Laboratory and Finite Element Analysis study of potential factors involved in the failure of proximal resin composite sandwich restorations

Sandwich restoration technique was one of the proposed procedures with the intention of improving marginal integrity of direct resin composite restorations, especially when the cervical margin is situated below the cemento-enamel junction. Co curing, which defined as the simultaneous photo polymerization of two different light activated restorative materials, claimed to decrease the internal stresses in resin composite sandwich restorations. The aim of this research project was to investigate the cause of failure with this type of restoration. This research included two approaches; experimental and numerical analysis. The experimental part included three experiments; (1) the main study which included 104 proximal RMGIC/RC sandwich restorations in premolar teeth using Fuji II and Herculite, to investigate the effect of the co-curing technique in comparison to separate curing on the presence of microleakage. Two further studies were undertaking using Typodont premolar teeth. (2)The first one was to investigate the effect of tooth angulation on the adhesive thickness. (3)The second one was to assess the effect of the application technique on the adhesive thickness Experimentally, it was found that: 1. there was no difference in microleakage between the two curing protocols, 2. the thickness of the adhesive layer was far thicker than the manufacturer’s recommendations of 10 microns, 3. angulation of the tooth during adhesive application may have an effect on adhesive pooling and lead to a thick adhesive layer, 4. adhesive application is a sensitive multi-stage procedure and further work may be needed to develop a consistently thin adhesive layer. The numerical approach was a Finite Element Analysis, 1. to investigate the stresses distribution in the sandwich restoration.2. to investigate the effect of a thick adhesive layer on stresses distribution. The FEA results revealed that, 1. the stresses generated in the sandwich restoration were within the limits for tensile, compressive and shear stresses for the dentine, enamel, adhesive, Fuji II and Herculite materials; 2. the addition of each new layer can relieve (reduce) the magnitude of the stresses induced by the curing of the previous layer, 3. polymerisation shrinkage of the thick adhesive layer generated high stresses at the tooth restoration interface. Conclusion: Polymerisation shrinkage of the thick adhesive layer was responsible for the marginal failure of the sandwich restoration.