Pelvic morphology decoupling from lumbar lordosis in chronic low back pain

To determine the relationship of lumbar lordosis tracking pelvic morphology in chronic low back pain (CLBP) patients and quantify any shift in determinants.

Standing lateral full-spine radiographs were obtained in 72 volunteers (49 male, 23 females; mean age 28.3 years) and 72 CLBP patients (28 male, 44 females; mean age 52.0 years; pain > 3 months). The posterior tangent pelvic incidence angle (PTPIA; a morphological constant quantifying pelvic shape) and angle of pelvic incidence (API) were measured as morphological indices while sacral base angle (SBA; a postural variable reflecting sacral inclination), pelvic tilt angle, L1-L5 lordosis, and T3-T10 kyphosis were measured as postural variables. Pearson correlations, ANCOVA, and multivariate ordinary least squares regression were conducted. Groups differed significantly in age (28.3 vs. 52.0 years) and sex composition (68% vs. 39% male); sensitivity analyses controlling for age and sex confirmed all primary findings were robust to these adjustments.

In volunteers, PTPIA correlated significantly with L1-L5 lordosis (r = -0.428, p < 0.001). In CLBP patients, this correlation was absent (r = -0.137, p = 0.251). A multivariate model (PTPIA + SBA + T3-T10 kyphosis) explained 32.5% of lordosis variance in volunteers (F = 10.90, p < 0.001; all three predictors significant), but 48.2% in CLBP patients (F = 21.05, p < 0.001), with PTPIA non-significant (p = 0.092) and SBA exhibiting an approximately three-fold amplified coefficient (-0.40 in normals vs. -1.32 in patients). SBA-lordosis partial correlations confirmed this amplification after controlling for other predictors (normals: partial r = -0.261, p = 0.027; patients: partial r = -0.625, p < 0.001). Based on normal volunteers, 44.4% of CLBP patients exhibited less lordosis than morphologically expected (mean deficit + 3.87°, p = 0.035).

CLBP patients demonstrate decoupling of lumbar lordosis from pelvic morphology, with three-fold amplification of the SBA-lordosis relationship. This biomechanical pattern is consistent with pelvic retroversion and its associated lordosis flattening. The lordosis deficit represents a clinically actionable radiographic outcomes target for individualized sagittal rehabilitation in CLBP.