Influence of cusp inclination and curvature on the in vitro failure and fracture resistance of veneered zirconia crowns

Abstract

The aim was to investigate the influence of cusp inclinations and curvatures on the failure behaviour and fracture resistance of veneered zirconia crowns. Five groups (n = 8/group) of zirconia-based molar crowns were fabricated by modifying cusp inclination (S, steep = 45°; M, medium = 30°; F, flat = 10°) and curvature (P, pointed; R, round; O, round-oblate). Combinations MP, MR, SP, SR and FO were investigated. All crowns were fixed on polymethylmethacrylate teeth, thermally cycled and mechanically loaded. Failures were monitored and fracture resistance was determined after ageing. Crowns were fractographically analysed with scanning electron microscopy. Chipping during thermal cycling and mechanical loading (TCML) was observed in groups MR (1×), SP (6×) and SR (3×). Defect sizes varied between 0.12 mm(2) (MR) and 17.28 mm(2) (SP). Failures started from occlusal contacts. Mean fracture varied significantly between 1,354.2 ± 360.0 N (SR) and 3,155.8 ± 444.4 N (FO). Cusp inclination and curvature influenced failures during TCML and fracture loads. Crowns with medium cusp inclination and pointed or round curvature are recommended for clinical application as they revealed high resistance to chipping. Steep inclination, especially in combination with pointed curvature, was shown to increase chipping rates and areas and to decrease fracture force. The results show the necessity of considering design parameters of the veneer in order to reduce failures of zirconia-based crowns.