Your knee joint is made up of bone, cartilage, ligaments and fluid. Muscles and tendons help the knee joint move. When any of these structures is hurt or diseased, you have knee problems. Knee problems can cause pain and difficulty walking.

Knee problems are very common, and they occur in people of all ages. Knee problems can interfere with many things, from participation in sports to simply getting up from a chair and walking. This can have a big impact on your life.

The most common disease affecting the knee is osteoarthritis. The cartilage in the knee gradually wears away, causing pain and swelling.

Injuries to ligaments and tendons also cause knee problems. A common injury is to the anterior cruciate ligament (ACL). You usually injure your ACL by a sudden twisting motion. ACL and other knee injuries are common sports injuries.

Treatment of knee problems depends on the cause. In some cases your doctor may recommend knee replacement.

Knee problems happen when you injure or develop disease in your knee and it can’t do its job. Your 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 your body. Flexibility, strength, and stability are needed for standing and for motions like walking, running, crouching, jumping, and turning.

What are joints?

The point at which two or more bones are connected is called a joint. Several kinds of supporting and moving parts, including bones, cartilage, muscles, ligaments, and tendons, help the knees do their job.

In all joints your:

• Bones are kept from grinding against each other by a lining called cartilage.
• Bones are joined to bones by strong, elastic bands of tissue called ligaments.
• Muscles are connected to bones by tough cords of tissue called tendons. Muscles pull on tendons to move joints.

Although muscles are not technically part of a joint, they’re important because strong muscles help support and protect your joints.

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.

What are the parts of the knee?

Like any joint, the knee is composed of:

• Bones.
• Cartilage.
• Ligaments.
• Tendons.
• Muscles.

Take a closer look at the different parts of the knee in the illustration below.

Bones and Cartilage

The knee joint is the junction of three bones:

• The femur, also known as the thigh bone or upper leg bone.
• The tibia, also known as the shin bone or larger bone of the lower leg.
• The patella or 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 cartilage in the knee joint includes:

• Articular cartilage, a tough elastic material that covers the ends of the three bones in the knee joint. Articular cartilage helps absorb shock and allows the knee joint to move smoothly.
• Menisci, two crescent-shaped discs of connective tissue that separate the bones of the knee. They are 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 (the kneecap) 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.

Injuries to the knee are common. Since this joint is primarily supported by muscles and ligaments, injuries to any of these structures will result in pain or knee instability. Injury to the posterior cruciate ligament occurs when the knee is flexed and the tibia is driven posteriorly, such as falling and landing on the tibial tuberosity or hitting the tibia on the dashboard when not wearing a seatbelt during an automobile accident. More commonly, injuries occur when forces are applied to the extended knee, particularly when the foot is planted and unable to move. Anterior cruciate ligament injuries can result with a forceful blow to the anterior knee, producing hyperextension, or when a runner makes a quick change of direction that produces both twisting and hyperextension of the knee.

A worse combination of injuries can occur with a hit to the lateral side of the extended knee (image). A moderate blow to the lateral knee will cause the medial side of the joint to open, resulting in stretching or damage to the tibial collateral ligament. Because the medial meniscus is attached to the tibial collateral ligament, a stronger blow can tear the ligament and also damage the medial meniscus. This is one reason that the medial meniscus is 20 times more likely to be injured than the lateral meniscus. A powerful blow to the lateral knee produces a “terrible triad” injury, in which there is a sequential injury to the tibial collateral ligament, medial meniscus, and anterior cruciate ligament.

Arthroscopic surgery has greatly improved the surgical treatment of knee injuries and reduced subsequent recovery times. This procedure involves a small incision and the insertion into the joint of an arthroscope, a pencil-thin instrument that allows for visualization of the joint interior. Small surgical instruments are also inserted via additional incisions. These tools allow a surgeon to remove or repair a torn meniscus or to reconstruct a ruptured cruciate ligament. The current method for anterior cruciate ligament replacement involves using a portion of the patellar ligament. Holes are drilled into the cruciate ligament attachment points on the tibia and femur, and the patellar ligament graft, with small areas of attached bone still intact at each end, is inserted into these holes. The bone-to-bone sites at each end of the graft heal rapidly and strongly, thus enabling a rapid recovery.

