RESEARCH AND EDUCATIONEffects of different cusp coverage restorations on the fracture resistance of endodontically treated maxillary premolars Tugba Serin Kalay, DDS, PhD,a Tahsin Yildirim, DDS, PhD,b and Mustafa Ulker, DDS, PhDc Endodontically treated teeth ABSTRACT (ETTs) are believed to be weak Statement of problem. Cusp coverage restorations for the restoration of endodontically treated and brittle because of the teeth represent a more conservative approach in terms of function and esthetics. However, limited extensive loss of coronal and scientific data are available regarding the optimum reduction design and thicknesses. radicular tooth structure.1-3 This Purpose. The purpose of this in vitro study was to evaluate the fracture resistance and fracture decreases fracture resistance patterns of cusp coverage restorations with different cusp reduction designs and reduction thickand increases cusp deflection nesses on endodontically treated maxillary premolars (ETMPs) with mesio-occluso-distal (MOD) under functional forces. Endcavities. odontic access cavities comMaterial and methods. One hundred sixty-five extracted intact human maxillary premolars were bined with mesio-occluso-distal divided into 11 groups: G1-10 (test groups) and G11 (intact group). In the test groups, all of the (MOD) cavity preparations teeth were restored with composite resin after canal treatments with MOD cavities. However, the dramatically increase cuspal cusps of the G1-9 teeth were reduced with combinations of different thicknesses (1.5, 2.5, and 3.5 mm) and designs (beveled, horizontal, and anatomic). The specimens were subjected to 105 deflection and lead to increased 4,5 cycles of 50 N mechanical loading. Next, the specimens were subjected to a compressive load at tooth fragility. a crosshead speed of 0.5 mm/min until fracture. The fractured specimens were analyzed to Clinical surveys show that determine the fracture pattern. Two-way ANOVA followed by the Fisher least significant ETTs with ideal coronal restodifference (LSD) test was used to analyze the interaction between groups. rations are crucial for long-term Results. The fracture resistance values increased with increases in the cusp reductions. The fracture clinic success. The majority of resistance values of the G6 (2.5 mm, anatomic) and G9 (3.5 mm, anatomic) groups were significantly ETTs are lost because of coronal greater than that of the MOD group. However, the G6, G8 (3.5 mm, horizontal), and G9 groups were restoration failures in the longcomparable with G11. The highest restorable fracture rates were observed in G6 and G9. 6 term. An ideal coronal restoConclusions. Cusp reduction design and thickness influenced the fracture resistance and fracture ration preserves the root canal patterns of cusp coverage restorations of ETMPs with MOD cavities. The teeth restored with system, supports the remaining anatomic cusp reduction designs with reduction thicknesses of at least 2.5 mm exhibited greater tooth structure, and restores fracture resistance and more frequent restorable fractures. (J Prosthet Dent 2016;-:---) 7 tooth function. Restorations of ETTs with adhesive techniques may conserve more tooth structure adhesive cusp coverage restorations increase the fracture and provide sufficient fracture strength.8-11 However, the resistance of endodontically treated posterior teeth direct composite resin restoration of extensive cavities compared with direct MOD composite resin restorations. cannot establish tooth strength, and a reliable alternative An early study reported that cusp reductions should be for restoration may be needed.12-14 at least 1.5 mm to reduce the stress values on the cusp Recently, several in vitro studies15-22 and clinical recoverage adhesive restorations,25 and later studies related 14,23,24 ports to cusp coverage restorations applied 1.5 to 3.5 mm have found that in the protection of cavities, a Research Assistant, Department of Restorative Dentistry, Faculty of Dentistry, Karadeniz Technical University, Trabzon, Turkey. Professor, Department of Restorative Dentistry, Faculty of Dentistry, Karadeniz Technical University, Trabzon, Turkey. c Associate Professor, Department of Restorative Dentistry, Faculty of Dentistry, Selcuk University, Konya, Turkey. b THE JOURNAL OF PROSTHETIC DENTISTRY 1 53 1184. Teeth prepared with anatomic cusp reduction designs and thicknesses of at least 2.07 G2 1.47 1621.34 ±139. The mesiodistal and buccolingual tooth sizes were measured with digital calipers.5-mm horizontal reduction 837. The working lengths were determined by subtracting 1 mm from this length. Caries-free human maxillary premolars