Dynamic change of pelvic incidence after long fusion to pelvis with S2-alar-iliac screw: a 2-year follow-up study

Introduction Pelvic incidence (PI) is a key morphological parameter that reflects the relation between the sacrum and iliac wings. It is well accepted that PI remains constant after reaching maturity. However, recent studies indicated that PI might be altered after lumbosacral fusion. Additionally, it remains uncertain on the long-term influence of long fusion to pelvis with S2-alar-iliac screw on PI in patients with adult spinal deformity (ASD). Study Design A retrospective study. Objective To investigate whether and how PI would change during the follow-up in ASD patients who underwent S2AI fixation and to identify factors associated with the change in PI. Methods We retrospectively reviewed all ASD patients who underwent spinal surgery using S2AI screws between November 2014 and January 2017 at our institution. Patients with minimum follow-up of two years were included. The following sagittal radiographic parameters were measured: PI, Lumbar lordosis (LL), pelvic tilt (PT), PI-LL, sagittal vertical axis (SVA) at pre-op, post-op and 2-year follow-up. According to the changes in PI at immediate post-operation, patients were classified into two groups; Group A: Changes of PI less than or equal 5 degrees and Group B: Changes of PI greater than 5 degrees. Results A total of 82 ASD patients (Group A: 32, Group B: 50; mean age of 53.5 +/- 12.6 years) with a mean follow-up period of 30.2 +/- 9.2 months were included in this study. At immediate post-operation, Group A showed no significant change in PI (45.7 degrees +/- 11.4 degrees to 45.3 degrees +/- 11.2 degrees, p = 0.749); while Group B had a significant decrease in PI (51.6 degrees +/- 14.5 degrees to 40.9 degrees +/- 14.0 degrees, p < 0.001). At the last follow-up, 48% patients (24/50) in Group B had a significant increase in PI (32.8 degrees +/- 6.4 degrees to 45.8 degrees +/- 11.2 degrees, p < 0.001). Intergroup analysis showed that Delta PI, post-op PI, post-op PT and age were significantly different between both groups. In addition, pre-op PI, post-op PI, post-op PT, post-op PI-LL were significantly correlated with Delta PI at last follow-up. Also, logistic regression analysis showed that post-op PI was the associated risk factor (OR = 0.865, p = 0.024) for PI-LL mismatch. Conclusion Our study showed that PI decreased in more than half of ASD patients immediately after spinal surgery using S2AI screws. Approximately 48% of them were able to recover during the 2-year follow-up. Lower pre-op PI, post-op PI and PT were found to be strongly associated with the return of PI. Thus, these current findings indicated that patients with a high PI at pre-operation should not be over-corrected to avoid PI-LL mismatch postoperatively.

Automated summary

This study investigated the changes in pelvic incidence (PI) in adult spinal deformity (ASD) patients who underwent S2-alar-iliac screw fixation. The results showed that PI decreased immediately after surgery in more than half of the patients, but approximately 48% were able to recover during the 2-year follow-up. Lower pre-op PI, post-op PI, and pelvic tilt were strongly associated with the return of PI. Therefore, it is recommended to avoid over-correction in patients with high pre-operative PI to prevent PI-LL mismatch postoperatively.