EFFECTS OF DIFFERENT ESTROGEN AND PROGESTIN REGIMENS ON THE MECHANICAL-PROPERTIES OF RAT FEMUR

The purpose of this study was to examine the effects of estrogen repla cement, in concert with three different progestin regimens, on the mec hanical properties of rat femoral cortical bone. Ninety-two 11-month-o ld female Sprague-Dawley rats were randomly divided into six groups an d were treated for a duration of 6 months. Group-1 rats were intact co ntrols, group-2 rats were ovariectomized controls, and groups 3-6 were ovariectomized and given continuous doses of estrogen with 5% estradi ol 17B silicone-rubber implants. Groups 4, 5, and 6 were also given di fferent doses of progestin (norethindrone): group 4 received a continu ous dose of 3 mu g per animal per day, group 5 received a cyclic dose of 6 mu g per animal per day for 14 days of a 28-day cycle, and group 6 received an interrupted dose of 3 mu g per animal per day for 3 days of a 6-day cycle. Femurs from each group were mechanically tested. Be nding stiffness was measured by nondestructive three-point bending tes ts and maximum torque capacity, by destructive torsion tests. Geometri cal properties and apparent density of cortical bone were also measure d. The significant differences were: the increases in elastic modulus (measured from the three-point bending stiffness) of group 5 (cyclic n orethindrone) compared with those of group 2 (ovariectomized controls) and group 3 (estrogen only) the increases in the size represented by the moment of inertia, the moment of the area, and medial-lateral widt h of group 2 compared with those of group 5; and the increases in appa rent density and decreases in moment of inertia of group 6 (interrupte d norethindrone) compared with those of group 2. Cyclic or interrupted treatment of progestin along with continuous treatment of estrogen af ter ovariectomy likely improves material properties of cortical bone, increases its density, and reduces the size of the bone compared with ovariectomized rats.