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Mechanical and Medical Aspects of an Implant-Bone Connection in the Femur: Comparison of two Solutions of Femur-Protheses

The investigation of the implant-bone connection is of significant surgical interest as the postoperative course is influenced by the primary stability of the connection. In addition to this achievement of secondary stability through ingrowth of bone is only possible if there is perfect fixation of the implant. The ingrowth of the bone is prevented by a peak pressure causing reactive bone resorption, as well as an incomplete fit which, throughlocal micromovements, leads to loosening of the connection. The aim of the configuration is an equal transfer of the load over a as wide as possible area. It is necessary to achieve primary stability, so that the patient can be mobilized as soon as possible without orthotic treatment. The dominant force when standing and walking is a pressure at the connection. However for some movements of the flexed knee causing a torsion moment an adequate torsion stability of the connection must be guaranteed. In this paper two different types of protheses, called “Viennese- and Bologna-Model” are investigated. The main difference between the two solutions is given by the kind of implantation and its influence to the stress distribution in the load transfer area. In the “Viennese Model” the transfer of pressure is carried out by four large ribs on a strong, conical-shaped implanthead which is relatively short. The ribs should fit accurately in the bone marrow after preparation with a drill so that the prothese can be put in without excessive force (stressless form-fit). In the “Bologna Model” five small, sharp ribs are carved into the inner cortical bonelayer to prevent rotation causing stresses alredy due to the implatation of it. In both cases the problems are coming up from the asymmetric form and the dimension of the femur. By the “Viennese Model” the fit of the protheses should be achieved by the preparation of the femur using a special tool whereas by the “Bologna Model” the fit is achieved by applying implantationforces. The stress distribution in the load-transfer areas were investigated using three-dimensional photoelasticity. Due the fact that the models consists of two parts with different mechanical and especially thermally properties, any thermal treatments have to be avoided. Therefore a new method for fixing the photoelastic fringes were used by applying gamma-radiation to partially polymerised Araldite B hardened with maleic acid anhydride.

femur; implant; photoelasticity; gamma radiation; araldite B; stress analysis

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