A critical analysis of the anterior-posterior radiographic anatomy of the ankle syndesmosis.

To determine the normal anatomic radiographic land-marks of the ankle syndesmosis, standardized anterior-posterior radiographs of the right ankle were performed on 40 male and 40 female volunteers. The average tibiofibular clear space was 3.8 mm in females, 4.6 mm in males, and 4.2 mm overall. The tibiofibular overlap measured 6.0 mm in females, 9.6 mm in males, and 7.8 mm overall. Due to this variability and the gender differences, we investigated the anatomy of the syndesmosis as ratios of the potentially variable values to fixed landmarks. The ratio of the tibiofibular overlap to the fibular width averaged 54% and the ratio of the tibiofibular clear space to the fibular width averaged 30%, with no statistically significant difference due to gender. Our data show that for 90% prediction intervals, the values are: (1) tibiofibular clear space less than 5.2 mm in women and 6.5 mm in men; (2) tibiofibular overlap of greater than 2.1 mm in females and 5.7 mm in males; (3) tibiofibular overlap:fibular width ratio greater than 24%; (4) tibiofibular clear space:fibular width ratio less than 44%. Additionally, using a linear regression model, a prediction of the tibiofibular overlap can be made when using the distance (in millimeters) from the incisura fibularis to the lateral tibial (LT) border: tibiofibular overlap = 0.862 x lateral tibia - 2.62 (P = .0001). Utilization of these values will help in the determination of posttraumatic disruption of the syndesmosis and postoperative assessment of mortise reduction.