6. What are the Main Structures to look for on MRI Scans of Posterolateral Corner Injuries?
The main structures to look for on MRI scans of the posterolateral corner of the knee are the fibular collateral ligament, the popliteus tendon, and the popliteofibular ligament as well as the biceps femoris attachments to the fibular head, the lateral capsule, and the iliotibial band.
We have validated with our research studies that injuries to the fibular collateral ligament, especially in chronic cases, are very difficult to diagnosis on MRI scans. In fact, only about 70% of the time an MRI can be accurate for injuries to the fibular collateral ligament. Thus, when there is some concern that the patient has increases in gapping and it appears that the FCL is intact, it may in actually be stretched out and nonfunctional and bilateral varus stress x-rays should be obtained to objectively diagnose this pathology.
Looking at the popliteus tendon involves assessment of its attachment site on the femur as well as its musculotendinous junction. These injuries are best determined acutely, because on chronic injuries the popliteus tendon can be stretched out and nonfunctional and they can look intact on an MRI scan.
The popliteofibular ligament is a very thin structure, so it is important to ensure that the MRI cuts pass through it before determining if there is an injury to either knot.
Assessment of the lateral capsule is important because when the lateral capsule is completely torn off the tibia, it can make the knee severely unstable.
Injuries to the biceps femoris attachment to the fibular head are particularly important because these injuries often are associated with injuries to the common peroneal nerve. In addition, even if the common peroneal nerve is intact or minimally injured, it can be malpositioned when there is an avulsion fracture (called an “arcuate fracture”) of the biceps femoris off the fibular head. This is very important to assess for prior to a surgical reconstruction to ensure that one is very careful in approaching the posterolateral corner of the knee because when the biceps femoris is avulsed because of the common peroneal nerve may be malpositioned with this injury pattern.
Finally, the iliotibial band attachment site at Gerdy’s tubercle on the tibia is also important to assess for. We have found the iliotibial band to be injured in only 3% of posterolateral corner injuries, so when this is injured, it is usually with a very severe posterolateral corner injury present.