Validation of a simple radiographic method to determine variations in pelvic and acetabular cup sagittal plane alignment after total hip arthroplasty

Background and purpose: Orientation of acetabular component, influenced by pelvic tilt, body position, and individual variations affects the outcome following total hip arthroplasty (THA). Currently available methods of evaluation are either imprecise or require advanced image processing. We analyzed intersubject and intrasubject variability of pelvic tilt, measured by sagittal sacral tilt (ST) and its relationship with acetabular component tilt (AT) by using a simple method based on standard radiographs. Materials and methods: ST was measured on lateral radiographs of pelvis including lumbosacral spine obtained in supine, sitting, standing, and lateral decubitus position for 40 asymptomatic THA patients and compared to computed tomography (CT) data obtained in supine position. AT was measured on lateral radiographs (measured acetabular tilt: MAT) in each position and compared to measurement of AT on CT and an indirectly calculated AT (CAT). Results: Mean ST changed from supine to sitting, standing, and lateral decubitus positions as follows: 26.5±15.5° (range 4.6-73.4°), 8.4±6.2° (range 0.6-24.5°), and 13.4±8.4° (range 0.1-24.2°; p<0.0001, p=0.002, p=0.006). The MAT on radiographs was not significantly different from the MAT measured on CT (p=0.002) and the CAT (p=0.06). There is a good correlation between change in ST and MAT in sagittal plane (r=0.93). Conclusion: Measurement of ST on radiographs is a simple and reliable method to track changes in pelvic tilt in different body positions. There is significant intersubject and intrasubject variation of ST and MAT with postural changes and it may explain causes of impingement or instability following THA, which could not be previously explained.