Introduction: Total hip arthroplasty (THA) is highly effective for advanced hip pathology, but postoperative instability remains a concern. The relationship among the spine, pelvis, and hip—the spinopelvic complex—influences acetabular orientation and implant stability. This registry-based study evaluated changes in spinopelvic alignment parameters before and after primary THA using standing and sitting radiographs.
Method: A consecutive series of 60 patients (32 males, 28 females; mean age 61±12 years; mean BMI 26.9±3.6 kg/m²) undergoing primary THA for osteoarthritis, avascular necrosis, developmental dysplasia, or trauma were included. We obtained standardized standing and sitting lateral spinopelvic radiographs preoperatively and postoperatively (2 months). Parameters measured included lumbar lordosis (LL), sacral slope (SS), pelvic tilt (PT), pelvic incidence (PI), sacro-acetabular angle (SAA), acetabular index (AI), proximal femoral angle (PFA), and combined sagittal index (CSI). Statistical comparisons used paired t-test or Wilcoxon signed-rank test.
Result: In the standing position, significant postoperative increases were observed for PT (12.3°±10.0° to 15.0°±9.5°; p=0.014), SAA (43.8°±24.0° to 70.8°±18.4°; p<0.001), AI (17.4°±13.9° to 39.2°±10.8°; p<0.001), and CSI (185.9°±17.9° to 207.7°±14.5°; p<0.001). In the sitting position, significant increases were found for SAA (69.5°±17.8° to 75.2°±11.1°; p<0.001) and AI (50.6°±17.9° to 62.2°±15.9°; p=0.001). Classical spinopelvic parameters, including LL, SS, and PI, did not change between positions. Subgroup analysis revealed that patients with a history of minor trauma (n=19) demonstrated significantly greater increases in acetabular index in the sitting position compared to those without trauma (25.7° vs. 5.0°; p=0.004).
Conclusion: THA induces significant postoperative changes in acetabular-related sagittal parameters (SAA, AI, CSI) rather than global spinopelvic morphology. The presence of minor trauma history appears to influence postoperative acetabular adaptation, particularly in the sitting position. These findings emphasize the importance of functional, posture-specific spinopelvic assessment in preoperative planning to enable personalized component positioning and reduce instability risk. These early postoperative adaptations suggest that THA induces rapid acetabular reorientation, though the long-term trajectory of these changes remains to be determined.