A biomechanical model was developed to investigate the immediate effect of the Boston brace on the spine and thorax of two scoliotic patients (12 year old females with identical lumbar curves of 37 degrees and thoracic curves of 32 degrees and 25 degrees). This model is an improvement of Stokes model and incorporates several modifications concerning the modelling of costo-vertebral and costo-transverse joints. Forces generated by braces on the thorax were measured by pressure sensors and used as input into the finite element model. The deformed models were compared to the geometry of the same patients wearing their brace and with Stokes model. Concordance of results supports the modelling method. Differences between Stokes and improved models were observed. However, because of the small amount of patients included in this study, it is not possible to conclude on the effect of the modelling improvements.