Preliminary approach to validate ultrasonographic features of the perientheseal sesamoid fibrocartilage in the diagnosis of early spondyloarthritis

Objective This study aimed to validate the sonographic features of the sesamoid fibrocartilage (SFC) as an imaging biomarker for spondyloarthritis (SpA) and to assess its diagnostic performance across two groups of rheumatologists with varying levels of ultrasound expertise. Methods A cross-sectional test-validation and observer-concordance study was conducted using a large archive of static images and short ultrasound videos of the Achilles tendon obtained between 2018 and 2025. Images corresponded to patients with SpA and to controls (non-SpA patients and healthy volunteers). All images were acquired before biological or local steroid therapy. Two observer tiers (experts and non-experts) with different levels of ultrasound experience evaluated three dichotomous SFC variables—continuity, linearity, and thickness—after a brief training module. Inter-observer agreement was analysed using kappa coefficients, considering patients and controls, and separately by diagnosis. Results A total of 75 images or videos from patients and 75 images from controls were collected. Expert observers showed the highest agreement for continuity ( κ = 0.963, 95% CI: 0.916–1.010), followed by thickness ( κ = 0.954, 95% CI: 0.901–1.007) and linearity ( κ = 0.894, 95% CI: 0.855–0.933). Non-expert observers showed similar findings, with continuity showing the greatest reproducibility ( κ = 0.924, 95% CI: 0.888–0.960). In the restricted analysis comparing patients with the 20 control records from 19 subjects referred for ultrasound for unrelated reasons, continuity and thickness remained the best-performing features, although perfect separation was no longer observed (expert κ : 0.82–0.90; AUC: 0.92–0.95). These findings confirm that part of the discrimination observed in the full dataset is attributable to selection constraints applied to patient images. Across disease subtypes, continuity and thickness were positive in 100% of psoriatic arthritis, radiographic axial spondyloarthritis and non-radiographic axial spondyloarthritis cases, while linearity showed slightly lower positivity (93%–97%). These uniformly high positivity proportions explain why pairwise kappa coefficients between subtypes were mathematically constrained and uninformative. Overall, continuity emerged as the most robust and reproducible ultrasonographic feature, while linearity showed greater dependence on observer experience. Conclusion Sonographic assessment of the SFC demonstrates strong discriminative power in identifying SpA, even when performed by observers with limited ultrasound experience. Thickness and loss of continuity are the most robust diagnostic features, supporting the hypothesis that SFC reflects enthesic pathological changes. Given its accessibility and reliability, SFC evaluation may be a promising, minimally invasive imaging biomarker for early or preclinical detection of SpA. Further studies should explore its applicability in other enthesic territories.