Load-bearing properties of minimal-invasive monolithic lithium disilicate and zirconia occlusal onlays: finite element and theoretical analyses

Abstract

Objective: The aim of this study was to test the hypothesis that monolithic lithium disilicate glass-ceramic occlusal onlay can exhibit a load-bearing capacity that approaches monolithic zirconia, due to a smaller elastic modulus mismatch between the lithium disilicate and its supporting tooth structure relative to zirconia.

Methods: Ceramic occlusal onlays of various thicknesses cemented to either enamel or dentin were considered. Occlusal load was applied through an enamel-like deformable indenter or a control rigid indenter. Flexural tensile stress at the ceramic intaglio (cementation) surface-a cause for bulk fracture of occlusal onlays-was rigorously analyzed using finite element analysis and classical plate-on-foundation theory.

Results: When bonded to enamel (supported by dentin), the load-bearing capacity of lithium disilicate can approach 75% of that of zirconia, despite the flexural strength of lithium disilicate (400MPa) being merely 40% of zirconia (1000MPa). When bonded to dentin (with the enamel completely removed), the load-bearing capacity of lithium disilicate is about 57% of zirconia, still significantly higher than the anticipated value based on its strength. Both ceramics show slightly higher load-bearing capacity when loaded with a deformable indenter (enamel, glass-ceramic, or porcelain) rather than a rigid indenter.

Significance: When supported by enamel, the load-bearing property of minimally invasive lithium disilicate occlusal onlays (0.6-1.4mm thick) can exceed 70% of that of zirconia. Additionally, a relatively weak dependence of fracture load on restoration thickness indicates that a 1.2mm thin lithium disilicate onlay can be as fracture resistant as its 1.6mm counterpart.

Figure 1

Ex vivo occlusal model. (a) Optical image of a cross-section through the mesial cusp region of a human mandibular second molar indicating the enamel thickness used in this study. BCT: Buccal cusp thickness (2.1 mm); ICS: Enamel thickness in the internal cusp slope region (1.8 mm). (b) Occlusal contact on the functional cusps of a mandibular molar. (c) and (d) Schematic representations of the box highlighted area in (b), used in finite element and analytical stress analyses for blunt loading of ceramic veneer bonded to enamel and dentin, respectively. The symbol P represents load, and R represents cementation surface radial cracks.

Figure 2

Details of the FE mesh generated from a ceramic/cement/enamel/dentin four-layer model loaded with a (a) rigid and (b) deformable indenter.

Figure 3

FEA-generated stresses in lithium disilicate onlays of thicknesses (a) 0.5 mm and (b) 1.0 mm cemented to enamel backed by dentin, loaded with a deformable indenter of radius, r = 3.2 mm, and elastic modulus, E = 95 GPa.

Figure 4

Comparison of ceramic fracture load versus thickness relationship generated by FEA simulation and theoretical analysis. Ceramic plates of various thicknesses were bonded to dentin and loaded at the top surface with a rigid indenter. Open symbols: FEA generated data; solid symbols: theoretical predictions. Triangles: lithium disilicate (LiDi); circles: zirconia (ZrO₂).

Figure 5

Load-bearing capacity of ceramics bonded to and supported by dentin loaded with a rigid indenter. (a) Ceramics luted to enamel backed by dentin (solid symbols) relative to those bonded to dentin alone (open symbols). Triangles: lithium disilicate (LiDi); circles: zirconia (ZrO₂). (b) Fracture load ratio between lithium disilicate and zirconia as a function of ceramic thickness and substrate materials. P_R-LiDi and P_R-ZrO2 denote fracture loads for lithium disilicate and zirconia, respectively. Solid circles: ceramics luted to enamel backed by dentin. Open triangles: ceramics luted to dentin. Dashed line: ceramics in freestanding flexure.

Figure 6

Load-bearing capacity of ceramics on enamel backed by dentin loaded with a deformable indenter (solid symbols) relative to a rigid indenter (open symbols). (a) Fracture loads for lithium disilicate (triangles) and zirconia (circles), and (b) fracture load ratio between lithium disilicate and zirconia as a function of ceramic thickness. Dashed line represents fracture load ratio for the two ceramics in freestanding flexure.