## 1 Introduction  
Adolescent idiopathic scoliosis (AIS) is an abnormal curvature of the spine that occurs in 1.7 to 2.9% of the general population (for AIS with a Cobb angle over 10\({}^{\circ}\)) [1, 2, 3, 4, 5]. Brace treatment is the most common non-surgical intervention for the management of AIS. It involves fitting a low-profile spinal orthosis around the patient's torso and pelvis with the aim of controlling curve progression. Traditionally, scoliosis braces are either created using a patient-specific plaster-cast or by using prefabricated modules that fit the patient geometry [6]. Currently, the most widely used braces for the treatment of AIS are thoracic lumbar sacral orthosis (TLSO) that have high patient acceptance and tolerance [7]. Although there is controversy surrounding the efficacy of bracing AIS patients, TLSOs have been found to decrease the risk of curve progression and can protect against the need for surgery in sketally immature AIS patients wearing a brace with high compliance [8, 9, 10, 11, 12, 13, 14]. The design of a plaster mould TLSO is divided in two steps. First, the creation of a positive mould from the cast of the patient's torso and, second, the rectification of the positive mould [15]. The rectification of the positive mould involves both removing and adding plaster in areas of the torso and pelvis to generate the desired mould geometry from which the brace shape is thermoformed. The rectification process is based on the orthotist's experience to achieve optimal in-brace spinal correction. Despite the wide use of this technique, there are downsides of manual mould rectification in that casting requires a significant amount of materials, tools, and time [16]. On average, it takes 60 min to cast a patient [17]. Furthermore, the geometry of the rectified mould is highly dependent on the orthotist expertise, and the manufacturing process involves the use of large equipment [18]. Recent technological advances have given the opportunity for alternative approaches to manual casting and rectification to be developed. These include employing computer-aided design/computer-aided manufacturing (CAD/CAM) and 3D imaging to generate a virtual geometry of the patient's torso [19; 20]. Three-dimensional imaging approaches, such as 3D scanning, present a promising technology for digital design and biomedical modelling applications using reverse engineering methods [21]. These methods can be used for digital rectification [22], positive mould manufacturing [15], and brace design [19; 23; 24], resulting in a reduction in the time required to generate a brace by 50% [18]. Several studies have compared braces developed using the plaster-casting method against CAD/CAM braces, finding either no significant difference [15; 22; 25; 26] or even better in-brace Cobb angle correction when using CAD/CAM methods [24]. The prospects of implementing a digital design process can go beyond the improvement of the geometry acquisition phase eliminating the need for casting. Manufacturing processes using additive manufacturing techniques present a promising solution to study the use of alternative materials to enhance the functionality, mechanical properties of the design [27], and use recycled materials [28]. Digital design of TLSOs allows for adaption of the design to create a new brace for the same patient when required by skeletal growth, compared to the plaster-casting method, for which a new cast must be completed to obtain the patient's geometry. Despite the implementation of CAD/CAM methods in the design process, the rectification process still requires input from an orthotist to ensure an effective brace design. This involves the orthotist using rectification software, adding, or removing material digitally. Although the input of the orthotist is likely to always be required, semi-automation of this process may lead to a more efficient and repeatable design processes. Before this is possible, a detailed analysis of the manual rectification process is required.