Spine - 2026-04-06 - Journal Article
Vertebral-Pelvic Angle Settling: Quantifying T4-L1-Hip Axis Changes Between Intraoperative and Postoperative Alignment for Adult Spinal Deformity.
Yahanda AT, Joseph K, Bui TT, Vogl S, Gupta VP, Pallotta NA, Neuman BJ, Hills J, Kelly MP, Ray WZ, Gupta MC, Molina CA
Topics
Key Takeaway
After ASD spinopelvic fusion, L1PA increases by ~3° and T4PA by ~4.4° between intraoperative prone and 6-week postoperative standing radiographs, a phenomenon termed 'VPA settling.'
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Summary
This study quantified changes in vertebral-pelvic angles (L1PA and T4PA) between intraoperative prone and 6-week postoperative standing radiographs in 216 ASD patients undergoing spinopelvic fusion to at least L2-pelvis. L1PA increased 2.94° (L1C group) and 3.43° (T4L1C group); T4PA increased 4.62° and 4.23°, respectively, with T4PA-L1PA concordance worsening in both groups (p≤0.007). Multivariable analysis identified preoperative C2PA, 3-column osteotomy, rod material, rod diameter, rod number, and lumbar interbody use as significant predictors of VPA change.
Key Limitation
Retrospective design with heterogeneous construct configurations prevents isolation of individual hardware variables (rod material, diameter, number) as independent contributors to VPA settling.
Original Abstract
STUDY DESIGN
Retrospective cohort.
OBJECTIVE
To evaluate changes in vertebral-pelvic angles (VPAs) between intraoperative and 6-week postoperative radiographs after fusions for adult spinal deformity (ASD).
SUMMARY OF BACKGROUND DATA
VPAs are measures of sagittal balance that are independent of posture. Their magnitude is expected to remain relatively fixed within spinal constructs between intraoperative and postoperative alignment. However, no studies have examined VPA fidelity between intraoperative prone and postoperative standing radiographs or analyzed potential factors influencing VPA shifts.
METHODS
We analyzed data for patients receiving spinopelvic fusions for ASD from at least L2-pelvis. Pre- and 6-week postoperative VPAs were measured on standing radiographs. Intraoperative VPAs were measured on prone radiographs obtained after screw tightening. Primary VPAs were L1-pelvic angle (L1PA) and T4-pelvic angle (T4PA). Uni- and multivariable regressions were performed to assess variables that may be associated with intra-to-postoperative VPA changes. Analyses were stratified by whether constructs controlled L1PA only (L1C;
UIV
L2-T6) or T4PA+L1PA (T4L1C;
UIV
T5 and above).
RESULTS
216 patients were analyzed (mean age 64.9±10.7 y, 68% female). 80 patients were L1C and 136 patients were T4L1C. Regardless of UIV, both L1PA (p≤0.0001) and T4PA (P<0.0001) increased on postoperative radiographs. L1PA changed by 2.94±2.5° and 3.43±2.5° for L1C and T4L1C, respectively. T4PA changed by 4.62±2.8° and 4.23±2.9° for L1C and T4L1C, respectively. T4PA-L1PA concordance similarly worsened (p≤0.007 for LIC and T4L1C). Multivariable analyses identified preoperative C2PA, 3-column osteotomy, rod material, rod diameter, rod number, and lumbar interbody use as significantly associated with ΔL1PA or ΔT4PA.
CONCLUSIONS
L1PA, T4PA, and T4PA-L1PA mismatch increased with upright posture postoperatively, even when stratifying by fusion length. Several variables were potentially related to these changes. These data support the notion of "VPA settling" whereby the realigned spine and construct cannot counteract some amount of reversion towards preoperative alignment. Anticipating these changes could have implications for intraoperative decision-making and alignment goals.
LEVEL OF EVIDENCE
III.