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° and more than 20°, and analyzed the same as the entire cohort.
Results: For the entire cohort, the median (IQR) standing lateral-center edge angle increased 17°, from a median of 21° (10°) to a median of 38° (8° [95% confidence interval (CI) 16° to 20°; p < 0.001). The median sitting lateral center-edge angle increased 17°, from a median of 18° (8°) to a median of 35° (8° [95% CI 14° to 19°]; p < 0.001). Standing pelvic incidence increased from 50° ± 11° to 52° ± 12° (mean difference 2° [95% CI 1° to 3°]; 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° (14°) to 60° (16°) (95% CI 1° to 5°; 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° to -2°; 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° of standing pelvic tilt demonstrated a median (IQR) 2° increase in pelvic incidence from median 43° (9°) to 45° (12° [95% CI 0.3° to 4°]; 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° 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.
Level of evidence: Level III, therapeutic study.
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