Understanding the superior clear space in the adult ankle

Background: The width of the medial clear space often is used to determine the integrity of the deltoid ligament, the primary medial stabilizer of the ankle joint. The normal clinical relationship of the superior clear space to the medial clear space is not well described. This investigation sought to determine if the superior clear space constitutes an accurate point of comparison for the medial clear space and a means for assessing ligamentous stability in an adult ankle. Methods: A retrospective review of consecutive ankle radiographs for a 4-month period of time was completed using a university-based radiology database. Using a digitally calibrated ruler, the widths of the medial and superior clear spaces were measured on the mortise view. These values were compared using a Student's t-test. Results: Digital radiographs of 564 consecutive ankles were reviewed retrospectively and 94 cases were without evidence of trauma, surgery, or degenerative disease. The medial and superior clear spaces were measured on the mortise view and found to be 2.7 mm (standard deviation 0.5; range 1.3 mm to 4.3 mm; 95% confidence interval 1.7 mm to 3.8 mm) and 3.6 mm (standard deviation 0.6; 2.0 to 5.3; CI 2.4 mm to 4.7 mm), respectively. The average absolute difference was 0.9 mm (standard deviation 0.5; -0.7 to 1.5; CI -0.1 mm to 1.8 mm) and in 92 of 94 ankles (98%), the superior clear space was greater than or equal to the medial clear space. Conclusions: Understanding the normal radiographic relationship of the superior and medial clear spaces may help in the diagnosis of ligamentous instability in the ankle and may obviate the need for additional diagnostic tests.