Musculoskeletal multibody modeling can offer valuable insight into aetiopathogenesis behind adolescent idiopathic scoliosis, which has remained unclear. However, the underlying model should represent anatomical joints with compatible kinematic constraints while allowing the model to attain scoliotic postures. This work presents an improved and kinematically determinate model including the whole spine and ribcage, which can attain typical scoliosis deformations of the thorax with compatible constraint strategy and simulate the interaction between all the bony segments of the ribcage and the spine. In the model, costovertebral/costotransverse joints were defined as universal joints based on reported anatomical studies.Articulations between ribs and the sternum were defined as spherical joints except in the ninth and tenth levels, which have one additional anteroposterior degree-of-freedom. The model is controlled by fifteen kinematic parameters including spinal rhythms and parameters relating to clinical metrics of scoliosis. These input values were measured from the bi-planar radiographs of a 17-year-old scoliosis patient with a right main thoracic curve of 33° Cobb angle. Dependent kinematic variables with clinical relevance were selected for validation purposes and compared with measurements from radiographs. The average errors of ribvertebra angles, rib-vertebra angle differences, and rib humps were 6.3° and 10.5°, and 8.7mm. The model appeared to reproduce the spine and rib deformation pattern conforming to radiographs, results in simulating the rib prominence, rib spread, rib-vertebra angles, and sternum orientation, therefore supporting the constraint definitions. The model can subsequently be used to investigate the kinetics of scoliosis and contribute to uncovering the aetiology.

The use of patient-specific biomechanical models offers many opportunities in the treatment of adolescent idiopathic scoliosis, such as the design of personalized braces. The first step in the development of these patient-specific models is to fit the geometry of the torso skeleton to the patient’s anatomy. However, existing methods rely on high-quality imaging data. The exposure to radiation of these methods limits their applicability for regular monitoring of patients. We present a method to fit personalized models of the torso skeleton that takes as input biplanar low-dose radiographs. The method morphs a template to fit annotated points on visible portions of the spine, and it relies on a default biomechanical model of the torso for regularization and robust fitting of hardly visible parts of the torso skeleton, such as the rib cage. The proposed method provides an accurate and robust solution to obtain personalized models of the torso skeleton, which can be adopted as part of regular management of scoliosis patients. We have evaluated the method on ten young patients who participated in our study. We have analyzed and compared clinical metrics on the spine and the full torso skeleton, and we have found that the accuracy of the method is at least comparable to other methods that require more demanding imaging methods, while it offers superior robustness to artifacts such as interpenetration of ribs. Normal-dose X-rays were available for one of the patients, and for the other nine we acquired low-dose X-rays, allowing us to validate that the accuracy of the method persisted under less invasive imaging modalities.