Shear force in the femoral neck affects clinical outcome of total hip arthroplasty

Case-reports of broken modular femoral necks have implied increased shear loading as the main culprit. The study aim was to determine whether total hip endoprostheses with modular femoral necks produced larger magnitudes of shear force, smaller leg length discrepancy and better WOMAC score in comparison to nonmodular implants. A single-surgeon series of unilateral uncemented primary total hip arthroplasties (50 modular ProfemurZ and 52 nonmodular Zweymuller) was compared retrospectively in hip force magnitudes computed with a previously validated static biomechanical model, radiographic changes before/after total hip arthroplasty, leg lengths and WOMAC. Modular implants ProfemurZ on average had larger shear force magnitudes in the femoral neck than nonmodular Zweymuller, but there was no significant difference in leg-length discrepancy or WOMAC score. In multivariate regression (adjusted for implant type, gender, age, BMI, leg length discrepancy) increase in shear force magnitude was an independent predictor of better WOMAC score, regardless of the implant type.