Impact direction from a fall influences the failure load of the proximal femur as much as age-related bone loss.

Recent studies have shown that factors related to fall biomechanics may play as important a role in the etiology of hip fracture as age-related bone loss. Motivated by finite element analyses that showed failure of the proximal femur to be sensitive to loading direction, our objective with the current investigation was to determine experimentally if changes in impact direction affect the failure load of the elderly proximal femur. Thirty-three cadaveric femurs were assigned randomly to three groups of 11 and tested at one of three loading angles, 0 degree, 15 degrees, or 30 degrees, representing a fall on the hip rolled slightly forward, to the side, or rolled slightly backwards, respectively. Femurs were scanned using dual-energy X-ray absorptiometry (DXA) to assess bone mineral density (BMD) and tested to failure in a fall loading configuration at a displacement rate of 100 mm/second. Using an analysis of covariance to adjust for total hip BMD, we found that failure load decreased by 24% as the loading angle changed from 0 degree to 30 degrees. This reduction in failure load is comparable to that associated with about 25 years of age-related bone loss after the age of 65. Therefore, the impact direction associated primarily with a fall is a critical determinant of hip fracture risk that is both independent of bone density and associated with fall biomechanics.