We found that osteonecrosis of the femoral head occurred in growing Wistar rats (WRs) when they were made to stand up during feeding. To observe the behavior of WRs and the histology of their femoral heads, 32 5-week-old male WRs were fed in a high cage (27 cm in height), and then 10, 10, and 12 WRs were killed at 9, 12, 15 weeks respectively. The total standing time in a day was around 2 hours at 6 weeks, and it decreased gradually as the WRs grew. Histological examination disclosed osteonecrosis in 3 femoral heads (15%) at 12 weeks and 8 femoral heads (33%) at 15 weeks.
Histological abnormalities in the growth plate of the lateral portion, where the nutrient vessels penetrated into the femoral heads from the lateral, were found in 4 femoral heads (20%) at 12 weeks and 8 femoral heads (33%) at 15 weeks. However, there were no osteonecrosis nor other abnormal findings in the femoral heads at 9 weeks. These results suggest that osteonecrosis resulted from repeated mechanical stress from standing.
