Comparison between bone density and bone strength in glucocorticoid-treated aged ewes

In order to assess if bone densitometry could be used as an indicator to evaluate bone fragility in short term studies performed on glucocorticoid-treated ewes, correlations between DXA measurements and biomechanical parameters obtained on the same bones were established in 27 aged ewes including sixteen animals treated with methylprednisolone 15 mg/day for 4 months and eleven untreated animals. DXA measurements were performed ex-vivo on HOLOGIC QDR-1000+ device. Biomechanical testings included a three-point bending test on the femur and a compression test on cylinders of cancellous bone excised from two lumbar vertebrae selected between L6 and L4. At the femoral site, bone mineral density was correlated with the bending stiffness (r = 0.65) and the ultimate bending strength (r = 0.64) whereas, at the vertebral site, biomechanical parameters failed to correlate with bone mineral density assessed by DXA. This apparent lack of correlation between vertebral bone mass and trabecular bone strength is mainly linked to anatomical characteristics of the ewe: in this species, the vertebral posterior arches, which consist mainly of cortical bone, are very large compared to the vertebral body and strongly influence the bone mineral density evaluated on the intact vertebra. This is not the case with other large animals, for instance non-human primates. In conclusion, DXA can give a good evaluation of bone strength for ewe femurs, but results must be interpreted carefully at the vertebral site due to the anatomical characteristics of this animal species.