July/August 2009, Volume 5, Issue 7
Published by AEGIS Communications
Failure Modes of Two Implant-Supported
Oxide-Ceramic Fixed-Partial-Denture Systems
Howard E. Strassler, DMD
Stappert CF, Cabrera M, Baldassarri B, et al. J Dent Res. 2008;88(Special Issue IADR abstracts): Abstract 119.
Objectives: To compare fatigue and damage-modes between aluminum-oxide and zirconium-oxide all-ceramic systems in implant-supported fixed-partial-dentures (FPDs) using off-axis mouth-motion fatigue loading.Methods: Zirconium-oxide abutments* (n = 36/group) were fabricated on NobelReplace* implant-replicas. Aluminum-oxide and zirconium-oxide bridge frameworks* (n=18/group) were CAD/CAM manufactured. Frameworks were veneered with hand-built up porcelains (*NobelBiocare). Abutments were screw-retained with 32 Ncm torque and FPDs were cemented and incubated in water at 37°C for at least 14 days. Material-specific step-stress-profiles were designed based on load-to-failure values (n = 2/group). Accelerated life testing was performed with distributed load immersed in water. Failure was defined by chipping or framework failure. Specimens were sectioned and inspected (polarized-specular-reflection-light-microscopy) at regular intervals to visualize crack propagation and failure mode. Reliability was compared using cumulative-damage step-stress analysis (Alta-7-Pro, Reliasoft).
Results: Crack propagation was observed in the veneering porcelain during fatigue for both systems. The majority of zirconium-oxide FPDs demonstrated porcelain chipping as dominant failure mode. Nevertheless, fatigue of the zirconium-oxide frameworks was observed. Aluminum-oxide FPDs showed more dramatic failure, but were less limited by early porcelain chipping. Weibull-stress-level-probability-curves were calculated and the reliability (2-sided at 90.0% confidence bounds) for 50K cycles and a 400N load indicated values of 0.97(0.99/0.77) for zirconium-oxide FPDs and 0.82(0.92/0.60) for aluminum-oxide FPDs.
Conclusions: Within the limitations of this study, mechanical-fatigue-test results demonstrated similar reliability for implant-supported aluminum-oxide and zirconium-oxide all-ceramic three-unit-FPDs under average clinical loads. A variation of failure modes is expected.
With the increased use of all-ceramic restorations with alumina and zirconia cores, there have been concerns that these materials may not have been adequately clinically tested before being recommended for use. The physical property testing of these silica, alumina, and zirconia-based ceramics have been typical strength testing. In some cases the clinical results were disappointing. The failure with fracture of veneer porcelain from the underlying zirconia, alumina, and metal substructure closely parallels what is seen when porcelain fractures intraorally.
This in vitro study uses a well-designed and proven protocol for testing porcelains. As practitioners we are placing and restoring implants as well as natural teeth with all-ceramic FPDs. There is a need to know how all-ceramic FPDs will perform. Based on this proven mechanical testing design, mechanical-fatigue test results demonstrated similar reliability for implant-supported, aluminum- oxide and zirconium-oxide, all-ceramic, three-unit FPDs under average clinical loads. Failure was defined as porcelain fracture. Similar to what is seen clinically, in most cases the failures were within the veneering porcelain. Additional investigation needs to be done to look at how framework substructure design contributes to the success or failure of all-ceramic FPDs.
About the Author
Howard E. Strassler, DMD
Professor and Director of Operative Dentistry
Department of Endodontics
Prosthodontics and Operative Dentistry
University of Maryland Dental School