Osteoarthritis (OA) is a progressive, chronic disease that causes the degeneration of joints, resulting in pain, stiffness and lameness in performance and pleasure horses of any age. This leading cause of equine lameness mainly affects the leg joints – hock, fetlock, pastern, coffin, stifle and knee – but can also involve the back and neck. It can result from a single traumatic incident, but it is more likely to be brought about by repetitive injurious incidents or mechanical strain.

Anatomy of a Joint

Essentially, OA is a breakdown in the cartilage of a synovial joint (i.e. a freely-moving joint). A synovial joint consists of the ends of two bones that are covered by smooth, supple articular cartilage, a connective tissue that allows the joint to move without friction while distributing pressure over the joint surface and acting as a shock absorber. The joint is covered by the capsule, a hard outer layer that connects the bones and provides protection. The interior lining of the joint capsule, the synovial membrane, produces synovial fluid that fills the joint capsule to provide lubrication and nutrients to the joint.

When Disease Strikes

Because articular cartilage has no blood or nerve supply, it is unable to heal or regenerate. It is also susceptible to break down under increases in joint stress. When cartilage begins to break down, inflammation sets in as a natural healing response. The synovial fluid alters in composition as a result, and excess fluids enter the joint. Without its normal lubrication, the cartilage degenerates even further. The bone is eventually exposed, becomes damaged and tries to heal itself by producing more bone. The ends become thickened and the joint deforms.

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