Influence of Fabrication Settings on the In Vitro Performance of Subtractively Manufactured Resin-Based Molar Crowns.

Purpose: To investigate the influence of milling parameters on the durability and fracture force of resinbased composite crowns during in vitro aging simulation.

Materials and methods: Identical molar crowns (n = 8 per group) were milled from resin-based composite crowns (Grandio, VOCO Dental) with different processing speeds (soft, normal, fast) or levels of detail (very high, high, low) from 98-mm disks. One group was milled wet to investigate the influence of cooling. The influence of polishing was tested in two groups. All crowns were adhesively bonded on standardized resin-based composite molars. Aging was performed with thermal cycling and mechanical loading (2 × 3,000 × 5°C/55°C, 2 minutes, H20 dist., 1.2 × 106 force at 50 N). Fracture forces were determined (v = 1 mm/minute, Z010, Zwick Roell).

Results: For statistical analysis, Pearson correlation, one-way ANOVA, and Bonferroni post hoc tests were used (α = .05). All crowns survived thermal cycling and mechanical loading (TCML) without any failures. The fracture values varied between 1,237.3 ± 326.7 N and 3,308.6 ± 655.8 N. Significant (P < .001) differences between the individual manufacturing approaches were detected. Failure was categorized as a fracture of the crown and partial loosening of the crown. No different failure pattern was observed between the tested systems.

Conclusions: A medium level of detail seems to be ideal to achieve the highest fracture forces. No relationship existed between machine speed and fracture force. Fracture force was not affected by wet grinding. In individual cases, polishing reduced crown fracture values due to reduced wall thickness.