Osteoarthritis (OA) is the most common type of arthritis. It most often affects weight-bearing joints in the knees and hips, along with the hands and spine. It can cause stiffness, joint pain and reduced movement. If OA is located in the hips or knees, it can impair an individual’s ability to stand, walk, climb and engage in other physical activities.

OA occurs in the joints, where two or more bones meet. It primarily develops in the joints with the most movement, such as hips, knees, feet and hands. These joints are known as cartilaginous and synovial joints. A synovial joint capsule is a space enclosed by the bones and adjoining ligaments. The outer layer of the capsule is formed by a fibrous membrane. The inside of the capsule is lined with synovial membrane and filled with synovial fluid, which provides lubrication. The ends of the bones encased in the capsule are cushioned in soft cartilage. The cartilage and synovial fluid permit the bones to move without rubbing against each other.

Cartilaginous joints do not have a synovial cavity, but have cartilage and fibrous discs between the bones to allow some movement. The vertebrae in the spine have cartilaginous joints.

Most OA damage is the result of cartilage degeneration. Cartilage is made up of three substances:

• Collagen. A fibrous protein common in skin and connective tissue.
  
  
• Proteoglycans. Strands of protein and sugar.
  
  
• Chondrocytes. Cells that usually help cartilage development, but can release enzymes (proteins that stimulate chemical reactions) that destroy collagen and other proteins.

Changes in the interaction of cartilage tissue lead to its deterioration. Chondrocytes can produce enzymes that destroy collagen and proteoglycans. Although the origin of this enzyme disruption is unknown, outside factors, such as years of stress on the joint, may be involved.

The disrupted interaction of chondrocytes, collagen and proteoglycans eventually leads the cartilage to break down. The smooth cartilage surface wears and frays. The bone underlying the cartilage is exposed and may form bony spurs called osteophytes. With less cartilage, more exposure of bone and ragged surfaces, the joint cavity becomes smaller and joint movement becomes painful or stiff.

OA is usually defined by this degenerative process. It was previously thought that the disease involved no inflammation, despite the fact that the word arthritis means inflammation in a joint. However, some recent studies have indicated that joints severely degenerated by OA have, in fact, shown some inflammation.

Joints commonly affected by OA include the fingers, spine and the weight-bearing joints of the hips, knees and feet. It is less common in the jaw, shoulders, elbows, wrists and ankles, unless there was a separate injury or stress in the area, such as an athletic injury or trauma. Although many forms of arthritis can affect other body systems, OA remains confined to the joints.

The course of OA varies for each person. Some individuals may experience a slight ache in one joint that never worsens. Other people may have morning stiffness or pain with exercise. In some cases, OA limits mobility and restricts the abilities of joints such as the fingers and knees. Most cases of OA are primary OA, which begin with no known origin. Secondary OA occurs after injury or overuse of a particular joint, such as the knee.

OA affects about 14 percent of Americans over age 24 and 34 percent over age 64, the Centers for Disease Control and Prevention (CDC) reported in 2008. That translates to 27 million adults in 2005, up from 21 million in 1990, a reflection of the aging population.

It is important to note that OA is not the same as rheumatoid arthritis (RA), a more severe, inflammatory form of arthritis that occurs in younger people. Although both forms of arthritis cause joint pain, there are several major differences:

• OA involves degeneration of joint cartilage. RA, an autoimmune disorder, involves inflammation of the membranes lining joints.
  
  
• OA may occur on one side (e.g., in one knee). RA usually occurs symmetrically, such as in both hands or both knees.
  
  
• OA affects only the joints. RA can affect other systems in the body, including the lungs, nerves or heart.

Nor should OA be confused with osteoporosis, another condition that commonly affects elderly people. Osteoporosis is a reduction in bone mass, commonly known as "thinning of the bones," that makes bones more brittle and easily broken. OA affects joints, not bone mass.