The effect of fabrication procedures and thermomechanical loading on the structural properties of screw-retained metal-ceramic implant restorations: An in vitro study.

Purpose: Metal-ceramic screw-retained implant restorations persist as a fundamental choice in specific clinical scenarios. Little is known about the effects of fabrication steps and aging on their structural properties. This study aimed to investigate how laboratory fabrication procedures and thermomechanical loading affect the structural properties of screw-retained metal-ceramic implant restorations.

Materials and methods: Ten screw-retained metal-ceramic restorations were conventionally cast using UCLA chromium-cobalt overcast abutments. After 500 cycles of thermocycling and 500,000 cycles of mechanical loading, changes in connection dimensions and rotational freedom (RF) were measured and compared at various fabrication steps and post-thermomechanical loading. A generalized estimating equations (GEE) model was employed to analyze trends across the studied time points within the fabrication stage and after thermomechanical loading, with LSD post-hoc tests applied for pairwise comparisons. The level of significance was set at α = 0.05.

Results: Significant changes were observed across the analyzed time points: the average hexagonal side length (L) decreased (p < 0.001), and the average hexagonal angle deformation (P) increased, with notable differences observed in most comparisons between different fabrication steps (p < 0.001). Short (T1) and long (T2) diagonals of the hexagon showed downward trends (p < 0.001), while concentricity (O) and RF increased (p < 0.001), except between porcelain firing and loading steps for RF (p = 0.637). Casting had the greatest impact on variations in O (93.33%), T1 (88.88%), and T2 (45%), while porcelain firing significantly affected L (71.42%), P (71.42%), with the greatest effect on RF (75.32%).

Conclusions: The fabrication processes and simulated clinical use adversely impacted the structural integrity and RF of abutments in screw-retained chromium-cobalt overcast implant restorations.