2314: Radiographic and Ultrasonographic Assessments of Sesamoid Index in Patients with Psoriatic Arthritis

Background/Purpose: The sesamoid index (SI), a radiographic measure of medial thumb sesamoid area, was originally described as a diagnostic tool in acromegaly¹ and later proposed as a tool to differentiate psoriatic arthritis (PsA) from other arthritides². We investigated the relationship between SI and disease type, sex, age, and disease duration using both radiographic and ultrasonographic measurements, and examined longitudinal changes.

Methods: We enrolled PsA (n=41), RA (n=41), and OA (n=40) patients in a cross-sectional study. SI was measured on hand X-rays (length × width) and by ultrasound in medial and lateral planes using a 20 MHz probe. For the longitudinal analysis, 17 patients with ≥2 serial X-rays were included: RA (n=9), PsA (n=5), and OA (n=3). Group differences in SI were analyzed using Kruskal-Wallis with Dunn’s test; sex differences by t-tests; and medial vs. lateral ultrasound SI by Wilcoxon signed-rank test. Spearman correlation assessed associations with age, BMI, and disease duration. Linear regression evaluated independent predictors, and mixed-effects modeling assessed longitudinal change.

Results: Radiographic SI was significantly higher in males across all groups (PsA: 31.8 vs. 18.2 mm², p=0.007; RA: 34.8 vs. 22.8, p=0.0016; OA: 28.7 vs. 22.8, p=0.011). SI correlated strongly with medial ultrasound SI (ρ = 0.64, p < 0.001) and moderately with lateral SI (ρ = 0.45, p < 0.001) (Figure 1). Lateral SI was larger than medial SI across all groups (p < 0.01). In a linear model (R² = 0.27), female sex predicted lower SI (β = -11.4 mm², p < 0.001), followed by age (β = +0.16 mm²/year, p = 0.046) and disease duration (β = +0.38 mm²/year, p = 0.022). In an interaction model (R² = 0.29), PsA showed a greater rate of SI increase per disease year vs. RA (β = +0.77 mm²/year, p = 0.042), with no effect in OA. BMI had no impact. Group diagnosis was not independently significant at baseline. A linear mixed-effects model of 47 radiographs from 17 patients showed distinct SI trajectories. PsA patients exhibited progressive SI growth from 18.2 mm² (95% CI: 5.6–30.8) at timepoint 1 to 33.2 mm² (95% CI: 19.5–46.9) at timepoint 4, while SI remained stable in RA (32.7–36.2 mm²) and OA (31.1–34.1 mm²) in Figure 2. This supports greater periosteal remodeling in PsA.

Conclusion: SI is higher in males and increases with age and disease duration. PsA demonstrates the most robust longitudinal SI growth, supporting its potential as a structural marker of disease progression. Ultrasound, particularly medial SI, correlates well with radiography and may serve as a non-radiographic alternative for tracking periosteal changes. Future studies with larger longitudinal cohorts are warranted to validate these findings.