Three-Dimensional Analysis of Disc and Vertebral Height and Their Role in Kyphosis Creation in Adolescent Idiopathic Scoliosis

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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 to 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 <0.001). 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: Five hundred sixty-four patients met inclusion (age 15 yr, 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, whereas both lengthened at T12. From T5 to T12, AC shortened 3.4 mm ( P <0.001) and MC lengthened 4.7 mm ( P <0.001). MC lengthening >10 mm was achieved in 30 (5%) cases. Longer fusions [OR 1.7 (1.2 to 2.4)] and a greater difference between anterior VB height and posterior VB height [OR 7.1, (3.1 to 16.2)] were associated with more lengthening. Anterior shortening was strongly correlated to more 3D kyphosis creation ( r =0.7, P <0.001). 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.