O1341 VARIATIONS IN MOTION BETWEEN OSSEOUS AND EXTRA-OSSEOUS TANTALUM BEADS IN RSA OF THE HIP

Abstract

Aims: To develop a threshold criteria for exclusion of unstable osseous beads in radiostereometric analysis (RSA) of total joint replacements. Methods: Bead movement was determined for radiologically identified extra-osseous beads which were assumed loose, and for intra-osseous beads as a stable control group. Analyses with a condition number greater than 100 were considered unreliable and rejected. The median rate of total motion over 2 years was used to identify a threshold that identifies unstable beads. Results: In 149 RSA hip study patients 44 extra-osseous beads that could be analysed were identified and 36 osseous control beads were randomly selected. The sensitivity and specificity for detecting extra-osseous beads was calculated for different thresholds of median bead motion (see ROC curve). An optimal threshold of 0.36mm was found to have a sensitivity of 89% and a specificity of 86% for identifying an extra-osseous bead.

Conclusions: Extra-osseous beads are more unstable than osseous beads and are should be removed from RSA analysis series. This new criteria allows automatic exclusion of unstable beads increasing the reliability of the RSA study.