Knee replacement surgery is a surgery to replace parts of your knee joint with new, artificial parts. You may need a knee replacement if you have knee damage that causes severe pain and difficulty doing daily activities, such as walking and climbing stairs. It is usually done when other treatments for knee pain haven't helped enough. The goal of a knee replacement is to relieve pain and help you move better.

People of all ages may have knee replacement surgery. But it is more common in older people. The decision whether to have surgery is based on your overall health and how much your knee bothers you.

Chondromalacia, also called chondromalacia patellae, refers to softening and breakdown of the articular cartilage of the kneecap. This disorder happens most often in young adults and can be caused by:

• Injury.
• Overuse.
• Misalignment of the patella.
• Muscle weakness.

Instead of gliding smoothly across the lower end of the thigh bone, the kneecap rubs against it, 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 injury happens when a blow to the kneecap tears off either a small piece of cartilage or a large fragment containing a piece of bone, known as osteochondral fracture.

• An anterior cruciate ligament (ACL) injury is usually caused by a sudden twisting motion in the knee when an athlete lands or steps.
• Young women are two to eight times more likely than young men to injure the ACL.
• Landing on the balls of your feet, rather than flat-footed, may help prevent ACL injuries.

Slippage of the FEMUR off the TIBIA.

The knee is a hinge joint and susceptible to injury from trauma, inflammation, infection, and degenerative changes. A knee effusion may result from acute or chronic conditions. Causes range from trauma or overuse to systemic disease. Understanding the basic anatomy and pathophysiology of knee effusions is important to make an accurate diagnosis. However, the most important information in making the diagnosis will be to correlate findings with a thorough history and physical exam.

A Baker Cyst, also known as a popliteal or parameniscal cyst, is a fluid-filled sac that forms in the posterior aspect of the knee, typically located between the semimembranosus and medial head of the gastrocnemius.

In adults, Baker's Cysts tend to form in association with degenerative conditions of the knee. One of the most common associations seen is secondary to degenerative meniscal tears. Inflammatory conditions and arthritides are also known causes of popliteal cyst formation. When the cyst forms in association with a meniscal tear, the meniscus serves as a one-way valve and the extruded synovial fluid localizes and consolidates to form a viscous, gel-like material.

In children, popliteal cysts more commonly arise as a primary condition. Unlike adults, popliteal cysts in children arise from a herniated posterior knee joint synovium/capsule.

The meniscus is a C-shaped piece of cartilage that acts like a pad between your femur or thigh bone and tibia or shin bone. You can easily injure it when the knee is twisted while bearing weight. You may have a partial or total tear. If the tear is tiny, the meniscus stays connected to the front and back of the knee. If the tear is large, the meniscus may be left hanging by a thread of cartilage. The seriousness of the injury depends on the location and the size of the tear.

Symptoms of meniscus injuries include:

• Pain, particularly when the knee is straightened.
• Swelling.
• Clicking sound in the knee.
• Locking of the knee joint.
• Weakness.

Sometimes if you injure your knee, but did not seek treatment, you can develop symptoms months or years later. Although symptoms of meniscal injury may disappear on their own, they frequently persist or return and require treatment.

The type of knee problem you have depends on the what part of the knee is injured or affected by disease.

Arthritis

There are some 100 different forms of arthritis, 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.

• Osteoarthritis is a form of arthritis. In this disease, your 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, which generally affects people at a younger age than does osteoarthritis, is an autoimmune disease. This means it happens because your immune system attacks areas of your body. In rheumatoid arthritis, the primary site of the immune system’s attack is your synovium, the membrane that lines the joint. This attack causes inflammation of your joint. It can lead to destruction of your cartilage and bone and, in some cases, muscles, tendons, and ligaments as well.
• Other rheumatic diseases, such as:
  • 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.
  • Reactive 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.

