Answer of June 2006

Clinical History:

Frontal radiograph; coronal and sagittal PD MR images of the knee of a 38 year old man with knee injury.

Diagnosis:

Posterolateral rotatory instability.

Discussion:

The Arcuate sign (on radiographs), signifies an avulsion fracture of the tip of the fibular head by the arcuate complex. The arcuate complex consists of the popliteofibular, arcuate, and fabellofibular ligaments. This avulsion fracture therefore indicates posterolateral rotatory instability of this patient's knee. Note that the biceps tendon inserts more laterally and distally on the side of the femoral head rather than at the tip.