More than 100 conditions can be classified as arthritis, including:

• Osteoarthritis (OA). The most common form of arthritis, usually seen among older people. OA affects only the joints, not any other systems in the body. The cartilage wears away or breaks down. The lack of cartilage and new growth of exposed bones (bone spurs) causes the bones to rub together. OA occurs most often in the knees, hips and spine, which bear much of the body's weight. It is also common in the hands and feet.

Trauma can also trigger OA. For example, an athletic injury that damages a meniscus (cartilage pad) may lead to OA in the knee.

• Rheumatoid arthritis (RA). Affects the membranes in the joint capsules and may attack other tissues and organs as well. RA inflames the synovial membranes (which cushion the joints) and causes joint swelling, pain and sometimes deformity. It is an autoimmune disorder, in which the body’s immune system attacks its own tissues.

RA usually occurs symmetrically (e.g., in both hands or both feet). RA inflammation may affect other body systems, such as the cardiac system, where it can be life-threatening. RA is much more common in women than in men. RA has periods of flare-ups followed by periods of remission.

• Juvenile arthritis. Several forms of arthritis occur in children. The most common is juvenile rheumatoid arthritis (JRA), which can affect one joint or many joints. JRA attacks the membranes of the joint capsules. About half of the children with JRA have it in four or fewer joints.

For some children, JRA extends into adult life. Other children actually have adult RA that begins at an early age. The development of systemic JRA later in life is known as adult-onset Still’s disease.

• Gout and pseudogout. Involve crystal deposits in one or more joints. In gout, an excess of uric acid builds up in a joint (often the big toe) causing pain. Gout can also be caused by an underexcretion of uric acid in the kidneys, or a combination of both. The first attack of gout is often acute and extremely painful. Gout is common and occurs more frequently in men. Pseudogout (chondrocalcinosis) involves deposits of calcium pyrophosphate dihydrate (CPPD) crystals in joints, usually the knee.
  
  
• Psoriatic arthritis. Autoimmune disorder affecting some people who have psoriasis, a disease that causes inflamed, scaly skin patches and discolored nails. People with psoriatic arthritis may develop painful, swollen joints with reduced motion, often in the fingers and toes. The arthritis may develop well after the initial psoriasis.
  
  
• Infectious arthritis (septic arthritis). Occurs when bacteria or viruses directly invade a joint and cause inflammation. The gonorrhea and staphylococcus bacteria are among the most common causes of this type of arthritis.

Lyme disease, an infection caused by tick bites, may lead to infectious arthritis if not treated promptly. Arthritis caused by Lyme disease is treated with antibiotics to address the underlying infection. Infectious arthritis may also develop after a traumatic injury that exposes a joint, or after joint surgery. It can destroy joints and is more common in people with RA, previous infections or previous joint surgery (e.g., arthroplasty).

• Reactive arthritis. Occurs when an infection elsewhere in the body causes inflammation in a joint. Infections in the gastrointestinal and urogenital systems may trigger reactive arthritis. The sexually transmitted disease chlamydia may create reactive arthritis in joints. Certain gastrointestinal infections, such as salmonella, may also trigger reactive arthritis. One type of reactive arthritis is Reiter’s syndrome, which includes inflammation of the joints, eyes (conjunctivitis) and urethra (urethritis).
  
  
• Ankylosing spondylitis. An uncommon condition that primarily affects the spine. As it progresses, it can also cause inflammation in other parts of the body, such as the joints between the ribs and spine and the joints of the hips, shoulders, knees and feet.

Many other conditions can cause arthritis-like pain, including lupus, TMJ disorder and chronic fatigue syndrome.