Three-dimensional evaluation of the dynamic interplay between pelvic anatomy, lower-limb compensation, and standing alignment in ASD

Abstract

BACKGROUND CONTEXT: Previous studies have shown that lower limbs play a crucial role in compensating for sagittal spinal malalignment. However, these studies primarily focused on lower limb parameters in the sagittal plane, mainly knee flexion, leaving compensatory mechanisms that might happen in the coronal or axial planes unexplored. PURPOSE: This study aimed to investigate factors associated with lower-limb recruitment in adult spinal deformity (ASD) patients. STUDY DESIGN/SETTING: Retrospective study of prospectively collected data. PATIENT SAMPLE: ASD patients who underwent full-body biplanar X-rays and 3D reconstruction of lower limbs and pelvis. OUTCOME MEASURES: Association between morphological parameters and compensatory mechanisms METHODS: The study included ASD patients with moderate to severe sagittal plane deformities. Classic 2D parameters included pelvic shift (PSh), knee flexion (KA), sacro-femoral (SFA), and ankle dorsiflexion (AA) angles for the lower limbs, as well as TPA, PT, PI, and PI-LL mismatch. 3D reconstructions were used to assess acetabular parameters (abduction, coverage, and anteversion), pelvic depth (PD: distance between the pubic symphysis and the sacral endplate), and knee varus/valgus angle. After univariate analysis, multiple linear regressions were performed to investigate associations between spinal deformity and lower limb 2D/3D parameters with and without accounting for spinal alignment. RESULTS: A total of 146 subjects (67±10 years) were included with a mean PI-LL of 25.1±16.1°, TPA 37.4±10.6°, PT 27±9.1°, and PD of 85.9±16.2mm. Lower limbs compensation consisted of a PSh 38.4±43.7mm, KA 6.9±7.9°, and AA of 5.8±4.1°. Pelvic depth significantly correlated with PI (r=0.6, p<0.001), PT (r=0.3, p<0.001), and SFA (r=0.2, p=0.02). In multivariate analysis considering the full-body parameters, ankle dorsiflexion (AA) was associated with PT, PSh, and KA (all p<0.001) but not with spinal alignment and correlated with increased knee varus angulation (p=0.01). Similarly, KA correlated with PT, SFA, and AA (all p<0.001) but not with spinal alignment. Those associations remained significant in multivariate analysis considering only the lower-limbs parameters. In addition, patients with high pelvic depth (>100mm) had greater pelvic shift and PT than low ones (<70mm): 29.4+49.1mm versus 54.8±41.7mm and 23.7±9.3° versus 32.4±9.4°. Finally, increased PT was associated with higher PI (p<0.001) and more vertical acetabular abduction (57.4±3.9° for PT<15°, vs 60.7±4.2° for PT > 25°, p=0.009). CONCLUSIONS: There was 3D analysis of the lower extremities that revealed significant multiplanar interplay in the setting of spinal deformity. Pelvic morphology including antero-posterior depth is associated with greater compensatory abilities such as pelvic translation and retroversion. Greater PT compensation in the sagittal plane is associated with a more vertical acetabulum in the coronal plane. Knee and ankle flexion were indirectly correlated with spinal alignment as they contributed to higher PT and pelvic shift. Consequently, their assessment is valuable for understanding how patients compensate for malalignment but should not be a primary consideration in the correction strategy. The sagittal and coronal alignment of lower limbs cannot be separated, as an increase in ankle and knee flexion angles is associated with greater genu varum. FDA Device/Drug Status: This abstract does not discuss or include any applicable devices or drugs.