Anatomy clinical correlates: Knee

The knee is one of the most complex joints in the human body, and along with the rest of the lower limb there are numerous ligamentous, muscular, and bony structures that are prone to injury. Oftentimes we can injure many of these structures at the same time, but injury to even one of these structures can affect how we walk, dance, or exercise. This video will go over all the relevant anatomy you ‘kneed’ to know in order to understand the clinical conditions affecting the knee.

First up, let’s discuss the Q angle - where “Q” stands for quadriceps. The Q angle is the angle measured between the femur and the tibia.

This angle is created by the femur’s diagonal placement within the thigh and by the tibia’s vertical placement in the leg. This angle is typically below 20 degrees and on average is higher in biologic females, and a normal Q angle allows the weight supported by the knee joint to be centered through the middle of the knee, in the knee’s intercondylar region.

When the Q angle increases over the normal range, it can lead to genu valgum or knock knees. With genu valgum, the increase in Q angle shifts the weight bearing center to the lateral compartment of the knee, which increases the quadriceps lateral pull and causes the medial collateral ligament to overstretch. This can cause joint misalignment and a predisposition for articular degeneration in the lateral compartment of the knee and subsequent gait abnormalities.

To remember the knock knee appearance of genu valgum, think of the ‘g’ as standing for ‘gum’ sticking the knees together!

Alternatively, when the Q angle is below normal range, a genu varum alignment can occur where the legs angulate away from the midline and create a bow legged appearance. This results in the weight bearing center being shifted through the medial compartment of the knee, stretching the lateral capsule lateral collateral ligament. So this time, there’s joint misalignment and a predisposition to articular degeneration in the medial compartment of the knee and subsequent gait abnormalities.

To remember this, think that in genu varum, the knees are far...um apart from each-other.

Risk factors for both genu valgum and genu varum is prior trauma, abnormal physiological development, and vitamin D deficiency, also known as rickets. However, both of these alignments can be physiological in infants, and resolve in adulthood.

Genu valgum and genu varum can be recognized on an x-ray.

Next up, there are knee dislocations, or tibiofemoral dislocations, which are considered a limb-threatening injury. That’s because they can lead to injury and compression of the neurovascular structures found in the popliteal fossa, specifically the popliteal artery.

Knee dislocations are usually caused by high energy traumas such as a car crash, where excessive force is directed to the proximal tibia,

resulting in the tibia being forced posterior in respect to the femur.

This results in a posterior dislocation causing disruption of multiple knee ligaments, such as the anterior and posterior cruciate ligaments and the medial and lateral collateral ligaments.

In a posterior dislocation, the tibia can cause damage to the popliteal artery resulting in ischemia and the potential for irreversible lower limb injury, which may necessitate amputation.

Patients will present with severe knee pain and likely the inability to weight bear or even move the knee joint properly. Management should be to immediately reduce the dislocated knee, followed by a thorough neurovascular exam, such as palpating the popliteal artery, posterior tibial artery, and dorsalis pedis artery. ABI and CT angiography may also be considered. If closed reduction is not possible, surgery is required.

Now, if you kneed a break, let’s see if you can remember what the Q-angle is? And can you identify genu valgum and genu varum on these images?

Moving on, let’s discuss iliotibial band syndrome, which refers to lateral knee pain during flexion and extension of the kee as well as tenderness over the lateral femoral epicondyle. This is thought to be due to an overuse injury of the iliotibial band, which is a fibrous fascial structure of the lateral leg that inserts onto the lateral tibia. During flexion and extension, the distal insertion of the iliotibial band irritates the lateral femoral epicondyle which it covers.

Let's look at the risk factors of iliotibial band syndrome. Runners and cyclists are at an increased risk of this syndrome, as well as those who have genu varum alignment.

Clinically, individuals present with pain over the iliotibial band course, particularly over the lateral femoral epicondyle, where activity modification and stretching can relieve the pain.

Ok, now let’s move on to some of the more famous injuries of the knee, the ones which seem to plague sports stars all over the world.

To start we have the medial and lateral collateral ligaments, also called the tibial and fibular collateral ligaments.

