root canal orifices was possible even from different visual angulations because the endodontic access was parallel with the enamel cut at 90 or more to the occlusal table (12, 15) (Fig. 1A–D). The extension was equally balanced between the buccal and lingual/palatal orifices. Then, teeth in the TEC, CEC, and NEC groups were scanned again using CBCT imaging as described earlier. Digital Imaging and JOE — Volume 43, Number 6, June 2017
Communications in Medicine data were moved to the MeVisLab image processing and visualization platform (MeVis Research, Bremen, Germany) to perform 3-dimensional surface rendering of the teeth and segmentation of the TEC, CEC, or NEC access (Fig. 2A–F). The percentage of volume of coronal enamel and dentin removed by TEC, CEC, and NEC access cavities and the total enamel and dentin crown volume for each type of tooth were calculated.
Endodontic Treatment Root canals were negotiated with size 10 K-type files (Flexofile; Dentsply Maillefer, Ballaigues, Switzerland) to the major apical foramen, and canals were instrumented to length with Mtwo nickeltitanium rotary instruments (Sweden & Martina, Padova, Italy), with a 16-mm working part, up to the #25 tip size and 0.06 taper file. During endodontic treatment, 5.25% sodium hypochlorite (Niclor 5; Ogna, Muggio, Italy) for irrigation was intermittently deposited using ProRinse side-vented 30-G needles (Dentsply Tulsa Dental Specialties, Tulsa, OK), and after instrumentation, the root canals were irrigated with 17% EDTA solution. The canals were dried with paper points and filled with gutta-percha (single-cone size 25, 0.06 taper) and a resin-based endodontic sealer (AH Plus; Dentsply De Trey, Konstanz, Germany). Afterward, the teeth were subjected to postoperative radiographs and CBCT imaging to evaluate the endodontic treatment. Fracture Strength of Endodontically Treated Teeth
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5.3 (0.3)b 4.7 (0.5)b 5.3 (0.3)b 5.7 (0.6)b 8.2a (0.9) 7.8a (0.9) 9.5a (1.0) 10.1a (0.8) 7.9a (0.9) 7.5a (1.4) 9.8a (0.7) 10.5a (0.9) 5.0 (0.8)b 5.1 (0.4)b 5.6 (0.5)b 5.6 (0.4)b 8.0a (0.5) 7.9a (0.8) 9.5a (0.9) 10.2a (0.5) 7.9a (0.3) 7.3a (0.5) 9.9a (0.3) 10.7a (1.0) 5.3 (0.7)b 4.9 (0.3)b 5.3 (0.2)b 5.7 (0.5)b CEC, conservative endodontic cavity; NEC, ‘‘ninja’’ endodontic cavity; TEC, traditional endodontic cavity. Similar lowercase letters in the same row indicate no statistically significant differences (P > .05).
8.1a (0.7) 7.8a (0.9) 9.6a (0.9) 10.2a (0.9) 8.0a (0.8) 7.2a (1.3) 9.7a (0.5) 10.6a (1.3) 7.8a (1.6) 7.6a (0.9) 9.4a (0.7) 10.3a (0.6) 8.1a (1.0) 7.4a (1.2) 9.7a (0.6) 10.7a (1.2) Upper Premolars Lower Premolars Upper molars Lower molars
5.2 (0.7)b 4.5 (0.4)b 5.4 (0.1)b 5.8 (0.4)b
Anatomic crown height BL
Occlusal surface
MD BL MD BL MD
Occlusal surface
Anatomic crown height
Occlusal surface
Anatomic crown height
NEC CEC
Anatomic crown height BL MD
Occlusal surface
Tooth type (n = 10)
Results The mean of the buccolingual and mesiodistal dimensions at the occlusal surface and the anatomic crown height of the tested teeth are presented in Table 1. No significant differences were found when comparing all teeth dimensions in the control and test groups for each type of tooth (P > .05). Table 2 shows the mean volume percentages of the coronal enamel and dentin removed by different access cavity designs in each tooth type. The mean load at fracture for teeth in the TEC group was significantly lower than the intact, CEC, and NEC groups (P < .05), whereas no difference was observed among the control, CEC, and NEC groups (P > .05) in all types of teeth (Table 3). Intact premolars had mostly cuspal chipping, whereas those with TEC, CEC, and NEC consistently had wall fractures extending below the cementoenamel junction. Molars in all the groups had mesiodistal fractures with a varying apical extent. The restorable fractures were significantly higher than the unrestorable ones in the intact teeth (P < .05), whereas the number of unrestorable fractures was higher than the restorable ones in the TEC, CEC, and NEC groups in every type of tooth (P < .05). No difference in the number of restorable or unrestorable fractures was observed for the TEC, CEC, and NEC groups in every type of tooth (P > .05).
TEC
Statistical Analysis The data were first verified with the Kolmogorov-Smirnov test for normal distribution and the Levene test for homogeneity of variances. Thus, they were statistically evaluated using analysis of variance and the Student-Newman-Keuls test for multiple comparisons (Prism 5.0; GraphPad Software Inc, La Jolla, CA), with the significance level established at 5% (P < .05).
