Plantar Flexion Influences Radiographic Measurements of the Ankle Mortise

BACKGROUND:The treatment of ankle fractures often depends on the integrity of the deltoid ligament. Diagnosis of a deltoid ligament tear depends on the measurement of the medial clear space. We sought to evaluate the impact of ankle plantar flexion on the medial clear space. METHODS:Mortise radiographs were made for twenty-five healthy volunteers, with the ankle in four positions of plantar flexion (0°, 15°, 30°, and 45°). Four observers measured the medial clear space and the superior clear space on each radiograph. The mean medial clear space at 0° was defined as the control, and the deviation of the medial clear space from the control value was calculated at 15°, 30°, and 45° of plantar flexion. The ratio of the medial clear space to the superior clear space was determined on all radiographs, and ratios that were false-positive for a deltoid ligament injury were identified. RESULTS:Fourteen male and eleven female volunteers were evaluated. The average increase in the medial clear space when ankle plantar flexion was increased from 0° to 45° was 0.38 mm (95% confidence interval, 0.18 to 0.58 mm). This increase was significant (p = 0.005). The average increase in the medial clear space was 0.04 mm when ankle plantar flexion was increased from 0° to 15° and 0.22 mm when it was increased from 0° to 30°. Neither of these changes was significant (p = 0.99 and 0.20). The prevalence of false-positive findings of deltoid injury based on the ratio of the medial clear space to the superior clear space increased as ankle plantar flexion increased, but this increase did not reach significance in our study group (p = 0.18). CONCLUSIONS:Plantar flexion of the ankle produces changes in radiographic measurements of the medial clear space. The potential for false-positive findings of deltoid disruption increases with increasing ankle plantar flexion. CLINICAL RELEVANCE:This study highlights the importance of the ankle being in a neutral position when radiographs are made to measure the medial clear space to assist in the decision of whether to perform nonoperative or operative treatment.