Development and validation of an automated and marker-free CT-based spatial analysis method (CTSA) for assessment of femoral hip implant migration

Background and purpose — We developed a marker-free auto- mated CT-based spatial analysis (CTSA) method to detect stem- bone migration in consecutive CT datasets and assessed the accu- racy and precision in vitro. Our aim was to demonstrate that in vitro accuracy and precision of CTSA is comparable to that of radiostereometric analysis (RSA). Material and methods — Stem and bone were segmented in 2 CT datasets and both were registered pairwise. The resulting rigid transformations were compared and transferred to an ana- tomically sound coordinate system, taking the stem as reference. This resulted in 3 translation parameters and 3 rotation param- eters describing the relative amount of stem-bone displacement, and it allowed calculation of the point of maximal stem migration. Accuracy was evaluated in 39 comparisons by imposing known stem migration on a stem-bone model. Precision was estimated in 20 comparisons based on a zero-migration model, and in 5 patients without stem loosening. Results — Limits of the 95% tolerance intervals (TIs) for accu- racy did not exceed 0.28 mm for translations and 0.20° for rota- tions (largest standard deviation of the signed error (SDSE): 0.081 mm and 0.057°). In vitro, limits of the 95% TI for precision in a clinically relevant setting (8 comparisons) were below 0.09 mm and 0.14° (largest SDSE: 0.012 mm and 0.020°). In patients, the precision was lower, but acceptable, and dependent on CT scan resolution. Interpretation — CTSA allows detection of stem-bone migra- tion with an accuracy and precision comparable to that of RSA. It could be valuable for evaluation of subtle stem loosening in clini- cal practice.