Hip joint prosthesis design: effect of stem introducers

The stress levels in the femoral component of a total hip prosthesis (Corin Taper Fit, Corin Medical Ltd, Cirencester, Gloucestershire, UK) were calculated by finite element (FE) analysis. This prosthesis has two holes drilled in the shoulder to engage a stem introducer. There were no unacceptable stress levels around these holes. Instead the maximum stresses were around the periphery of the shaft of the stem, as has been observed for FE analyses of conventional designs. Three prostheses were also subjected to cyclic mechanical testing (peak load 2.3 kN) according to the appropriate British Standard. The holes were examined for cracks, before and after testing, by stereomicroscopy. All three specimens were able to withstand 5 million loading cycles with no evidence of damage. Thus it is possible to design a femoral component with holes in the shoulder, to accommodate a stem introducer, without creating unacceptable stress concentrations.