THE AGED MALE-RAT AS A MODEL FOR HUMAN OSTEOPOROSIS - EVALUATION BY NONDESTRUCTIVE MEASUREMENTS AND BIOMECHANICAL TESTING

Effects of androgen deficiency and androgen replacement on bone densit y, as measured with dual-energy X-ray absorptiometry (DXA) and single photon absorptiometry (SPA), cortical ratio (cortical thickness/outsid e bone diameter x 100), and biomechanical proper-ties were evaluated i n 14-month-old (1 month after orchiectomy (orch) or sham-operation) an d in 17-month-old (4 months after orch or sham) male rats. Whole femor al bone mineral content (BMC) and density (BMD) measured with DXA were not significantly decreased 1 month after orch. Whole femoral BMC and BMD were 10% and 8% lower in 4 months after orch (P < 0.01 and P < 0. 001, respectively). This decrease was prevented by testosterone replac ement. There was an excellent correlation (R = 0.99) between whole fem oral BMC and femoral ash weight. Selective scanning of cortical and ca ncellous sites of the femur showed that both cancellous and cortical B MC and BMD were significantly decreased 4 months after orch. SPA of th e right tibia confirmed a 7% decrease in cancellous BMC and BMD 4 mont hs after orch (preventable by testosterone) but not in cortical BMD an d BMC. Femoral cortical ratio decreased with age (47 +/- 2 in 14-month -old and 40 +/- 2 in 17-month-old sham rats versus 63 +/- 1 in 6-month -old male rats) due to a continuously enlarging femoral shaft. Androge n deficiency resulted in an even greater decrease of the cortical rati o 4 months after orch (36 +/- 2 in 17-month-old orch rats) that was ag ain prevented by testosterone (47 +/- 3). These changes in femoral cor tical, cancellous density, and cortical ratio did not affect biomechan ical properties of the femur as evaluated by torsion testing. The lack of an effect on bone biomechanics was most likely due to the protecti on afforded by an increased femoral shaft diameter. We conclude that 4 months after orch, aged male orch rats had a lower femoral cortical a nd cancellous density and a lower cortical ratio without decrease of b iomechanical properties of the femoral shaft. Testosterone replacement was effective not only in preventing the decrease of cancellous and c ortical density but also in preventing the age-related thinning of the femoral cortex.