AGE-DEPENDENCE OF BONE MATERIAL QUALITY SHOWN BY THE MEASUREMENT OF FREQUENCY OF RESONANCE IN THE ULNA

In women before and after the age of peak bone mass, identical values of bone mineral density (BMD) can be obtained. However, there is a much higher incidence of osteoporotic fractures in older women. We investigated whether a deterioration of bone material quality with increasing age might contribute to this phenomenon. Material properties of bone tissue can be characterized by the modulus of elasticity, which is correlated to the square of sound transmission velocity. In this study, sound transmission velocity was determined in cortical bone by measuring the frequency of resonance in the ulna in the direction of the bone's longitudinal axis and correcting the values by multiplying by ulna length. Validation of this method indicated acceptable reproducibility: interobserver variability determined as the mean coefficient of variation was 1.82%. In a clinical study, 21 young women (22.5 +/- 1.2 years old) were compared with 21 middle-aged women (52.9 +/- 2.7 years old). Pairs were matched that had identical values of BMD in the nondominant forearm at a location representing mainly cortical bone (SPA). The product of ulna length and frequency of resonance in the ulna in the younger women was found to be 61.4 +/- 5.8 m/second, and in the middle-aged women 55.7 +/- 4.5 m/second. The difference was highly significant with P < 0.005. Our results confirm recent findings indicating a deterioration of bone material quality independent of BMD with increasing age. As shown by comparing ulna width at the site of measurement of bone mineral density between both groups of women, the deterioration of bone material quality in ulnar cortical bone with increasing age might at least in part be functionally compensated by an increase of the moment of inertia due to greater bone width [8, 10].