Thoracolumbar junction orientation: its impact on thoracic kyphosis and sagittal alignment in both asymptomatic volunteers and symptomatic patients

Purpose: The thoracolumbar junction (TLJ) has not been explored in regard to its contribution to global sagittal alignment. This study aims to define novel sagittal parameters of the TLJ and to assess their roles within global sagittal alignment. Methods: Included for cross-sectional, retrospective analysis were asymptomatic volunteers and symptomatic patients who had undergone operation for adult spinal deformity. Unique sagittal parameters of the TLJ were measured using the midline of the T12–L1 disk space: The TLJ orientation [TLJO; thoracolumbar tilt (TLT) and slope (TLS)]. Thoracic kyphosis (TK; T5–12), C7–S1 sagittal vertical axis (SVA), lumbar lordosis (LL; L1–S1), sacral slope (SS), pelvic tilt (PT), and pelvic incidence (PI) were measured. Continuous variables were compared using the independent t test. Pearson correlations examined relationships between the parameters in each group. The asymptomatic TK was calculated using the measurement of the asymptomatic volunteer’s TLJO by linear regression. Results: One hundred fifteen asymptomatic volunteers and 127 symptomatic patients were included. Only LL among the lumbopelvic parameters correlated with TK (asymptomatic volunteers: r = − 0.42; symptomatic patients: r = − 0.40). All the pelvic parameters have no direct correlation with TK in both groups. TLJO had stronger correlation with TK [asymptomatic volunteers: r = − 0.68 (TLS), r = 0.41 (TLT); symptomatic patients: r = − 0.56 (TLS), r = 0.44 (TLT)] than the lumbopelvic parameters. TLS correlated with LL (asymptomatic volunteers: r = 0.78; symptomatic patients: r = 0.73). Most pelvic parameters correlated with TLJO except for PI. The asymptomatic TK was estimated by the derived formula: 20.847 + TLS × (− 1.198). Conclusion: The TLJO integrates the status of the lumbopelvic sagittal parameters and simultaneously correlates with thoracic and global sagittal alignment. Graphic abstract: These slides can be retrieved under Electronic Supplementary Material. [Figure not available: see fulltext.].