2D/3D registration for measurement of implant alignment after total hip replacement.

Measurements of cup alignment after total hip replacement (THR) surgery are typically performed on postoperative pelvic radiographs. Radiographic measurement of cup orientation depends on the position and orientation of the pelvis on the X-ray table, and its variability could introduce significant measurement errors. We have developed a tool to accurately measure 3D implant orientation from postoperative antero-posterior radiographs by registering to preoperative CT scans. The purpose of this study is to experimentally and clinically validate the automatic CT/X-ray matching algorithm by comparing the X-ray based measurements of cup orientation with direct 3D measurements from postoperative CT scans. The mean measurement errors (+/- stdev) found in this study were 0.4 degrees +/-0.8 degrees for abduction and 0.6 degrees +/- 0.8 degrees for version. In addition, radiographic pelvic orientation measurements demonstrated a wide range of inter-subject variability, with pelvic flexion ranging from -5.9 degrees to 11.2 degrees.