Changes of cancellous bone mass in rat mandibular condyle following ovariectomy

Changes in cancellous bone of the rat mandibular condyle following estrogen deficiency were histomorphometrically examined with 120-day-old female Fischer rats. Sixty-four animals were either ovariectomized bilaterally (ovx) or subjected to sham surgery (sham), and eight from each group were killed at 7, 14, 30, and 60 days after surgery. Seven intact animals were killed on day 0. Before killing, tetracycline and calcein were administered to all animals. Following histological observation, bone histomorphometry of the mandibular condyle was done using a confocal laser scanning microscope and an image analyzer. The sampling site was divided into two regions for analysis: (1) a "subchondral region," formed by the region connected to cartilage; and (2) a "central region," formed by the region beneath the former. The changes in these two regions were analyzed separately. In the sham group's condyle, the bone volume of the subchondral and central regions increased with the passage of time, although the bone turnover became low. This bone gain could be due to the effects of growth and the mechanical stimulus by occlusal load. In the subchondral region of the ovx group's condyle, the bone volume decreased significantly at 7 days, but recovered to reach approximately the same value as the sham group from 14 days onward. In the central region of the ovx group's condyle, the bone volume was unchanged, but revealed a significantly lower value than that of the sham group at 60 days (p < 0.01). Thus, ovariectomy inhibited bone gain, which was observed in the sham group's condyle even though there was no bone loss. On the other hand, the trabecular separation in the ovx's condyle of both the subchondral and central regions increased considerably and small marrow cavities interconnected to form a large bone marrow. Therefore, the ovx rat mandibular condyles dynamically altered their structures under the effects of estrogen deficiency and occlusal loads. Consequently, estrogen deficiency induced transient subchondral bone loss and recovery, whereas, in the central region, it inhibited bone gain. This suggests that mechanical loading modulates the normal ovx-induced bone loss found in other parts of the skeleton.