Contribution a l’amelioration de la qualite des etats de surfaces des protheses orthopediques

Abstract

Commonly, knee prostheses are composed of two metal parts fixed respectively on femur and tibia, and a third one called intercalary in plastic material. The femoral part is the most complex one to realize due to its complex shape and its material, which is generally difficult to machine. To reduce the volume of bone cut during the operation, the thickness of this part is in constant reduction. This thickness reduction induces a decrease of the safety factor before the prosthesis failure, increasing the importance of the nominal thickness ensuring. During the manufacturing process, of these components distortions appear on roughcast workpiece geometry. The contact between the femoral component and the intercalary is nonconforming; this property allows a geometric variation of the surface finish of these parts without a reduction of the prostheses function. Thus, prosthesis manufacturers choose to ensure the nominal thickness of the prosthesis by removing a constant thickness on the roughcast workpiece. This operation is generally carried out by an abrasive process realized manually by an operator. The aim of this thesis work is to contribute to the automation of these operations by providing a method to adapt the machining toolpaths at geometrical variations of the target surface. The aim of this research work is to adapt a machining toolpath computed on a nominal model to remove a constant thickness on a roughcast measured surface. The proposed method starts with an alignment step of the measured surface on the nominal toolpath using an ICP algorithm. Subsequently, the nominal toolpath is deformed to remove the desired thickness of the measured rough surface defined in presented case by a STL model. Naturally, discontinuities of this type of model induce the apparition of pattern for the STL on the adapted toolpath and thus on the machined workpiece. Subsequently, to limit this problem and to improve the quality of realized surface, it is proposed a toolpath smoothing method. To validate theoretical developments of this work, tests were carried out on a five-axis machine for roughing of femoral components of a unicompartmental knee prosthesis.