A 14-year-old healthy female presented with pain to the medial aspect of her left sole for 1 year in duration. Pain occurred with first step in the morning, as well as after running, and disappeared within 15 min. She was diagnosed with a flat foot at another clinic, but the use of an orthotic device aggravated the pain.
The patient had no history of any trauma or other diseases. Her body mass index was 32.1 kg/m2. Physical examination revealed a soft mass located on the medioplantar surface of the left foot. There was no numbness of the sole and a negative Tinel's sign; however, tenderness was detected at the distal end of the tarsal tunnel. The pain in the medial aspect of her left sole occurred when she attempted ankle dorsiflexion and hindfoot eversion. The weightbearing exam revealed flattening of the arch consistent with pes planus (Figure 1).
Radiographs demonstrated a longitudinal arch of normal height and a soft-tissue enlargement. Magnetic resonance imaging (MRI) and computed tomography (CT) scans revealed significant hypertrophy of the abductor hallucis muscle and slight hypertrophy of the flexor digitorum brevis muscle in the left foot (Figure 2). A nerve conduction velocity test was not performed because the patient and her parents did not provide consent.
The patient was diagnosed with unilateral hypertrophy of the abductor hallucis muscle with entrapment of the medial plantar nerve. Two injections of triamcinolone acetonide and lidocaine at the point of tenderness every other month resulted in resolution of the pain. She experienced no restriction in her activities of daily living except for running 20 months later.