Drawings of the six types of vertebral anomaly as defined by , with explanation of the different types . Figure 1 has been reproduced with kind permission of Springer Science and Business Media. Type 1 is characterized by an individual surplus half vertebral body. It is mostly round or oval and localized between two adjoining vertebral bodies. In the course of time, it often merges with one or both adjacent vertebral bodies. In the thoracal region, the surplus vertebral body is associated with an additional rib and regular vertebral arch oval. Type 2 can be a wedge-shaped hemivertebra or a wedge-shaped vertebra. It mostly has a triangular configuration. It is not associated with an additional rib at the thoracic spine. Subtype 2a (wedge-shaped hemivertebra) represents a hemivertebra for which the ontogenesis of the other side of the vertebral body as well as of the neural tube has completely failed. Subtype 2b (designated by a wedge-shaped vertebra) is a hemivertebra in which one side of the vertebral body and of the neural tube is underdeveloped. Type 3 is characterized by multiple hemivertebra that can be round, oval, or wedge-shaped. Type 4 is defined by appearance of multiple hemivertebra that are fused on one side (so called unilateral bar). There is a further differentiation into two subtypes. Subtype 4a is characterized by multiple hemivertebrae with one-sided fusion of vertebral bodies and of the posterior elements of the vertebral body on the concave side. Subtype 4b is characterized by multiple wedge-shaped vertebrae with developing one-sided fused vertebrae. Type 5 represents balanced hemivertebrae, localized in such a way that the deforming effects are neutralized by each other. In this way, an extreme scoliosis form can be avoided. Type 6 is defined by posterior hemivertebrae that lead to a progressive kyphosis rather than a scoliosis. A kyphosis arises when the anterior part of the vertebral body does not develop as an independent unit.