Hypothesis 2 - Osteochondritis dissecans of the Femoral Condyle

Endochondral ossification of the epiphyseal surface of long bones results in a surface of articular cartilage covering subchondral bone.  The articular cartilage acts to distribute focal compressive forces over the entire bone surface.  The epiphyses of the long bones begin as a cartilaginous template and this is transformed into bone by endochondral ossification.  Any dysfunction in this process is termed osteochondrosis.  This often results in a thickened articular cartilage, and areas of focal weakness.  Many etiologies have been hypothesized for this abnormality.  Too high a plane of nutrition, genetics, and hormonal disturbances are often implicated. 

There exist two common sequelae to this disease.  In joints where the main force is compressive, subchondral bone cysts may occur.  In joints where shear forces are more common, such as the stifle joint, osteochondritis dissecans is more likely.  Osteochondritis dissecans is a defect in the articular surface that results in a flap of cartilage separating from the underlying subchondral bone.  This cartilage may or may not be calcified, so the appearance of a "joint mouse" on radiographs is not guaranteed.  The accompanying inflammatory response causes joint effusion, stretching the fibrous joint capsule, resulting in pain and lameness.

Bonnie has many of the predisposing factors for OCD.  She is an obese Labrador retriever.  (Labs tend to show a higher incidence of OCD.)  Her genetic background is unknown, so we cannot look at her parents or siblings to see if they developed osteochondrosis.  In Bonnie's case, the likely precipitating factor would be her weight.  Even with normal movement, the forces that her joints are experiencing may increase to a pathologic level.  A large focal force on an area of increased retained cartilage could cause a fissure and thus a flap to form.

To diagnose OCD, radiographs are necessary.  One looks for an irregular subchondral bone density, surrounded by a zone of increased bone density, increased joint fluid, and possibly the presence of a cartilaginous flap (if it is ossified).