It has been estimated that as much as 20 percent of the feline population over one year of age has DJD. The unifying theme in DJD is degeneration and destruction of articular cartilage – the cartilage looses its elasticity and softening occurs. Fissures can form and result in fibrillation and cell death. The altered chondrocytes release mediators (enzymes and other factors) that cause the cartilage to break itself down in a vicious cycle of degeneration. Osteochondritis dissecans (OCD) can occur in the shoulder, elbow, stifle or hock joint in the cat and can cause joint inflammation and secondary DJD at an early age.
The importance of the anatomy and disease process of DJD becomes meaningful when discussing the action of many of the newer drug therapies. Normal articular cartilage covers the bone on both sides of a joint and provides nearly friction-free motion of the joint. It also provides a "shock absorbing" protection to the joint and associated bones. When the articular cartilage structure is altered, the biomechanical properties of the joint change.
Normal articular cartilage is made up of cartilage cells (chondrocytes), an extracellular matrix and water. The chondrocytes manufacturer much of the extracellular matrix. The matrix is made up of microscopic fibers called collagen, which provides a structural support for the cartilage matrix and a complex biochemical "goo" called proteoglycan. The chemical chondroitin sulfate makes up much of the proteoglycan.
The tissue surrounding the joint is called the joint capsule and it contains a thick fluid (hyaluronic acid) that is partly responsible for joint lubrication. The joint capsule becomes inflamed with DJD and the quality of the joint fluid decreases, which causes more changes to the cartilage.
There are a number of congenital orthopedic diseases that occur in the cat that can lead to DJD at an early age. Many of these are related to the osteochondrosis syndrome:
Fragmented medial coronoid process (FCP) of the elbow produces secondary elbow DJD in cats as young as six months of age.
Ununited anconeal process (UAP) can produce severe elbow DJD.
Joint trauma can also lead to secondary DJD, including any fracture that involves a joint surface. Joint fractures need to be reduced and stabilized precisely to prevent the occurrence of DJD. Any incongruity during healing will result in degeneration. Hip and elbow fractures occur fairly frequently. A traumatic dislocation of a joint can produce severe DJD if not treated appropriately. Cats are susceptible to ligament injuries, in particular the cranial cruciate ligament (CCL) of the knee, which is the most common injury to the canine stifle. Cranial cruciate rupture causes variable amounts of DJD.