knee

knee A joint that permits flexibility in the middle of the lower extremity. The thigh bone (femur) meets the large shin bone (tibia) to form the main knee joint. This joint has an inner (medial) and an outer (lateral) compartment. The kneecap (patella) joins the femur to form a third joint, called the patellofemoral joint. The patella protects the front of the knee joint. The knee joint is surrounded by a joint capsule, with ligaments strapping the inside and outside of the joint (collateral ligaments) as well as crossing within the joint (cruciate ligaments). The collateral ligaments run along the sides of the knee and limit its sideways motion. The anterior cruciate ligament (ACL) connects the tibia to the femur at the center of the knee and functions to limit the tibia’s rotation and forward motion. The posterior cruciate ligament (PCL), located just behind the ACL, limits the backward motion of the tibia. All these ligaments provide stability and strength to the knee joint. A thickened cartilage pad between the two joints (meniscus) is formed by the femur and tibia. The meniscus acts as a smooth surface for the joint to move on. It serves to evenly load the surface during weight bearing, and it also aids in disbursing joint fluid for joint lubrication. The knee joint is surrounded by fluid-filled sacs called bursae, which serve as gliding surfaces to reduce friction of the tendons. Below the kneecap is a large tendon (patellar tendon) that attaches to the front of the tibia bone. Large blood vessels pass through the area behind the knee, which is called the popliteal space. The large muscles of the thigh move the knee. In the front of the thigh, the quadricep muscles extend the knee joint. In the back of the thigh, the hamstring muscles flex the knee. The knee also rotates slightly under the guidance of specific muscles of the thigh. The knee is critical to normal walking and is a weight-bearing joint. Knee pain can be caused by a number of factors, including injury, inflammation of the bursa (bursitis), strain, and problems with the sciatic nerve, which runs from the lower back to the knee. See also bursitis; patellofemoral syndrome; sciatica.