Control
Fracture Test The 120 teeth in the TEC, CEC, and NEC groups and the 40 teeth (n = 10/type) kept intact were mounted on brass rings with the roots embedded in self-curing resin (SR Ivolen; Ivoclar Vivadent, Schaan, Lichtenstein) up to 2 mm apical to the cementoenamel junction as reported in a previous study (19). The 160 tooth specimens were placed in a custom-made water bath and mounted in a mechanical material testing machine (LR30 K; Lloyd Instruments Ltd, Fareham, UK) (19). The teeth were loaded at their central fossa at a 30 angle from the long axis of the tooth. The continuous compressive force at a crosshead speed of 0.5 mm/min was applied using a 6-mm-diameter ball-ended steel compressive head. The loads at which the teeth were fractured, indicated by the software of the load testing machine, were recorded in newtons. The fractured specimens were examined under a stereomicroscope (SZR- 10; Optika, Bergamo, Italy) to determine the fracture levels. Fracture patterns were classified as ‘‘restorable’’ when the failures were above the level of bone simulation (site of fracture above the acrylic resin) and ‘‘unrestorable’’ when the failures were extending below the level of bone simulation (site of fracture below the acrylic resin) (20) (Fig. 3A–D).
Groups
Teeth Restoration The enamel and dentin of the access cavity were cleaned and etched with 37% phosphoric acid for 30 and 15 seconds, respectively; rinsed for 30 seconds with a water/air spray; and gently air dried to avoid desiccation. A light-polymerizing primer bond adhesive (XP Bond; Dentsply International, York, PA) was applied, gently air thinned, and exposed to light-emitting diode polymerization for 30 seconds. At the end, the access cavities were restored with direct composite restorations (CeramX mono, Dentsply International).
TABLE 1. Mean and (Standard Deviation) of the Mesiodistal (MD) and Buccolingual (BL) Dimensions and the Anatomic Crown Height (Measured at the 4 Sides of the Tooth) of the Tested Teeth in Each Group
Basic Research—Technology
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Basic Research—Technology fatigued tests may not reflect complete root strain patterns for the complex chewing process (22). Access cavities were restored with bonded resin composite to simulate clinical procedures and facilitate loading tests (22). Restoration of endodontic access cavities may restore the fracture strength of teeth up to 72% of that of intact teeth (22, 30). The same expert operator performed all specimen preparation procedures in order to avoid different results caused by different operator skills. In this study, the TEC group presented lower fracture strength than the control, CEC, and NEC groups. These results are in agreement with a previous study in which the teeth were tested without any restoration, which is different from clinical practice (19). The results of the present study are in agreement and corroborate reports that showed improved fracture strength of teeth because of dentin preservation obtained by cavity size reduction (9, 31, 32). In addition, no difference in the fracture strength was observed among the CEC, NEC, or control groups in all tested teeth. These results, relative to CEC, are in agreement with a previous study (19). Despite the fact that our results related to NEC cannot be directly compared with previous reports, it is not surprising that teeth in the NEC group showed no difference in fracture strength compared with the control and CEC groups because of the minimally invasive access cavity designs of NEC. However, in a recent study, the CEC cavity did not increase the fracture strength of restored maxillary molars in comparison with ones prepared with TEC, suggesting no apparent benefit of CEC in this regard (22). This contrasting finding is probably because of the differences in the methodology of that study including the type of teeth considered (only maxillary molars were reported to be subjected to fracture more than mandibular ones [33]); the techniques and materials used for endodontic and restoring procedures; and the method used to assess the fracture strength (teeth were cyclically fatigued and subsequently loaded to failure [22]). Although CEC improved fracture strength more than TEC, it could increase the risks of inefficient canal instrumentation and the occurrence of procedural errors as previously reported (19). However, a recent study showed that CECs in maxillary molars did not appear to impact instrumentation efficacy (22). No studies have investigated the quality of endodontic procedures using NEC. In addition, the ideal access cavity would allow complete removal of pulp tissue, debris, and necrotic materials. However, the smaller the access cavity, the higher the risk of bacterial contaminations and the possibility of missing some root canal orifices (22, 34). The results of the present study showed a higher number of restorable fracture patterns in intact teeth than in the ones prepared with TEC, CEC, or NEC. These findings are in agreement with a previous report (35). Furthermore, the majority of the teeth prepared with TEC, CEC, or NEC showed unrestorable fracture patterns with no significant difference among the different access cavity designs.
TABLE 2. The Volume Percentage (Mean and Standard Deviation) of the Coronal Enamel and Dentin Removed in Teeth with Different Access Cavity Designs Including Traditional, Conservative, and ‘‘Ninja’’ Access Coronal dentin and enamel volume removed (% of total crown volume) Tooth type (n = 10) Upper premolars Lower premolars Upper molars Lower molars
TEC
CEC
NEC
22.15 (3.71)a 13.43 (3.12)b 23.89 (3.04)a 15.17 (3.67)b 19.27 (3.82)a 11.03 (2.81)b 16.48 (3.47)a 7.31 (3.33)b
5.13 (0.76)c 6.07 (0.54)c 5.92 (0.75)c 4.81 (0.82)c
CEC, conservative endodontic cavity; NEC, ‘‘ninja’’ endodontic cavity; TEC, traditional endodontic cavity. Similar lowercase letters in the same row indicate no statistically significant differences (P > .05).
