Bones and Cartilage
The knee joint is the junction of three bones: the femur (thigh bone or upper leg bone), the tibia (shinbone or larger bone of the lower leg), and the patella (kneecap). The patella is 2 to 3 inches wide and 3 to 4 inches long. It site 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 ends of the three bones in the knee joint are covered with articular cartilage, a touch, elastic material the helps absorb shock and allows the knee joint to move smoothly. Separating the bones of the knee are pads of connective tissue called menisci. The menisci are two crescent-shaped discs called meniscus 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 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 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.