Study Design

This comparative, retrospective, and nonselective study analyzed existing data recorded prospectively as part of routine clinical care. All procedures were performed in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Ethical approval from the relevant Institutional Review Board was obtained (IRB number OS-RGDS-2017-10-001).

Study Population

Datasets were obtained for 370 consecutive feet (189 patients) referred to a specialized orthopedic surgery center in our institution between July 2016 and October 2018, who underwent a WBCT investigation. Weight bearing cone beam CT was performed on patients requiring comparative weight bearing imaging when adequate imaging had not already been provided elsewhere. Indications to perform a WBCT were the same as a conventional comparative bilateral radiographic assessment.

History of CLAI was based on medical history, clinical symptoms, physical examination, and imaging studies including radiography (weight-bearing anteroposterior and lateral views, varus stress test, radiological anterior drawer test), arthro-CT, or MRI and ultrasonography, according to each patient’s follow-up. For this study we included patients who had reported at least three repeated episodes of ankle “giving way” in the previous 6 months, complaining of ankle instability associated with pain or not, with at least one prior initial ankle sprain requiring active treatment. Patients who had pain and discomfort but no history of ankle giving way were not diagnosed with CLAI. Clinical examination was performed by a senior orthopedic surgeon and was independent from hindfoot alignment measurements.

Out of 378 feet investigated through WBCT, one foot from a patient with medial ankle instability, one with syndesmotic instability, and those (six) who had a history of hindfoot realignment surgery or traumatic malalignment were excluded. Feet were divided into two groups: (i) patients with CLAI and (ii) all other patients. Out of 370 feet, 43 (12%) had CLAI, in 34 patients (18%). No extra investigation other than the standard of care in our institution was required in this study.

Investigations and Measurements

WBCT scans were performed using a PedCAT™ unit (CurveBeam LLC, 175 Titus Ave, Suite 300, Warrington, PA 18976, USA) installed in the outpatient department. The datasets were obtained using the following cone beam scanner settings: voxel size, 0.37 mm; field of view diameter, 350 mm; field of view height, 200 mm; exposure time, 9 sec; and total scan time, 54 sec. The datasets were extracted from the existing database, containing the 3D image data and patient demographics including age, sex, body mass index (BMI), and history of CLAI and other clinical conditions. Datasets were screened using the manufacturer’s visualization (CubeView™) and measurement (Talas®) softwares, and FAO was calculated using specific software, as described previously [12]. FAO is a semiautomated tool that measures offset between the center of the ankle joint and the center of the foot weight-bearing surface (Fig. 8.1). The offset is given as a percentage to normalize its value according to foot length. In the first published study, normally aligned feet had a FAO value of 2.3 ± 2.9% [12]. We also recorded the calcaneal offset (CO) and hindfoot angle. CO represents the distance (in mm) between a theoretically neutral position of the calcaneus (in terms of mechanical lever arm at the level of the ankle) and the actual position of the calcaneus. Hindfoot angle is an angle whose endpoints are the apex of the center of the talar dome projected on the ground plane (as the vertex), the ideal position of the calcaneus, and the actual position of the calcaneus, which is comparable to mainstream hindfoot angles used on 2D traditional radiographs.

Statistical Analysis

Data are reported as mean, standard deviation, and range values (min-max). Univariate analysis was conducted to compare patients with and without CLAI against the following variables: sex (Fisher test) and age and BMI (Student’s t-test). To compare feet with and without CLAI against FAO, generalized mixed model was used to take into account correlation between the 2 feet of the same patient, unadjusted and adjusted on age and sex. A subgroup analysis was performed to assess the relationship between (1) a negative FAO and clinical varus and (1) a negative FAO and diagnosis of CLAI (Fisher test). p = 0.05 was considered significant. All statistical analyses were performed by an independent statistician using SAS for Windows (Version 9.4; SAS Institute Inc.).