Impact of Overdried Preparation and Thermocycling on the Fracture of CAD–CAM Hybrid Ceramic Occlusal Veneer Restorations

Abstract

Aim: To study the impact of overdried preparations prior to cementation with a self-adhesive resin cement and thermocycling on the fracture of hybrid ceramic occlusal veneer restorations. Materials and methods: Sixty extracted maxillary molars were mounted and sectioned to remove the coronal structure 4 mm above the cementoenamel junction, leaving a flat area of exposed dentin and peripheral enamel. Ultrathin occlusal veneers with 0.3 mm central fossa thickness were milled from CAD-CAM hybrid ceramic (Vita Enamic). Teeth were randomly divided into three groups. After a selective enamel etching with phosphoric acid, groups I and II were blot-dried using cotton pellets; group III was desiccated with pressurized air for 15 seconds prior to cementation with a self-adhesive resin cement (Rely-X Unicem). Groups II and III were thermocycled between 5/55°C for 5000 cycles. The restored teeth were loaded axially until fracture. Fracture patterns were classified as fracture in the veneers only or fracture involving the tooth structure. Results: The fracture strengths (mean ± standard deviation) were 1672 ± 585, 1789 ± 722, and 1586 ± 711 N for groups I (control), II (thermocycled), and III (overdried and thermocycled), respectively. No statistically significant differences were indicated (one-way ANOVA, p = 0.6426). Fracture patterns were significantly different (the Kruskal-Wallis rank sum test for multiple independent samples, p = 0.01315, followed by the Dunn post hoc test), where group III had more fractures in the veneer only than groups I and II. Conclusion: Thermocycling of samples and overdrying of preparations did not show a significant difference in failure strength. However, fracture patterns indicated more veneer-only fractures under desiccated conditions, suggesting compromised restoration bonding.