Arcuate sign | Radiology Signs

Answer:

The arcuate sign is an avulsion fracture of the fibular styloid, the tip of the fibular head, seen as a small horizontally orientated elliptical fragment on the frontal knee radiograph. Its significance is that the fragment carries the insertion of the arcuate ligament complex, so it indicates injury to the posterolateral corner, the structures comprising the lateral collateral ligament, the popliteus tendon, the popliteofibular ligament and the arcuate and fabellofibular ligaments. MRI studies show that an arcuate sign is accompanied by cruciate ligament injury, most often of the posterior cruciate ligament, in the large majority of cases, and by injury to two or more posterolateral structures. Recognition matters because untreated posterolateral corner injury is a principal cause of failure of cruciate reconstruction: residual varus and external rotational instability places abnormal load on the graft. The fragment is small and easily overlooked, is best seen on the frontal view, and should trigger dedicated MRI including coronal oblique images through the popliteofibular ligament. Associated findings include a common peroneal nerve injury, which occurs in a significant minority and must be assessed clinically, and a lateral meniscus or biceps femoris injury. Acute repair or reconstruction within about three weeks gives better results than delayed surgery.

Why is it called so?

The sign is named for the arcuate ligament complex, whose attachment is avulsed with the fibular styloid fragment.

Pathophysiology

The posterolateral corner resists varus angulation, external rotation of the tibia and posterior translation. A blow to the anteromedial knee, or hyperextension with varus, loads these structures, and because the arcuate complex and the fabellofibular and popliteofibular ligaments converge on the fibular styloid, the bone at that confluence fails in tension before the ligaments tear in substance. The horizontal orientation of the fragment reflects the direction of pull. The same force commonly disrupts the posterior cruciate ligament, which is why the two injuries are so often combined, and stretches the common peroneal nerve as it winds around the fibular neck, explaining the associated foot drop.

Alternative names: Fibular styloid avulsion; arcuate complex avulsion fracture

Other associated named signs: Segond fracture of the lateral tibial rim and the conjoint tendon avulsion of the fibular head

References

1. Huang GS, Yu JS, Munshi M, Chan WP, Lee CH, Chen CY, et al. Avulsion fracture of the head of the fibula (the “arcuate” sign): MR imaging findings predictive of injuries to the posterolateral ligaments and posterior cruciate ligament. AJR Am J Roentgenol. 2003;180(2):381-7. PMID: 12540438.