The role of femoral stem extension in total knee arthroplasty

Although stemmed femoral components often are used in revision total knee a rthroplasty, no quantitative comparison of the relative stability of a femo ral component with and without a stem has been performed previously, A radi ostereophotogrammetric analysis was performed to determine the influence of stem extension on mechanical stability in a laboratory experiment, In addi tion, the contribution of impacted morselized bone graft used for reconstru ction of bone defects to stability was determined. Ten fresh frozen distal femoral specimens, of which the bone mineral density was measured, were pre pared to fit a cemented femoral component with an uncemented stem. A cyclic axial load of 750 N was applied to the medial part of the femoral componen t. The loading test was repeated after creation of a standard, unicondylar, uncontained medial bone defect, after reconstruction of the defect with im pacted morselized bone graft, after disconnecting the stem from the compone nt, and after removal of the morselized bone graft. A significant differenc e was found in rotation and translation of the femoral component with the s tem and after its functional removal. The femoral component rotated signifi cantly into varus and internal rotation, and the tip of the stem translated significantly laterally after disconnection of the stem, A linear correlat ion between hone mineral density in the femoral neck and the radiostereopho togrammetric motion data was absent, but a bone mineral density threshold l evel of 0.55 g/cm(2) was present, below which stability decreased considera bly, Morselized bone graft provided only a minor contribution to stability compared with a stem.