Organ: Knee

Knee: Joint connecting upper legs and lower legs
Introduction: The knees provide stable support for the body and allow the legs to bend and straighten. Both flexibility 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. Any of these parts can be involved in pain or dysfunction. 1

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

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.


There are two groups of muscles at the knee. The quadriceps muscle comprises four muscles on the front of the thigh that work to straighten the leg from a bent position. The hamstring muscles, which bend the leg at the knee, run along the back of the thigh from the hip to just below the knee. Keeping these muscles strong with exercises such as walking up stairs or riding a stationary bicycle helps support and protect 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:

  • The medial collateral ligament (MCL) provides stability to the inner (medial) part of the knee.

  • The lateral collateral ligament (LCL) provides stability to the outer (lateral) part of the knee.

  • The anterior cruciate ligament (ACL), in the center of the knee, limits rotation and the forward movement of the tibia.

  • The posterior cruciate ligament (PCL), also in the center of the knee, limits backward movement of the tibia.

Other ligaments are part of the knee capsule, which 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. 1

Condition count: 16 ; see list below.
Organ subtypes: kneecap (0)
Organ types: Leg (70)
Number: 2 knees
Related organs: thigh (11), foot (41), ankle (8), toes (17), big toe (3)
Main condition: knee conditions
Organs: all organs

Diseases list: The following list of medical conditions have 'Knee' or similar listed as an affected body part in our database:


  • Arthritis ... knee


  • Chondromalacia ... knee


  • Dislocation ... knee


  • Gout ... knee


  • Iliotibial Band Syndrome ... knee


  • Knee conditions ... knee
  • Knee sprain ... knee
  • Knee tendinitis ... knee


  • Meniscus injury ... knee


  • Osgood-Schlatter Disease ... knee
  • Osteoarthritis ... knees
  • Osteochondritis Dissecans ... knee
  • Osteosarcoma ... knee


  • Plica Syndrome ... knee


  • Reiterís syndrome ... knees
  • Rheumatoid arthritis ... knees

    1. excerpt from Questions and Answers About Knee Problems: NIAMS

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