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Anatomy of the Knee

An inside look at the structure of the knee.

The knee is the joint where the bones of the lower and upper legs meet. The largest joint in the body, the knee moves like a hinge, allowing you to sit, squat, walk or jump.

The knee consists of three bones:

  • femur – the upper leg bone, or thigh bone
  • tibia – the bone at the front of the lower leg, or shin bone
  • patella – the thick, triangular bone that sits over the other bones at the front of the knee, or kneecap.

The ends of the bones are covered with a layer of cartilage, a slick, elastic material that absorbs shock and allows the bones to glide easily against one another as they move.

Between the tibia and femur bone are two crescent-shaped pads of connective tissue that reduce friction and disperse the weight of the body across the joint. They are:

  • The lateral meniscus, situated at the outside of the knee.
  • The medial meniscus, situated on the inside of the knee.

The bones are held together by a joint capsule, which consists of two distinct layers – an outer layer of dense connective tissue and an inner membrane, called the synovium, which secretes a fluid to lubricate the joint. 

The outer layer of the capsule is attached to the ends of the bones and is supported by these ligaments and tendons:

  • quadriceps tendon, which attaches the quadriceps to the patella
  • medial collateral ligament (MCL),  which gives stability to the inner part of the knee
  • lateral collateral ligament (LCL), which stabilizes the outer part of the knee
  • anterior cruciate ligament (ACL), which is located in the center of the knee and prevents excessive forward movement of the tibia
  • posterior cruciate ligament (PCL), which is located in the center of the knee and prevents excessive backward shifting of the knee.

Two groups of muscles support the knee. They are:

  • Hamstrings – muscles on the back of the thigh, which run from the hip to just below the knee and work to bend the knee.
  • Quadriceps – four muscles on front of the thigh that run from the hip to the knee and straighten the knee from a bent position.

Knee Pain Causes

Arthritis and Diseases that Affect the Knee

From cartilage breakdown to infections in the joint, problems that may be to blame for painful knees.

Virtually any form of arthritis can affect the knee, and cause pain and swelling. These include:

Osteoarthritis.   The most common form of arthritis, osteoarthritis is a chronic condition characterized by the breakdown of the cartilage that cushions the ends of the bones where they meet to form joints. The breakdown of cartilage causes the bones to rub against each other, causing stiffness, pain and loss of movement in the joint. The knee is one of the joints most commonly affected by osteoarthritis.

Rheumatoid arthritis.   Rheumatoid arthritis is a chronic inflammatory disease of the joints which occurs when body’s immune system – which normally protects us from infection – mistakenly attacks the synovium, the thin membrane that lines the joints. The result can be joint damage, pain, swelling, inflammation, loss of function and disability. In addition to the knees, rheumatoid arthritis, commonly affects the hands, wrists, feet, elbows and ankles.

Juvenile arthritis. Juvenile arthritis is the term used to describe arthritis when it begins at age 16 or before. There are several different types of juvenile arthritis, including juvenile rheumatoid arthritis (JRA) and juvenile idiopathic arthritis (JIA). Many can cause pain and swelling of the knee.

Gout. Gout is a form of arthritis that occurs when excess uric acid, a bodily waste product circulating in the bloodstream, is deposited as needle-shaped monosodium urate crystals in tissues of the body, including the joints. For many people, the first symptom of gout is excruciating pain and swelling in the big toe – often following a trauma, such as an illness or injury. Subsequent attacks may occur off and on in other joints, primarily those of the foot and knee, before becoming chronic. 

Reactive arthritis. Reactive arthritis is a chronic form of arthritis that often occurs following an infection of the genital, urinary or gastrointestinal system. Features of reactive arthritis include inflammation and swelling of the joints (primarily the knee, sacroiliac joints and joints of the feet), eyes and structures within the gastrointestinal or genitourinary tracts, such as intestines, kidneys or bladder.

Lyme disease.  Lyme disease is an infectious disease characterized by a skin, rash, joint swelling and flu-like symptoms. The disease is caused by the bite of a tick infected with a bacterium called B. burgdorferi. The knee is often the first joint affected by Lyme disease.

