Finite Element Evaluation of Primary Stability of Teeth Implants
Starting from a CT data set of a 63 years old male patient the geometry of the lower mandible has to be extracted. In figure 1 CT sample slices of the patient's skull are shown and in figure 2 the skull geometry is depicted. Using segmentation techniques the lower mandible is reconstructed as displayed in figure 3. The green part is the region that is used for this analysis.
figure 1: sample CT slices figure 2: reconstructed skull geometry
figure 3: reconstructed lower madible
The implant used in this project is a conical endosseous implant and is placed as a Incisor tooth at the ventral part of the mandible. The implant, which is shown in figure 4, consists of a conical part with a screw thread, an abutment (above), a crown (left) and the bar for the rotational stability. The crown is created to apply the load more physiologically. As mentioned two models , one with and one without bar, were created. The length of the bar is chosen in that way, that it can be removed after the implant got osseointegrated. In figure 5 the CAD model of mandible and implant is shown, the bar is slightly overhanging the ventral part of the mandible.
figure 4: CAD model of the implant figure 5: CAD model of mandible and implant
Based on the CAD model the finite element model is generated. To simulate physiological correct interface behaviour a layer of 3D contact elements is inserted between implant and bone. In figure 6 both finite element models are shown in a exploded view, the contact layer is colored yellow. Both models consist of about 20.000 nodes and 100.000 tetrahedra each. As Dirichlet boundary conditions the models are fixed at the medial and dorsal ends. The crown is loaded with about 45N as shown in figure 7. This load for a single tooth was calculated according to biting force statistics of people in this age.
figure 6: finite element models with and without bar
figure 7: finite element with load
As a measure for the rotation of the implant the relative inplane displacements in the contact layer were evaluated, which is shown in figure 8. The implant without bar underlies higher inplane displacements. The micromotion peak value in the model with bar is 20% smaller than without bar. This is an important factor for the osseointegration of the implant and indicates the better primary stability.
figure 8: inplane displacement in the contact layer