DUAL-ENERGY X-RAY ABSORPTIOMETRY IN SHEEP - EXPERIENCES WITH IN-VIVO AND EX-VIVO STUDIES

As different large animal models of osteopenia and osteoporosis are ex plored, the use of DXA to rapidly, non-invasively and accurately estim ate BMD will become widespread. We used DXA in live sheep and cadaveri c material and the areas of trabecular bone that are most accessible o n a simple, repeatable basis in the sheep were the lumbar vertebrae (L 4-L6/L5-L7), the CAL and the DR. We performed dual-energy X-ray absorp tiometry (DXA) using an Hologic QDR 1000-W bone densitometer to measur e bone mineral density (BMD) at various regions of interest in anesthe tized sheep and cadaveric specimens of sheep. In vivo measurements of L4-L6/L5-L7, the calcaneus (GAL) 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 repositione d between 10 scans of each bone. In another study, ex-vivo BMD measure ments of the proximal and distal femur, proximal tibia, and proximal h umerus were performed on isolated bones of 45 ewes of similar age. Exc ised vertebrae were scanned on the Hologic QDR 1000-W and on a Lunar D PX (at another site) and the data were compared. Correlations of BMD b etween individual vertebrae in anesthetized sheep were excellent (r = 0.944-0.843; P < 0.0001). Correlation between BMD of individual verteb rae 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 < .00 01). 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 p roximal tibia) was good (r = 0.772-0.449 P < 0.0001). In vivo precisio n 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 prec ision of BMD of the proximal and distal femur, proximal tibia, and pro ximal humerus was always < 1.0%. The correlation between BMD data from the Hologic QDR1000-W and from the Lunar DPX was positive and strong (r = 0.942, P < .0001), with the latter reading slightly higher densit ies. Unfortunately, we found other regions such as the proximal and di stal femur, proximal tibia and proximal humerus were accessible on a r epeatable basis only on cadaveric material. It remains to be determine d if BMD changes in the areas investigated correlate with bone fragili ty, an endpoint that investigators and regulatory agencies now conside r important. It is likely that DXA will become the standard for BMD me asurements in large animals such as the sheep.