Digital dentistry has revolutionized single-day dental prosthetic treatments, significantly improving efficiency and precision. Compared to conventional convection furnaces, high-speed sintering furnaces drastically reduce sintering time from several hours to minutes. However, variations in sintering time and temperature may alter material characteristics, affecting the dimensional stability and fit of fixed dental prosthetics. Previous studies on internal fit and marginal adaptation have yielded conflicting results, primarily focusing on single-unit restorations.

This study investigates the effects of sintering protocols and yttria content on the internal fit and marginal adaptation of 3-unit monolithic zirconia fixed dental prostheses (FDPs). FDPs were fabricated using 3Y-TZP, 4Y-TZP, and 5Y-TZP zirconia under three sintering protocols: conventional sintering (CS), high-speed sintering (HS), and an optimized speed sintering (OS) protocol developed in-house to improve translucency while maintaining efficiency. A silicone impression method was used to quantify marginal and internal discrepancies.

Results indicate that both sintering protocol and yttria content significantly affect adaptation. 3Y-PSZ exhibited the smallest marginal and internal gaps across all conditions, with CS yielding the highest precision. 4Y-PSZ demonstrated intermediate adaptation, while 5Y-PSZ showed the largest discrepancies, particularly under HS, where some gaps exceeded clinically acceptable thresholds. A statistically significant interaction between yttria content and sintering protocol was observed, with adaptation accuracy decreasing as yttria content increased. HS resulted in the largest variation, highlighting its limitations in maintaining precise adaptation, while OS provided a more clinically acceptable balance between speed and fit.

These findings emphasize the need for material-specific sintering strategies. CS remains the most reliable method for optimal adaptation, particularly for zirconia with higher translucency. OS presents a viable alternative for achieving clinically acceptable results with reduced processing time, making it suitable for chairside workflows. However, HS, while time-efficient, introduces greater dimensional instability, necessitating further optimization.

This study contributes to the growing body of knowledge in digital dentistry, offering valuable insights into material selection and sintering protocols for zirconia FDPs. By refining sintering parameters, future advancements may enhance chairside zirconia restoration workflows, improving efficiency, precision, and long-term clinical outcomes.