Measuring the Medial Clear Space in Uninjured Adult Ankles: Agreement With CT and Interobserver Reliability of Common Measurement Methods.

Background: The medial clear space (MCS) is a radiographic proxy for the ankle mortise used to determine the need for surgical intervention for ankle fractures. Several measurement methods have been described but no consensus exists on which method is most appropriate to guide clinical care. Our objective was to assess commonly used measurement methods to determine which was most accurate and reliable for measuring the MCS on nonweightbearing and weightbearing studies. Methods: Imaging of 34 morphologically normal, uninjured patients was reviewed. Paired radiographs and computed tomography (CT) scans, 17 nonweightbearing and 17 weightbearing, were measured by 5 observers. Three different methods were used to measure the MCS on each radiograph and 1 method on CT. CT was used as the "true" value of the MCS. Two-sided P tests were used to compare each radiographic MCS measurement to the CT MCS value. Cronbach alpha was calculated for each MCS measurement method to determine the interobserver correlation. Results: Measuring the MCS halfway between the superior medial talar dome and the tip of the medial malleolus, perpendicular to the tibiotalar joint, was the most accurate on weightbearing imaging (no difference in radiographic method 2 vs CT, P = .09). There was a significant correlation between any nonweightbearing radiograph measurement and CT (all P < .001). Interobserver reliability was found to be good to excellent for nonweightbearing MCS measurement (ICC 0.82-0.97), whereas weightbearing imaging demonstrated good reliability (ICC 0.77-0.9). Conclusion: Any of the 3 tested measurement methods can be used when analyzing nonweightbearing radiographs. Measuring the MCS halfway between the superior medial talar dome and the tip of the medial malleolus, perpendicular to the tibiotalar joint, should be used when examining weightbearing imaging. To support standardization, this method may be reasonable to use, particularly on weightbearing radiographs. Level of Evidence: Level III, retrospective comparative study.