Scoliosis is a medical condition in which a person's spinal axis has a three-dimensional deviation. Although it is a complex three-dimensional deformity, on an X-ray, viewed from the rear, the spine of an individual with scoliosis can resemble an "S" or a "?", rather than a straight line. 
Scoliosis is typically classified as either congenital (caused by vertebral anomalies present at birth), idiopathic (cause unknown, sub-classified as infantile, juvenile, adolescent, or adult, according to when onset occurred), or secondary to a primary condition.
 Secondary scoliosis can be the result of a neuromuscular condition (e.g., spina bifida, cerebral palsy, spinal muscular atrophy, or physical trauma) or syndromes such as Chiari malformation.
 Recent longitudinal studies reveal that the most common form of the condition, late-onset idiopathic scoliosis, is physiologically harmless and self-limiting even without treatment. Older beliefs that idiopathic scoliosis progresses into severe (cardiopulmonary) disability by old age have been refuted by later studies. The rarer forms of scoliosis pose risks of complications.