-Age of the outbreak
According to the age, we distinguish the following forms of idiopathic scoliosis:

• Infantile………….0-3 years

• Juvenile ………...4 years – beginning of puberty (on the average 10 years in women and 12 years in men)

• Teenage …beginning of puberty – bone maturation (10-16 years)

-The infantile variety (2%) (which must not be confused with the congenital one) usually appears in the first year of life; uncommon, in most cases it is a thoracic curve which affects mainly men, by far more frequent in England, clearing up spontaneously without any treatment (resolving structural scoliosis); in a few cases, it progresses in the growth up to seriously rigid structural curves with an unfavourable prognosis (fig. 3): besides the presence of the hump, the clinical test shows a – sometimes serious – deformation of the head and face (plagiocephaly) with the skull in the form of a parallelogram and a flattening of the forehead, which is consequently set back (fig. 4)

-The juvenile variety (5-10 %) appears most frequently between 6 and 8 years, stopping during the evolution and re-evolving at the beginning of puberty.

-The teenage variety ( 80-85 % ) coincides with the beginning of the pubertal growth (10 years in women and 12 years in men on the average), evolving in this period, which is called precisely of the rapid pubertal growth of the backbone, lasting approx. 4-5 years; it is during this time interval when the sexual development takes place, i.e. the menarche in a girl and the change in the tone of the voice in a boy as well as the osseous maturity of the vertebral column.

-Seat and morphology of the curves
Form the clinical and radiological point of view, idiopathic scoliosis can have various types of curves which diversify in (fig. 5):

• Simple thoracic with apex T8 or T9;

• Simple thoracolumbar with apex T11 or T12 or L1

• Simple lumbar with apex L2 or L3;;

• Double thoracolumbar with two curves of opposite direction;
  
• Double thoracic (described by Moe), uncommon;
• Some others, which, however, are very uncommon.

The aesthetic deformity varies according to the curve: it is more serious in the thoracic and simple thoracolumbar curve, less serious in the curves of the double thoracolumbar, where the humps, the shoulders and the pelvis balance each other and the only alteration present is the shortening of the trunk.