Investigation of the effect of repeated firing on the marginal fit and optical properties of zirconia crowns

In recent years, zirconia has emerged as one of the most favored ceramic materials for prosthetic rehabilitation. To achieve clinical success, zirconia should be influenced minimally by laboratory processes after manufacturing of it. The objective of this study is to investigate the effects of repeated firing - a commonly performed procedure following production - on the marginal fit and optical properties of implant-supported single unit monolithic zirconia. In this study, a maxillary model incorporating an implant analog positioned in tooth number 24 was utilized. A digital impression post was positioned to align with the analog component within the model, and digital impressions were obtained using an intra oral scanner Trios 3 (3Shape, Denmark), in accordance with the scanning protocols recommended by the manufacturer. Digital designs for the crown and abutment were created based on the scanning data using a design software called Exocad (Align Technology Inc., USA). Following the milling process, a total of 12 zirconia crowns (IPS E.max ZirCAD Prime, Schaan, Liechtenstein) and one zirconia ti-base (Evosoft, Turkey) abutment were fabricated (n=12). The sintered crowns were then placed sequentially on individual abutments and examined under a stereomicroscope (Leica Model M80, Germany) at x 16 magnification, capturing images from the defined buccal, palatinal, mesial, and distal surfaces for each crown. Color and translucency measurements were conducted using the Vita Easyshade V (VITA Zahnfabrik, Bad Säckingen, Germany), a clinical spectrophotometer. To evaluate color and translucency parameters, the CIEDE2000 color difference (ΔE00) was measured under optical contact, employing a consistent color background of ideal black, white, and grey for each material. Measurements were repeated after glaze firing (T1), the third firing (T2), and the fifth firing (T3). For the statistical analysis of the obtained data, Repeated Measures ANOVA (F and Greenhouse-Geisser statistics) was employed when normal distribution was present; otherwise, the Friedman test was applied. The results indicated statistically significant differences in marginal fit measurements among the buccal, palatal, and mesial surfaces (p < 0.05), while no significant difference was observed for the distal surface (p > 0.05). Although ΔE00 color measurement findings did not reveal statistically significant differences (p > 0,05), significant differences were noted in the l* and b* values (p < 0,05). Additionally, statistically significant differences were identified in translucency measurement results (p < 0,001). The findings of this study, suggest that the repeated firing process may have an impact on the fit and esthetic qualities of monolithic zirconia restorations. Therefore, it is recommended to minimize the number of firing processes applied to restorations whenever feasible. Keywords: Dental marginal adaptation, zirconium, color