**5. Application of FEA in restorative dentistry**

Restorative dentistry refers to the diagnosis and integrated management of diseases of the teeth and their supporting structures and rehabilitation of the dentition for functional and esthetic requirements of an individual. Restorative dentistry. It is a broader term encompasses the dental specialties of endodontics, prosthodontics, and periodontics.

Many newer materials have been developed owing to the increasing interest in the field of esthetic dental restorations. In order to minimize the stress concentration of the restorative materials and to decrease the incidence of restorative failure; physical properties like modulus of elasticity should be near or equal to that of the natural dental tissue. Due to the lack of proper understanding on the biomechanical principles of the materials involved in restorative procedure, lead too many detrimental effects causing a restorative failure. Therefore, in order to know the behaviour of materials and dental tissue, biomechanical studies are very crucial [6, 7].

Goel et al*.,* in 1991 investigated stress variation in the enamel and dentin adjacent to the Dentinoenamel Junction (DEJ) on FEM of maxillary first premolar. The results suggested that, because of mechanical interlocking between enamel and dentin in the cervical region is weaker than in other regions of the DEJ, enamel in this region may be susceptible to belated cracking that could eventually contribute to the development of cervical caries than other areas of tooth [8].

Rees in 2002 examined the effect of varying position of an occlusal load on the stress contour in the cervical region of a lower second premolar using a 2-D plane strain FEM. A 500 N load was applied vertically to either of the cusp tips or in various positions along the cuspal inclines. He found that, loads applied to the inner aspects of the buccal or the lingual cuspal inclines produced maximum principal stress values of up to 358 MPa, which is exceeding the known failure stresses for enamel [9].

Ausiello et al*.,* in 2002 conducted a 3-D FEA study to identify the thickness and flexibility of the teeth adhesively restored with resin-based material. No difference in the stress relief between the application of a thin layer of more flexible adhesive with low elastic modulus and thick layers of less flexible adhesive of high-elastic modulus was found. They observed a relatively small cuspal deformation in all the models with increased cusp-stabilizing effect of ceramic inlays compared to composite restorations [10].

Ausiello et al*.,* in 2004 investigated the composite inlay restored class-II MOD cavities the effect of differences in the resin-cement elastic modulus on stresstransmission to ceramic or resin-based during vertical occlusal loading. They found better stress dissipation in indirect composite resin-inlays. Glass ceramic inlays transferred stresses to the resin cement and adhesive layer [11].

Magne et al., in 2006 described a rapid method of generating FE models of dental structures and restorations. They evaluated five models: natural tooth, mesial-occlusal (MO), and mesial-occlusal-distal (MOD) cavities, MO, and MOD endodontic access preparations and found a progressive loss of cuspal stiffness in MO to MOD to endodontic access, as there is loss of tooth structure with these type of restorations. The natural tooth and the tooth with the MOD ceramic inlay retained 100% cuspal stiffness [7].

Ichim et al., in 2007 investigated the influence of the elastic modulus (*E*) on the failure of cervical restorative materials (Glass ionomer cement (GIC) and composite) and identified an *E* value that minimizes the mechanical failure under clinically realistic loading conditions. They found that the materials used in non-carious cervical lesions are unsuitable for restorations as they are less resistance to fracture and suggested that the elastic modulus of a restorative material to be in the range of 1 GPa [2].

Asmussen et al., in 2008 analyzed the stresses generated in tooth and restoration by occlusal loading of Class-I and Class-II restorations restored with resin composite; suggested that the occlusal restorations of resin composite should have a high modulus of elasticity in order to reduce the risk of marginal deterioration [12].

Coelho et al*.,* in 2008 conducted a study to test the hypothesis that micro-tensile bond strength values are inversely proportional to dentin-to-composite adhesive layer thickness through laboratory mechanical testing and FEA. They found microtensile bond for Single Bond as increased adhesive layer thickness did not reduce Clear fil SE Bond strength [13].

Magne and Oganesyan in 2009 measured cuspal flexure of intact and restored maxillary premolars with MOD porcelain, and composite-inlay restorations and occlusal contacts (in enamel, at restoration margin, or in restorative material). They found a relatively small cuspal deformation in all the models and an increased cuspstabilizing effect of ceramic inlays compared with composite ones [9].

#### **5.1 Dental composites**

Composites are the resin restorative materials developed to overcome the disadvantages of amalgam restorations, which are unaesthetic and toxic. Composites are filled resins, exhibit high compressive strength, abrasion resistance, ease of application, and high translucency. FEA has been in use to analyze stresses generated in teeth and restorations. It is a proven useful tool in understanding biomechanics of tooth and the biomimetic approach in restorative dentistry [14].

Lee et al*.,* in 2007 conducted a study to measure the cusp deflection by polymerization shrinkage during composite restoration for MOD cavities in premolars, and examined the influence of cavity dimension, C-factor, and restoration method on the cusp deflection. They found that, the cusp deflection increased with increasing cavity dimension and C-factor and suggested the use of an incremental filling technique or an indirect composite inlay restoration to reduce the cuspal strain [15].

Choi et al*.,* in 2011 analyzed the disintegration of a dental composite restoration around the margin due to contraction stress by measuring the circumferential strain on the outer surface of a ring-type dental substrate. They found increase in the marginal gap size representing the increase in the number of cracking's along the margin due to polymerization contraction [16].

Jongsma et al*.,* in 2011 studied to find out the rationale of using whether 60% increase in push-out strength with a two-step cementation procedure of fiber posts is equivalent to the layering technique of composite restorations or not. They found two-step cementation of fiber posts lead to a decrease in internal stresses in the restoration, resulted in higher failure loads and less microleakage [17].
