Radiostereometric analysis of sacroiliac joint movement and outcomes of pelvic joint fusion.

Background Different techniques have been used to quantify the movement of sacroiliac (SI) joints. These include radiostereometric analysis (RSA), but the accuracy and precision of this method have not been properly evaluated and it is unclear how many markers are required and where they should be placed to achieve proper accuracy and precision. Purpose The purpose of this study was to test accuracy and precision of RSA, applied to the SI joint, in a phantom model and in patients. Methods We used a plastic phantom attached to a micrometer to obtain a true value of the movement of the SI joint and compared this value with the measured value obtained by RSA; the difference represented the accuracy. The precision of the system was measured by double examination in the phantom and in six patients, and was expressed by a limit of significance (LOS). We analyzed different marker distributions to find optimal marker placement and number of markers needed. Results The accuracy was high and we identified no systematic errors. The precision of the phantom was high with a LOS less than 0.25 and 0.16 mm for all directions, and in patients, the precision was less than 0.71 for rotations and 0.47 mm translations. No markers were needed in the pubic symphysis to obtain good precision. Conclusions The accuracy and precision are high when RSA is used to measure movement in the SI joint and support the use of RSA in research of SI joint motion.