Lupus. Lupus, or systemic lupus erythematosus, is a chronic autoimmune disease, meaning the body's immune system creates antibodies that attack healthy tissues. In addition to causing inflammation in the knee and other joints, lupus can affect many organs, including the skin, heart, lungs, and kidney.

Ankylosing spondylitis. Ankylosing spondylitis is a form of arthritis that primarily affects the spine, causing inflammation in the spine that can lead to chronic pain and stiffening of the spine. In some people, particularly younger people, it begins with pain and swelling in the knee rather than the spine.

Psoriatic arthritis.Psoriatic arthritis is a form of arthritis accompanied by the skin disease psoriasis. The skin disease often precedes the arthritis; in a small percentage the joint disease develops before the skin disease.

Infectious arthritis.  Also called septic arthritis, infectious arthritis refers to arthritis that is caused by an infection within the joint. Infectious arthritis is often cause by bacteria that spread through the bloodstream to the joint. Sometimes it is caused by viruses or fungi.

Knee Injury

Common and some not-so-common injuries of this vulnerable joint.

The knee is one of the joints most prone to injury. Its structure and many components put it at risk of many types of injuries, which can result in knee pain or loss of function.

Sometimes a knee injury happens suddenly as a result of the knee being hit, fallen on, twisted or moved beyond its intended range of motion.  Sudden knee injuries are common among athletes and may result in tears to one of three major ligaments of the knee –  the anterior cruciate ligament (ACL), medial collateral ligament (MCL) or posterior cruciate ligament (PCL) – or the menisci, crescent-shaped wedges of cartilage within the knee designed to distribute your body weight across the joint.

At other times, knee injuries happen slowly. For example, a problem such as a leg-length discrepancy or arthritis in the hip that causes you to walk awkwardly can throw off the alignment of the knee, leading to damage. Constant stress to the knee – from sports or jobs that require bending and lifting, for example – can cause joint cartilage to wear down over time.

The following are some of the more common knee injuries:

Meniscal Injuries. Menisci can be torn when the knee is bent and then twisted, such as turning to hit a tennis ball. If the outside of the knee is hit, during contact sports, for example, the ligaments can be torn as well. Meniscal injuries that are not repaired increase the risk of developing osteoarthritis years later.

Anterior Cruciate Ligament and Posterior Cruciate Ligament Injuries.  A sudden twisting motion or change in direction can lead to injury of the anterior cruciate ligament, whereas the posterior cruciate ligament is more likely to be damaged from direct impact, such as being tackled in football. Medial cruciate ligament injuries are often the result of a direct blow to the outside of the knee. The anterior cruciate ligament is the ligament most commonly injured.

Tendon Injuries. Ranging from inflammation (tendinitis) to ruptures, tendon injuries can result if you overwork or over-stretch your tendon. Activities that can injure tendons include running, jumping, dancing and squatting, especially to lift heavy items.

Bursitis. Some injuries to the knee can lead to inflammation of the bursae, small fluid-filled sacs that normally cushion the knee and reduce friction between the joint and surrounding ligaments and tendons. Injury to bursa can lead to swelling, warmth, pain and stiffness.

Loose bodies. Sometimes an injury to the knee can cause a piece or pieces of bone or joint cartilage to break off into the joint space. These loose bodies may interfere with joint movement and cause pain.  

Osgood-Schlatter disease. Usually affecting preteen and young teenage boys, this condition is caused by repetitive stress on the upper area of the tibia, where the bone is growing. In children with this condition, the patellar tendon (which connects the knee cap and tibia) becomes inflamed and may even tear away from the tibia.

Dislocated kneecap. This occurs when an injury causes the patella, or kneecap, to move out of position. The movement of the kneecap is always visible and, often, intensely painful.

Iliotibial band syndrome. This syndrome occurs when a band of tissue rubs against the outer portion of your femur (thigh bone), causing sharp, burning pain on the outer side of the knee. Although  this can result from a direct injury to the knee, often the cause is the stress of long-term use, such as long-distance running.

