The Effect of Direct Vertebral Rotation on the Uninstrumented Lumbar Curve in Thoracic Adolescent Idiopathic Scoliosis

Summary: One hundred six patients with thoracic adolescent idiopathic scoliosis were analyzed to determine the effect and direction of direct vertebral rotation of the lowest instrumented vertebra (LIV) on uninstrumented lumbar curve. In lumbar modifier B and C, screws at LIV have to be rotated opposite to the direction of screw rotation of main thoracic curve for better spontaneous lumbar curve correction. However, in modifier A, screws at LIV have to be rotated the same direction as screw rotation of main thoracic curve.

Objectives: To determine the effect and direction of direct vertebral rotation (DVR) of the lowest instrumented vertebra (LIV) on uninstrumented lumbar curve according to lumbar modifier.

Materials and Methods: One hundred six patients with thoracic adolescent idiopathic scoliosis treated by pedicle screw fixation and rod derotation were retrospectively analyzed. All patients who had a distal fusion level from T11 to L1 were divided into two groups; No-DVR group (n=52) treated without DVR vs DVR group (n=54) treated with DVR. Each group was divided into subgroups according to lumbar modifiers; No-DVR-A (n=23), B (n=14) and C (n=15) group vs DVR-A (n=26), B (n=16) and C (n=12) group. The DVR-A group was subdivided into two groups based on direction of screw rotation of LIV compared to that of main thoracic curve; first opposite to the direction (DVR-A-O group) and second the same direction (DVR-A-S group). In DVR-B and C groups, the screws of LIV were rotated in the opposite direction to screw rotation of main thoracic curve.

Results: There was no significant difference in the preoperative curve characteristics between the No- DVR and DVR groups. In the No-DVR group the preoperative lumbar curve of 34 ± 10° was corrected to 16 ± 9° (56% correction). In the DVR group the preoperative lumbar curve of 34 ± 9° was corrected to 11 ± 6° (69% correction). The DVR-A-S group showed better coronal correction of lumbar curve compared with No-DVR-A group. However, DVR-A-O group had the higher incidence of adding-on deformity. DVR-B and C group showed better spontaneous correction of lumbar characteristics than simple rod derotation groups.

Conclusions: In lumbar modifier B and C, screws at LIV have to be rotated opposite to the direction of screw rotation of main thoracic curve. However, in modifier A, screws at LIV have to be rotated the same direction as screw rotation of main thoracic curve.