Cartilage and joint capsules are the two major components of joints. By understanding their biology, we can best prepare ourselves to fight off long term damage to our horses’ joints.
Articular cartilage (or joint cartilage) is a slowly metabolizing tissue covering the ends of bones, and provides for relatively friction-free movement between the opposing bones.
A joint capsule provides stability to a joint and its synovial lining makes hyaluronan or hyaluronic acid, which helps provide lubrication to the interior of the joint lining tissues. The joint capsule has a heavy nerve presence, and is the tissue that ‘hurts’ when a joint is injured or becomes inflamed.
All of the tissues that comprise the joint anatomically are what allow the joint to actually function as an organ within the body. Nutrients gain access to the joint via the synovial fluid and wastes are carried away in a similar manner. Cartilage cell (chondrocytes) are continually making new cartilage matrix components (mainly glycosaminoglycans and collagen) while concurrently senescent or older components are broken down by tissue enzymes and replaced by these new entities. Through these metabolic mechanisms, joint homeostasis is maintained.
When a joint becomes injured, the synovial lining cells secrete inflammatory mediators in response. At the same time, other substances are released that attempt to quell the inflammatory insult. Cartilage cells also increase production of matrix components, and part of the inflammatory response increases destruction of some matrix components already present. During inflammation the requirements for some nutrients needed by cells may increase as the joint attempts to regain homeostasis. If the joint insult is not extreme, the responses made by the joint will control the local inflammation, toxic byproducts will be broken down and removed, cartilage cells will manufacture more matrix components, and homeostasis will be restored. With a greater injury to the joint, or with continued or prolonged inflammation, homeostasis is not restored. In this situation, persistent inflammation leads to ongoing joint swelling, and nutrition to joint tissues is impaired. Ultimately destruction of cartilage matrix ingredients exceeds production, and cartilage matrix is lost.
Microscopic View of Normal Cartilage Matrix
Microscopic View of Cartilage with Partially Depleted Matrix
The goal of many treatments for joint injury is to decrease inflammation and halt this process. A really good treatment plan also includes nutrients required by the joint to make new matrix tissue when those tissues are stressed. By providing for this, we are enabling the joint to have the greatest chance for successful recovery and avoid long-term damage.