Automated Estimation of Hip Prosthesis Migration: a Feasibility Study

A common complication associated with hip arthoplasty is prosthesis migration, and for most cemented compo- nents a migration greater than 0.85 mm within the first six months after surgery, are an indicator for prosthesis failure. Currently, prosthesis migration is evaluated using X-ray images, which can only reliably estimate migra- tions larger than 5 mm. We propose an automated method for estimating prosthesis migration more accurately, using CT images and image registration techniques. We report on the results obtained using an experimental set-up, in which a metal prosthesis can be translated and rotated with respect to a cadaver femur, over dis- tances and angles applied using a combination of positioning stages. Images are first preprocessed to reduce artefacts. Bone and prosthesis are extracted using consecutive thresholding and morphological operations. Two registrations are performed, one aligning the bones and the other aligning the prostheses. The migration is estimated as the difference between the found transformations. We use a robust, multi-resolution, stochastic optimization approach, and compare the mean squared intensity differences (MS) to mutual information (MI). 30 high-resolution helical CT scans were acquired for prosthesis translations ranging from 0.05 mm to 4 mm, and rotations ranging from 0.3?to 3?. For the translations, the mean 3D registration error was found to be 0.22 mm for MS, and 0.15 mm for MI. For the rotations, the standard deviation of the estimation error was 0.18?for MS, and 0.08?for MI. The results show that the proposed approach is feasible and that clinically acceptable accuracies can be obtained. Clinical validation studies on patient images will now be undertaken.