- ./OUR:\AL Of BOF\E AND 'lIi':ERAL RESEARCH \ olumc II. ;\umber J. 1996 Science. Inc. The Effect of Impact Direction on the Structural Capacity of the Proximal Femur During Falls CATHERINE M. FORD.' TONY M. KEAVENY.: and WILSON C. HAYES' ABSTRACT As "ith any structure, the structural capacity of the proximal femur depends on the applied loads and these can vary as a function of impact direction during a fall. However, despite its potential importance in hip fracture risk assessment. the relative importance of impact direction is unknown. To investigate the role of impact direction in hip fracture. we developed a detailed finite element model of the proximal femur. We analyzed four loading configurations that represent a range of possible falls on the greater trochanter. Our results indicate that a change in the angle between the line of action of the applied force and the axis of the femoral neck from 0' (representing a direct lateral impact) to 45" (representing a posterolateral impact) reduced structural capacity by 26%. This weakening of the femur with changes in impact direction is comparable to the weakening associated with 2-3 decades of age-related bone loss. Our result elucidates the independent contribution of fall mechanics to hip fracture risk by identifying an aspect of the fall (the direction of impact) that is an important determinant of fall severity. The results can also be incorporated into a refined clinical method for assessment of hip fracture risk that accounts for the complex interactions between fall severity and bone fragility. (J Bone Miner Res 1996;11:377-383) INTRODUCTION T HE SUCCESSFUL DESIGN of intervention efforts to reduce hip fracture incidence depends on a sound understand- ing of the dominant factors that contribute to hip fracture etiology. The reduction of bone mineral density (BMD) at the hip with age,o,2) coupled with the demonstrated asso- ciations between bone density and femoral strength,<3-8) demonstrate that age-related bone loss contributes to in vivo hip fracture risk. Clinical evidence that the frequency of hip fracture increases as BMD declines below a densitometric fracture threshold supports this contention. (9) However, since over 90% of hip fractures are the result of falls(lO,ll) and fewer than 5% of falls result in hip fracture,(12.l3) the mechanics of falling must also contribute substantially to hip fracture risk, In this regard, both a fall to the side{l4) and impact near the hip(15.16) have been shown to be sig- nificant predictors of hip fracture risk, regardless of BMD. Elucidation of the mechanisms by which fall mechanics influence hip fracture risk, and incorporation of this knowl- edge into fracture risk assessment techniques, should result in an improved ability to measure hip fracture risk in the clinical setting. Hip fracture risk can be quantified using a factor of risk, defined as the ratio of the force applied to the proximal femur divided by the structural capacity of the femur.(l7·18) Intrinsic factors that affect the structural capacity include femoral geometry and material properties of the bone tis- sue. The direction of impact is an extrinsic factor that can also affect the structural capacity, because of the femur's complex geometry and inhomogeneous material properties. Although the role of bone density as a detenninant of hip fracture has been extensively investigated ex vivoY-S) the contribution of loading direction has been largely ignored. Differences in loading direction may partly explain why inves- tigators have reported structural capacities for fall loading conditions which differ by as much as a factor of three to (Table 1), Although comparison of these ex tOrthopaedic Biomechanics Laborarory, Department of Orthopaedic Surgery, Charles A. Dana Research Institute, Harvard Thorndike Laboratory, Beth Israel Hospital and Harvard Medical SchooL Boston. Massachusetts, U.S,A. Biomechanics Laboratory, Department of Mechanical Engineering. University ofCaJifornia. Berkeley. California. U.S.A. and Department of Orthopaedic Surgery, University of California. San Francisco. California, U.S.A. 377