Topic updated: December 2012

Knee Injuries & Disorders

Knee problems are very common, and they occur in people of all ages. This booklet contains general information about several knee problems. It includes descriptions and a diagram of the different parts of the knee. Individual sections of the booklet describe the symptoms, diagnosis, and treatment of specific types of knee injuries and conditions. Information on how to prevent these problems is also provided.

What Do the Knees Do? How Do They Work?

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. 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 Causes Knee Problems?

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. We’ll describe some of the most common forms of arthritis and their effects on the knees a bit later in the 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 throws 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. We’ll discuss some of the most common knee injuries later in this booklet, but first we’ll take a look at the structure of the knee joint.

What Are the Parts of the Knee?

Like any joint, the knee is composed of bones and cartilage, ligaments, tendons, and muscles.

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 (knee cap). 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 leg 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. One pad is called a meniscus (muh-NISS-kus). The plural is menisci (muh-NISS-sky). The menisci are divided into two crescent-shaped discs 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 extend the leg. Four ligaments connect the femur and tibia and give the joint strength and stability:

  1. The medial collateral ligament, which runs along the inside of the knee joint, provides stability to the inner (medial) part of the knee.
  2. The lateral collateral ligament, which runs along the outside of the knee joint, provides stability to the outer (lateral) part of the knee.
  3. The anterior cruciate ligament, in the center of the knee, limits rotation and the forward movement of the tibia.
  4. 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.

How Are Knee Problems Diagnosed?

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 discover the limits of movement and the location of pain. The patient may be asked to stand, walk, or squat to help the doctor 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 (CAT) 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.

Read about all the different knee injuries and disorders at the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) Information Clearinghouse.

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