Physiologic widening of the medial clear space: What's normal?

Background: Literature has validated the use of stress radiographs for evaluation of ankle stability. However, to our knowledge no study has reported the amount of physiological widening that occurs with manual external rotation stress test in uninjured ankles. The purpose of this study was to assess the amount of medial clear space widening that occurs with a manual external rotation stress test in uninjured ankles.

Methods: A cohort of adult patients undergoing operative fixation of unstable ankle fractures were prospectively enrolled to have their contralateral ankle undergo manual external rotation stress examination. Fluoroscopic images of the unaffected ankle were performed in the OR. A non-stressed mortise view and manual external rotation stress view were obtained with a standardized marker to correct for magnification differences. The images were de-identified, presented in a randomized order and reviewers who were blinded. Each reviewer measured the medial clear space.

Results: Thirty fluoroscopic images on fifteen patients were obtained. The mean medial clear space on the non-stressed mortise view was 3.1 mm (SD-0.69; Range 1.9 to 4.2, 95% CI [2.75, 3.45]) versus a mean of 3.2 mm (SD-0.71; Range 2.0 to 4.7, 95% CI [2.94, 3.66]) in the stressed mortise view group. Inter-rater reliability was excellent between all observers for medial clear space (ICC-0.88; CI [0.78, 0.94]).

Conclusions: Our results support the previous literature and allow us to advocate for ankle fractures with >5 mm medial clear space after external rotational stress to be considered unstable. Additionally, ankles with a medial clear space between 4 and 5 mm, instability should be considered only if lateral shift is > 2 mm on stress examination. Our data shows that no physiologically healthy ankles widened beyond these established cut-offs before or after the manual external rotation stress.