Effect of 1alpha-vitamin D3 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 1alpha-vitamin D3 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 1alpha-vitamin D3 (0.5 microgram/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 1alpha-vitamin D3 therapy. The combined therapy of 1alpha-vitamin D3 and estrogen was less effective than 1alpha-vitamin D3 alone, but better than estrogen therapy alone, suggesting interactive effects between both therapies. We conclude that 1alpha-vitamin D3 treatment of ovariectomized rats improves bone mechanical competence, which might be partially related to alterations in both bone mass and size.