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Fracture resistance of endodontically treated molars restored with extensive composite resin restorations
Fracture resistance of endodontically treated molars restored with extensive composite resin restorations Gianluca Plotino, DDS,a Laura Buono, DDS,b Nicola M. Grande, DDS,c Vincenzo Lamorgese, MD, DDS,d and Francesco Somma, MD, DDSe Catholic University of Sacred Heart, Rome, Italy Statement of problem. When cuspal coverage is required, there is no evidence that indirect composite resin restorations are superior to direct restorations in terms of biomechanical behavior. Purpose. The purpose of this in vitro study was to compare the fracture resistance of cusp-replacing direct and indirect composite resin restorations in endodontically treated molars. Material and methods. Forty-five human mandibular molars were selected and divided into 3 groups (n=15): DIR specimens, restored with direct composite resin (Estelite Sigma) restorations; IND specimens, restored with indirect composite resin (Estelite Sigma) restorations, and control specimens, which remained intact. Endodontic treatment was performed using NiTi ProTaper rotary instruments, and teeth were filled using lateral condensation of gutta-percha and sealer. Extensive Class II MO cavities were prepared, and the 2 mesial cusps were reduced, allowing a 2-mm layer of composite resin. All teeth were prepared to the same dimensions, considering reasonable human variation. Specimens were loaded to failure and the fracture loads were recorded (N). The mode of fracture was determined using a stereomicroscope and classified as favorable or unfavorable failure. The data were subjected to a Kruskal-Wallis test, multiple-comparison Mann-Whitney test, and a chi-square test (α=.05). Results. Significant differences (P<.001) were observed between the control group and both DIR and IND groups. However, no significant difference was found between the DIR and IND groups. The chi-square test did not show a significant difference in the frequencies of favorable/unfavorable failure modes among the 3 groups (P=.981). Conclusions. No significant difference was observed in the fracture resistance of endodontically treated molars restored to original contours with an extensive cusp-replacing direct or indirect composite resin restoration. (J Prosthet Dent 2008;99:225-232)
Clinical Implications
Within the limitations of this study, cusp-replacing direct and indirect composite resin restorations presented similar resistance to fracture under simulated occlusal loads and may be a viable treatment option for endodontically treated molars with a guarded prognosis.
Esthetic dentistry continues to evolve through innovation in bonding systems, restorative materials, and conservative preparation de-
signs. Increased use of composite resin materials for the restoration of the posterior dentition has drawn attention to technological advances in
Assistant Professor, Department of Endodontics. PhD student, School of Dentistry. c Assistant Professor, Department of Endodontics. d Private practice, Rome, Italy. e Chair and Professor, Department of Endodontics. a
b
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this field. A stable and durable bond between dental materials and tooth substrates is important from both a mechanical and esthetic perspec-
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Volume 99 Issue 3 tive.1 Such materials not only seal the margin,2 but several studies have also shown that the use of adhesive materials can reduce the weakening effect of preparation designs.3,4 In fact, cavity preparation procedures for dental restorations are a primary factor in most cuspal fractures,5 especially for endodontically treated teeth.6,7 Adhesive resin-based restorative techniques for posterior endodontically treated teeth can be either in the form of direct composite resin restorations or composite resin inlays/onlays. Each of these restorative procedures has unique advantages and disadvantages. The preferred technique is not obvious, considering that the clinical wear of composite resin inlays is expected to equal the wear resistance of direct posterior composite resin restorations.8 There is also no scientific evidence to support manufacturers’ claims that extraoral postpolymerization improves the wear characteristics and the mechanical properties of the material.9,10 Recently developed composite resins are superior to previous versions with regard to wear resistance and color stability.11,12 However, the primary shortcoming of composite resins, polymerization shrinkage, remains a concern.13 In posterior preparations, especially when the cervical margin is located in dentin, the polymerization shrinkage effects can be significant, producing marginal defects and gaps despite careful application.14 This result facilitates microleakage, which could promote secondary caries, marginal discoloration, and, in vital teeth, pulpal irritation and postoperative sensitivity.15 To minimize the development of stresses, it is important to use incremental placement techniques, in which the composite resin is applied in thin or oblique layers and then polymerized throughout the cusps.16 The composite resin inlay systems were introduced for large defects, with the aim of overcoming some of the problems associated with directly placed posterior composite resin restorations, such as the polym-
erization shrinkage that occurs when using conventional incremental techniques.17 The primary advantages of indirectly placed composite resin inlays and onlays are the minimization of polymerization stress due to the extraoral method of polymerization, better control of anatomic form and proximal contacts, and improved surface finish.12,18 However, the unresolved problem with indirectly placed inlays/onlays is the bond between the composite resin cement and the restoration.19 Adhesive systems with direct composite resin restorations provide superior bond strengths when compared to indirect restorations.20-22 Furthermore, it has been stated that direct composite resin restorations are preferred over indirect composite resins because they preserve more sound tooth structure.23 The marginal seal and fracture resistance of the restorative materials are important factors for the long-term performance of posterior composite resin restorations.24 It has been shown that the resulting weakening of the tooth due to restorative procedures increases with the reduction of tooth structure.2527 The literature is contradictory regarding the strengthening effect of bonded restorations on weakened teeth. Several in vitro studies demonstrated that directly bonded restorations increased the fracture resistance of teeth.2,3 However, others evaluated teeth restored with bonded restorations and showed fracture strengths similar to those of teeth with the same unrestored cavity preparation.28-30 Conversely, recent reports indicate that tooth-color adhesive restorative materials may be promising alternatives for cusp-replacement restorations even in endodontically treated teeth with extensive loss of tooth structure.31,32 The need for cuspal coverage has always been considered an indication for the placement of an onlay/overlay because, in this instance, extensive direct restorations are technically difficult to perform.33-35 However, when
The Journal of Prosthetic Dentistry
cuspal coverage is required, there is no evidence that indirect restorations are superior to direct restorations in terms of biomechanical behavior. 36-39 In fact, the few studies that demonstrated the effectiveness of these techniques only compared them on the basis of microleakage.40,41 The purpose of this in vitro study was to compare the fracture resistance of extensive direct and indirect composite resin restorations in endodontically treated molars. The null hypothesis tested was that there is no difference in the resistance to fracture and the mode of failure between direct and indirect composite resin restorations in endodontically treated molars prepared with an extensive loss of tooth structure.
