The Effect of Flexor Hallucis Longus Morphology on Os Trigonum Syndrome

Background: Os trigonum (OT) is an accessory bone at the posterior ankle that can lead to posterior ankle impingement. The flexor hallucis longus (FHL) muscle is a potential contributor to this pathology as it passes through the fibro-osseous tunnel behind the medial malleolus adjacent to the talus. Purpose: To compare the morphology of the FHL muscle belly between patients with symptomatic and asymptomatic OT. Study Design: Cross-sectional study; Level of evidence, 3. Methods: We performed a retrospective analysis of pediatric patients with an OT who underwent ankle magnetic resonance imaging at a single center from 2015 to 2023. We collected descriptive data, imaging indications, and the following radiographic measurements: distance from the distal end of the tibia to the FHL muscle musculotendinous junction (MTJ) (absolute FHL MTJ distance); ratio of absolute MTJ distance to the distance from the tibia to the tarsal tunnel (relative FHL MTJ distance); axial area of the FHL muscle belly or tendon at the midpoint of the OT/Stieda process; and axial and sagittal areas of the OT. A 1-tailed independent t test was used to compare continuous variables, guided by the Levene test for equality of variances. Results: The FHL was positioned significantly lower in the symptomatic group (n = 14) than in the asymptomatic group (n = 21), when represented by both absolute ( P = .044) and relative FHL MTJ distances ( P = .037). The relationship between symptoms and sex was significant ( P = .024). The relationships were also significant between symptoms FHL MTJ distance/patient height ( P = .049) and FHL MTJ distance/patient body mass index ( P = .041). There was no significant difference in age, FHL axial area, or OT length, area, and volume between groups ( P > .05). Conclusion: These findings suggest that the FHL muscle may play a role in the pathogenesis and clinical presentation of OT syndrome. We propose that the evaluation and treatment of OT syndrome should include the assessment and management of the FHL muscle, and that future studies should explore the mechanisms underlying the association between the FHL muscle and OT syndrome.