Comparison of Loading Behavior of Femoral Stems of Ti-6Al-4V and Cobalt-Chromium Alloys: A Three-Dimensional Finite Element Analysis

Large stiff, small moderately stiff, and small flexible femoral total hip components cemented in the proximal femur have been investigated using a three-dimensional finite element model and validated using experimental strain gage techniques. A physiological load of three times body weight (2000 N), 18 deg from vertical was utilized to compute stresses in the bone, cement, and metal construct.

For a well-fixed total hip replacement, the results show stem stresses to be between 5 and 20 percent of the reported fatigue limits for the metals analyzed. Cement stresses in the proximal-lateral region were negligible for all stems tested. In looking at the failure stress for cement, the proximal-medial cement stresses were seen to be only 4 to 6 percent of the reported compression failure criteria and the distal stresses only 5 to 10 percent of tensile failure criteria.

The proximal-medial bone stress was 30 percent of normal for a titanium alloy small stem, 21 percent of normal for a moderately stiff small stem, and only 6 percent of normal for a large stiff stem. Distal bone stress on the medial cortex was 39 percent higher for the stiff stem than for the flexible stem and 20 percent higher laterally.

In viewing these results in light of the stem design or femoral component failure criteria, the cement does not appear to be the weak link for the well-fixed total hip replacement, regardless of the stem design. If failure initiation does not begin within the cement in a well-fixed hip, physiological changes in the bone may be responsible for loosening at the cement-bone interface, and the cement may then be prejudiced. Some advantages of load transfer to bone are gained with the use of flexible stems, but the significance of these changes can only be confirmed or disputed with detailed clinical and radiographic follow-up.