Methods: Six hand-veneered 3-unit zirconia fixed dental prostheses (FDPs) were manufactured using a slow cooling (NobelBiocare, Gothenburg, Sweden). Each FDP was sectioned parallel to the occlusal plane for Vickers indentations (n=143; 9.8 N peak load; 5 s dwell time). Tests were performed in the veneer of porcelain-fused-to-zirconia specimens (bilayers, n=4) and monolithic porcelain specimens (without core, n=2). Indentations were performed with sharp corners oriented perpendicular (radial) and parallel (transverse) to the veneer/core interface. Residual stresses (σR) were estimated using the following formula: σR= (K1c*(1-((c0/c1)^(3/2))))/(φ*(c1^(1/2))) where ψ (=1.24): crack geometry factor; c0 and c1: indentation crack lengths in unstressed (monolith) and stressed (bilayer) materials, respectively; K1c: fracture toughness of porcelain).
Results: The crack lengths for the bilayers were 67±12 μm (mean±SD) and 52±8 μm in the transverse and radial directions, respectively (p<0.001). The crack lengths in the monolithic porcelain were 64±8 and 64±7 μm in transverse and radial directions, respectively. Compared to unstressed porcelain monoliths, indentation cracks in bilayers were significantly longer (p=0.006) in the transverse but shorter (p=0.001) in the radial directions. The above equation predicts a hoop compressive stress (~40 to 50 MPa) and a radial tensile stress (~10 MPa) in the bulk of porcelain veneers.
Conclusion: Our results demonstrate the presence of a residual radial tensile stress in the porcelain of veneered zirconia prostheses. We contend that this radial tensile stress is responsible for the large clinical chips and fractures observed in veneered zirconia restorations.
Keywords: Bioengineering, Biomaterials, Biomechanics, Implants and Porcelain systems