Influence of Margin Designs on Crack Initiation of High Translucency Monolithic Zirconia Crowns.

PURPOSE: Zirconia crowns often crack at the margin. This study determined the loads and the times at which cracks are initiated in high-translucency monolithic zirconia crowns with different margin designs. MATERIALS AND METHODS: A total of 90 crowns were fabricated from Zirconia blanks. The fabricated crowns had different margin thicknesses (light-chamfer, C(L) and heavy-chamfer, C(H) ) and collar heights (no-collar, N(C) ; low-collar, L(C) ; high-collar, H(C) ). They were grouped as C(L) N(C) , C(L) L(C) , C(L) H(C) , C(H) N(C) , C(H) L(C) , and C(H) H(C) (15 crowns/group). The crowns were seated on a metal model and loaded vertically through round end punch (Phi = 10 mm) at 0.2 mm/min crosshead speed until cracks began to be seen. Videos of the crack initiation were recorded at the rate of 50 frames/second. Load-initiated cracks and durability time were compared for significant differences using analysis of variance. RESULTS: The mean +/- standard deviation values of load (N) and time (s) taken to initiate cracks were 3190 +/-775, 212 +/-47 for C(L) N(C) ; 2754 +/-1109, 180 +/-42 for C(L) L(C) ; 2887+/-832, 191+/-27 for C(L) H(C) ; 4082 +/-896, 241 +/-36 for C(H) N(C) ; 4180 +/-1029, 220 +/-28 for C(H) L(C) ; 4119 +/-1124, 222 +/-39 for C(H) H(C) . This indicates that the thickness of the margin has a significant influence on load-withstanding crack initiation capacity and durability time (p < 0.05). No significant impact of collar height was observed on either load-withstanding capacity or durability time (p > 0.05). No interaction was observed among these factors. CONCLUSION: Heavy chamfer margin provided a stronger zirconia crown than the light chamfer margin, but both of them were capable of withstanding crack-initiated load higher than the theoretical maximum masticatory force. The presence or absence of a collar did not have any impact on the crack initiation. Fabrication of zirconia crowns with either a heavy or light chamfer margin and with or without the presence of a collar should be generated by considering the relevant emergence profile.