BACKGROUND: Gold alloy inlays have a long history of application in the repair of posterior dental defects. As the gold standard for inlay repair, its excellent mechanical properties and biocompatibility have long been recognized. However, there is no consensus on the design of gold alloy inlays, selection of adhesives, stress analysis of the bonding interface, and design and assessment of resistance and retention forms of the remaining tooth tissues. Further investigation is beneficial to increase the long-term success in gold alloy inlays.
OBJECTIVE: To establish a three-dimensional finite element model of mesio-occluso-distal cavity (MOD) gold inlay of the third molar in the right mandible, and after the use of 3M RelyX Unicem and vario-link resin adhesive to bond the gold alloy inlay, to explore the stress distribution and characteristics of bonding interface and dental tissues with different cavity depth.
METHODS: Micro-CT scanning, Mimics, Goemagic Studio, NX 10.0 and other software were used to establish a three-dimensional finite element model of MOD gold inlay with different cavity depths and adhesives. Profiles for different models are as follows: Model A: cavity depth, 1 mm; cavity width, 3 mm; adhesive, 3M RelyX Unicem resin adhesive. Model B: cavity depth, 2 mm; cavity width, 3 mm; adhesive, 3M RecessX Unicem resin adhesive. Model C: cavity depth, 4 mm; cavity width, 3 mm; adhesive, 3M RelyX Unicem resin adhesive. Model D: cavity depth, 1 mm; cavity width, 3 mm; adhesive, vario-link resin adhesive. Model E: cavity depth, 2 mm; cavity width, 3 mm; adhesive, vario-link resin adhesive. Model F: cavity depth, 4 mm; cavity width, 3 mm; adhesive, vario-link resin adhesive. With the grid of ANSYS Workbench software, the stress distribution of models at 10 N•mm torque under lingual 45° 175 N and lingual 90° 600 N were analyzed.
RESULTS AND CONCLUSION: Minimal equivalent stress of the tooth root was found in the model with cavity depth of 2 mm under lingual 45° 175 N, which was suitable for gold alloy inlays and vario-link adhesive repair. Under lingual 90° 600 N, the equivalent stress of the tooth root was higher in model F than model C; the maximum root stress in model A was higher than that in model A; the equivalent stress of the tooth root was higher in vario-link models than 3M RelyX Unicem models. When the cavity depth was equal to 4 mm, a higher destructive stress of the tooth root produced on the vario-link bonding interface. In each model, the stress mainly concentrated on the root bifurcation zone, roof of the medullary cavity, gingival wall, and the 1/3 of the mesial or distal root. These findings suggest that the size and area of stress concentration are different under different cavity depth, adhesives and loading conditions. The shallow (< 1 mm) or deep (> 4mm) MOD cavity design should be avoided. Both 3M RelyX Unicem and vario-link are adhesives for gold alloy inlay. However, further clinical trials are needed to verify the results of three-dimensional finite element models.