The geometry of the device is considered all-important and is epitomised by the high-tech precise fit of uncemented CAD-CAM femoral hip stems. During the late 1980s a unique Hip Design Workstation was developed by Professor Walker at the Centre for Biomedical Engineering. By digitising key anatomical landmarks on a pair of bi-planar radiographs the 3 dimensional shape of the proximal femoral canal can be generated. Using this 3D mesh of the internal surface of the femoral cortex, the optimal femoral stem design is calculated. Manipulation of the software enables the designer to correct the design for retro or anteversion of the femoral neck. Additional design features can be incorporated and can be selected from a menu of choices including anti-rotation cutting flutes, and the presence, the size and position of a collar. On completion of the on-screen design, automated software produces the machining code for the computer controlled milling machines that are capable of producing patient specific femoral hip stems within one hour with extreme precision.

Over 2,100 femoral hip stems produced for both primary and revision joint replacement have in general been for indications such as CDH, JRA and the more severe failed standard hip arthroplasties. Concurrent mathematical modelling studies using finite element analysis combined with dual energy x-ray absorptiometry (DEXA) and migration studies have shown that optimising the fit and fill of the proximal femoral canal by incorporating a lateral flare permits more natural physiological loading and preferential adaptive bone remodelling and osseointegration. The migration studies showed that after 24 months the amount of axial sinkage for both the primary and revision custom was less than that of the gold standard of cemented Charnley and Stanmore replacements (Walker, 2000). Furthermore, the DEXA studies of the revision patients identified that after 4 years bone density was maintained within 12% of the immediate postoperative values.