Although, the history of orthotic treatment for idiopathic scoliosis goes back more than fifty years, the mechanism of curve control by spinal orthosis is still controversial. Hypothetical explanations have been provided but few, if any, have been tested clinically. This study aims at the biomechanical evaluation of a spinal orthosis (Milwaukee brace) in order to improve understanding about the mechanism of curves control in orthotic movement. From the results of the study, the change of the interface pressure between the patient's body and thoracic pad, and the tension of the thoracic strap were highly correlated (r = 0.84) as patients performed different lying postures and daily activities. Lying on the thoracic pad is found to have the highest correctional force among different lying postures that may be favourable for preventing curve deterioration. The findings indicate that an increase in tension of the thoracic strap will increase the interface pressure on the thoracic pad and thus increase the resultant force exerted on the patient's body by the thoracic pad. Care must be taken as an excessive strap tension will increase discomfort and restrict body shifting exercises. The results also suggest that in scoliosis with thoracic lordosis, a short outrigger (small pulling angle of the thoracic strap) should be used as it will decrease the anteriorly directed force component so as to prevent exaggerating the thoracic lordosis.