Polymerization and loading stress distribution in adhesive resin-based composite class II restorations

P. Ausiello, PhD

Thanks to the introduction of bonding techniques in the second half of the last century, restorative dentistry underwent a major improvement. Until then, retention was based, at the cost of sacrificing much sound tooth structure, on macro-scale undercuts in the preparation of the cavity and sealing of exposed  dentin was left in the limited adaptation capability of restorative materials to tooth structure. 

At the same time, the well-established metal alloy silver-amalgam was gradually replaced by resin-based composites as first choice for restoring decayed teeth. Unfortunately, the combination of resin-based materials and bonding forms also one of the main pit-falls in today’s dentistry.

The resins’ shrinkage during polymerization and their mismatching modulus withhold a secure wall-to-wall integrity as the arising stresses often exceed the bond strength. Demanding and complicated techniques have been introduced to offer resistance to premature debonding. One of the most successful techniques is to introduce some flexibility in the restoration as a stress absorber. As the restorative material itself has to withstand considerable biting forces and wear, it cannot be too flexible. The usual solution to obtain flexibility within the restored system is to employ a low-module material underneath  the strong and hard superficial restorative material. It was the purpose of this study to determine on theoretical and experimental basis the optimal values for layer thickness and stiffness to ensure a durable adhesive interface during polymerization shrinkage and mastication. As reliable and strong bonding also might restore some of the lost coherence of the decayed tooth, one of the most critical restorations, a deep mesio-occlusal-distal cavity in an endodontically treated premolar, was studied.
A computerized model was formulated, in which the great diversity of material properties and complex geometry of teeth were simulated.

By doing so, the analysis of the complicated stress distribution was enabled and study of the simultaneous interaction of the many variables came into reach. A 3-demensional solid model of a human maxillary premolar was prepared and exported into a 3D-Finite Element Model. Additionally, a generic Class II mesio-occlusal-distal cavity preparation and restoration was simulated in the Finite Element Model by a proper choice of the mesh volumes. A validation procedure of the Finite Element Model was executed based on comparison of theoretical calculations and experimental data. Different rigidities were assigned to the adhesive system and restorative materials. Two different stress conditions were simulated: (a) stresses arising from the polymerization shrinkage and (b) stresses resulting from shrinkage stress in combination with vertical occlusal loading.

This preliminary study by 3D Finite Element Model on adhesively restored teeth with a Class II mesio-occlusal-distal cavity indicated that Young’s modulus values of the restorative materials play an essential role in the success of the restoration. Premature failure due to stresses arising from polymerization shrinkage and occlusal loading can be prevented by proper selection and combination of materials.

From this study, it may be concluded that hybrid resin composites in combination with bonding systems are the materials of first choice to restore endodontically treated teeth if full coverage by cast metals is not indicated. 

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