Influence of preparation, fitting, and cementation on the vitro performance and fracture resistance of CAD/CAM crowns

Objectives: To investigate debonding and stability of CAD/CAM composite crowns as a function of (a) preparation design, (b) fitting parameters of the milling process, and (c) type of cementation. Methods: Extracted human molars were prepared providing either retentive design (R) or no retention (NR). After digitalization, full-contour crowns were milled using either optimal (OF) or reduced (RF) fitting parameters. A total of 112 crowns were milled from the composite materials Lava Ultimate (L) and Grandio Blocs (G) and a ceramic reference. The crowns were either cemented with self-adhesive cement (SE) or were adhesively bonded (A). After water storage, thermal cycling and mechanical loading was performed. Restorations which failed during storage or TCML were analyzed using scanning electron microscopy and surviving restorations were loaded to fracture. Results: L crowns survived only with R/OF/A conditions. No debondings were observed for G crowns with OF/A conditions as well as R/OF/SE conditions. Surviving L crowns showed mean fracture values of 1227 N (NR/RF/A) and 1534 N (R/OF/A), and for surviving G crowns mean fracture values of 2021 N (R/OF), 1872 N (R/RF), 2242 N (NR/OF), and 2070 N (NR/RF) were identified. Conclusions: Retentive preparation design and/or optimal fitting reduced the number of debondings. For composite restorations, adhesive cementation should be preferred