Human joints: Their function and problems
By Dr. John Aronen 

Articular cartilage is essential in a joint.

There are many statements in life that we do not like to hear. One of these is being told that we have become a candidate for a total joint replacement.

To understand how this can apply to seemingly healthy handball players, one first has to understand the structural make-up of a healthy joint.

Our bodies were designed with joints to allow bones to move or articulate on one another. Some joints, such as the shoulder, are designed for mobility more than stability. Others, such as the hip, are designed for stability more than mobility. Either way, joints allow us the motion we enjoy.

Ideally, a joint would be pain-free, provide and maintain a set level of a healthy fluid to function as a lubricant, have an extremely low coefficient of friction to minimize wear-and-tear changes, and be problem-free and pain-free throughout our lives.

Even at rest, forces are constantly placed on joints by the muscles that move them. Weight-bearing joints, such as the hip, knee and ankle, experience significantly greater forces than non-weight-bearing joints.

Bone has an abundance of pain fibers. Thus, if joints consisted solely of bone on bone, every joint would be a source of constant pain. That’s why the parts of bones that move on each other are covered with articular cartilage.

Whereas bone has an abundance of pain fibers, articular cartilage has no pain fibers. (Articular cartilage also is referred to as chondral, so the bone under the articular cartilage is called subchondral bone.)

In covering the subchondral bone, articular cartilage can effectively serve as an intermediary between the subchondral bone and the forces placed on a joint. This cartilage absorbs the forces placed on a joint so the underlying bone does not have to.

If the subchondral bone, which has an abundance of pain fibers, does not experience any of the forces placed on the joint, the joint will be pain-free.

Every joint has a capsule that encloses it, making the joint a confined space. The capsule is lined with synovial cells.

There are two types of synovial cells. The first, Type A cells, are secretory. These cells produce and secrete the synovial fluid that functions as an extremely effective lubricant for the joint. Synovial fluid is much more effective than any lubricant produced by man.

We also have a second type of cells, called Type B cells, added to the lining of the capsule. The Type B cells are phagocytic, meaning they remove the broken-down lubricant from the joint.

So we produce our own healthy lubricant with the Type A cells and remove the broken-down lubricant with the Type B cells. Through the combined actions of the Type A and B cells, our joints maintain a set level of healthy synovial fluid.

When articular cartilage is in pristine condition, it is super smooth. Besides having no pain fibers, articular cartilage also has no blood supply, which means:

• Articular cartilage lacks a source of nourishment.
• Articular cartilage cannot heal or regenerate.

   

  The first problem is solved because the surface of the articular cartilage is filled with microscopic holes. These tiny holes allow the synovial fluid to enter the cartilage and provide nourishment. The synovial fluid also coats the surface of the articular cartilage. The combination of the super-smooth surface of articular cartilage coated by the synovial fluid results in a coefficient of friction of almost zero between the articulating surfaces of the joint.

  The second problem, that articular cartilage cannot regenerate, will be addressed in a subsequent article.