Missed on X-ray, Found on CT: A Retrospective Study on the Diagnostic Yield and Clinical Consequences of Occult Posterior Malleolus Fractures in Tibial Shaft Fractures

Background Posterior malleolus fractures (PMFs) are common in distal tibial shaft fractures yet are frequently occult on plain radiographs. Accurate preoperative characterization on CT may improve surgical planning and functional recovery. Methods We conducted a retrospective cohort study of consecutive adults (≥18 years) with mid- or distal tibial shaft fractures treated at a major trauma center (January 2022-December 2024). Demographics, imaging, fracture characteristics, management, and outcomes were abstracted. PMF detection was compared across radiography, CT, and intraoperative findings. Predictors of a posterior malleolus (PM) fragment being missed on radiography were evaluated with multivariable logistic regression. Among operatively treated cases, the effect of fixation strategy on postoperative full weight‑bearing (FWB) was analyzed with a logistic model including fixation, PM status, and their interaction; pairwise, covariate‑adjusted contrasts (emmeans with Tukey correction) compared strategies. An exploratory random forest provided permutation‑importance rankings. Findings PMFs were present in 147/387 fractures (38.0%), more often in women (60.5%), closed injuries (76.9%), and distal fractures (98.0%). Radiography identified 116/147 PMFs (78.9%), and CT 120/147 (81.6%); 29/147 (19.7%) fragments were missed on radiographs but detected on CT or intraoperatively. Missed fragments were most frequent in undisplaced and intermediate‑sized (33-50%) fragments; CT identified all posteromedial and >50% fragments. In adjusted analyses, fracture morphology was the principal correlate of radiographic omission: oblique patterns were less likely to be missed (adjusted odds ratio 0.18, 95% CI 0.04-0.60), whereas other demographic and injury variables were not significant. Among operatively managed fractures, intramedullary nailing was associated with the highest likelihood of achieving FWB and was statistically superior to plate and external fixation in pairwise, covariate‑adjusted comparisons; the relative ranking of fixation methods did not differ by PM involvement. Neither PMF location (posteromedial, posterolateral, or undisplaced) nor fragment size independently predicted FWB, infection, or mal‑/non‑union. Conclusions Approximately two in five distal tibial shaft fractures harbour a PM fragment, and nearly one in five PMFs are occult on initial radiographs, particularly when undisplaced or of intermediate size. CT modestly increases detection and ensures complete characterisation of posteromedial and large fragments, supporting a low threshold for pre‑operative CT in distal tibial fractures.