Study design: Retrospective analysis of spine and rods 3D shape in the 2 years after adolescent idiopathic scoliosis (AIS) posterior surgical instrumentation.
Objective: To evaluate spine and rod shape 3D changes in the 2 years after AIS instrumentation with different rod materials.
Summary of background data: Postoperative loss of correction has been reported with modern segmental instrumentation. The postoperative 3D rod shape changes of different rod materials and their implication in postoperative loss of correction have never been studied.
Methods: A total of 42 Lenke 1 AIS cases who underwent surgery all with pedicle screws and 5.5 mm rods of three different materials were retrospectively reviewed (14 cases for each of the three rod material subgroups: titanium, cobalt-chromium, stainless steel). 3D reconstructions of the spine were performed using biplanar radiographs preoperatively, 1-week postoperatively (1WPO), and 2 years follow-up (2YFU). Rods 3D shape also was reconstructed at 1WPO and 2YFU. The spine and rods shapes were analyzed and compared between time points.
Results: An average of 1.93 implant per vertebra was used. The main thoracic curve (61° ± 9°) was corrected on average by 75% (15° ± 6°, P < 0.01) with no change at 2YFU (17° ± 7°, P = 0.14). The apical vertebral rotation (23° ± 7°) was corrected by 44% (13° ± 9°, P < 0.01) with no change at 2YFU (14° ± 9°, P = 0.64). The thoracic kyphosis (24° ± 12°) remained unchanged (P = 0.78). The orientation of the planes of maximal curvature with respect to the sagittal plane of the main thoracic curve (39° ± 15°) and of the rods (concave: 28° ± 23°, convex: 12° ± 10°) at 1WPO was unchanged at 2YFU (all P > 0.05). Rod curvature and deflection also remained unchanged (all P > 0.05). 3D curve correction was maintained in the 2YFU for all rod materials subgroups (all P > 0.05). The lumbar lordosis changed from 1WPO (47° ± 8°) to 2YFU (56° ± 9°, P < 0.01). At preoperatively, 57% of the cases had a balanced posture (sacral vertical axis/sacral femoral distance ≤0.5 and sacral vertical axis ≤0), 33% at 1WPO and 79% at 2YFU.
Conclusion: There was no significant 3D shape change of the instrumented thoracic spine or of the rods postoperatively for any of the segmental pedicle screw constructs with titanium, stainless steel, and cobalt-chromium rods. Patients overall unbalanced sagittal posture documented immediately after surgery came back to balanced 2 years after surgery.
Level of evidence: 3.