Biomechanical Principles and Design Details of the Thrust Plate Prosthesis

Abstract

As is probably the case for all substantially successful innovations, the remarkably unique design of the thrust plate prosthesis (TPP) was not born of a sudden whim or a capricious desire to merely produce a hip joint replacement that radically differed from all other commonly known types, but emerged from an elaborate biomechanical investigation on the loosening of conventional, intramedullary anchored hip prosthesis shafts. Therefore, to appreciate the principles on which the TPP is based, it is necessary to first review some of the earlier work done on this matter before proceeding to the TPP itself. This article therefore begins with a description of the experimental stress analysis that was carried out in the middle of the 1970s on models of the human pelvis and femur. The observations made during these early studies and the conclusions drawn from them virtually forced Arnold Huggler and myself to seek for a practicable solution that would overcome some of the biomechanical weaknesses of conventional prostheses. While considering all possible means of attaching a ball head to the proximal femur and bearing in mind that a successful prosthesis would be one that loads the bone in its immediate vicinity as physiologically as possible, we were suddenly captured by the simple idea of transmitting the hip joint force by means of a thrust plate, or washer, to the resected neck of the femur. From the birth of this idea, on 12 March 1976, up to the first implantation of a prototype in a 42-year-old man on 1 Februar 1978, various model investigations were carried out to test the mechanical behaviour of the prosthesis. This also involved reflections pertaining to the biocompa- tiblity of the materials chosen. Hence, the second part of this article deals with the basic design features of the TPP.