**7. Application of FEA in prosthodontics and implantology**

The branch of dentistry pertaining to the restoration and maintenance of oral function, comfort, appearance, and health of the patient by the restoration of natural teeth and/or the replacement of missing teeth and craniofacial tissues with artificial substitutes. FEA helps in studying the stress patterns and their distribution between the tooth and the material used in restoring the natural or missing tooth/ teeth structure and predicting the favorable outcome with least chance of failure.

Zarone et al*.,* in 2005 conducted a study on maxillary central incisor, the influence of tooth preparation design restored with alumina porcelain veneer on the stress distribution under functional load. They suggested the use of chamfer with palatal overlap design when restoring with porcelain veneers as it restored the natural distribution of stress than window technique [9].

FEA has been extensively used in implant dentistry to predict the biomechanical behaviour of various dental implant designs, as well as the effect of clinical factors for predicting the clinical success. Stress patterns in implant components and surrounding bone are well studied. The achievement of any FE study depends on the accuracy of simulating structures used. They are the material properties of implant and bone, surface characteristics and geometry of the implant and its components, loading method and support conditions, and the biomechanical behaviour of implant-bone interface. The prime difficulty in simulating the living tissues and the responses to the applied load can be successfully achieved with the use of advanced imaging techniques [36].

FEA gives an in-depth idea about the patterns of stress in the implant and more importantly in the peri-implant bone and this helps in the betterment of the implant design and implant insertion techniques. Several studies had been put forward on the effect of material properties of implant, implant number, size (length and diameter), thread profile, and on the quality and quantity of surrounding bone on stress distribution. The stresses of various kinds such as von Mises stress, maximum shear stress, maximum and minimum principal stress are used to assess the mechanical stress on the bone, implant, and bone-implant interface. Amongst, von Mises stress is most frequently and mainly used scalar-valued stress invariant to evaluate the yielding, and or failure behavior of dental materials. While minimum principal stress gives an idea on the compressive stress, maximum principal stress gives on tensile stress. Principal stress is used to study both ductile and brittle properties of a bone [36].

Siegele and Soltesz in 1989 conducted a study using implants of various shapes to evaluate the patterns of stress generation in the jawbone found that different shapes produced different stress patterns and conical implant showed higher stress than screw shaped and cylindrical implants [2].

Mailath et al*.,* in 1989 evaluated the stress values at the level of bone while placing implants with different designs and shapes (cylindrical and conical). They found more desirable stress patterns in the cylindrical implants than conical implants, large implant diameters provides more favorable stress distributions and implant materials should have a modulus of elasticity of at least 110,000 N/mm2 . Slipping between implants and cortical bone is desirable [37].

Geng et al*.,* in 2001 did literature review on the application of FEA in implant dentistry. They advised the use of advanced digital imaging technique for preparing the models with high accuracy, considering anisotropic and non-homogenous material and simulating the exact boundary conditions and mimicking the implant and its components [7].

Chun et al., in 2002 found that the square thread shape filleted with a small radius was more effective in stress distribution than other dental implants used in the analyses also maximum effective stress decreased not only as screw pitch decreased gradually but also as implant length increased [38].

Himmlova et al*.,* in 2004 conducted a study by taking implants of various lengths and diameters to evaluate the stress values produced at implant-bone interface. They found maximum stress at the collar of the implant and an increase in the implant diameter decreased the maximum von Mises equivalent stress around the implant neck more than an increase in the implant length [39].

Ding et al., in 2009 conducted a study on immediate loading implants showed that the masticatory force around the implant neck was decreased with increased diameter of an implant. Several studies found higher risk of bone resorption occurring in the implant neck region. By using FEM, authors could able to compare the elastic modulus and deformation with different types of bone, and implant materials which helps clinicians to better understand the process of bone remodeling, and for further improvements in surgical techniques [40].

Eraslan et al., in 2009 evaluated the effects of different implant thread designs on stress distribution characteristics at supporting structures. Four different thread-form configurations for a solid screw implant was prepared with supporting bone structure. V-thread, buttress, reverse buttress, and square thread designs with a 100-N static axial occlusal load applied to occlusal surface of abutment to calculate the stress distribution. They found that the implant thread forms has no effect on von Mises stress distribution in the supporting bone, but produced dissimilar compressive stress intensities in the bone [7].

Dos Santos et al*.,* in 2011 conducted a study to evaluate the influence of height of healing caps and the use of soft liner materials on the stress distribution in peri-implant bone during masticatory function in conventional complete dentures during the healing period in submerged and non-submerged implants. They found non-submerged implants with higher values of stress concentration and soft liner materials gave better results. They stated that use of soft liners with submerged implants to be the most suitable method to use during the period of osseointegration [41].

Demenko et al*.,* in 2011 emphasized that, selecting an implant size is one of the important factor in determining the load bearing capacity. The most common reason of mandibular implant supported overdenture failure was peri-implantitis due to the loss of osseointegration without any sign of infection [42].

The increase risk of mechanical failure can occur with the increase in crown to implant ratio, which was substantiated by many FE studies. A study by Verri et al*.,* in 2014 found an oblique loading induced high stress on the abutment screw when the crown:implant ratio was 1.5:1 which is in agreement with the study done by Urdaneta et al*.,* in 2010 on correlation between screw loosening, fracture of prosthetic abutments, and crown to implant height [36].

#### **7.1 Prosthesis for maxillectomy or hemi-mandiblectomy**

FEA is important in predicting the success of implant supported prosthetic rehabilitation of maxillectomy patients. In case of maxillary or partial mandibular resection patients, FE models can be used to simulate the resection areas and biomechanics of maxillary obturator or mandibular partial or implant supported prosthesis can be studied. de Sousa and Mattos in 2014 conducted a study to evaluate the stability and functional stress caused by implanted-supported obturator prostheses in simulated maxillary resections of an edentulous maxilla corresponding to Okay Classes Ib, II, and III, with no surgical reconstruction. They found that the implant-supported obturator prostheses tended to rotate toward the surgical resection site, the region where there is no osseous support. As the osseous support and the numbers of implants and clips diminished, the tensile and compressive stresses in the gingival mucosa and in the cortical bone increased. They concluded that the osseous tensile and compressive stresses resulting from the bar-clip retention system for Okay Classes Ib, II, and III maxillectomy may not be favorable to the survival rate of implants [36].
