Abstract
In an attempt to study the effect of mechanical loading on a developing hip joint, the hip joints of young rabbits were fixed in a flexed, abducted and externally rotated position admitting of some motion and subsequently were examined for changes macroscopically, histologically and roentogenologically as well as by utilizing tetracycline labelling technique, contactmicroradiography and microangiography. The results were summarized as follows:
i) There were flattening of the articular head of the femur, steepening and enlargment of the posterior portion of the acetabular roof, retroversion of the femoral neck and thickening of the joint capsule noted. These changes were considered to represent dysplasia occurring to sustain competence of the joint in such an unusual position. The dysplasia of the head of the femur was preceded by the deformation of the acetabular roof.
ii) Histological changes were most prominent in the epiphyseal plate, followed by the articular cartilage. The acetabular roof and the articular cartilage of the femur, which constitute the articulation, showed degenerative and atrophic changes without demonstrable pressure necrosis. The epiphyseal plate of the femoral head responded to the exercised pressure with gradual enchondral dysostotic changes.
iii) The first 5-week period was marked by degenerative and atrophic changes of a mild degree, while the subsequent periods were characterized by regressive degeneration of increasing intensity.
iv) Tetrcycline labelling and microradiography provided biophysical evidence of impaired ossification.
v) Whereas the fixation of a young hip joint in non-functional position causes the joint to develop in the direction of forming an articulation which is competent for such unusual conditions, its fixation in a position which allows the movement of the joint diminishes the risk of necrosis of the head of the femur.
