Nondestructive measurement of bone mineral in femurs from ovariectomized rats.

One hundred ninety-eight rats were ovariectomized (OX) or sham-ovariectomized (shOX) at 100 days of age. Groups were killed at 35, 70, 100, 125, 180, 270, 360, and 540 days postsurgery. Bone mineral content (BMC) of the right femur was assayed on a dual photon absorptiometer (DPA) optimized for human spine and whole body measurements. Three regions were studied: the distal measuring 0.8 cm, the proximal measuring 0.88 cm, and the diaphysis, the remainder. The DPA technique accurately showed the ash content (r = 0.96), with a precision error of 3-5%. Whole femoral BMC was 4.3-11.1% lower in OX than shOX rats, with significant differences from 35-180 days. By 35 days, distal femoral BMC declined 6% in OX rats and rose 12% in shOX rats. Distal femoral BMC was 11.3-17.5% lower in OX than shOX rats, with significant differences at all times except 540 days. Femoral diaphyseal BMC of OX and shOX rats did not differ at any time. The relative distal femoral osteopenia which appeared by 35 days in OX rats did not worsen during the next 17 months. A DPA suited for human BMC studies is also accurate for BMC determination in bones with 250-500 mg of mineral. It is less precise for this purpose than dedicated instruments using single photon absorptiometry. However, enough precision exists to monitor the development of relative osteopenia in OX rats. Osteopenia in OX rats is confined to a region containing appreciable cancellous bone. Its self-limiting nature suggests the existence of an estrogen-dependent quantum of cancellous bone in female rats. The adult rat model is accurate for cancellous bone of the adult human, but inaccurate for cortical bone.