This cadaveric biomechanical study used sequential ligament sectioning in 10 fresh-frozen knees to establish millimeter-based radiographic thresholds for diagnosing isolated FCL tears and grade-III PLC injuries. Varus stress was applied two ways — a standardized 12-Nm moment and clinician-applied force — at 20° of knee flexion, with lateral compartment gapping measured on digital fluoroscopic images. Three observers across training levels assessed reproducibility of the measurements.
Before this paper, no objective millimeter-based thresholds existed for interpreting varus stress radiographs in PLC injuries — clinicians relied on physical exam alone or grading systems (IKDC, AMA) that, as this paper shows, systematically underclassify lateral-sided instability.
When you order a varus stress radiograph on a knee with suspected posterolateral injury, use these thresholds referenced to the contralateral side: ~2.7 mm increase suspects isolated FCL tear; ~4.0 mm increase suspects grade-III PLC injury.
If gapping approaches 6–8 mm, assume combined cruciate involvement and plan accordingly. Unrecognized PLC instability is a well-documented cause of ACL and PCL graft failure. These measurements can be made reliably by anyone reading a digital PACS image, making varus stress radiographs a practical, cost-effective tool for preoperative planning and postoperative follow-up of PLC reconstructions.
This cadaveric biomechanical study used sequential ligament sectioning in 10 fresh-frozen knees to establish millimeter-based radiographic thresholds for diagnosing isolated FCL tears and grade-III PLC injuries. Varus stress was applied two ways — a standardized 12-Nm moment and clinician-applied force — at 20° of knee flexion, with lateral compartment gapping measured on digital fluoroscopic images. Three observers across training levels assessed reproducibility of the measurements.
Before this paper, no objective millimeter-based thresholds existed for interpreting varus stress radiographs in PLC injuries — clinicians relied on physical exam alone or grading systems (IKDC, AMA) that, as this paper shows, systematically underclassify lateral-sided instability.
When you order a varus stress radiograph on a knee with suspected posterolateral injury, use these thresholds referenced to the contralateral side: ~2.7 mm increase suspects isolated FCL tear; ~4.0 mm increase suspects grade-III PLC injury.
If gapping approaches 6–8 mm, assume combined cruciate involvement and plan accordingly. Unrecognized PLC instability is a well-documented cause of ACL and PCL graft failure. These measurements can be made reliably by anyone reading a digital PACS image, making varus stress radiographs a practical, cost-effective tool for preoperative planning and postoperative follow-up of PLC reconstructions.