Degenerative Joint Disease (DJD) Degenerative Joint Disease (DJD), often referred to as osteoarthritis, is a very common cause of lameness in performance horses. Most equine training strategies involve the prevention and/or maintenance of some form of joint disease. Lack of performance as a result of joint disease often precedes overt lameness and/or radiographic changes. What structures make up a joint? The synovial or diarthrodial joint consists of: • articulating surfaces of bone (called subchondral bone) • cartilage layer (overlying the subchondral bone) • a fibrous joint capsule • synovium (consisting of synovial membrane and fluid ) • associated supporting ligaments • The combination of a smooth cartilage surface and thick synovial fluid provides a low-friction environment through which the articulating surfaces can move freely and independently of each other. What makes synovial fluid so thick and slippery? Within synovial membrane and fluid is a large molecule called hyaluronan. Hyaluronan is the primary component of synovial fluid which gives it its thick and slippery characteristics. Since minimizing friction is critical to normal joint function, the presence of normal hyaluronan within a joint is very important. Lack of hyaluronan within synovial fluid increases friction associated with the synovial membrane and articular surfaces, therefore predisposing cartilage to excessive wear and erosion. What causes joint inflammation (synovitis/arthritis)? Joint inflammation is a consequence of joint injury and/or instability. Instability can occur as a result of compromise of the supporting structures of the joint (such as the collateral ligaments), loss of articular cartilage (as may occur in osteochondrosis), overuse, and/or increased age. What causes joint pain? The majority of the nerve endings lie within the fibrous capsule and supporting ligaments of the joint. There is very little sensitivity in the cartilage and synovium. The three primary causes of joint pain, therefore, are: 1. Joint effusion (i.e. increased fluid within the joint) resulting in distension/stretching of the joint capsule, 2. Fibrosis/scarring of the joint capsule (often referred to as capsulitis), and 3. Inflammation within the associated supporting ligaments of the joint (called desmitis) Eliminating excess synovial fluid, inflammation within the fibrous capsule, and inflammation within the supporting ligaments will usually produce a comfortable joint. How does degenerative joint disease (DJD, osteoarthritis) occur? The degenerative cycle is initiated as a result of joint instability and/or injury. Inflammation of the synovial membrane (a condition called synovitis) occurs secondarily to joint instability/trauma. Inflammatory cells within the synovial membrane and fluid do two things: 1. Their presence increases the hydrostatic pressure within the joint and 2. They release catabolic enzymes (lysozymes) and other chemical mediators into the joint. The increased hydrostatic pressure results in the influx of fluid into the joint (synovial effusion), distension (stretching) of the joint capsule, and pain. The enzymes released by the inflammatory cells degrade hyaluronan (the molecule that is involved in normal lubrication of the joint) as well as the cartilage layer. Compromise and eventual erosion of the cartilage layer results in loss of articular congruency, which in turn increases joint instability. Pain also accentuates the activity/release of harmful chemical mediators into the joint. Increased joint instability, the persistence of chemical mediators within the joint, and pain all result in an increased inflammatory reaction. Fox Run Equine Center
Posted on: Sat, 03 Jan 2015 14:00:01 +0000
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