Effects of preparation design on the marginal and internal fit of CAD-CAM overlay restorations: A μCT evaluation

Statement of problem The effects of different finish line designs on the seating accuracy of partial indirect restorations are unclear. Purpose The purpose of this in vitro study was to evaluate the influence of different preparation designs on the marginal and internal discrepancy of lithium disilicate computer-aided design and computer-aided manufacture (CAD-CAM) partial indirect restorations before and after thermomechanical aging by using 3-dimensional (3D) microcomputed tomography (mu CT). Material and methods Forty human molars were divided according to the preparation design and their location relative to the tooth survey line: SO: rounded shoulder occlusal to the survey line; CO: chamfer occlusal to the survey line; SA: rounded shoulder apical to the survey line; CA: chamfer apical to the survey line (n=10). Ceramic onlays were luted using a universal resin cement (RelyX Universal + Scotchbond Universal Plus). The specimens underwent thermomechanical loading in a mastication simulator (1 200 000 mechanical and 5000 thermal cycles). The marginal and internal discrepancy was evaluated with mu CT before and after aging. The volume of cement and internal voids was evaluated with 3-dimensional imaging. The marginal discrepancy (MD), absolute marginal discrepancy (AMD), axial gap (AG), and occlusal gap (OG) were quantified using 2-dimensional analyses. Data were statistically analyzed using a 2-way ANOVA and the Tukey post hoc test (alpha=.05). Results Preparation designs apical to the survey line resulted in a larger volume of cement compared with those occlusal to the survey line (P<.05). No differences were observed in the internal voids among the groups immediately or after aging (P>.05). Both preparation designs and aging had a significant influence on the MD, AMD, AG, and OG (P<.05). Specifically, CB exhibited significantly greater values for AMD, MD, AG, and OG compared with all other preparations (P<.05). CA and SA performed significantly worse than CO and SO in terms of AMD and MD (P<.005). Conclusions Preparations located occlusal to the survey line, particularly CO, demonstrated superior results with respect to both marginal and internal adaptations. CA exhibited the least favorable adaptation among the preparations investigated. Aging had a detrimental impact on the marginal adaptation of the restorations, regardless of the specific preparation design used.