Normative segmental cervical lordosis distribution and sagittal vertical axis variations with T1 slope: Defining the...

The T1 slope (TS) is a major determinant of cervical sagittal alignment, yet its relationship to lordosis distribution and sagittal vertical axis (SVA) in asymptomatic adults remains insufficiently defined. This study evaluates how TS influences cervical lordosis segmentation, apex location, and SVA.

Standing lateral cervical radiographs from 85 asymptomatic adults (mean age 50.5 ± 29.1 years; 45 males, 40 females) obtained between September 2024 and September 2025 were analyzed. Participants were grouped as Low-TS (<20°, n = 32), Mid-TS (20°-30°, n = 41), and high-TS (>30°, n = 12). Total cervical lordosis (TCL, OC7) was divided into upper cervical lordosis (UCL, occiput-C2) and subaxial cervical lordosis (SCL, C2-C7), with SCL further separated into upper SCL (USCL, C2-C5) and lower SCL (LSCL, C6-C7) components. SVA was measured at C2-C5 and C6-C7, and lordosis apex was determined from posterior tangent intersections. Analyses included analysis of variance, Chi-square, Spearman correlations, and polynomial regression (P < 0.05).

TS correlated with TCL (r = 0.526, P < 0.001), SCL (P < 0.001), and C2-C7 SVA (r = 0.265, P = 0.017), but not UCL (P = 0.983). With increasing TS, TCL rose (low: 24.3° to high: 35.2°, P < 0.001), driven by USCL (P = 0.002); UCL remained constant. UCL contribution decreased (P = 0.006), and LSCL contribution was unchanged (P = 0.198). The apex shifted caudally with rising TS (P = 0.016). C2-C7 SVA increased due to higher C6-C7 SVA (P = 0.020). Polynomial modeling confirmed a curvilinear TS-USCL relationship (R² = 0.38).

TS drives a predictable caudal redistribution of cervical lordosis in asymptomatic adults, primarily through subaxial modulation, while upper cervical alignment remains preserved to maintain horizontal gaze.