The ability of mouthguards to protect veneered teeth: A 3D finite element analysis.

BACKGROUND/AIMS: Professional and amateur athletes might have veneer restorations. The aim of this study was to investigate the protective effect of mouthguards on veneered anterior restorations. METHODS: A nonlinear dynamic analysis was performed to simulate conditions during an impact with or without a custom-made mouthguard. Using a computer-aided design (CAD) software, a slice of a human maxilla was designed containing an upper right central incisor. The model was composed of mucosa, cortical bone, trabecular bone, periodontal ligament, dentin, enamel, and pulp tissue. The enamel was prepared (feather design), restored with an indirect veneer (1.0 mm thickness), and duplicated to simulate both conditions with or without a mouthguard (4 mm thickness). Both models were subdivided into finite elements using the computer-aided engineering (CAE) software. Frictionless contacts were used, and an impact was simulated in which a rigid sphere hit the model at 1 m s-1 . Fixation was defined at the base of the bone. The elastic modulus of the veneer was assessed by using five different restorative materials (resin composite, hybrid ceramic, zirconia-reinforced lithium silicate, lithium disilicate, and zirconia). Von Mises stress, minimal principal stress, and maximum principal stress (in MPa) were obtained and plotted for visual comparison. RESULTS: Von-Mises results showed higher stress concentrations in the veneer's cervical labial region for models without a mouthguard. Observing the quantitative results for each model, the highest compressive (709 MPa) and tensile (58 MPa) stresses occurred in the situation without a mouthguard with a zirconia veneer, while the lowest occurred in resin composite veneer with a mouthguard (8 and 5 MPa). The mouthguard was able to reduce the stresses in the tooth structure and it also reduced the risk of fracture in all conditions. CONCLUSIONS: Mouthguards were beneficial in reducing the effects of dental trauma regardless of the restorative material used to manufacture the indirect veneer, since they act by dampening the generated stresses during the trauma event. Equal impact stresses on a mouthguard will lead to higher stresses in veneered teeth with more rigid restorative materials leading to a less protective effect.

Fatigue resistance of simplified CAD-CAM restorations: Foundation material and ceramic thickness effects on the fatigue behavior of partially- and fully-stabilized zirconia.

OBJECTIVE: To evaluate the fatigue failure load, number of cycles until failure and survival probability of partially (PSZ) and fully-stabilized (FSZ) polycrystalline zirconia disc shaped specimens with different thicknesses adhesively cemented onto foundations with distinct elastic moduli. METHODS: Disc-shaped specimens (n = 15, Ø = 10 mm; thickness = 1.0 and 0.7 mm) of CAD/CAM PSZ and FSZ blocks were adhesively cemented onto discs with different foundations (Ø = 10 mm; thickness = 2.0 mm) made from epoxy resin, composite resin or Ni-Cr metallic alloy. The cemented assemblies were subjected to fatigue testing using a step-stress approach (600-2800 N; step-size of 100 N; 10,000 cycles per step; 20 Hz) and the data was submitted to specific statistical tests (α = 0.05). Fractography and finite element (FEA) analyzes were also performed. RESULTS: PSZ and FSZ presented higher fatigue failure load, number of cycles until failure and survival probabilities when cemented onto metallic alloy. All PSZ specimens survived the fatigue test when cemented onto Ni-Cr alloy (100% probability of survival at 2800 N; 230,000 cycles). Regardless of the foundation type, PSZ had better fatigue behavior than FSZ. For thickness, thinner PSZ restorations underperformed when bonded to softer foundations, while FSZ groups and groups bonded to metallic foundations had no statistical difference. SIGNIFICANCE: The foundation material strongly influences the fatigue performance of PSZ and FSZ restorations, which presented mechanical behavior improvements when bonded to a metallic foundation. PSZ restorations showed better fatigue behavior than FSZ, while the ceramic thickness only influenced PSZ restorations bonded to softer foundations.

Laminated ceramics with elastic interfaces: a mechanical advantage?

