Comparison of the fracture strength of CAD/CAM oclusal venery restorations according to different material thickness and preparation design

Occlusal Veneers are frequently used as they are minimally invasive restorations. Lithium disilicate ceramics are the first choice of materials for occlusal veneers and zirconia reinforced lithium silicate ceramics are recently introduced commercially. However, there is no scientific study regarding the multi aspect comparison of these two materials yet. The aim of this study is to comparatively investigate the effects of; two different ceramic materials (Celtra-Duo, IPS E.max CAD) two different thicknesses (1mm & 3mm) and three different tooth preparations (full shoulder, buccal shoulder and anatomical occlusal reduction) as variables on the fracture resistance of occlusal veneer restorations. 120 caries-free mandibular human molars were collected and divided into 12 groups (n=10) for the study. Tooth preparations were made with the same burr set by the same dentist. Restorations were produced by using CEREC software and were cemented with an adhesive resin cement. Samples were subjected to mechanical and thermal aging with the help of a chewing simulator and a thermos-cycle unit (1 200 000 mechanical dynamic and 5000 thermal cycles (5-55 ̊C). No cracks or fractures in any of the samples were observed after aging. Static compressive loads were applied by a Universal Testing Machine to determine compressive strengths. A stereo-microscope was used to determine the types of fractures. A three-way analysis of variance was used for statistical evaluation. Bonferroni correction was applied for multiple comparison. The evaluation of fracture types among groups were made with the Chi-Square Test. The results revealed that; Thickness and material groups did not present any significant differences, whereas only full shoulder and anatomical occlusal reduction groups showed a significant difference among preparation type groups. Anatomical occlusal reduction samples, in both thicknesses and for both materials showed the highest fracture resistance. 3mm thick samples, regardless of material and preparation types, presented statistically significantly more unrepairable fracture type.