Discussion One of the most important causes of fractures in root-filled teeth is the loss of tooth structure. The preparation of the endodontic access cavity following the TEC principals was reported as the second largest cause of loss of tooth structure (20). Thus, a proper and reduced endodontic access design could improve the prognosis for an endodontically treated tooth (21). Recently, CEC and NEC were proposed to reduce the fracture risk in endodontically treated teeth (15, 19). However, clinically, these approaches can mainly be performed on intact teeth that are going to be treated endodontically. This clinical scenario does not seem to occur frequently, representing only 8% of the cases treated by the authors in the last 5 years (G. Plotino et al, unpublished data, 2016). Until now, in the literature, the fracture strength of teeth with CEC and NEC access was investigated in a few studies (19, 22) and no studies, respectively. For this reason, the fracture strength of endodontically treated teeth with TEC, CEC, or NEC access cavity was tested in the present study. The use of mature, intact maxillary and mandibular molars and premolars was a priority to avoid the effects of different amounts of tooth structure loss (22). Anterior teeth were not tested in this investigation because no differences between TEC and CEC fracture strength in these teeth were reported (19). Although premolars and molars are subjected to a different occlusal force in the clinical situation (23), in this study the same loading force was applied to standardize the procedure (19). Fracture resistance was assessed with a mechanical testing machine as in other studies (19, 24–26). A 30 inclination angle was used because teeth are most vulnerable to fracture when eccentric forces are applied (27), reaching the failure point at lower loads when compared with the axial fracture loads of other studies (28, 29). However, loading to fracture methodology used for in vitro analyses does not accurately reflect intraoral conditions in which failures occur primarily because of fatigue. In the same way, axial cyclically
TABLE 3. Load at Fracture (Mean Standard Deviation) and Type of Fracture, ‘‘Restorable’’ (R) or ‘‘Unrestorable’’ (U), for Intact Teeth (Control) or Traditional, Conservative, or ‘‘Ninja’’ Access Assessed after the Static Test Using a Mechanical Material Testing Machine Load at fracture (N)
Type of fracture Control
Tooth type (n = 10) Upper premolars Lower premolars Upper molars Lower molars
Control
TEC a
913 (188) 1006 (313)a 1172 (598)a 1572 (639)a
CEC b
498 (250) 704 (310)b 810 (425)b 923 (393)b
NEC a
821 (324) 929 (384)a 1143 (506)a 1401 (495)a
a
805 (204) 945 (267)a 1170 (432)a 1459 (278)a
TEC
CEC
NEC
R
U
R
U
R
U
R
U
a
b
b
a
b
a
b
7a 7a 7a 7a
7 7a 8a 7a
3 3b 2b 3b
2 3b 3b 2b
8 7a 7a 8a
3 2b 3b 2b
7 8a 7a 8a
3 3b 3b 3b
CEC, conservative endodontic cavity; NEC, ‘‘ninja’’ endodontic cavity; TEC, traditional endodontic cavity. Similar lowercase letters in the same row indicate no statistically significant differences (P > .05).
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Basic Research—Technology Within the limitations of this study, it can be concluded that conservative endodontic access cavities such as CEC and NEC increased the fracture strength of teeth compared with those with TEC. The ultraconservative NEC access did not improve the fracture strength of teeth with CEC access. Moreover, restored CEC and NEC did not reduce the fracture strength, but they did influence the fracture pattern of intact teeth. Further clinical studies are necessary to determine the efficacy of instrumentation, difficulties during endodontic procedures and longterm prognosis of endodontically treated maxillary and mandibular molars and premolars with CEC or NEC.
Acknowledgments The authors thank Giusy La Rosa from the University of Catania for the support in the sketches of teeth with CEC and NEC accesses. The authors deny any conflict of interests related to this study.