MATERIAL AND METHODS Forty-five recently extracted human mandibular molars with completely formed apices, without caries or visible fracture lines, were selected from a tooth bank. The selection of specimens was based on the teeth having similar bucco-lingual (BL) and mesio-distal (MD) dimensions, as determined with a digital caliper (Mitutoyo, Tokyo, Japan). All external debris was removed with a hand scaler, and the teeth were stored individually in buffered saline plus 0.5% thymol (Carlo Erba, Milan, Italy) at 37°C. Cleaned specimens were carefully inspected under a stereomicroscope (Stemi SV6; Carl Zeiss SpA, Arese, Italy) at x30 magnification to detect cracks in the teeth. Specimens that did not meet the criteria were replaced. The product (mm2) of the BL and MD dimensions was determined. On the basis of this value, the 45 specimens were sequenced according to decreasing values, and alternating specimens were subsequently allocated to 3 groups of 15 teeth each, so that the average tooth size in each group was as equal as possible to minimize the influence of size and shape variations on the results. Tooth dimensions were
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March 2008 assessed with 1-way analysis of variance (ANOVA) to determine significant differences between groups. The control group contained teeth that remained intact; the teeth of the other 2 groups were subjected to the endodontic and restorative procedures. Two preliminary radiographs were made in bucco-lingual and mesio-distal directions to determine root canal anatomy. Endodontic treatment was performed using NiTi rotary instruments (ProTaper; Dentsply Maillefer, Ballaigues, Switzerland). Five percent sodium hypochlorite was used for irrigation during the endodontic treatment. A 17% EDTA solution (EDTA 17%; OGNA Laboratori Farmaceutici, Milan, Italy) was used after the last instrument, followed by a final flush with saline solution. The canals were dried with paper points (Dentsply Maillefer) and all roots were obturated with laterally condensed guttapercha (Dentsply Maillefer) and resinbased endodontic sealer (Topseal; Dentsply Maillefer). The access opening was sealed with an elastic lightpolymerizing provisional restorative material (Fermit; Ivoclar Vivadent, Schaan, Liechtenstein) to protect the endodontic filling material from leakage of the saline storage media. The teeth were then stored in buffered saline plus 0.5% thymol at 37°C for 1 week to ensure complete polymerization of the sealer. One operator made all of the preparations and restorations. The enamel and dentin of the access cavity were etched with 37% phosphoric acid (3M ESPE, St. Paul, Minn) for 40 seconds and 20 seconds, respectively, rinsed for 20 seconds with an air/water spray, and gently air-dried to avoid dessication. The primer (Scotchbond Multi-Purpose Primer; 3M ESPE) was applied with a microbrush to the tooth surface for 20 seconds and then air-dried for 5 seconds. Light-polymerizing adhesive (Scotchbond MultiPurpose Adhesive; 3M ESPE) was applied with another microbrush, the excess was gently air-thinned, and the surface was exposed to an LED-po-
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lymerization unit with an intensity of 800 mW/cm2 (Starlight pro; Mectron SpA, Carasco, Italy) for 40 seconds. A second layer of adhesive was applied with the same protocol. Subsequently, the access cavity was filled with a dual-polymerizing composite resin (Virage Dual; Sweden & Martina, Padua, Italy). Class II MO cavities were prepared with a water-cooled high-speed handpiece and a bur kit (Universal Set; Intensiv, Grancia, Switzerland) that was replaced after 5 preparations. All teeth were prepared as closely as possible to the same size using a periodontal probe and standard burs (Universal Set; Intensiv) to measure the depth
and width (Figs. 1 and 2). The pulpal floor was prepared at a depth of 4 mm from the occlusal cavosurface margin, and the 2 mesial cusps were reduced to allow for a 2-mm layer of composite resin to ensure adequate bulk. The proximal box was located 1 mm coronal to the cemento-enamel junction. Its width corresponded to one third of the distance between the buccal and lingual surfaces of the teeth at the point of the height of the contour, and its axial depth was 1.5 mm. The buccal-lingual width of the occlusal portion of the cavity preparation corresponded to two thirds of the distance between the 2 sound cusps, while the occlusal portion of
1 Schematic illustration of mesial view of preparation design. A is distance between buccal and lingual surfaces of teeth at point of maximum circumference; 1/3 A is one third of distance between buccal and lingual surfaces of teeth at point of maximum circumference.
2 Schematic illustration of occlusal view of preparation design. A is distance between buccal and lingual surfaces of teeth at point of maximum circumference; 1/3 A is one third of distance between buccal and lingual surfaces of teeth at point of maximum circumference; B is distance between 2 sound distal cusps; 2/3 B is two thirds of distance between 2 sound distal cusps.
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Volume 99 Issue 3 the cavity preparation extended mesio-distally to include the distal occlusal fossa, thus preserving the distal marginal ridge. The axial wall length was 1.5 mm. The angles of divergence of the preparation walls were approximately 5-15 degrees, and the internal line angles were rounded. Occlusal finish lines were not bevelled. Clinical and laboratory procedures were standardized as follows. Fifteen teeth were restored with direct composite resin restorations (DIR). Immediately following that, the foundation was placed and the cavity was prepared. Fifteen teeth were restored with indirect composite resin restorations (IND); a week later, the foundation was placed and the cavity was prepared. For teeth restored with direct composite resin restorations (DIR), adhesive procedures of the preparation (enamel, dentin, and foundation) were repeated as described above. A matrix retainer system (Tofflemire matrix; Miltex Inc, York, Pa) was used and changed for each restoration. The matrix was tightened and held by finger pressure against the gingival margin of the cavity, so that the preparations could not be overfilled at the gingival margin. The composite resin (Estelite Sigma; Tokuyama Dental Corp, Tokyo, Japan) was placed using the oblique incremental technique,42,43 and each increment was no more than 1.5 mm to ensure adequate polymerization. Each increment was polymerized for 40 seconds (20 seconds of slow-rise function repeated 2 times) with an LED-polymerizing unit with a power light intensity of 800 mW/ cm2 (Starlight pro; Mectron SpA) in contact with the occlusal surface of the tooth. The external layer was polymerized after placement of a glycerine gel (DeOx; Ultradent Products Inc, South Jordan, Utah) to maintain an anaerobic environment to permit complete polymerization of the resin surface. The composite resin restorations were formed with A3 shade in order to simulate dentin, and a final thin layer of 0.5-1 mm of A1 shade to
