Why Are Knee Injuries So Common?

Knee injuries are extremely common across the board, not just in athletes. While many people recognize this, it's difficult to understand why this is the case if you don't already have previous knowledge of anatomy and medicine. In this article, we would like to briefly discuss the anatomy of the knee and factors that lead to its relative vulnerability.

Anatomy of the Knee Joint

Bones

The knee joint is a synovial modified hinge joint formed by the articulation between the femur (thigh bone), which is the largest bone in the body, and the tibia (shin bone), as well as the patella (knee cap), which sits over the front of the knee joint. In fact, the knee joint is the largest joint in the body. There is another smaller shin bone on the lateral aspect of the leg called the fibula, but this bone is doesn't form an actual articulating surface of the joint.

Unlike the ball-and-socket hip joint where the bones fit together like a 3D puzzle, the surfaces of the tibia and the femur don't match up well, as the distal end of the femur has a rounded surface and the proximal end of the tibia is relatively flat. This allows for less restricted movement, but at the end of the day, this comes at the cost of stability. Fortunately, there are some other common structures around the knee joint that help stabilize the joint.

Below is an illustration of a front view of the right knee joint. In this illustration, the patella (kneecap) has been reflected (flipped up)​ in order to provide a better view of the articulating surfaces and the intrinsic ligaments - the ACL and PCL (more on those further down in the article).

Meniscus

The meniscus is a figure-8 shaped ring of cartilage that sits on top of the flat surface of the tibia, which help to deepen the surface for the femur. It also provides a buffer between the bones in order to avoid painful and damaging bone-on-bone contact. By distributing forces over a larger area, the knee joint is protected from typical forces that occur during walking and running.

Ligaments

Ligaments are found around all the joints in the body and serve as passive structures that help stabilize the joint. In the knee, there are ligaments both outside and inside the joint capsule. They are strong bands of slightly elastic tissue that cross the joint, acting almost like bungee cords to keep the knee in place.

The two ligaments found inside the joint capsule of the knee are the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). They primarily prevent the bones from moving back and forth with respect to each other. The external ligaments are the medial collateral ligament (MCL) and the lateral collateral ligament (LCL), which are found on the sides of the knee joint, and they prevent varus and valgus forces on the knee.

Muscles

Muscle are also powerful stabilizers of the knee joint. The hamstrings and quadriceps, among other muscles, cross the joint and control any motion that occurs at the knee. In this sense, the muscles are dynamic stabilizers, while the ligaments are passive stabilizers, and they work in tandem. For example, the quadriceps muscle and the PCL both help prevent poster translation of the tibia with respect to a fixed femur.

Common Injuries of the Knee

Bones

Injuries to the bones of the knee joint are less common than soft tissue injuries, but when they do occur, it's usually to the patella or proximal tibia in the form of a bruise or fracture. A fracture to the patella usually results from an external impact directly on the knee cap. The same can be true for the proximal tibia, but fractures in that area can also result from things like a fall where the joint is compressed.

Meniscus

Injuries to the meniscus are quite common, especially in sports. The meniscus is quite strong, but is more prone to injury when subjected to high twisting or torsional forces, such as when pivoting, planting, and quickly changing direction. Furthermore, with age, the meniscus gradually becomes damaged through general wear and tear, making itself more vulnerable to injury.

A torn meniscus can be quite painful and debilitating. Depending on the exact tear, the knee may lock up completely even without high levels of pain. Meniscus tears are difficult to treat because the cartilage is mostly avascular, so it doesn't have the same healing capacity of a tissue that receives a constant blood supply. The outer regions receive the most blood, hence the name "Red Zone", with this zone typically healing the fastest. As blood supply decreases, such as in the "White Zone", the healing process will take longer. Treatment will usually consist of rest, ice and heat, and physical therapy. In more severe cases, surgery may be required to repair the damaged tissue.

Ligaments

Ligaments are also commonly injured, especially in sports. Contact sports have a lot of knee ligament injuries from external impacts, but they can also happen in non-contact situations as well. Sports that involve a lot of jumping, such as basketball and volleyball, or sports with a lot of cutting, such as soccer, have particularly high rates of knee ligament injuries.

The ACL is definitely the most commonly injured ligament in the knee, and often results from hyperextension, or also a twisting motion of the knee, the latter of which can occur when your knee twists while planting your foot in a change of direction. A knee brace for an ACL tear will act to limit or prevent these types of motions from occurring. All this being said, it's also fairly common for multiple structures to be injured at one time. For example, ACL injuries are commonly seen paired with MCL and/or meniscus injuries, all of which can occur during twisting or an awkward external impact.

When an ligament in the knee is sprained or fully torn, it can be difficult for the patient to pinpoint where the injury is based on feel alone, as well as the extent of the injury. Doctors, physiotherapists, athletic therapists, and other medical professionals can usually test the condition of the knee ligaments by physically moving the knee joint in different ways. They may test the uninjured knee first to gain a sense of the patient's normal anatomy and joint laxity, and upon subsequent testing of the injured joint, they can deduce which structures are injured and how badly by detecting more laxity in particular directions. For example, check out the video demonstration of the Lachman Test, one of the most popular tests for an ACL tear, courtesy of the SMU Athletic Therapy Department.

