How Low Is Low? A Normative Study Defining Flexor Hallucis Longus Anatomic Location

Flexor hallucis longus (FHL) is implicated in pathologies involving the posteromedial ankle—especially if muscle belly fibers extend into the tarsal tunnel. Low lying FHL has been defined as muscle fiber presence distal to the tibia. Normative location and relative position of the muscle belly throughout maturity remain unexplored. PURPOSE: We aimed to define the normative location of the FHL muscle belly throughout maturity. METHODS: Retrospective review was conducted of patients who underwent ankle MRI for pathology unrelated to posteromedial ankle at one institution from 2015-2023. We collected demographics, indications, and radiographic measurements: regional maturity (distal tibial physeal status given its weight-bearing role), musculotendinous junction (MTJ) distance from end of tibia to end of FHL muscle belly, relative MTJ: ratio of MTJ to the distance from tibia to tarsal tunnel, and axial area of the FHL muscle belly or tendon at the midpoint of an os trigonum when present or the same axial slice relative to the talus. RESULTS: 72 patients (40 females) underwent 78 MRIs (mean age 13.6 ± 2.7 years, range: 8.2-19.0). Indications for MRI were most often secondary to osteochondral lesion of the talus [OLT] (54/78). 72/78 FHLs were low lying, with average relative MTJ of 0.60 ± 0.37. Simple linear regression demonstrated age as a significant predictor of MTJ, with FHL muscle belly progressing distally by 0.972 mm/year (p = 0.005). Gender was not statistically significant in relation to MTJ, neither by itself nor as a control in the regression of MTJ with respect to age. A significant difference in MTJ, relative MTJ, MTJ/height, and FHL axial area was observed between subjects with open vs closing/closed tibial physes with respective means of 9.9 ± 7.9 vs 16.1 ± 7.8 mm (p < 0.001), 0.45 ± 0.34 vs 0.74 ± 0.34 (p < 0.001), 0.07 ± 0.05 vs 0.10 ± 0.05 mm (p = 0.007), and 30.6 ± 27.7 vs 48.1 ± 43.9 mm2 (p = 0.038). CONCLUSION: Most patients in this study met previously defined criteria for low lying FHL. Low lying FHL in children and adolescents may not be low lying after all. Further, results of this study indicate that FHL muscle may progress distally through chronologic maturity. These results indicate younger patients with compressive pathology may worsen without intervention.