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15. Belograd M. The Genious 2 is coming. Available at: http://www.dentinaltubules.com/ videos/ninja-access-a-new-access-concept-in-endodontics. Accessed September 18, 2016. 16. Pedulla E, Genovese C, Campagna E, et al. Decontamination efficacy of photon initiated photoacoustic streaming (PIPS) of irrigants using low-energy laser settings: an ex vivo study. Int Endod J 2012;45:865–70. 17. Paque F, Ganahl D, Peters OA. Effects of root canal preparation on apical geometry assessed by micro- computed tomography. J Endod 2009;35:1056–9. 18. Patel S, Rhodes J. A practical guide to endodontic access cavity preparation in molar teeth. Br Dent J 2007;203:133–40. 19. 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. 20. Rezaei Dastjerdi M, Amirian Chaijan K, Tavanafar S. Fracture resistance of upper central incisors restored with different posts and cores. Restor Dent Endod 2015; 40:229–35. 21. Ikram OH, Patel S, Sauro S, et al. Micro-computed tomography of tooth tissue volume changes following endodontic procedures and post space preparation. Int Endod J 2009;42:1071–6. 22. 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. 23. Ogawa T, Suzuki T, Oishi N, et al. Tactile sensation and occlusal loading condition of mandibular premolars and molars. Odontology 2011;99:193–6. 24. Luthria A, Srirekha A, Hegde J, et al. The reinforcement effect of polyethylene fibre and composite impregnated glass fibre on fracture resistance of endodontically treated teeth: an in vitro study. JConserv Dent 2012;15:372–6. 25. Pradeep P, Kumar VS, Bantwal SR, Gulati GS. Fracture strength of endodontically treated premolars: an in-vitro evaluation. J Int Oral Health 2013;5:9–17. 26. 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. 27. ElAyouti A, Serry MI, Geis-Gerstorfer J, et al. Influence of cusp coverage on the fracture resistance of premolars with endodontic access cavities. Int Endod J 2011;44:543–9. 28. de Freitas CR, Miranda MI, de Andrade MF, et al. Resistance to maxillary premolar fractures after restoration of class II preparations with resin composite or ceromer. Quintessence Int 2002;33:589–94. 29. Ortega VL, Pegoraro LF, Conti PC, et al. Evaluation of fracture resistance of endodontically treated maxillary premolars, restored with ceromer or heat-pressed ceramic inlays and fixed with dual-resin cements. J Oral Rehabil 2004;31:393–7. 30. Hamouda IM, Shehata SH. Fracture resistance of posterior teeth restored with modern restorative materials. J Biomed Res 2011;25:418–24. 31. Assif D, Nissan J, Gafni Y, et al. Assessment of the resistance to fracture of endodontically treated molars restored with amalgam. J Prosthet Dent 2003;89:462–5. 32. Al-Omiri MK, Al-Wahadni AM. An ex vivo study of the effects of retained coronal dentine on the strength of teeth restored with composite core and different post and core systems. Int Endod J 2006;39:890–9. 33. Zadik Y, Sandler V, Bechor R,, et al. Analysis of factors related to extraction of endodontically treated teeth. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2008;106:31–5. 34. Caicedo R, Clarck S, Rozo L, et al. Guidelines for access cavity preparation in endodontics. Available at: http://www.devosendo.nl/uploads/pdf/116_Guidelines% 20for%20access%20cavity.pdf. Accessed September 18, 2016. 35. Hansen EK, Asmussen E. Cusp fracture of endodontically treated posterior teeth restored with amalgam: teeth restored in Denmark before 1975 versus after 1979. Acta Odontol Scand 1993;51:73–7.
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Fracture Strength of Endodontically Treated Teeth with Different Access Cavity Designs Gianluca Plotino, DDS, PhD,* Nicola Maria Grande, DDS, PhD,† Almira Isufi, DDS, PhD, MSc,* a, DDS, PhD,§ Rossella Bedini, DSc, PhD,‡ Pietro Ioppolo, DpHS,‡ Eugenio Pedull Gianluca Gambarini, MD, DDS,* and Luca Testarelli, DDS, PhD* Abstract Introduction: The purpose of this study was to compare in vitro the fracture strength of root-filled and restored teeth with traditional endodontic cavity (TEC), conservative endodontic cavity (CEC), or ultraconservative ‘‘ninja’’ endodontic cavity (NEC) access. Methods: Extracted human intact maxillary and mandibular premolars and molars were selected and assigned to control (intact teeth), TEC, CEC, or NEC groups (n = 10/group/type). Teeth in the TEC group were prepared following the principles of traditional endodontic cavities. Minimal CECs and NECs were plotted on conebeam computed tomographic images. Then, teeth were endodontically treated and restored. The 160 specimens were then loaded to fracture in a mechanical material testing machine (LR30 K; Lloyd Instruments Ltd, Fareham, UK). The maximum load at fracture and fracture pattern (restorable or unrestorable) were recorded. Fracture loads were compared statistically, and the data were examined with analysis of variance and the Student-Newman-Keuls test for multiple comparisons. Results: The mean load at fracture for TEC was significantly lower than the one for the CEC, NEC, and control groups for all types of teeth (P < .05), whereas no difference was observed among CEC, NEC, and intact teeth (P > .05). Unrestorable fractures were significantly more frequent in the TEC, CEC, and NEC groups than in the control group in each tooth type (P < .05). Conclusions: Teeth with TEC access showed lower fracture strength than the ones prepared with CEC or NEC. Ultraconservative ‘‘ninja’’ endodontic cavity access did not increase the fracture strength of teeth compared with the ones prepared with CEC. Intact teeth showed more restorable fractures than all the prepared ones. (J Endod 2017;43:995–1000)
Key Words Conservative access cavity, endodontic access cavity, fracture resistance, ‘‘ninja’’ cavity, traditional endodontic cavity
O
ne of the most imporSignificance tant steps for successCEC and NEC access was proposed to reduce ful endodontic treatment is fracture risk of endodontically treated teeth. Teeth access cavity preparation. with CEC and NEC showed similar fracture The traditional endodonstrength, which was higher than that of teeth with tic cavity (TEC) design for traditional endodontic access. different tooth types has remained unchanged for decades, and only minor modifications have been done (1). However, the removal of tooth structure needed for access cavity preparation may undermine the strength of the tooth to fracture under functional loads (2, 3). Extraction is the most frequent consequence of fracture of endodontically treated teeth (4–6). Extended preparation of endodontic access cavities critically reduces the amount of sound dentin (7–10) and increases the deformability of the tooth (8), compromising the strength to fracture of endodontically treated teeth (7). Recently, conservative endodontic cavity (CEC) preparation (11, 12) to minimize tooth structure removal and preserve some of the chamber roof and pericervical dentin was reported in the literature. This sound dentin preservation could be achieved with the help of cone-beam computed tomographic (CBCT) imaging to identify all the canals (13, 14). This could improve the fracture strength of endodontically treated teeth (11). Following this concept, an extreme conservative approach has recently been proposed, which is conventionally known as ‘‘ninja.’’ This technique may improve the fracture strength of endodontically treated teeth (15). To date, there are no studies comparing CEC access cavity preparation with ultraconservative ‘‘ninja’’ endodontic cavity (NEC) access. Therefore, the purpose of this study was to investigate the fracture strength of endodontically treated teeth with a TEC, CEC, or NEC access cavity.
