Knee Anatomy

crossedlegsThis week’s ANATOMY FOCUS is the ever elusive and sometimes tricky knee.

This knobby pair of joints are often an enthusiastic topic of conversation amongst yogis as it seems everyone knows somebody who’s either injured a meniscus or torn an ACL, or done “something” to it.

Let’s start by looking at the bigger picture of the knee joint. To understand the inherent problem with the knee requires that you look at its function in our most common activity, no, not sitting, walking! Imagine your body without a knee. Frankenstein comes to my mind with a sideways waddle and swing of the big heavy leg to get it in front of the other. We would not get around very well or very quickly for that matter.

The knee is the middle joint of the leg that is, it sits between the ankle and the hip joint. It is the connection between the tibia (shin bone) below and the femur (thigh bone) above. At the other end of the tibia is the ankle joint and at the other end of the femur is the hip joint where the femur connects to the pelvis. The tibia and femur are the two longest bones in the body and that makes for more work in the knee.

Ya see, all joints could be classified as levers and when a lever has a long arm it means there is a potential to create more power or force with that lever. This is a good thing because the knee has a lot to do with regulating how we move and run which requires a lot of force. At the same time, that means that joint has to be able to handle the force created by the two longest bones in the body acting on one another.

Therefore the knee has to be strong to handle that force and at the same time it has to be flexible to handle the variety of movements that occur both below at the ankle joint as well as above at the hip joint. This makes its functions somewhat contradictory in nature.

Because the knee is located between these two joints it can become a regulator of movement at the other two joints. While standing the three joints of the leg become a functional chain where movement at one joint influences movement of the others. The knee in particular cannot move while the feet are on the ground without the ankle and hip joint also moving.

Bending the knees in standing does a few things, because the hips have to lower, the bending of the knees lowers your center of gravity, and can bring you back into balance if you have lost it. This is particularly useful transitioning between poses. Although the pose itself may require you to have straight knees, transitioning in and out of the poses with a slightly bent knee can give you a very strong sense of your feet and legs, which are the foundation of standing poses.

Bending the knees also changes the length of the hamstrings, which attach to your ischial tuberosities (sit bones) on the bottom of your pelvis. Because of there attachment to the pelvis the hamstrings can influence the ability of the pelvis to tilt anteriorly (pubic bone going down) and posteriorly (pubic bone going up). When the knees are bent the pelvis has the ability to tilt anteriorly more freely which can make all the difference in the world in a forward bend or a downward facing dog.

The flexibility of the hamstrings can have an effect on the low back, sacroiliac joint as well as the overall posture of an individual. Perhaps a deeper discussion in another article will talk about this crazy trio and the hips and pelvis.

ANATOMICAL DESIGN AND ACTIONS

The knee is normally classified as a hinge joint, which means that it simply opens (extension, straightening the knee) and closes (flexion – bending the knee). However, you may remember from the “Sitting in Meditation” article two issues ago that the knee also rotates. This action takes place between the end of the distal (bottom) end of the femur and the proximal (top) end of the tibia when the knee is flexed ten degrees or more. Technically it is the tibia itself that is doing the rotation against the femur, which is staying in one place. This is a very natural and important action that is required for walking, running and a variety of other movements at this joint.

This action happens in two possible directions, either medial (tibia rotating inward) or lateral (tibia rotating outward) rotation. We find that the average person has the ability to do lateral rotation of the tibia at a bent knee somewhere around 45-50 degrees if assisted say, with your hands or other generator of force, and about 30-35 degrees of medial rotation if assisted.

It is often during these rotational movements of the bent knee that injuries occur, and most commonly it is the medial or inward rotation of the bent knee that does it. Of course, injury to the knee can occur at other times doing the simplest of activities. Skiing, although not a simple activity is a very common place where twisting of the knee is quite common and meniscus and ACL (anterior cruciate ligament) tears happen all the time.

LIGAMENTS OF THE KNEE

ligamentsLike every other joint in the body, ligaments are there to allow for and restrict movements of those joints. The knee of course is no different. There are four main ligaments in and around the knee that allow for and restrict the movements of the joint.

Two of the main ligaments of the knee are called the co-lateral ligaments. Both of these ligaments connect the femur above to the tibia below. One is on the medial (inner) side of the two bones and the other exactly opposite on the lateral (outside) surface of the joint.

These two straps like ligaments are primarily concerned with preventing the tibia from slipping sideways under the femur ends.

In addition these two ligaments help control rotation of a bent knee in conjunction with the cruciate ligaments that we’ll talk about in a moment. For the most part the ligaments are slackened when the knee is bent but if rotation happens at that bent knee they will be tightened again by that movement and can be injured when doing movements like lotus, and leg behind head.

cruciates1The other two ligaments that we’re going to talk about are the cruciate ligaments. The word cruciate literally means crossed. And when you look at these two ligaments, from almost any perspective, they cross one another. These two ligaments, instead of being on the outsides of the knee are actually on the inner part of the knee keeping the femur and tibia closely held together.

They are named the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). If you look at the associated picture you will see that on the tibia the ACL attaches to the front part (anterior) of the tibia and the PCL attaches to the back part (posterior) of the tibia, hence their names.

These are very strong ligaments and prevent and allow for movements at the knee joint. The ACL becomes more taught during two actions primarily, the first is an anterior displacement of the tibia under the femur. In other words, it keeps the tibia from sliding forward under the femur. The other main action this ligament prevents is too much medial or inward rotation of the flexed knee. As the tibia rotates medially this ligament begins to become more taught as it is lengthened. This is often why an extreme medial rotation is the main cause of ACL tears.

The PCL is most simply the opposite of the ACL. Its main purpose is to restrict the tibia from sliding backwards under the femur. In an extended knee it allows almost no room for give in the tibia and is the main force holding together those hyper-extended knees. In its most simplistic understanding you could say that the PCL is a restrictor of lateral rotation but it seems to play a minor role in this, but a role nonetheless. It’s not the norm to find injury to this ligament, although it is of course possible.

CONCLUSION

We hopefully now have a better picture of how the knee is held together from its deepest layers and how these affect its ability to handle the tremendous force and need for adaptability in the knee joint.

Source: David Keil, Yoganatomy

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