Femoral shaft strains during daily activities: Implications for atypical femoral fractures

Background: Atypical femoral fractures are low-energy fractures initiating in the lateral femoral shaft. We hypothesized that atypical femoral fracture onset is associated with daily femoral strain patterns. We examined femoral shaft strains during daily activities. Methods: We analyzed earlier calculations of femoral strain during walking, sitting and rising from a chair, stair ascent, stair descent, stepping up, and squatting based on anatomically consistent musculoskeletal and finiteelement models from a single donor and motion recordings from a body-matched volunteer. Femoral strains in the femoral shaft were extracted for the different activities and compared. The dependency between femoral strains in the lateral shaft and kinetic parameters was studied using multi-parametric linear regression analysis. Findings: Tensile strain in the lateral femoral shaft varied from 327 με (squatting) to 2004 με (walking). Walking and stair descent imposed tensile loading onthe lateral shaft, whereas the other activities mainly imposed tensile loads on the anterior shaft. The multi-parametric linear regression showed a moderately strong correlation between tensile strains in the lateral shaft and the motion kinetic (joint moments and ground reaction force) in the proximal (R² = 0.60) and the distal shaft (R² = 0.46). Interpretation: Bone regions subjected to tensile strains are associated with atypical femoral fractures. Walking is the daily activity that induces the highest tensile strain in the lateral femoral shaft. The kinetics of motion explains 46%-50% of the tensile strain variation in the lateral shaft, whereas the unexplained part is likely to be attributed to the way joint moments are decomposed into muscle forces.