Influence of different cavity preparation designs on fracture resistance of onlay and overlay restorations using different CAD/CAM materials.

PURPOSE: To evaluate fracture strength of different preparation designed onlay and overlay restorations produced by Computer Aided Design/Computer Aided Manufacturing (CAD/CAM) and three different new ceramic-polymer blocks.

METHODS: Ninety-one extracted human mandibular molars were used in this study. Preparations were made as (1) Only functional cusps reduction, (2) Only functional cusps reduction+rounded shoulder finish line, (3) All cusps reduction, or (4) All cusps reduction+rounded shoulder finish line. Then these four cavity types were restored with three different materials: VITA Enamic, GC Cerasmart, Lava Ultimate. Thirteen groups were created by added control group (n= 7). Adhesive cementation was achieved by using a dual cured composite resin adhesive cement RelyX Ultimate. All samples were subjected to thermocycling for 5,000 cycles in water baths between 50°C and 55°C. The fracture resistance of specimens was determined under compressive loads along the long axis of the restored teeth at 0.5 mm/minute crosshead speed until fracture with a universal test machine. Fracture types of restored groups were evaluated. For fracture resistance data, one-way ANOVA; for correlation between material type and preparation design, two-way ANOVA were used. Percentage table and Kruskal-Wallis test were performed for analysis of fracture types.

RESULTS: There was no significant difference between all the groups (one-way ANOVA, P> 0.05). Group 4 showed statistically higher fracture strength values than Group 1 and no significant difference was found among other preparation designs by two-way ANOVA that compared all types of preparation designs regardless of material difference (P< 0.05).

CLINICAL SIGNIFICANCE: The results of the present study could help clinicians regarding which materials and type of preparations should be selected for onlay and overlay restorations produced with newly introduced ceramic-polymer CAD/CAM materials.