Photo Credit: The Daily Mail UK
Welcome to the introduction to the knee. The knee is a hinge joint where the femur (thigh bone) meets your tibia (lower leg). Hinge joints primarily move in one plane (like a door hinge), however they do have a little bit of play in the coronal and even transverse planes to allow for the types of movements that humans do (and to allow for correction and injury prevention). The knee also contains a sesamoid bone, the patella (or kneecap). A sesamoid bone is a bone that develops inside a tendon or ligament. This bone serves to protect the articular surface of the joint and also increases the leverage that the patellar ligament can exert on the femur based on its angle. There are four main ligaments in the knee to keep it from buckling front to back or side to side. Six muscles or muscle groups also cross the knee joint, as does one thick, long, fibrous band.
Structure and Function
As stated above, the knee is a hinge joint. Most of its movement will result in flexion or extension. The little bit of rotation that occurs in the knee occurs when bending the knee from a locked or fully extended position. The tibia (lower leg) must rotate inwardly to allow knee flexion to occur (or the femur must rotate slightly outwardly on the tibia to allow for knee flexion). The muscle that performs this unlocking action is called the popliteus. It runs from the lateral epicondyle of the femur to the posterior portion of the proximal tibia, and is one of the deepest muscles in this part of the body.
The play that the knee has side to side allows us to walk on uneven terrain without injury and can even allow for some amount of give with impact before injury occurs. The ligaments that prevent too much movement in this are called the medial collateral ligament (MCL) and the lateral collateral ligament (LCL). These are common areas of sprains, which we’ll get to a little bit later. In addition to these two ligaments on the side of the knee, the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) run criss-cross, one in the front of the knee (ACL) and one in the back of the knee (PCL). These prevent the tibia (lower leg) from falling forward or backward. These four ligaments help to attach the thigh to the lower leg and help to maintain proper alignment.
Photo Credit: Myers Sports Medicine and Orthopaedic Center
There are two other main soft tissue structures in the knee, which help to absorb shock. They are called menisci. Each knee has one medially and one laterally. This is another common area of injury, as tears can occur from impact or from twisting (something we do quite often in dance). The medial meniscus is more commonly injured, whether acutely or due to degenerative changes. This is partly due to the fact that the lateral meniscus moves more freely in the joint than the medial meniscus. The medial meniscus may also be injured in concert with the MCL, as the MCL attaches to the medial meniscus. This is not the case on the lateral side. But lateral meniscus tears are more common with ACL injuries than are medial meniscus tears.
As for the muscles that cross the knee joint, the two largest groups are the quadriceps on the front of the thigh and the hamstrings in the back of the thigh. The quadriceps contain four different muscles that originate on the pelvis or the femur itself, and attach to the tibia at the tibial tuberosity via the patellar ligament (or commonly referred to as the patella tendon, which contains your kneecap). The work together to straightern and extend your knee. The hamstrings originate on the ischial tuberosity (commonly referred to in dance class as the sits bones) and run along the back of the leg until they separate (two on the inside and one on the outside). The two on the inside (semimebranosus and semitendinosus) attach to the tibia and your outermost hamstring (biceps femoris) attaches to the fibula. The hamstrings work in concert to bend or flex the knee.
In addition to these larger muscle groups, you also have popliteus (previously mentioned), gastrocnemius, gracilis, and the IT Band crossing the knee joint. The gastrocnemius is your most superficial calf muscle. This helps you do your relevés. It also helps to bend the knee a little. Gracilis is the only muscle of your adductor group that crosses the knee on the inside of your leg. It originates off of your pubic bone, and inserts on your tibia with semitendinosus in a wonderful spot known as pes anserine (goose’s foot). One other muscle joins them to insert here, called sartorius (the tailor’s muscle and the longest muscle in the body), which comes from part of your pelvis and crosses the hip and knee joint before attaching here. Finally, the IT band, which is a thick, fibrous extension of a hip muscle known as TFL, comes down the outer part of the thigh and inserts on the lateral epicondyle of the tibia. For those out there that aren’t as big on your anatomy, it essentially goes into your knee joint, and can be a common area of irritation in the outer knee for all types of athletes.
That wraps up our overview of the structure and function of the knee. Stay tuned next week as we begin our journey into common knee injuries. First up: meniscus tears.