Effect of cavity preparation design and ceramic type on the stress distribution, strain and fracture resistance of CAD/CAM onlays in molars

Abstract Objective This study aimed to evaluate the effect of the cavity preparation and ceramic type on the stress distribution, tooth strain, fracture resistance and fracture mode of human molar teeth restored with onlays. Material and Methods Forty-eight molars were divided into four groups (n=12) with assorted combinations of two study factors: BL- conventional onlay preparation with boxes made from leucite ceramic (IPS-Empress CAD, Ivoclar Vivadent); NBL- conservative onlay preparation without boxes made from leucite ceramic; BD- conventional onlay preparation with boxes made from lithium disilicate glass ceramic (IPS e.max CAD, Ivoclar Vivadent); NBL- conservative onlay preparation with boxes made from lithium disilicate glass ceramic cuspal deformation (µS) was measured at 100 N and at maximum fracture load using strain gauge. Fracture resistance (N) was measured using a compression test, and the fracture mode was recorded. Finite element analysis was used to evaluate the stress distribution by modified von Mises stress criteria. The tooth strain and fracture resistance data were analyzed using the Tukey test and two-way ANOVA, and the fracture mode was analyzed by the chi-square test (α=0.05). Results The leucite ceramic resulted in higher tooth deformation at 100 N and lower tooth deformation at the maximum fracture load than the lithium disilicate ceramic (P<0.001). The lithium disilicate ceramic exhibited higher fracture resistance than the leucite ceramic (P<0.001). The conservative onlay resulted in higher fracture strength for lithium disilicate ceramic. Finite element analysis results showed the conventional cavity preparation resulted in higher stress concentration in the ceramic restoration and remaining tooth than the conservative onlay preparation. The conservative onlays exhibited increased fracture resistance, reduced stress concentration and more favorable fracture modes. Conclusion Molars restored with lithium disilicate CAD-CAM ceramic onlays exhibited higher fracture resistance than molars restored with leucite CAD-CAM ceramic onlays.


Introduction
Ceramic restorations are improved because of their increased translucency and light transmission. 1 Another advantage includes minimal tooth reduction compared with metal ceramics; minimal thermal conductivity; mimic natural dentition because they have desirable properties, including their physical and mechanical properties; excellent biocompatibility to periodontal tissues; reduced plaque accumulation compared with composite resin; and less susceptibility to metal allergies. 2,3 When an indirect restoration is selected as the treatment option for posterior teeth, the clinician must determine the configuration of the cavity preparation. 4, 5 Several designs have been proposed for preparing all-ceramic resin-bonded posterior restorations, 6,7 as guided by the particular mechanical and structural characteristics of ceramic restorative materials. 8 The primary causes of failure of ceramic inlay or onlay restorations are cohesive bulk fractures and marginal deficiencies, 9 which manifest clinically as marginal discoloration and secondary caries. 10 Tooth preparation designs for posterior ceramic restorations have been based on traditional cast metal restoration designs, but with more occlusal tooth reduction and with a slightly increased taper. 4 These preparations may involve the removal of considerable tooth structure. 11 As more structure is removed, higher tooth strain and lower fracture resistance may occur. 5 The increased tooth structure loss may increase cuspal flexure, thereby reducing the tooth fracture resistance, or open the restoration-tooth interface. 12 However, it has been demonstrated that cusp recovery results in fewer failures, likely increasing the longevity of posterior ceramic restorations. 6 Recently, minimally invasive cavity preparations for posterior indirect restorations were demonstrated to present the benefit of conservation of tooth structure, as well as improved stress distribution. 13 However, the performance of posterior restoration is also material dependent. 14,15 Due to the continuous advancements in dental ceramics and innovative manufacturing techniques, the following question arises: could traditional preparation guidelines for ceramic onlays be modified in terms of minimally invasive therapy?
Several all-ceramic systems, such as leucite and lithium disilicate CAD-CAM systems, have two major recent developments: dentine bonding and stronger all-ceramic crown systems. 16 Ceramic inlays and onlays can be manufactured in a laboratory or milled chairside from ceramic blocks using CAD/CAM technology. 17 The restorations prepared with indirect technique with CAD/ CAM system in case of more extensive loss of dental structure can be preferred because of their better fracture resistance, esthetic looks, implementation in a single visit, and shorter intraoral working time. 18 This system shows good clinical performance; however, it depends on the material and its indication in fixed prostheses of one or more elements. 15  if such stresses become excessive and exceed the elastic limit, structural failure may result. 26 To the best of the authors' knowledge, to date, no study has integrally analyzed the failure of minimal cavity preparations for posterior teeth with different ceramic compositions in comparison with conventional cavity preparation designs. Therefore, this study aimed to analyze the biomechanical performance of onlays made from leucite and lithium disilicate-reinforced ceramics in CAD/CAM restorations associated with both conventional cavity preparations and minimal preparations without occlusal and proximal boxes. The null hypothesis was that the ceramic type and cavity preparation design have no effect on the remaining tooth strain, stress distribution, fracture resistance and fracture mode of molars restored with onlays.