OBJECTIVES: As CAD/CAM technologies improve we question whether adhesive lamination of ceramic materials could offer mechanical advantages over monolithic structures and improve clinical outcomes. The aim was to identify whether an adhesive interface (a chemically cured resin-cement) would influence the biaxial flexure strength (BFS) and slow-crack growth in a machinable dental ceramic. METHODS: Monolithic and adhesively laminated (with a chemically cured dimethacrylate resin-cement) feldspathic ceramic discs of identical dimensions were fabricated. BFS testing was performed on the Group A monolithic specimens (n = 20), on Group B laminated specimens with the adhesive interface positioned below the neutral bending axis (n = 20) and Group C laminated specimens with the adhesive interface positioned above the neutral bending axis (n = 20). To study subcritical crack growth additional laminated specimens received controlled indentations and were exposed to thermo-mechanical fatigue. BFS data was analysed using parametric statistics (α = 0.05). Fractographic analyses were qualitatively assessed. RESULTS: No significant differences between the mean BFS data of Groups A and B were observed (p = 0.92) but the mean BFS of Group C was slightly reduced (p < 0.01). Lamination reduced the stiffness of the structure and fractographic analysis demonstrated that energy consuming crack deflection occurred. Thermo-mechanical fatigue caused subcritical extension of radial cracks associated with indentations adjacent to the adhesive interface. Crack growth was limited to parallel to the interface and was arrested or deflected in a direction normal to the interface. CONCLUSIONS: Ceramic lamination increased the damage tolerance of the structure and could limit or arrest subcritical crack growth at regions near the 'interlayer'. CLINICAL SIGNIFICANCE: Lamination of a dental ceramic with a polymeric 'interlayer' could offer toughening effects which could potentially delay or arrest sub-critical crack growth at regions near the interface and thereby improve restoration longevity.

The strength of sintered and adhesively bonded zirconia/veneer-ceramic bilayers.

OBJECTIVES: Recently all-ceramic restorative systems have been introduced that use CAD/CAM technology to fabricate both the Y-TZP core and veneer-ceramic layers. The aim was to identify whether the CAD/CAM approach resulted in more favourable stressing patterns in the veneer-ceramic when compared with a conventionally sintered Y-TZP core/veneer-ceramic. METHODS: Nominally identical Vita VM9 veneer-ceramic disc-shaped specimens (0.7mm thickness, 12mm diameter) were fabricated. 20 specimens received a surface coating of resin-cement (Panavia 21); 20 specimens were bonded with the resin-cement to fully sintered Y-TZP (YZ Vita Inceram Vita) discs (0.27mm thickness, 12mm diameter). A final series of 20 Y-TZP core/veneer-ceramic specimens were manufactured using a conventional sintering route. Biaxial flexure strength was determined in a ball-on-ring configuration and stress at the fracture origin calculated using multilayer closed-form analytical solutions. Fractography was undertaken using scanning electron microscopy. The experimental test was simulated using Finite Element Analysis. Group mean BFS were compared using a one-way ANOVA and post hoc Tukey tests at a 95% significance level. RESULTS: Resin cement application resulted in significant strengthening of the veneer-ceramic and further significant strengthening of the veneer-ceramic (p<0.01) occurred following bonding to the Y-TZP core. The BFS calculated at the failure origin for conventionally sintered specimens was significantly reduced when compared with the adhesively bonded Y-TZP/veneer-ceramic. CONCLUSIONS: Under the test conditions employed adhesive cementation between CAD/CAM produced Y-TZP/veneer-ceramic layers appears to offer the potential to induce more favourable stress states within the veneer-ceramic when compared with conventional sintered manufacturing routes. CLINICAL SIGNIFICANCE: The current investigation suggests that the stressing patterns that arise in all-ceramic restorations fabricated using CAD/CAM for both the core and veneer-ceramic layers differ from those that occur in conventionally sintered bilayer restorations. Further work is required to ascertain whether such differences will translate into improved clinical outcomes.