THE EFFECT OF DIFFERENT HORMONE REPLACEMENT THERAPY REGIMENS ON THE MECHANICAL-PROPERTIES OF RAT VERTEBRAE

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 lumbar vertebrae. Ninety-two Sprague-Dawley rats (11 months old) were divided into six groups for treatment. The f irst group was an intact control, the second group (OVX) was ovariecto mized only, and the third group (estrogen-only) was ovariectomized and received continuous estrogen through a 17 beta-estradiol implant. The remaining groups were ovariectomized and received estrogen and proges tin (norethindrone, NET) therapy; 3 mu g of NET was injected daily (es trogen plus continuous NET), or 6 mu g of NET was injected for 14 cons ecutive days of a 28-day cycle (estrogen plus cyclic NET), or for 3 co nsecutive days of a 6-day cycle (estrogen plus interrupted NET). The a nimals were sacrificed after 6 months, and the vertebrae were dissecte d out. The vertebral processes of the fourth lumbar vertebrae were rem oved, and the density of the vertebral bodies was determined. They wer e then subjected to compression testing. We found that all three estro gen/progestin regimens maintain bone density and all mechanical proper ties at a level indistinguishable from the control. However, the cycli c and continuous NET treatment results were, with the exception of den sity, also indistinguishable from those of the ovariectomized group. T he estrogen plus interrupted NET group on the other hand, has a signif icantly greater compressive modulus and density than the ovariectomize d 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 equ ivalent to those resulting from the continuous or cyclic progestin reg imens.