Corrective Capacity of Monaxial versus Multiaxial Thoracic Pedicle Screws in Adolescent Idiopathic Scoliosis

• a - Medtronic Sofamor Danek

Background: Thoracic pedicle screws provide improved curve correction over hook and wire or hybrid constructs for adolescent idiopathic scoliosis. 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.

Purpose: To compare correctional capacity (chest wall/vertebral body derotation) of monaxial versus multiaxial pedicle screws in a matched cohort.

Materials and Methods: We retrospectively reviewed the preoperative and final postoperative follow-up radiographs of an age and curve-matched cohort of 30 Lenke Type I curves: 15 treated with monaxial and 15 treated with multiaxial pedicle screw constructs. All patients had a minimum two-year follow-up, and no patient had a thoracoplasty or rib resection. The average age at surgery was 13 yrs., 9 mos. (range, 12+0 - 15+1), with no difference between groups (p=0.39). Evaluation included coronal proximal thoracic (PT), main thoracic (MT) and thoracolumbar-lumbar (TL-L) Cobb anges and side-benders, apical vertebral bodyrib ratio (AVB-R: ratio of linear measures from left and right apical body to lateral rib), apical rib spread distance at the lateral transverse process (ARSD-TP: difference of the sums of the intercostal distances at the five periapical segments measured at the lateral TP), ARSD-MC (difference of the sums of the intercostal distances at the five periapical segments measured at the mid-clavicular line), and the sagittal rib hump difference (RH).

Results: For the monoaxial group, the preoperative main thoracic (MT) curve was 53.3° (range, 45-62°, SE 1.5), with an average side-bender of 32.7° (39% flexibility). For the multiaxial group, the main thoracic curve was 51.7° (range, 46-68°, SE 1.9, p = 0.52), with an average side-bender of 30.5° (38% flexibility, p= 0.59). Follow-up averaged 69 months (range, 49-98) for the monaxial group and 41 months (range, 25-50) for the multiaxial group (p <0 .0001). There was no statistical difference in the preoperative PT or TL-L curves respectively (p="" 0.18, 0.52); sagittal measurements of T2-T5, T5-T12, T2-12, T10-L2, T12-S1; RH, as well AVB-R, ARSD-TP, and ARSD-MC - all p> 0.18. An average of 7.6 (SE 0.2) and 7.8 (SE 0.6) levels were fused in the monaxial and multiaxial groups, respectively (p =0.77). Postoperatively, curve correction for the proximal thoracic, main thoracic, and thoracolumbar-lumbar curves was similar (all p > 0.39). However, there was significantly greater correction of the rib hump deformity for monaxial screws (68% vs. 33%, p = 0.0001). Additionally, there was a trend towards greater correction of the AVB-R ratio (81% vs. 65%, p = 0.10) and the ARSD-TP (79% vs. 52%, p = 0.11) for monaxial screws. There were no neurologic deficits or complications in either group.

Conclusions: Monaxial screws provide superior corrective capacity (derotation) as noted by statistically significant differences in the apical vertebral body-rib ratio (AVB-R), the apical rib spread difference at the TP (ARSD-TP), and the sagittal rib hump (RH). This most likely is due to the greater rod-screw rigidity of the monaxial screws, which do not permit screw head pivot prior to maximum rod engagement, as well as the necessity of moving the rod to the screw as opposed to rotating the screw head to the rod.

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