Material and methods
Teeth selection and cavity preparation In this in vitro study, forty-eight freshly extracted mandibular molars were selected with the approval

Coronal Deformation (CD)
The tooth deformation values (strain) for the two ceramic restorations and the two cavity preparations at 100 N are shown in Table 2. Two-way ANOVA showed ceramic type factor (P=0.005) had significant effects on tooth deformation; however, the cavity preparation factor (P=0.426) interaction between the two study factors had no significant effect (P=0.258). The coronal deformation values (strain) for the two ceramic restorations and the two cavity preparations at the maximum fracture load are shown in Table 2.
Two-way ANOVA showed the ceramic type (P=0.020) had a significant effect on fracture resistance; however, the cavity preparation (P=0.426) and the interaction between the two study factors had no significant effect Fracture resistance and fracture mode The fracture resistances in N for the two ceramic restorations and the two cavity preparations are shown in Table 3  Fracture modes: I, fractures involving a small portion of the coronal tooth structure; II, fractures involving a small portion of the coronal tooth structure and cohesive failure of the restoration; III, fractures involving the tooth structure, cohesive and/or adhesive failure of the restoration, and root involvement that can be restored in association with periodontal surgery; and IV, severe root and crown fracture, necessitating extraction of the tooth The fracture mode distributions are shown in Figure   3. The chi-square test showed the lithium disilicate ceramic resulted in a more severe fracture mode, irrespective of the type of cavity preparation. When the load is within the elastic limit of the restored tooth, the structural integrity is not affected.

Finite element analysis
When the tooth structure is removed, more cusp strain is observed, requesting more of the interfaces, and then may reduce the fracture resistance. 12 In the presence of the higher levels of the stress concentration factors and high load applied on the occlusal surface, the concentrated stress may result in crack formation and propagation, causing fracture and structural failure. Although the IPS e-max CAD has greater elastic modulus and stiffness than IPS empress CAD, 17 at a maximum fracture load, the samples restored with lithium disilicate exhibited greater tooth structure deformation than the samples restored with leucite. This may be because the load In this study, the lithium disilicate ceramic groups had significantly higher fracture resistance than the leucite ceramic restoration groups, irrespective of the type of cavity preparation. This may be due to the higher elastic modulus and fracture strength. 17 The disilicate ceramic can support higher load, absorbing greater amounts of energy inside the ceramic material before fracture. This aspect is very important in the new paradigm that determines conservative occlusal reduction. Therefore, for occlusal reconstruction in patients with bruxism, lithium disilicate may be preferable. Analyzing the fracture modes in addition to fracture resistance is important. The findings of this study may be explained by the higher stiffness of lithium disilicate, which reaches the yield strength and leads to the fracture of the remaining tooth structure.
The lesser deformation of lithium disilicate is caused by the higher elastic modulus and, therefore, leads to support greater deformation of the remaining tooth structure, resulting in a higher percentage of complex fracture. The maximum preservation of healthy tooth structure and the use of restorative materials with mechanical properties similar to dental structure may promote a greater longevity of the tooth-restoration complex.
Effect of cavity preparation design and ceramic type on the stress distribution, strain and fracture resistance of CAD/CAM onlays in molars