Computer assisted determination of acetabular cup orientation using 2D-3D image registration

Background 2D-3D image-based registration methods have been developed to measure acetabular cup orientation after total hip arthroplasty (THA). These methods require registration of both the prosthesis and the CT images to 2D radiographs and compute implant position with respect to a reference. The application of these methods is limited in clinical practice due to two limitations: (1) the requirement of a computer-aided design (CAD) model of the prosthesis, which may be unavailable due to the proprietary concerns of the manufacturer, and (2) the requirement of either multiple radiographs or radiograph-specific calibration, usually unavailable for retrospective studies. In this paper, we propose a new method to address these limitations. Methods A new formulation for determination of postoperative cup orientation, which couples a radiographic measurement with 2D-3D image matching, was developed. In our formulation, the radiographic measurement can be obtained with known methods so that the challenge lies in the 2D-3D image matching. To solve this problem, a hybrid 2D-3D registration scheme combining a landmark-to-ray 2D-3D alignment with a robust intensity-based 2D-3D registration was used. The hybrid 2D-3D registration scheme allows computing both the post-operative cup orientation with respect to an anatomical reference and the pelvic tilt and rotation with respect to the X-ray imaging table/plate. The method was validated using 2D adult cadaver hips. Results Using the hybrid 2D-3D registration scheme, our method showed a mean accuracy of 1.0 +/- 0.7 degrees (range from 0.1 degrees to 2.0 degrees.) for inclination and 1.7 +/- 1.2 degrees (range from 0.0 degrees to 3.9 degrees) for anteversion, taking the measurements from postoperative CT images as ground truths. Conclusions Our new solution formulation and the hybrid 2D-3D registration scheme facilitate estimation of postoperative cup orientation and measurement of pelvic tilt and rotation.