Effect of thoracic and pelvic anteroposterior diameters on spinal sagittal alignment

Purpose: This study aimed to analyze the effect of the thoracic anteroposterior diameter (TAPD) and pelvic anteroposterior diameter (PAPD) on global sagittal alignment in asymptomatic patients with normal sagittal alignment. Patient sample: The study investigated 2042 adult patients who initially presented at our hospital with a hip and knee problem without history of symptoms related to the entire spine. Only 57 patients with normal global sagittal alignment (C2–7 sagittal vertical axis (SVA) and C7–S1 SVA of <10 mm) were considered. Methods: The whole-spine standing lateral radiographs were obtained to analyze the following parameters: pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), lumbar lordosis (LL), thoracic inlet angle (TIA), T1 slope, cervical spinal parameters (angle of C0–2, C2–7, and C0–7), TAPD, and PAPD. Statistical analysis was performed using Pearson correlation coefficients and multiple regression analyses. Results: All the parameters showed a normal distribution. TAPD had a significant relationship with thoracic kyphosis (TK; r = 0.458), TIA ( r = 0.677), and C0–2 angle ( r = 0.294) but no significant relationship with T1 slope and other cervical parameters. PAPD had a significant relationship with PI ( r = 0.309) and PT ( r = 0.463) but no significant relationship with LL, SS, and TK. The multiple regression analysis showed that TIA = 21.974 + 0.405 (TK) + 0.188 (TAPD) ( p < 0.0001). Conclusions: TAPD and PAPD are associated with TIA, TK, C0–2 angle, PI, and PT, all of which act as key factors in spinal sagittal alignment. Although they did not directly correlate with other cervical parameters, T1 slope, and LL, TAPD and PAPD might have indirect effects on cervical and lumbar spinal sagittal alignment through their relationships with TIA, TK, and PI.