Plica syndrome. This condition occurs when bands of synovial tissue, called plica, are irritated by overuse or injury. Symptoms may include knee pain, swelling, locking and weakness.

Knee Pain Diagnosis

Diagnosing Knee Arthritis and Other Problems

From blood work to X-rays, tests your doctor uses to find the cause of your knee pain.

Diagnosing knee arthritis and other problems will begin with a physical exam and medical history. During the medical history your doctor will ask you questions, such as:

  • What symptoms are you experiencing?
  • Are your symptoms worse after activity or rest? Are there certain activities that make them worse – or better?
  • How long have you been experiencing knee symptoms?
  • Do you recall an having an illness or accident around the time your symptoms began?
  • Are you experiencing symptoms in any joints besides the knee(s)?
  • Do any of your family members have arthritis or other knee problems?
  • Do you have other medical problems that could be causing your symptoms?

During the physical exam your doctor will look at and touch your knee, and any other joints you may be experiencing problems with. Your doctor will be looking for areas with tenderness, pain or swelling, as well as indications the joint may be damaged.  Your doctor may also ask you to stand up and move your knee joint. This will show how well you can move it. Depending on the findings of the history and exam, your doctor may order lab tests and imaging tests to help make or confirm a diagnosis.

Lab Tests for Knee Diagnosis

Tests that tell what's going on inside your body.

Often a sample of blood or joint fluid drawn from the knee can help your doctor confirm a diagnosis. For example, a blood test showing high blood levels of rheumatoid factor – an antibody that acts against the blood component gamma globulin – or antibody called anti-cyclic citrullinated peptide antibody (anti-CCP) may suggest rheumatoid arthritis. High levels of antinuclear antibodies (ANAs), abnormal antibodies directed against the cells' nuclei, could suggest lupus or another inflammatory disease.  A blood test that detects an immune response to the infectious agent that causes Lyme disease could be helpful in confirming a diagnosis of that disease.

Tests of fluid drawn from the knee joint with a needle may reveal crystals of uric acid, confirming a diagnosis of gout, or bacteria, suggesting that joint inflammation is caused by an infection.  

X-rays, MRIs and Other Imaging Tests for Knee Diagnosis

Tests that tell what's going on inside your body.

Imaging tests can allow your doctor to see inside your joint painlessly. The most common imaging tests to diagnose knee problems are:

X-ray (radiography). A standard X-ray is a simple test in which an X-ray beam (a form of electromagnetic radiation) is passed through the knee to create a two-dimensional picture of the bones that form the joint. Your doctor can use X-rays to view:

  • joint alignment. Problems with alignment can cause or worsen arthritis-related changes in the joint.
  • joint space. Narrowing of the space between the two bones, which are normally covered by cartilage, can be a sign of arthritis and its severity.
  • bone spurs. Bony overgrowths at the joint are a sign of osteoarthritis.
  • fractures.

Computerized axial tomography (CAT) scan. Also called   a computed tomography (or CT) scan, this noninvasive test combines X-ray equipment with sophisticated computers to produce multiple images of the knee. The computer can combine the individual images to create a three-dimensional view of the knee. CT scans show soft tissues such as ligaments and muscles more clearly than traditional X-rays, so they are more useful for diagnosing certain knee problems, such as a torn meniscus.

Magnetic resonance imaging (MRI).   This procedure uses a strong magnet linked to a computer to create a picture of the knee joint in black and white and shades of gray. Because an MRI shows the soft tissues as well as the bones, it is particularly useful for diagnosing injuries to the cartilage, tendons, ligaments tendons and menisci, as well as areas of swelling.

Knee Pain Treatment

Exercises to Strengthen the Knee and Relieve Pain

How exercise can help your knees, and moves you need to try.

Exercise is one of the best things you can do to help your knees by helping maintain range of motion and strengthening the muscles that support them. Research shows that even relatively minor increases in the strength of the quadriceps – the muscles that run along the front of the thigh – can help reduce the risk of knee osteoarthritis and its progression, and reduce pain. Exercise can be helpful for other forms of arthritis, too, because it strengthens the muscles that support the joint. Proper exercise also may reduce the risk of knee injury and, if you need knee surgery, it will make recovery easier.

