Accuracy of using radiographs for custom hip stem design.

If the shape of the femoral canal could be predicted with sufficient accuracy for the design of custom femoral stems, this would reduce the cost and provide a simpler design method than using computed tomography scans. Five groups of femurs were used for the study. The first two groups were used to determine an average femur shape, described by 25 transverse sections. In the next group of femurs, the shape was predicted from radiographs by distorting the shape of the average femur to conform to the radiographic outlines. The profiles were accurate to within 1 mm on average, the larger errors being in the trochanteric regions. In the proximomedial region, and in the distal canal, the accuracy was better than 0.3 +/- 0.8 mm. When femoral stems were designed from the actual and predicted canals, there was minimal difference in the sections of the two stems. The stems had a particular geometric form with a straight distal end, and flares at the proximomedial and proximoanterior locations; however, when stems designed by either method were inserted into canals prepared by distal reaming and proximal rasping, there was an error in fit in certain regions of approximately 1 mm. It was concluded that for femurs without serious abnormality of shape, a biplane radiographic method was sufficiently accurate for the prediction of canal shape and for the design of a standard type of custom uncemented stem.