1504. Changes in Pelvic Incidence after Adult Deformity Surgery

Authors: Christian B. Ricks, MD; Adam Kanter (Pittsburgh, PA)

Objective

The authors challenge the dogma that pelvic incidence is a fixed parameter by investigating changes after spinal deformity surgery.


Methods

A retrospective chart review was performed on 29 patients who underwent sagittal deformity surgery at UPMC.  Measurements of the pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), and lumbar lordosis (LL) were performed using Surgimap software and confirmed with conventional manual measurements. Oswestry Disability Index (ODI) and Visual Analog Scale (VAS) were prospectively gathered.  A paired t-test was used to compare changes in pelvic incidence.


Results

29 patients, averaging 60.4 years, preoperative ODI 49.4, VAS 7.3, PI 60.0, PT 28.9, and PI-LL mismatch of 25.4 underwent surgical correction fusing an average of 10.6 spinal levels.  Changes in PI compared to preoperative values were statistically significant for those undergoing pelvic fixation (n=14) at 6 months (p=.04) and 24 months (p=.05), representing an average of 2 degrees.  Those without pelvic fixation  (n=15) did not have statistically significant changes in PI.  Among those fused to pelvis, iliac screws (n=3) did not show significant changes in PI, while those with S2-alar screws did at 6 months (p=.01) and 24 months (p=.02).  Those who required a reoperation within 5 years had a statistically significant change in PI (p=.04).  Analyzing the correlation between postoperative change in PI among S2-alar fused patients showed an R= -.44 for ODI and R= -.63 for PI-LL mismatch.


Conclusions

Pelvic incidence changes in patients undergoing sagittal deformity correction with S2-alar screws.  This may represent shear forces across the sacro-iliac joint which change from a standing to prone position, and become immobilized with S2-alar fixation.   This is supported by reasonable correlation with preoperative PI-LL mismatch, and may be used to help predict those who will require eventual revision surgery.