The Various Stress Patterns of Press-Fit, Ingrown, and Cemented Femoral Stems

Finite-element analysis was used to study the general differences in load-transfer mechanisms and stress patterns of cemented, fully ingrown, proximally ingrown, and smooth press-fitted femoral stems in total hip arthroplasty (THA). Identical stems were used for the noncemented configurations and a similar stem shape for the cemented configurations. In each model, bone properties and loading characteristics were equal. Stem elastic moduli were varied so that the effects of cobalt-chromium-molybdenum (CoCrMo) and titanium as different stem materials could be assessed. The load-transfer mechanism is similar for all bonded configurations but differs dramatically for unbonded stems, e.g., press-fit designs. In the bonded configurations, interface stress concentrations occur on the proximal and distal sides. Stress value depends on stem rigidity, with higher proximal stress occurring in cemented stems and higher distal stress in noncemented stems. In the press-fit stem, the interface stresses are affected more by stem shape as a geometric entity and less by stem rigidity. Considering possible postoperative failure mechanisms, such as interface loosening and cortical bone loss, titanium is expected to produce better results in noncemented stems and CoCrMo in cemented stems. Cortical stress shielding as a qualitative phenomenon is caused by all stems, particularly in the calcar region. Quantitatively, stress-shielding effects differ with each type of fixation used. Stress-shielding effects are severe in fully ingrown stems and milder in cemented stems because of the differences in stem rigidity. The proximally ingrown stem falls between the fully ingrown and cemented stems in regard to stress shielding because stress transfer is more evenly distributed along the stem and concentrated at the lower coated edge. The press-fit stem provokes calcar stress shielding only. In the midstem region the stresses in the cortex are even greater than in the natural case.