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 glucocor ticoid-treated ewes, correlations between DXA measurements and biomech anical parameters obtained on the same bones were established in 27 ag ed ewes including sixteen animals treated with methylprednisolone 15 m g/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 te st on cylinders of cancellous bone excised from two lumbar vertebrae s elected between L6 and L4. At the femoral site, bone mineral density w as correlated with the bending stiffness (r = 0.65) and the ultimate b ending strength (r = 0.64) whereas, at the vertebral site, biomechanic al parameters failed to correlate with bone mineral density assessed b y DXA. This apparent lack of correlation between vertebral bone mass a nd trabecular bone strength is mainly linked to anatomical characteris tics of the ewe: in this species, the vertebral posterior arches, whic h consist mainly of cortical bone, are very large compared to the vert ebral body and strongly influence the bone mineral density evaluated o n the intact vertebra. This is not the case with other large animals, for instance non-human primates. In conclusion, DXA can give a good ev aluation of bone strength for ewe femurs, but results must be interpre ted carefully at the vertebral site due to the anatomical characterist ics of this animal species.