M1 MAGNET 4.0

M2 MAGNET 4.5

Fig. 1  Generation of key point for mandibular   bone

Fig. 2  Line plot for mandibular   bone

24 mm and width of 5 mm was modeled (Figs. 1, 2). The bone was modeled on D2 bone according to the classifica- tion given by Misch [5]. A Computer Tomography scan of the mandible was used to model the bone by plotting the key points on a graph and generating the same key points on the ANSYS Software 8 [6]. This was in accordance with the study conducted by Meijer et al. [4] where similar results were obtained on loading the entire mandible or a section of it at the interformainal region.  Hence  only  a section was modeled for the study. The implant was modeled using appropriate dimensions as given by the manufacturer [mastero implant system Biohorizon]. The implant was modeled having length of 9 mm and width of 4 mm [7]. The surface of the simulated implant was threaded and the thread pitch was 0.4 mm. The inner diameter of the implant was 3.2 mm. The final number of threads that were present on the generated implant was 9 (Fig.  3).

When the material properties—Young’s modulus (stress/strain) and Poisson’s ratio (lateral strain/longitudi- nal strain) were assigned, the simulated finite element model will behave like the actual   prototype.

The ball attachment was modeled to be 2.5 mm [7] in diameter with a cuff height of 1 mm and an overall length of 4 mm [7] for the first model (Fig. 4) and 4 mm diameter with cuff height of 1 mm and an overall length of 4.75 mm for the second model as specified by the manufacturer [7] [Maestro implant system Biohorizon]. The silicone O-ring attachment is an O-shaped member with an inner radius and an outer radius. The first model had an inner radius  of

1.25 mm and an outer radius of 4 mm. The second model had an inner radius of 2 mm and an outer radius of 4 mm. The magnet attachment was modeled to be of two diameters. The first magnet had a diameter of 4 mm [8] and length of 1.5 mm. The magnetic attraction of the magnet is 800 g.  The  second Magnet  attachment  had  a diameter of

4.5 mm [8] and length of 1.7 mm. The magnetic attraction of the magnet was 910 g (Fig. 5), both the magnetic attachment was based on the Dyna magnetic system   [8].

The mucosa was modeled over the cortical bone with a uniform thickness of 2 mm. A section of the overdenture over the implant had been modeled. It consisted of an acrylic denture base and acrylic teeth. All materials used in this model were considered to be homogeneous isotropic and linearly elastic [10] (Table  2).

Processing and Meshing

All preprocessed models should be processed to convert geometrical data into graphical representations. Once the graphical representations of the finite element model were obtained, meshing was done. The procedure of desiccating the finite element model into elements of equal size is called Meshing.  The entire  array of elements and     nodes

Fig. 3  Line plot for model B1 and   M1

Fig. 4  Area Plot of model B1 with ball    attachment

Fig. 5  Area plot of model M1 with magnet    diameter

formed by meshing is called a Mesh. Tetrahedral (three- dimensional solid state structure with 10 nodes) elements were used because they were more harmonious with the design  structure  and  hence  will  produce  more  accurate

Table 2   Material properties [9]

Young’s modulus (Mpa) Poission’s ratio

Cortical bone