MATERIALS AND METHODS

The tensional analyse was effected thanks to a specific program for calculus of Finished Elements. The basic geometry to construct the model is shown in fig. xx and xx; it's obvious the simulation of a case of perfect osteo-integration, in fact in both models the screws are completely surrounded by the cortical bone. The geometry of the considered elements is equal to the real one, but the cortical bone has everywhere a thickness equal to 1 mm. The volumes showed in the previous figure have been subdivided in 49000 tetrahedrons in the case of bracket implants and in 15000 tetrahedrons in the case of traditional implants (the higher simplicity of geometry allowed to utilise bigger tetrahedrons). The finished element is represented by the tetrahedrons with 10 nodes where the nodes are placed on the hits of tetrahedron and in correspondence of the half of each side (the nodes are the points of space where the tensions and deformations are effectively calculated). The volumes showed in different colours in the figure xx correspond to different materials:

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trabecolous bone:
it has been considered a bone of medium quality with an elastic modulus equal to 300 Mpa;
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cortical bone:
according the values of literature (xx), an elastic modulus equal to 11000 Mpa has been imposed;
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implanted bone:
with equal characteristics to the cortical bone, necessarily;
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implants:
the elastic modulus of titanium is equal to 100000 Mpa;
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teeth:
in the model they have been modelled by an homogeneous beam with an elastic modulus equal to 110000 Mpa.


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All materials were considered homogeneous because it was simpler, even if it's known that the bone has a high orthotropic behaviour; we retain that this approximation wasn't able to invalidate the results obtained through the comparison between the two different modalities of operation. The modelled portion of bone was completely considered bound in correspondence of the two extreme vertical areas (both on the left and the right). We wanted to consider two heavy conditions of load where there is a force localised in the half of the considered bone segment; in the 1st case the force is only vertical, equal to 200 N, in the 2nd case there is also an horizontal component equal to 40 N.

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