simulate enamel. The matrix was removed, and to ensure that the deepest parts of the interproximal box had been polymerized adequately, each restoration was further polymerized for 60 seconds from the buccal aspect and 60 seconds from the lingual aspect of the box. After polymerization, specimens were finished and polished with rubber cups and points (Identoflex; KerrHawe SA, Bioggio, Switzerland). For prepared teeth to be restored with indirect composite resin restorations (IND), impressions with a vinyl polysiloxane (Aquasil; Dentsply Caulk, Milford, Del) were made using a custom-made impression tray. The impressions were poured with a vacuum-mixed type IV stone (FujiRock EP; GC Italia Srl, San Giuliano Milanese, Italy) and separated from the dies after 1 hour. After separation, the cast was carefully evaluated to ensure that the finish line was entirely visible, and that there were no distortions, air bubbles, or undercuts, prior to sending the cast to the dental laboratory. The dies were coated with separating medium (Tenatex wax; Kemdent, Swindon, UK) and onlays were fabricated with the same composite resin and technique used for the direct restorations. Onlays were further polymerized in a light-heat polymerization oven (LaborluxL 300W; Micerium SpA, Avegno, Italy) for 10 minutes. Each restoration was verified for fit accuracy and adjusted accordingly, then finished with a fine diamond rotary cutting instrument (Intensiv FG; Intensiv). Both the internal surfaces of the onlays and the teeth were airborne-particle abraded with 50-µm silica-coated aluminium-oxide particles (Special sand, Kumapan; Consorzio Onda, Grugliasco, Italy). Then the teeth were treated, as previously described for the DIR specimens, with etching, primer, and bonding agents. The onlays, after the airborne-particle abrasion, were cleaned with ethyl alcohol (95% vol), and silane and bonding agents were applied. The same dualpolymerizing composite resin (Virage
The Journal of Prosthetic Dentistry
Dual; Sweden & Martina) used for the foundation procedure was used as a luting agent. The composite resin was then placed on the tooth, the onlay was seated in place, and the excess cement was removed with a brush. Cavosurface margins were coated with a glycerine gel (DeOx; Ultradent Products Inc) to permit complete polymerization of the luting agent. Each restoration, for the first 10 seconds held under load, was polymerized with an LED-polymerizing unit (Starlight pro; Mectron SpA) from the occlusal, facial, and lingual directions for 20 seconds in each direction, 3 times each (for a total of 1 minute in each direction). After complete polymerization, specimens were finished with carbide finishing burs (Dentsply Maillefer) to remove excess cement, then repolished with rubber cups and points (Identoflex; KerrHawe SA). Root surfaces were marked 3 mm below the crown margin to simulate the biologic width and covered with 0.3-mm-thick wax (Tenatex wax; Kemdent). Specimens were then embedded in autopolymerizing acrylic resin (Ortho-Jet; Lang Dental Mfg Co, Wheeling, Ill) surrounded by a cylindrical-shaped plastic mold (IKEA; Rome, Italy), with the long axis of the tooth parallel to that of the cylinder. After the first signs of polymerization, teeth were removed from the resin blocks, and the wax on the root surfaces was removed using a hand instrument. Light-body silicone-based impression material (Aquasil Ultra LV; Dentsply Caulk) was injected into the resin base, and the teeth were reinserted into the resin base. Thus, the standardized silicone layer that simulated the periodontal ligament were created.44,45 All specimens were stored in buffered saline plus 0.5% thymol (Carlo Erba) at 37°C for 1 week before undergoing the testing procedure. Specimens were mounted in a jig that allowed loading at the central fossa with a lingual orientation in the axio-occlusal line at a 15-degree angle to the long axis of the tooth. The
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March 2008 choice of this angulation was based on anatomic observation.46 Continuous compressive force at a crosshead speed of 1.6 mm/s was applied in a universal load testing machine (LR30K; Lloyd Instruments Ltd, Fareham, UK) using a 6-mm-diameter steel ball (Fig. 3). The fracture loads were determined in Newtons (N), and the modes of fracture were recorded and classified by 2 independent observers using a stereomicroscope (Stemi SV6; Carl Zeiss SpA). Favorable failures were defined as repairable failures, including adhesive failures, above the level of bone simulation. Unfavorable failures were defined as nonrepairable failures, including (vertical) root fractures, below the level of bone simulation.47 Disagreements were resolved by discussion between the 2 observers. The data were analyzed using statistical software (SPSS 11.0; SPSS Inc, Chicago, Ill). Data were subjected to a Kruskal-Wallis test to determine significant differences in failure loads among groups. When the KruskalWallis test indicated a significant difference, multiple comparisons were performed using the Mann-Whitney test to determine which group differed from the others. Percentages were determined for the mode of failure, and statistical evaluation was completed using a chi-square test to determine significant differences in the mode of failure among groups. A preset alpha level of .05 was used for all statistical analyses.
RESULTS Mean (SD) bucco-lingual and mesio-distal dimensions of the teeth were 9.94 (0.46) mm and 11.25 (0.50) mm for DIR specimens, 9.86 (0.42) and 11.12 (0.54) mm for IND specimens, and 9.97 (0.50) and 11.30 (0.53) mm for control specimens, respectively. The mean sizes of the teeth in the 3 groups were not significantly different for bucco-lingual (P=.797) or mesiodistal (P=.627) dimensions. The Kruskal-Wallis test showed
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3 Simulated occlusal loading using 6-mm-diameter steel sphere placed on central fossa with lingual orientation in axio-occlusal line at 15-degree angle to long axis of mandibular molar tooth.
Table I. Percentage (frequency) of mode of failure for all groups (n=15) Group
Favorable
Unfavorable
DIR (direct)
33% (n = 5)
67% (n = 10)
IND (indirect)
33% (n = 5)
67% (n = 10)
Intact teeth
40% (n = 6)
60% (n = 9)
that there were significant differences among the groups in their resistance to fracture under load (P=.001). The Mann-Whitney test showed significant differences (P<.001) between the control group and both DIR and IND groups. No significant difference was found between DIR and IND groups (P=.512). The specimens fractured, respectively, at a mean (SD) failure load of 1421.4 (319.5) N and 1367.8 (266.3) N. The mean fracture strength of the control group was 2451.3 (569.9) N. Teeth restored with direct and indirect restorations had a decreased fracture resistance of 42% and 44%, respectively, compared to intact teeth. Almost 65% of failures for all groups were unfavorable (DIR, 67%; IND, 67%; control group, 60%). Disagreements between the 2 independent observers were resolved by discussion for 2 specimens, because the location of the fracture line was difficult to define with respect to the level of bone simulation. The chi-square test
did not show a significant difference in frequencies of favorable/unfavorable failure modes between the 3 groups (P=.984) (Table I). All failures of the restored teeth were fractures of the composite resin restorations in combination with tooth material (cohesive failures); no purely adhesive failures were observed.