Chondromalacia

Chondromalacia, also called chondromalacia patellae, refers to softening and breakdown of the articular cartilage of the kneecap. This disorder happens most often in young adults and can be caused by:

• Injury.
• Overuse.
• Misalignment of the patella.
• Muscle weakness.

Instead of gliding smoothly across the lower end of the thigh bone, the kneecap rubs against it, 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 injury happens when a blow to the kneecap tears off either a small piece of cartilage or a large fragment containing a piece of bone, known as osteochondral fracture.

Meniscal Injuries

The menisci can be easily injured by the force of rotating the knee while bearing weight. A partial or total tear may occur when a person quickly twists or rotates the upper leg while the foot stays still. For example, when dribbling a basketball around an opponent or turning to hit a tennis ball. If the tear is tiny, the meniscus stays connected to the front and back of the knee; if the tear is large, the meniscus may be left hanging by a thread of cartilage. The seriousness of a tear depends on its location and extent.

Cruciate Ligament Injuries

Cruciate ligament injuries are sometimes referred to as sprains. They don’t necessarily cause pain, but they are disabling. The anterior cruciate ligament is most often stretched, torn, or both by a sudden twisting motion. For example, when the feet are planted one way and the knees are turned another. The posterior cruciate ligament is most often injured by a direct impact, such as in an automobile accident or football tackle.

Medial and Lateral Collateral Ligament Injuries

The medial collateral ligament is more easily injured than the lateral collateral ligament. The cause of collateral ligament injuries is most often a blow to the outer side of the knee that stretches and tears the ligament on the inner side of the knee. Such blows frequently occur in contact sports such as football or hockey.

Tendon Injuries

Knee tendon injuries range from tendinitis, which is inflammation of a tendon to a ruptured or torn tendon. If a person overuses a tendon, it can stretch and become inflamed. This can happen during activities such as:

• Dancing.
• Cycling.
• Running.

Tendinitis of the patellar tendon is sometimes called “jumper’s knee” because in sports that require jumping, such as basketball, the muscle contraction and force of hitting the ground after a jump strain the tendon. After repeated stress, the tendon may become inflamed or tear.

Osgood-Schlatter Disease

Osgood-Schlatter disease is a condition caused by repetitive stress or tension on part of the growth area of the upper tibia (the apophysis). It is characterized by inflammation of the patellar tendon and surrounding soft tissues at the point where the tendon attaches to the tibia. The disease may also be associated with an injury in which the tendon is stretched so much that it tears away from the tibia and takes a fragment of bone with it. The disease most commonly affects active young people, particularly boys between the ages of 10 and 15, who play games or sports that include frequent running and jumping.

Iliotibial Band Syndrome

Iliotibial band syndrome is an inflammatory condition caused when a band of tissue rubs over the lateral condyle, or outer bone, of the knee. Although iliotibial band syndrome may be caused by direct injury to the knee, it is most often caused by the stress of long-term overuse, such as what sometimes occurs in sports training and, particularly, in running.

Osteochondritis Dissecans

Osteochondritis dissecans results from a loss of the blood supply to an area of bone underneath a joint surface and usually involves the knee. The affected bone and its covering of cartilage gradually loosen and cause pain. This problem usually arises spontaneously in an active adolescent or young adult. It may be caused by a slight blockage of a small artery or to an unrecognized injury or tiny fracture that damages the overlying cartilage. A person with this condition may eventually develop osteoarthritis.

Lack of a blood supply can cause bone to break down, known as osteonecrosis. The involvement of several joints or the appearance of osteochondritis dissecans in several family members may indicate that the disorder is inherited.

