How Low is Low? A Normative Study Defining Flexor Hallucis Longus Anatomic Location From Late Childhood Through Adolescence.

Abstract

The flexor hallucis longus (FHL) muscle and tendon are implicated in pathologies involving the posteromedial ankle-especially if the muscle belly extends distally into the tarsal tunnel. Clinicians often include FHL tenosynovitis and a low-lying FHL muscle belly as contributors to posterior ankle impingement syndrome. We aimed to define the spectrum of normative location and relative position of the FHL muscle belly during late childhood and adolescence. A retrospective review was conducted of patients who underwent ankle magnetic resonance imaging (MRI) for pathology unrelated to the posteromedial ankle at a single institution from 2015 to 2023. Demographics (age, sex, race, ethnicity, height, and weight) and imaging indications were collected. FHL musculotendinous junction (MTJ) location was first defined by its distance from the most distal aspect of the posterior tibia (absolute MTJ distance). It was then set relative to the length from the same point on the distal tibia to the point at which FHL tendon dives anteriorly into the tarsal tunnel (relative MTJ distance). Using the axial slice at the mid-talus, the cross-sectional area of the FHL was measured. Descriptive statistics, univariate analyses, and regression were performed. Sixty-eight patients (38 females, aged 8 to 18 y) underwent 72 ankle MRIs. The mean MTJ distance was 13.5±8.3 millimeters (mm) with 72/79 cases considered "low-lying" by current criteria. Simple linear regression demonstrated age as a significant predictor of MTJ distance with FHL muscle belly progressing distally by 1.0 mm/year with increasing age (P=0.004). Statistically significant differences in FHL tendon and FHL tendon/muscle belly cross-sectional areas, absolute and relative MTJ distances, MTJ distance/patient height, and MTJ distance/talar height ratios were observed between skeletally immature versus mature subjects. Our results show an age- and maturation-dependent normative difference in positioning of the FHL MTJ, suggesting that the currently used single cutoff value for "low-lying" FHL may not adequately distinguish normal from pathology when used in children and adolescents. Level IV.