with two roots and two canals (extracted for orthodontic reasons) were used.80 ±455.17 ±188.30 ±173.81 1228. Rotary instruments were used with a torque-limited engine (X-Smart.5-mm horizontal reduction 1039.24 ±207. an impression of each tooth was made with a heavy-body silicone impression material (Optosil.37 ±235.28 These fractures cannot be restored.5-mm beveled reduction 870. Fracture loads (N) Group (G) (n=15) MOD Cavity Characteristics Fracture Loads (N) Mean ±SD Minimum Maximum G1 1. This study protocol was approved by the Ethical Research Committee (2014. and all procedures were performed by 1 researcher (T.64 1009. and the majority of these situations result in the extraction of the tooth.5-mm beveled reduction 961.97 ±199.27 Unrestorable subgingival or vertical cusp fractures are observed more commonly in ETTs with extensive intracoronal restorations.36 G9 3. fractures. and the restorative procedures.29.b 599. the cavity design.62 G5 2.82 ±216.64 G8 3.05). Dentsply Maillefer).144) of the Karadeniz Technical University.88b 694. and teeth with similar dimensions were selected.76b 449.5-mm anatomic reduction 1110. diamond rotary cutting instruments (Dentsply Maillefer) were used in a high-speed handpiece under copious airwater cooling. Standardized canal enlargements were performed with an engine-driven rotary NiTi system up to F3 for the apical preparation using a crown-down technique.09b 602.40 G4 2.31-34 The purpose of this study was to determine the influences of various cusp reduction designs and thicknesses on the in vitro fracture resistances of ETMPs with MOD cavities to identify the optimum reduction thickness and design and to evaluate whether these various cavity designs affect the restorable fracture rate. limited scientific data are available regarding the optimum reduction thickness and the effects of different cusp reduction designs on the tooth-restoration complex. size 15 K files (Dentsply Maillefer) were inserted until their tips could be seen at the apical foramen.81 G10 No reduction 777.5-mm beveled reduction 851. Additionally. The null hypothesis tested was that the reduction thickness and design would have no influence on the fracture resistance and fracture pattern of the cusp coverage restoration of endodontically treated maxillary premolars with MOD cavities.75b 524. Dentsply Maillefer).15.75a 792.3. THE JOURNAL OF PROSTHETIC DENTISTRY Serin Kalay et al . Endodontic and restorative procedures were performed in groups 1 to 10.2 Volume - Issue - MATERIAL AND METHODS Clinical Implications Adhesive cusp coverage MOD restorations are a more conservative treatment than traditional crowns for endodontically treated maxillary premolar teeth.05a 751.). Before the preparations.5-mm horizontal reduction 885. For the endodontic cavity preparations. cuspal reductions with a single type of reduction design17-19 or applied different reduction designs with a single reduction thickness.49 1264. The selected teeth were assigned to 11 groups of 15 teeth each (Table 1) and were stored in 0.47 G3 1.13b 555.21 G7 3.K.25a. The cavity dimensions were measured with digital calipers.2% sodium azide solution.00 1343. or defects and internal-external resorption were excluded from the study.64 2194.26 However. The buccal and lingual walls of the occlusal isthmus were prepared parallel to each other.5-mm anatomic reduction 906.79 1570.10 ±262.28 ±214.79b 434. cusp fractures are among the major reasons for the extractions of ETTs. Heraeus Kulzer GmbH) for use as an anatomic guide to obtain an original form while the restoration was applied.17 Different superscript letters represent significantly differences identified by 1-way ANOVA with post hoc Tamhane tests (P<.50 G6 2. The endodontic treatments were performed using NiTi rotary instruments (ProTaper.20.76a 938. After extirpating the pulp.52 ±209.33 G11 Intact teeth 1640. specifically.5 mm exhibited greater fracture resistance and more frequent restorable fractures.S. the remaining tooth structure. MOD cavities were prepared without proximal steps at the cemento-enamel junction (CEJ) and with an occlusal isthmus width of one-half of the buccolingual distance.5-mm anatomic reduction 1085.30 Studies have reported that many factors influence the locations and directions of root fractures. Teeth with cracks.86b 423.49 1498. Table 1.65 1087.41 1187.13. the cavities were filled with a posterior composite resin (P60.35 and the roots of the teeth were then embedded in acrylic resin up to 3 mm below the CEJ using cylindrical blocks. 3M ESPE). and gently airdried. 