A joint is the point where two or more bones are connected. With a few exceptions (in the skull and pelvis, for example), joints are designed to allow movement between the bones and to absorb shock from movements like walking or repetitive motions. These movable joints are made up of the following parts:
Cartilage: a hard but slippery coating on the end of each bone. Cartilage, which breaks down and wears away in osteoarthritis, is described in more later on.
Joint capsule: a tough membrane sac that encloses all the bones and other joint parts.
Synovium (sin-O-vee-um): a thin membrane inside the joint capsule that secretes synovial fluid.
Synovial fluid: a fluid that lubricates the joint and keeps the cartilage smooth and healthy.
A Healthy Joint
In a healthy joint, the ends of bones are encased in smooth cartilage. Together, they are protected by a joint capsule lined with a synovial membrane that produces synovial fluid. The capsule and fluid protect the cartilage, muscles, and connective tissues.
A Joint With Severe Osteoarthritis
With osteoarthritis, the cartilage becomes worn away. Spurs grow out from the edge of the bone, and synovial fluid increases. Altogether, the joint feels stiff and sore.
Ligaments, tendons, and muscles are tissues that surround the bones and joints, and allow the joints to bend and move. Ligaments are tough, cord-like tissues that connect one bone to another. Tendons are tough, fibrous cords that connect muscles to bones. Muscles are bundles of specialized cells that, when stimulated by nerves, either relax or contract to produce movement.
Cartilage: The Key to Healthy Joints
Cartilage is 65 to 80 percent water. The remaining three components - collagen, proteoglycans, and chondrocytes - are described below.
• collagen (KAHL-uh-jen): A family of fibrous proteins, collagens are the building blocks of skin, tendon, bone, and other connective tissues.
• proteoglycans (PRO-tee-uh-GLY-kanz): Made up of proteins and sugars, strands of proteoglycans interweave with collagens and form a mesh-like tissue. This allows cartilage to flex and absorb physical shock.
• chondrocytes (KAHN-druh-sytz): Found throughout the cartilage, chondrocytes are cells that produce cartilage and help it stay healthy as it grows. Sometimes, however, they release substances called enzymes that destroy collagen and other proteins. Researchers are trying to learn more about chondrocytes.
How Do You Know if You Have Osteoarthritis?
Although osteoarthritis can occur in any joint, most often it affects the hands, knees, hips, and spine (either at the neck or lower back). Different characteristics of the disease can depend on the specific joint(s) affected. For information on the joints most often affected by osteoarthritis, please see the following descriptions below:
Hands: Osteoarthritis of the hands seems to have some hereditary characteristics; that is, it runs in families. If your mother or grandmother has or had osteoarthritis in their hands, you’re at greater-than-average risk of having it too. Women are more likely than men to have hand involvement and, for most, it develops after menopause. When osteoarthritis involves the hands, small, bony knobs may appear on the end joints (those closest to the nails) of the fingers. They are called Heberden’s (HEBerr-denz) nodes. Similar knobs, called Bouchard’s (boo-SHARDZ) nodes, can appear on the middle joints of the fingers. Fingers can become enlarged and gnarled, and they may ache or be stiff and numb. The base of the thumb joint also is commonly affected by osteoarthritis.
Knees: The knees are among the joints most commonly affected by osteoarthritis. Symptoms of knee osteoarthritis include stiffness, swelling, and pain, which make it hard to walk, climb, and get in and out of chairs and bathtubs. Osteoarthritis in the knees can lead to disability.
Hips: The hips are also common sites of osteoarthritis. As with knee osteoarthritis, symptoms of hip osteoarthritis include pain and stiffness of the joint itself. But sometimes pain is felt in the groin, inner thigh, buttocks, or even the knees. Osteoarthritis of the hip may limit moving and bending, making daily activities such as dressing and putting on shoes a challenge.
Spine: Osteoarthritis of the spine may show up as stiffness and pain in the neck or lower back. In some cases, arthritis-related changes in the spine can cause pressure on the nerves where they exit the spinal column, resulting in weakness or numbness of the arms and legs.
The Warning Signs of Osteoarthritis
• stiffness in a joint after getting out of bed or sitting for a long time
• swelling in one or more joints
• a crunching feeling or the sound of bone rubbing on bone
About a third of people whose x-rays show evidence of osteoarthritis report pain or other symptoms. For those who experience steady or intermittent pain, it is typically aggravated by activity and relieved by rest.
What Is Osteoarthritis?
Osteoarthritis mostly affects cartilage (KAR-til-uj), the hard but slippery tissue that covers the ends of bones where they meet to form a joint. Healthy cartilage allows bones to glide over one another. It also absorbs energy from the shock of physical movement. In osteoarthritis, the surface layer of cartilage breaks down and wears away. This allows bones under the cartilage to rub together, causing pain, swelling, and loss of motion of the joint. Over time, the joint may lose its normal shape. Also, small deposits of bone - called osteophytes or bone spurs - may grow on the edges of the joint. Bits of bone or cartilage can break off and float inside the joint space. This causes more pain and damage.
People with osteoarthritis usually have joint pain and some movement limitations. Unlike some other forms of arthritis, such as rheumatoid arthritis, osteoarthritis affects only joint function and does not affect skin tissue, the lungs, the eyes, or the blood vessels.
In rheumatoid arthritis, the second most common form of arthritis, the immune system attacks the tissues of the joints, leading to pain, inflammation, and eventually joint damage and malformation. It typically begins at a younger age than osteoarthritis, causes swelling and redness in joints, and may make people feel sick, tired, and uncommonly feverish.
Although osteoarthritis is more common in older people, younger people can develop it - usually as the result of a joint injury, a joint malformation, or a genetic defect in joint cartilage. Both men and women have the disease. Before age 45, more men than women have osteoarthritis; after age 45, it is more common in women. It is also more likely to occur in people who are overweight and in those with jobs that stress particular joints.
As the population ages, the number of people with osteoarthritis will only grow. By 2030, 20 percent of Americans - about 72 million people - will have passed their 65th birthday and will be at high risk for the disease.