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.

What Causes Knee Pain and Problems?

There are two general kinds of knee problems: mechanical and inflammatory.

Mechanical Knee Problems: Some knee problems result from injury, such as a direct blow or sudden movements that strain the knee beyond its normal range of movement. Other problems, such as osteoarthritis in the knee, result from wear and tear on its parts.

Inflammatory Knee Problems: Inflammation that occurs in certain rheumatic diseases, such as rheumatoid arthritis and systemic lupus erythematosus, can damage the knee.

Joint Basics

The point at which two or more bones are connected is called a joint. In all joints, the bones are kept from grinding against each other by padding called cartilage. Bones are joined to bones by strong, elastic bands of tissue called ligaments. Tendons are tough cords of tissue that connect muscle to bone. Muscles work in opposing pairs to bend and straighten joints. While muscles are not technically part of a joint, they're important because strong muscles help support and protect joints.

What Are the Parts of the Knee?

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

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 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: