Influence of Milling Parameters on the In-vitro Performance of Zirconia Molar Crowns.

PURPOSE: To investigate the influence of milling parameters on the durability of zirconia molar crowns during in-vitro aging-simulation and on fracture force. MATERIALS AND METHODS: Identical molar crowns (n=8 per group) were milled from IPS emax ZirCAD Prime (Ivoclar Vivadent, Schaan, Liechtenstein; n=8 per group) with different processing speed (soft, normal, fast) or level of details (very high, high, low) from 98 mm discs. All crowns were adhesively bonded on standardized resin-based composite molars. Artificial aging was performed with thermal cycling and mechanical loading (2x3000x5°C/55°C, 2 min, H2O dist., 1.2x106 force 50 N). Fracture forces were determined (v = 1 mm/min, Z010, Zwick, Germany). Statistics were performed by using Pearson-correlation, one-way ANOVA, and Bonferroni post-hoc-tests (α=0.05). RESULTS: No crown failures occurred during TCML. The fracture forces ranged between 1551.1 ± 225.0 N (SV) and 2178.6 ± 479.0 N (SL). Significant (p<0.001) differences between groups were identified in ANOVA comparisons. Significant differences (p=0.006) were identified between high and low level of detail. No correlation was observed for fracture force and the processing speed (Pearson: 0.153/p=0.199) or the level of detail (0.124/0.301). Tests showed intermediate subject effects for the level of detail (p=0.007) and the level of detail*processing speed (p=0.002). CONCLUSION: The fracture forces were only slightly influenced by the different machining parameters of the milling machine. A low level of detail in combination with a soft machining speed appears to be ideal for achieving the highest fracture forces.