1. Surgical technique

All surgeries were performed using the pedicle screw instrumentation system. Fusion was usually performed from one level proximal to the upper-end vertebra to one level caudal to the lower end vertebra [^5,7]. Single thoracic curves (Lenke type 1) were treated with selective thoracic fusion and double thoracic curves (Lenke type 2) by double thoracic fusion. In the DVR group, the distal fusion level was T11 in four patients, T12 in 24, and L1 in 26. In the No-DVR group, the distal fusion level was T11 in five patients, T12 in 27, and L1 in 20.

The technique of the surgical procedure in the DVR groups (Fig. 1) was as follows. After posterior exposure and facetectomy, pedicle screws were inserted segmentally on the correction side (thoracic concavity) and every other or third vertebra on the support side (thoracic convexity). A precontoured rod was inserted into the correction side and rotated in a clockwise direction (from the cephalad view) without any compression or distraction. Specially designed screw derotators (4–8 derotators) were connected to the heads of the pedicle screws in the apical and periapical vertebrae on the correction side. After rod derotation, the connected screw derotators were rotated to improve the rotation of the apical and periapical vertebrae, opposite to the rod derotation. In one or two uppermost instrumented vertebrae, screws were rotated to correct the rotation of the upper uninstrumented curve, usually clockwise and opposite to the apical and periapical screw rotation. Screw rotation in the LIV was dependent on the preoperative lumbar modifier. For lumbar modifier A, screw rotation in the LIV was in the opposite direction (Fig. 3) or same direction (Fig. 4) to that of thoracic DVR, as stated above. In lumbar modifiers B and C (Fig. 5), the screw in the LIV was rotated clockwise in the direction opposite to that of the thoracic DVR to correct vertebral rotation (counter-clockwise) of the lumbar curve. In modifier C, the screws in two LIVs were rotated with greater torque. After locking the concave rod in the corrected position, the other rod contoured to the corrected curve was inserted into the convex side and locked in place. Intraoperative portable radiographs were taken to evaluate the instrumented thoracic curve and uninstrumented lumbar curve. If the coronal and rotational corrections of the lumbar curve were inadequate, both screws in the LIV were unlocked and rotated further to obtain better correction. In lumbar modifier C, the screws in the two lowermost instrumented vertebrae were used for DVR. After finishing DVR, the two rods were usually joined by two transverse connectors. Then, posterior fusion was performed with bone graft.