Fracture Resistance of Ceramo-metal Restorations, Variously Retained to implants

Objective: To compare fracture resistance of screw-retained, cement-retained, and the combined screw- and cement-retained metal-ceramic (MC) implant-supported crowns.

Methods: Thirty standardized molar-shaped MCs were fabricated using UCLA patterns and retained to 3i implant fixtures (3i Implant Innovations Inc.) embedded in acrylic resin blocks as follows; SRC: screw-retained with the access hole in the centre of the occlusal surface; CRP: zinc phosphate (ZP) cement-retained with no access hole; CSC: combined ZP cement and screw-retained. After thermal cycling, compressive loading was made by the Instron machine (2mm/min). Mean values of the load at fracture were compared and visual inspection, digital imaging, and inverted microscope examined modes of failure.

Results: loads at fracture were as follows: SRC: 1721 ± 593 N; CRP: 3707 ± 1086 N; CSC: 1700 ± 526 N. SRC and CSC had a significantly lower resistance than CRP (1-way ANOVA and Post Hoc Tukey's test, p<0.05). All groups demonstrated a cohesive fracture in the porcelain. The area involved in fracture was more extensive in the SRC, and CSC than CRP. CRP fractures extended to the middle third and involved only one axial surface of the restorations SRC had fractures extending to the cervical margin. Conclusions: fracture loads were greater than clinically anticipated loads. Cement-retained restorations had the highest resistance compared to screw retained or the combined cement and screw retained restorations. The metal-ceramic bond proved to be efficient and predictable in all groups with failures being judged as repairable.