Materials and Methods Specimen Selection and Preparation After ethics approval, 160 recently extracted intact human maxillary and mandibular molars and premolars from a white population with completely formed apices were used. The exclusion criteria for the tested teeth were the presence of caries, previous restoration, and visible fracture lines or cracks. After a debridement with hand scaling instruments and cleansing with a rubber cup and pumice, the teeth were stored in individually numbered containers with 0.1% thymol solution at 4 C until used and during all the time between the different phases of the experiment in order to prevent their dehydration.
From the *Department of Endodontics, La Sapienza University of Rome, Rome, Italy; †Catholic University of Sacred Heart, Rome, Italy; ‡Technologies and Health Department, Istituto Superiore di Sanita, Rome, Italy; and §Department of General Surgery and Surgical-Medical Specialties, University of Catania, Catania, Italy. Address requests for reprints to Dr Eugenio Pedulla, Via Cervignano, 29, 95129, Catania, Sicily, Italy. E-mail address: [email protected] 0099-2399/$ - see front matter Copyright ª 2017 American Association of Endodontists. http://dx.doi.org/10.1016/j.joen.2017.01.022
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Figure 1. (A–D) Sketches with an (A–C) occlusal view and a (D) sagittal view of access cavity designs of a first mandibular molar. (A–D) A traditional access cavity (black line dashed), (A, C, and D) conservative access cavity (green), and (B–D) ultraconservative ‘‘ninja’’ access cavity (red). Comparison of the 3 access cavity designs in the (C) occlusal and (D) sagittal view, respectively. The sagittal view shown as a conservative access cavity maintains a robust amount of pericervical dentin. B, buccal; D, distal; L, lingual; M, mesial.
Forty maxillary first molars with 3 separate roots, 40 mandibular first molars with 2 separate roots, 40 maxillary first premolars with 2 separate roots, and 40 mandibular first single-rooted premolars were selected based on similar dimensions. The anatomic crown height was measured from the occlusal surface to the cementoenamel junction on all 4 sides of the teeth; buccolingual and mesiodistal (MD) dimensions were measured at the occlusal surface. Tooth measurements were taken with a digital caliper (Digimatic 500; Mitutoyo, Kanagawa, Japan). Specimens were subsequently assigned to 4 groups (n = 10) for each tooth type. Therefore, the following homogenous groups were created based on the averages of tooth dimensions in order to minimize the influence of size and shape variations on the results:
1. 2. 3. 4.
Group A: the control group, which included teeth that were left intact Group B: the TEC group Group C: the CEC group Group D: the NEC group
TEC, CEC, and NEC cavity accesses of all teeth were drilled with size 856 diamond burs (Komet Italia srl, Milan, Italy) mounted on a highspeed handpiece with water cooling (16). Teeth in the TEC, CEC, and NEC groups were mounted in a custom-made device (17) and imaged with a CBCT scanner (Kodak 9000 3D; Carestream Health, Inc, Marnela-Vallee, France) with a spatial resolution of 200 mm; the scans were used to plan TEC, CEC, and NEC outlines. Teeth in the TEC group
Figure 2. (A–F) CBCT 3-dimensional reconstructions and segmentations of lower molars prepared with different access cavity designs in (A–C) the sagittal view and (D–F) the axial view at the occlusal surface. (A and D) A traditional access cavity (purple), (B and E) conservative access cavity (green), and (C and F) ultraconservative ‘‘ninja’’ access cavity (red) are segmented on CBCT reconstructions.
