The purpose of this study was to examine the effects of estrogen replacement, in concert with three different progestin regimens, on the mechanical properties of rat lumbar vertebrae. Ninety-two Sprague-Dawley rats (11 months old) were divided into six groups for treatment. The first group was an intact control, the second group (OVX) was ovariectomized only, and the third group (estrogen-only) was ovariectomized and received continuous estrogen through a 17β-estradiol implant. The remaining groups were ovariectomized and received estrogen and progestin (norethindrone, NET) therapy;​ 3 μg of NET was injected daily (estrogen plus continuous NET), or 6 μg of NET was injected for 14 consecutive days of a 28-day cycle (estrogen plus cyclic NET), or for 3 consecutive days of a 6-day cycle estrogen plus interrupted NET). The animals were sacrificed after 6 months, and the vertebrae were dissected out. The vertebral processes of the fourth lumbar vertebrae were removed, and the density of the vertebral bodies was determined. They were then subjected to compression testing.

We found that all three estrogen/progestin regimens maintain bone density and all mechanical properties at a level indistinguishable from the control. However, the cyclic and continuous NET treatment results were, with the exception of density, also indistinguishable from those of the ovariectomized group. The estrogen plus interrupted NET group on the other hand, has a significantly greater compressive modulus and density than the ovariectomized group. In conclusion, with respect to the ovariectomized group, the estrogen plus interrupted NET treatment resulted in a superior density and compressive modulus. The remaining mechanical properties were equivalent to those resulting from the continuous or cyclic progestin regimens.