Does the Thickness and Translucency of CAD/CAM Blocks Affect the Polymerization of Resin Cements with Different Structures?

This study evaluated the effects of CAD/CAM material thickness and translucency on the Vickers microhardness and bottom-to-top hardness ratio of adhesive and self-adhesive resin cements. Eighty specimens (n = 10 per group) were prepared using RelyX Ultimate and RelyX U200. Resin cements were light-activated through lithium disilicate-reinforced CAD/CAM blocks (Initial LiSi Block) with two thicknesses (2 and 4 mm) and two translucency levels (high translucency [HT] and low translucency [LT]). Vickers microhardness was measured on the top and bottom surfaces after 24 h, and the bottom-to-top hardness ratio was calculated as a relative hardness-based indicator of polymerization-related mechanical performance. Increasing block thickness from 2 to 4 mm significantly reduced the bottom-to-top hardness ratio for both resin cements under both translucency conditions (p ≤ 0.008). High-translucency blocks produced significantly higher microhardness values than low-translucency blocks for both cements at both thicknesses (p = 0.001). The effect of thickness on microhardness varied according to cement type and translucency. For RelyX Ultimate, the bottom-to-top hardness ratio was significantly higher under 2 mm HT than 2 mm LT conditions (p = 0.006). Within the limitations of this study, greater CAD/CAM thickness and lower translucency were associated with less favorable polymerization-related mechanical performance.