Syndesmosis injuries often occur during a supination-external rotation of the foot with weight-bearing and account for 1%–20% of all ankle sprains [1,2]. They are recognized as serious injuries with long-term impacts on athletes [3]. However, these injuries are often overlooked in cases without ankle fractures owing to diagnostic difficulties [4]. Chronic pain and instability may persist during delayed diagnosis, thus increasing the risk of ankle osteoarthritis [5]. Therefore, an accurate method for diagnosing syndesmosis injuries should be developed in the clinical setting.

Previously, plain radiographs have been utilized to evaluate syndesmosis injuries. This evaluation is relatively easy to confirm with short imaging time, however, its interpretation has been problematic due to the difficulty of standardizing ankle rotation and lack of reliability [6]. Subsequently, magnetic resonance imaging (MRI) and computed tomography (CT) have been often employed for the diagnostic imaging of syndesmosis injuries. MRI can robustly evaluate signal changes associated with the anterior inferior tibiofibular ligament (AITFL) and interosseous injuries [7]. Further, it can be used to diagnose the widening and subluxation of the tibiofibular joint [7]. CT can simultaneously evaluate fractures that often occur with syndesmosis injuries. However, these imaging methods are expensive and pose the risk of radiation exposure. In contrast, ultrasonography (US) does not pose such challenges and it is a user-friendly diagnostic tool. US allows for a quantitative evaluation; therefore, it can assess syndesmotic instability by measuring the anterior tibiofibular gap (ATFG), which is the distance between the tibia and fibula along the AITFL. US assessment of ATFG along the AITFL has been applied in previous studies because of its higher sensitivity and specificity compared to MRI for diagnosis of the ligament injury [8,9]. Despite an increasing number of reports using US for syndesmosis injuries, they predominantly comprise measurements in the non-weight-bearing position or cadaveric studies [[10], [11], [12]]. Since syndesmosis injuries occur in the weight-bearing position, in vivo dynamics of the distal tibiofibular joint should be evaluated in the standing posture. Malhotra et al. reported that the ATFG varies with and without weight-bearing on CT [13]. Consideration of US methods of evaluating ATFG may help reduce the risk of missing syndesmosis injuries without fractures. However, no US study has been performed to date. This gap necessitates quantifying ATFG changes with and without weight-bearing using US in healthy adults.

This study aimed to investigate the possibility of a US-based quantitative analysis of the ATFG, with and without weight-bearing. We hypothesized that the ATFG can be assessed using US under weight-bearing conditions and that its values are wider than those obtained under non-weight-bearing conditions.