DISCUSSION The results of the present study support the null hypothesis that there is no difference in the resistance to fracture and the mode of failure between direct and indirect composite resin restorations in endodontically treated molars prepared with an extensive loss of tooth structure. Numerous studies have been conducted to determine the ideal method to restore endodontically treated teeth. Endodontic treatment is considered to weaken teeth, resulting in increased susceptibility to fracture. Consequently, authors suggest that
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Volume 99 Issue 3 cuspal coverage with cast restorations is necessary for predictable restorative success of endodontically treated posterior teeth.7 It has been shown that the resulting weakening of teeth due to restorative procedures increases with the reduction of tooth structure.25-27 According to Reeh et al,6 endodontic procedures have only a small effect on the tooth, reducing the relative rigidity by 5%, which is contributed entirely by the access opening. Restorative procedures and, particularly, the loss of marginal ridge integrity, were the greatest contributors to loss of tooth rigidity. The loss of 1 marginal ridge resulted in a 46% loss in tooth rigidity, and an MOD preparation resulted in an average loss of 63% in relative cuspal rigidity. Metal onlays and crowns have traditionally been recommended for large restorations, including cusp coverage. More recently, the use of indirect composite resin techniques has been indicated as well.34,35 However, biomechanically, there is no evidence that indirect composite resin restorations are superior to direct restorations, and there are few longitudinal studies on the clinical behavior of extensive composite resin restorations.33,36-39 Complex direct composite resin restorations exhibit durability and have been shown to have sufficient strength to withstand occlusal forces and protect the remaining tooth structure.8,33 Clinical evidence suggests that the longevity of direct composite resin posterior restorations is equal to that of indirect composite resin posterior restorations.11 Nevertheless, there is sparse long-term information concerning the longevity of cusp-replacing composite resin restorations.33 The results of the present study demonstrated that there are no differences in the in vitro fracture resistance of extensive direct and indirect composite resin restorations. These results are in agreement with those of Kuijs et al,51 who reported no differences in fracture strength between
direct and indirect composite resin restorations in premolars. Furthermore, previous studies reported no significant differences in resistance to cuspal fracture between direct posterior composite resin restorations and composite resin inlays,3,49 confirming that biomechanically, no difference exists between direct and indirect placement of composite resin. In the present study, no differences were found in the mode of failure of restored teeth. These results are in accordance with Kuijs et al,51 who reported no differences in the failure mode between direct and indirect composite resin restorations. In the present study, all failures of the restored teeth were cohesive fractures regardless of the type of restoration; no pure adhesive failures were observed. These results are somewhat in contrast with those of Kuijs et al,51 who reported more combined cohesive and adhesive fractures for indirect restorations than the direct composite resin restorations, which demonstrated more adhesive fractures. These findings corroborate the clinical findings that fracture tendency of direct composite resin restorations is similar to that of inlay/onlay restorations.8 Both direct and indirect restorations had a decrease in fracture resistance, respectively, of 42% and 44%, compared to intact teeth. These results are in agreement with other studies reporting that restored teeth had a significantly lower resistance to fracture.3,28-30 This confirms that cavity preparation reduces the rigidity of teeth and that the restorative process, even when adhesive techniques are associated with cuspal coverage, is not able to restore the resistance to load to the level of nonrestored, noncarious molars.3,6,29,50 The results of the present study suggest that the rationale for the use of direct composite resin restorations could be extended to teeth with a large amount of lost tooth structure. Although extensive restorations are technically difficult to perform using direct techniques, more expensive re-
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storative procedures may not be the first choice for treating severely damaged posterior teeth with a poor prognosis for endodontic or periodontal reasons. In these situations, extensive composite resin restorations may be placed as an intermediary restoration. The restorations may later be used as a foundation for a subsequent definitive restoration, providing increased longevity at low cost and preserving tooth structure.8,18 Considering the increased treatment time and cost to produce crowns and indirect onlays, the advantage of the direct placement of composite resin restorations may be considered as a viable treatment option for posterior endodontically treated molars with uncertain prognosis requiring coverage of 1 or 2 cusps and having the cervical margin situated in enamel. Other advantages of a direct composite resin restoration as definitive treatment for posterior endodontically treated molars include that dental laboratory support is eliminated and that the restoration is placed in a single visit. The limitations of this study must be recognized. The experimental methods used for in vitro analyses do not accurately reflect intraoral conditions. There are a number of factors that may interfere with resistance to fracture, such as the differences between specimens, tooth embedment method, type and direction of load application, crosshead speed, and simulation of thermal or mechanical fatiguing. The continually increasing load applied to the teeth in this study is not typical of the type of loading that occurs in clinical conditions, in which failures occur primarily due to fatigue. Future research in this area should use cyclic loading and other fatiguing simulation to more accurately reproduce the clinical environment. Additional clinical studies are necessary to determine the long-term prognosis for extensive direct composite resin restorations of endodontically treated molars.
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March 2008 CONCLUSIONS Within the limitations of this in vitro study, endodontically treated molars prepared with an extensive loss of tooth structure and restored to their original contours with direct composite resin restorations presented a resistance to fracture under simulated occlusal load not significantly different than that of indirect composite resin restorations. Restored teeth had a decrease in fracture resistance compared to intact teeth. Furthermore, no differences were found in the mode of failure of the restored and intact teeth.
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508. 12.Ferracane JL. Is the wear of dental composites still a clinical concern? Is there still a need for in vitro wear simulating devices? Dent Mater 2006;22:689-92. 13.Ferracane JL. Developing a more complete understanding of stresses produced in dental composites during polymerization. Dent Mater 2005;21:36-42. 14.Dietschi D, Scampa U, Campanile G, Holz J. Marginal adaptation and seal of direct and indirect Class II composite resin restorations: an in vitro evaluation. Quintessence Int 1995;26:127-38. 15.Brannstrom M. Communication between the oral cavity and the dental pulp associated with restorative treatment. Oper Dent 1984;9:57-68. 16.Nikolaenko SA, Lohbauer U, Roggendorf M, Petschelt A, Dasch W, Frankenberger R. Influence of c-factor and layering technique on microtensile bond strength to dentin. Dent Mater 2004;20:579-85. 17.Burke FJ, Watts DC, Wilson NH, Wilson MA. Current status and rationale for composite inlays and onlays. Br Dent J 1991;170:269-73. 18.Donly KJ, Jensen ME, Triolo P, Chan D. A clinical comparison of resin composite inlay and onlay posterior restorations and castgold restorations at 7 years. Quintessence Int 1999;30:163-68. 19.Hisamatsu N, Tanoue N, Yanagida H, Atsuta M, Matsumura H. Twenty-four hour bond strength between layers of a highly loaded indirect composite. Dent Mater J 2005;24:440-6. 20.Jayasooriya PR, Pereira PN, Nikaido T, Tagami J. Efficacy of a resin coating on bond strengths of resin cement to dentin. J Esthet Restor Dent 2003;15:105-13. 21.Kitasako Y, Burrow MF, Nikaido T, Harada N, Inokoshi S, Yamada T, et al. Shear and tensile bond testing for resin cement evaluation. Dent Mater 1995;11:298-304. 22.Burrow MF, Nikaido T, Satoh M, Tagami J. Early bonding of resin cements to dentin-effect of bonding environment. Oper Dent 1996;21:196-202. 23.Tyas MJ, Anusavice KJ, Frencken JE, Mount GJ. Minimal intervention dentistry--a review. FDI Commission Project 1-97. Int Dent J 2000;50:1-12. 24.Hickel R, Manhart J. Longevity of restorations in posterior teeth and reasons for failure. J Adhes Dent 2001;3:45-64. 25.Fokkinga WA, Kreulen CM, Vallittu PK, Creugers NH. A structured analysis of in vitro failure loads and failure modes of fiber, metal, and ceramic post-and-core systems. Int J Prosthodont 2004;17:476–82. 26.Panitvisai P, Messer HH. Cuspal deflection in molars in relation to endodontic and restorative procedures. J Endod 1995;21:5761. 27.Mondelli J, Steagall L, Ishikiriama A, de Lima Navarro MF, Soares FB. Fracture strength of human teeth with cavity preparations. J Prosthet Dent 1980;43:419-22. 28.Stampalia LL, Nicholls JI, Brudvik JS, Jones DW. Fracture resistance of teeth with resin-bonded restorations. J Prosthet Dent 1986;55:694-8. 29.St-Georges AJ, Sturdevant JR, Swift EJ Jr,
