The T4-L1 Hip Axis Describes Sagittal Alignment in Operative Adolescent Idiopathic Scoliosis

Retrospective cohort.

To examine preoperative sagittal alignment using vertebral pelvic angles (VPA) in adolescent idiopathic scoliosis (AIS).

VPA describe sagittal spine shape accounting for magnitudes and distributions of sagittal curvatures. Normal adults stand with a C2Tilt (Odontoid-Hip axis) near 0° irrespective of pelvic incidence (PI) and an L1 pelvic angle (L1PA)=0.5*PI - 21(r2=.6). AIS is associated with thoracic hypokyphosis, though the spine "shape" has not been described.

A registry was queried for preoperative AIS patients with standardized, standing radiographs including C2 and the hips. VPA,vertebral tilts and PI were measured for all patients. Global balance was assessed by C2Tilt. Linear regression analyzed relationships between pelvic incidence and (1) L1PA, (2) lumbar lordosis (LL), and (3) T4 pelvic angle (T4PA). R2 measured strength of model fit. The correlation and the difference between T4PA and L1PA were calculated. The relationship between L1PA/T4PA and coronal Cobb angles were evaluated, controlling for PI.

331 subjects were examined, 258 (78%) female, mean age 16.3 years (SD: 1.7), median Risser 4 (IQR: 4-5). Median C2 Tilt was 0.9° (IQR: -1.2°, 3.4°). LL did not have a meaningful relationship with PI alone (r2=.1) while L1PA had a strong relationship with PI (r2=.55, L1PA=0.5*Pelvic Incidence -19±7°). L1PA and T4PA had a nearly perfect, positive correlation (r=.89, P<.001) with a tendency for T4 to be more dorsal than L1 with increasing PI. There were no relationships between thoracic/lumbar Cobb angles and L1PA.

Sagittal alignments (as measured by L1PA/T4PA) and balance (as measured by C2 Tilt) in patients with AIS are similar to normal standing adults. L1PA and T4PA are strongly related to pelvic incidence, offering a reliable, patient-specific framework for description of the sagittal plane that moves beyond generic targets like thoracic kyphosis or lumbar lordosis.