A Closer Look at Equine Joint Inflammation

the bone underlying the articular cartilage. In contrast degeneration can occur from normal forces on abnormal cartilage. This occurs in joints with the normal wear of articular cartilage that comes with aging in joints affected by osteochondrosis which is the failure of the bone that underlies the articular cartilage to mineralize properly or on cartilage that has been damaged by synovitis inflammation of the synovial lining.
Osteoarthritis can be defined as the point at which the repair processes of the joint are overwhelmed by the destructive processes. It can affect horses of any age and is thought to account for about 60 percent of lameness problems in horses. There is a continuum of disease severity that occurs in osteoarthritis and it is likely that most of the components of the joint are involved. There is generally no central inflammatory role in the osteoarthritis that occurs in horses this is similar to many types of osteoarthritis in humans but is very different from the human rheumatoid arthritis which does have a central inflammatory role.
As osteoarthritis takes hold in a joint whether it be from a single incident of injury or chronic injury over time inflammatory mediators are released from the synovial membrane. These inflammatory mediators may include prostaglandins cytokines and matrix metalloproteinases and they contribute to the degradation of the cartilage. The synovial membrane becomes edematous and swollen and increases the amount of blood vessels present which can increase the amount of joint fluid referred to as effusion and the amount of protein in the joint fluid. The inflammatory mediators contribute to the degeneration of the articular cartilage making it less able to withstand normal loads. 
The joint capsule synovial membrane tendons ligaments and bone will all contribute to the pain the horse feels in osteoarthritis. Injury to the joint capsule causes it to form a fibrous repair tissue which is not the same as the original tissue and can lead to a decreased range of motion for the joint. Sclerosis an increase in bone density osteolysis bone destruction new bone production a narrowing of the joint space and bone fragments can be seen on radiographs in many cases. However the level of pain and the degree of radiographic change do not always correlate a significant number of horses will show pain despite relatively normal radiographs. This makes sense when you consider the number of soft tissue structures which are not imaged