EFFECTS OF STEM LENGTH ON MECHANICS OF THE FEMORAL HIP COMPONENT AFTER CEMENTED REVISION

Bone loss in the proximal femur at the time of revision hip arthroplas ty for a failed primary cemented femoral component can substantially r educe the stability of the revision stem. Use of an extended-length fe moral component has been suggested to aid in achieving long-term fixat ion; however, the optimal stem length is unknown. A three-dimensional finite element model of a Charnley-type revision femoral component in a sclerotic shell of cortical bone devoid of cancellous bone was devel oped, and five different stem lengths ranging from 140 to 273 mm were used. The interface between the sclerotic bone and cement mantle consi sted of fibrous tissue. Distal to the sclerotic bone, bonding was allo wed between the cement and bone. Relative motion between the cement an d bone was reduced substantially when the stem extended beyond the ori ginal defect. Maximum principal stresses in the proximal cement mantle decreased from 7.7 to 5.5 MPa, but cement stresses near the distal ti p increased from 7.9 to 10.7 MPa when the stem just bridged the defect . Further increases in stem length reduced the distal cement stresses. Increases beyond two femoral diameters had a minor effect on changes in relative motion, cement mantle stresses, and stresses across the ce ment-bone interface. The results suggest that a femoral component that extends beyond the area of cancellous bone defect by two femoral diam eters will be most effective in minimizing stresses and motion that co uld be associated with clinical loosening of the cemented revision. A shorter stem that just bridges the cancellous bone defect left from th e primary procedure may not provide adequate distal fixation due to hi gh cement-bone shear stresses.