Evaluation of methods of measuring wear of polyethylene acetabular components intotal hip arthroplasty

Abstract

Background: The introduction of highly cross-linked, wear resistant polyethylene acetabularcomponents for clinical use in total hip replacements has intensified the interest in and need for highlyaccurate methods of measuring wear from clinical radiographs. There are several methods currentlyavailable for measuring the relative displacement of the femoral head into the acetabular componentresulting from the combination of plastic deformation and wear of the polyethylene bearing surface.However, there is no standard method available for evaluating the accuracy and precision of thesevarious methods and therefore no way of objectively comparing them. This thesis was designed todevelop an experimental standard practice for this purpose. The second purpose of the thesis was toperform two clinical studies designed to evaluate wear of polyethylene using different methods as wellas evaluate various parameters which may affect the quality of the calculated results.Specific aim: The goal of this research thesis was to explore a variety of variables and conditions whichmay affect a clinical radiographic study of femoral head penetration by first developing and using aphantom model of a total hip replacement and ultimately by evaluating two series of clinical radiographsfor two groups of patients who have received total hip replacements.Method: A physical hip model which used actual THR components was constructed which couldsimulate three dimensional migrations of the femoral head into the acetabular component due to creepand wear of the polyethylene component. A displacement protocol was developed consisting ofseventeen steps designed to simulate the three dimensional magnitude of femoral head displacementknown to occur in vivo. This phantom was used for the three research papers which evaluated, underideal circumstances, several variables known to affect RSA and non-RSA radiographic studies.The first clinical study compared the results of two methods of measuring femoral head penetrationusing the same group of THR patients. The second clinical study compared the results using radiographstaken with the patients standing vs. lying supine.Results: Using the phantom, change from conventional hard copy radiographs to digital DICOM imageswas validated. The results using the UmRSA software were superior to those using the RSA-CMSmethod. Different bead placement methods and configurations were validated. In further development ofthe Hip Analysis Suite program, the results using oblique projection of the hip were comparable to theuse of the standard A/P and lateral films under ideal experimental conditions. While the clinical resultsusing the Hip Analysis Suite software were similar to those obtained using the UmRSA software interms of the pattern of wear over time, the magnitude of the measured femoral head penetration was30% higher. In an RSA study of femoral head penetration, the evaluation of supine vs. standingradiographs showed comparable results.Conclusions: The results of this thesis enable comparisons of the effect of changing differentparameters which can be useful in developing an experimental design. The phantom model has to be auseful tool in evaluating and developing analysis software, validation of new research centers, and as atraining tool for new users. The clinical studies provide insight into the relative comparison of twoanalysis methods under practical conditions and have examined the need and practicality of requiringstanding radiographs for clinical wear studies.