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.

Table 1 | Demographic characteristics of the enrolled patients

Figure 1. Correlation between Sesamoid Index (SI) measured by Radiographic XRAY and Ultrasound (US). Radiographic SI was calculated as maximum length x width on X-ray. Medial and lateral SI on US was derived from the longitudinal (B, E) and transverse (C, F) measurements respectively. Careful adjustment of the ultrasound probe is necessary to avoid edge shadowing artifacts on the transverse view of the medial sesamoid (white arrow in C). The Spearman correlation between radiographic SI and ultrasonographic medial SI was better than the correlation for lateral SI.

Figure 2. (A) Estimated marginal means of X-ray sesamoid index (SI) over time by disease group. PsA patients showed a progressive increase in SI from timepoint 1 to 4, while RA and OA patients exhibited minimal change. Error bars represent 95% confidence intervals. (B) Representative radiographs from a PsA patient showing progressive enlargement of the medial sesamoid bone over time. (C) Serial radiographs from a RA patient showing stable SI measurements across timepoints, with minimal change in sesamoid size.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/radiographic-and-ultrasonographic-assessments-of-sesamoid-index-in-patients-with-psoriatic-arthritis/