Osteoarthritis occurs when the cushioning substance between the bones in a joint—called cartilage—breaks down, increasing bone-on-bone friction and causing inflammation, impaired mobility, and pain. Often, the pain is worse later in the day. The type and severity of symptoms vary widely from one patient to the next. They may be chronic and debilitating or intermittent. The most common form of arthritis, osteoarthritis can be caused by aging, genetics, previous injury, or previous or existing disease. As the body ages, the cartilage accumulates more water and the protein content diminishes. Eventually, flaking and fissuring occur. In severe cases of osteoarthritis, there is a complete loss of cartilage. In people under the age of 45, osteoarthritis occurs more frequently in men. In people older than 55, however, the condition is more common among women. Commonly affected bodily regions include the hands, feet, spine, and large weight-bearing joints, including the knees and hips. If the cause of the arthritis has been identified—for example, if a simultaneously occurring disease is to blame—then it is referred to as secondary osteoarthritis. If the exact cause is unknown, the condition is called primary osteoarthritis. Treatment for osteoarthritis may include rest, lifestyle changes such as diet or weight control, use of supportive devices, administration of pain relief medications or injections, and participation in a physical therapy program to improve mobility and range of motion and strengthen the muscles that support the affected joints. In some severe cases, surgery may be considered as a treatment option.