The replacement of the arthritic human knee joint by an artificial implant (or total knee arthroplasty) is currently a very successful medical intervention. A large
majority of the patients enjoys relief of pain and restoration of function for at least ten to fifteen years. As a consequence, it has also become a very popular treatment that is now considered as the treatment of choice in almost all patients with severe knee problems. Thus the number of knee replacements increases from year to year.
However, even in the most successful medical intervention, such as total knee arthroplastie, some failures will always occur. In view of the large number of patients involved, even small failure rates amount to large numbers of people experiencing a disappointing result and even undergoing a revision operation.
Since failure of a knee prosthesis will usually become apparent by a phenomenon called “aseptic loosening”, one of the design issues that obviously needs further attention is the fixation of the prosthesis to the underlying bone. One of the ideas is the fixation of knee prostheses by bone growing into a porous coating. Current research on joint replacements therefore focuses on the solution of the problem of bone ingrowth. In this thesis, the results are presented of some experiments that contribute to this endeavours.
The requisites for bone ingrowth are quite challenging. It seems that bone ingrowth is only possible when the prosthesis is rigidly fixed to the bone initially or else that the coating material for the prosthesis has bioactive capacities. In order to improve the initial stability of implants, one must obviously be able to measure relative movements of the prosthesis with respect to bone. Therefore, the development of a set-up for in vitro tests that enables a complete three-dimensional analysis of the relative movement of a prosthesis subjected to physiological loads was the first goal of the experimental part of this thesis. We used this set-up to analyse a
clinically used knee implant and to evaluate the influence of fixation aides and bone quality on the initial stability.
One of the so-called bioactive materials that stimulate bone ingrowth even in the absence of optimal mechanical conditions is hydroxyapatite. We used the sheep as an
animal model to investigate the influence of hydroxyapatite on bone ingrowth in a knee prosthesis.