Ultrasound measurements of the os calcis. Side differences and prediction of bone density in 39 persons.

We measured the ultrasound Stiffness Index (SI) of the os calcis bilaterally with the Achilles Ultrasound Bone Densitometer in 30 women and 9 men, aged 53 (31-76) years. Lumbar spine BMD (percent of mean per age group) was measured with a DXA bone densitometer. Supplementary BMD measurements of the hip and the nondominant radius were made in 29 of the 39 persons; 11 of them had had a unilateral total hip arthroplasty (THA), the rest were healthy control subjects. SI values were in the range of 78-138 percent. Large individual differences between the right and the left os calcis were seen, even in healthy controls (CV 6.3 percent), although no differences between the means of the two sides were found. The prediction of SI of one os calcis from that of the other was inaccurate (SEE 6 percent). The SI of the dominant os calcis correlated significantly to the lumbar spine BMD, to the hip BMD and to the non-dominant radius BMD. In the group with unilateral THA the individual SI side-differences were larger (CV 8 percent; SEE 9 percent), but no systematic difference between the means of the operated and non-operated sides was found. We conclude that there are large random individual differences between the SI of the right and the left os calcis and recommend measurement of both sides for classification of one individual.