The effect of strain rate on the failure stress and toughness of bone of different mineral densities

Abstract

The risk of low energy fracture of the bone increases with age and osteoporosis. This paper investigates the effect of strain rate and mineral level on the peak stress and toughness of whole ovine bones.40 fresh ovine femurs were subjected to 3-point bending at high (17.14s−1) and low (8.56×10−3s−1) strain rates with or without a controlled amount of demineralisation. Mineral removal was achieved by ultrasonically assisted exposure in Ethylene diamine tetra-acetic acid (EDTA). The ultimate stress for whole bones of normal mineral content was 200MPa at the high rate of strain and 149MPa at the low rate of strain.With changes in bone mineral levels such as may occur in osteomalacia and osteoporosis, the change in toughness varied at different strain rates; a mean value of 3.7±1.4MJ/m3 was obtained for the toughness of normal quality whole bone tested at slow loading rate and a reduction of approximately 25% was observed in the demineralised whole bone specimens at the slow loading rate (mean 2.8±0.9MJ/m3). When tested at the high loading rate there was a negligible difference in the toughness between the two (2.0±0.6MJ/m3) mineral levels.This indicated that there was a strain rate dependant effect for the mineral density, and that the removal of mineral alone did not explain all of the reduction in mechanical properties that occur with age or disease. Thus, the reduction in mechanical properties at high strain rates was likely to be due to other phenomena such as increased porosity or reduced collagen quality, rather than loss of mineral.With decreasing mineral levels, as measured by DEXA in clinical practice, the increased fracture risk is dependent on the velocity of the impact. Thus the estimates of increased fracture risk given clinically for a lower DEXA value should be different for high and low energy injuries.