The knee is the joint where the bones of the upper leg meet the bones of the lower leg, allowing hinge-like movement while providing stability and strength to support the weight of the body. Flexibility, strength, and stability are needed for standing and for motions like walking, running, crouching, jumping, and turning.

Several kinds of supporting and moving parts, including bones, cartilage, muscles, ligaments, and tendons, help the knees do their job (see "Joint Basics"). Each of these structures is subject to disease and injury. When a knee problem affects your ability to do things, it can have a big impact on your life. Knee problems can interfere with many things, from participation in sports to simply getting up from a chair and walking.

Knee problems can be the result of disease or injury.

• Disease A number of diseases can affect the knee. The most common is arthritis. Although arthritis technically means "joint inflammation," the term is used loosely to describe many different diseases that can affect the joints. Some of the most common forms of arthritis and their effects on the knees are described a bit later in this booklet.
• Injury Knee injuries can occur as the result of a direct blow or sudden movements that strain the knee beyond its normal range of motion. Sometimes knees are injured slowly over time. Problems with the hips or feet, for example, can cause you to walk awkwardly, which throw off the alignment of the knees and leads to damage. Knee problems can also be the result of a lifetime of normal wear and tear. Much like the treads on a tire, the joint simply wears out over time. This booklet discusses some of the most common knee injuries, but first describes the structure of the knee joint.

Like any joint, the knee is composed of bones and cartilage, ligaments, tendons, and muscles. Take a closer look at the different parts of the knee in the diagram below.

Bones and cartilage: The knee joint is the junction of three bones: the femur (thigh bone or upper leg bone), the tibia (shin bone or larger bone of the lower leg), and the patella (kneecap). The patella is 2 to 3 inches wide and 3 to 4 inches long. It sits over the other bones at the front of the knee joint and slides when the knee moves. It protects the knee and gives leverage to muscles.

The ends of the three bones in the knee joint are covered with articular cartilage, a tough, elastic material that helps absorb shock and allows the knee joint to move smoothly. Separating the bones of the knee are pads of connective tissue called menisci (pronounced men-NISS-sky). The menisci are two crescent-shaped discs, each called a meniscus (pronounced men-NISS-kus), positioned between the tibia and femur on the outer and inner sides of each knee. The two menisci in each knee act as shock absorbers, cushioning the lower part of the leg from the weight of the rest of the body as well as enhancing stability.

Muscles: There are two groups of muscles at the knee. The four quadriceps muscles on the front of the thigh work to straighten the knee from a bent position. The hamstring muscles, which run along the back of the thigh from the hip to just below the knee, help to bend the knee.

Tendons and ligaments: The quadriceps tendon connects the quadriceps muscle to the patella and provides the power to straighten the knee. The following four ligaments connect the femur and tibia and give the joint strength and stability:

• The medial collateral ligament, which runs along the inside of the knee joint, provides stability to the inner (medial) part of the knee.
• The lateral collateral ligament, which runs along the outside of the knee joint, provides stability to the outer (lateral) part of the knee.
• The anterior cruciate ligament, in the center of the knee, limits rotation and the forward movement of the tibia.
• The posterior cruciate ligament, also in the center of the knee, limits backward movement of the tibia.

The knee capsule is a protective, fiber-like structure that wraps around the knee joint. Inside the capsule, the joint is lined with a thin, soft tissue called synovium.

Doctors diagnose knee problems based on the findings of a medical history, physical exam, and diagnostic tests.

Medical History: During the medical history, the doctor asks how long symptoms have been present and what problems you are having using your knee. In addition, the doctor will ask about any injury, condition, or health problem that might be causing the problem.

Physical Examination: The doctor bends, straightens, rotates (turns), or presses on the knee to feel for injury and to determine how well the knee moves and where the pain is located. The doctor may ask you to stand, walk, or squat to help assess the knee's function.

