[Remodeling of the acetabulum after experimental hip joint dislocation--an animal experiment study of the rabbit]

Little is known about the remodeling potential of the acetabulum after dislocation and relocation of the hip joint. Most clinical data rely on radiographic measurements in a clinical setting, with only few singular reports on histologic changes at the time of surgery. This study was undertaken to examine the remodeling potential of the rabbit acetabulum after different length of hip dislocation and to gain insight into the histology of the remodeling process involved.

The right knee joint was immobilized in 28 matched New Zealand white rabbits, causing dislocation of the associated hip joint. Assessment of the joint status was made by serial X-rays in a reproducible standardized way, allowing measurement of the Acetabular Index (AI). The untreated left hip joints served as controls in the histologic sections and also allowed to develop an age matched scale of normal acetabular development for radiographic assessment. Dislocation was unsuccessful in 2 rabbits, 9 animals were sacrificed with the joint dislocated, and in 13 animals a closed reduction was performed and maintained in a Petrie type position for 10 days. Reduction was unsuccessful in 4 cases. The reduced group was sacrificed after 4 to 12 weeks, the point of harvesting being determined by no further change in the serial standardized radiographic measurements of the AI. All surgical procedures were done under the supervision of a veterinary surgeon and followed an approved protocol.

In the time observed, the normal values of the Acetabular Index drop from 30 degrees to 20 degrees over a time of 3 months. In animals with a dislocated hip the AI increased to 40 degrees, which was completely reversible and reached normal values in 4 weeks, if reposition was done at this time (group A). Later reposition did improve the AI, but compared to normal age matched values, the AI remained elevated (> 30 degrees), demonstrating a residual dysplasia (group B). The histologic sections of the control left joints showed normal hyaline cartilage with physiologic vertical stacking of the cartilage zones. In the dislocated group the vertical stacking was lost, with chondrocytes demonstrating a fibro-cartilaginous metaplasia. There was hypocellularity with loss of nuclear staining. These changes were completely reversed in the group A, but remained in group B.

This study demonstrates that an early reduction will not only allow for a normalisation of the radiographic values, but that the histologic changes as they develop during dislocation can completely be reversed. Long standing dislocation will lead to a permanent tissue metaplasia similar to the one seen in compression testing of joint cartilage, therefore loosing the potential for remodeling. Similar processes may occur in the human situation and the findings here underline the necessity for early reduction to allow for a complete remodeling of the hip joint.