The standard clinical measurement for adolescent idiopathic scoliosis is the Cobb angle, measured from the end-plates of the end vertebral bodies in a standing radiograph. This measurement of anterior column structures describes the anterior spinal deformity. The posterior spinal deformity can be described by the “spinous process angle,” measured from a curve joining the tips of the spinous processes. A computer model, and a radio-graphic study of Cobb angle, spinous process angle and vertebral rotation show that adolescent idiopathic scoliosis results in larger angulations of the anterior elements than posterior elements. This helps to explain some of the inherent limitations of posterior instrumentation, including Cotrel-Dubousset instrumentation, and of noninvasive posterior surface measurement systems.