The arcuate ligament is the Y-shaped posterolateral knee capsular reinforcement arising from the fibular head and fanning over the popliteus muscle to the posterior capsule. Together with the LCL, popliteus, and popliteofibular ligament it forms the posterolateral corner. The Arcuate sign — an avulsion fracture of the fibular head on plain radiograph — indicates posterolateral corner injury requiring surgical assessment.
| Origin | Posterior fibular head |
|---|---|
| Insertion | Posterior knee capsule — fans out superiorly over the popliteus muscle |
| Actions | Posterolateral knee capsular reinforcement; resists external tibial rotation |
|---|
The Arcuate sign (small fibular head bony avulsion) on a knee radiograph after trauma is pathognomonic of posterolateral corner injury involving the arcuate ligament and popliteofibular ligament. Its presence should prompt MRI to assess the full extent of PLC injury and guide reconstruction planning.
Fibular head avulsion fracture indicating arcuate ligament and PLC injury requiring MRI assessment and early PLC reconstruction.
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