To validate the surgical principles by analyzing the cause of a correction loss after pedicle subtraction osteotomy in patients with a sagittal imbalance in the radiological aspects, retrospectively.

Forty cases were analyzed radiographycally according to the presence (Group A) or absence (Group B) of the lower mobile segment and anterior column support performed (Group A2) or not (Group A1), respectively.

The overall correction loss of the total fused part appeared to increase in group A (p=0.025) and the degree of lordosis of the osteotomy site showed an almost zero correction loss in group A and B. No statistic difference of the correction loss of the upper segment of the osteotomy site between the two groups was found (p=0.138). The correction loss of the lower segment of osteotomy site increased statistically more in goup A (p=0.014). The correction loss in group A occurred more in the lower segment than in the upper segment and the correction loss of the lower segment in group A1 appeared to have a better correlation than group A2 (p=0.012).

Correction loss occurs at the intervertebral disc of the mobile segment. It is considered desirable to have anterior column support to all lower segment of the osteotomy site, because the correction loss increases more in the lower segment than in the upper segment.