Impact of Weber B Fracture Displacement on Medial Clear Space Volume: How Much Displacement Matters?

CATEGORY: Ankle; Sports; Trauma INTRODUCTION/PURPOSE: Numerous radiographic studies examining Weber B fibular fractures demonstrate no change in tibiotalar contact area until the deltoid ligament is disrupted. In contrast, more recent studies using weight-bearing 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 due to 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 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 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 3cm and 5 cm proximally, and 3) lateral clear space volume were calculated. Area under the receiver operating characteristic (ROC) curve 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's 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 <0.001). When compared to the uninjured side, fibular displacement up to 2cm and up to 4cm increased lateral MCS volume by 37.1% (0.23cm3) and 51.8% (0.3cm3), respectively. Based on ROC analysis and Youden's J statistic, the optimal threshold for differentiating stable from an unstable Weber B fracture was 2.3 mm of displacement (AUC 0.81; sensitivity 77.8%; specificity 80%). The control group showed no difference in any of the measurements between the two sides. CONCLUSION: When using WBCT, 3D MCS volume increases significantly with Weber B fibular fractures, even if 1D distance measurements at the medial clear space are reassuring. This is especially true with fibular displacement greater than 2.3mm and may have an important role in the management of SER II ankle fractures.