Strength requirements for internal and external prostheses

Throughout the history of development of joint replacement implants and external prostheses there have been mechanical failures due to a discrepancy between material strength, cross-sectional characteristics and the loads developed in normal or abnormal function by the patient utilising the device. Particularly for internal prostheses attention is being paid at the present time to wear characteristics and the requirements for the articulating surfaces and the volume of wear particles produced during tests simulating the use of the device within the patient. The particular importance of the wear particles is that they seem to be associated with accelerated resorption of bone at areas essential for successful fixation of the implant within it. This article will consider joint replacements at the knee and hip and external prostheses for the leg. If failure due to external trauma is ignored the loads to be considered in testing standards correspond in implants to the muscular and ligamentous forces related to the forces developed between ground and foot and to the bending moments in the structure of leg prostheses. Generally it can be assumed that the treatment of the patient following trauma is more easily accomplished and more likely to be successful if the prosthesis has failed and not the bony structure of the patient. However, the author is unaware that these devices have ever been designed to have lower intrinsic strength than the anatomical structures to which they are connected; indeed in many cases particularly for implants they are much stronger than the bone to which they are connected. The major difficulty in rational design of prosthetic devices has been uncertainty about the importance of occasionally applied loads of a high value relative to those on a frequent basis and also to the frequency of application of these overloads. In this paper consideration is given to methods of determination of load systems relevant to the mechanical performance of implanted joint replacements at the hip and the knee and external prostheses for leg amputees. New data are presented relating to walking, other daily activities and the corresponding frequency of occurrence of these. Loading data on implants obtained by various biomechanical models is compared and related to the loads actually measured by implanted transducers. The philosophy of the standardised test load systems and the performance requirements is reviewed.