## 2 Materials and Methods
### Study Design and Measurement Protocol  
A prospective, observational study of 42 selected frontal spinal radiographs of patients with AIS, with an equivalent image quality and no defects (e.g., image noise or inadequate voltage), was performed. Radiographic images were collected from an image repository retrospectively, during the routine medical care of AIS patients. Our study followed the ethical guidelines of the World Medical Association Declaration of Helsinki, as revised in 2013. The study was considered exempt from requiring  
Figure 1: Schematic description of the anatomical references selected by the observer and the distances calculated by the software. D: width of the vertebral body. d: distance from the centre of the pedicle to the side of the vertebral body. \(\varphi\): angle of axial vertebral rotation. Adapted with permission from Nature/Springer, European Spine Journal, [PERSON], [PERSON] [PERSON], [PERSON]. A review of methods for quantitative evaluation of axial vertebral rotation (2009). ethical approval as complete and irreversible anonymisation of the images did not involve data processing. The radiographs were natively in digital format (in DICOM format, with 283.46 pixels/mm resolution). The selected radiographs showed mild scoliosis (with curves between 11\({}^{\circ}\) and 20\({}^{\circ}\), 7 cases), moderate scoliosis (between 21\({}^{\circ}\) and 40\({}^{\circ}\), 19 cases), and severe scoliosis (more than 40\({}^{\circ}\), 16 cases). The radiographs belonged to 34 females and 8 males, with an average age of 14.9 years (SD 5.5; range 9 to 37 years). In each radiograph, each observer identified the vertebra in the most rotated major curve and the most rotated vertebra in the minor or major compensatory curve, if any, resulting in a total of 74 vertebrae in the 42 radiographs (T3, 3; T4, 6; T6, 2; T7, 8; T8, 9; T9, 8; T10, 5; T11, 1; T12, 2; L1, 12; L2, 16; L3, 2. 44 thoracic and 30 lumbar vertebrae total). The study was carried out with twelve independent evaluators with different experience levels in using [PERSON]'s method. A 5 h briefing was held before the software measurements, with comprehensive information on the study and training in the software that was used. Each observer measured the 42 X-rays with the software on three occasions, which were separated by one month. The sequence of presentation of the radiographs was randomly assigned in each of the measurement rounds by the study coordinator, who kept the randomisation key confidential. Since the number of thoracic (44) and lumbar (30) vertebrae were different, in order to have the same number of cases, the following 30 thoracic vertebrae, numbered as follows, were randomly selected: V3, V6, V8, V13, V15, V16, V19, V20, V21, V26, V30, V34, V36, V39, V47, V48, V50, V54, V55, V57, V59, V60, V61, V62, V63, V65, V66, V68, V71, V74. To establish a gold standard (Table 1), a specialist in orthopaedic surgery, a specialist in physical medicine and rehabilitation (both with more than 35 years of professional experience in the field of the spine, as well as regular users of the [PERSON] method), and the engineer who designed the software measured together, as well as on the same computer, all the vertebrae. This gold standard made it possible to compare the observers' AVR values that were obtained for each vertebra (the average of each set of 36 measurements obtained by the 12 observers).