A simplified radiographic method for measuring bone-component motion in total knees.

Abstract

Measurement of the relative motion between an implant and the surrounding bone over different time periods is valuable for assessing and comparing the component stability and predicting the potential future outcome. The RSA method, where small beads are implanted in the bone adjacent to the component, can measure implant-bone position to an accuracy of about 0.1 mm. However, the method involves special radiographic views and analytical software, not readily available. For purposes of component assessment on a more routine basis and for multi-centre trials, a method was developed where standard A-P and M-L radiographs were used. Computer software was written which estimated the out-of-plane rotations of the component relative to the plane of the film, and then carried out corrections to enhance the accuracy of calculation of the bead heights relative to the component. The theoretical errors were shown to be less than 0.04 mm for the expected range of out-of-plane rotations. When radiographs of components in simulated bones were taken at a range of rotations, the 95% confidence limits for axial displacement were found to be less than +0.3 mm, and for rotation in the plane to be 0.6 degrees. This indicated that the method was useful for studying knee components where the sinkage could reach a range of 0.5-2 mm in a 2 year period.