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Injury to the posterolateral complex of the knee 

Introduction/anatomy 

The knee is composed of several structures. In terms of bones, the knee region as a whole consists of the femur, tibia, fibula and patella. Around this are different muscles, the meniscus and different ligament structures.

Together, this provides the ability to move, shock absorption, stability and direction in the knee. At the knee we find the following larger band structures;

  • anterior cruciate ligament (VKB)
  • posterior cruciate ligament (AKB)
  • lateral collateral ligament (LCL)
  • medial collateral ligament (MCL)
  • posteromedial complex (PMC)
  • posterolateral complex (PLC)

In an injury where the forces are great enough, there can be actual damage of the ligaments in or around the knee. In this blog, we will specifically address injury to the posterolateral complex. 

The posterolateral complex of the knee consists of several ligaments (ligaments) and tendons. The most important are the popliteal tendon, the lateral collateral ligament, and the popliteo-fibular ligament.

An injury to the posterolateral complex causes instability symptoms. This usually occurs after a fall or twisting of the knee, usually during sports. In most cases there is a combined injury. This means that not only the posterolateral complex is damaged but also other structures in the knee such as the anterior cruciate ligament. Other structures that we usually see in combination with injury to the posterolateral complex are damage to the meniscus or posterior cruciate ligament. More than 70% of all cases with posterolateral instability involve combined injury. 

Symptoms that may indicate posterolateral injury include;

  • Tingling in the foot (in ⅓ patients). 
  • Pain on the outside of the knee
  • hematoma (in the acute phase)
  • unstable feeling in the knee

Missing posterolateral injury for example anterior cruciate ligament or posterior cruciate ligament injury at the physiotherapist or orthopedist has a huge impact on rehabilitation. Missing posterolateral instability is one of the biggest reasons for resection of the anterior or posterior cruciate ligament after surgery. Diagnosing injury to the posterolateral complex quickly is therefore very important and has a great impact on the success of your rehabilitation. If this injury is properly diagnosed and treated within two weeks of its onset, in many cases it can recover well and chronic instability can be prevented. 

Within physiotherapy, it is very important to know how the knee problems arose, what movement was made and what symptoms this caused. We will always test the posterolateral complex in an injury where there is possible damage to the outside of the knee. In these cases, we know that there is also a chance of an anterior or posterior cruciate ligament injury. So this will always be included in the examination of the knee.

A variety of tests can be used to make a statement about posterolateral complaints:

  • Varus stress test in 0 degrees and 30 degrees knee flexion 
  • Dial test in 30 degrees and 90 degrees of knee flexion
  • Figure-4 test 
  • Rotation recurvatum test 
  • Posterolateral drawer test

At the varus test we grasp the leg in 0 degrees or 30 degrees of knee flexion. In doing so, we will apply varus pressure to the knee joint (we push the lower leg inward relative to the upper leg. When we perform this test in 30 degrees of knee flexion we assess whether the lateral collateral ligament is intact. A small "gapping" may be present during this test. We assess whether the gap present is larger than normal or strongly enlarged compared to the other knee. We also ask about the occurrence of recognizable pain symptoms. The varus stress test in 0 degrees also tests the posterolateral complex of the knee.

At the dial test we have the patient lie on the stomach with the legs relaxed. Then we place the knee in 30- or 90-degree flexion and bring the ankle into what we call dorsiflexion as much as possible. By doing this, we ensure that we are moving cleanly from the knee during our test. After this preparation, the physical therapist rotates the knee outward as far as possible and we look for a difference in rotation between the left and right leg. The test is deemed positive if a left-right difference is measurable of >10 degrees. If an enlarged rotation is present at 30 degrees of flexion but not in 90 degrees, we assume an isolated posterolateral problem. If this difference is also present in 90 degrees of knee flexion, we can assume that the posterior cruciate ligament is also involved in the problem. In addition, it is important to test the anterior cruciate ligament in case of a positive dial test. This because a rupture of the ACL can also cause a larger rotation in the dial test. This can cause a difference of up to 7 degrees and give a distorted view on this test. 

The figure-4 test We perform by having the patient lie on his or her back on the treatment couch. We then place the leg that is causing pain bent over the other leg to create figure 4. This test is positive if a recognizable aching sensation occurs on the outside of the knee. As you have read by now, the posterolateral complex consists of several structures including the popliteus. The idea behind the figure-4 test is that pain can occur if damage is present in the fibers between the tendon of the popliteus and the lateral meniscus. This causes a kind of sub-luxation of the lateral meniscus inward during this test. Normally this is stopped by the fibers of the popliteus inhibiting the meniscus. 

The rotation recurvatum test is performed in a reclining or slow sitting position. The legs are stretched out on the bench and completely relaxed. Next, the physical therapist grabs you by the big toes and lifts the feet off the bench. This is done to see if there is an enlarged extension of the knee and enlarged exorotation of the tibia relative to the other side. An enlarged extension and exorotation indicates injury to VKB/AKB in conjunction with the PLC. 

At the posterior drawer test We have the patient lie on the treatment couch with the knee flexed 80 degrees and rotated outward approximately 15 degrees. The physical therapist then performs a posterior translation of the knee where the test is positive if no cord phenomenon is observed and the tibia allows for an increased movement relative to the opposite body side. This test is very reliable in creating insight and both increasing and decreasing the posterior probability of a PLC injury.

Furthermore, we look at the direction of rotation of the tibia in different tests. There is an important difference between the direction of rotation in a posterolateral problem versus a posteromedial problem. With a PLC injury, we will see that with an exorotation movement of the tibia, we will see the outer (lateral) side of the tibia rotate away in the knee. With a PMC injury, we will see the inner side (medial side) of the tibia rotate forward. 

Additional imaging studies are necessary when the above tests are positive. An X-ray may be taken to rule out a possible (avulsion) fracture (broken bone) and an MRI of the knee is taken to assess whether the ligaments, meniscus and other structures are intact.

Injury to the posterolateral complex can be classified into several degrees:

Grade 1 means that there is a partial rupture with minimal instability

Grade 2 means a partial rupture with instability where an endpoint in the varus stress is present (<10mm instability and an increase in exorotation at the Dial test in 30 degrees of less than 10 degrees).

At grade 3 there is a complete rupture without end point at varus stress (>10mm instability and >10 degrees of exorotation at Dial test in 30 degrees). 

Treatment of the posterolateral complex can be treated with regular physical therapy (maximum grade 2 injury) or by surgery (grade 3 injury or in combination with other injuries). When surgery is not performed, the leg is put in a cast in a stretched position of the knee.

Surgery gives the best results when operated on within 3 weeks of the point of origin. We also see better results with reconstruction than with repair surgery. This can involve using tissue taken from elsewhere in your own body (autograft). Another choice is to use donor tissue (allograft). Usually the choice is made to use the hamstring tendons or tissue from the iliotibial band. After surgery, you will only be allowed to load 50% for at least the first 4 weeks. In addition, you will have to wear a brace 24 hours a day for up to 12 weeks that has a flexion lock built in. This ensures that you cannot flex beyond 120 degrees. 

Total rehabilitation lasts six months to a year. In general, people with a sedentary job can resume their activities after a few weeks. Someone with a more active profession will have to be a bit more patient.

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