High Preoperative T1 Slope is a Marker for Global Sagittal Malalignment

Fares Ani; Ethan W Ayres; Diann Woo; Dennis Vasquez-Montes; Avery Brown; Haddy Alas; Edem J Abotsi; Cole Bortz; Katherine E Pierce; Tina Raman; Micheal L Smith; Yong H Kim; Aaron J Buckland; Themistocles S Protopsaltis

doi:10.1097/BSD.0000000000001734

High Preoperative T1 Slope is a Marker for Global Sagittal Malalignment

Clin Spine Surg. 2025 Jan 6. doi: 10.1097/BSD.0000000000001734. Online ahead of print.

Affiliations

¹ Department of Orthopedic Surgery, NYU Langone Health, New York, NY.
² Department of Orthopedic Surgery, Penn Medicine, Philadelphia, PA.
³ Department of General Surgery, NYU Langone Health.
⁴ Department of General Surgery, Columbia University, New York, NY.

PMID: 39760389
DOI: 10.1097/BSD.0000000000001734

Abstract

Study design: Retrospective cohort study.

Objective: To develop parameter thresholds obtainable from cervical radiographs that correlate with concomitant thoracolumbar malalignment.

Summary of background data: T1 slope (T1S) is typically discussed in the context of cervical deformity and correlated with health-related quality of life outcomes. Prior research suggests that T1S is related to global alignment; however, a definition for "high" T1S has not been established. Most patients undergoing cervical surgery do not undergo full-spine imaging; therefore, obtaining a parameter associated with thoracolumbar malalignment from cervical radiographs would be beneficial.

Methods: A database of preoperative adult spinal deformity (ASD) patients was analyzed. Measures obtained from standing lateral radiographs included T1S, thoracic kyphosis (TK), sagittal vertical axis (SVA), T1-pelvic angle (TPA), pelvic tilt (PT), and pelvic incidence minus lumbar lordosis (PI-LL). Decision tree analysis was then used to determine the T1S corresponding to published thresholds for high TK (40 degrees), SVA (40 mm), TPA (25 degrees), and PT (25 degrees). Alignment between high and normal T1S patients was compared.

Results: Two hundred twenty-six preoperative patients were included (mean: 58±16 y 62%F). Larger T1S was correlated with greater SVA (r=0.365), TPA (r=0.302), TK (r=0.606), and PT (r=0.230) (all P<0.001). Decision tree analysis yielded a threshold of 30 degrees for high T1S, which 50% of patients had. Compared with patients with T1S <30 degrees, those with T1S >30 degrees had higher TK (41.5 vs. 25.8 degrees), SVA (78.7 vs. 33.7 mm), TPA (27.6 vs. 18.3 degrees), and PT (26.3 vs. 20.8 degrees), and PI-LL (18.2 vs. 11.7 degrees) (all P<0.05). Seventy-nine percent of patients with high T1S had high TK (T1S <30=13%), 69% had high SVA (T1S <30=38%), 66% had high TPA (T1S <30=37%), 60% had PT >25 degrees (T1S <30=42%), and 47% had PI-LL >20 degrees (T1S <30=34%) (all P<0.05).

Conclusion: Higher T1S was associated with worse global alignment. T1S was most strongly associated with TK. A T1S=30 degrees corresponded to high TK, SVA, TPA, and PT thresholds. Therefore, surgeons should consider obtaining full-spine radiographs if a T1S >30 degrees is present on cervical imaging.