Arthritis is a painful condition that affects one or more joints. More than 100 conditions may be classified as arthritis.

Most forms of arthritis are degenerative or rheumatic conditions, meaning they involve some sort of inflammation, usually of the soft tissue (cartilage) or membranes surrounding the body’s joints. Other arthritic conditions may primarily involve inflammation of supporting structures, such as muscles, ligaments or tendons. In time, the inflammation may also cause swelling, redness and difficulty moving the joints.

Arthritis comes from the Greek words for joint inflammation. However, the most common arthritic condition, osteoarthritis, involves degeneration of cartilage at the joints, but usually not inflammation.

Arthritis occurs primarily in the joints. In human anatomy, a joint is where two or more bones meet. There are several types of joints. Some joints do not move very much or at all, such as where bones in the skull meet. Arthritis occurs mostly in cartilaginous and synovial joints, such as those in the knees, hips, hands and feet, which can move.

The cartilaginous and synovial joints are formed by the juncture of bones and their surrounding ligaments. The space enclosed by the ligaments and adjoining bones forms a cavity called the joint capsule. The outer layer of the capsule is formed by a fibrous membrane that may include the ligament. The inside of the capsule is lined with synovial membrane (synovium). This membrane secretes synovial fluid, which fills the joint capsule and 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.

Joints affected by arthritis do not function smoothly. In some cases, the cartilage degenerates and irritates the bones. Small bony outgrowths form, swelling the bone and decreasing the space within the joint capsule. In other cases, the synovial membranes or cartilage become inflamed and swell. As a result, the joint may swell and become painful to use. Regardless of the origin (degeneration or inflammation), joints can become stiff, painful and swollen, eventually impairing their function.

Some forms of arthritis also affect other parts of the body, such as the skin, urinary tract, eyes or heart.

Arthritis develops in many ways. Osteoarthritis, the most common form, usually develops with age. Years of weight and repetitive stress on the joints cause the cartilage to degenerate. Sudden trauma to a joint, such as from a sports injury, can ultimately lead to osteoarthritis. Other forms of arthritis develop at any age (even among children) as the result of causes including infection, such as Lyme disease, or as an autoimmune disorder, such as rheumatoid arthritis (RA), where the body mistakenly attacks its own tissues.

Some people experience arthritis as stiffness in a hip or knee joint. For others, the pain may start in one joint and spread to others and include stiffness, swelling and redness. In some cases, arthritis causes stiffness upon rising in the morning. For other people, the pain worsens after they use the affected joint, such as during exercise. All these symptoms vary depending upon the kind of arthritis.

Arthritis can be debilitating. Many of the mobility problems elderly people experience are the result of osteoarthritis. RA, which can begin at any age, may swell and damage joints enough to cause deformity. Although there are ways to stop or delay the progression of many forms of arthritis, there is generally no cure. Treatment focuses on controlling pain, increasing the mobility of the joint, halting progression of the disease and improving quality of life.

About 46.4 million Americans, including 294,000 children, had physician-diagnosed arthritis in 2003, the U.S. Centers for Disease Control and Prevention (CDC) estimated in 2007. That's a 25 percent increase from 1997, due largely to the aging of the population, Arthritis is the nation's leading cause of disability and in 2003 cost $128 billion in medical expenses and lost productivity, the CDC says .