Fracture resistance of monolithic zirconia restorations after endodontic access cavity preparation: Effect of repair method

objective: The present study was performed to assess the effect of restoration of access cavity with composite resin filling on fracture resistance of multilayered zirconia crowns. Materials and Methods: Thirty-three sound human mandibular first molars were prepared and restored with three types of monolithic zirconia crowns: High-Translucent Multilayered Plus (HTML), Yttria Multilayered (YML), and Ultra-Translucent Multilayered (UTML) (N=11 each). Standardized access cavities were prepared through the occlusal surfaces, endodontic treatment was completed, and each access opening was repaired using incremental composite resin. The specimens were subjected to thermocycling (10,000 cycles between 5 °C and 55 °C) and cyclic mechanical loading (240,000 cycles at 50 N) prior to single-load fracture testing. Data were statistically analyzed using one-way ANOVA and Tukey post-hoc tests. Results: There was significant interaction (P=.001) of crown material and repair method fracture resistance. High-Translucent Multilayered Plus (HTML) showed the highest fracture value(2132.14±282.86N) followed by Yttria Multilayered (YML) (1665.72±220.98N) and UTML multilayered zirconia (1133.12±150.33N) Conclusions: Both the type of zirconia and the selected repair protocol significantly influence the fracture resistance of monolithic zirconia crowns following endodontic access cavity preparation. High-Translucent Multilayered Plus zirconia demonstrated the greatest overall fracture strength, particularly when repaired with direct composite resin. Ultra-Translucent Multilayered zirconia exhibited the lowest resistance to fracture. These differences highlight the critical role of microstructural composition and yttria concentration in determining the mechanical performance of zirconia restorations after repair.