The posterolateral corner is the region on the outer side at the back of the knee.
This illustration shows the back of the right knee, deep to the hamstrings muscles.
One of the key structures of the posterolateral corner is the popliteus muscle and its tendon. The muscle arises on the tibia bone and wraps around the back of the tibia to attach via its tendon on the outer aspect of the femur at the side of the knee joint. The popliteus tendon has an intimate relationship to the back of the lateral meniscus.
This next illustration is of the same region. We have simply added in the capsule of the knee - which encloses the joint and makes it water-tight.
But look carefully at the region of the posterolateral corner. Here there is a deficiency in the capsule where the poplilteus tendon actually goes into the joint space itself. This means that injuries in this region - the posterolateral corner - often involve the capsule as well as the tendon and ligament. That part of the capsule that loops over the popliteus tendon is referred to as the arcuate ligament.
When the surgeon looks into the joint from the front during arthroscopy the camera is within the capsule. The surgeon is normally able to see the popliteus tendon as it passes inside the joint. If it is ruptured a careful surgeon should be able to see this during arthroscopy, and should at least note the absence of the tendon at the back of the lateral meniscus.
If we rotated the two illustrations above, so that one looks at the right knee from the outer aspect, you can see yet another of the important structures of the postero-lateral corner - the lateral collateral ligament (also called the fibular collateral ligament). This spans the outside of the joint from the fibula bone to the femur, and is an important stabilising structure. You can see that it is outside the capsule, so the surgeon will not be able to see during arthroscopy if it is broken.
There are other important structures in this region, but these are the main ones.
Open access article - Recondo JA, Salvador E, Villanúa JA, Barrera MC, Gervás C, Alústiza JM.Lateral stabilizing structures of the knee: functional anatomy and injuries assessed with MR imaging.Radiographics. 2000 Oct;20 Spec No:S91-S102.