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Figure 3. (A–D) Representative pictures of fractured molars for different groups. (A) Intact teeth had more restorable fractures than teeth prepared with (B) TEC, (C) CEC, and (D) ultraconservative NEC, which had unrestorable fractures often.
were prepared following the principles of TECs as previously reported (1, 18). In the CEC group, premolars were accessed 1 mm buccal to the central fossa, and cavities extended apically, maintaining part of the chamber roof and lingual shelf. Molars were accessed at the mesial quarter of the central fossa, and cavities extended apically and distally while maintaining part of the chamber roof. Mesiodistal, buccolingual, and circumferential pericervical dentin removal was minimized to ensure the maintenance of the part of the chamber roof compatible with the localization of all root canal orifices from the same visual angulation (11, 12, 19). This was caused by the shape of preparation; the occlusal enamel was only beveled at 45 (12). The extension was not balanced equally between the buccal and palatal orifices but rather slightly favored the buccal orifice (11). In the NEC group, premolar and molar teeth were accessed in the same way as the teeth in the CEC group, but the chamber roof was maintained as much as possible. The access ‘‘ninja’’ outline was derived from the oblique projection toward the central fossa of the root canal orifices on the occlusal plane. By doing this, localization of all the root canal orifices was possible even from different visual angulations because the endodontic access was parallel with the enamel cut at 90 or more to the occlusal table (12, 15) (Fig. 1A–D). The extension was equally balanced between the buccal and lingual/palatal orifices. Then, teeth in the TEC, CEC, and NEC groups were scanned again using CBCT imaging as described earlier. Digital Imaging and JOE — Volume 43, Number 6, June 2017
Communications in Medicine data were moved to the MeVisLab image processing and visualization platform (MeVis Research, Bremen, Germany) to perform 3-dimensional surface rendering of the teeth and segmentation of the TEC, CEC, or NEC access (Fig. 2A–F). The percentage of volume of coronal enamel and dentin removed by TEC, CEC, and NEC access cavities and the total enamel and dentin crown volume for each type of tooth were calculated.
Endodontic Treatment Root canals were negotiated with size 10 K-type files (Flexofile; Dentsply Maillefer, Ballaigues, Switzerland) to the major apical foramen, and canals were instrumented to length with Mtwo nickeltitanium rotary instruments (Sweden & Martina, Padova, Italy), with a 16-mm working part, up to the #25 tip size and 0.06 taper file. During endodontic treatment, 5.25% sodium hypochlorite (Niclor 5; Ogna, Muggio, Italy) for irrigation was intermittently deposited using ProRinse side-vented 30-G needles (Dentsply Tulsa Dental Specialties, Tulsa, OK), and after instrumentation, the root canals were irrigated with 17% EDTA solution. The canals were dried with paper points and filled with gutta-percha (single-cone size 25, 0.06 taper) and a resin-based endodontic sealer (AH Plus; Dentsply De Trey, Konstanz, Germany). Afterward, the teeth were subjected to postoperative radiographs and CBCT imaging to evaluate the endodontic treatment. Fracture Strength of Endodontically Treated Teeth
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5.3 (0.3)b 4.7 (0.5)b 5.3 (0.3)b 5.7 (0.6)b 8.2a (0.9) 7.8a (0.9) 9.5a (1.0) 10.1a (0.8) 7.9a (0.9) 7.5a (1.4) 9.8a (0.7) 10.5a (0.9) 5.0 (0.8)b 5.1 (0.4)b 5.6 (0.5)b 5.6 (0.4)b 8.0a (0.5) 7.9a (0.8) 9.5a (0.9) 10.2a (0.5) 7.9a (0.3) 7.3a (0.5) 9.9a (0.3) 10.7a (1.0) 5.3 (0.7)b 4.9 (0.3)b 5.3 (0.2)b 5.7 (0.5)b CEC, conservative endodontic cavity; NEC, ‘‘ninja’’ endodontic cavity; TEC, traditional endodontic cavity. Similar lowercase letters in the same row indicate no statistically significant differences (P > .05).
8.1a (0.7) 7.8a (0.9) 9.6a (0.9) 10.2a (0.9) 8.0a (0.8) 7.2a (1.3) 9.7a (0.5) 10.6a (1.3) 7.8a (1.6) 7.6a (0.9) 9.4a (0.7) 10.3a (0.6) 8.1a (1.0) 7.4a (1.2) 9.7a (0.6) 10.7a (1.2) Upper Premolars Lower Premolars Upper molars Lower molars
5.2 (0.7)b 4.5 (0.4)b 5.4 (0.1)b 5.8 (0.4)b
Anatomic crown height BL
Occlusal surface
MD BL MD BL MD
Occlusal surface
Anatomic crown height
Occlusal surface
Anatomic crown height
NEC CEC
Anatomic crown height BL MD
Occlusal surface
Tooth type (n = 10)
Results The mean of the buccolingual and mesiodistal dimensions at the occlusal surface and the anatomic crown height of the tested teeth are presented in Table 1. No significant differences were found when comparing all teeth dimensions in the control and test groups for each type of tooth (P > .05). Table 2 shows the mean volume percentages of the coronal enamel and dentin removed by different access cavity designs in each tooth type. The mean load at fracture for teeth in the TEC group was significantly lower than the intact, CEC, and NEC groups (P < .05), whereas no difference was observed among the control, CEC, and NEC groups (P > .05) in all types of teeth (Table 3). Intact premolars had mostly cuspal chipping, whereas those with TEC, CEC, and NEC consistently had wall fractures extending below the cementoenamel junction. Molars in all the groups had mesiodistal fractures with a varying apical extent. The restorable fractures were significantly higher than the unrestorable ones in the intact teeth (P < .05), whereas the number of unrestorable fractures was higher than the restorable ones in the TEC, CEC, and NEC groups in every type of tooth (P < .05). No difference in the number of restorable or unrestorable fractures was observed for the TEC, CEC, and NEC groups in every type of tooth (P > .05).
