Cartilage

The hard, smooth, slippery tissue that covers the ends of bones where they meet to form a joint. Cartilage allows the bones in a joint to move easily against each other and acts as a “shock absorber” for the joint. In people with osteoarthritis (OA), the cartilage in a joint breaks down, causing symptoms of pain and stiffness.

There are three basic types of cartilage: hyaline cartilage, elastic cartilage, and fibrocartilage. Hyaline cartilage is the hard and translucent cartilage in the joints. It is also found in the nose and the larynx (voice box) and between the ribs and the breastbone. Elastic cartilage is found in the outer part of the ear, the auditory canals, and the epiglottis (the flap of tissue that keeps food from entering the windpipe). Fibrocartilage, which is very tough and can withstand considerable pressure, is found in the joints most subject to stress, such as the vertebrae in the spine.

Cartilage is composed of water and substances called chondrocytes, collagen, and proteoglycans. These substances form a matrix that gives cartilage its structure and allows it to function. Chondrocytes are cells that manufacture collagen and proteoglycans. Collagen is a protein that gives cartilage its strength. Proteoglycans, which are composed of proteins and sugar, help move fluid in and out of the cartilage to absorb shock. When you use a joint, the pressure pushes fluid out of the proteoglycans so that the cartilage compresses. Once the pressure is taken off the joint, the proteoglycans absorb the fluid again so that the cartilage can return to its original size and shape. Unlike other connective tissue, cartilage has no blood supply. The chondrocytes are nourished by the nutrients and oxygen in the fluid that the cartilage absorbs. Because cartilage is nourished indirectly, it grows and repairs itself more slowly than other types of tissue.

As people age, the cartilage in their joints loses its elasticity and can become brittle. Cracks may appear, and in time pieces of cartilage may break off from the bone. By the age of 70, almost everyone shows signs of cartilage degeneration on an x-ray of the knees, but many people do not experience pain or stiffness. When cartilage breakdown does cause pain or stiffness in a joint, the diagnosis is OA. OA is most common in the knees, the hips, the joints of the upper and lower spine, and certain joints in the hands and feet.

There is still much that is not known about the cause or causes of cartilage degeneration. Mechanical wear on the joint is widely thought to play a major role. Researchers note that people who are heavier are at higher risk for OA in their weight-bearing joints. However, scientists continue to investigate chemicals in the body that may also be involved in cartilage breakdown.

Rheumatoid arthritis (RA) and other types of inflammatory arthritis can cause cartilage to break down as well. In RA, the immune system mistakenly attacks the tissue that lines the joint capsule, kicking off an inflammatory process that can also erode the bone and cartilage in a joint.

Researchers have developed many techniques that attempt to repair eroded cartilage, especially cartilage in the knee. In one procedure, small fractures, known as microfractures, are made in the bone underlying the cartilage in an effort to stimulate cartilage growth. In another procedure, called osteochondral autograft transplantation, cartilage is removed from a non–weight-bearing part of a joint and grafted onto a part of the joint where cartilage has eroded. Still another procedure, known as autologous chondrocyte implantation, removes cartilage from a joint and grows it in a laboratory before reinserting it into the joint (usually the knee). Although these techniques are not as common as others for OA, such as joint replacements, they have been gaining popularity in recent years. They are generally best suited for younger people with small areas of cartilage loss, not widespread arthritis.

Robert S. Dinsmoor is a medical writer and editor based in Massachusetts.

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