The effects of synthetic conjugated estrogens, a (cenestin) on trabecular bone structure and strength in the ovariectomized rat model.

This study evaluated the effect of Cenestin, a synthetic conjugated estrogens product, on the maintenance of trabecular bone microarchitecture, bone strength, and of bone turnover in the ovariectomized (ovx) rat model. Two doses of Cenestin were chosen in an attempt to approximate the equivalent human oral doses of 0.3 mg and 0.625 mg. Forty-nine 6-month-old retired female breeder Sprague-Dawley rats were randomly assigned to one of four groups: (1) sham-operated + vehicle; (2) ovx + vehicle; (3) ovx + day 1 post-ovariectomy Cenestin (8.12 mg/kg); (4) ovx + day 1 post-ovariectomy Cenestin (16.24 mg/kg) for 8 weeks. Trabecular structure of the right proximal tibia of each rat was imaged noninvasively by microCT. A compression test to induce a tibial plateau fracture was performed to determine the mechanical properties of the proximal tibia. Urine was collected on days 0, 14, 28, 42 and 56 and serum on days 0, 28 and 56 to assess biochemical markers of bone turnover including deoxypyridinoline crosslinks and osteocalcin. Both biochemical markers of bone turnover were analyzed by ELISA. Trabecular bone mass, structure, and connectivity density in the Cenestin-treated groups did not differ statistically ( p>0.05) from those of the sham-operated + vehicle-treated rats, but all were significantly higher ( p<0.05) than in the ovx + vehicle-treated rats. Structure Model Index, a measure of trabecular plate morphometry, in Cenestin-treated rats maintained a more equal mix of plate- and rod-like structures similar to the sham group, whereas the ovx group had predominantly rod-like trabeculae. Fracture load in the Cenestin (16.24 mg/kg) treated group was 31% ( p<0.01) higher than in the sham + vehicle-treated group and 61% ( p<0.05) higher than in the ovx + vehicle-treated group. Both the sham-operated + vehicle-treated and Cenestin-treated groups showed significantly lower urinary deoxypyridinoline crosslink excretion at all timepoints and serum osteocalcin at day 56 compared with the ovx + vehicle-treated group. In summary, Cenestin maintained trabecular bone microarchitecture and strength in an ovariectomized rat model of estrogen deficiency.

The temporal changes of trabecular architecture in ovariectomized rats assessed by MicroCT.

To study the short- and long-term effects of estrogen deficiency on trabecular bone, three-dimensional measurements of proximal tibiae of ovariectomized rats were performed by micro-computed tomography (MicroCT). New three-dimensional (3D) techniques were employed to characterize the trabecular architecture from 0 to 110 days post-ovariectomy (OVX). These new methods no longer assume a plate or rod model of bone, but calculate trabecular thickness, separation, and number and their distribution by placing maximal spheres into the 3D representation of the structure. The model type of bone was quantified with the Structure Model Index (SMI). Utilizing these methods we found a rapid loss of trabecular bone in the first week after OVX. After the first week bone mass declined further, although the rate of loss was lower. In addition there was a complete change in model type from plate-like to rod-like within 7 days post-OVX, and then a very constant SMI after 12 days. After an initial thinning of trabecular structure, further bone loss seems to occur through removal of trabeculae, while the trabecular plate thickness remains constant. The heterogeneity of the network could be quantified by intra-individual standard deviation of local separations, which showed a stair-like progression, with a plateau between 12 and 60 days post-OVX. This study provides new insights into ovariectomy-related changes in cancellous bone structure evaluated by 3D MicroCT. In addition, these data suggest that the rapid change of model from plate-like to rod-like post-OVX may potentially introduce biases in the parameters that are determined using model-based algorithms, and these biases may modify the impact of age-related or therapeutic changes.