Fatty Muscle Atrophy: Prevalence in the Hindfoot Muscles on MR Images of Asymptomatic Volunteers and Patients with Foot Pain

Purpose: To determine prevalence and degree of fatty muscle atrophy in plantar foot muscles in asymptomatic volunteers and in patients with foot pain. Materials and Methods: Institutional review board approval and informed consent were obtained. The prevalence and degree of fatty muscle atrophy were evaluated with magnetic resonance imaging in the abductor digiti minimi (ADM), flexor digitorum brevis (FDB), abductor hallucis (AH), and quadratus plantae (QP) muscles in 80 asymptomatic volunteers (mean age, 48 years; range, 23-84 years) and 80 patients with foot pain (mean age, 48 years; range, 20-86 years). Muscles were characterized as normal (grade 0) or as having mild (grade 1) or substantial (grade 2) fatty atrophy by two readers separately. Results of visual grading for both readers were compared by using the Mann-Whitney test. Associations between age and degree of fatty muscle atrophy were assessed by using the Kruskal-Wallis test. Results: Readers 1 and 2 found substantial fatty atrophy of the ADM muscle in four (5%) and five (6%) volunteers, respectively, and in three (4%) and nine (11%) patients, respectively. One reader diagnosed substantial fatty atrophy of the AH muscle in three (4%) volunteers and of the FDB muscle in two (2%) volunteers. Prevalence for the QP muscle varied between 0% and 1%. An association between age and degree of fatty atrophy of the ADM muscle was found for volunteers by both readers and for patients by reader 1 (P < .01). Conclusion: Prevalence of fatty muscle atrophy of the ADM muscle― classically considered to represent entrapment neuropathy―is between 4% and 11 % in both asymptomatic volunteers and patients with foot pain, and it increases with age.