This study aimed to evaluate the fracture strength of implant-supported screw-retained monolithic zirconia crowns compared to porcelain-fused-to-metal (PFM) crowns cemented onto Ti-base abutments. The null hypothesis proposed no statistically significant difference between the two materials in terms of fracture strength and fatigue resistance. Given that bite force is influenced by prosthetic type, patient-specific factors (age, gender), tooth location, and opposing dentition, understanding the mechanical performance of different restorative materials under load is vital for predicting long-term success. Fatigue testing is particularly important in assessing how restorations withstand the repetitive, lower-magnitude forces of daily use. While the single load to failure (SLF) test can determine the maximum force a restoration can endure, it does not replicate clinical loading conditions and should therefore be limited to preliminary analysis rather than clinical prediction. In this study, 54 internal hexagon implants (4.2 mm platform, 10 mm length) were embedded in acrylic resin blocks and positioned at platform level following ISO 14801:2016 guidelines. Each implant received a digitally designed crown for the lower right second premolar (#45), fabricated as either zirconia (n=27) or PFM (n=27), bonded to Ti-base abutments using resin cement, and torqued to 32 Ncm. A universal testing machine applied perpendicular force at a crosshead speed of 1 mm/min until failure occurred. Post-fracture analysis was conducted with micro-computed tomography (micro-CT) and scanning electron microscopy (SEM) to examine the fracture characteristics. The zirconia crowns displayed sharp, sudden fracture peaks with complete failure typically occurring at the midline, reflecting the brittle nature of monolithic zirconia. Conversely, PFM specimens demonstrated smoother force curves and gradual failure patterns, indicating the development of micro-cracks in the porcelain and underlying metal before structural collapse. Although zirconia crowns fractured at lower forces, they exhibited minimal deflection and limited crack propagation, whereas PFM crowns withstood higher forces before catastrophic failure. However, the higher load tolerance of PFM restorations may transmit more stress to the underlying implant and screw assembly, potentially leading to mechanical complications in clinical scenarios. SEM and micro-CT analysis confirmed that while zirconia crowns fractured cleanly, the supporting implant and screw components remained undamaged. PFM specimens, although mechanically robust, pose challenges in reconstruction following porcelain fracture due to the more complex nature of failure. In conclusion, a statistically significant difference in fracture strength exists between the two materials. PFM crowns are more resistant to mechanical loading before failure, while zirconia crowns, though prone to sudden fracture, offer less risk to the implant and screw structure. These findings highlight the trade-offs between material strength and failure behavior in selecting restorative options for implant-supported prostheses, and emphasize the importance of considering not only peak load tolerance but also the mode of failure and its clinical implications.
Ειρήνη Δ. Ζαμπάρα (Wed,) studied this question.