The hinge joint between the femur (thighbone) and tibia (shin). The patella (kneecap) lies across the front of the joint. Two protective discs of cartilage called menisci (see meniscus) cover the surfaces of the femur and tibia to reduce friction. Bursas (fluid-filled sacs) are present above and below the patella and behind the knee. External ligaments on each side of the joint provide support. Cruciate ligaments within the joint prevent overstraightening and overbending of the knee. The quadriceps muscles on the front of the thigh straighten the knee; the hamstring muscles at the back of the thigh bend it. Knee injuries are common. They include ligament sprains, torn meniscus, dislocation of the patella, and fracture of any of the bones in the joint. Chondromalacia patellae is common in adolescents.


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