Pelvic tilt makes acetabular cup navigation inaccurate

Background Modern navigation techniques allow precise positioning of the acetabular cup relative to the anterior pelvic plane. Variations in pelvic tilt will affect the resulting spatial orientation of the cup. Methods We measured pelvic tilt in 30 volunteers with an inclinometer combined with an ultrasonographic position measurement system. A mathematical algorithm was developed to calculate the resulting cup position measured on standard radiographs, depending on pelvic tilt. Results Average pelvic tilt at rest was -4 degrees in the lying position and -8 degrees in the standing position, and ranged from -27 degrees to +3 degrees. Pelvic reclination of 1 degrees will lead to functional anteversion of the cup of approximately 0.7 degrees. Interpretation Pelvic tilt makes navigation systems referring to the anterior plane inaccurate.