Clinical history A warmblood 6 months of age was presented lame on the right hind limb. The stifle joint was swollen and warm on palpation and the patella fit loosely. Lateral, oblique, and caudocranial radiographs were made of both stifle joints. Affected joint Radiographic diagnosis A failure of development or a destruction of the lateral trochlear ridge of the femur. The lesion had a irregular interface between the surface of the ossifying growth center and adjacent cartilage. Synovial swelling was prominent with radiopaque debris in the surrounding tissues. No swelling of the femorotibial joint was present as indicated by joint capsule protrusion caudally. This was with the knowledge that over 1/2 of the joints have a communication between the femoropatellar joint and the medial sac of the femorotibial joint. The articular surface of the patella on the conventional views appeared unaffected although poorly visualized. Skyline views of the femoropatellar joint (not shown) suggested a roughening of the articular surface of the patella. It was not possible to determine if the roughened surface represented an additional primary osteochondrosis or only represented secondary changes resulting from contact with the injured articular surface of the trochlea. Affected joint - arrows identify the margin of the missing trochlea Differential diagnosis The bony lesion could represent destruction of previously normally formed bone or could represent a failure of the bony maturation to take place. The first suggests an osteomyelitis while the second suggests an osteochondrosis. It would be expected that an osteomyelitis of this magnitude would also include a secondary septic arthritis with lesions throughout the joint. In particular, the medial ridge of the trochlea and the articular surface of the patella might be affected. An osteochondrosis of this magnitude is unusual, but could appear as a solitary lesion without radiographic change seen in other parts of the joint at this time. Later,changes of secondary arthrosis that could be identified radiographically would be expected to develop. The debris in the joint capsule supports the diagnosis of an OCD lesion that has minimal fragmentation. Normal limb Arthroscopy Arthroscopy was performed and the lesion was diagnosed as a severe osteochondrosis. A sample of the joint fluid returned a very low cell count atypical for an infectious lesion. Diagnosis Radiographic evaluation of the opposite limb failed to assist in the diagnosis. The presence of symmetrical lesions would have suggested a developmental disease, i.e. OCD. The finding of a normal joint supports a monoarticular lesion and includes either infectious or developmental etiologies. The radiographic changes more strongly supported a diagnosis of OCD. Failure to identify an increased white cell count in the synovial fluid made an infective arthritis unlikely. This, plus the arthroscopic examination confirmed the diagnosis of an OCD lesion. Comments Care must be taken to avoid diagnosing roughened bony margins seen in normal endochondral ossification of the trochlear ridges of young foals as OCD lesions. In this patient, the focal lesion is so large that normal ossification cannot be considered in the differential diagnosis. OCD lesions in the trochlea are often seen on the lateral-tomedial radiograph but the caudolateral-to-craniomedial oblique projections are of value in determination of the depth of the lesion on the trochlear ridge.
