Dual-energy X-ray absorptiometry in sheep: Experiences with in vivo and ex vivo studies

As different large animal models of ostepenia and osteoporosis are explored, the use of DXA to rapidly, non-invasively and accurately estimate BMD will become widespread. We used DXA in live sheep and cadaveric material and the areas of trabecular bone that are most accessible on a simple, repeatable basis in the sheep were the lumbar vertebrae (L4-L6/L5-L7), the CAL and the DR. We performed dual-energy X-ray absorptiometry (DXA) using an Hologic QDR 1000-W bone densitometer to measure bone mineral density (BMD) at various regions of interest in anesthetized sheep and cadaveric specimens of sheep. In vivo measurements of L4-L6/L5-L7, the calcaneus (CAL) and distal radius (DR) in 48 intact 3 to 5-year-old ewes (same breed) were performed. Correlations between the different bones were investigated. In an in vivo precision study, BMD of L3-L6/L7, CAL and DR was determined with one animal repositioned between 10 scans of each bone. In another study, ex-vivo BMD measurements of the proximal and distal femur, proximal tibia, and proximal humerus were performed on isolated bones of 45 ewes of similar age. Excised vertebrae were scanned on the Hologic ODR 1000-W and on a Lunar DPX (at another site) and the data were compared. Correlations of BMD between individual vertebrae in anesthetized sheep were excellent (r = 0.944− 0.843; P< 0.0001). Correlation between BMD of individual vertebrae and CAL was good (r = 0.677−0.630), while correlation between BMD of individual vertebrae and DR was also good (r = 0.551−0.507; P< .0001). Correlation between BMD of CAL and DR was good (r = 0.440, P = 0.0025). Correlation between BMD of vertebrae of anesthetized sheep and cadaveric specimens (proximal and distal femur, proximal humerus and proximal tibia) was good (r = 0.772−0.449 P<0.0001). In vivo precision of BMD for the lumbar spine was 1.4–4.3%, and 1.5% and 3.5% for CAL and DR respectively. In the ex-vivo study, the correlation between the BMD of the individual bones was also strong (P<.0001). Ex-vivo precision of BMD of the proximal and distal femur, proximal tibia, and proximal humerus was always <1.0%. The correlation between BMD data from the Hologic QDR 1000-W and from the Lunar DPX was positive and strong (r = 0.942, P<.0001), with the latter reading slightly higher densities. Unfortunately, we found other regions such as the proximal and distal femur, proximal tibia and proximal humerus were accessible on a repeatable basis only on cadaveric material. It remains to be determined if BMD changes in the areas investigated correlate with bone fragility, an endpoint that investigators and regulatory agencies now consider important. It is likely that DXA will become the standard for BMD measurements in large animals such as the sheep.