Effects of variation of cement thickness on bone and cement stress at the tip of a femoral implant.

With the resurgence of the use of bone cement in total hip arthroplasty, a renewed concern in techniques or designs that may reduce cement fixation failure has arisen. Analysis of the stresses at the tip of the prosthesis may suggest strategies to reduce loosening. Using a three-dimensional finite element model this study analyzed stresses in the bone, cement, and prosthesis near the tip of a femoral component as a function of cement thickness. A section of an idealized circular femoral shaft with implant prosthesis and cement was modeled with loading conditions representing the stance phase of gait. Increasing cement thickness is predicted to significantly reduce stress in the cement mantle of a femoral implant. Tensile stress is reduced by fifty percent while shear stress is reduced at least twelve percent. Peak tensile stresses occur on the medial side at the tip of the prosthesis in a transverse direction, indicating likelihood of failure due to debonding. Local shear stress peaks also occur at the tip. Shear stresses in the cement mantle are in the same range as the tensile stresses and must be considered when analyzing the possible modes of failure. However, the mode of failure in shear is complex, and shear strength of the stem-cement interface is unknown at present.