Residual stresses in porcelain-veneered zirconia prostheses

Abstract

Objectives: Compressive stress has been intentionally introduced into the overlay porcelain of zirconia-ceramic prostheses to prevent veneer fracture. However, recent theoretical analysis has predicted that the residual stresses in the porcelain may be also tensile in nature. This study aims to determine the type and magnitude of the residual stresses in the porcelain veneers of full-contour fixed-dental prostheses (FDPs) with an anatomic zirconia coping design and in control porcelain with the zirconia removed using a well-established Vickers indentation method.

Methods: Six 3-unit zirconia FDPs were manufactured (NobelBiocare, Gothenburg, Sweden). Porcelain was hand-veneered using a slow cooling rate. Each FDP was sectioned parallel to the occlusal plane for Vickers indentations (n = 143; load = 9.8 N; dwell time = 5s). Tests were performed in the veneer of porcelain-zirconia specimens (bilayers, n=4) and porcelain specimens without zirconia cores (monolayers, n = 2).

Results: The average crack lengths and standard deviation, in the transverse and radial directions (i.e. parallel and perpendicular to the veneer/core interface, respectively), were 67 ± 12 μm and 52 ± 8 μm for the bilayers and 64 ± 8 μm and 64 ± 7 μm for the monolayers. These results indicated a major hoop compressive stress (~40-50 MPa) and a moderate radial tensile stress (~10 MPa) in the bulk of the porcelain veneer.

Significance: Vickers indentation is a powerful method to determine the residual stresses in veneered zirconia systems. Our findings revealed the presence of a radial tensile stress in the overlay porcelain, which may contribute to the large clinical chip fractures observed in these prostheses.

Figure 1

Clinical and laboratory fractures in porcelain-veneered zirconia prostheses (Procera, Nobel Biocare, Gothenburg, Sweden). a) Clinically fractured zirconia FDP: a hand-veneered zirconia upper right central incisor crown of a Procera Implant Bridge fractured by porcelain chipping six months after restoration. Protrusive contact was documented on the incisal edge of the crown. b) Laboratory fractured zirconia FDP: an over-pressed zirconia Procera Implant Bridge loaded under mouth-motion step-stress accelerated life testing fractured by buccal chipping of the porcelain.

Figure 2

Schematic plan view showing specimen sectioning direction and the incipient surface for Vickers indentation. a) Two cuts (dashed lines) were made parallel to the occlusal plane; one surface (blue arrows) was indented. ZA: zirconia abutment; ZC: zirconia core; PV: porcelain veneer; and I: implant. b) The incipient surface of bilayer specimens, including two abutments, a zirconia framework, and a porcelain veneer. Indentations were performed with the two orthogonal axes parallel and perpendicular to the zirconia/porcelain interface so that crack length in both transverse (T) and radial (R) directions could be measured.

Figure 3

Schematic plan view of fracture patterns produced by Vickers diamond pyramid indentation on porcelain. Crack lengths were measured from the center of the indentation impression to the crack tips for all directions. a) In unstressed material, cracks emanating from the four corners of the impression have identical length (c₀). b) In stressed material, the crack lengths (c₁) can vary depending on the nature and direction of the residual stresses. For cracks propagating near-perpendicular to a major component of tension, c₁ > c₀. For cracks extending near-perpendicular to the major component of compression, c₁ < c₀.

Figure 4

Representative crack patterns of Vickers indentation on porcelain with and without a zirconia framework. a) For porcelain/zirconia bilayered specimens, crack lengths in the transverse direction (T) parallel to the zirconia/porcelain interface are significantly longer than those in the radial direction (R). b) For monolithic porcelain specimens, crack lengths in all directions are identical.