Femoral Stem Design and Cement Mantle Stress

Abstract

This chapter will describe how the design of femoral components of cemented total hip replacements (THR) affects the stresses generated in the cement mantle. Intuitively, one would expect that cement stresses, and therefore failure, are affected by prosthetic design. However, the failure process of a cemented THR is complex, governed by component as well as patient- and surgeon-dependent variables. The question is whether there is any evidence that prosthetic design does affect the longevity of cemented THR reconstructions. An answer to this question can be found in the Scandinavian Hip Registers [1–3] which have produced a large amount of data about risk-factors that determine survival of THR reconstructions. The Registers show that significantly different survival rates are produced by different hip systems. Hence, it is very likely that prosthetic design does affect clinical outcome. The Registers also provide data about relatively small changes in the design of the hip systems and their effects on prosthetic survival. The Swedish Register [2] provides an example of how a prosthetic collar may affect prosthetic survival. With a prosthetic collar, the results of the Scan Hip prosthesis were considerably better than without it. This should not be considered as a general rule, but one that does apparently apply to the Scan Hip system. Another example of a relatively small change which affected clinical outcome is the alternative surface finish applied to the Exeter hip system a decade ago. By changing the surface finish of the femoral component from matt to polished the survival rates improved from about 88% after 9 years to about 96% after the same time period [2]. Hence, it can be concluded that different prosthetic designs produce different survival rates and that relatively small changes to a design may have a significant effect on the durability of THR reconstructions.