Diagnostic Tests: Depending on the findings of the medical history and physical exam, the doctor may use one or more tests to determine the nature of a knee problem. Some of the more commonly used tests include:

• X ray (radiography): A procedure in which an x-ray beam is passed through the knee to produce a two-dimensional picture of the bones.
• Computerized axial tomography (CT) scan: A painless procedure in which x rays are passed through the knee at different angles, detected by a scanner, and analyzed by a computer. CT scan images show soft tissues such as ligaments or muscles more clearly than do conventional x rays. The computer can combine individual images to give a three-dimensional view of the knee.
• Bone scan (radionuclide scanning): A technique for creating images of bones on a computer screen or on film. Before the procedure, a harmless radioactive material is injected into your bloodstream. The material collects in the bones, particularly in abnormal areas of the bones, and is detected by a scanner.
• Magnetic resonance imaging (MRI): A procedure that uses a powerful magnet linked to a computer to create pictures of areas inside the knee. During the procedure, your leg is placed in a cylindrical chamber where energy from a powerful magnet (rather than x rays) is passed through the knee. An MRI is particularly useful for detecting soft tissue damage.
• Arthroscopy: A surgical technique in which the doctor manipulates a small, lighted optic tube (arthroscope) that has been inserted into the joint through a small incision in the knee. Images of the inside of the knee joint are projected onto a television screen.
• Joint aspiration: A procedure that uses a syringe to remove fluid buildup in a joint to reduce swelling and relieve pressure. A laboratory analysis of the fluid can determine the presence of a fracture, an infection, or an inflammatory response.
• Biopsy: A procedure in which tissue is removed from the body and studied under a microscope.

There are many diseases and types of injuries that can affect the knee. These are some of the most common, along with their diagnoses and treatment.

Arthritis: There are some 100 different forms of arthritis,1 rheumatic diseases, and related conditions. Virtually all of them have the potential to affect the knees in some way; however, the following are the most common.

The National Institute of Arthritis and Musculoskeletal and Skin Diseases Information Clearinghouse has separate publications on the different forms of arthritis mentioned in this section. See the end of this publication for contact information.

Osteoarthritis: Some people with knee problems have a form of arthritis called osteoarthritis. In this disease, the cartilage gradually wears away and changes occur in the adjacent bone. Osteoarthritis may be caused by joint injury or being overweight. It is associated with aging and most typically begins in people age 50 or older. A young person who develops osteoarthritis typically has had an injury to the knee or may have an inherited form of the disease.

Rheumatoid arthritis: Rheumatoid arthritis, which generally affects people at a younger age than does osteoarthritis, is an autoimmune disease. This means it occurs as a result of the immune system attacking components of the body. In rheumatoid arthritis, the primary site of the immune system's attack is the synovium, the membrane that lines the joint. This attack causes inflammation of the joint. It can lead to destruction of the cartilage and bone and, in some cases, muscles, tendons, and ligaments as well.

Other rheumatic diseases include:

• Gout: An acute and intensely painful form of arthritis that occurs when crystals of the bodily waste product uric acid are deposited in the joints

• Systemic lupus erythematosus (lupus): An autoimmune disease characterized by destructive inflammation of the skin, internal organs, and other body systems, as well as the joints

• Ankylosing spondylitis: An inflammatory form of arthritis that primarily affects the spine, leading to stiffening and in some cases fusing into a stooped position

• Psoriatic arthritis: A condition in which inflamed joints produce symptoms of arthritis for patients who have or will develop psoriasis

• Infectious arthritis: A term describing forms of arthritis that are caused by infectious agents, such as bacteria or viruses. Prompt medical attention is essential to treat the infection and minimize damage to joints, particularly if fever is present.

Symptoms: The symptoms are different for the different forms of arthritis. For example, people with rheumatoid arthritis, gout, or other inflammatory conditions may find the knee swollen, red, and even hot to the touch. Any form of arthritis can cause the knee to be painful and stiff.