Thompson JY. Fracture resistance of prepared teeth restored with bonded inlay restorations. J Prosthet Dent 2003;89:551-7. 30.Allara FW Jr, Diefenderfer KE, Molinaro JD. Effect of three direct restorative materials on molar cuspal fracture resistance. Am J Dent 2004;17:228-32. 31.Krejci I, Duc O, Dietschi D, de Campos E. Marginal adaptation, retention and fracture resistance of adhesive composite restorations on devital teeth with and without posts. Oper Dent 2003;28:127-35. 32.Van Meerbeek B, Vargas M, Inoue S, Yoshida Y, Peumans M, Lambrechts P. Adhesives and cements to promote preservation dentistry. Oper Dent 2001;26(Suppl 6):119-44. 33.Van Nieuwenhuysen JP, D’Hoore W, Carvalho J, Qvist V. Long-term evaluation of extensive restorations in permanent teeth. J Dent 2003;31:395-405. 34.Magne P, Dietschi D, Holz J. Esthetic restorations for posterior teeth: practical and clinical considerations. Int J Periodontics Restorative Dent 1996;16:104-19. 35.Blank JT. Scientifically based rationale and protocol for use of modern indirect resin inlays and onlays. J Esthet Dent 2000;12:195-208. 36.Plasmans PJ, Creugers NH, Mulder J. Longterm survival of extensive amalgam restorations. J Dent Res 1998;77:453-60. 37.Robbins JW, Summit JB. Longevity of complex amalgam restorations. Oper Dent 1988;13:54-7. 38.Smales RJ. Longevity of cusp-covered amalgams: survivals after 15 years. Oper Dent 1991;16:17-20. 39.Smales RJ, Hawthorne WS. Long-term survival of extensive amalgams and posterior crowns. J Dent 1997;25:225-7. 40.Pioch T, Staehle HJ, Duschner H, GarciaGodoy F. Nanoleakage at the composite-dentin interface: a review. Am J Dent 2001;14:252-8. 41.Alani AH, Toh CG. Detection of microleakage around dental restorations: a review. Oper Dent 1997;22:173-85. 42.Liebenberg WH. Successive cusp build-up: an improved placement technique for posterior direct resin restorations. J Can Dent Assoc 1996;62:501-7. 43.Liebenberg WH. The axial bevel technique: a new technique for extensive posterior resin composite restorations. Quintessence Int 2000;31:231-9. 44.Ottl P, Hahn L, Lauer HCh, Fay M. Fracture characteristics of carbon fibre, ceramic and non-palladium endodontic post systems at monotonously increasing loads. J Oral Rehab 2002;29:175–83. 45.Heydecke G, Butz F, Hussein A, Strub JR. Fracture strength after dynamic loading of endodontically treated teeth restored with different post-and-core systems. J Prosthet Dent 2002;87:438–45. 46.Okeson JP. Management of temporomandibular disorders and occlusion. 3rd ed. St Louis: Elsevier Health Sciences; 1993. p.69-71. 47.Larson TD, Douglas WH, Geistfeld RE. Effect of prepared cavities on the strength of teeth. Oper Dent 1981;6:2-5. 48.Kuijs RH, Fennis WM, Kreulen CM, Roeters
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Volume 99 Issue 3 FJ, Verdonschot N, Creugers NH. A comparison of fatigue resistance of three materials for cusp-replacing adhesive restorations. J Dent 2006;34:19-25. Epub 2005 Jun 2. 49.Dalpino PH, Francischone CE, Ishikiriama A, Franco EB. Fracture resistance of teeth directly and indirectly restored with com-
posite resin and indirectly restored with ceramic materials. Am J Dent 2002;15:38994. 50.Reeh ES, Douglas WH, Messer HH. Stiffness of endodontically-treated teeth related to restoration technique. J Dent Res 1989;68:1540-4.
Corresponding author: Dr Gianluca Plotino Via Eleonora Duse, 22 00197 Rome ITALY Fax: +39068072289 E-mail: [email protected] Copyright © 2008 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Clinical Implications
Within the limitations of this study, cusp-replacing direct and indirect composite resin restorations presented similar resistance to fracture under simulated occlusal loads and may be a viable treatment option for endodontically treated molars with a guarded prognosis.
Esthetic dentistry continues to evolve through innovation in bonding systems, restorative materials, and conservative preparation de-
signs. Increased use of composite resin materials for the restoration of the posterior dentition has drawn attention to technological advances in
Assistant Professor, Department of Endodontics. PhD student, School of Dentistry. c Assistant Professor, Department of Endodontics. d Private practice, Rome, Italy. e Chair and Professor, Department of Endodontics. a
b
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this field. A stable and durable bond between dental materials and tooth substrates is important from both a mechanical and esthetic perspec-
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Volume 99 Issue 3 tive.1 Such materials not only seal the margin,2 but several studies have also shown that the use of adhesive materials can reduce the weakening effect of preparation designs.3,4 In fact, cavity preparation procedures for dental restorations are a primary factor in most cuspal fractures,5 especially for endodontically treated teeth.6,7 Adhesive resin-based restorative techniques for posterior endodontically treated teeth can be either in the form of direct composite resin restorations or composite resin inlays/onlays. Each of these restorative procedures has unique advantages and disadvantages. The preferred technique is not obvious, considering that the clinical wear of composite resin inlays is expected to equal the wear resistance of direct posterior composite resin restorations.8 There is also no scientific evidence to support manufacturers’ claims that extraoral postpolymerization improves the wear characteristics and the mechanical properties of the material.9,10 Recently developed composite resins are superior to previous versions with regard to wear resistance and color stability.11,12 However, the primary shortcoming of composite resins, polymerization shrinkage, remains a concern.13 In posterior preparations, especially when the cervical margin is located in dentin, the polymerization shrinkage effects can be significant, producing marginal defects and gaps despite careful application.14 This result facilitates microleakage, which could promote secondary caries, marginal discoloration, and, in vital teeth, pulpal irritation and postoperative sensitivity.15 To minimize the development of stresses, it is important to use incremental placement techniques, in which the composite resin is applied in thin or oblique layers and then polymerized throughout the cusps.16 The composite resin inlay systems were introduced for large defects, with the aim of overcoming some of the problems associated with directly placed posterior composite resin restorations, such as the polym-
erization shrinkage that occurs when using conventional incremental techniques.17 The primary advantages of indirectly placed composite resin inlays and onlays are the minimization of polymerization stress due to the extraoral method of polymerization, better control of anatomic form and proximal contacts, and improved surface finish.12,18 However, the unresolved problem with indirectly placed inlays/onlays is the bond between the composite resin cement and the restoration.19 Adhesive systems with direct composite resin restorations provide superior bond strengths when compared to indirect restorations.20-22 Furthermore, it has been stated that direct composite resin restorations are preferred over indirect composite resins because they preserve more sound tooth structure.23 The marginal seal and fracture resistance of the restorative materials are important factors for the long-term performance of posterior composite resin restorations.24 It has been shown that the resulting weakening of the tooth due to restorative procedures increases with the reduction of tooth structure.2527 The literature is contradictory regarding the strengthening effect of bonded restorations on weakened teeth. Several in vitro studies demonstrated that directly bonded restorations increased the fracture resistance of teeth.2,3 However, others evaluated teeth restored with bonded restorations and showed fracture strengths similar to those of teeth with the same unrestored cavity preparation.28-30 Conversely, recent reports indicate that tooth-color adhesive restorative materials may be promising alternatives for cusp-replacement restorations even in endodontically treated teeth with extensive loss of tooth structure.31,32 The need for cuspal coverage has always been considered an indication for the placement of an onlay/overlay because, in this instance, extensive direct restorations are technically difficult to perform.33-35 However, when
The Journal of Prosthetic Dentistry
cuspal coverage is required, there is no evidence that indirect restorations are superior to direct restorations in terms of biomechanical behavior. 36-39 In fact, the few studies that demonstrated the effectiveness of these techniques only compared them on the basis of microleakage.40,41 The purpose of this in vitro study was to compare the fracture resistance of extensive direct and indirect composite resin restorations in endodontically treated molars. The null hypothesis tested was that there is no difference in the resistance to fracture and the mode of failure between direct and indirect composite resin restorations in endodontically treated molars prepared with an extensive loss of tooth structure.
