Marginal discrepancy and fracture load of thermomechanically fatigued crowns fabricated with different CAD-CAM techniques

Purpose: To assess the effect of CAD-CAM technique (monolithic, CAD-on, or CAD-on cemented) and thermomechanical fatigue on the marginal discrepancy and fracture load of ceramic crowns.

Material and methods: A total of 90 brass master dies were fabricated to investigate marginal adaptation and fracture load. A mandibular first molar crown's median measurements were loaded into CAD software and divided into 2 crown design groups: Monolithic (M)(IPS e.max zirCAD)(n = 30) or CAD-on core (IPS e.max zirCAD) and lithium disilicate veneer (IPS e.max CAD) (n = 60). The crowns and cores were milled, seated on their respective dies, and marginal discrepancy values were measured by using microcomputed tomography. After veneers were milled, the cores in the veneer groups were divided into 2 groups; veneers bonded with fusion glass-ceramic (IPS e.max CAD Crystal) in CAD-on group (CO) and CAD-on cemented group (CO-C) where veneers were cemented (RelyX U200) onto cores (CO-C). The marginal discrepancy measurements were remade and the crowns were subjected to thermomechanical fatigue (TMF) by using a chewing simulator and thermocycling (5-55°C, 1,200,000 cycles). Marginal discrepancy measurements were repeated and the crowns were subjected to fracture load test by using a universal test device. Data were analyzed statistically by analysis of variance (ANOVA) and Tukey's honestly significant difference test (α = 0.05).

Results: All crown groups had similar marginal discrepancy before veneering. Veneering and cementation on die increased the marginal discrepancy of crowns in cemented CAD-on group. Thermomechanical fatigue increased the marginal discrepancy of both CAD-on groups. Monolithic crown group had the lowest marginal discrepancy after thermomechanical fatigue (p < 0.001), and the highest fracture load (p < 0.001) CONCLUSIONS: Fabrication technique affected the marginal fit and fracture load of CAD-CAM crowns after thermomechanical fatigue. All crowns survived the thermomechanical fatigue test without dislodgement or fracture. Monolithic crowns had the best fit and highest fracture load after fatigue testing. The CAD-on systems had similar marginal discrepancies, and static loading reproduced veneer chipping.