The Gorilla lip sign: Diagnostic utility of anteroposterior X-ray for pediatric talocalcaneal coalition

Background: Talocalcaneal coalition (TCC) is highly prevalent but diagnostically challenging due to the subtalar joint's complex anatomy, leading to frequent oversights on conventional radiography. Existing signs (e.g., C-sign, talar beaking) rely primarily on lateral views, with variable sensitivity. This study aims to introduce and validate the "Gorilla lip" sign on anteroposterior (AP) radiographs as a complementary diagnostic marker for TCC. Methods: In this retrospective study, AP ankle radiographs of 30 TCC-confirmed feet (CT/surgery; positive group) and 33 normal feet (CT-confirmed; negative group) from children aged 9–18 years were analyzed. The "Gorilla lip" sign was defined by sustentaculum tali abnormalities: (1) osseous protuberance ≥50% medial malleolus height, (2) narrowed and rough articular surface, or (3) absent joint space. Three blinded observers (varying experience) independently assessed randomized images twice (2-month interval). Sensitivity, specificity, likelihood ratios (PLR/NLR), and inter-/intraobserver reliability (Fleiss/simple kappa) were calculated. Results: The "Gorilla lip" sign demonstrated high diagnostic accuracy. Across two evaluations by three observers, sensitivity ranged from 77.0% to 93.0%, and specificity ranged from 85.0% to 100%. Positive likelihood ratios were strong (ranging from 5.28 to infinite), indicating a high probability of TCC when the sign is present, while low negative likelihood ratios (ranging from 0.07 to 0.24) suggest its absence is a strong indicator to rule out TCC. The sign showed good interobserver reliability (Fleiss kappa: 0.65-0.70) and good to excellent intraobserver reliability (kappa: 0.67-0.87). Conclusion: The "Gorilla lip" sign is a highly sensitive, specific, and reproducible radiographic marker for diagnosing TCC on anteroposterior ankle X-rays. Its application can complement traditional lateral view signs, enhancing diagnostic confidence and potentially reducing the need for more advanced and costly imaging modalities.