We studied femoral head lesions resulting from applying excessive mechanical stress to the hip joints of ordinary growing Wistar Kyoto rats by forcing them to stand. Twenty rats were fed in high and low cages 5-15 weeks after birth. High rat cages were prepared with the feeding apparatus placed up high so that the rats had to stand on their hindlimbs to feed. In contrast, the rats in low cages could not stand up. Histological examination of the rats at 15 weeks showed that osteonecrosis of the femoral head and ossification disturbance occurred frequently in the high cage group. The incidence of osteonecrosis was 40% (eight of 20 femoral heads) and that of ossification disturbance was 10% (two of 20 femoral heads). In the femoral heads with these lesions, localized cartilaginous abnormalities were found frequently in the lateral portion of the femoral head, where the feeding vessels of the femoral head penetrate the epiphyseal nucleus. These findings were rarely seen in the low cage group. These results indicate that standing was a causative factor in osteonecrosis of the femoral head in growing rats.

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