Influence of pelvic sagittal tilt on 3-dimensional bone coverage in total hip arthroplasty: a simulation analysis

Background: In total hip arthroplasty with computer navigation assistance, cup orientation is generally determined according to the coordinate system relative to the functional pelvic plane (FPP). However, there is a large difference in the cup anteversion between a posterior pelvic tilt relative to the computed tomography (CT) table in the sagittal plane and anterior pelvic tilt, even when the cup is set at the same orientation angle according to each FPP. The present study analysed this difference from the viewpoint of 3-dimensional (3D) coverage of the acetabular component (3D coverage) to determine how the 3D acetabular coverage is altered with changes in pelvic sagittal tilt. Methods: We analysed 3D-simulated acetabular coverage measured in 3D pelvic models reconstructed from the preoperative CT data of 50 patients. In each patient, we created 5 pelvic models with a sagittal tilt of 10 degrees increments between 20 degrees anterior tilt and 20 degrees posterior tilt relative to the CT table. Results: We found that 3D coverage decreased as the pelvis tilted posteriorly. Particularly, there were significant differences between the pelvis with 20 degrees anterior tilt and that with neutral tilt (p < 0.001). There was also a difference between the pelvis with neutral tilt and that with a 20 degrees posterior tilt (p < 0.01). The mean decrease in 3D coverage between the pelvis with neutral tilt and that with 20 degrees posterior tilt was 7.2 +/- 1.6%. Conclusions: We found that 3D coverage differed among pelvis with different sagittal tilts when the cup orientation angle was determined according to FPP.