Diagnosis: The doctor may confirm the diagnosis by conducting a careful history and physical examination. Blood tests may be helpful for diagnosing rheumatoid arthritis, but other tests may also be needed. Analyzing fluid from the knee joint, for example, may be helpful in diagnosing gout. X rays may be taken to determine loss or damage to cartilage or bone.

Treatment: Like the symptoms, treatment varies depending on the form of arthritis affecting the knee. For osteoarthritis, treatment is targeted at relieving symptoms and may include pain-reducing medicines such as aspirin or acetaminophen; nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen; or, in some cases, injections of corticosteroid medications directly into the knee joint. Other treatments for the pain of knee osteoarthritis include injections of hyaluronic acid substitutes and the nutritional supplements glucosamine and chondroitin sulphate. (For more information about the use of these two supplements, see "Research Highlights")

People with diseases such as rheumatoid arthritis, ankylosing spondylitis, or psoriatic arthritis often require disease-modifying antirheumatic drugs (DMARDs) or biologic response modifiers (biologics) to control the underlying disease that is the source of their knee problems. These drugs are typically prescribed after less potent treatments, such as NSAIDs or intra-articular injections, are deemed ineffective.

DMARDs are a family of medicines that may be able to slow or stop the immune system from attacking the joints. This in turn prevents pain and swelling. DMARDs typically require regular blood tests to monitor side effects. In addition to relieving signs and symptoms, these drugs may help to retard or even stop joint damage from progressing. However, DMARDs cannot fix joint damage that has already occurred. Some of the most commonly prescribed DMARDs are methotrexate, hydroxychloroquine, sulfasalazine, and leflunomide. Biologic response modifiers, or biologics, are a new family of genetically engineered drugs that block specific molecular pathways of the immune system that are involved in the inflammatory process. They are often prescribed in combination with DMARDs such as methotrexate. Because biologics work by suppressing the immune system, they could be problematic for patients who are prone to frequent infection. They are typically administered by injection at home or by an intravenous infusion at a clinic. Some commonly prescribed biologics include etanercept, adalimumab, infliximab, and anakinra.

People with any type of arthritis may benefit from exercises to strengthen the muscles that support the knee and from weight loss, if needed, to relieve excess stress on the joints.

If arthritis causes serious damage to a knee or there is incapacitating pain or loss of use of the knee from arthritis, joint surgery may be considered. Traditionally, this has been done with what is known as a total knee replacement. However, newer surgical procedures are continuously being developed that include resurfacing or replacing only the damaged cartilage surfaces while leaving the rest of the joint intact.

Chondromalacia Chondromalacia (pronounced KON-dro-mah-LAY-she-ah), also called chondromalacia patellae, refers to softening of the articular cartilage of the kneecap. This disorder occurs most often in young adults and can be caused by injury, overuse, misalignment of the patella, or muscle weakness. Instead of gliding smoothly across the lower end of the thigh bone, the kneecap rubs against it, thereby roughening the cartilage underneath the kneecap. The damage may range from a slightly abnormal surface of the cartilage to a surface that has been worn away to the bone. Chondromalacia related to injury occurs when a blow to the kneecap tears off either a small piece of cartilage or a large fragment containing a piece of bone (osteochondral fracture).?

Symptoms: The most frequent symptom of chondromalacia is a dull pain around or under the kneecap that worsens when walking down stairs or hills. A person may also feel pain when climbing stairs or when the knee bears weight as it straightens. The disorder is common in runners and is also seen in skiers, cyclists, and soccer players.

Diagnosis:Your description of symptoms and an x ray usually help the doctor make a diagnosis. Although arthroscopy can confirm the diagnosis, it's not performed unless conservative treatment has failed.

