Influence of repeated milling bur use on the marginal adaptation of monolithic zirconia crowns.

PURPOSE: This in vitro investigation was designed to assess the vertical marginal discrepancies in monolithic zirconia crowns manufactured using CAD-CAM technology, specifically focusing on the effects of progressive deterioration associated with the cumulative use of tungsten carbide instruments. MATERIALS AND METHODS: Fifty-six monolithic zirconia crowns were manufactured utilizing a dry milling protocol. Milling was continued without bur replacement, and production proceeded with the same bur set even after the system issued a bur replacement warning after the 27th restoration. These restorations were then allocated into four distinct groups (n = 14 per group) based on the milling sequence. Marginal gap measurements were performed with a stereomicroscope, and measurements were obtained from the four surfaces of each crown. To evaluate statistical significance among the groups, a Welch analysis of variance (ANOVA) was conducted, followed by Games-Howell procedures for post hoc pairwise comparisons. RESULTS: Significant variations in vertical marginal adaptation were detected among the tested cohorts (F = 68.5, P < .001). The mean ± SD (µm) values were as follows: G1 = 34.78 ± 5.89; G2 = 63.13 ± 14.53; G3 = 82.54 ± 17.61; and G4 = 99.69 ± 21.66. With the exception of Groups 3 and 4, which demonstrated statistically comparable outcomes (P = .126), all other pairwise comparisons yielded significant disparities. The marginal gap increased in parallel with the production sequence (r = 0.86, P < .001). CONCLUSION: As the milling bur usage cycle increased, the marginal gap increased; however, the difference in marginal discrepancy between the later production groups (Group 3 and Group 4) was no longer statistically significant.