What Are Posterolateral Corner Ligaments
The posterolateral corner is a complex of ligaments and structures on the outer side of the knee that stabilizes the joint against varus stress and external rotation. Key structures include the lateral collateral ligament, the popliteus tendon and ligament, the popliteofibular ligament, the fabellofibular ligament, and the biceps femoris insertion, as detailed by the American Academy of Orthopaedic Surgeons. These ligaments work together with the cruciate ligaments to prevent abnormal opening of the knee and rotational instability.
Injury mechanisms typically involve a forced varus moment, hyperextension, or combined external rotation and adduction, often seen in high-energy trauma, contact sports, or dashboard injuries in motor vehicle collisions. The incidence of isolated posterolateral corner injuries is lower than that of anterior cruciate ligament tears, but they frequently occur together with cruciate or other ligament damage, and missed diagnosis can lead to graft failure and chronic instability, according to research summarized by the National Institutes of Health.
Grades, Diagnosis, and Imaging
Grades of Injury
Posterolateral corner injuries are graded from I to III based on ligament stretch, partial tear, or complete disruption, with grade III often involving multiple structures and a positive varus stress test at zero and thirty degrees of flexion. Associated findings can include peroneal nerve palsy, joint effusion, and soft tissue swelling, which require prompt evaluation to avoid long-term functional deficits.
Imaging and Diagnostic Tools
Standard X-rays are used first to rule out fractures and avulsion injuries, while stress radiographs can quantify varus gapping. Magnetic resonance imaging is the primary soft tissue modality, with dedicated posterolateral corner sequences improving sensitivity for tears of the popliteus complex and fabellofibular ligament, as described by the Radiological Society of North America. In equivocal cases, stress ultrasound or diagnostic arthroscopy may be used to confirm the diagnosis and assess concomitant intra-articular pathology.
Treatment Pathways and Outcomes
Nonoperative Management
Grade I and II injuries without significant instability are managed with bracing, protected weight bearing, and progressive physical therapy focusing on quadriceps and hamstring strength, neuromuscular control, and range of motion. Nonsteroidal anti-inflammatory drugs and cryotherapy are used for pain and swelling control, and return to sport is considered when strength, stability, and functional tests are symmetric.
Surgical Options and Rehabilitation
Grade III injuries or cases with persistent varus instability are treated with anatomic reconstruction using autografts or allografts, often in a staged approach if vascular or neurological injury is present. Rehabilitation protocols emphasize protected motion early, gradual loading, and a return-to-sport progression that typically spans six to nine months, with outcomes influenced by the quality of graft fixation, concomitant ligament reconstruction, and adherence to therapy, as reported by the American Orthopaedic Society for Sports Medicine.