Dual phenotypes of sagittal spinopelvic balance in post-poliomyelitis patients: an imaging study

Abstract Background Patients with post-poliomyelitis syndrome experience significantly higher dislocation rates following total hip arthroplasty (THA) than the general population. The sagittal hip–spine relationship—the dynamic interplay between spinopelvic alignment and functional acetabular orientation—is a principal determinant of THA stability and is relevant to dislocation risk. Sagittal spinopelvic compensatory mechanisms in patients with neuromuscular disorders remain poorly characterized. This study characterized sagittal spinopelvic balance and sit-to-stand postural dynamics in post-poliomyelitis patients and explored their implications for THA. Methods This single-center cross-sectional study consecutively enrolled 71 patients with post-poliomyelitis syndrome who underwent dual-position (standing and sitting) EOS imaging and bilateral lower-extremity computed tomography (CT). Spinopelvic parameters were measured in both positions, including pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), lumbar lordosis (LL), pelvic femoral angle (PFA), sagittal vertical axis (SVA), and spinosacral angle (SSA). Standing PI–LL mismatch was used to classify sagittal phenotypes. CT measurements included femoral anteversion (FA) and paraspinal muscle characteristics. Between-group comparisons were adjusted for age using analysis of covariance, and Spearman correlation analysis was used to examine the strength of association between PI and other sagittal parameters within each phenotype group. Results Among 71 patients (mean age 43.9 ± 12.8 years, 66.2% female), 59% exhibited a hyperlordotic phenotype and 41% a balanced phenotype. Compared with the balanced group, the hyperlordotic group had lower PI (41.2° vs 47.6°, P = 0.015), greater affected-side FA (25.0° vs 14.6°, P = 0.010), and larger limb length discrepancy (55.6 mm vs 33.3 mm, P = 0.043). In the standing position, the hyperlordotic group showed significantly lower PT (− 3.74° vs 3.74°, P < 0.001), greater LL (58.93° vs 52.50°, P = 0.024), and lower PFA (169.15° vs 180.40°, P < 0.001). Despite these differences, both groups converged to a similar alignment in sitting. During sit-to-stand transitions, the hyperlordotic group demonstrated lower hip user index (57% vs 73%, P = 0.002) and greater relative pelvic movement (39% vs 27%, P = 0.033). PI showed strong correlations with standing alignment parameters. Conclusions Ambulatory post-poliomyelitis patients exhibit a characteristic standing posture featuring anterior PT and lumbar hyperlordosis, without a flatback phenotype. PI appears to be the principal anatomical factor associated with sagittal phenotype, while residual muscle strength supports active compensation. Standing differences are largely functional, converging in sitting. The combination of low PI, high FA, and excessive pelvic mobility challenges conventional THA static safe zone concepts, underscoring the need for dynamic, position-specific preoperative planning in this population.