Variability in Radiographic Medial Clear Space Measurement of the Normal Weight-Bearing Ankle

Abstract

Medial clear space (MCS) width on mortise radiographs of the ankle is commonly used by clinicians for determining the competence of the deltoid ligament in the Weber B supination-external rotation ankle fracture. Significant variability exists in the current literature regarding methods of obtaining this measure and definition of a normal measure in comparison with a pathologic state. Seventy-three paired bilateral ankle mortise radiographs that were without ankle pathology were retrospectively reviewed. MCS width at two separate locations (oblique and perpendicular) and superior clear space (SCS) were measured on digital radiographs. A Student's t test was used to compare mean values. Mean values (± SD) were 3.2 (± 0.7)~mm for MCS oblique, 2.6 (± 0.7)~mm for MCS perpendicular, and 3.3 (± 0.6)~mm for SCS. A significant difference (p < .001) existed for all three measures between males and females. MCS oblique was statistically different than MCS perpendicular (p < .001) for all patients and for males and females independently. The mean difference between paired bilateral radiographs was 0.3 (± 0.2)~mm for MCS oblique, 0.6 (± 0.6)~mm for MCS perpendicular, and 0.2 (± 0.2)~mm for SCS. MCS width has variability based on the location chosen for measurement and gender. Contralateral radiographic comparison of MCS should be routinely used to identify pathologic widening versus normal anatomic variation. Use of single threshold values for MCS width as an operative indicator may produce a false-positive diagnosis of deltoid incompetence in Weber B supination-external rotation ankle fractures and possibly lead to unnecessary surgery.