The long-term results of conventional cemented total hip replacement show a deterioration with time, such that beyond 10-15 years the failure rate may rise to as high as 40%. In younger more active patients, a failure rate of 50% at only 5 years has been reported. Revision surgery is difficult, most often leads to a result which radiographically at least will be much shorter-lived than the primary case, and sometimes involves serious bone loss jeopardizing the future viability of the entire reconstruction. Neither surface replacement nor bone ingrowth is seen as the answer. However there has now been 30 years of clinical experience with non-cemented fermoral components such as the Austin-Moore and while the failure rates are not lower than with cemented stems, a large proportion of the failures are associated with acetabular protrusion, while in many other failures, a poor fit of the stem in the canal is implicated. The loss of bone associated with failure and removal is generally much less than with cemented stems. It is proposed that a stem designed to be a close fit in specific load-bearing areas, a """"""""Close-Fit Stem"""""""", may well provide the answer to reliably obtaining a durable result.
Our aims are to determine the required sizes and shapes of such a stem and to test the fit and the load transfer in vitro, and the biological response in vivo. Color computer graphics will be used to model the shape and size ranges of the femoral canal while a stem fit program will determine the number of stems required to specified accuracies. Closeness of fit will be tested on actual bones using sectioning techniques. Bone strains and stem-bone shear movements will be compared for cemented stems, the Austin-Moore, a metallic close-fit stem, and close-fit stems with a polymeric coating. These designs will be tested in an ovine model, to observe the differences in bone response and to test whether the bone appears to adapt more satisfactorily to the Close-Fit Stems. For possible augmentation of closeness of fit and encasement of a Close-Fit Stem with new load-bearing bone, osteogenic stimulating demineralized bone powder (DBP) will be used as a filler. This work will provide valuable data of the viability of a Close-Fit Stem. If such a scheme is successful, it will have an invaluable place in hip surgery and have far-reaching applications even in the older population.
