Stress Radiographs after Ankle Fracture: The Effect of Ankle Position and Deltoid Ligament Status on Medial Clear Space Measurements

OBJECTIVE:This study was designed to determine 1) how ankle position affects the medial clear space by using stress radiographs, 2) which medial clear space measurement, overall width or increase in width, better predicts deep deltoid ligament disruption after Weber type-B distal fibular fracture, and 3) what value of medial clear space is most predictive of deep deltoid ligament disruption after Weber type-B distal fibular fracture. DESIGN:Cadaveric fracture model. SETTING:Biomechanics laboratory. INTERVENTION:Fluoroscopic mortise views were taken of 6 fresh cadaveric ankles mounted in a fixture permitting both positioning in neutral flexion, dorsiflexion, and plantarflexion, and the application of internal and external rotational forces. After destabilizing the ankles according to the supination-external rotation mechanism of Lauge-Hansen, repeat radiographs were taken with the same combination of flexion and applied rotational stress. MAIN OUTCOME MEASURE:Radiographic measurements of medial clear space width and changes in medial clear space were made. RESULTS:A medial clear space of ≥5 mm on radiographs taken in dorsiflexion with an external rotational stress was most predictive of deep deltoid ligament transection after distal fibular fracture. In dorsiflexion-external rotation, medial clear spaces of ≥4 mm yielded lower specificity and positive predictive value, whereas ≥6 mm yielded lower sensitivity and negative predictive value. All other stress conditions and increases in medial clear space of 2 or 3 mm were less predictive. CONCLUSIONS:Ankle stress radiographs taken in dorsiflexion-external rotation were most predictive of deep deltoid ligament disruption after distal fibular fracture. Under this stress condition, a medial clear space of ≥5 mm was the most reliable predictor of deep deltoid ligament status.