Relationship among radiographic ankle medial clear space, sex, and height

The ankle medial clear space (MCS) is frequently measured to evaluate ankle stability after an injury. However, controversy exists regarding a threshold size that distinguishes a normal MCS from an abnormal MCS. A retrospective radiographic review of mortise ankle radiographs in the uninjured ankle was performed, with the goal of defining the relationship among patient height, sex, and radiographic ankle MCS. Forty-nine patients with normal mortise radiographs and with information on patient height available in the electronic medical chart were identified for inclusion. For men, mean±standard deviation (in millimeters) was 3.3±0.8 for MCS perpendicular (MCSp), 3.8±0.7 for MCS oblique (MCSo), and 3.8±0.5 for superior clear space (SCS). For women, mean±standard deviation was 2.3±0.6 for MCSp, 2.9±0.5 for MCSo, and 3±0.4 for SCS. Univariate analysis showed that all 3 variables (MCSp, MCSo, and SCS) were statistically different when men were compared with women (P<.0001). Bivariate regression models showed statistically significant (P<.001) positive relationships between each of the measures of clear space and height. In multivariate analysis, female sex alone was associated with a decrease in clear space. When evaluating isolated lateral malleolus fractures, clinicians should consider the patient's height and sex when measuring MCS and SCS to determine deltoid ligament competence. These data suggest that men and people of tall stature are at risk for a false-positive diagnosis of deltoid ligament rupture when previously published threshold MCS and SCS values, such as 4 mm or 5 mm, are used for diagnosis and operative indication.