AUTOMATED TEST FOR TANTALUM BEAD INSTABILITY IN RSA OF THE HIP

Abstract

The use of roentgen stereophotogrammetric analysis (RSA) in the assessment of total hip arthroplasty is well recognised as an accurate technique in the measurement of small movements of implants. The technique requires the insertion of tantalum beads into a stable location in the bone at the time of surgery. Failure of bead insertion leads to unstable extra-osseous beads that require to be excluded from the analysis. Previous studies have reported an incidence of extra-osseous beads in the proximal femur of between 2% and 13%. In order to further improve the accuracy of the RSA technique, we have developed a test criterion for exclusion of unstable osseous beads in RSA of total joint replacements.

Using specifically developed software each bead’s movement was determined relative to the rest of the beads in the bone segment. The bead movement was determined for radiologically identified extra-osseous beads, which were assumed loose, and for the remaining intra-osseous beads which were suitable for analysis. Analyses with a condition number greater than 100 were rejected. The rate of motion was calculated from consecutive examinations. Unstable beads were identified as those having a median rate greater than a given threshold. The sensitivity and specificity for detecting extra-osseous beads was calculated for different thresholds of median bead motion.

In 149 RSA hip study patients, 43 extra-osseous beads that could be analysed were identified and a group of 36 osseous beads were selected as a control group. This resulted in an optimum threshold of 0.36mm that gave a sensitivity of 89% and a specificity of 86% for detecting unstable (extra-osseous) beads. The remaining 1428 beads, which were assumed to be osseous, were then tested with this threshold, which gave a sensitivity of 84% and a specificity of 79%.

The median extra-osseous bead rate of movement at 6, 12, 18 and 24 months were 2.24, 0.78, 1.03 and 1.31mm respectively and for osseous beads were 0.27, 0.19, 0.18 and 0.19mm. As both groups of beads appear to show a “bedding in” period, with a higher median bead movement in the first 6 months, the test was repeated with the first 6 month period excluded from the criteria. An optimal threshold of 0.37mm was found to have a sensitivity of 73% and a specificity of 87% for identifying an extra-osseous bead.

While most radiographically classified osseous beads identified as unstable may be false-positives, it is probable that some are extra-osseous but not visibly so on radiographs. The specificity of this technique is likely to be further improved with the increased precision from digital scanning techniques. Tantalum beads in general appear to be relatively unstable in the first 6 months, consistent with the expected osteo-intergration of the bead. This new criterion for bead instability allows automatic exclusion of unstable beads increasing the reliability of the RSA technique both in future studies and retrospectively to existing data.