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

Cemented fixation of hip replacements is the elective choice of many orthop aedic surgeons. The cement is an acrylic polymer which grouts the prosthese s 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 curin g bone cement which can be used to predict pressures achieved during insert ion of the femoral stem. The viscosity of the cement is a vital property de termining the cement flow and hence cement interdigitation into bone. The a pparent viscosities, eta (a), Of three commercial bone cements were determi ned with respect to time by extrusion of the curing cement through a parall el 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 s imple parallel cylinder, (b) a tapered conical mandrel and (c) an actual fe moral prosthesis, the latter models being complicated by extensional effect s as annular areas increase. Predicted pressures were close to those measured experimentally, maximum pr essures being in the range 10 160 kPa which may be compared with a threshol d 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 re ference to preparation of the medullary canal. It is proposed that these mo dels may assist retrospective studies of failed components and contribute t o implant selection, or to making informed selection from options in custom hip prosthesis designs to achieve optimum cement pressurization.