Effects of loading rate on strength of the proximal femur

Abstract

Results from previous quasi-static mechanical tests indicate that femurs from elderly subjects fail in vitro at forces 50% below those available in a fall from standing height. However, bone is a rate-dependent material, and it is not known whether this imbalance is present at rates of loading which occur in a fall. Based on recent data on time to peak force and body positions at impact during simulated falls, we designed a high rate test of the femur in a loading configuration meant to represent a fall on the hip. We used elderly (mean age 73.5±7.4 (SD) years) and younger adult (32.7±12.8 years) cadaveric femurs to investigate whether (1) the strength, stiffness, and energy absorption capacity of the femur increases under high rate loading conditions; (2) elderly femurs have reduced strength, stiffness, and energy absorption capacity compared with younger adult femurs at this loading rate; and (3) densitometric and geometric measures taken at the hip correlate with the measured fracture loads. Femurs were scanned using dual-energy X-ray absorptiometry (DXA) and then tested to failure in a fall loading configuration at a displacement rate of 100 mm/second. The fracture load in elderly and younger adult femurs increased by about 20% with a 50-fold increase in displacement rate. However, energy absorption did not increase with displacement rate because of a twofold increase in stiffness at the higher loading rate. Age-related differences in strength and energy absorption capacity were consistent with those found previously for a displacement rate of 2 mm/second. There were moderate to strong correlations between fracture load and DXA variables, with the best correlation provided by cross-sectional area (r²=0.77) and bone mineral density (BMD) (r²=0.72) at the femoral neck. Our results indicate that, even at rates of loading applied during a fall, the estimated impact force in a fall on the hip is 35% greater than the average fracture load of the elderly femur. Moreover, the relationship we found between femoral neck BMD and fracture load indicates that an increase in femoral neck BMD of more than 20% would be required to raise the strength of the femur to the level of the impact load. As clinical trials of pharmacologic interventions have demonstrated increases in BMD of only a few percent at best, our results emphasize the continuing need for intervention strategies that focus on fall prevention and on reducing the severity of those falls that do occur.