Acetabular component position significantly influences the rebalancing of pelvic sagittal inclination following total hip arthroplasty in patients with Crowe type III/IV developmental dysplasia of the hip

Aims Sagittal lumbar pelvic alignment alters with posterior pelvic tilt (PT) following total hip arthroplasty (THA) for developmental dysplasia of the hip (DDH). The individual value of pelvic sagittal inclination (PSI) following rebalancing of lumbar- pelvic alignment is unknown. In different populations, PT regresses in a linear relationship with pelvic incidence (PI). PSI and PT have a direct relationship to each other via a fixed individual angle angle gamma. This study aimed to investigate whether the new PI created by acetabular component positioning during THA also has a linear regression relationship with PT/PSI when lumbar- pelvic alignment rebalances postoperatively in patients with Crowe type III/ IV DDH. Methods Using SPINEPARA software, we measured the pelvic sagittal parameters including PI, PT, and PSI in 61 patients with Crowe III/IV DDH. Both PSI and PT represent the pelvic tilt state, and the difference between their values is angle gamma (PT = PSI + angle gamma). The regression equation between PI and PT at one year after THA was established. By substituting angle gamma, the relationship between PI and PSI was also established. The Bland- Altman method was used to evaluate the consistency between the PSI calculated by the linear regression equation (ePSI) and the actual PSI (aPSI) measured one year postoperatively. Results The mean PT and PSI changed from preoperative values of 7.0 degrees (SD 6.5 degrees) and-8.0 degrees (SD 6.7 degrees), respectively, to 8.4 degrees (SD 5.5 degrees) and-4.5 degrees (SD 5.9 degrees) at one year postoperatively. This change shows that the pelvis tilted posteriorly following THA. In addition, when lumbar- pelvic alignment rebalanced, the linear regression equation between PI and PT was PT = 0.45 x PI- 10.5 degrees, and PSI could be expressed as PSI = 0.45 x PI- 10.5 degrees- angle gamma. The absolute difference between ePSI and aPSI was less than 5 degrees in 55 of 61 patients (90.16%). Conclusion The new PI created by the positioning of the acetabular component significantly affects the PSI when lumbar- pelvic alignment changes and rebalances after THA in patients with Crowe III/IV DDH.