1A). Fracture patterns after fracture resistance tests. and anatomic) (Table 1). The fractured specimens were analyzed to determine the fracture patterns according to the location of the fracture as follows (Fig. Nova Tic). A. The teeth were placed into the impression guides previously made for each tooth. Subsequently. and glass-ionomer cement (Ketac Molar Easymix.37 (Vega Chewing Simulator. 3M ESPE) was applied to the tooth surface with a microbrush and then air-dried for 5 seconds. The canals were dried with paper points (Dentsply Maillefer). after the reduction of the cusps parallel to the occlusal plane.- 2016 Five percent sodium hypochlorite was used for irrigation during the endodontic preparation. Teeth in the G11 group were left intact as the control group.5. The compressive load was applied parallel to the long axis of the tooth with a 6-mm-diameter stainless steel antagonist placed in the center of the tooth with contacts only on the buccal and lingual cusps. 3M ESPE) was then inserted into this area as a coronal restoration. and 3. type II. the surface was exposed to a light-emitting diode (LED)polymerization unit (Elipar S10. Kerr Corp) was applied to the roots to simulate the periodontal ligament. 2. Kerr Corp). After the restorations were finished. After the artificial aging procedures. the cusp reduction was prepared parallel to the occlusal plane without bevels. and all roots were obturated with ProTaper F3 gutta percha (Dentsply Maillefer) and endodontic sealer (AH Plus. In the anatomic reduction design.5 mm/min in a universal testing machine (Instron 3382. the specimens were subjected to a compressive load at a crosshead speed of 0. such that the cavities could not be overfilled at the margins and the reduced cusps could be restored to their original forms. Cusp reduction designs and cavity dimensions. A single-component bonding agent (Adper Single Bond 2. 1B): type I. Water-based liquid latex (Rubber-Sep. The prepared surfaces were selectively etched (enamel for 30 seconds. The mechanical loading was applied to the center of the occlusal surface Serin Kalay et al 3 Anatomic A B 3 mm Horizontal A=2B Beveled A I II III B Figure 1.5 Hz in a mastication simulation machine36. the cusp reduction was prepared parallel to the natural cusp ridge. In the beveled reduction design. The coronal part of the gutta percha material was removed 2 mm apically to the CEJ. Dentsply Maillefer). Instron Corp). B. bevel preparations were performed in an opposite angle to the natural cusp ridge. rinsed for 20 seconds with an air/water spray. the teeth were polished with rubber cups and points (Identoflex. teeth in the G10 group were only prepared with MOD cavities. and each increment was polymerized for 20 seconds. horizontal. The composite resin was placed using the oblique incremental technique. then. The reduction thicknesses were evaluated at the reference point (cusp tip) with digital calipers (Fig. after the endodontic treatments and MOD cavity preparations. in contact with both cusp ridges with a stainless steel antagonist with a rounded end that was 6 mm in diameter. dentin for 15 seconds) with 37% phosphoric acid (Scotchbond Etchant. THE JOURNAL OF PROSTHETIC DENTISTRY . After the endodontic treatments. 3M ESPE). coronal fractures involving small amounts of enamel/composite resin (restorable).38 The specimens were maintained in a humid environment throughout the mastication simulation. The force required for fracture was recorded in newtons. All of the specimens were submitted to 105 cycles of the application of 50-N loading forces at a frequency of 0. 3M ESPE). In the horizontal reduction design.5 mm) and designs (beveled. In the reduction groups (G1-9).5. both cusps of the teeth were reduced with combinations of different thicknesses (1. 05 for all tests). the teeth in the MOD group fractured with nonrestorable patterns. the 1-way ANOVA revealed significant differences (P<.7 (4) 73. 2. A power analysis revealed a power of 93. conversely.5-mm reduction thicknesses. The thicknesses of the 1. All of the reduction groups exhibited higher fracture resistances than the MOD group.5-mm reductions were compared (Fig. 3).(n) Restorable Group (G) Type I G1 (1. the roots of the teeth were covered with latex liners.7 (7) 0 13.3 (2) 86.0 (3) 26. the width of the MOD cavity isthmus was prepared to be one half of the buccolingual distance.5-mm anatomic).5. DISCUSSION In the present study.7 (1) 33. Simulations of periodontal ligaments can influence not only the fracture load values but also the fracture patterns. 