In the most commonly occurring form of scoliosis in clinical practice, adolescent idiopathic scoliosis, there is no apparent malformation of the vertebrae. The process of segmentation that gives rise to somites, followed by resegmentation giving rise to vertebrae, and the subsequent formation of the vertebrae all appear normal. Abnormal formation of the vertebrae, for example, in some of the skeletal dysplasias that give rise to platyspondyly, may not necessarily be accompanied by scoliosis. In conditions with segmentation defects of the vertebrae (SDV), scoliosis is a frequent feature, though to a very variable and unpredictable degree. These conditions frequently present as