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The velocity of ultrasound in human cortical bone has been measured in vivo by a pulse echo technique. The velocity is reduced in a small group of older females, compared with a small group of young males. It is suggested that this change results from a proportionately greater decrease in the elastic constant of the bone, compared with its density. This method measures a new combination of physical properties of bone, which may have important clinical significance in the assessment of the diagnosis and course of osteoporosis.