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.
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.
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 (400 MPa) being merely 40% of zirconia (1000 MPa). 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.
When supported by enamel, the load-bearing property of minimally invasive lithium disilicate occlusal onlays (0.6–1.4 mm thick) can exceed 70% of that of zirconia. Additionally, a relatively weak dependence of fracture load on restoration thickness indicates that a 1.2 mm thin lithium disilicate onlay can be as fracture resistant as its 1.6 mm counterpart.
Esthetics and preservation of tooth structure are the primary driving forces in modern restorative dentistry . Bonded feldspathic porcelain and glass-ceramic veneers have become the treatment of choice for restorations of anterior teeth, with proven long-term success . Superior esthetics, diminished gingival inflammation, and reduced risk for secondary caries are cited as the benefits of these restorations . However, fracture of bonded ceramics becomes a concern when considering the same treatments for posterior teeth. This is particularly the case with restorations covering the entire occlusal surface. To meet the load-bearing requirements, the current recommendation for posterior porcelain restoration thickness is 1.5–2.0 mm . With the development of stronger ceramics, thinner more conservative restorations can be made to meet the posterior load requirements.
The rehabilitation of patients with increased chewing forces due to bruxism or parafunctions represent a particular challenge in restorative dentistry . In most studies on all-ceramic restorations, patients with parafunctions were excluded due to the increased risk of fracture. Restorations with metal occulsal surfaces were the standard of care for these patients; however, these restorations did not meet the esthetic demands of patients .
Zirconia, the strongest and toughest of all dental ceramics, with a flexural strength of 800–1200 MPa and a fracture toughness of 6–8 MPa m 1/2 , meets the mechanical requirements for high stress-bearing posterior restorations. Zirconia, however, is a non-adhesive restoration and has limited translucency and shade options. Lithium disilicate glass-ceramics, the strongest and toughest of the glass-ceramics available, have moderate flexural strength (360–440 MPa) and fracture toughness (2.5–3 MPa m 1/2 ) , yet have excellent translucency and shade matching options . These glass ceramics can be bonded to enamel and dentin through hydrofluoric acid etching and silanization. Additionally, glass ceramics prevent excessive wear of the opposing dentition due to their similar modulus and hardness to enamel. Therefore, it would be most desirable if lithium disilicate glass-ceramics could be found to have similar load-bearing capacities to zirconia through fine tuning of application parameters such as ceramic thickness, bonding substrate, and cement modulus and thickness.
The thickness of the restoration is dictated by the amount of tooth preparation required to remove disease, obtain natural looking contours, rehabilitate occlusion, promote periodontal health, and/or to meet the physical requirements of the restorative material. Thinner, conservative occlusal veneers and onlays provide the advantage of enamel bonding which offers superior bond strength as compared to dentin . However, thin ceramics have low fracture resistance. One way to overcome this is to reduce the modulus ratio between the ceramic and tooth support (enamel and/or dentin) . Thus, there exists an optimal ceramic thickness and ceramic/substrate modulus ratio that yields excellent load-bearing capacity of ceramic restorations. This study compares the load-bearing capacity of monolithic lithium disilicate onlays of various thicknesses to their zirconia counterparts, when bonded onto enamel or dentin, and loaded with a deformable or rigid indenter using finite element stress analysis and the classical plate-on-foundation theory.
Materials and methods
Ex vivo occlusal model
We shall first establish an occlusal model for stress analysis in posterior ceramic restorations using classical plate-on-foundation theory and finite element analyses (FEA). Ceramic occlusal veneers and onlays are bonded to and supported by enamel or dentin ( Fig. 1 ). While chewing, high flexural tensile stresses may develop on the ceramic intaglio (cementation) surface directly below the loaded area ( Fig. 1 b) . When these tensile stresses exceed the flexural strength of the ceramics, radial fracture occurs in the ceramic subsurface, at its interface with the cement. Clinically, this radial fracture can cause the bulk failure of all-ceramic restorations . The stress solution in a ceramic restoration is complex and is best solved by finite element methods. Thus, FEA utilizing axis-symmetrical models of multilayer structures loaded with a blunt indenter ( Fig. 1 c and d) have been widely used to determine the flexural tensile stresses in the ceramic restorations . The accuracy and reliability of such FEA simulation have been validated by experimentation and theory . Below we outline the key points of the plate-on-foundation theory and describe the details of the FEA method utilized in this study. But first, we define the parameters of the multilayer models used for our FEA and theoretical analyses.