DOI: 10.1148/radiol.2233991817
(Radiology 2002;223:756-757.)
© RSNA, 2002
The C Sign1
Seong Hyun Kim, MD
1 From the Department of Radiology, Samsung Medical Center, Sungkyunkwan University School of Medicine, 50 Irwon-Dong, Kangnam-Ku, Seoul 135-710, Korea. Received October 7, 1999; revision requested November 16; revision received July 5, 2000; accepted July 21. Address correspondence to the author (e-mail: kimshyun@unitel.co.kr).
Index terms: Ankle, abnormalities, 4641.1499, 4641.486 • Calcaneus, 4642.486 • Signs in Imaging • Talus, 4641.486
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APPEARANCE
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The talocalcaneal C sign (1) is a continuous, C-shaped line that extends from the talus to the sustentaculum tali and can be seen on lateral radiographs of the ankle (Figure, part a).
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(a) Lateral radiograph of the right ankle of a 34-year-old man shows the talocalcaneal C sign, which suggests subtalar coalition (upper black arrows and white arrow = medial outline of talar dome, lower black arrows = posteroinferior outline of sustentaculum tali). The coalition occurred posterior to the middle subtalar joint. (b) Coronal computed tomographic (CT) scan in the same patient shows the osseous coalition of the medial aspect of the posterior subtalar joint (arrow). (c) Sagittal reformatting of b shows the coalition (arrow) posterior to the middle subtalar joint. The linear lucency anterior to the coalition is the middle subtalar joint that is not fused and that is seen in a.
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(a) Lateral radiograph of the right ankle of a 34-year-old man shows the talocalcaneal C sign, which suggests subtalar coalition (upper black arrows and white arrow = medial outline of talar dome, lower black arrows = posteroinferior outline of sustentaculum tali). The coalition occurred posterior to the middle subtalar joint. (b) Coronal computed tomographic (CT) scan in the same patient shows the osseous coalition of the medial aspect of the posterior subtalar joint (arrow). (c) Sagittal reformatting of b shows the coalition (arrow) posterior to the middle subtalar joint. The linear lucency anterior to the coalition is the middle subtalar joint that is not fused and that is seen in a.
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View larger version (141K):
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(a) Lateral radiograph of the right ankle of a 34-year-old man shows the talocalcaneal C sign, which suggests subtalar coalition (upper black arrows and white arrow = medial outline of talar dome, lower black arrows = posteroinferior outline of sustentaculum tali). The coalition occurred posterior to the middle subtalar joint. (b) Coronal computed tomographic (CT) scan in the same patient shows the osseous coalition of the medial aspect of the posterior subtalar joint (arrow). (c) Sagittal reformatting of b shows the coalition (arrow) posterior to the middle subtalar joint. The linear lucency anterior to the coalition is the middle subtalar joint that is not fused and that is seen in a.
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EXPLANATION
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The talocalcaneal C sign can be seen in patients with subtalar coalition on lateral ankle radiographs. The anatomic-pathologic basis for a talocalcaneal C sign on lateral ankle radiographs is the bony bridge that extends from the talar dome to the sustentaculum tali, in combination with a prominent inferior outline of the sustentaculum tali (Figure). The talocalcaneal C sign can also be seen in the absence of synostosis of the posterior subtalar joint in patients with a subtalar syndesmosis or synchondrosis, which lies in a plane that is not parallel with the x-ray beam. A posterior interruption of the talocalcaneal C sign occurs in patients with radiolucent syndesmosis or synchondrosis of the medial part of the posterior subtalar joint that is parallel with the x-ray beam, or in patients with coalition of the middle subtalar joint without involvement of the posterior subtalar joint. The C sign may frequently be posteriorly interrupted in the absence of synostosis of the posterior facet; therefore, the sign has lower specificity. Subtalar coalition is frequently accompanied by a dysplastic sustentaculum tali, which may not cast a well-defined lower interface to contribute to the inferior aspect of the C (1).
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DISCUSSION
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Tarsal coalition, an abnormal bridging across two or more tarsal bones, is a well-recognized cause of foot pain and restricted mobility in adolescents and young adults. While the cause is uncertain, the most widely accepted theory suggests a lack of differentiation and segmentation of embryonic mesenchyme (2). In the general population, the true frequency of tarsal coalition is unknown but is less than 1% (1). Calcaneonavicular coalition is the most common type of coalition, followed by talocalcaneal coalition. Synostosis, synchondrosis, and syndesmosis occur in subtalar joints of the coalition. The middle subtalar joint is the most common site of subtalar coalition, but coalition may also occur in the posterior subtalar joint (1).
On standard radiographs, primary signs of subtalar coalition (ie, narrowing and subchondral sclerosis of the posterior subtalar joint and absence of the middle subtalar joint and sinus tarsi) may be subtle or even absent in up to 50% of patients (3). Its secondary signs (eg, talar beaking, ball-and-socket deformity of the tibiotalar joint, broadening of the talar lateral process, and concave undersurface of the talar neck), which are nonspecific, may be absent. Therefore, subtalar coalition can easily be missed on conventional radiographs (3,4).
Lateur et al (1) reported that in a study of 33 patients with subtalar coalition, only the C sign was positive in 32 of 32 true-positive cases; the sensitivity and specificity were 86.6% and 93.3%, respectively, with 32 true-positive cases, one true-negative case, no false-positive cases, and two false-negative cases. The talocalcaneal C sign is subtle in cases of subtalar coalition with dysplastic or rounded sustentaculum tali, since the lower part of the talocalcaneal C is less prominent on lateral ankle radiographs because the x-ray beam strikes the inferior surface of the dysplastic or rounded sustentaculum tali tangentially over a shorter distance. Aplastic sustentaculum tali in patients with achondroplasia may render the C sign absent. A positional artifact of the foot may cause a false-positive C sign in patients without subtalar coalition (1). The interrupted C sign may sometimes be seen with a valgus hindfoot or with inexact lateral x-ray beam angulation. CT is helpful in confirming subtalar coalition and establishing its extent, especially in patients with an interrupted C sign (5). Magnetic resonance imaging is ideally suited for differentiating syndesmosis and synchondrosis (6,7).
In summary, the C-shaped line formed by the medial outline of the talar dome and the posteroinferior outline of the sustentaculum tali on lateral radiographs of the ankle is a useful sign in the diagnosis of subtalar coalition.
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FOOTNOTES
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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REFERENCES
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Resnick D. Additional congenital or heritable anomalies and syndromes. In: Resnick D, eds. Diagnosis of bone and joint disorders. 3rd ed. Philadelphia, Pa: Saunders, 1995; 4294-4301.
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Resnick D. Talar ridges, osteophytes and beaks: a radiographic commentary. Radiology 1984; 151:329-332.[Abstract/Free Full Text]
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Sarno RC, Carter BL, Bankoff MS, Semine MC. Computed tomography in tarsal coalition. J Comput Assist Tomogr 1984; 8:1155-1160.[Medline]
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Mund PL, Vellet DA, Levin MF, Helms CA. Current status of magnetic resonance imaging of the ankle and the hindfoot. Can Assoc Radiol J 1992; 43:19-30.[Medline]
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Wechsler RJ, Schweitzer ME, Deely DM, Horn BD, Pizzutillo PD. Tarsal coalition: depiction and characterization with CT and MR imaging. Radiology 1994; 193:447-452.[Abstract/Free Full Text]
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C Sign for Diagnosis of Talocalcaneal Coalition
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