TEC
Statistical Analysis The data were first verified with the Kolmogorov-Smirnov test for normal distribution and the Levene test for homogeneity of variances. Thus, they were statistically evaluated using analysis of variance and the Student-Newman-Keuls test for multiple comparisons (Prism 5.0; GraphPad Software Inc, La Jolla, CA), with the significance level established at 5% (P < .05).
Control
Fracture Test The 120 teeth in the TEC, CEC, and NEC groups and the 40 teeth (n = 10/type) kept intact were mounted on brass rings with the roots embedded in self-curing resin (SR Ivolen; Ivoclar Vivadent, Schaan, Lichtenstein) up to 2 mm apical to the cementoenamel junction as reported in a previous study (19). The 160 tooth specimens were placed in a custom-made water bath and mounted in a mechanical material testing machine (LR30 K; Lloyd Instruments Ltd, Fareham, UK) (19). The teeth were loaded at their central fossa at a 30 angle from the long axis of the tooth. The continuous compressive force at a crosshead speed of 0.5 mm/min was applied using a 6-mm-diameter ball-ended steel compressive head. The loads at which the teeth were fractured, indicated by the software of the load testing machine, were recorded in newtons. The fractured specimens were examined under a stereomicroscope (SZR- 10; Optika, Bergamo, Italy) to determine the fracture levels. Fracture patterns were classified as ‘‘restorable’’ when the failures were above the level of bone simulation (site of fracture above the acrylic resin) and ‘‘unrestorable’’ when the failures were extending below the level of bone simulation (site of fracture below the acrylic resin) (20) (Fig. 3A–D).
Groups
Teeth Restoration The enamel and dentin of the access cavity were cleaned and etched with 37% phosphoric acid for 30 and 15 seconds, respectively; rinsed for 30 seconds with a water/air spray; and gently air dried to avoid desiccation. A light-polymerizing primer bond adhesive (XP Bond; Dentsply International, York, PA) was applied, gently air thinned, and exposed to light-emitting diode polymerization for 30 seconds. At the end, the access cavities were restored with direct composite restorations (CeramX mono, Dentsply International).
TABLE 1. Mean and (Standard Deviation) of the Mesiodistal (MD) and Buccolingual (BL) Dimensions and the Anatomic Crown Height (Measured at the 4 Sides of the Tooth) of the Tested Teeth in Each Group
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Basic Research—Technology fatigued tests may not reflect complete root strain patterns for the complex chewing process (22). Access cavities were restored with bonded resin composite to simulate clinical procedures and facilitate loading tests (22). Restoration of endodontic access cavities may restore the fracture strength of teeth up to 72% of that of intact teeth (22, 30). The same expert operator performed all specimen preparation procedures in order to avoid different results caused by different operator skills. In this study, the TEC group presented lower fracture strength than the control, CEC, and NEC groups. These results are in agreement with a previous study in which the teeth were tested without any restoration, which is different from clinical practice (19). The results of the present study are in agreement and corroborate reports that showed improved fracture strength of teeth because of dentin preservation obtained by cavity size reduction (9, 31, 32). In addition, no difference in the fracture strength was observed among the CEC, NEC, or control groups in all tested teeth. These results, relative to CEC, are in agreement with a previous study (19). Despite the fact that our results related to NEC cannot be directly compared with previous reports, it is not surprising that teeth in the NEC group showed no difference in fracture strength compared with the control and CEC groups because of the minimally invasive access cavity designs of NEC. However, in a recent study, the CEC cavity did not increase the fracture strength of restored maxillary molars in comparison with ones prepared with TEC, suggesting no apparent benefit of CEC in this regard (22). This contrasting finding is probably because of the differences in the methodology of that study including the type of teeth considered (only maxillary molars were reported to be subjected to fracture more than mandibular ones [33]); the techniques and materials used for endodontic and restoring procedures; and the method used to assess the fracture strength (teeth were cyclically fatigued and subsequently loaded to failure [22]). Although CEC improved fracture strength more than TEC, it could increase the risks of inefficient canal instrumentation and the occurrence of procedural errors as previously reported (19). However, a recent study showed that CECs in maxillary molars did not appear to impact instrumentation efficacy (22). No studies have investigated the quality of endodontic procedures using NEC. In addition, the ideal access cavity would allow complete removal of pulp tissue, debris, and necrotic materials. However, the smaller the access cavity, the higher the risk of bacterial contaminations and the possibility of missing some root canal orifices (22, 34). The results of the present study showed a higher number of restorable fracture patterns in intact teeth than in the ones prepared with TEC, CEC, or NEC. These findings are in agreement with a previous report (35). Furthermore, the majority of the teeth prepared with TEC, CEC, or NEC showed unrestorable fracture patterns with no significant difference among the different access cavity designs.
