Accessory Navicular on MRI in an Adult Ankle MRI Referral Cohort (N = 1988): Prevalence, Subtypes, and Edema Correlates.

Purpose: To determine the prevalence and subtype distribution of accessory navicular (AN) on adult ankle MRI and to evaluate MRI factors associated with AN-related bone marrow edema (BME); bilaterality was explored in patients with available contralateral MRI data. Methods: In this retrospective cross-sectional study, consecutive adult patients (≥18 years) who underwent ankle MRI between January 2022 and January 2025 at a single institution in Van, eastern Türkiye, were identified from the institutional archive. Two blinded readers assessed AN presence, classified subtypes using Coughlin criteria, measured maximal ossicle dimension for types 1-2, and recorded BME on fluid-sensitive sequences. Prevalence with 95% CIs was estimated, and multivariable logistic regression assessed factors associated with BME in patients with types 1-2. Results: Among 1988 unique patients (mean age: 42.2 ± 14.2 years; 42.6% male and 57.4% female), AN prevalence was 24.4% (95% CI: 22.6-26.4%), and type 2 predominated (50.2%). In the subgroup of AN-positive patients with available contralateral MRI data, AN was bilateral in 61 of 69 patients (88.4%). Interobserver agreement was excellent for AN detection and subtype classification (κ = 1.00 and κ = 0.98). Among types 1-2 included in the BME analysis (n = 385), BME occurred in 81 patients (21.0%). In the multivariable model, the adjusted ORs for BME were 4.46 for type 2 morphology, 1.24 per 1 mm increase in maximal dimension, 2.08 for female sex, and 0.71 per 10-year increase in age; concomitant os trigonum was not associated with BME. Conclusions: AN was common on ankle MRI, and type 2 was most strongly associated with BME; however, because this was a retrospective, cross-sectional, symptom-blinded study, causality and clinical correlation could not be established. Clinical Significance: Type 2 AN, particularly with a larger ossicle dimension, should prompt careful evaluation for MRI-detected BME, which should be interpreted as an imaging finding requiring clinical correlation.