Mechanical axial instability of segmental pedicle screw instrumentation for adolescent idiopathic scoliosis: a retrospective cohort study of tulip screw versus dual locking cup instrumentation

Abstract

Background and purpose: The effects of axial instability in the rod-screw interface resulting in axial slippage between screws and rods are largely unknown. We aimed to assess the incidence of axial slip and loss of correction by comparing tulip screw versus dual locking cup in spinal instrumentations of patients treated with posterior spinal fusion for adolescent idiopathic scoliosis (AIS). We also aimed to assess whether axial slip would affect health-related quality of life.

Methods: This study consists of 194 patients who underwent posterior spinal fusion for AIS during 2012-2022. All patients had a minimum of 2 years' follow-up. There were 98 patients treated with segmental tulip pedicle screw instrumentation and 96 patients with segmental dual locking cup constructs. Axial slip was defined as ≥ 2 mm and was assessed by measuring the rod exceeding the last pedicle screw and the distance between the 2 lowest screws on the same rod. Loss of correction was assessed by comparing postoperative and 2-year radiographic measurements. Health-related quality of life was assessed using the SRS-24 questionnaire.

Results: Axial slip occurred only between the lowest instrumented vertebra and the vertebra above it on the convex side of the deformity. At 2 years of follow-up, axial slippage of 2 mm or more was observed more often in the dual locking group, which was observed in 24 (25%) patients in the dual locking cup group and 11 (11%) patients in the tulip group (risk ratio [RR] 2.2, 95% confidence interval [CI] 1.2-4.4). Minimum of 10° loss of major curve correction was found in 1 (1%) patient in the tulip group and 9 (9%) patients in the dual locking group (RR 9.1, CI 1.2-100).

Conclusion: Axial slip was significantly less frequent in the tulip group than in the dual locking cup group. This suggests that tulip screw instrumentation may offer superior mechanical stability in posterior spinal fusion for AIS. Axial slip was not associated with health-related quality of life outcomes.

Figure 1

(A) Tulip screw with set screw inserted to secure the rod. (B) Dual locking cup screw with outer locking cup lifted over inner cup to secure the rod.

Figure 2

(A) Upper round part of a transition rod. (B) Lower rail rod part of a transition rod. Like rail rods, sagittally reinforced rods have a larger sagittal diameter than round rods but with an oval transverse area.

Figure 3

Flowchart.

Figure 4

(A) 14-year-old girl with 54° isolated right thoracic adolescent idiopathic scoliosis (Lenke 1BN). (B) Dual locking cup instrumentation from T3 to T12 using cobalt chromium transition range-rail rods. Gradually increasing axial slip > 5 mm between T11 and LIV resulting in coronal malalignment can be observed on the (C) 6-month and (D) 2-year radiographs. (D) Note the shortening of the residual rod below the right T12 pedicle screw.

Figure 5

(A) 15-year-old boy with 51° left thoracolumbar adolescent idiopathic scoliosis (Lenke 6CN). (B) Dual locking cup instrumentation from T4 to L3 using cobalt chromium transition range-rail rods. Note the axial slip > 5 mm between L2 and LIV resulting in loss of lumbar curve correction on the (C) 6-month and (D) 2-year radiographs. (D) Progressive shortening of the residual rod below the left L3 pedicle screw can be observed.

Figure 6

(A) 15-year-old girl with 52° right thoracic adolescent idiopathic scoliosis (Lenke 2AN). (B) Instrumentation from T2 to L2 using tulip screws with sagittal reinforced cobalt chromium rods. Axial slip of 2 mm between the L1 and LIV on the (D) 2-year radiograph, which showed minimal increase during follow-up and resulted in minor loss of major curve correction and coronal malalignment in comparison with (B) postoperative and (C) 6-month radiographs.