Correlation between sacro-femoro-pubic angle and spine-pelvic parameters in patients with Lenke types 1, 5, and 6 adolescent idiopathic scoliosis

Abstract

BACKGROUND: The sacro-femoro-pubic angle, as a coronal plane alternative to the pelvic tilt angle, has an unclear association in different Lenke classifications of adolescent idiopathic scoliosis, limiting clinical application of the sacro-femoro-pubic angle in classification-specific assessment. OBJECTIVE: To explore the correlation between the sacro-femoro-pubic angle and spine-pelvic parameters (pelvic incidence, pelvic tilt, sacral slope, lumbar lordosis, thoracic kyphosis, and main Cobb angle) in patients with Lenke types 1, 5, and 6 adolescent idiopathic scoliosis, and to establish classification-specific regression models. METHODS: Clinical and imaging data of 191 adolescent idiopathic scoliosis patients were retrospectively analyzed, including 60 with Lenke type 1, 65 with Lenke type 5, and 66 with Lenke type 6. Pearson or Spearman correlation analysis was used to assess the correlation between left and right sacro-femoro-pubic angles and pelvic incidence, pelvic tilt, sacral slope, lumbar lordosis, thoracic kyphosis, Cobb angle, and sagittal vertical axis. Statistically significant parameters were subjected to univariate linear regression analysis to construct linear regression models. RESULTS AND CONCLUSION: In Lenke type 1 patients, the right sacro-femoro-pubic angle was significantly negatively correlated with pelvic tilt (r=-0.366, P=0.028), and both left and right sacro-femoro-pubic angles were significantly positively correlated with lumbar lordosis (r=0.429, P=0.007; r=0.460, P=0.007). In Lenke type 5 patients, both left and right sacro-femoro-pubic angles were significantly positively correlated with lumbar lordosis (r=0.415, P=0.007; r=0.400, P=0.007). In Lenke type 6 patients, both left and right sacro-femoro-pubic angles were significantly negatively correlated with pelvic tilt (r=-0.385, P=0.007; r=-0.376, P=0.014). Univariate regression models quantified these associations (R²=0.13-0.21), with the largest slope for the sacro-femoro-pubic angle-lumbar lordosis regression in Lenke type 5 (0.85-0.89), and slopes of -0.52 to -0.54 for the sacro-femoro-pubic angle-pelvic tilt regression in Lenke type 6. These findings indicate that the associations between sacro-femoro-pubic angle and spine-pelvic parameters are classification-specific: Lenke types 1 and 5 predominantly show positive correlations with lumbar lordosis, while Lenke type 6 is characterized by bilateral negative correlations with pelvic tilt.

1. Introduction

Adolescent idiopathic scoliosis is a three-dimensional spinal deformity of unknown cause that occurs around puberty, characterized by lateral curvature (Cobb angle ≥10°) with vertebral rotation [1]. As one of the most common spinal deformities in adolescents, its global prevalence ranges from 0.47% to 5.2%, with a significant gender disparity (female-to-male ratio up to 8:1) and onset during growth spurts. Adolescent idiopathic scoliosis not only affects posture but can also impair cardiopulmonary function due to thoracic deformity, threatening long-term quality of life [2-4].

The sacro-femoro-pubic angle, introduced by Blondel et al. [5], is defined as the angle formed by the midpoint of the superior endplate of the sacrum, the center of the acetabulum, and the superior border of the pubic symphysis. Studies in healthy populations have confirmed that the sacro-femoro-pubic angle can serve as a coronal plane surrogate for pelvic tilt, with a clear linear regression relationship. Its application has been extended to patients with ankylosing spondylitis, normal children, and middle-aged and elderly populations, showing significant correlations with pelvic sagittal parameters and its utility as a supplementary indicator for trunk balance [6-8].

In the spine-pelvic parameter system, pelvic incidence, pelvic tilt, and sacral slope are core indicators for assessing pelvic morphology and sagittal balance, with a clear anatomical relationship (pelvic incidence = pelvic tilt + sacral slope). Abnormal values of these parameters are closely related to the progression of adolescent idiopathic scoliosis and trunk imbalance [9]. Pelvic tilt, in particular, influences overall spinal alignment through a sagittal coupling mechanism; anterior or posterior tilt abnormalities can trigger compensatory changes in lumbar lordosis and thoracic kyphosis, leading to sagittal imbalance. Studies have confirmed that thoracic kyphosis in adolescent idiopathic scoliosis patients is associated with pelvic tilt, underscoring the importance of these parameters in understanding the deformity.