Treatment will often involve rest, followed by physical therapy to gradually strengthen the knee joint while allowing the ligament to heal. However, if the ligament is sprained to a degree where reconstructive surgery may be an option, chances are that some form of imaging, such as magnetic resonance imaging (MRI), will be required to confirm the true extent of the injury. The reason you might not have an MRI immediately is because there is usually a huge wait list, running an MRI scan is highly technical, and of course, it's expensive.

Muscles and Tendons

We mentioned in our previous section about general knee anatomy that muscles can act as dynamic stabilizers of the knee in addition to actually being responsible for voluntary movement at the knee. While this is true, it is also worth noting that tendons are included in this as well. Tendons blend into the muscle itself and connect the muscle to bone. So while they are basically the same overall structure, the muscle is the strong tissue that you can voluntarily contract, and you can picture the tendon as a piece of rope. When the muscle contracts, this will cause it to shorten and the tendon ("rope") will pull on the bone to initiate movement (or stabilization). Even when we are in a simple standing position, muscle are constantly active in order for us to maintain balance, constantly correcting for subtle shifts in stature.

Both muscles and tendons can be sites of injuries. As opposed to ligament sprain, injuries to the muscles or tendons are called strains. You may have heard of terms like a torn muscle or a pulled hamstring before. These are both examples of muscle strains.

At the knee joint, the quadriceps and hamstrings are common sites of injury. They both involve components that cross the knee AND hip joints, so depending on the movement, they sometimes have to handle lots of forces while being in very stretched positions. Therefore, muscle strains are usually non-contact injuries, and are simply a result of the muscle taking on too much load too quickly, or also from repetitive stress that gradually breaks down a muscle in a certain location. Strains can be quite painful, as it can be fairly difficult to limit movement of muscles, especially in the lower body.

In the image below, we have overlaid an extremely simplified depiction of the quadriceps and hamstring muscles, or at least the portions that cross both the hip and knee joints. We have provided a color gradient where red represents contracting/shortening of the muscle, while blue represents stretching/lengthening the muscle. ​In reality, muscles can still contract while being stretched, which is known as an eccentric contraction, so really all of the muscle is contracting to some degree, but hopefully this gives you a general idea of the different forces a single muscle can be subjected to. The hamstring is commonly injured in athletes who do a lot of power running in a stance that combines back flexion with powerful flexion and extension at the knee.

Treatment for muscle strains can vary depending on the extent of the injury. However, one thing is certain, and that's the effect of prompt treatment. If you delay seeking treatment, you can risk further injury to the area, or at the very least, will be subjected to pain longer than you have to be. 

During the first couple days after sustaining the injury, it is usually recommended to completely rest the area. Simply put, you don't want to do anything that reproduces the pain. During this time of rest, ice is highly important, as it will decrease the swelling and can also help with pain. Elevating the affected limb can be a good option, and compression apparel can also help to reduce the swelling and support the injured area.

After this first couple days, it is usually advised to swap out ice for heat. Using a heat pack or hot water bottle (or whatever you have) will help promote blood flow in the area, which can facilitate proper healing, as well as reduce the sensation of pain. The reason you don't want to do this immediately is because increasing blood flow can also promote swelling, so you want to have that taken care of first with the ice. After the initial rest period, a physiotherapist can work with you to massage the area as well as practice range of motion. Eventually, you may start to incorporate specific exercises into your treatment plan to gradually re-strengthen the muscle while it heals.

Surgery is usually avoided for muscle strains, except when it comes to the most severe cases. A full muscle rupture will lead to a complete disconnection between the muscle and the bone, so this obviously has to be repaired. For instance, the Achilles tendon in the ankle is a common site for complete tendon rupture. You can imagine how much your ability to move and walk around would be affected if this structure was essentially removed.

At the knee, full ruptures occur more often at the quadriceps tendon, also known as the patellar tendon. This tendon is an extension of the quadriceps muscle. As it travels down over the front of the knee, the patella acts to provide mechanical leverage for the tendon. This is a highly effective way at gaining a mechanical advantage, but it also isolates that portion of the quadriceps tendon under heavy loads. A sudden and high force can cause a tear to the patellar tendon, or can also result int he form of an avulsion fracture, which is when the tendon rips off from the bone taking a small piece of bone with it. These injuries would require surgery.

Summary

We started this article with the question of why knee injuries are so common. We briefly addressed this question and followed up by discussing some of the structures and their typical injuries in a little more detail. Hopefully, you can see that the shear complexity of the knee joint lends itself to injury simply due to the amount of anatomical structures found in the area, as well as the relative incongruity of the articulating surfaces of the joint.

If you reading this looking for ways to help yourself out with a knee injury, please feel free to check out our various articles on different types of knee braces. There is something for every type of injury, and often times these can be extremely helpful devices to not only get you back to your old self more quickly, but to also help prevent future injury down the road.

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