Stability and bone preservation in custom designed revision hip stems

Three types of uncemented femoral stems were designed for patients having r evision hip surgery, with the goals of promoting axial stability and preser ving proximal bone stock. These stems were made individually using computer design and manufacturing technology. Various design features were examined using nonlinear finite element analysis, All stems had lateral, medial, an d anterior flares in the proximal region, proximal hydroxyapatite coating, and a collar. Based on a published classification system, the three designs were found suitable for variously encountered cavitary defects. For cases involving small amounts of bone destruction, a primary type of stem was use d, With severe cases, an extended polished stem was used. For the worst cas es, an extended stem with longitudinal cutting flutes and complete hydroxya patite coating was necessary, The axial migration was measured radiographic ally for a 2-year period. The migration rates were comparable with those se en in cemented primary and in custom primary hydroxyapatite coated stems. D ual energy xray absorptiometry data were obtained during a 4-year postopera tive period. Average bone density in all regions was maintained within 12% of the immediate postoperative values. It was concluded that the proposed s ystem for treating patients needing revision hip surgery showed desirable p roperties that were comparable to primary hip replacements.