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Statement of the problem. Selection of the proper prosthetic restorative material is still a dilemma facing most of the clinicians. Purpose. The aim of this study was to evaluate the effect of different framework materials on the stress distribution in a posterior three-unit fixed dental prosthesis. Materials and methods. One 3D finite element model was specially prepared by scanning a model of a fixed dental prosthesis (FDP) in the posterior region. The model had missing maxillary first bicuspid and prepared abutments, that representing maxillary cuspid and second bicuspid. The model was simulated as restored by three different materials; Nickel-Chromium (Ni-Cr), Zirconia and Bio-HPP (PEEK). The model was subjected to compressive vertical load of 200N applied at the central fossa of the pontic. Von Mises stress values were determined. Results. Three linear static analyses were carried out. Stress and deformation fields generated under applied load were compared. Location of both maximum Von Mises stress and maximum total deformation were found on the abutment finish line towards the pontic. Conclusion. Special attention should be considered to the finish line contact with fixed dental prosthesis to avoid stress concentration. Tooth may be insensitive to fixed dental prosthesis materials. (3908) Mazen A. Attia E.D.J. Vol. 64, No. 4 forces and its esthetic outcome. (4) For many years, metal-ceramic restorations are considered the gold standard in the prosthetic field because of their predictable structural performance, versatility, and low cost. (5) Despite the success of metal-ceramic restorations, zirconia-ceramic restorations have been introduced as an alternative option to metalceramics inposterior three-unit (FDPs) because of their excellent biocompatibility, superior mechanical properties and enhanced esthetics;(6-8) however, fracture of the framework especially in the connector area and chipping of the veneering ceramic are considered the main drawback of these restorations. (9, 10) In contrast to ceramics, BioHPP; (bioactive high performance polymer containing 20% ceramic fillers) based on polyetheretherketone (PEEK) has been introduced in dentistry.(11) This thermoplastic material is characterized by excellent biocompatibility, good wear resistance, chemical stability, low weight and adequate mechanical properties enabling it to be a suitable alternative to ceramic restorations. (12, 13) The modulus of elasticity of this material is 4GPa, close to that of bone, enabling it to act as a shock absorber; thus, reducing the forces transmitted to the restoration and the tooth root accordingly.(14) The stress distribution in a (FDP) depends mainly on its material properties and geometric configuration. (15) The elastic modulus of the material is an important parameter for evaluation of aprosthetic restoration. Ideally, a restorative material should have a modulus of elasticity close to that of the tooth structure to obtain a more uniform stress distribution. However; the tooth consists of enamel and dentin, both of which have very different composition and elasticities. (16) According to Möllers et al. (9) the framework design and ma terial properties of the superstructure had a significant role in stress distribution. Borb et al.(10) also revealed that stress distribution was influenced by the type of ceramic used for the infrastructure. Several techniques have been used to investigate the effect of dental restorative materials on stress distribution in dental prosthesis such as strain gauge analysis, photoelastic stress analysis, laser beam analysis and finite element analysis. (9, 17) Finite element analysis (FEA) has been employed by many investigators due to its accuracy, efficiency, time saving and low cost to evaluate mechanical behavior in complex structures such as implants, onlays, crowns and (FDPs).(18) In this approach, the selected structure is divided into smaller elements which can be easily calculated on a computer. When an element is under pressure, based on the pre-definedmechanical properties of each element, its shape will be changed .These elements are connected to each other via nodesand, as a result, the total structure is affected. Becauseof the complexity of both the prosthesis and the tooth structure, the resulting changeson each surface caused by the applied forces can be determinedby adopting finite element analysis.(19) The aim of this study was to evaluate the effect of different framework materials on the stress distribution in posterior three-unit fixed dental prosthesis. MATERIALS AND METHODS: A plaster model based on a clinical condition with a missing maxillary first bicuspid was prepared. The maxillary cuspid and maxillary second bicuspid were prepared to the dimensions of 4.5 mm height; assuming 2 mm flat occlusal reduction, with a uniform 120 degrees circumferential chamfer finish line of 1.2 mm width, and an axial taper angle of 6 degrees. A 3D finite element model was constructed by 3D scanning a (simulating FDP). The (FDP) geometry was acquired by using 3D scanner (Roland Modela-model MDX-15-Roland DG Corporation of Hamamatsu, Japan) and computer graphics A THREE-DIMENSIONAL FINITE ELEMENT ANALYSIS (3909) program (Roland ́s Dr. PICZA 3TM software), utilizing Roland Active Piezoelectric Sensor. Such scanner produced data file containing a cloud of points coordinates (Figure 1). An intermediate, software was required (Rhino 3.0 McNeelinc., Seattle, WA, USA) to trim a newly created surface by the acquired points. Finally, the (FDP) outer surface was closed and filled from its bottom to generate volume representing solid (FDP). Then, the solid (FDP) geometry was exported to finite element program as STEP file format. The same process was repeated for supporting bone and prepared abutments. Cement layer of 40 μm was created by scaling the prepared abutments prior to using set of Boolean operations (subtract, cut, ... etc.) to keep the cement layer only. The materials used in this study were assumed to be homogenous, isotropic and to possess linear elasticity, and its properties were listed in (Table 1). All the components (tooth, cement layer, FDP) of the model were exported as STEP files, and imported into finite element package ANSYS Workbench version 16 (ANSYS Inc., Canonsburg, PA, USA) to be assembled and analyzed. The parabolic tetrahedral element was used meshing the model. That increasing the mesh density will improve the results accuracy for the discrete model. Mesh density of all model components is presented in (Table 2. Figure 2) illustrate the model of (FDP) on ANSYS screen, while (Figure 3) showed meshed component. The solid modeling and finite element analysis (linear static analysis) were performed on Workstation HP Z820, with Dual Intel Xeon E52660, 2.2 GHz processors, 64GB RAM. Three runs were performed, using three different (FDP) materials. A compressive load of 200 N was applied on the central fossa of the pontic, while the model base was fixed as a boundary condition. TABLE (1) Material properties imported to the Finite Element program Material Young’s modulus [MPa] Poisson’s ratio Base: Dentin 18,600 0.31 Resin cement (RelyX) (40 μm Cement Layer) 8,000 0.30

EFFECT OF MATERIAL TYPE ON THE STRESS DISTRIBUTION IN POSTERIOR THREE-UNIT FIXED DENTAL PROSTHESIS: A THREE-DIMENSIONAL FINITE ELEMENT ANALYSIS