Development of surface replacement prostheses for the proximal interphalangeal and metacarpo phalangeal joints
There were two main aims of the project. A surface replacement prosthesis was previously designed at the University of Durham for the metacarpo-phalangeal joint (MCPJ). Tools were required to assess the joint prosthesis pre-operatively and postoperatively. The areas of assessment which the author was involved in were joint stiffness and a self-assessment questionnaire. The Durham arthrograph had previously been used for many clinical trials to assess joint stiffness objectively. However, the computer system was not portable. Hence a new computer system was developed, in Lab VIEW, for a lap-top computer. Ten normal individuals were assessed to validate the system. A questionnaire was also developed for patients to self-assess the performance of their joints. The questionnaire assessed parameters such as range of movement, hand strength, stiffness and pain on visual analogue scales. The difficulty in performing activities of daily living were assessed on simple descriptive scales. The second part of the project was to develop a surface replacement prosthesis for the proximal interphalangeal joint (PIPJ). Since there was inadequate information in the literature on the architecture of the PEPJ bearing surfaces and phalangeal bone shafts, a detailed study was performed on the bones from 83 PIPJs. Proximal and middle phalangeal bones were dissected, modelled in bone cement, sectioned and shadowgraphed. The shadowgraphs were measured and models of the proximal phalangeal heads were produced. These models were then used to design four PIPJ surface replacement prostheses over a range of sizes which covered 97.6% of the sample population of PIPJs. It was proposed that the MCPJ and PIPJ prostheses would be made entirely from cross-linked polyethylene (XLPE). Hence wear tests on pin-on-plate apparatuses were carried out to investigate the wear characteristics of XLPE-on-XLPE compared with other biomaterial combinations. The wear of XLPE-on-XLPE was comparable with UHMWPE-on-stainless steel. XLPE-on-stainless steel wore 10 times faster than XLPE-on-XLPE, and UHMWPE-on-UHMWPE wore 100 times faster than XLPE-on- XLPE. Hence it was concluded that all XLPE joint prostheses were feasible as far as the wear considerations were concerned.