Fig. 22

One of the 3C compatible curve patterns is the single main thoracic curve going down into the lumbar region. We can describe this as a “long-low main thoracic curve,” confirming that the apex is still in the main thoracic region, and we call it A1 type. The second radiological criterion is the imbalance of the “transitional point” to the convex thoracic side according to the central sacral line. The transitional point (TP) corresponds to the middle point between the lower end vertebra of the main thoracic curve and the upper end vertebra of the caudal curve, in this case just a functional lumbar counter-curve, which has the real apex at S1. Thus, for this particular curve pattern, the TP is in the middle of the lower end vertebra of the main thoracic curve. To be A1 type, L5 has to be horizontal and neutral; L4 can be already tilted to the convex thoracic side and could be also mildly rotated to that side, but most frequently is horizontal and neutral as well; L3 must be already tilted to the convex thoracic side, and usually with a mild rotation to the same side. It is better not to classify A1 when L3 is horizontal and neutral, even with the main thoracic curve being long and with a low apex. The lower end vertebra is used to be L1 or L2, so L3 could be considered a vertebra of the upper part of a lumbar curve but it is not, because it is rotated to the convex thoracic side, being L4 or sometimes L5 the first “neutral” vertebra. It can be and it is often combined with a structural proximal thoracic curve (D modifier)