Treatment: Many doctors recommend that people with chondromalacia perform low-impact exercises that strengthen muscles, particularly muscles of the inner part of the quadriceps, without injuring joints. Swimming, riding a stationary bicycle, and using a cross-country ski machine are examples of good exercises for this condition. Electrical stimulation may also be used to strengthen the muscles.

Increasingly, doctors are using osteochondral grafting, in which a plug of bone and healthy cartilage is harvested from one area and transplanted to the injury site. Another relatively new technique is known as autologous chondrocyte implantation (ACI). It involves harvesting healthy cartilage cells, cultivating them in a lab, and implanting them over the lesion.

If these treatments don't improve the condition, the doctor may perform arthroscopic surgery to smooth the surface of the cartilage and "wash out" the cartilage fragments that cause the joint to catch during bending and straightening. In more severe cases, surgery may be necessary to correct the angle of the kneecap and relieve friction between it and the cartilage, or to reposition parts that are out of alignment.

There are more than 100 different forms of arthritis, symptoms vary according to the form of arthritis. Each form affects the body differently.

Arthritic symptoms generally include swelling and pain or tenderness in one or more joints for more than two weeks, redness or heat in a joint, limitation of motion of a joint, early morning stiffness and skin changes, including rashes.

After an examination by your primary care doctor, he or she may refer you to a rheumatologist, an orthopaedic surgeon, or both. A rheumatologist specializes in nonsurgical treatment of arthritis and other rheumatic diseases. An orthopaedic surgeon, or orthopaedist, specializes in nonsurgical and surgical treatment of bones, joints, and soft tissues such as ligaments, tendons, and muscles.

You may also be referred to a physiatrist. Specializing in physical medicine and rehabilitation, physiatrists seek to restore optimal function to people with injuries to the muscles, bones, tissues, and nervous system.

Minor injuries or arthritis may be treated by an internist (a doctor trained to diagnose and treat nonsurgical diseases) or your primary care doctor.

Some knee problems, such as those resulting from an accident, cannot be foreseen or prevented. However, people can prevent many knee problems by following these suggestions:

• Before exercising or participating in sports, warm up by walking or riding a stationary bicycle, then do stretches. Stretching the muscles in the front of the thigh (quadriceps) and back of the thigh (hamstrings) reduces tension on the tendons and relieves pressure on the knee during activity.

• Strengthen the leg muscles by doing specific exercises (for example, by walking up stairs or hills or by riding a stationary bicycle). A supervised workout with weights is another way to strengthen the leg muscles that support the knee.

• Avoid sudden changes in the intensity of exercise. Increase the force or duration of activity gradually.

• Wear shoes that fit properly and are in good condition. This will help maintain balance and leg alignment when walking or running. Flat feet or overpronated feet (feet that roll inward) can cause knee problems. People can often reduce some of these problems by wearing special shoe inserts (orthotics).

• Maintain a healthy weight to reduce stress on the knee. Obesity increases the risk of osteoarthritis of the knee.

Ideally, everyone should get three types of exercise regularly:

• Range-of-motion exercises to help maintain normal joint movement and relieve stiffness.

• Strengthening exercises to help keep or increase muscle strength. Keeping muscles strong with exercises, such as walking up stairs, doing leg lifts or dips, or riding a stationary bicycle, helps support and protect the knee.

•Aerobic or endurance exercises to improve function of the heart and circulation and to help control weight. Weight control can be important to people who have arthritis because extra weight puts pressure on many joints. Some studies show that aerobic exercise can reduce inflammation in some joints.

Aerobic or endurance exercises to improve function of the heart and circulation and to help control weight. Weight control can be important to people who have arthritis because extra weight puts pressure on many joints. Some studies show that aerobic exercise can reduce inflammation in some joints.

If you already have knee problems, your doctor or physical therapist can help with a plan of exercise that will help the knee(s) without increasing the risk of injury or further damage. As a general rule, you should choose gentle exercises such as swimming, aquatic exercise, or walking rather than jarring exercises such as jogging or high-impact aerobics.