The medial collateral ligament, or MCL, is a flat band that extends from the medial epicondyle of the femur to the medial condyle of the tibia. This ligament is commonly injured during sports that undergo twisting motions, like basketball. Additionally, injury can occur after a direct blow to the lateral knee while the foot is planted as this results in a valgus stress, causing stretching of the medial surface of the knee. Individuals typically present with swelling, bruising, and joint line tenderness of the medial knee. Of note, the medial collateral ligament does have attachment points to the medial meniscus, therefore injury to this structure can also cause injury to the medial meniscus which we will talk about soon!

To test for MCL injury, a valgus stress test is done. Here, the individual lies on their back with the knee slightly flexed. One hand is positioned on the lateral surface of the knee placing gentle inward pressure, and the other hand placed around the medial ankle applying lateral force in order to create a valgus stress on the knee, opening up the medial compartment.

If the medial compartment widens and has increased laxity, this is indicative of medial collateral ligament damage as the ligament provides medial knee stability.

Going forward, or rather laterally, is the lateral collateral ligament or LCL. The ligament extends distally from the lateral epicondyle of the femur to the lateral surface of the fibular head. Isolated Injury to the lateral collateral ligament or LCL is rare, and it is more commonly injured along with other structures of the posterolateral knee such as the lateral meniscus.

Individuals usually present after trauma to the anteromedial or medial aspect of the knee

which results in varus stress. This frequently occurs during sports such as football, rugby, or wrestling. Individuals clinically develop lateral knee pain, swelling, bruising, and instability.

LCL injuries can be diagnosed clinically using the varus stress test, which when the individual lies on their back with the knee slightly flexed. One hand is positioned on the medial surface of the knee placing gentle outward pressure, and the other hand placed on the lateral ankle applying medial force in order to create a varus stress on the knee opening up the lateral compartment. If the lateral compartment widens and has increased laxity, this is indicative of lateral collateral ligament damage as the ligament .provides lateral knee stability.

Imaging is often not done for MCL and LCL injuries, however MRI can be done to classify the severity of the injury, as surgical management is considered for a severe injury.

The anterior cruciate ligament, or ACL, is also frequently injured in athletes. ACL tears can occur due to both high energy mechanisms such as a motor vehicle collision, and non contact or low energy mechanisms, which often occurs as a sudden internal rotation when the foot is planted, as well as a rapid deceleration or direction changes often seen in sports.

The ACL functions mainly to prevent anterior displacement of the tibia in relation to the femur, and also functions to resist rotational forces of the knee, and resist varus and valgus forces. Therefore, a direct blow to the anterior femur can result in ACL injury

as this causes posterior translation of the femur relative to the tibia, which is the same as anterior translation of the tibia relative to the femur!

Clinically, ACL injuries present with pain, instability, and the potential for severe swelling

due to rupture of the ACL vascular supply which can present as rapid-onset hemarthrosis of the knee. Non contact injuries may also present with an audible "pop".

Injury can be diagnosed clinically with a positive Lachman test and anterior drawer tests. The Lachman test is done by flexing the knee to 30 degrees and stabilizing the distal femur with one hand while pulling the proximal tibia anteriorly with the other hand, all to produce anterior translation of the tibia. An intact ACL should limit anterior translation and have a distinct endpoint. WIth an ACL injury, there’s increased translation compared with the uninjured knee and a vague endpoint.

Next, in the anterior drawer test the individual is supine and the knee flexed at 90 degrees with the foot flat on the bed. With the foot flat and secured to the table, the examiner grasps the leg around the calf with the thumbs placed on the tibial tuberosity and attempts to translate the tibia anteriorly. The test is positive if there is laxity or excess anterior translation of the tibia compared to the contralateral side.

To confirm the diagnosis of a ruptured ACL, an MRI is usually done. A sagittal view of the knee with an intact ACL will show continuous fibers from their attachment points, where a ruptured ACL will show fiber discontinuity between the attachment points.

Finally, we have the posterior cruciate ligament or PCL, which normally prevents posterior translation of the tibia relative to the femur,

and is usually injured after a direct blow to the anterior tibia.

This can happen during a car crash or, as with most other ligaments, it can also be damaged during sports, usually when an athlete, like a gymnast, falls on their flexed knee while the foot is plantarflexed.

Symptoms of a PCL injury include pain, instability, potential swelling of the knee, and may also present with a ‘popping’ sensation.