MATERIAL AND METHODS Forty-five recently extracted human mandibular molars with completely formed apices, without caries or visible fracture lines, were selected from a tooth bank. The selection of specimens was based on the teeth having similar bucco-lingual (BL) and mesio-distal (MD) dimensions, as determined with a digital caliper (Mitutoyo, Tokyo, Japan). All external debris was removed with a hand scaler, and the teeth were stored individually in buffered saline plus 0.5% thymol (Carlo Erba, Milan, Italy) at 37°C. Cleaned specimens were carefully inspected under a stereomicroscope (Stemi SV6; Carl Zeiss SpA, Arese, Italy) at x30 magnification to detect cracks in the teeth. Specimens that did not meet the criteria were replaced. The product (mm2) of the BL and MD dimensions was determined. On the basis of this value, the 45 specimens were sequenced according to decreasing values, and alternating specimens were subsequently allocated to 3 groups of 15 teeth each, so that the average tooth size in each group was as equal as possible to minimize the influence of size and shape variations on the results. Tooth dimensions were
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March 2008 assessed with 1-way analysis of variance (ANOVA) to determine significant differences between groups. The control group contained teeth that remained intact; the teeth of the other 2 groups were subjected to the endodontic and restorative procedures. Two preliminary radiographs were made in bucco-lingual and mesio-distal directions to determine root canal anatomy. Endodontic treatment was performed using NiTi rotary instruments (ProTaper; Dentsply Maillefer, Ballaigues, Switzerland). Five percent sodium hypochlorite was used for irrigation during the endodontic treatment. A 17% EDTA solution (EDTA 17%; OGNA Laboratori Farmaceutici, Milan, Italy) was used after the last instrument, followed by a final flush with saline solution. The canals were dried with paper points (Dentsply Maillefer) and all roots were obturated with laterally condensed guttapercha (Dentsply Maillefer) and resinbased endodontic sealer (Topseal; Dentsply Maillefer). The access opening was sealed with an elastic lightpolymerizing provisional restorative material (Fermit; Ivoclar Vivadent, Schaan, Liechtenstein) to protect the endodontic filling material from leakage of the saline storage media. The teeth were then stored in buffered saline plus 0.5% thymol at 37°C for 1 week to ensure complete polymerization of the sealer. One operator made all of the preparations and restorations. The enamel and dentin of the access cavity were etched with 37% phosphoric acid (3M ESPE, St. Paul, Minn) for 40 seconds and 20 seconds, respectively, rinsed for 20 seconds with an air/water spray, and gently air-dried to avoid dessication. The primer (Scotchbond Multi-Purpose Primer; 3M ESPE) was applied with a microbrush to the tooth surface for 20 seconds and then air-dried for 5 seconds. Light-polymerizing adhesive (Scotchbond MultiPurpose Adhesive; 3M ESPE) was applied with another microbrush, the excess was gently air-thinned, and the surface was exposed to an LED-po-
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lymerization unit with an intensity of 800 mW/cm2 (Starlight pro; Mectron SpA, Carasco, Italy) for 40 seconds. A second layer of adhesive was applied with the same protocol. Subsequently, the access cavity was filled with a dual-polymerizing composite resin (Virage Dual; Sweden & Martina, Padua, Italy). Class II MO cavities were prepared with a water-cooled high-speed handpiece and a bur kit (Universal Set; Intensiv, Grancia, Switzerland) that was replaced after 5 preparations. All teeth were prepared as closely as possible to the same size using a periodontal probe and standard burs (Universal Set; Intensiv) to measure the depth
and width (Figs. 1 and 2). The pulpal floor was prepared at a depth of 4 mm from the occlusal cavosurface margin, and the 2 mesial cusps were reduced to allow for a 2-mm layer of composite resin to ensure adequate bulk. The proximal box was located 1 mm coronal to the cemento-enamel junction. Its width corresponded to one third of the distance between the buccal and lingual surfaces of the teeth at the point of the height of the contour, and its axial depth was 1.5 mm. The buccal-lingual width of the occlusal portion of the cavity preparation corresponded to two thirds of the distance between the 2 sound cusps, while the occlusal portion of
1 Schematic illustration of mesial view of preparation design. A is distance between buccal and lingual surfaces of teeth at point of maximum circumference; 1/3 A is one third of distance between buccal and lingual surfaces of teeth at point of maximum circumference.
2 Schematic illustration of occlusal view of preparation design. A is distance between buccal and lingual surfaces of teeth at point of maximum circumference; 1/3 A is one third of distance between buccal and lingual surfaces of teeth at point of maximum circumference; B is distance between 2 sound distal cusps; 2/3 B is two thirds of distance between 2 sound distal cusps.
