Pathology of Joints and Related Structures

Since synovial joints depend on free movement for effective functioning, anything that inter­feres with their mobility can produce gait abnormalities. Disorders affecting joints are commonly due to injuries, infections, or inflammations.

A dislocation, also known as a luxation, is a condition in which articular surfaces undergo a significant loss of congruency (less severe dis­locations are subluxations). Dislocation of a joint nearly always includes stretching or tearing of ligaments, and if the dislocation is severe enough, the joint capsule also may tear. The usual treatment for dislocation consists of replacing the joint in its normal position (reduc­ing the luxation), surgical repair of severely dis­rupted elements if necessary, and external support and/or fixation. Reduction may be dif­ficult unless the animal is fully anesthetized so that all muscles are relaxed. Early treatment is important to prevent the joint cavity from filling with connective tissue. Because of the excessive stretching or tearing of ligaments, recovery from a dislocation may be less satisfac­tory and take longer than recovery from a pro­perly treated fracture.

occasionally, a reasonably functional joint, called a false joint, may develop at the site of a chronic luxation. In a false joint, the fibrous connective tissue that develops in response to the inflammation and abnormal stresses at the luxation may provide enough stability to permit considerable movement, even though no joint capsule or cartilage develops as in true joints. False joints may also form at a fracture site if the ends of the fracture are not immobilized.

Fractures, or breaks of bones, may affect one or more of the segments making up a joint. Fractures within or close to a joint are often difficult to reduce adequately, since even minor discontinuities in the articular surface can lead to severe arthritic changes.

A sprain is a condition in which the liga­ments are stretched, but the joint does not remain subluxated following removal of the displacing force. The term strain is sometimes used in place of sprain, although strain is used more frequently to denote excessive stretching of a muscle or tendon. Although a considerable amount of swelling may follow a sprain, the affected joint usually recovers spontaneously if rested adequately.

Lacerations such as those from barbed wire may extend into a joint cavity, causing loss of synovial fluid and exposing the interior of the joint to the external environment. A condition of this nature may be serious and difficult to treat. The danger is not from the loss of syno­vial fluid (which is rapidly replaced), but from infection of the joint cavity. Synovial fluid is a good medium for bacterial growth, and the many recesses of most joint cavities make drainage and treatment of an infected joint dif­ficult. There is danger of permanent damage to the articular cartilage from infection.

Puncture wounds to joints may result from penetration by a sharp object such as a nail, wire, or wood sliver. Such wounds are espe­cially dangerous; since the wound is often not obvious, it usually drains poorly, and the envi­ronment into which the bacteria are introduced is frequently anaerobic (not exposed to air). In such conditions, a puncture wound can pre­cipitate a particularly severe infection.

Infection may also reach the joint by way of blood or lymph. Erysipelas of swine and umib- lical infections arising from a variety of bacteria in foals frequently result in joint infections. Certain viral illnesses, for example caprine arthritis-encephalitis (CAE) virus of goats, can cause nonseptic arthritis of multiple joints.

Arthritis, or more correctly, osteoarthritis, is inflammation of the components of a joint, causing swelling and pain; it usually accompa­nies each of the conditions previously men­tioned.

Table 6-1. Pathologic Disorders of Joints and Related Structures

includes loss of articular cartilage, erosion of underlying bone, and the development of bone spurs (enthesiophytes or osteophytes) around the margins of the joint. The obvious enlarge­ments associated with high and low ringbone in horses are due to the osteophytes associated with the osteoarthritic pastern and coffin joints, respectively. These and other disorders affecting joints or related structures (mostly in horses) are listed in Table 6-1.

Osteoarthritis is commonly treated with nonsteroidal anti-inflammatory drugs (e.g., aspirin, phenylbutazone), which reduce the pain associated with inflammation but do not reverse DJD. Recently, much anecdotal and some clinical evidence has supported the use of chondroprotective agents (e.g., chondroitin sulfate, glucosamine). These substances, many of which are used as food additives, may inhibit formation of destruc­tive enzymes and prostaglandins in the dis­eased joints and increase production of the molecular constituents of synovial fluid and cartilage.