Locating implants with respect to the bone in diagnostic X-ray images of the pelvis

This thesis is about the assessment of the migration of artificial hip sockets in clinical x-ray images. “Migration”means the displacement of the bony implant with respect to the bone in time. It is seen as the most objective indicator for an implant loosening, which is the main problem in hip replacement. The data source are the standard anterior-posterior pelvic radiographs which are usually acquired for the post-operative examination of the replacement. According to the general case, it is assumed that no computer tomograms of the pelvis are available. Due to the lack of 3D information of the bone, only the components of the migration parallel to the plane of the radiographic film can be observed. It is assumed that a migration of only 0.5 to 1.0 millimeter in the first two postoperative years implies an increased risk of later loosening. This correlation is especially important if a new type of cup or implantation technique is introduced. It means that a precise assessment of migration allows for an early judgement of the fixation properties and for the prevention of the use of implants with a high tendency to loose in many patients. Hence, a useful method for measuring migration should allow for the observation of implant displacements in the sub-millimeter range. The precision of the previously proposed methods for cup migration assessment was analysed theoretically considering the errors entering the measurement. Standard deviations in the range of 0.6mm to 3.0mm resulted. Based on the theoretical analysis, a computerized system for a more precise assessment of the migration was designed. In this system, which is named XIMIT, the sensitivity of the measurement towards the variable position and orientation of the pelvis at exposure is minimized. The most relevant of the remaining sources of error is the uncertainty in determining the position of the necessary bony landmarks and of the implant in the image. It is reduced by the use of image analysis tools. The precise localization of the bony landmarks is achieved by applying a state-of-the-art template matching algorithm. The user roughly determines the position of a landmark in all images of a pelvis and chooses the template, i.e. a region of interest containing the respective bony feature in the first post-operative radiograph. The algorithm finds the exact landmark coordinates in the later radiographs by geometrically fitting the bony structures in these images to the template. The implant is located based on the principle of Analysis-by-Synthesis. Using the CAD model of the artificial hip socket, a synthetic radiograph of the cup depending on its approximate 3D position and orientation in the x-ray system is generated. The exact 3D parameters are recovered by matching the synthetic image to the original one while virtually translating and rotating the cup.