How does thoracic scoliosis surgery affect thoracolumbar spinal flexibility and lumbar intradiscal pressure? An in vitro study confirming the importance of the rib cage.

Purpose: To evaluate effects of spinal and rib osteotomies on the resulting spinal flexibility for surgical correction of thoracic scoliosis and to explore effects of posterior fixation on thoracolumbar segmental range of motion and lumbar intervertebral disc loading.

Methods: Six fresh frozen human thoracolumbar spine and rib cage specimens (26-45 years, two female / four male) without clinically relevant deformity were loaded with pure moments of 5 Nm in flexion/extension, lateral bending, and axial rotation. Optical motion tracking of all segmental levels (C7-S) and intradiscal pressure measurements of the lumbar spine (L1-L5) were performed (1) in intact condition, (2) after Schwab grade 1, (3) Schwab grade 2, and (4) left rib osteotomies at T6-T10 levels, as well as (5) after posterior spinal fixation with pedicle screw-rod instrumentation at T4-L1 levels.

Results: Schwab grade 1 and 2 osteotomies did not significantly (p > 0.05) affect spinal flexibility, whereas left rib osteotomies significantly (p < 0.05) increased segmental ranges of motion at upper and lower levels in flexion/extension and at treated levels in lateral bending. Posterior fixation caused significantly (p < 0.05) increased range of motion at upper adjacent thoracic and mid-lumbar levels, as well as significantly (p < 0.05) increased intradiscal pressure at the lower adjacent level.

Conclusion: Low effects of Schwab grade 1 and 2 osteotomies question the impact of isolated posterior spinal releases for surgical correction maneuvers in adolescent idiopathic scoliosis, in contrast to additional concave rib osteotomies. High effects of posterior fixation potentially explain frequently reported complications such as adjacent segment disease or proximal junctional kyphosis.