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What is the diagnosis?

Proximal fibula avulsion fracture, also known as an “arcuate sign,” highly indicative for a posterior lateral corner (PLC) injury.

Image 1 and 2. Plain radiographs of the injured lower extremity. Author’s own images.

• Pearl: The fracture occurs at the site of insertion of the arcuate ligament complex and is usually associated with cruciate ligament injury (~90% of cases) [1]. The differential for this fracture includes the very similar appearing “Segond Fracture” and lateral fibular head fracture. The Arcuate sign is found from an avulsion of the most proximal aspect of the fibula. Orientation of the fracture line in the Arcuate sign is generally transverse across the proximal fibula, while the lateral proximal fibula avulsions are typically obliquely oriented. 

What is the mechanism of injury?

Two common mechanisms: 

• Direct force to anteromedial tibia during knee extension results in posterolateral subluxation of the tibia in external rotation 

• Sudden knee hyperextension with tibial internal rotation 

What physical exam findings are expected?

A knee effusion can be expected with lateral knee tenderness to palpation. Significant laxity of the lateral knee at the LCL will be noted with the varus stress test. The Dial test is used to evaluate PLC injury; it is performed with the patient prone, knees flexed to both 30 and 90 degrees. If there is greater than 10 degrees of external rotation asymmetry (compared to the uninjured side) at 30 degrees of knee flexion, a PLC injury is suspected. If there is greater than 10 degrees of external rotation asymmetry at both 30 and 90 degrees of knee flexion, this is indicative of a combined PLC and PCL injury.

Which imaging modalities can be used?

Plain radiograph (AP and lateral, as above) is the initial modality of choice, usually showing a small <1 cm avulsion fracture involving the styloid process of the fibula displaced superiorly and medially. AP films with slight internal rotation of the tibia give the best radiographic picture [3]. MRI cross-sectional imaging can show cruciate ligament injuries (PCL), bone edema, collateral ligament injuries, meniscal tears, and popliteus injury. Ultrasound of the proximal fibula may be utilized to show a hypoechoic defect in the hyperechoic bone cortex. 

What is the management in the ED?

Initial management includes pain control and radiographs. Patients with an arcuate sign and concern for PLC/PCL injury should be placed in a hinged knee brace locked in extension to provide complete knee stability.

When do you consult Orthopedics?

If not diagnosed acutely, PLC injuries may lead to posterolateral instability, which is challenging to correct and may result in failed cruciate ligament reconstruction [1,3]. Orthopedics consultation is not necessary in the ED; however close follow up with Orthopedic Surgery as outpatient is recommended to prevent these complications.