Effect of veneering material type and thickness ratio on flexural strength of bi-layered PEEK restorations before and after thermal cycling

OBJECTIVE: The purpose of this study was evaluating the biaxial strength of bi-layered PEEK restorations before and after aging using different veneering materials in different thickness ratios. MATERIAL AND METHODS: Ninety specimens of thickness 1.5 mm were divided into three groups according to their veneering material. Group (CAD LD): BioHPP discs veneered with CAD milled lithium disilicate (n=30), group (CAD C): BioHPP discs veneered with CAD milled composite (n=30), and group (LC): BioHPP discs veneered with conventionally layered composite (n=30). Each group was subdivided into 3 subgroups (n=10) according to the different thickness ratios between the core and the veneering material (T(C:)T(V)). Subgroup 1: T(C:)T(V)=1:0.5, subgroup 2: T(C:)T(V)=0.7:0.8, and subgroup 3: T(C:)T(V)=0.5:1. Half of the specimens of each subgroup were subjected to thermocycling, and the bi-axial flexural strength of all specimens was tested before and after aging. Three-way ANOVA followed by Bonferroni’s post hoc test were used for data analysis. The significance level was set at P ≤ 0.05. RESULTS: Material, thickness ratio, and aging all had a significant effect on biaxial flexural strength. (LC) group had the highest biaxial flexural strength. T(C:)T(V)=0.5:1 showed the lowest biaxial flexural strength. All groups showed significant decrease in biaxial flexural strength after aging. CONCLUSIONS: Veneering material for PEEK together with the thickness ratio between the core and veneering material greatly affect the flexural strength of bi-layered restorations. Thermocycling negatively impacts the flexural strength of PEEK bi-layered restorations. CLINICAL SIGNIFICANCE: According to the results of that study, PEEK cores are best veneered with conventionally layered composite with core to veneering thickness ratio being 1:0.5.