Age-associated differences in cervical sagittal alignment parameters among asymptomatic adults: a cross-sectional reference study.

PURPOSE: While age-adjusted alignment goals are established for the lumbar spine, analogous clinically actionable tools are lacking for cervical sagittal assessment. This study aimed to derive and internally assess simple, age- and sex-specific formulas for key cervical parameters-thoracic inlet angle minus cervical lordosis (TIA-CL), neck tilt (NT), and cervical sagittal vertical axis (cSVA)-in asymptomatic adults. METHODS: This retrospective cross-sectional study analyzed 632 rigorously screened asymptomatic adults (268 males, 364 females; mean age 54.8 ± 16.6 years) who underwent neutral lateral cervical radiography. Radiographic parameters (TIA, C2-C7 lordosis, NT, cSVA) were measured using a validated protocol. All angular parameters (TIA, CL, TIA-CL, and NT) are reported in degrees (°), whereas cSVA is reported in millimeters (mm). Sex-stratified linear regression models quantified age-related associations for TIA-CL, NT, and cSVA, with internal resampling via 1,000-iteration bootstrap resampling (CI width < 0.15 indicating good stability). RESULTS: All three parameters showed significant positive associations with age (all P < 0.05). The derived simplified formulas were: TIA-CL ≈ 56 + 0.07 × Age (R2 = 0.010), NT ≈ 40 + 0.16 × Age (R2 = 0.111), and cSVA ≈ 15 + 0.10 × Age (R2 = 0.045). Age-associated differences were more pronounced for NT before age 50 (P = 0.005) and for cSVA after age 50 (P = 0.028). Males had significantly higher baseline NT and cSVA than females (P = 0.009 and P < 0.001). Bootstrap resampling demonstrated internal consistency of the model estimates (all 95% CI widths < 0.15). CONCLUSION: We provide age- and sex-specific reference formulas for NT and cSVA, and an age-specific formula for TIA-CL. These intuitive equations offer a practical reference for comparing an individual's alignment against age-associated population estimates.