Welcome to the posterolateral corner (PLC) of the knee: The Dark Sleepy Hollow of post ACL knees.

Although perhaps more commonly thought to be found in PCL injuries, i personally cannot tell you how many cases of ACL repair I have seen over the years that turned into a failed surgical response because damage and laxity in the posterolateral corner of the knee was missed. 
 I have sent enough knees back to surgeons with detailed explanations of a discovered PLRI (posterolateral rotatory instability), some impressed that it was found, others dismissing it (and eventually surgery by another doctor). These are frustrating cases and they cannot be missed.  One must not just assess for the ACL tear, post event tear is an optimal time to determine if there is BOTH a positive drawer phenomenon and a pivot shift. The majority of PLC injuries do not occur in isolation and are part of a more complex injury pattern that typically involves other vital supporting structures. Do not dismiss the restraining capabilities of the capsular and non-capsular secondary restraints in this far corner of the knee. Finding the pivot shift after the ACL reconstruction is just too late, you must catch it before it heads to surgery and make sure the surgeon knows that the posterolateral corner restraints were also trashed. They likely need repaired as well. otherwise the client will have a great tight drawer test post surgery but will have rotational instability, which is arguably worse if you ask me.  If you find PLRI on the exam make a strong note of it on the MRI request, be sure the radiologist has the clinical functional info in mind when they get the static images coming up on the screen.
Too many clinicians do not know how to assess this area, and the pivot shift phenomenon is also overlooked and misunderstood. If you have never likely had someone walk you through what a positive pivot shift feels like on a ACL knee you will not know what it feels like in a post ACL reconstruction that is failing rehab.
"Although rare, posterolateral corner (PLC) injuries can result in sustained instability and failed cruciate ligament reconstruction if they are not diagnosed. The anatomy of the PLC was once thought to be perplexing and esoteric-in part because of the varying nomenclature applied to this region in the literature, which added unnecessary complexity. "- Rosas
"More recently, three major structures have been described as the primary stabilizers of the PLC on the basis of biomechanical study findings: the lateral collateral ligament, popliteus tendon, and popliteofibular ligament. " 

Do not miss this one gang. Know how to test and feel for PLRI, you will find it if you start looking for it. And, you will likely fail in rehabilitating these knees, it usually need surgical correction of that corner.

- Dr. Shawn Allen, one of the gait guys

Radiographics. 2016 Oct;36(6):1776-1791. Unraveling the Posterolateral Corner of the Knee. Rosas HG1.