Vertebral-Pelvic Angle Settling: Quantifying T4-L1-Hip Axis Changes Between Intraoperative and Postoperative...

Area of Science:

• Spine surgery
• Orthopedics
• Radiographic analysis

Background:

• Vertebral-pelvic angles (VPAs) measure sagittal balance and are expected to remain stable post-fusion.
• No studies have assessed VPA changes between intraoperative prone and postoperative standing radiographs in adult spinal deformity (ASD) patients.
• Potential factors influencing VPA shifts require investigation.

Purpose of the Study:

• To evaluate changes in vertebral-pelvic angles (VPAs) between intraoperative and 6-week postoperative radiographs after fusions for adult spinal deformity (ASD).
• To identify factors associated with VPA shifts from intraoperative to postoperative alignment.

Main Methods:

• Retrospective cohort analysis of 216 patients undergoing spinopelvic fusions for ASD (minimum L2-pelvis).
• Intraoperative VPAs measured on prone radiographs; postoperative VPAs measured on standing radiographs.
• Analyses stratified by construct type (L1PA control vs. T4PA+L1PA control); uni- and multivariable regressions used to identify associated variables.

Main Results:

• Both L1-pelvic angle (L1PA) and T4-pelvic angle (T4PA) significantly increased postoperatively (p≤0.0001).
• L1PA increased by 2.94° (L1C) and 3.43° (T4L1C); T4PA increased by 4.62° (L1C) and 4.23° (T4L1C).
• T4PA-L1PA concordance worsened postoperatively (p≤0.007); preoperative C2PA, osteotomy, rod characteristics, and interbody use were associated with VPA changes.

Conclusions:

• Postoperative upright posture leads to increased L1PA, T4PA, and T4PA-L1PA mismatch, termed "VPA settling."
• This phenomenon suggests a tendency for the spine/construct to revert towards preoperative alignment.
• Understanding VPA settling is crucial for optimizing intraoperative decision-making and achieving alignment goals in ASD surgery.