Load-bearing capacity of lithium disilicate and ultra-translucent zirconias

Abstract

Objective: The aim of this study was to evaluate the load-bearing capacity of monolithic lithium disilicate (LiDi - IPS e.max CAD) and novel ultra-translucent zirconia restorative systems of various compositions: 5Y-PSZ (5 mol% yttria-partially-stabilized zirconia) and 4Y-PSZ (4 mol% yttria-partially-stabilized zirconia); relative to a 3Y-TZP (3 mol% yttria-stabilized zirconia) control.

Materials and methods: Experiments were carried out with 10 disc specimens (Ø12 ×1 mm) per ceramic material. The zirconia intaglio surface (as machined) was sandblasted (50 µm Al₂O₃ at 2 bar), while LiDi was etched with 5% HF for 20 s. The ceramic discs were then adhesively bonded onto a dentin-like substrate (G10, a high-pressure fiberglass material) using Multilink Automix cement and Monobond Plus primer, producing a ceramic/cement/dentin-like substrate trilayer structure. The bonded specimens were stored in water for 3 days at 37 °C prior to a Hertzian indentation flexural radial fracture test. The plate-on-foundation theory was used to validate the load-bearing capacity of the trilayer systems based on the flexural tensile stress at the ceramic intaglio (cementation) surface-a cause for bulk fracture of ceramic onlays.

Results: The experiment data showed that, when bonded to and supported by a dentin-like substrate, the load-bearing capacity of LiDi (872 N) is superior to the 5Y-PSZ (715 N) and can even reach that of 4Y-PSZ (864 N), while 3Y-TZP still holds the highest load-bearing capacity (1195 N). Theoretical analyses agree with experimental observations. The translucency of 5Y-PSZ approaches that of LiDi, which are superior to both 4Y-PSZ and 3Y-TZP.

Conclusions: When adhesively bonded to and supported by dentin, lithium disilicate exhibits similar load-bearing properties to 4Y-PSZ but much better than 5Y-PSZ.

Keywords: Elastic modulus; Flexural strength; Layer thickness; Lithium disilicate; Load-bearing capacity; Ultra-translucent zirconia.

Figure 1.

Schematic of the Hertzian load-to-fracture test configuration used to determine the critical load of radial crack.

Figure 2.

The zirconias are polycrystalline ceramics, with (a) 3Y-TZP having average crystal size of 0.54 ± 0.04 μm, (b) 4Y-PSZ 0.95 ±0.03 μm, and (c) 5Y-PSZ 1.33 ± 0.06 μm. (d) IPS e.max CAD is a glass ceramic composed of ~70% lithium disilicate and lithium phosphate crystals of 2 to 4 µm average size.

Figure 3.

Representative stereo optical microscopy images show surface features of (a) 3Y-TZP and (c) 5Y-PSZ discs fractured at 1010 N and 774 N, respectively. Images were obtained using reflected light illumination. Arrows indicate the contact area, demonstrating the absence of near-contact induced top surface cone cracks. (b) and (d) are images of the same specimens, but acquired using transmitted light illumination to reveal the far-field flexural induced radial cracks initiated at the cementation surface of the ceramic discs. Images were taken on ceramic discs that have been “peeled off” of their dentin analog composite substrate after the Hertzian load-to-fracture test.

Figure 4.

Theoretical and experimental load-bearing capacity of zirconia and lithium disilicate for the trilayer systems—ceramic/cement/dentin-like substrate. Different upper and lower-case letters indicate a statistical difference among materials for theory and experiment, respectively.

Figure 5.

Digital photograph illustrating the comparative translucency of the four materials at 1 mm thickness. Average (standard deviation) of translucency parameter (TP) and contrast ratio (CR). Different letters indicate statistical difference among materials, within each property.