3-Dimensional Analysis of Disc and Vertebral Height and their Role in Kyphosis Creation in Adolescent Idiopathic Scoliosis

Study Design: Retrospective Objective: Describe three-dimensional (3D) changes after Adolescent Idiopathic Scoliosis (AIS) reconstruction, with attention to anterior column (AC) shortening and middle column (MC) lengthening. Summary of Background Data: Relative elongation of the AC, particularly the disc, is a common feature of AIS. 3D correction of deformity requires creation of thoracic kyphosis (TK). Methods: An AIS registry was queried for patients treated with posterior-only instrumented fusion, with pre/postoperative biplanar radiographs and 3D spine models. MATLAB script calculated heights of the anterior disc/vertebral body (VB) and posterior disc/VB for each segment from T1-L5 in the plane of each disc/VB. The respective disc/VB heights were summed to calculate AC length and MC length. AC and MC lengths of the instrumented segments were compared before and after surgery using paired t-tests with a Bonferroni correction (p<.001). Linear regression examined factors related to greater MC lengthening. The relationships between AC/MC lengths and 2D/3D TK were analyzed using Pearson correlations. Results: 564 patients met inclusion (Age 15yrs, Female 82%; Major Curve 58°; 3D TK 3°, Lenke 1 44%, Risser 3/4/5 81%). Mean number of levels fused was 11, LIV T12/L1 50%, postoperative major curve 16°, 3D TK 23°. The AC shortened and the MC lengthened at all levels from T5 to T11, while both lengthened at T12. From T5-T12, AC shortened 3.4mm (p<.001) and MC lengthened 4.7mm (p<.001). MC lengthening >10mm was achieved in 30 (5%) cases. Longer fusions (OR 1.7 [1.2-2.4]) and a greater difference between anterior VB height and posterior VB height (OR 7.1, [3.1-16.2]) were associated with more lengthening. Anterior shortening was strongly correlated to more 3D kyphosis creation (r=0.7, p<.001). Conclusion: 3D kyphosis creation requires shortening of the AC and lengthening of the MC through the discs. After the posterior longitudinal ligament (MC) is taut, a discectomy may be required for further 3D kyphosis creation.