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Volume 99 Issue 3 the cavity preparation extended mesio-distally to include the distal occlusal fossa, thus preserving the distal marginal ridge. The axial wall length was 1.5 mm. The angles of divergence of the preparation walls were approximately 5-15 degrees, and the internal line angles were rounded. Occlusal finish lines were not bevelled. Clinical and laboratory procedures were standardized as follows. Fifteen teeth were restored with direct composite resin restorations (DIR). Immediately following that, the foundation was placed and the cavity was prepared. Fifteen teeth were restored with indirect composite resin restorations (IND); a week later, the foundation was placed and the cavity was prepared. For teeth restored with direct composite resin restorations (DIR), adhesive procedures of the preparation (enamel, dentin, and foundation) were repeated as described above. A matrix retainer system (Tofflemire matrix; Miltex Inc, York, Pa) was used and changed for each restoration. The matrix was tightened and held by finger pressure against the gingival margin of the cavity, so that the preparations could not be overfilled at the gingival margin. The composite resin (Estelite Sigma; Tokuyama Dental Corp, Tokyo, Japan) was placed using the oblique incremental technique,42,43 and each increment was no more than 1.5 mm to ensure adequate polymerization. Each increment was polymerized for 40 seconds (20 seconds of slow-rise function repeated 2 times) with an LED-polymerizing unit with a power light intensity of 800 mW/ cm2 (Starlight pro; Mectron SpA) in contact with the occlusal surface of the tooth. The external layer was polymerized after placement of a glycerine gel (DeOx; Ultradent Products Inc, South Jordan, Utah) to maintain an anaerobic environment to permit complete polymerization of the resin surface. The composite resin restorations were formed with A3 shade in order to simulate dentin, and a final thin layer of 0.5-1 mm of A1 shade to
simulate enamel. The matrix was removed, and to ensure that the deepest parts of the interproximal box had been polymerized adequately, each restoration was further polymerized for 60 seconds from the buccal aspect and 60 seconds from the lingual aspect of the box. After polymerization, specimens were finished and polished with rubber cups and points (Identoflex; KerrHawe SA, Bioggio, Switzerland). For prepared teeth to be restored with indirect composite resin restorations (IND), impressions with a vinyl polysiloxane (Aquasil; Dentsply Caulk, Milford, Del) were made using a custom-made impression tray. The impressions were poured with a vacuum-mixed type IV stone (FujiRock EP; GC Italia Srl, San Giuliano Milanese, Italy) and separated from the dies after 1 hour. After separation, the cast was carefully evaluated to ensure that the finish line was entirely visible, and that there were no distortions, air bubbles, or undercuts, prior to sending the cast to the dental laboratory. The dies were coated with separating medium (Tenatex wax; Kemdent, Swindon, UK) and onlays were fabricated with the same composite resin and technique used for the direct restorations. Onlays were further polymerized in a light-heat polymerization oven (LaborluxL 300W; Micerium SpA, Avegno, Italy) for 10 minutes. Each restoration was verified for fit accuracy and adjusted accordingly, then finished with a fine diamond rotary cutting instrument (Intensiv FG; Intensiv). Both the internal surfaces of the onlays and the teeth were airborne-particle abraded with 50-µm silica-coated aluminium-oxide particles (Special sand, Kumapan; Consorzio Onda, Grugliasco, Italy). Then the teeth were treated, as previously described for the DIR specimens, with etching, primer, and bonding agents. The onlays, after the airborne-particle abrasion, were cleaned with ethyl alcohol (95% vol), and silane and bonding agents were applied. The same dualpolymerizing composite resin (Virage
The Journal of Prosthetic Dentistry
Dual; Sweden & Martina) used for the foundation procedure was used as a luting agent. The composite resin was then placed on the tooth, the onlay was seated in place, and the excess cement was removed with a brush. Cavosurface margins were coated with a glycerine gel (DeOx; Ultradent Products Inc) to permit complete polymerization of the luting agent. Each restoration, for the first 10 seconds held under load, was polymerized with an LED-polymerizing unit (Starlight pro; Mectron SpA) from the occlusal, facial, and lingual directions for 20 seconds in each direction, 3 times each (for a total of 1 minute in each direction). After complete polymerization, specimens were finished with carbide finishing burs (Dentsply Maillefer) to remove excess cement, then repolished with rubber cups and points (Identoflex; KerrHawe SA). Root surfaces were marked 3 mm below the crown margin to simulate the biologic width and covered with 0.3-mm-thick wax (Tenatex wax; Kemdent). Specimens were then embedded in autopolymerizing acrylic resin (Ortho-Jet; Lang Dental Mfg Co, Wheeling, Ill) surrounded by a cylindrical-shaped plastic mold (IKEA; Rome, Italy), with the long axis of the tooth parallel to that of the cylinder. After the first signs of polymerization, teeth were removed from the resin blocks, and the wax on the root surfaces was removed using a hand instrument. Light-body silicone-based impression material (Aquasil Ultra LV; Dentsply Caulk) was injected into the resin base, and the teeth were reinserted into the resin base. Thus, the standardized silicone layer that simulated the periodontal ligament were created.44,45 All specimens were stored in buffered saline plus 0.5% thymol (Carlo Erba) at 37°C for 1 week before undergoing the testing procedure. Specimens were mounted in a jig that allowed loading at the central fossa with a lingual orientation in the axio-occlusal line at a 15-degree angle to the long axis of the tooth. The
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March 2008 choice of this angulation was based on anatomic observation.46 Continuous compressive force at a crosshead speed of 1.6 mm/s was applied in a universal load testing machine (LR30K; Lloyd Instruments Ltd, Fareham, UK) using a 6-mm-diameter steel ball (Fig. 3). The fracture loads were determined in Newtons (N), and the modes of fracture were recorded and classified by 2 independent observers using a stereomicroscope (Stemi SV6; Carl Zeiss SpA). Favorable failures were defined as repairable failures, including adhesive failures, above the level of bone simulation. Unfavorable failures were defined as nonrepairable failures, including (vertical) root fractures, below the level of bone simulation.47 Disagreements were resolved by discussion between the 2 observers. The data were analyzed using statistical software (SPSS 11.0; SPSS Inc, Chicago, Ill). Data were subjected to a Kruskal-Wallis test to determine significant differences in failure loads among groups. When the KruskalWallis test indicated a significant difference, multiple comparisons were performed using the Mann-Whitney test to determine which group differed from the others. Percentages were determined for the mode of failure, and statistical evaluation was completed using a chi-square test to determine significant differences in the mode of failure among groups. A preset alpha level of .05 was used for all statistical analyses.
RESULTS Mean (SD) bucco-lingual and mesio-distal dimensions of the teeth were 9.94 (0.46) mm and 11.25 (0.50) mm for DIR specimens, 9.86 (0.42) and 11.12 (0.54) mm for IND specimens, and 9.97 (0.50) and 11.30 (0.53) mm for control specimens, respectively. The mean sizes of the teeth in the 3 groups were not significantly different for bucco-lingual (P=.797) or mesiodistal (P=.627) dimensions. The Kruskal-Wallis test showed
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3 Simulated occlusal loading using 6-mm-diameter steel sphere placed on central fossa with lingual orientation in axio-occlusal line at 15-degree angle to long axis of mandibular molar tooth.
