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 rats can be increased by 1 alpha-vitamin D-3 therapy. The repercussion of this therapy on bone mechanical competence is far less clear. Therefore, the objective of this study was to examine the mechanical properties of the shaft femur in ovariectomized rats (3 months old) receiving 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 after ovariectomy or starting 3 months later. Torsional testing was performed 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 importance of the therapies on bone mechanical properties. Bone stiffness increased with age. Ovariectomy improved bone mechanical parameters until 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 after 1 alpha-vitamin D-3 therapy. The combined therapy of 1 alpha-vitamin D-3 and estrogen was less effective than 1 alpha-vitamin D-3 alone, but better than estrogen therapy alone, suggesting interactive effects between both therapies. We conclude that 1 alpha-vitamin D-3 treatment of ovariectomized rats improves bone mechanical competence, which might be partially related to alterations in both bone mass and size.