Longitudinal changes in ultrasound parameters of the calcaneus

We examined with a median follow-up of 1.4 years (range 1.0-2.0 years) the rates of change per year in ultrasound parameters of the calcaneus. Speed of sound (SOS), Broadband ultrasound attenuation (BUA) and Stiffness were measured twice in 543 subjects (224 men) participating in the Rotterdam Study. SOS fell by -2.5 m/s per year in both sexes (95% CI -4.0 to -1.1 m/s per year in men and -3.6 to -1.4 m/s per year in women). Stiffness decreased by -0.62 (-1.33 to 0.09) per year in men and -0.66 (-1.24 to -0.08) per year in women. In men the rate of change in SOS and Stiffness tended to increase with age. BUA did not change significantly during follow-up in either sex. The prospectively assessed rates of loss differed considerably from those observed cross-sectionally, especially for SOS in men (cross-sectional -0.7 m/s per year, longitudinal -2.5 m/s per year). There was substantial variation between individuals both in changes per year in SOS and in changes per year in BUA. With a median follow-up time of 1.4 years, approximately 27% of the variation in the rate of change for SOS could be explained by measurement error while for BUA this was approximately 9% and for Stiffness 11%. Only a small percentage of subjects had changes larger than could be accounted for by measurement error (SOS: men 26.8%, women 21.6%; BUA: men 28.5%, women: 38.8%; Stiffness: men 32.6%, women 35.1%). The latter may limit the use of ultrasound measurements as a follow-up tool in individuals rather than in populations.