Intaglio surface polishing and adjustment by grinding of 4YSZ crowns: Effect on internal/marginal adaptation and load-bearing capacity under fatigue.

Abstract

This study aimed to evaluate the effect of CAD/CAM milling and internal adjustment procedures, as well as subsequent intaglio surface polishing applied after both processes, on the internal/marginal adaptation and fatigue behavior of simplified zirconia crowns. Fifty 4YSZ crowns were subjected to five surface conditions (n = 10): CTRL^- (milled without additional surface treatment), CTRL⁺ (milled with additional surface treatment), POL (milled + intaglio surface polishing + surface treatment), ADJ (internal adjustment + surface treatment), and ADJ + POL (internal adjustment + intaglio surface polishing + surface treatment). As surface treatment to enhance adhesion, the crowns were subjected to air-abrasion with 45 μm aluminum oxide. The only group that was not surface treated was the CTRL^-. 4YSZ crowns thickness was measured before and after internal adjustment by grinding and intaglio surface polishing. Following the treatments, the internal/marginal adaptation of the 4YSZ crowns was assessed using the replica technique. After, the crowns were cemented onto epoxy resin substrates and the cyclic fatigue test started by 100 N for 10,000 cycles at 20 Hz, followed by load increments of 100 N every 10,000 cycles until reaching 500 N, after which the increment was reduced to 50 N per 10,000 cycles, until failure (crack). Changes in crown thickness before and after internal adjustment by grinding and intaglio surface polishing were evaluated using paired t-tests (α = 0.05). Internal/marginal adaptation, fatigue failure load and cycles for failure data were by Tukey's post-hoc test (α = 0.05) to compare the effects of different surface conditions. Internal adjustment by grinding significantly reduced crown thickness (P < 0.001), while intaglio surface polishing did not affect thickness (P = 0.168). Internal/marginal adaptation was assessed using the Kruskal-Wallis test, followed by pairwise comparisons, with POL showing the best adaptation, ADJ the highest misfit, and ADJ + POL demonstrating partially improved adaptation compared to ADJ, with intermediate to low discrepancy values depending on the evaluated region. Additionally, Kaplan-Meier survival analysis with log-rank (Mantel-Cox) tests (α = 0.05) was performed to evaluate the survival of each group under cyclic fatigue. POL exhibited the highest fatigue behavior, followed by CTRL+, ADJ, and ADJ + POL, while CTRL- showed the lowest performance. Internal adjustment by grinding increased internal/marginal discrepancies and showed intermediate fatigue behavior. Intaglio surface polishing after CAD/CAM milling improved both internal/marginal adaptation and fatigue behavior, whereas when performed after internal adjustment, it partially improved adaptation without enhancing fatigue behavior.