Ankle sprains are the most common type of ankle trauma in daily life and sports activities [1]. The anterior talofibular ligament (ATFL) is injured in 80 % of cases. In addition, up to 40 % of patients develop chronic ankle instability one year after injury [1]. The quantitative evaluation of ankle instability provides essential information in determining the indications for surgery, evaluating postoperative results, and comparing the effects of different surgical techniques [2]. Stress ultrasonography has attracted attention for quantifying ankle instability. In stress sonography, instability is quantified by measuring the ATFL length before and after the anterior drawer or other types of stress such as internal rotation of the ankle. Stress can be applied manually [[3], [4], [5], [6], [7], [8]] or using devices [9,10]. Manual stress is preferred for clinical studies involving a large number of patients because it can be performed easily in an examination room [[6], [7], [8]]. Furthermore, stress devices are not always available [5].

Operator dependence is an issue in ankle stress sonography, particularly when manual stress techniques are used. The difference in the stress technique is a factor that influences measurement variability [3,4]. Indeed, various methods such as ankle anterior drawer, internal rotation, plantarflexion, and their combinations have been reported [[3], [4], [5], [6], [7], [8],11]. Furthermore, stress can be applied by pushing the lower leg posteriorly against the fixed ankle instead of pulling the ankle anteriorly [12]. Other factors, including patient position (knee and ankle flexion angles), amount of stress applied, image depiction, and measurement method of the acquired image, can contribute to variability [9,10]. Studies have attempted to standardize sonography techniques and manual examinations to reduce the variability in other body regions [13,14]. Ankle stress sonography involves a unique challenge because the procedure requires standardization of both the sonographic technique and stress maneuver. No study has explored whether standardization reduces inter-examiner variability in quantitative stress sonography of the ankle.

Previous studies reporting the inter-examiner reliability of ankle stress sonography have several methodological issues. First, only two to four examiners trained in specific research were included in the study [4,5,8]. Because ankle ligament injuries are common, many physicians have the opportunity to perform stress sonography. Second, these studies have assessed the reliability using only subjects with an intact ATFL [4,5,8]. However, the measurement variability among examiners was greater in ATFL-injured patients than in ATFL-intact patients [15]. Therefore, a study involving a larger number of examiners and patients with ATFL injuries is necessary to clarify the inter-examiner variability experienced in clinical practice.

This study aimed to clarify whether standardization of sonographic techniques reduces inter-examiner variability in the quantitative evaluation of ankle stress sonography. We hypothesized that the variance would decrease after standardization.