Pelvic Position and Movement during Hip Arthroplasty

Acetabular component orientation is influenced not only by the orientation at which the surgeon implants the component but also the orientation of the pelvis at the time of impaction. Hence, the orientation of the pelvis at set-up and its movement during the operation are critically important. Using a validated photogrammetric technique, during 67 hip arthroplasties, we measured how 3 surgeons orientated the patients’ pelvis, how much the pelvises moved and what effect these had on final cup orientation. Pelvic orientation at setup varied widely (Mean/SD; tilt: 8°/16°, obliquity: -4°/6°, rotation: -8°/7°). Significant differences in pelvic positioning were detected between surgeons (p<0.001). The mean angular movement of the pelvis between set-up and cup implantation was 9° (SD:6°). Factors influencing pelvic movement included surgeon, approach (posterior>lateral), procedure (resurfacing>THR) and type of support (p<0.001). Although on average surgeons achieved their desired cup orientation, there was considerable variability (2SD=16°) in cup orientations. We conclude that error in positioning the patient at set-up and movement of the pelvis during the operation account for much of the variability in cup orientation. In order to improve reliability of achieving optimal cup orientation improved methods of positioning and holding the pelvis are required.