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Introduction

Resin-based composite materials are widely used in indirect adhesive restorations particularly in class II MOD cavities . A promising advantage is the well known possibility to bond an inlay using modern adhesive materials . In addition, indirect restorations rather than direct restorations could help in reducing the polymerization shrinkage, which is basically restricted to the thin composite cement layer . The polymerization shrinkage of the resin-based composites represents one of the main causes of leakage in dental restorations. This is also influenced by different parameters, such as the material properties, cavity configuration factor (C-factor) , cavity size, presence or absence of enamel at the cavity margins, and the dentin quality, morphology, and location . Residual shrinkage stress distributions in molars after composite restoration have been investigated by means of 3D finite element analysis (FEA) and compared to shrinkage strains measured by the strain gauge experiments . In indirect restoration method, the polymerization of the resin composite occurs only in a laboratory oven, outside the oral cavity. Thus, the composite resin inlay restoration has improved mechanical properties . However, with regard to restorative materials, considerable disadvantages are also reported. First, there is a great mismatch between the mechanical properties of the employed filling materials and dental tissues. Furthermore, the occlusal loading of adhesively restored class II MOD restorations did not lead to equal fracture strength and comparable fracture or stress pattern , depending on the intensity of the masticatory loading, on the contact-points or on the contact-area of the applied load, on the used test . Also, repeated functional loading will determine fatiguing of the restored tooth . Crack growth and propagation inside the restored teeth under occlusal fatigue loading have been investigated in simulated conditions by means of 3D FEA, reproducing a standard load application . However, cohesive or adhesive cracks and fracture patterns are not always similar. Thus, it is still difficult to understand and to investigate their propagation and where and when was the origin. The influence of the cavity size and design of class II MOD indirect restorations have been frequently considered in terms of mechanical resistance. Cavity materials angles (CMA) can be differently squared to achieve a better stress distribution during mastication, also avoiding the stress concentration along the occlusal margins, as well as to reduce or to eliminate microcrack developments . It is well reported that dental research may be expensive and ethically questionable if carried out on live subjects. On the other hand, the use of virtual models and simulation, can contribute to perform interesting investigations reducing the costs of in vitro and in vivo experiments and improving the benefits . Accordingly, in the adhesive and restorative dentistry a great interest has been focused on CAD-FEM modeling and several studies focused on the stresses at the tooth-restoration interface, as well as on the transfer of occlusal forces through molars and on the tooth deformation . In this context, the study investigated the stresses induced in a mandibular molar during clenching and chewing of morsels with different values of Young’s modulus. As reported in the literature, teeth experience a force whose magnitude is dependent on the food bolus stiffness . The mastication force is distributed over the bolus surface, which is in contact with the mandibular and maxillary teeth when the bolus is compressed . During mastication the gliding phase does not always occur. Anyway, it starts when the total compression of the food bolus occurs (i.e., tooth-to-tooth contact), continuing until the start of the preparatory phase (i.e., the jaw begins to open). At contact, the mastication force is concentrated in the occlusal contact area . Accordingly, in the current research a 3D model of an untreated lower molar was developed and analyzed. Over the last decade, great efforts have been made in the development of process and analysis techniques as well as in the design of polymer networks for dentistry and tissue engineering applications . So, the FEM model was soon generated. Finally, three different class II MOD indirect cavities were modeled and restored with a resin-based composite material. The variability of the masticatory function, depending on the contact between tooth surface and food bolus, was considered. The influence of both different MOD cavity shape and stiffness on the stress and strain distributions was assessed.

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Materials and methods