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Anatomy of the human knee

The remarkable human knee is one of the most complex and important joints in the human body. It is built to simultaneously embody the attributes of strength and flexibility and is put to near constant use to facilitate the performance of a myriad of actions. Your knee has to be sturdy enough to support the impact of your upper body weight while walking or running, yet flexible enough for you to fold yourself into the lotus position for a meditation session for example. The knee joint is mostly made up of components like bone, muscle, ligaments, tendons, cartilage… and each of the component parts of the knee serves a distinct function. The bones of the knee joint provide support, the surrounding muscles furnish the human with the ability to control his/her movements, the ligaments and tendons within the knee joint provide stability, and the cartilage provides cushioning to absorb the shock from the constant pounding associated with daily movement and exercise. When healthy, the components of the knee work together so that we can walk, run, jump, and change direction instinctively. As a matter of fact, the knee works so well when healthy, that many of us do not fully appreciate its importance until something goes awry. I can say that with confidence because I for one certainly didn’t appreciate the benefit of having two perfectly functional knees until I injured one of them.

Knee support is provided by the bones

Knee support is provided by the bones.
Knee support is provided by the bones.
The bony structures that make up the knee joint are: the patella (kneecap), the proximal end of the tibia, and the distal end of the femur. In plain english, the “proximal end of the tibia” just means the end of your tibia that is closest to your head while the “distal end of the femur” means the end of your thigh bone that is closest to your feet. The patella or kneecap is the most mobile bone in the knee joint, and is essential for controlled extension of the knee. Interestingly, the kneecap acts like a sort of track that keeps some of the very important tendons in your knee from sliding all over the place and compromising the structural integrity of your leg when you bend or extend it. The tibia or shin bone (which connects the knee to the ankle) and the femur or thigh bone (which connects the hip to the knee) both serve to support the weight of the body while standing, walking, running, etc. At first glance, human bone might look like a lifeless structural entity but In truth, your bones are very much alive and can be strengthened in response to good nutrition and controlled stress placed on them through exercise. That’s why an NFL running back who squats 300+ lbs four times a week generally has much higher bone density than the average person. You know how you watch all those violent collisions on Sunday Night Football and wonder how they don’t get injured more often? Well the elevated bone density/thickness of the average NFL player probably has a lot to do with it, making them less vulnerable to injury compared to the average joe. So when your doctor or personal trainer advises you to add weight training to your exercise routine, you now know that their reasons for suggesting it are legit. You certainly don’t want to be prone to broken bones and torn ligaments now do you? Just please make sure that you have been taught the correct technique before you attempt to lift any heavy weights on your own.

Ligaments and tendons stabilize the knee joint

Ligaments of the human knee joint ACL: Anterior cruciate ligament PCL: Posterior cruciate ligament LCL: Lateral collateral ligament MCL: Medial collateral ligament
Ligaments of the human knee joint
ACL: Anterior cruciate ligament
PCL: Posterior cruciate ligament
LCL: Lateral collateral ligament
MCL: Medial collateral ligament
Ligaments and tendons are tough bands of fibrous tissue that serve the main purpose of stabilizing the knee joint. A ligament connects bone to bone, while tendons connect muscle to bone. These thick fibrous bands of tissue are constantly tasked with preventing buckling or excessive hyperextension of the knee. The ligaments and tendons in the knee joint are: ACL (anterior cruciate ligament), LCL (lateral collateral ligament), MCL (medial collateral ligament), PCL (posterior cruciate ligament), the quadriceps tendon, and the patellar tendon. Each knee ligament and tendon has a distinct set of functions as noted below:

  • The ACL resists excessive twisting of the knee and keeps the tibia from sliding too far forward under the femur when a human being in motion comes to an abrupt stop.
  • The PCL keeps the femur from sliding too far forward over the tibia.
  • The MCL functions to prevent the knee joint from buckling inwards due to an impact on the outside of the knee.
  • The LCL functions to prevent the knee joint from collapsing outwards due to sudden impact to the inside of the knee.
  • The patellar tendon and quadriceps tendon are both connected to the kneecap and are crucial for controlled bending and extension of the leg.

You’re probably correctly thinking – “Whoa that is a lot of different ligaments and tendons in the same knee that are each tasked with a unique set of functions… and how the heck does the knee work so perfectly without any of those ligaments getting tangled up with each other?” The answer to that questions is that the magic of nature has found a way to ensure that these function specific ligaments and tendons work together to make the working whole far greater than the sum of the parts. Even more amazing is to think that the knee in its finished form wasn’t built in a utilitarian sense per se with nails, hammers, and screws, but rather it slowly developed as you grew up in accord with a developmental program that was embedded in you the moment you dad’s sperm partnered with your mother’s ovum to form you.

Although all these ligaments are important for proper knee function, it is possible to live life without some of them. In other words a torn knee ligament as an isolated injury might make life difficult and perhaps painful, but it certainly will not kill you. Many people live their lives with no ACL and manage to function just fine. As a matter of fact, Dejuan Blair doesn’t have an ACL in either knee but he managed to play basketball at the highest level for many years. Having said that, I would like to emphasize here that Dejuan is an outlier. For most people, playing a demanding sport like professional basketball with no ACLs would mean frequent and painful knee buckling episodes served with a side order of very nasty falls. For this reason, it is highly recommend to fix any torn knee ligaments or tendons in an effort to restore maximum stability to the knee.

The natural cushion of the knee: meniscus

The meniscus of the knee provided support and cushioning for the bones of the knee joint.
The meniscus of the knee provided support and cushioning for the bones of the knee joint.
The menisci (plural for meniscus) act as the major shock absorbers of the knee joint. They are made of tough cartilaginous tissue and generally adhere to the contours of the top of the tibia. Both menisci are situated right at the joint line. The lateral meniscus exists in the outer reaches of the knee capsule, while the medial meniscus exists in the part of the knee capsule that is closer to the midline of your body. The main function of the meniscus is to ensure that the tibia and femur do not continually collide due to the impact of walking or running. This would become very painful because the bones – femur and tibia – would effectively be engaged in a constant war of attrition and eventually wear each other out. The menisci also lend stability to the entire knee joint by gently wedging in the rounded edges of the femur and tibia. The importance of the meniscus is further highlighted by the fact that modern science hasn’t really found a substitute that works just as well as the one nature dreamed up. Let’s put the fact immediately above into perspective by comparing a torn ligament to a damaged meniscus. If you have a torn knee ligament, your surgeon can always give you another one from a cadaver or from your own knee. This is NOT the case with the meniscus. Orthopedic surgeons often say that they try everything in their power and within reason to preserve as much of the meniscus as possible during corrective knee surgeries.

The human body is a truly remarkable feat of engineering. You won’t find anything else on this planet that is as effective and versatile when it comes to the performance of physical and mental tasks. Even some of its constituent parts that aren’t considered as “sexy” in popular culture such as the human knee are still marvels to behold and study. Think about it for a second… how did you and I as fertilized eggs in our mother’s respective wombs know that we’d need a knee someday and how did we go about growing it in the exact right spot? Puzzling right? Yeah I know… I feel the same way at times. So the next time you catch yourself on the verge of falling into a depressive state, stop and remind yourself of how awesome the body you have been carrying around for X number of years is. Lord knows we all need to heed that sort of advice more often. Till next time my friends, take care of yourselves and each other.

This article is an excerpt from “In Kneed of Repair… Getting through ACL reconstruction”. A comprehensive interactive eBook designed to guide you through ACL reconstruction and recovery. You can buy the full iBook for iPad or Macintosh on the Apple iBookstore.

Without Wax
Oyolu B.C. Ph.D.
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