Effect of multiple firings on the marginal fit of monolithic zirconia crowns: An in vitro study

Statement of problem. Speed sintering was introduced to save chair time and produce monolithic zirconia restorations in a single visit. Multiple firings are usually required clinically for both speed -sintered and conventionally sintered monolithic zirconia crowns. However, the effects of multiple firings on the marginal fit of speed-sintered and conventionally sintered monolithic zirconia crowns are unknown.Purpose. The purpose of this in vitro study was to investigate the effect of multiple firings on the marginal fit of speed-sintered and conventionally sintered monolithic zirconia crowns.Material and methods. Twenty conventionally sintered and 20 speed-sintered monolithic zirconia crowns were milled, sintered, and repeatedly fired by using conventional sintering and speed sintering furnaces. The absolute marginal discrepancy of the crowns was measured with a measuring microscope at x100 magnification after sintering (T0) and after the first (T1), second (T2), and third firings (T3). Two-way repeated-measures ANOVA was used to detect the impact of multiple firings on the absolute marginal discrepancy of conventionally sintered and speed -sintered monolithic zirconia crowns and the differences between the 2 materials (a=.05).Results. Multiple firings improved the absolute marginal discrepancy of conventionally sintered and speed-sintered monolithic zirconia crowns (P<.001). The absolute marginal discrepancy of conventionally sintered monolithic zirconia crowns at T2 and T3 was significantly smaller than that at T1 (P=.008 and 0.016, respectively), and the absolute marginal discrepancy of speed -sintered monolithic zirconia crowns at T2 was significantly smaller than that at T1 (P=.015). The speed-sintered monolithic zirconia crowns had a better marginal fit than conventionally sintered monolithic zirconia crowns (P=.008). No significant interaction was found between the multiple firings and material types on the absolute marginal discrepancy of monolithic zirconia crowns (P=.914).Conclusions. Multiple firing cycles can significantly improve the marginal fit of conventionally sintered and speed-sintered monolithic zirconia crowns. The speed-sintered monolithic zirconia crowns have a better marginal fit (both vertically and horizontally) than the conventionally sintered monolithic zirconia crowns. (J Prosthet Dent 2023;130:897-901)