105 cycles of mechanical loading were applied at a frequency of 0.3 (11) G4 (2.001) among all groups.5-mm anatomic and 3. No significant differences between any of the reduction designs were observed at the 1.5-mm thickness exhibited a significantly higher fracture resistance than the 1.5-mm thicknesses. G8 (3.5-mm anatomic) 26. the fracture resistances of ETMPs restored with various cusp coverage restorations were evaluated.5-mm beveled) 6.5-mm anatomic) 6. Regarding the horizontal design. Regarding the anatomic design. Additionally.5-mm anatomic) group relative to the intact tooth group. and G9 (3.02% for this study.5-mm beveled) 0 33. Significantly higher fracture resistance values were observed in 2 reduction groups (2. similar fracture resistance values were observed in the G6 (2.36 Maxillary premolars were used in this study because their anatomy is susceptible to cusp deflection and fracture under occlusal loads. vertical root/coronal fractures greater than 1 mm below than CEJ (nonrestorable) (Table 2).35 For these reasons.5-mm horizontal) 0 20.5-mm anatomic) compared with the MOD group.5-mm and 3. In the reduction groups (G1-9).0 (12) G6 (2.0 (3) 73.5-.7 (4) 13. Changes in the mechanical properties of a restoration under masticatory load and fatigue failures after a period of clinical use are important.and 2. A Tamhane test revealed differences between the reduction groups and the control group (Table 1).6 (4) 46.9 The removal of tooth structure for endodontic and restorative procedures increases cuspal deflection and susceptibility to fracture and also affects the restoration type. prevent additional tissue loss and exhibit more homogeneous distributions Serin Kalay et al . the 2.5-mm beveled) Type II Nonrestorable Type III 6. which is close to the masticatory cycle in vivo.7 (10) G2 (1.3 (11) G3 (1.3 (5) 66.5-mm anatomic) 20.5-mm thickness group.and 3.3 (2) G10 MOD) G11 (Intact teeth) coronal cusp fractures involving the dentin at a maximum of 1 mm below the CEJ (restorable). Two-way ANOVA followed by the Fisher least significant difference (LSD) test was used to analyze the interaction between the cusp reduction thicknesses and the designs of the reduction groups (a=.3 (5) 66.7 (10) G8 (3.5.7 (1) 6. The statistical analyses were performed with the Shapiro-Wilk test for normal distributions and 1-way analysis of variance (ANOVA) followed by the Tamhane tests.5-mm horizontal) 0 6.21 In our study.7 (4) 53.0 (3) 80. and the results are graphically displayed in the box plots in Fig. A masticatory simulator was used to simulate the clinical conditions that occur during mastication with the intention of mimicking at least 1 year of in vivo clinical use. Many factors may influence in vitro results. Regarding the control and reduction groups.37 In the present study. The 2-way ANOVA followed by the Fisher LSD tests indicated differences in terms of the interaction between reduction thickness and reduction design. Restorations of ETTs with adhesive methods support the remaining tooth structure.0 (9) 26. The intact teeth predominantly fractured with restorable patterns.5-mm horizontal) 0 33. the anatomic reduction design exhibited a significantly THE JOURNAL OF PROSTHETIC DENTISTRY - Issue - higher fracture resistance compared with the horizontal and beveled groups.3 (8) G7 (3. Restorations are thought to undergo 1 000 000 active stress cycles in 20 years. the highest restorable fracture rates were observed in the 2.5-. thus. furthermore. 2.6 (4) 66.3 (5) 60. No significant differences among the reduction thicknesses were observed with the beveled reduction design. 80% of the fractures in the anatomic reduction design group exhibited type I patterns (Table 2). the 3.5-mm horizontal). and 3.7 (1) 20. Cusp coverage becomes necessary when the width of the cavity isthmus is greater than two thirds of the intercuspal distance or one half of the buccolingual distance. Different designs were compared. in vitro experiments should represent the intraoral environment.0 (9) G5 (2.5 Hz. type III. Regarding the 2.5-mm and 3.4 Volume Table 2. Distribution of fracture patterns for each group Fracture Pattern % .5-mm thickness groups exhibited significantly higher fracture resistances than the 1. RESULTS The data exhibited normal distributions according to the Shapiro-Wilk tests.7 (13) 60.7 (10) G9 (3.5-mm thickness.7 (4) 26.5-mm anatomic reduction groups. of occlusal forces. In the present study.5 Group Beveled P = . 