"A continuous C shaped arc is seen on a lateral ankle radiograph which is formed by the medial outline of the talar dome and posteroinferior aspect of the sustentaculum tali."
J Pediatr Orthop. 2014 Dec;34(8):814-9. doi: 10.1097/BPO.0000000000000188.
C sign: talocalcaneal coalition or flatfoot deformity?
Moraleda L1, Gantsoudes GD, Mubarak SJ.
C sign is used to alert the physician of the possible presence of talocalcaneal coalition (TCC), so that advanced imaging can be ordered. The purpose of this study was to know the prevalence of the C sign among patients with TCC and its relationship to the presence of a TCC or to hindfoot alignment.
Retrospective reviews of the presence of C sign in radiographs of 88 feet with TCC (proved by computed tomography scan or surgical findings) and 260 flexible flatfeet were conducted. C sign was classified as complete and interrupted (types A, B, and C). The interobserver variability of the C sign was studied. Seven radiographic parameters were measured to analyze the relationship of these measurements with the presence or absence of the C sign.
C sign was present in 68 feet (77%) with TCC: 14.5% complete and 62.5% interrupted (26% type A, 19.5% type B, and 17% type C). C sign was present in 116 flatfeet (45%), all of them interrupted (0.4% type A, 5.5% type B, and 39% type C). The talo-first metatarsal angle, the talohorizontal angle, the calcaneal pitch, the calcaneo-fifth metatarsal angle, and the naviculocuboid overlap presented a more pathologic value when a C sign was present. The κ-value for the presence of a C sign was 0.663.
The so-called true C sign (complete or interrupted type A) indicates the presence of a TCC and it is not related to flatfoot deformity. However, it is only present in 41% of the cases. The interrupted C sign is much more likely to be related to flatfoot deformity than to the presence of a TCC, specifically when a type C is found.