Comparison of Vertical Marginal Discrepancy in High and Low Translucent Monolithic Zirconia Crowns in Repeated Firing Cycles.

Objective:  Increase in vertical marginal discrepancy (VMD) during repeated firing cycles and its clinical outcomes is a major concern for high and low translucent monolithic zirconia crowns. The purpose of this in vitro study was to evaluate and compare VMD in high and low translucent monolithic zirconia crowns in repeated firing cycles.

Material and methods:  To perform this study, 10 monolithic zirconia crowns made by computer-aided design and computer-aided manufacturing method were used in two groups of five with high and low translucency, which were designed on Zimmer tissue-level implant abutment. The crowns in each group were randomly numbered from 1 to 5 and each underwent 1, 3, and 5 firing cycles. After completing each mentioned cycle, the VMD was measured at eight predetermined points on abutment by optical microscope and their average was recorded for each sample. Data analysis was done by SPSS 22 software through repeated-measure analysis of variance, paired t-test, and t-test with a 5% significance threshold.

Results:  A total of 240 measurements were made for the VMD, which, due to the presence of five samples in each translucency group and eight examined points in each sample, was finally summed up to six averages for each translucency group in the mentioned three stages of firing cycles. The averages for the low-translucency group after 1, 3, and 5 firing cycles were 76.86, 85.02, and 90.55 μm, respectively, and for the high-translucency group after 1, 3, and 5 firing cycles were 80.38, 87.33, and 97.78 μm, respectively. The average VMD of the samples regardless of the translucency level after 1, 3 and 5 firing cycles was calculated as 78.62, 86.18, and 94.16 μm, respectively.

Conclusion:  This study found that VMD increased with repeated firing cycles, with no significant difference between high- and low-translucency zirconia crowns. Repeated firings significantly raised VMD, but all values remained within clinically acceptable limits, supporting the suitability of both translucency types for clinical use.