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 1227N (NR/RF/A) and 1534N (R/OF/A), and for surviving G crowns mean fracture values of 2021N (R/OF), 1872N (R/RF), 2242N (NR/OF), and 2070N (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.

Clinical Significance: Preparation design, fitting parameters as well as the type of cementation impact the performance of composite crowns. Retentive preparation design and/or good fitting are mandatory to avoid debonding of composite restorations; adhesive cementation should be preferred in any case.