The knee is a hinge joint that has a simple purpose. It needs to flex (bend) or extend (straighten) to allow the body to perform many activities, like running, walking, kicking, and sitting. Imagine standing up from a chair if your knees couldn't bend.
While there are four bones that come together at the knee, only the femur (thighbone) and the tibia (shinbone) form the joint itself. The head of the fibula (strut bone on the outside of the leg) provides some stability, and the patella (kneecap) helps with joint and muscle function. Movement and weight-bearing occur where the ends of the femur called the femoral condyles match up with the top flat surfaces of the tibia (tibial plateaus).
There are two major muscle groups that are balanced and allow movement of the knee joint. When the quadriceps muscles on the front of the thigh contract, the knee extends or straightens. The hamstring muscles on the back of the thigh flex or bend the knee when they contract. The muscles cross the knee joint and are attached to the tibia by tendons. The quadriceps tendon is special, in that it contains the patella within its fibers. The patella allows the quadriceps muscle/tendon unit to work more efficiently. The quadriceps tendon is renamed the patellar tendon from the kneecap to its attachment in the tibia.
The stability of the knee joint is maintained by four ligaments, thick bands of tissue that stabilize the joint. The medial collateral ligament (MCL) and lateral collateral ligament (LCL) are on the sides of the knee and prevent the joint from sliding sideways. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) form an "X" on the inside of the knee and prevent the knee from sliding back and forth. These limitations on knee movement allow the knee to concentrate the forces of the muscles on flexion and extension.
Inside the knee, there are two shock-absorbing pieces of cartilage called menisci (singular meniscus) that sit on the top surface of the tibia. The menisci allow the femoral condyle to move on the tibial surface without friction, preventing the bones from rubbing on each other. Without this cartilage covering, the friction of bone on bone would cause inflammation, or arthritis.
Bursas surround the knee joint and are fluid-filled sacs that cushion the knee during its range of motion. In the front of the knee, there is a bursa between the skin and the kneecap called the prepatellar bursa and another above the kneecap called the suprapatellar bursa (supra=above).
Each part of the anatomy needs to function properly for the knee to work. Acute injury or trauma as well as chronic overuse cause inflammation and its accompanying symptoms of pain, swelling, redness, and warmth.
Medically Reviewed by a Doctor on 3/13/2015
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