BACKGROUND: Arthritis is the leading cause of disability among Americans over age 15, according to the Arthritis Foundation. The condition is second only to heart disease as a cause of work disability. Knee pain caused by arthritis can be caused by one of three forms of arthritis. In osteoarthritis -- the most common form of arthritis in the knee -- the joint cartilage gradually wears away. Rheumatoid arthritis is an inflammatory type of arthritis that can destroy joint cartilage. Post-traumatic arthritis often develops after a knee injury, but presents itself similarly to osteoarthritis. Usually, arthritis pain develops gradually. Patients with knee arthritis often notice the joint becoming stiff and swollen, making it difficult to bend or straighten the knee. Patients often report the pain being worse in the morning and causing a feeling of weakness in the joint that can lead to "locking."

WHO'S AT RISK? The risk of most types of arthritis increases with age and body weight. Women who are older than 50 are more likely to develop knee arthritis than men, and some evidence suggests genetics may play a role. Other factors that increase a person's chances of developing osteoarthritis of the knee include previous trauma; repetitive stress to the joint like kneeling, squatting or lifting at least 55 pounds regularly; and participating in high-impact sports like soccer and tennis.

TREATMENT: The American Academy of Orthopaedic Surgeons says only one in four people with knee osteoarthritis need surgery. Nonsurgical treatments include losing weight, special exercises, medication and supportive devices like shoe inserts, braces and canes. When surgery is necessary, surgeons may turn to arthroscopic surgery to clean out the knee or repair torn cartilage; osteotomy to cut the shinbone or thighbone to improve the knee joint's alignment; or knee arthroplasty, which involves replacing severely damaged knee joint cartilage with metal and plastic. For some people with limited cartilage loss, cartilage grafting is an option. In cartilage grafting, cartilage is either transplanted from another part of the body into the knee or a sample of knee cartilage is removed and cultured in a lab.

FIXING YOUNG KNEES: Although arthritis most often affects middle-aged and older people, some patients begin having knee pain much younger. Treatments specifically for this younger population are emerging, including partial knee resurfacing. The procedure involves removing only damaged cartilage and minimal bone and implanting a small replacement. Surgeons at the Cleveland Clinic use a system called UniCAP, which treats damaged surfaces on the inside or outside of the knee as well as under the kneecap.