Comparison between model-based RSA and an AI-based CT-RSA: an accuracy study of 30 patients

Abstract

Background and purpose: Radiostereometry (RSA) is the current gold standard for evaluating early implant migration. CT-based migration analysis is a promising method, with fewer handling requirements compared with RSA and no need for implanted bone-markers. We aimed to evaluate agreement between a new artificial intelligence (AI)-based CT-RSA and model-based RSA (MBRSA) in measuring migration of cup and stem in total hip arthroplasty (THA).

Patients and methods: 30 patients with THA for primary osteoarthritis (OA) were included. RSA examinations were performed on the first postoperative day, and at 2 weeks, 3 months, 1, 2, and 5 years after surgery. A low-dose CT scan was done at 2 weeks and 5 years. The agreement between the migration results obtained from MBRSA and AI-based CT-RSA was assessed using Bland-Altman plots.

Results: Stem migration (y-translation) between 2 weeks and 5 years, for the primary outcome measure, was -0.18 (95% confidence interval [CI] -0.31 to -0.05) mm with MBRSA and -0.36 (CI -0.53 to -0.19) mm with AI-based CT-RSA. Corresponding proximal migration of the cup (y-translation) was 0.06 (CI 0.02-0.09) mm and 0.02 (CI -0.01 to 0.05) mm, respectively. The mean difference for all stem and cup comparisons was within the range of MBRSA precision. The AI-based CT-RSA showed no intra- or interobserver variability.

Conclusion: We found good agreement between the AI-based CT-RSA and MBRSA in measuring postoperative implant migration. AI-based CT-RSA ensures user independence and delivers consistent results.

Figure 1

Patient flowchart.

Figure 2

Stem MBRSA and AI-based CT-RSA migration. Mean y-translation (superior-inferior), y-rotation (internal-external), z-rotation (valgus-varus), and total translation results for stems (with coordinate system aligned with stem). MBRSA results with day 1 or 2 weeks as postoperative reference (green and blue lines, respectively). AI-based CT-RSA migration values from 2 weeks to 5 years represented with red markers. Whskers are CI.

Figure 3

Cup MBRSA and AI-based CT-RSA migration. Mean y-translation (superior-inferior), z-rotation (decreased-increased inclination), and total translation results for cups. See Figure 2 for details.

Figure 4

Bland–Altman plots for stem x, y, z-translations (medial-lateral, superior-inferior, anterior-posterior), x, y, z-rotations (anterior-posterior, internal-external, valgus-varus), and total translation. Limits of agreement (LOA) are shown as dotted lines. The mean difference (bias) is shown as a solid blue line.

Figure 5

Bland–Altman plots for cup x, y, z-translations (medial-lateral, superior-inferior, anterior-posterior), x, y, z-rotations (anterior-posterior, internal-external, decreased-increased inclination), and total translation. Limits of agreement (LOA) are shown as dotted lines. The mean difference (bias) is shown as a solid blue line.