A Fractographic Study of Clinically Retrieved Zirconia-Ceramic and Metal-Ceramic FPDs

Objectives: A recent 3-year clinical study of randomized controlled three- to five-unit metal-ceramic and zirconia-ceramic fixed partial dentures (FPDs) revealed that veneer chipping and fracture in zirconia-ceramic systems occurred more frequently than those in metal-ceramic systems. This study seeks to elucidate the underlying mechanisms responsible for this clinical observation using a quantitative fractographic analysis.
Methods: Thirty-six zirconia-ceramic and thirty-one metal-ceramic FPDs were randomly placed in fifty-nine patients who had good general and periodontal health with no obvious signs of bruxism. Impressions of FPDs were taken from the patients at the end of a mean observation period of 40.3 ± 2.8 months. Epoxy replicas were produced from these impressions. All replicas were gold coated, and inspected under the optical microscope and scanning electron microscope (SEM). Fracture origin, fracture path, damage modes, and stress states were determined. Critical load for the onset of fracture was estimated based on the principles of fracture mechanics.
Results: Among 31 metal-ceramic FPDs, 25 survived, 5 had small chippings and 1 had large chipping. Out of 36 zirconia-ceramic FPDs, 24 survived, 2 had small chippings, 9 had large chippings, and 1 exhibited delamination. Detailed SEM examinations revealed that fracture initiated from the wear facet at the occlusal surface in all cases, irrespective of the type of restoration.
Conclusions: Metal-ceramic and zirconia-ceramic FPDs all fractured from veneer chipping with the origin of fracture located in an occlusal wear facet. The relatively low fracture toughness and high residual tensile stress in the porcelain veneer of zirconia-based restorations contributed to the higher chipping rate and larger chip size in zirconia-ceramic FPDs relative to their metal-ceramic counterparts. The low veneer/core interfacial toughness of porcelain-veneered zirconia resulted in a higher incidence of delamination in zirconia-ceramic FPDs relative to metal-ceramic FPDs.