Table I. Percentage (frequency) of mode of failure for all groups (n=15) Group
Favorable
Unfavorable
DIR (direct)
33% (n = 5)
67% (n = 10)
IND (indirect)
33% (n = 5)
67% (n = 10)
Intact teeth
40% (n = 6)
60% (n = 9)
that there were significant differences among the groups in their resistance to fracture under load (P=.001). The Mann-Whitney test showed significant differences (P<.001) between the control group and both DIR and IND groups. No significant difference was found between DIR and IND groups (P=.512). The specimens fractured, respectively, at a mean (SD) failure load of 1421.4 (319.5) N and 1367.8 (266.3) N. The mean fracture strength of the control group was 2451.3 (569.9) N. Teeth restored with direct and indirect restorations had a decreased fracture resistance of 42% and 44%, respectively, compared to intact teeth. Almost 65% of failures for all groups were unfavorable (DIR, 67%; IND, 67%; control group, 60%). Disagreements between the 2 independent observers were resolved by discussion for 2 specimens, because the location of the fracture line was difficult to define with respect to the level of bone simulation. The chi-square test
did not show a significant difference in frequencies of favorable/unfavorable failure modes between the 3 groups (P=.984) (Table I). All failures of the restored teeth were fractures of the composite resin restorations in combination with tooth material (cohesive failures); no purely adhesive failures were observed.
DISCUSSION The results of the present study support the null hypothesis that there is no difference in the resistance to fracture and the mode of failure between direct and indirect composite resin restorations in endodontically treated molars prepared with an extensive loss of tooth structure. Numerous studies have been conducted to determine the ideal method to restore endodontically treated teeth. Endodontic treatment is considered to weaken teeth, resulting in increased susceptibility to fracture. Consequently, authors suggest that
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Volume 99 Issue 3 cuspal coverage with cast restorations is necessary for predictable restorative success of endodontically treated posterior teeth.7 It has been shown that the resulting weakening of teeth due to restorative procedures increases with the reduction of tooth structure.25-27 According to Reeh et al,6 endodontic procedures have only a small effect on the tooth, reducing the relative rigidity by 5%, which is contributed entirely by the access opening. Restorative procedures and, particularly, the loss of marginal ridge integrity, were the greatest contributors to loss of tooth rigidity. The loss of 1 marginal ridge resulted in a 46% loss in tooth rigidity, and an MOD preparation resulted in an average loss of 63% in relative cuspal rigidity. Metal onlays and crowns have traditionally been recommended for large restorations, including cusp coverage. More recently, the use of indirect composite resin techniques has been indicated as well.34,35 However, biomechanically, there is no evidence that indirect composite resin restorations are superior to direct restorations, and there are few longitudinal studies on the clinical behavior of extensive composite resin restorations.33,36-39 Complex direct composite resin restorations exhibit durability and have been shown to have sufficient strength to withstand occlusal forces and protect the remaining tooth structure.8,33 Clinical evidence suggests that the longevity of direct composite resin posterior restorations is equal to that of indirect composite resin posterior restorations.11 Nevertheless, there is sparse long-term information concerning the longevity of cusp-replacing composite resin restorations.33 The results of the present study demonstrated that there are no differences in the in vitro fracture resistance of extensive direct and indirect composite resin restorations. These results are in agreement with those of Kuijs et al,51 who reported no differences in fracture strength between
direct and indirect composite resin restorations in premolars. Furthermore, previous studies reported no significant differences in resistance to cuspal fracture between direct posterior composite resin restorations and composite resin inlays,3,49 confirming that biomechanically, no difference exists between direct and indirect placement of composite resin. In the present study, no differences were found in the mode of failure of restored teeth. These results are in accordance with Kuijs et al,51 who reported no differences in the failure mode between direct and indirect composite resin restorations. In the present study, all failures of the restored teeth were cohesive fractures regardless of the type of restoration; no pure adhesive failures were observed. These results are somewhat in contrast with those of Kuijs et al,51 who reported more combined cohesive and adhesive fractures for indirect restorations than the direct composite resin restorations, which demonstrated more adhesive fractures. These findings corroborate the clinical findings that fracture tendency of direct composite resin restorations is similar to that of inlay/onlay restorations.8 Both direct and indirect restorations had a decrease in fracture resistance, respectively, of 42% and 44%, compared to intact teeth. These results are in agreement with other studies reporting that restored teeth had a significantly lower resistance to fracture.3,28-30 This confirms that cavity preparation reduces the rigidity of teeth and that the restorative process, even when adhesive techniques are associated with cuspal coverage, is not able to restore the resistance to load to the level of nonrestored, noncarious molars.3,6,29,50 The results of the present study suggest that the rationale for the use of direct composite resin restorations could be extended to teeth with a large amount of lost tooth structure. Although extensive restorations are technically difficult to perform using direct techniques, more expensive re-
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storative procedures may not be the first choice for treating severely damaged posterior teeth with a poor prognosis for endodontic or periodontal reasons. In these situations, extensive composite resin restorations may be placed as an intermediary restoration. The restorations may later be used as a foundation for a subsequent definitive restoration, providing increased longevity at low cost and preserving tooth structure.8,18 Considering the increased treatment time and cost to produce crowns and indirect onlays, the advantage of the direct placement of composite resin restorations may be considered as a viable treatment option for posterior endodontically treated molars with uncertain prognosis requiring coverage of 1 or 2 cusps and having the cervical margin situated in enamel. Other advantages of a direct composite resin restoration as definitive treatment for posterior endodontically treated molars include that dental laboratory support is eliminated and that the restoration is placed in a single visit. The limitations of this study must be recognized. The experimental methods used for in vitro analyses do not accurately reflect intraoral conditions. There are a number of factors that may interfere with resistance to fracture, such as the differences between specimens, tooth embedment method, type and direction of load application, crosshead speed, and simulation of thermal or mechanical fatiguing. The continually increasing load applied to the teeth in this study is not typical of the type of loading that occurs in clinical conditions, in which failures occur primarily due to fatigue. Future research in this area should use cyclic loading and other fatiguing simulation to more accurately reproduce the clinical environment. Additional clinical studies are necessary to determine the long-term prognosis for extensive direct composite resin restorations of endodontically treated molars.
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March 2008 CONCLUSIONS Within the limitations of this in vitro study, endodontically treated molars prepared with an extensive loss of tooth structure and restored to their original contours with direct composite resin restorations presented a resistance to fracture under simulated occlusal load not significantly different than that of indirect composite resin restorations. Restored teeth had a decrease in fracture resistance compared to intact teeth. Furthermore, no differences were found in the mode of failure of the restored and intact teeth.
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Corresponding author: Dr Gianluca Plotino Via Eleonora Duse, 22 00197 Rome ITALY Fax: +39068072289 E-mail: [email protected] Copyright © 2008 by the Editorial Council for The Journal of Prosthetic Dentistry.
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