The normal size and shape of the spine is different from person to person and no-one's spine is completely straight.
When viewed from the side the spine has four normal curves (see "Your Back"). The cervical region has a lordotic posture with the mid portion further forward than the upper and lower regions. The thoracic region has a kyphotic profile. This is the reverse of a lordosis with the mid portion curved towards the back. The lumbar region is again lordotic, and the profile of the spine is completed by the kyphotic profile of the sacrum and coccyx.
Any of these curves may be or become exaggerated for a variety of reasons. In the thoracic region the kyphosis is usually less than 40°, but some "normal" individuals with have a thoracic kyphosis of up to 60°. Where the kyphosis is excessive (> 90°) there can be limitation of respiratory function, but this in an uncommon situation.
Patients who suffer from inflammatory conditions such as Ankylosing Spondylitis have a tendency to develop a progressive, and in some cases severe kyphotic deformity of all regions of the spine. This type of deformity can cause quite marked functional limitations, and for sever cases surgery can be very effective way of resorting a more normal spinal contour and will literally improve these patients outlook on life.
The spine is usually straight when viewed from either in front or from behind. Where there is a deviation from this normal posture there may be a scoliosis.
True scoliosis is not just a curve to the side but the vertebrae at the apex of the curve are also twisted or rotated which affects the ribs and results in the typical clinical features of unilateral rib or scapular prominence.
Spinal deformities are either postural (flexible and not fixed) or structural. Postural deformity usually develops as a result of pain or poor muscular support and corrects with attention to the underlying problem. A structural deformity on the other hand is one where secondary structural changes develop in the skeleton that limits the degree of correction that can be achieved.