Marginal fit, internal adaptation, and thermocycling effects on the mechanical and optical properties of graphene CAD-CAM and zirconia crowns for primary molars: An in-vitro study

Abstract

Aim To evaluate the marginal fit and internal adaptation of graphene-reinforced CAD-CAM crowns compared with prefabricated zirconia crowns, and to assess the influence of thermocycling on their mechanical and optical properties. Materials and methods In this in vitro comparative study, typodont mandibular second primary molars were prepared using a standardized protocol and allocated into two groups: graphene-reinforced CAD-CAM (G-CAM) crowns and prefabricated zirconia crowns. Marginal fit was assessed using a stereomicroscope at 40× magnification, and internal adaptation was evaluated using a silicone replica technique combined with three-dimensional digital analysis. For fracture resistance, crowns were tested under pre- and post-thermocycling conditions using a universal testing machine. Disc specimens were prepared separately for surface roughness, microhardness, and color stability; within each parameter, the same specimens were evaluated before and after thermocycling. Artificial aging was performed using 5000 thermocycles (5°C-55°C). Statistical analysis included Shapiro–Wilk and Levene's tests, independent and paired t-tests, and two-way ANOVA ( P < 0.05). Results G-CAM crowns showed significantly lower marginal discrepancy and internal gap values than zirconia crowns ( P ≤ 0.002). Fracture resistance was higher in G-CAM crowns before and after thermocycling, with a significant reduction observed in both groups ( P < 0.05). Surface roughness, microhardness, and color stability were unaffected by thermocycling ( P > 0.05), with ΔE values within clinically acceptable limits. Conclusion Under in vitro conditions, G-CAM crowns demonstrated superior marginal fit, internal adaptation, and fracture resistance compared with zirconia crowns. Thermocycling had minimal effect on surface and optical properties, though clinical studies are needed to confirm long-term performance.