Pose estimation of artificial hip joint using a single radiographic projection

Total hip joint arthroplasty is commonly used in cases of osteoarthritis and rheumatoid arthritis. A hip joint prosthesis consists of a cup and a stem. The relative pose (position and orientation) of the prosthesis, when the patient is standing naturally, is planned pre-operatively; however, there is no guarantee that the pre-operatively planned pose will be accurate. Prosthesis component malposition, which is inadequate positioning of the acetabular or femoral component, results in limited joint movement and a large prosthetic joint contact area, and this can cause dislocation of the femoral head from the cap. Because the range of motion after total hip joint arthroplasty is determined by the relative pose of the cup and the stem, it is important to know the relative pose after hip arthroplasty. Several pose estimation methods using single-plane fluoroscopy or conventional roentgenography have been proposed. Because a conventional radiograph system is commonly used at small cost, a method using conventional radiograms is desirable for use in a clinical setting. In this study, a pose estimation method using conventional radiography is proposed, and an in vitro experiment is performed to evaluate its accuracy. The rotational errors of the relative pose of the cup with respect to the stem were smaller than 2.3 degrees.