TABLE 2. The Volume Percentage (Mean and Standard Deviation) of the Coronal Enamel and Dentin Removed in Teeth with Different Access Cavity Designs Including Traditional, Conservative, and ‘‘Ninja’’ Access Coronal dentin and enamel volume removed (% of total crown volume) Tooth type (n = 10) Upper premolars Lower premolars Upper molars Lower molars
TEC
CEC
NEC
22.15 (3.71)a 13.43 (3.12)b 23.89 (3.04)a 15.17 (3.67)b 19.27 (3.82)a 11.03 (2.81)b 16.48 (3.47)a 7.31 (3.33)b
5.13 (0.76)c 6.07 (0.54)c 5.92 (0.75)c 4.81 (0.82)c
CEC, conservative endodontic cavity; NEC, ‘‘ninja’’ endodontic cavity; TEC, traditional endodontic cavity. Similar lowercase letters in the same row indicate no statistically significant differences (P > .05).
Discussion One of the most important causes of fractures in root-filled teeth is the loss of tooth structure. The preparation of the endodontic access cavity following the TEC principals was reported as the second largest cause of loss of tooth structure (20). Thus, a proper and reduced endodontic access design could improve the prognosis for an endodontically treated tooth (21). Recently, CEC and NEC were proposed to reduce the fracture risk in endodontically treated teeth (15, 19). However, clinically, these approaches can mainly be performed on intact teeth that are going to be treated endodontically. This clinical scenario does not seem to occur frequently, representing only 8% of the cases treated by the authors in the last 5 years (G. Plotino et al, unpublished data, 2016). Until now, in the literature, the fracture strength of teeth with CEC and NEC access was investigated in a few studies (19, 22) and no studies, respectively. For this reason, the fracture strength of endodontically treated teeth with TEC, CEC, or NEC access cavity was tested in the present study. The use of mature, intact maxillary and mandibular molars and premolars was a priority to avoid the effects of different amounts of tooth structure loss (22). Anterior teeth were not tested in this investigation because no differences between TEC and CEC fracture strength in these teeth were reported (19). Although premolars and molars are subjected to a different occlusal force in the clinical situation (23), in this study the same loading force was applied to standardize the procedure (19). Fracture resistance was assessed with a mechanical testing machine as in other studies (19, 24–26). A 30 inclination angle was used because teeth are most vulnerable to fracture when eccentric forces are applied (27), reaching the failure point at lower loads when compared with the axial fracture loads of other studies (28, 29). However, loading to fracture methodology used for in vitro analyses does not accurately reflect intraoral conditions in which failures occur primarily because of fatigue. In the same way, axial cyclically
TABLE 3. Load at Fracture (Mean Standard Deviation) and Type of Fracture, ‘‘Restorable’’ (R) or ‘‘Unrestorable’’ (U), for Intact Teeth (Control) or Traditional, Conservative, or ‘‘Ninja’’ Access Assessed after the Static Test Using a Mechanical Material Testing Machine Load at fracture (N)
Type of fracture Control
Tooth type (n = 10) Upper premolars Lower premolars Upper molars Lower molars
Control
TEC a
913 (188) 1006 (313)a 1172 (598)a 1572 (639)a
CEC b
498 (250) 704 (310)b 810 (425)b 923 (393)b
NEC a
821 (324) 929 (384)a 1143 (506)a 1401 (495)a
a
805 (204) 945 (267)a 1170 (432)a 1459 (278)a
TEC
CEC
NEC
R
U
R
U
R
U
R
U
a
b
b
a
b
a
b
7a 7a 7a 7a
7 7a 8a 7a
3 3b 2b 3b
2 3b 3b 2b
8 7a 7a 8a
3 2b 3b 2b
7 8a 7a 8a
3 3b 3b 3b
CEC, conservative endodontic cavity; NEC, ‘‘ninja’’ endodontic cavity; TEC, traditional endodontic cavity. Similar lowercase letters in the same row indicate no statistically significant differences (P > .05).
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Basic Research—Technology Within the limitations of this study, it can be concluded that conservative endodontic access cavities such as CEC and NEC increased the fracture strength of teeth compared with those with TEC. The ultraconservative NEC access did not improve the fracture strength of teeth with CEC access. Moreover, restored CEC and NEC did not reduce the fracture strength, but they did influence the fracture pattern of intact teeth. Further clinical studies are necessary to determine the efficacy of instrumentation, difficulties during endodontic procedures and longterm prognosis of endodontically treated maxillary and mandibular molars and premolars with CEC or NEC.
Acknowledgments The authors thank Giusy La Rosa from the University of Catania for the support in the sketches of teeth with CEC and NEC accesses. The authors deny any conflict of interests related to this study.
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JOE — Volume 43, Number 6, June 2017