Monaxial versus multiaxial thoracic pedicle screws in the correction of adolescent idiopathic scoliosis

Study Design. Radiographic outcome analysis following thoracic fusion of Lenke Type I adolescent idiopathic scoliosis (AIS) curves with segmental pedicle screw fixation. Objective. To compare the correctional capacity of monaxial versus multiaxial pedicle screws in a matched cohort of AIS patients. Summary of Background Data. Thoracic pedicle screws provide improved curve correction over hook and wire or hybrid constructs for AIS. Further, both monaxial and multiaxial screws are available, with each offering certain advantages over the other. However, different screw types have not been evaluated against each other. Methods. We retrospectively reviewed the preoperative and final postoperative follow-up radiographs of an age- and curve-matched cohort of 35 consecutive Lenke Type I AIS patients. Fifteen were treated with monaxial and 20 were treated with multiaxial pedicle screw constructs. All patients had a minimum 2-year follow-up. The average age at surgery was 14 years 4 months (range, 12-17 years) in the monaxial group and 13 years 8 months (12-16 years) in the multiaxial group. Evaluation included coronal proximal thoracic (PT), main thoracic (MT), and thoracolumbar/lumbar (TL/L) Cobb angles and flexibility indexes, regional sagittal curvature, the sagittal apical rib hump (RH) deformity, the apical vertebral body-rib ratio (AVB-R: ratio of linear measures from left and right apical body to lateral rib), and the apical rib spread distance (ARSD, difference of the sums of the intercostal distances at the five periapical segments measured at the lateral transverse process). Results. There was no statistically significant difference with regard to the preoperative PT curves, MT curves, TL/L curves, flexibility indexes, regional sagittal curvature, AVB-R, or ARSD. The preoperative rib humps were significantly greater in the monaxial screw group (42.4 mm vs. 34.7 mm; P=0.02). Postoperative follow-up averaged 59.9 months (range, 24-98 months) for the monaxial group and 38.0 months (range, 24-55 months) for the multiaxial group (P<0.0001). An average of 7.7 vertebral levels were fused in the monaxial group compared with 7.2 levels in the multiaxial group (P=0.39). After surgery, both constructs provided excellent instrumented correction of the MT curves (64.9% vs. 60.0% for the monaxial and multiaxial groups, respectively; P=0.33), as well as good spontaneous correction of the PT (41.3% vs. 40.5%; P=0.92) and TL/L curves (55.4% vs. 51.7%; P=0.66). Monaxial screws demonstrated significantly greater absolute (13.9 mm vs. 25.2 mm; P<0.0001) and relative (66.1% vs. 24.7%; P<0.0001) correction of the apical RH. Additionally, AVB-R (77.9% vs. 54.1%; P=0.0007) and ARSD (82.8% vs. 69.9%; P=0.04) corrections were significantly greater in the monaxial group. There were no neurologic deficits or major complications in either group. Conclusion. Both monaxial and multiaxial thoracic pedicle screws provide excellent coronal deformity correction for thoracic fusion of main thoracic AIS. Monaxial screws provide superior derotation and restoration of thoracic symmetry as noted by significantly greater correction of the AVB-R, RH, and ARSD.