Are SER-II Ankle Fractures Actually Anatomic? Bilateral CT Demonstrates Subtle Non-Anatomic Mortise & Syndesmotic Alignment in the Setting of Apparent Normal Radiographs

CATEGORY: Ankle; Trauma INTRODUCTION/PURPOSE: Current treatment of ankle fractures is predicated on 1-2 mm of talar shift leading to abnormal contact pressures. Determination of anatomic alignment has conventionally been based on plain radiographs. The ability of x-ray to detect subtle mortise displacement however is unknown. The purpose of this investigation is to determine if CT scan can detect malalignment undetectable utilizing plain radiography when assessing injuries assumed to have anatomic mortise alignment. The findings have implications for utilizing higher resolution diagnostic imaging to assess malalignment and challenging the validity of previous literature in which conclusions of anatomic mortise alignment were determined based on plain radiography. METHODS: 24 patients with stress-negative, stable SER-II type ankle fractures were included. Contralateral ankle mortise radiographs were obtained in addition to static and gravity stress radiographs of the injured side. Bilateral CT scans were performed once patients were deemed to have achieved clinical/radiographic healing. Measurements of the medial clear space (MCS) and syndesmosis were performed by two blinded orthopedic surgeons at two different time points. Intraclass correlation coefficients (ICC) were calculated to evaluate the intra- and inter-rater reliability and consistency of measurements. Statistical analyses were performed to determine any differences between x-ray and CT scan measurements to detect discrepancies between the injured and normal ankle measurements. Statistical significance was defined as a two-sided alpha error of less than 0.05 (p < 0.05). RESULTS: Despite plain radiographs demonstrating no statistical side-to-side differences and achieving apparent final anatomic alignment in the majority of the cohort, CT MCS on coronal, axial planes and syndesmosis measurements were significantly different when comparing the injured to the normal contralateral ankle by on average of about 0.5mm. There was also excellent intra and interobserver agreement achieved for the measurements. CONCLUSION: CT is superior to plain radiography in detecting subtle mortise/syndesmotic displacement and side-to-side differences undetectable by x-ray. These findings challenge conventional assumption of anatomic alignment for SER II injuries and reliance on x-rays to assess anatomic alignment. While injured ankles uniformly demonstrated non-anatomic alignment as compared to the uninjured side, the clinical implications of subtle malalignment are unknown. However, broad reaching implications of this preliminary study include challenging the validity of x-ray to detect possibly clinically-relevant mortise malalignment in patients presenting with isolated lateral malleolus fractures.