Cement keyhole fixation of the acetabular component of a total hip replacement
Loosening of the cemented acetabular component of a total hip replacement is 2-3 times more common than femora stem failure. Cement keyholes drilled into the acetabulum have been recommended to improve this fixation but little is known of the optimum sizes and locations of these holes. This study investigated the diameter, depth and number of keyholes to be drilled to maximise the failure torque in a model system. A two-pronged approach was used; mechanical testing and finite element (FE) analysis. A Taguchi experimental design was used to identify the most significant factors and to predict the best configuration of keyholes within the constraints of the acetabular dimensions. One hole at each of the pubic, iliac and ischial sites, of 12 mm diameter and 6 mm depth, was found to be the optimum configuration. The failure torque was most strongly dependent on the hole diameter in the pubic region, decreased with increasing hole depth and was not sensitive to the number of holes. Both two dimensional and three dimensional FE analyses of a single cement keyhole showed that the lowest von Mises stress and the best distribution of shear stress was for the hole of 12 mm diameter and 6 mm depth. The results of the two approaches used here are in excellent agreement. The final stage of this study investigated the optimisation of the keyhole shape and the design of a suitable drill bit for surgical use.