Weightbearing CT Assessment of Medial Clear Space Volume Changes in Weber B Fractures: A Preliminary Analysis.

Background: The clinical treatment of Weber B fibular fractures has primarily focused on the symmetry of the medial clear space. Numerous radiographic studies demonstrate no change in tibiotalar contact area until the deltoid ligament is disrupted. In contrast, more recent studies using weightbearing CT scan (WBCT) and 3D volume analysis suggest that medial clear space (MCS) volume can in fact increase in the setting of a Weber B fibular fracture even if initial radiographs are reassuring, possibly because of a loss of fibular buttress. This study aims to evaluate the ability of WBCT to quantify the impact of isolated Weber B fractures on MCS volume (3D), as well as understand the implication of increasing fibular displacement among patients with a symmetric MCS distance (1D) on initial radiographs.

Methods: The study group included 18 patients with unilateral Weber B ankle fractures who underwent preoperative bilateral foot and ankle WBCT. The control group consisted of 60 patients with forefoot/midfoot conditions without ankle injury who underwent similar imaging. Measurements on WBCT images included (1) MCS distance; (2) syndesmotic area; (3) anterior, middle, and posterior distal tibiofibular distance; (4) fibular rotation; (5) distance from fibular tip to plafond; and (6) fibular fracture displacement. Additionally, volumetric measurements included (1) MCS volume, (2) syndesmotic joint volume from the tibial plafond extending to 3 cm and 5 cm proximally, and (3) lateral clear space volume were calculated. Area under the receiver operating characteristic (ROC) curve (AUC) analysis and Delong test were used, and optimal cutoff values to distinguish between stable and unstable Weber B fractures without syndesmotic instability and MCS widening were determined using Youden J statistic.

Results: Among patients with unilateral Weber B ankle fractures, all WBCT measurements showed no side-to-side difference in any parameter, except MCS volume (P values < .001). When compared to the uninjured side, fibular displacement up to 2 mm and up to 4 mm was associated with MCS volume increases of 37.1% and 51.8%, respectively, compared with the contralateral uninjured side. Based on ROC analysis and Youden J statistic, a fibular fracture displacement of 2.3 mm was identified as the optimal threshold associated with a significant increase in MCS volume (AUC, 0.81; sensitivity, 77.8%; specificity, 80%). The control group showed no side-to-side difference in any of the measurements.

Conclusion: In this preliminary study using WBCT, 3D MCS volume measurements showed statistical differences in Weber B fibular fractures compared with the contralateral side, even when 1D distance measurements appeared symmetric. A displacement threshold of 2.3 mm was identified through ROC analysis, although the clinical significance of these volumetric changes remains unclear and requires validation through clinical outcome studies.

Level of evidence: Level III, comparative diagnostic study.