Marginal fit of CAD/CAM lithium disilicate occlusal veneers with two preparation designs: a scanning electron microscope quantitative evaluation

Aim: The use of all-ceramic conservative occlusal veneers is becoming more popular in the clinical setting thanks to the recent improvement of the mechanical and esthetical properties of the restorative materials. Furthermore, the option of CAD/CAM fabrication may constitute a further advantage for both clinicians and patients. Since little is known about the influence of the occlusal veneer preparation design on the marginal fit of the final restoration, the present study compared the marginal fit of two preparation designs. Methods: Sixteen maxillary molars were selected from a pool of freshly extracted teeth and received 1-mm cusp reduction by using silicon indexes as reference. The teeth were randomly divided into two groups identified by the preparation design: conservative full-coverage occlusal veneer with a 90° rounded shoulder margin (n=8) and marginal chamfer (n=8). At the end of the preparation process, the exposed dentine surfaces were immediately sealed with a self etch adhesive system. Lithium disilicate restorations (IPS e.max CAD) were produced with the Cerec 3 CAD/CAM system. The intaglio surface of the occlusal veneers was conditioned with hydrofluoric acid and silane, while the tooth surface underwent silicatization, enamel etching, and adhesive system application. The occlusal veneers were luted with Variolink II cement. The specimens were embedded into acrylic resin simulating the periodontal ligament with a layer of vinyl polysiloxane; then, they were chewing machine (1,250,000 cycles, 1 Hz, 5-55°C). A quantitative marginal fit evaluation was performed observing at the scanning electron microscope resin replicas of the specimens obtained before and after the thermomechanical aging simulation. For each replica, a single trained blinded operator measured the marginal gap along 120 equidistant points of the adhesive interface. Collected data were tested for the assumptions for using parametric tests. The marginal fit measured at different experimental time points was compared within the same group with a paired sample t-test, while the two groups were compared at the same time point with an independent-sample t-test (p<0.05). Results: At baseline, the occlusal veneer with a 90° rounded shoulder margin group showed a mean marginal gap equal to 103.83±54.97 μm, while the conservative preparation design with a marginal chamfer 120.43±58.92 μm. The difference between the two preparation designs in terms of marginal fit was not significant at any experimental time point. Thermomechanical aging caused a slight not significant reduction of the marginal gap in both groups. Conclusion: The present study on extracted molars demonstrated that a new minimally invasive preparation for CAD/CAM lithium disilicate occlusal veneers with marginal chamfer is capable of marginal fit comparable to that of a standard conservative preparation.