Abstract
Measurement of polyethylene (PE) wear in total hip joint replacement (THJR) is performed by measuring change in the position of the femoral head on post-operative radiographs. Early methods used manual measurement with calipers and concentric circles, while more recent techniques involve the use of computer assisted technology. RSA, while mainly used for measuring component migration, can also be used for measurement of PE wear. The aim of this paper is to describe two new methods for measuring PE wear;
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A completely automated measurement (which eliminates user error and is 100% reproducible).
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A method currently under development which uses artificial intelligence to match CAD models to radiographs, enabling measurement of both PE wear and prosthesis migration.
For the Automated Measurement Technique (AMT), software has been developed which locates the centre of the acetabular cup and femoral head on both the anteroposterior and lateral radiographs. No user input is required. Accuracy is ± 0.16 mm. Clinically, it has been used in a double-blinded randomized controlled trial (RCT) comparing conventional with cross-linked PE. For the Model Matching Technique (MMT), two pieces of software are combined, Ray-Tracing technology (used in the generation of animated movies), and the Genetic Algorithm (a branch of Artificial Intelligence). CAD models of an acetabular cup and femoral head are matched to post-operative films to position them in 3D space. Change in position of these models over time represents PE wear. CAD models of the patients’ pelvis and femur (built from CT scans) can be similarly used to measure femoral and acetabular component migration.
The AMT was used to measure the PE wear of 116 patients enrolled in a prospective RCT comparing conventional and cross-linked PE. At a follow-up of two to four years, cross-linked PE showed statistic ally significant lower PE wear than the conventional material. A cadaver pelvis and femur has been used to analyse accuracy of the MMT for measurement of component migration. Preliminary results show an accuracy of ± 0.22mm for component migration. The accuracy of PE wear measurement appears to be significantly less than this.
The development of new bearing surfaces to reduce wear in THJR requires new techniques of in-vivo wear measurement. These two new techniques should give important information on the performance of new bearings, and possibly allow measurement of clinical component migration without the need for bead implantation.
Correspondence should be addressed to Associate Professor N. Susan Stott, Orthopaedic Department, Starship Children’s Hospital, Private Bag 92024, Auckland, New Zealand.
