Development of a computer model to predict pressure generation around hip replacement stems.

Cemented fixation of hip replacements is the elective choice of many orthopaedic surgeons. The cement is an acrylic polymer which grouts the prostheses into the medullary cavity of the femur. Cement pressure is accepted as a significant parameter in determining the strength of cement/bone interfaces and hence preventing loosening of the prostheses. The aim of this work was to allow optimal design of the intramedullary stem of a hip prosthesis through knowledge of the flow characteristics of curing bone cement which can be used to predict pressures achieved during insertion of the femoral stem. The viscosity of the cement is a vital property determining the cement flow and hence cement interdigitation into bone. The apparent viscosities, nu(a), of three commercial bone cements were determined with respect to time by extrusion of the curing cement through a parallel die of known geometry under selected pressures. Theoretical models were developed and implemented in a computer program to describe cement flow in three models each of increasing complexity: (a) a simple parallel cylinder, (b) a tapered conical mandrel and (c) an actual femoral prosthesis, the latter models being complicated by extensional effects as annular areas increase. Predicted pressures were close to those measured experimentally, maximum pressures being in the range 10-160 kPa which may be compared with a threshold of 76 kPa proposed for effective interdigitation with cancellous bone. The theoretical model allows the prosthesis/bone geometry of an individual patient to be evaluated in terms of probable pressure distributions in the medullary cavity during cemented fixation and can guide stem design with reference to preparation of the medullary canal. It is proposed that these models may assist retrospective studies of failed components and contribute to implant selection, or to making informed selection from options in custom hip prosthesis designs to achieve optimum cement pressurization.