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

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.

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.

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).

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.