Injections and Procedures for Knee Pain

What to try if oral medications don't provide enough relief and you're not ready for surgery.

When oral medications do not relieve knee pain, but you're not to the point of pursuing knee surgery, one of the following injections or procedures may help.

Hyaluronic acid supplements – Although not technically medications, these substances are injected into knee joints to supplement naturally occurring hyaluronic acid. In healthy joints hyaluronic acid acts as a shock absorber and lubricant, allowing joints to move smoothly over each other. However, the acid appears to break down in people with osteoarthritis. Injecting it into a joint may lessen pain and inflammation. The injections are given weekly for three or five weeks, depending on the product (examples are Synvisc and Hyalgan). A small amount of joint fluid is removed first to make room for the hyaluronic acid.

Corticosteroid Injections – Doctors sometimes inject corticosteroids directly into the knee joint for quick relief of pain and inflammation. Their benefits may last anywhere from a few days to more than six months. While the injections bring targeted relief to the joint and lack many of the side effects of oral corticosteroid medications, they are not without risks. Repeated knee injections may actually contribute to cartilage breakdown. For that reason your doctor will likely put a limit on the number of injections you can receive.

Read a report from the British Medical Journal on corticosteroid injections for knee osteoarthritis.

Arthrocentesis – Also called joint fluid aspiration, arthrocentesis is removal of joint fluid through a hollow needle inserted into the joint space of the knee. Although the purpose of removing joint fluid from the knee is usually so that it can be tested in the lab, removing excess fluid can also quickly ease pain and swelling. Often after withdrawing fluid, doctors use the same puncture site where the fluid was removed to inject a corticosteroid preparation and/or anesthetic into the knee joint to further relieve pain and inflammation.

Medications for Treatment of Knee Pain and Arthritis

What you need to know about the many medications used for knee problems.

Nonsteroidal anti-inflammatory drugs. There are more than a dozen different drugs in this category, some of which are available without a prescription. Nonsteroidal anti-inflammatory drugs, or NSAIDs, are used to help ease pain and inflammation of arthritis and injuries. They are used for all forms of arthritis.

Medications for Treatment of Knee Pain and Arthritis

Pain relievers, or analgesics, are an important part of treatment for many knee problems. If your knee pain is due to an inflammatory form of arthritis, other medications may be necessary to control the disease in your knee and elsewhere in your body. The medication used will depend largely on the specific condition or form of arthritis you have. The types of medications commonly used in arthritis treatment are:

Analgesics. Analgesics are among the most commonly used drugs for many forms of arthritis. They may also be used to relieve pain from knee injuries and surgery. Unlike nonsteroidal anti-inflammatory medications, or NSAIDs, which target both pain and inflammation, analgesics are designed purely for pain relief. For that reason, they may be safe for people who are unable to take NSAIDs due to allergies or stomach problems, for example. They’re also an appropriate, and possibly safer, choice for people whose arthritis causes pain but not inflammation.

Corticosteroids. These quick-acting drugs, similar to the hormone cortisone made by your own body, are used to control inflammation. If knee inflammation is due to a systemic inflammatory form of arthritis, your doctor may prescribe oral corticosteroids.

Disease modifying anti-rheumatic drugs.  Disease-modifying anti-rheumatic drugs (DMARDs) are drugs that work slowly to modify the course of autoimmune disease. Different DMARDs may be useful for a number of different forms of arthritis including rheumatoid arthritis, lupus, ankylosing spondylitis and psoriatic arthritis.

Gout medications. Some medications for gout are designed to reduce levels of uric acid in the blood to prevent future attacks of joint pain and inflammation. Others are designed to relieve the pain and inflammation of an acute attack. Many people with gout take both types of medication.
Biologic response modifiers. The newest category of medications used for rheumatoid arthritis and a few other inflammatory forms of arthritis are the biologic agents. There are currently eight such agents approved by the Food and Drug Administration. Each blocks a step in the inflammation process without suppressing the entire immune system. In addition to rheumatoid arthritis, certain biologic agents may be used in juvenile arthritis, ankylosing spondylitis and psoriatic arthritis.

