# Precision, Accuracy, and Observer Reliability of Computed Tomography‐Based Radiostereometric Analysis in Measuring Femoral Stem Migration In Vitro

## TL;DR

This study evaluates the precision and reliability of using CT scans to measure tiny movements of hip implants in lab conditions.

## Contribution

The study introduces CT-based RSA as a reliable alternative for measuring implant micromotion in total hip arthroplasty research.

## Key findings

- CT-RSA achieved sub-tenth-millimeter precision in measuring femoral stem translations.
- Observer reliability was excellent with ICC values ranging from 0.91 to 0.99.
- Accuracy of measurements was within 0.03–0.14 mm on transformed axes.

## Abstract

CT‐based radiostereometric analysis (CT‐RSA) is a method of measuring implant micromotion using low‐dose CT scans. We evaluated the precision, accuracy, and observer reliability of CT‐RSA for measuring femoral stem translation in THA in vitro. A cementless femoral stem was implanted in a synthetic femur mounted on a calibrated micrometer platform. Controlled translations were applied along each orthogonal axis across 5 series, generating 90 CT data sets. Only translational micromotions were imposed and analyzed. Precision was calculated from double examinations—repeated scans of the same displacement acquired in a different series—yielding 60 pairs. The femoral stem, head, and bone were segmented, the head's geometric center was defined, and translations were analyzed. Because the platform was fixed 90° to the femoral neck (CCD 135°), data were reported both on native axes, and after a 45° counterclockwise rotation to the conventional RSA frame. Precision was estimated from double examinations (SD × t), accuracy from RMSE versus micrometer readings (RMSE × t), and reliability from intraclass correlation coefficients (ICC) by two blinded observers. On native axes, precision ranged 0.06–0.08 mm and accuracy 0.09–0.13 mm. On transformed axes, precision ranged 0.06–0.07 mm and accuracy 0.03–0.14 mm. Intraobserver reliability was excellent (mean ICC 0.99 for all axes), as well as interobserver reliability (mean ICC 0.91–0.98). CT‐RSA achieved sub‐tenth‐millimeter precision with high repeatability and excellent observer agreement. These findings support CT‐RSA as a viable alternative for early migration measurement in THA research. Clinical validation with low‐dose protocols and predefined quality criteria remains warranted.