Three-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 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.