Enhanced subfibular ossicle diagnosis: CT-MRI integration of morphology and ligament attachments

To assess the diagnostic value of CT and MRI in distinguishing nonunited avulsion fracture (NAF) of the lateral malleolus from os subfibulare (OSF).

In this retrospective study, 114 subfibular ossicles (SFOs) in 108 patients were evaluated by CT and MRI for shape, margin, size, CT attenuation, spatial orientation, and anatomical relations. Surgical, arthroscopic, and follow-up findings served as reference standards. Logistic regression and receiver operating characteristic (ROC) analyses assessed diagnostic performance.

NAFs were associated with higher incidences of lateral ankle pain (92.3% vs. 71.4%, p = 0.003) and instability (29.2% vs. 10.2%, p = 0.014) than OSF. CT showed that NAF had a more irregular shape, rougher margins, higher attenuation, and anteroposterior orientation compared to OSF (all p < 0.05). MRI revealed ligamentous attachment predominated in NAF (72.3%), while OSF mostly showed discontinuity or apposition (91.8%, p < 0.001). The combined CT model achieved an AUC of 0.782, accuracy of 74.3%, sensitivity of 91.9%, and specificity of 56.8%. MRI-based ligamentous attachment MRI strict criterion yielded an AUC 0.821, with a sensitivity of 72.3% and a specificity of 91.8%, while the MRI inclusive criterion was 0.752, 95.4% and 55.1%. Ossicle size correlated with symptom severity (p < 0.001).

MRI is highly effective for differentiating NAF from OSF by directly visualizing SFO-ligament attachments, while CT provides complementary morphological detail. The combined use of CT and MRI delivers robust diagnostic performance, supporting clinical decision-making in lateral malleolar lesions.

CT and MRI can distinguish a nonunited avulsion fracture from os subfibulare. Especially, MRI can visually display the relationship between SFO and the lateral collateral ligament.

Distinguishing chronic lateral malleolus avulsion fractures from congenital accessory bones remains diagnostically challenging. MRI directly visualizing ligament attachment to the bone fragment best differentiates fractures from accessory bones. Combining MRI and CT findings provides comprehensive evidence for clinical management decisions.