Influence of substructure design, veneer application technique, and firing regime on the in vitro performance of molar zirconia crowns.

Abstract Objectives The aim of this in vitro study was to evaluate the influence of substructure design, veneer application technique, and firing regime on the failure and fracture resistance of molar zirconia crowns. Methods Six groups ( n  = 8/group) of zirconia crowns were fabricated in simple core (SC) or anatomically reduced (AR) design, veneered with different feldspathic or glass ceramic materials, and defined according to the application technique and firing regime (LT: layering technique; LT_L: LT with long-term cooling; PT: press technique; DV: digital veneering technique). The following groups were investigated: SCLT, ARLT, SCLT_L, SCPT, ARPT, ARDV. Crowns were adhesively bonded to polymethylmethacrylate abutment teeth and subjected to thermal cycling (TC: 2 × 3000 × 5°/55°) and mechanical loading (ML: 1.2 × 10 6 ; 50 N; 1.6 Hz) in a chewing simulator with metal-ceramic molar crowns as antagonists. Failures were monitored and fracture resistance determined after aging. Data were statistically analyzed (one-way analysis of variance, ANOVA; post hoc Bonferroni, α  = 0.05). Crowns were subjected to scanning electron microscopy for fractographic failure analysis. Results Failures (chipping, cracks) during TCML were observed in groups SCLT (2×), ARDV (2×) and SCLT_L (1×). Defect sizes varied between 3.5 mm (SCLT: crack) and 30.0 mm 2 (SCLT_L: chipping). Mean (SD) fracture forces ranged between 1529.0 (405.2) N for SCPT and 2372.3 (351.8) N for ARDV. Significance The failure frequency of veneered zirconia crowns could be reduced by using anatomically reduced substructures, the press veneering technique, and an adapted cooling protocol. Fracture resistance increased with use of anatomically reduced substructures and the digital veneering technique.