Precision of dual-energy X-ray absorptiometry of the knee and heel: methodology and implications for research to reduce bone mineral loss after spinal cord injury.

STUDY DESIGN

Methodological validation of dual-energy x-ray absorptiometry (DXA)-based measures of leg bone mineral density (BMD) based on the guidelines of the International Society for Clinical Densitometry.

OBJECTIVES

The primary objective of this study was to determine the precision of BMD estimates at the knee and heel using the manufacturer provided DXA acquisition algorithm. The secondary objective was to determine the smallest change in DXA-based measurement of BMD that should be surpassed (least significant change (LSC)) before suggesting that a biological change has occurred in the distal femur, proximal tibia and calcaneus.

SETTING

Academic Research Centre, Canada.

METHODS

Ten people with motor-complete SCI of at least 2 years duration and 10 people from the general population volunteered to have four DXA-based measurements taken of their femur, tibia and calcaneus. BMDs for seven regions of interest (RIs) were calculated, as were short-term precision (root-mean-square (RMS) standard deviation (g cm), RMS-coefficient of variation (RMS-CV, %)) and LSC.

RESULTS

Overall, RMS-CV values were similar between SCI (3.63-10.20%, mean=5.3%) and able-bodied (1.85-5.73%, mean=4%) cohorts, despite lower absolute BMD values at each RIs in those with SCI (35%, heel to 54%, knee; P<0.0001). Precision was highest at the calcaneus and lowest at the femur. Except at the femur, RMS-CV values were under 6%.

CONCLUSIONS

For DXA-based estimates of BMD at the distal femur, proximal tibia and calcaneus, these precision values suggest that LSC values >10% are needed to detect differences between treated and untreated groups in studies aimed at reducing bone mineral loss after SCI.