Background There are few data on the impact of periacetabular osteotomy (PAO) on sagittal spinopelvic alignment. Prior studies have attempted to delineate the relationship by performing measurements on AP radiographs and using mathematical models to determine changes in postoperative pelvic tilt. This information is clinically significant to a surgeon when evaluating acetabular/pelvic position intraoperatively and understanding spinopelvic alignment changes postoperatively; therefore, radiographic changes from PAO should be described in more detail. Questions/purposes In this study, we asked: (1) Does the performance of PAO result in consistent changes in spinopelvic alignment, as measured on EOS radiographs? (2) Does this differ for unilateral versus bilateral PAOs? (3) Does this differ in the setting of a mobile spine versus an immobile spine? (4) Does this differ based on preoperative pelvic tilt? Methods Mean preoperative and at least 1-year postoperative (15 +/- 8 months from surgery, minimum 11 months, maximum 65 months) EOS hip-to-ankle standing and sitting radiographs for 55 patients in a prospectively collected registry who underwent PAO with a single surgeon from January 1, 2019, to January 11, 2022, were measured for pelvic incidence, pelvic tilt, sacral slope, lumbar lordosis, lateral center-edge angle, L1 pelvic angle, and pubic symphysis to the sacroiliac index. Normality was assessed and paired sample t-tests (normally distributed data) or Wilcoxon signed rank tests (not normally distributed data) were utilized to assess if any measurements changed from preoperative to postoperative. Patients were then divided based on whether they had unilateral or bilateral dysplasia and unilateral or bilateral surgery, and these subgroups were analyzed the same way as the entire cohort. Two more subgroups were then formed based on lumbar mobility, defined as a change in sitting-to-standing lumbar lordosis less or greater than 1 SD from the population mean preoperatively, and the subgroups were analyzed the same way as the entire cohort. Finally, two additional subgroups were formed, preoperative standing pelvic tilt less than 10 degrees and more than 20 degrees, and analyzed the same as the entire cohort. Results For the entire cohort, the median (IQR) standing lateral-center edge angle increased 17 degrees, from a median of 21 degrees (10 degrees) to a median of 38 degrees (8 degrees [95% confidence interval (CI) 16 degrees to 20 degrees; p < 0.001). The median sitting lateral center-edge angle increased 17 degrees, from a median of 18 degrees (8 degrees) to a median of 35 degrees (8 degrees [95% CI 14 degrees to 19 degrees]; p < 0.001). Standing pelvic incidence increased from 50 degrees +/- 11 degrees to 52 degrees +/- 12 degrees (mean difference 2 degrees [95% CI 1 degrees to 3 degrees]; p = 0.004), but there were no changes for other measured parameters. There were no changes in any of the spinopelvic parameters for patients with unilateral dysplasia receiving a unilateral PAO, but patients with bilateral dysplasia who underwent bilateral PAOs demonstrated an increase in pelvic incidence from 57 degrees (14 degrees) to 60 degrees (16 degrees) (95% CI 1 degrees to 5 degrees; p = 0.02) and a decrease in pubic symphysis to sacroiliac index from 84 mm (24 mm) to 77 mm (23 mm) (95% CI -7 degrees to -2 degrees; p = 0.007). Patients with mobile lumbar spines preoperatively did not exhibit any changes in sagittal spinopelvic alignment, but patients with immobile lumbar spines preoperatively experienced several changes after surgery. Patients with less than 10 degrees of standing pelvic tilt demonstrated a median (IQR) 2 degrees increase in pelvic incidence from median 43 degrees (9 degrees) to 45 degrees (12 degrees [95% CI 0.3 degrees to 4 degrees]; p = 0.03), but they did not experience any other changes in sagittal spinopelvic alignment parameters postoperatively. Patients with preoperative pelvic tilt more than 20 degrees did not experience any change in sagittal spinopelvic parameters. Conclusion PAO increases pelvic incidence, potentially because of anterior translation of the hip center. There were no changes in other spinopelvic parameters postoperatively except after bilateral PAO. Additionally, patients lacking spine mobility preoperatively, indicated by a minimal change in lumbar lordosis between standing and sitting positions, may experience several changes in spinopelvic alignment, including increased mobility of their spine after PAO. This may be because of decreased compensatory spine splinting after increasing acetabular coverage, but further research including patient-reported outcomes is warranted.