Knee Surgery

Knee replacement, arthroscopy, other surgical options for knee problems

When pain is not helped by other treatments, many people find knee surgery an option well worth pursuing.

Total joint replacement. The most common knee surgery is total knee replacement, a procedure in which the damaged knee joint is removed and replaced with prosthesis of metal, ceramic and/or plastic components. The knee is the most common replaced joint. Knee replacement is an option when irreparable joint damage interferes with function and causes constant pain that is not alleviated by more conservative therapies.

Learn more about knee replacement surgery from the American Academy of Orthopaedic Surgeons.

Arthroscopy is a minimally invasive knee surgery performed by inserting a lighted scope and narrow instruments through small incisions in the skin over the knee. Arthroscopic surgery can be used for the following:

  • to remove or repair torn menisci
  • to repair a torn cruciate ligament
  • to trim torn pieces of joint cartilage
  • to remove loose bodies
  • to remove an inflamed joint lining (synovectomy).

While knee arthroscopy is a procedure that is helpful for many people, there is controversy over the usefulness of some arthroscopic procedures.

Osteotomy. If the damage to your knee is mostly limited to one section, your doctor may recommend a surgery called osteotomy. In an osteotomy, the surgeon reshapes the bones and repositions them to take the weight off of the damaged part of the knee. In doing so it can help relieve pain and improve function, particularly for someone who is not ready to have a total knee replacement. An osteotomy may also be used to correct a broken knee that hasn't healed properly.

Knee Care

Lose Weight to Relieve Knee Pain

Good advice for losing weight and how doing so can help your knees.

Obesity is the number one preventable risk factor for osteoarthritis of the knee. Excess weight strains the knees, causing cartilage to wear away. In addition, fat cells are believed to produce inflammatory cytokines that contribute to arthritis. If you are overweight, even modest weight loss can reduce the risk of osteoarthritis and its progression. Weight loss will also reduce some of the stress of the knee, which can be helpful, regardless of the problem.

Ways to Ease Knee Pain and Get Around

Braces, canes and other helpful devices and tips for managing knee pain.

Medications and injections don't always relieve pain completely. And if you have surgery, you'll likely need some help getting around while you heal. At times when you need extra help with pain relief or mobility, here are some techniques and devices worth trying.

Canes. If placing weight on your knee causes pain, your doctor or physical therapist may recommend a cane. There are many different types of canes. The most common type has a single tip, but if you have trouble balancing, your doctor may recommend a quad, or four-point cane. Your doctor can advise the best way to use your cane, but generally you should hold it in the hand opposite the painful knee.

Knee Braces. Braces made of metal, plastic, foam and elastic material are designed to stabilize and protect the knee. Braces may be used to limit movement of the knee during recovery from an injury or surgery or relieve knee osteoarthritis pain by helping correct alignment and redirecting the body's weight from the affected compartment of the knee joint.

Electrical Stimulation. Electrical stimulation of muscle tissue (called neuromuscular electrical stimulation) around the knee may be useful for strengthening the muscles that support the joint and relieving pain in and around the joint.  A number of studies have shown effectiveness of the treatment, specifically on the symptoms of knee osteoarthritis. 

Preventing Knee Problems

Same measures used to treat knee problems can be used to prevent them in the first place.

Many of the same measures used to treat knee problems can be used to prevent them in the first place. For example, staying physically active and maintaining a healthy weight – or losing weight if you are overweight – can relieve knee pain and reduce the risk of knee osteoarthritis.
Doing exercises to strengthen leg muscles and taking proper precautions against sports injuries can prevent problems both now and down the road.

The following tips can help:

  • Stretch and warm up before exercise. Cool down afterwards.
  • Don't overdo it. If you experience knee pain when exercising, stop and cool down.
  • Wear properly fitting shoes.
  • Avoid running on hard surfaces like asphalt and concrete when possible.
  • Avoid twisting your knees.
  • Wear protective sports equipment.
  • Stop exercising if you injure your knee and seek treatment.