Model-based Roentgen Stereophotogrammetric Analysis (RSA) of polyethylene implants.

Model-based Roentgen Stereophotogrammetric Analysis (RSA) is able to measure the migration of metallic prostheses with submillimeter accuracy through contour-detection and 3D surface model matching techniques. However, contour-detection is only possible if the prosthesis is clearly visible in the radiograph; consequently Model-based RSA cannot be directly used for polymeric materials due to their limited X-ray attenuation; this is especially clinically relevant for all-polyethylene implants. In this study the radiopacity of unicompartmental Ultra-High Molecular Weight Polyethylene (UHMWPE) knee bearings was increased by diffusing an oil-based contrast agent into the surface to create three different levels of surface radiopacity. Model-based RSA was performed on the bearings alone, the bearings alongside a metallic component held in position using a phantom, the bearings cemented into a Sawbone tibia, and the bearings at different distances from the femoral component. For each condition the precision and accuracy of zero motion of Model-based RSA were assessed. The radiopaque bearings could be located in the stereo-radiographs using Model-based RSA an accuracy comparable to metallic parts for translational movements (0.03 mm to 0.50 mm). For rotational movements, the accuracy was lower (0.1 to 3.0). The measurement accuracy was compared for all the radiopacity levels and no significant difference was found (p=0.08). This study demonstrates that contrast enhanced radiopaque polyethylene can be used for Model-based RSA studies and has equivalent translational measurement precision to metallic parts in the superior-inferior direction.