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Implants

I 27 research_ Single molar restoration I implants1_2013 neering method that allows investigators to assess stresses and strains within a solid body.10-13 FEA provides calculation of stress- es and deformations of each element alone and the net of all elements. A finite element model is constructed by breaking a solid ob- ject into a number of discrete elements that are connected at common nodal points. Each element is assigned appropriate material properties that correspond to the properties of the structure to be modelled. Boundary conditions are applied to the model to stimu- late interactions with the environment.14 This model allows simulated force application to specific points in the system, and it provides the resultant forces in the surrounding struc- tures. FEA is particularly useful in the evalu- ation of dental prostheses supported by im- plants.13-16 Two models were subjected to FEA study to compare between a wide implant restoration versus the two implant restora- tion of lower first molar. _Material and Methods Three different parts were modelled to simulate the studied cases; the jaw bones, implant/abutment assembly, and crown. Two of these parts (jaw bone and implant/abut- ment) were drawn in three dimensions by commercial general purpose CAD/CAM soft- ware “AutoDesk Inventor” version 8.0. These parts are regular, symmetric, and its dimen- sions can be simply measured with their full details. On the other hand, crown is too compli- cated in its geometry therefore it was not possible to draw it in three dimensions with sufficient accuracy. Crown was modelled by using three-dimensional scanner, Roland MDX-15, to produce cloud of points or trian- gulations to be trimmed before using in any other application. The second phase of difficulty might ap- pear for solving the engineering problem, is importing and manipulating three parts one scanned and two modelled or drawn parts on a commercial FE package. Most of CAD/ CAM and graphics packages deal with parts as shells (outer surface only). On the other hand the stress analysis required in this study is based on volume of different materials.3 Therefore set of operations like cutting vol- Fig. 4a Fig. 4b Fig. 4c Fig. 5 Fig. 6a Fig. 6b Fig. 4a_Radiographic view of wide implants used to restore missing lower first molars.1, 24 Fig. 4b_Buccal view of 2 standard 20-degree abutments on 3.5 mm Astra Tech implants for restoration of mandibular right first molar.1, 24 Fig. 4c_Radiographic view of the restoration.1, 24 Fig. 5_Crown, implants and bone assembled in a model (FEA software). Figs. 6a & b_Von Mises stress on crown (a) wide implant; (b) two implants. Tab. 1_Material Properties.