フィクスチャーの本数および埋入様式がインプラント構造体および周囲骨構造に与える力学的影響

―下顎臼歯欠損を想定した実験的研究―

抄録

Mechanical stress due to occlusal force is one of the most important factors that will decide the success or failure of oral implant treatment.
Excessive load ont the superstrucures will cause not only the destruction of implant components but also bone resorption or micro-fracture of the surrounding bone. A treatment plan based on the knowledge of the behavior of implant components and surrounding bone structures under occlusal force and masticational movement of the jaws should be made.
As for the effect of number and placement alignment of implant fixtures on the stress formation caused by the implant, several estimations based on clinical treatment results have already been reported. Furthermore, some finite element methods, that is, qualitative analysis, on the strain distribution around implant fixtures have been studied using two-dimensional models. However, a quantitative mechanical analysis using an actual model and strain gauges had not been carried out.
Thus, in this paper, we describe experiments conducted with epoxy-resin replicate models of edentulous mandible to determine the safest way to place implant fixtures. Two-or threefixture implants of Branemark system implants® were placed with the fixtures in a straight line of offset line for three fixtures, and in an extension-type or long span-type configurarion for two fixtures, at the posterior portion of replica models. The fixtures were designated F1, F2, and F3 for medial, central, and distal portions, respectively. Superstructures were fabricated from a silver palladium alloy to have a flat-type or roof-rype occlusal surface. A static load was applied to the part of the superstructucture above each of the three points, and then the surface strains at the corresponding abutment and surrounding bone and the internal strain of the corresponding fixture were measured with strain gauges.
The results and conclusions were as follows:
1. In the extension-type model, when a load applied to the extension portion of the superstructure, a large compression strain was measured on the surface of the central abutment;and a large tensile strain, at the medial abutment. These results indicad the possibility of deformation of the superstructure by such a loading condition.
2. With respect to the surface strain around the fixtures on the replica when the roof-type superstructure was supported by the long span-type fixtures, at the lingual site of F 1 under a load at the medial portion and at the lingual site of F 3 under a load at the distal portion, the strain surpassed 1,820 ,μ strain which is reported as the threshold level for bone resorption. Further, when the support was made with the extension-type fixtures, strain larger than the bone-bearable stress were measured equally at the lingual site of F 1 under a load at the central portion and at the lingual portion of F2 under a load at the distal portion. The results suggeed the possibility of bone resorption around the fixtures at the portions mentioned above.
3. As for the strain inside the fixtures, the largest compression strain was measured in the fixture beneath each loading point. The strains inside the fixtures were found to be smaller in the three-fixture models than in the two-fixture ones. In conclusion, it was concluded that the models having three fixtures are more suitable for use than those having two fixtures.