2. Studies related to cusp coverage restorations of extensive MOD cavities have applied different cusp reduction designs. These authors used only a single type of reduction design and reported that thick composite resin overlays increased the fatigue resistances of endodontically treated premolars compared with thin overlays. which would lead to a favorable distribution of occlusal forces and transfer to the tooth structure when a compressive load is applied.5. Mondelli et al18 and Magne et al17 evaluated cusp coverage restorations with reductions parallel to the cusp incline. and these findings agree with those of other studies.5 500 750 1000 1250 1500 1750 Load (00) Figure 2. Additionally. and 3.14. Comparison of reduction groups with identical reduction designs and different reduction thicknesses. The results of the present study support the existence of differences in the resistance to fracture and the mode of failure between cusp coverage restorations of ETMPs with MOD cavities according to various reduction thicknesses and designs. In the present study. The 2. 3.803 P = .021 P = .17.5 mm) to obtain the optimum reduction thickness and design within the limitations of the study.5mm anatomic groups exhibited significantly increased ETMP fracture resistances compared with the MOD group. 3 different cusp reduction designs (anatomic.26 Moreover. The anatomic cusp reduction design significantly strengthened the teeth.14. but the fracture resistances of such restorations remain significantly below those of intact teeth.108 P = .5 mm) on the in vitro fatigue resistances and failure modes of endodontically treated premolars. overlays. and rounded shoulder preparation designs and reported that onlays with rounded shoulder margins exhibited favorable stress distributions between the restorations and the tissues. finding that cusp reduction increased fracture resistance.5-mm anatomic.009 2. When the results of the cusp fracture patterns were assessed.528 1. Dejak et al15 evaluated different onlay designs with horizontal.046 P = .23-26 the literature remains unclear regarding which cusp reduction thicknesses and designs are optimal for weakened ETTs. Cusp reduction was found to increase the fracture resistances of ETMP restorations with extensive MOD cavities compared with restoration without cusp coverage.5 P = .5 P = .546 3.5-mm anatomic groups were statistically similar to those of the intact teeth.18-20. some studies have reported that cusp coverage restorations improve fracture resistance compared with MOD restorations.779 Horizontal P = . Krifka et al20 evaluated cusp coverage methods with a horizontal reduction design of the cusps. for the restoration of ETTs with large structural losses represents a more conservative approach in terms of function and esthetics.20.5 P = .5 Horizontal 3.223 1. 2. the greatest proportion of restorable fractures among the reduction groups was also observed in the anatomic reduction groups. this finding might be attributable to the Serin Kalay et al THE JOURNAL OF PROSTHETIC DENTISTRY . Comparison of reduction groups with identical reduction thicknesses and different reduction designs.5 Anatomic P = .5 P = . and 3.009 1.5 Anatomic Horizontal Beveled 2016 3. The present study confirmed these results. the fracture resistances of the restored premolars with various cusp reduction designs were different.5 P = .01 Beveled 1.18.5-mm anatomic and 3. Mondelli et al18 investigated the influences of 2-mm overlay thicknesses in a single type of reduction design on the fracture resistances and fracture modes of endodontically treated premolars with MOD cavities and found that the cusp coverage resulted in fracture resistances similar to those of sound teeth. beveled.8.5.843 Beveled 500 750 1000 1250 1500 P = . such as onlays. Figure 3. and beveled) were applied at 3 different thicknesses (1.9 Magne et al17 evaluated the influences of overlay thickness (1.18 Although earlier studies reported that bonded cusp coverage restorations improve fracture resistance. and 3. This finding is most likely due to the axial direction of the cusp reduction design. and endocrowns.150 2.5-mm horizontal.5 Anatomic - 3.5.632 1750 Load (00) Group P = . Box plots of the load to fracture test (N).5 P = .854 P = .14.15.22 The use of bonded cuspal coverage restorations.00 Horizontal P = .5. The results from the 2.743 2. horizontal.313 Anatomic 2. Box plots of load to fracture test (N).5 P = . Watts DC. Chang YH.54:345-74. Thickness of CAD-CAM composite resin overlays influences fatigue resistance of endodontically treated premolars. Fracture resistance of root filled premolar teeth restored with direct composite resin with or without cusp coverage. Rocha Ada C.32:283-6. Oper Dent 2008. 24. - Issue - either full cast coverage or with direct composite restoration.36: 194-203. 16.45:524-9. Fracture strength of teeth restored with ceramic inlays and overlays. Vieira GF. Krejci I. Lin CL. 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