Comparison of matching procedures applied on conventional X-ray images of total hip arthroplasty

In this work we compare the results of two matching methods for the alignment of x-ray images of Total Hip Arthroplasty (THA). The application of Singular Value Decomposition (SVD) on these type of images may encounter some problems related to the identification of the marker points. We developed an alternative iterative procedure, based on the linearization of the set of equations which define the affine transformations. The two methods, SVD vs. iterative, have been applied to match five couples of x-ray images of patients implanted with different commercial THA devices. The repeated tests show average fits usually better for the iterative method, with values of misalignments less than 2 pixels. The greatest improvement coming from the iterative methods is due to its robustness: it has a two order of magnitude higher repeatability, and it is much less sensitive to the operator input error.