DOI: 10.1148/radiol.2442042160
(Radiology 2007;244:620-621.)
© RSNA, 2007
The Arcuate Sign1
William M. Strub, MD
1 From the Department of Radiology, University of Cincinnati, 234 Goodman St, ML 0761, Cincinnati, OH 45267. Received December 21, 2004; revision requested February 4, 2005; revision received February 9; final version accepted March 2. Address correspondence to the author (e-mail: williamstrub{at}hotmail.com).
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APPEARANCE
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The arcuate sign is a finding that is best demonstrated at standard anteroposterior radiography of the knee. The sign consists of a linear lucency through the head of the fibula, representing a fracture (Figure) (1).
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EXPLANATION
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The arcuate sign indicates an avulsion fracture of the head of the fibula at the insertion site of the arcuate ligament complex (Figure) and is considered to be a sign indicative of injury to the posterolateral corner of the knee (1,2). The arcuate sign indicates an injury to the arcuate ligament complex and does not refer to the appearance of the fracture line itself. The arcuate ligament complex is not a single structure but a combination of several structures that form an arcuate (arched) appearance (3). The arcuate ligament complex has a lateral limb that merges with the lateral capsule of the knee and a medial limb that ascends over the popliteus muscle.
The mechanisms of injury that can cause this avulsion fracture are a direct varus force with the tibia in external rotation or sudden hyperextension of the knee with the tibia internally rotated (2), either of which may subsequently result in posterolateral subluxation (4). The anterolateral aspect of the knee is stabilized by the iliotibial tract and the joint capsule. Posterolaterally, the knee is stabilized by the arcuate ligament complex: the lateral collateral ligament, biceps femoris tendon, popliteus muscle and tendon, popliteal meniscal ligament, popliteofibular ligament, oblique popliteal and fabellofibular ligaments, and the lateral gastrocnemius muscle (5).
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DISCUSSION
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Posterolateral complex injuries of the knee are uncommon and usually occur in conjunction with a cruciate ligament injury (6). Initially, many of these patients are identified as having an isolated anterior cruciate ligament deficiency, and the posterolateral arcuate ligament complex injury is often overlooked and not recognized until it has become a chronic problem (6,7). When the posterolateral arcuate ligament complex injury is not recognized in its acute stage, chronic posterolateral instability results (2) and reconstructive surgery may be difficult (6). Therefore, it is essential to identify the injury as early as possible.
Patients with isolated injuries often have a history of minor trauma and resultant chronic pain in the posterolateral aspect of the knee that may be compounded by knee instability in extension and hyperextension (2). Initial physical examination may demonstrate only mild tenderness and ecchymosis, and isolated injuries to the posterolateral aspect of the knee may also go unrecognized (2).
At conventional anteroposterior radiography, the avulsed fragment is horizontally oriented (8), frequently less than 1 cm in length (2,8), and displaced medially and superiorly to the styloid process of the fibula (2). If the avulsed fragment involves the lateral aspect of the fibular head, the fragment will be larger (1.5–2.5 cm) and displaced a greater distance (2–4 cm) (2). The avulsed bony fragment is usually attached to the fibular collateral ligament, the biceps femoris tendon, or the conjoined tendon (9).
In summary, the arcuate sign, which can be seen on standard anteroposterior radiographs of the knee, is useful in diagnosing trauma to the posterolateral corner of the knee.
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FOOTNOTES
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Author stated no financial relationship to disclose.
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References
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