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Posterolateral Corner Injuries
The posterolateral corner “PLC” of the knee is becoming increasingly recognized as an extremely important structure to maintain the stability of the knee joint.
PLC injuries occur with hyperextension, varus load and tibial external rotation. So the most common mechanism is a posterolaterally directed blow to the anteromedial tibia when the knee is hyperextended. PLC injuries are commonly associated with injury to other ligaments (ACL, PCL, LCL) and occur in isolation in <5% of cases. If suspected make sure to check for other ligamentous injuries.
Since this injury can be missed and is associated with significant disability it is important to test for it. This YouTube video, http://youtu.be/bnXaTdvZZ6o, demonstrates several examination techniques that can identify the injury.
References
Wheeless Online Ortho Reference http://www.wheelessonline.com/ortho/posterolateral_rotary_instability_of_the_knee