Osteonecrosis induced by standing in growing Wistar Kyoto rats

Wistar Kyoto (WKY) rats bred in ordinary rat cages rarely develop osteonecrosis of the epiphyseal nucleus of the femoral head (OENFH), a pathological condition with findings similar to those of early-stage Perthes' disease in humans. OENFH occurs frequently in spontaneously hypertensive rats. Interestingly, when rats are forced to stand upright on the hindlimbs at feeding time, OENFH is frequently encountered.

Experiment I investigated the stage of growth development at which osteonecrosis might be generated. Forty male WKY rats 5 weeks of age were kept and fed in high (27 cm height) rat cages. Twenty femora from 10 WKYs per observation time were resected under deep ether anesthesia at 9, 12, 15, and 20 weeks. Histologic examination of the femoral head focused on OENFH and abnormalities of the growth plate. Experiment II defined the period after postnatal week 5 when OENFH was generated while standing upright. The standing posture was lessened up to postnatal weeks 9 and 12 using a low (10 cm height) rat cage and then reenforced at feeding time with a high rat cage until 15 weeks. Forty femora from the 20 WKYs were then resected as in experiment I.

The onset of OENFH was observed only at 12 and 15 weeks of age in experiment I, revealing that OENFH occurred specifically during this growth period. In experiment II the osteonecrosis was minimal. Abnormalities in the growth plates of experiment II animals were less pronounced than those of rats killed 15 weeks after birth in experiment I.

Repetitive mechanical stress on the femoral heads from 5 to 9 weeks of age played an important role in the etiology of osteonecrosis in WKYs. This etiological finding could provide a clue to the pathogenesis and prevention of Perthes' disease in humans.