Stability of osteochondral fragments of the femoral condyle: magnetic resonance imaging with histopathologic correlation in an animal model.

The stability of surgically induced osteochondral fragments of the femoral condyle was examined by magnetic resonance imaging (MRI) using T1- and T2-weighted spin echo sequences in 7 dogs; contrast-enhanced T1-weighted spin-echo sequences were also obtained. Animals were sacrificed between the 34th and 196th day after surgery. MR images were compared with the histopathologic findings. Two loose and five stable fragments were found after injection of contrast medium. With the loose fragments, a well-defined line of high signal intensity between the fragment and the epiphysis showed marked enhancement. Histological examination revealed vascularized granulation tissue at the interface. Stable fragments also showed a similar, but irregularly defined line on plain sequences, but no enhancement after injection of contrast medium; histological examination showed no granulation tissue at the interface but intact bone trabeculae within the completely repaired fracture. Fibrocartilaginous repair at the articular cartilage surface also showed enhancement. Contrast-enhanced MR imaging allowed an exact delineation of the line of separation of unstable osteochondral fragments in this animal model with differentiation from a similar line occurring in stable fragments. However, this interface line in relation to stable fragments could not be explained histologically and probably reflects differences of binding or distribution of protons in healing osteochondral fragments.