EFFECT OF 1-ALPHA-VITAMIN-D-3 AND ESTROGEN THERAPY ON CORTICAL BONE MECHANICAL-PROPERTIES IN THE OVARIECTOMIZED RAT MODEL

It is well documented that both bone mass and size of ovariectomized r ats can be increased by 1 alpha-vitamin D-3 therapy. The repercussion of this therapy on bone mechanical competence is far less clear. There fore, the objective of this study was to examine the mechanical proper ties of the shaft femur in ovariectomized rats (3 months old) receivin g estrogen (0.25 mg/kg . week) and/or 1 alpha-vitamin D-3 (0.5 mu kg . day). The medication was given during 6 months starting immediately a fter ovariectomy or starting 3 months later. Torsional testing was per formed from which the parameters strength, stiffness, maximum angular displacement, and energy absorbing capacity (toughness) were derived. Multiple regression models were generated to estimate the relative imp ortance of the therapies on bone mechanical properties. Bone stiffness increased with age. Ovariectomy improved bone mechanical parameters u ntil 6 months postovariectomy, whereas estrogen treatment resulted in similar mechanical properties as those in intact age-matched controls. A significant improvement of all mechanical parameters was observed a fter 1 alpha-vitamin D-3 therapy. The combined therapy of 1 alpha-vita min D-3 and estrogen was less effective than 1 alpha-vitamin D-3 alone , but better than estrogen therapy alone, suggesting interactive effec ts between both therapies. We conclude that 1 alpha-vitamin D-3 treatm ent of ovariectomized rats improves bone mechanical competence, which might be partially related to alterations in both bone mass and size.