Background
The Selective thoracic fusion (STF) may be associated with risk of postoperative coronal decompensation, lumbar decompensation and adding-on phenomenon, which can lead to persistence of the lumbar curve and consequently to deviation of the trunk. Therefore, the STF is the most debatable issue as the optimal surgical correction in adolescent idiopathic scoliosis with Lenke 1C curves.
Methods
A total of 30 patients with adolescent idiopathic scoliosis with Lenke 1C curves who underwent STF between 1996 and 2017 were included. Minimum follow-up duration was five years. We analyzed the incidence of coronal decompensation, lumbar decompensation, distal adding-on phenomenon and trunk shift in these patients for radiographic adverse event. Clinical outcome was assessed by using the Scoliosis Research Society (SRS)-22r scores.
Results
The mean age at the time of surgery was 13.8±2.9 years. The mean follow-up duration was 80.4±12.3 months. The Cobb’s angle for main thoracic curve improved by 59.6% (p<0.001), and also The Cobb’s angle for thoracolumbar/ lumbar curve improved by 40.5% comparing preoperative and postoperative values (p<0.001). There was significant improvement in the Cobb’s angle for main thoracic and Thoracolumbar/lumbar curve comparing preoperative and last follow-up values (p<0.001). At last follow-up, the coronal balance was 10.3 ± 9.1 that significant improved from the immediate postoperative value (p=0.033). The incidence of coronal decompensation, lumbar decompensation, adding-on and trunk shift in our cohort was 16.7%, 10.0%, 13.3% and 10.0% respectively. The average SRS score at last follow-up in patients with radiographic adverse events was 4.3±0.5. That of patients without adverse events was 4.4±0.6. All domains between patients with and without adverse events had no statistical significance difference.
Conclusions
Selective thoracic fusion in Lenke 1C curves have acceptable risk of coronal decompensation, lumbar decompensation, distal adding-on, trunk shift. However, no revision surgery was required in these patients after long term follow-up. Therefore, STF in Lenke 1C curves seems to be enough.

Spinal sagittal balance is influenced by thoracic kyphosis, lumbar lordosis, as well as the position and angle of the pelvis. The abnormal position of sagittal imbalance causes easy fatigue and pain on the back muscles, so that the body voluntarily takes action to compensate for the imbalance. Compensatory mechanism occurs over several steps. It begins primarily in the mobile lumbar segments. When sagittal imbalance occurs due to decreased lumbar lordosis, the disc space at the mobile segments is hyperextended, allowing the C7 plumb to be restored within near normal range. As patients are older, subsequent degenerative disc changes develops at the hyperlordotic segments leading to loss of lumbar lordosis, which results in sagittal imbalance again. If the compensation in the spine is no longer able to restore the sagittal imbalance, posterior rotation of pelvis can reestablishes sagittal balance. This finding shows that the rotation of pelvis is important in the compensatory mechanisms. Another thing to consider is that compensation gets involved in the actual lumbar lordosis. During the compensation process rotation of pelvis determines the sagittal imbalance. This is also very difficult to estimate how much the rotation of pelvis is, because the rotation is variable depending on the position. Pelvic incidence (PI) is a suitable index to determine how much correction of lumbar lordosis requires, as this does not change depending on the position of the pelvis. Patients with high PI are able to compensate the sagittal imbalance more than them with low PI. As PI is the sum of pelvic tilt and sacral slope, PI is closely related to lumbar lordosis. Patients with high PI needs more correction of lumbar lordosis to restore sagittal imbalance than the patients with low PI.