Etiology:  Osteoarthritis is a progressive degenerative condition which occurs when the articular cartilage wears down over time.

Incidence:  Incidence of osteoarthritis is moderate and usually occurs in rats greater than one year of age.  Fischer 344 rats appear to be more commonly and more severely affected.  There is no known sex predilection.

Clinical Signs:  Usually there are no clinical signs.

Pathology:  Grossly, lesions are not usually apparent.  Histologically, there is erosion of the articular cartilage and chondromucoid degeneration of the matrix.  Tibiotarsal joints, medial femoral condyles and sternum are most commonly affected.

Diagnosis:  Diagnosis is made based on histopathologic findings.