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

Ceramic restorations are improved because of their increased translucency and light transmission. 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.

When an indirect restoration is selected as the treatment option for posterior teeth, the clinician must determine the configuration of the cavity preparation. 

Several designs have been proposed for preparing all-ceramic resin-bonded posterior restorations, as guided by the particular mechanical and structural characteristics of ceramic restorative materials.

The primary causes of failure of ceramic inlay or onlay restorations are cohesive bulk fractures and marginal deficiencies, which manifest clinically as marginal discoloration and secondary caries. 

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. 

These preparations may involve the removal of considerable tooth structure. As more structure is removed, higher tooth strain and lower fracture resistance may occur.

The increased tooth structure loss may increase cuspal flexure, thereby reducing the tooth fracture resistance, or open the restoration-tooth interface. However, it has been demonstrated that cusp recovery results in fewer failures, likely increasing the longevity of posterior ceramic restorations. 

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. 

However, the performance of posterior restoration is also material dependent. 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.

Ceramic inlays and onlays can be manufactured in a laboratory or milled chairside from ceramic blocks using CAD/CAM technology. 

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. 

This system shows good clinical performance; however, it depends on the material and its indication in fixed prostheses of one or more elements. However, the use of these materials is extremely technique sensitive. 

CAD/CAM ceramic materials are manufactured under optimized conditions, which can minimize voids and volume defects. The fracture rate for CAD/CAM posterior ceramic restorations is suggested to be related to design aspects of the restoration and to the composition of the ceramic. 

Fracture resistance tests have been used to predict the failure of ceramic restorations under influence of the preparation design. Nondestructive experimental methodologies, such as the strain gauge test, and finite element analysis should be combined with conventional mechanical tests to better explain the failure of the ceramic restorations. 

The stresses generated by bite loading cause structural strain; if such stresses become excessive and exceed the elastic limit, structural failure may result. 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. 

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