The curves allow the human spine to better stabilize the body in the upright position.The upper cervical spine has a curve, convex forward, that begins at the axis (second cervical vertebra) at the apex of the odontoid process or dens and ends at the middle of the second thoracic vertebra; it is the least marked of all the curves. This inward curve is known as a lordotic curve. The thoracic curve, concave forward, begins at the middle of the second and ends at the middle of the twelfth thoracic vertebra. Its most prominent point behind corresponds to the spinous process of the seventh thoracic vertebra. This curve is known as a kyphotic curve. The lumbar curve is more marked in the female than in the male; it begins at the middle of the last thoracic vertebra, and ends at the sacrovertebral angle. It is convex anteriorly, the convexity of the lower three vertebrae being much greater than that of the upper two. This curve is described as a lordotic curve. The sacral curve begins at the sacrovertebral articulation, and ends at the point of the coccyx; its concavity is directed downward and forward as a kyphotic curve. The thoracic and sacral kyphotic curves are termed primary curves, because they are present in the fetus. The cervical and lumbar curves are compensatory, or secondary, and are developed after birth. The cervical curve forms when the infant is able to hold up its head (at three or four months) and sit upright (at nine months).
Surgery
Controversy has arisen regarding the selection and planning of surgical procedures, the timing of surgery and about what constitutes a favourable outcome. 1. Selection and planning
Some ophthalmologists, notably Ing  and Helveston, favour a prescribed approach often involving multiple surgical episodes whereas others prefer to aim for full alignment of the eyes in one procedure and let the number of muscles operated upon during this procedure be determined by the size of the squint. 2. Timing and outcome
This debate relates to the technical anatomical difficulties of operating on the very young versus the possibility of an increased potential for binocularity associated with early surgery. Infants are often operated upon at the age of six to nine months of age and in some cases even earlier at three or four months of age. Some emphasize the importance of intervening early such as to keep the duration of the patients abnormal visual experience to a minimum. Advocates of early surgery believe that those who have their surgery before the age of one are more likely to be able to use both eyes together post-operatively. A Dutch study (ELISSS) compared early with late surgery in a prospective, controlled, non-randomized, multicenter trial and reported that:

Children operated early had better gross stereopsis at age six as compared to children operated late. They had been operated more frequently, however, and a substantial number of children in both [originally-recruited] groups had not been operated at all. Other studies also report better results with early surgery, notably Birch and Stager and Murray et al.