MRI of posterolateral knee stabilizers: diagnosis and reporting considerations

The posterolateral corner (PLC) of the knee comprises a complex arrangement of anatomical and biomechanical structures. Owing to their variability, small size, and the inconsistent terminology found in the literature, these structures have historically been referred to as the "dark side of the knee". This review aims to summarize the relevant anatomy and MR anatomy of the PLC stabilizers, illustrate key MRI findings in acute and chronic injuries, and provide practical considerations for structured diagnosis and reporting. The main stabilizers of this region include the lateral collateral ligament (LCL), the popliteofibular ligament (PFL), and the popliteus myotendinous complex. Together, these elements provide resistance against varus forces and external tibial rotation, with additional compensatory roles in the presence of cruciate ligament insufficiency. Injury mechanisms are diverse, commonly involving direct high-energy trauma, hyperextension, or rotational-varus-hyperextension stress in sports-related activities. Although PLC lesions represent nearly one-third of all ligamentous injuries of the knee, isolated involvement is uncommon, with frequent associations with posterior and anterior cruciate ligament tears. Accurate imaging evaluation, particularly with magnetic resonance imaging (MRI), is fundamental for timely diagnosis, guiding appropriate management, and reducing the risk of chronic posterolateral instability, cruciate graft failure, and progression to osteoarthritis.