Plica Syndrome

Plica syndrome occurs when plicae, the bands of synovial tissue, are irritated by overuse or injury. Synovial plicae are the remains of tissue pouches found in the early stages of fetal development. As the fetus develops, these pouches normally combine to form one large synovial cavity. If this process is incomplete, plicae remain as four folds or bands of synovial tissue within the knee. Injury, chronic overuse, or inflammatory conditions are associated with this syndrome.

The symptoms of your knee problem depend on the type of injury or disorder.

Arthritis

The symptoms of osteoarthritis in the knee are:

• Pain.
• Stiffness.

The symptoms of rheumatoid arthritis and gout in the knee can include:

• Pain.
• Stiffness.
• Swelling.
• Redness.
• Hot to the touch.

Chondromalacia

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.

Meniscal Injuries

Symptoms of meniscus injuries include:

• Pain, particularly when the knee is straightened.
• Swelling.
• Clicking sound in the knee.
• Locking of the knee joint.
• Weakness.

Sometimes if you injure your knee, but did not seek treatment, you can develop symptoms months or years later. Although symptoms of meniscal injury may disappear on their own, they frequently persist or return and require treatment.

Cruciate Ligament Injuries

The symptoms for cruciate ligament injuries may include:

• Hearing a popping sound
• Buckling of the leg when you try to stand on it.

Medial and Lateral Collateral Ligament Injuries

The symptoms for medial and lateral collateral ligament injuries may include:

• Feeling a pop and the knee may buckle sideways.
• Pain.
• Swelling.

Tendon Injuries

The symptoms for tendon injury or disorder can include:

• Pain.
• Inflammation.
• Swelling

Osgood-Schlatter Disease

The symptoms of Osgood-Schlatter disease are usually:

• Pain below the knee that usually worsens with activity and is relieved by rest.
• A bony bump below the knee cap that is painful when pressed.

Iliotibial Band Syndrome

The symptoms of iliotibial band syndrome can include:

• Pain at the side of the knee, which can travel up the side of the thigh.
• Feeling a snap when the leg is bent and then straightened.
• An ache or burning sensation at the side of the knee during activity.

Osteochondritis Dissecans

The symptoms of osteochondritis dissecans can include:

• Pain, which may be sharp if the cartilage breaks off.
• Weakness.
• Locking knee joint.

Plica Syndrome

The symptoms of plica syndrome can include:

• Pain.
• Swelling.
• Clicking sensation in the knee
• Weakness
• Locking of the knee joint.

Knee pain can have many different causes, including sudden injury, such as a torn ligament; overuse; or an underlying condition. Injuries can affect many different parts of the knee: the bones, cartilage, and ligaments that form the joint as well as the tendons, ligaments, and bursae (fluid-filled sacs) that surround the joint.

Injuries to the knee may result in a torn meniscus (shock-absorbing knee cartilage), bursitis (inflammation of the bursae), and patellar tendinitis (inflammation of knee tendons).

Mechanical problems can include dislocated knee cap and loose pieces of bone or cartilage in the joint. Arthritis, including osteoarthritis (degenerative arthritis), gout, and rheumatoid arthritis, can also create knee pain. Risk factors include overweight or obesity, structural abnormalities, weak or inflexible muscles, certain sports, and previous injury. Diagnosis of knee pain involves physical inspection of the knee and its range of motion and structural integrity along with imaging tests. Treatment options include medication, physical therapy, injections, and surgery.

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 your knee to produce a two-dimensional picture of your bones.
• Computerized axial tomography (CT) scan. A painless procedure in which x-rays are passed through your 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 your knee.
• Ultrasound. A technique that uses sound waves to produce images of the soft tissue structures within and around your knee. A small, hand-held scanner is placed on and around the skin of the knee, which may be moved into different positions throughout the exam.
• Magnetic resonance imaging (MRI). A procedure that uses a powerful magnet linked to a computer to create pictures of areas inside your 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 your 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 your knee. Images of the inside of the knee joint are projected onto a television screen.
• Joint aspiration. A procedure that a doctor performs by removing fluid buildup in your 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 your body and studied under a microscope.