The Relationship Between Flexor Hallucis Longus Profile And Os Trigonum Syndrome

Os trigonum (OT) is an accessory bone that develops at the posterior talus. OT syndrome occurs when that accessory bone compresses adjacent soft tissues, commonly the flexor hallucis longus (FHL) muscle and/or tendon. While OT has been associated with FHL tenosynovitis and rupture, less is known about the relationship between FHL morphology and posterior ankle bony anomalies such as OT and Steida process (SP). PURPOSE: This study aimed to determine whether the location and size of the FHL muscle belly are related to OT/SP symptomatology. METHODS: This was a retrospective MRI review of patients with OT/SP conducted at a single institution from 2015-2023. We collected demographic information, imaging indications, and radiographic measurements which included 1. regional maturity (distal tibial physis), 2. axial area of the FHL muscle belly or tendon at the midpoint of the OT/SP (subjects) or at the same axial MRI cut relative to the talus (controls), 3. axial and sagittal cross-sectional areas of the OT, 4. musculotendinous junction (MTJ) distance: distal extent of the FHL muscle belly to the distal extent of the tibia, and 5. relative MTJ: ratio of MTJ to the distance from tibia to tarsal tunnel. Treatment invasiveness was used as a proxy for symptomatic severity. Descriptive statistics were performed. T-testing and Mann Whitney U were performed as appropriate. RESULTS: 35 patients (25 OT, 10 SP) met study inclusion criteria. 14 patients were symptomatic, with 5 requiring conservative management or analgesic injections, and 9 requiring surgical excision. Of note, 4/9 surgical patients had synovectomy or FHL muscle belly debridement documented. All patients who underwent analgesic injection experienced at least partial resolution of symptoms. Symptomatic OT/SP patients had a significantly more distal FHL MTJ than their asymptomatic counterparts with medium effect size (p = .044, Hedge’s g = .594). While not statistically significant, patients with more severe symptoms had a more distal FHL MTJ and a relatively larger FHL axial area than their less symptomatic counterparts. CONCLUSIONS: The results of this study suggest that FHL muscle belly size and location may play a role in the presence and severity of symptoms in patients with osseous etiologies of posterior ankle impingement.