Anatomy of a Joint
To understand how lameness occurs, you must be familiar with healthy joint
function and how inflammation causes damage to the joint. Three
components are most important to maintaining normal
joint action: the joint
capsule, synovial fluid
(also called synovia) and articular cartilage.
The joint capsule is fibrous tissue that connects the bones of the joint and
serves to contain the joint fluid. The inner surface of the joint capsule is
lined with the synovial membrane, a connective tissue thick with blood
vessels that produces synovial fluid.
Synovial fluid consists of blood plasma
components and sodium
hyaluronate. This fluid is clear, lubricating and semi-solid. When a joint
is under physical load, the amount of synovial fluid increases in the stressed
joint. Normal volume is reached after a few hours' rest.
Articular cartilage covers the bone ends and is subjected to greater mechanical
stress than any other joint tissue. The condition of the articular cartilage is
a determining factor when assessing a joint's potential for working properly.
Articular cartilage consists of collagen,
proteoglycans, sodium hyaluronate and a great deal of water (60 to 80
percent).
Collagen, which is about 50 percent of the dry material of the cartilage,
absorbs, equalizes and dampens the mechanical load put on the articular
cartilage. Adult animals have a limited ability to produce new collagen. This
limited capacity for healing contributes to the seriousness of arthritis,
particularly for high-performance animals.
Proteoglycans add elasticity to articular cartilage.
Water molecules, bound by the network of sodium hyaluronate chains, help
maintain the health and function of the joint.
Cartilage cells, called chondrocytes,
are somewhat sparsely located in the cartilage mass. They have neither blood vessels nor lymph vessels within
easy reach for their nutrition.
The synovial fluid provides a supply of nutrients through a natural pumping
action as the cartilage is loaded and unloaded. When the joint is under load,
the elastic cartilage, including the network of sodium hyaluronate,
compresses. Part of its liquid content, including degraded products, is
expelled into the synovial fluid. When the joint is relieved, the cartilage
works to regain its volume, drawing in liquid and nutrients, especially
glucose.
This exchange of nutrients and waste is required to maintain
functional cartilage, because the joint does not have access to nutrients from
the bloodstream.
Should the joint suffer trauma, inflammation usually begins in the synovial
membrane. Inflammation of the synovial membrane is called
synovitis.
When a joint is inflamed, leukocytes (white blood cells) are activated and
migrate into the joint cavity, where they release destructive
free radicals and
proteolytic enzymes. These enzymes and free radicals erode the tissue
surrounding the joint and damage the articular cartilage. The damage may be
permanent, because articular cartilage has only a limited capacity for
healing.
Research has shown that degradation of the proteoglycans occurs in the
inflammatory process and is a significant part of joint degeneration. As the
proteoglycans degrade and leave the cartilage, the elasticity of the cartilage
also decreases. Consequently, the pumping mechanism that carries the
nutrients between the synovial fluid and the chondrocytes becomes less
efficient. With fewer nutrients, the chondrocytes cannot replenish the
cartilage matrix as quickly as it is degraded. If this process is not stopped in
time, the degeneration proceeds to a point of no return. Early diagnosis,
swift treatment and rest are crucial.
Highlighted terms are defined in Glossary.
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