Anomalous flexor digitorum superficialis muscle belly presenting as a mass within the palm

Anomalies of the flexor digitorum superficialis muscle are extremely uncommon and usually present as a painful mass or pseudotumour within the palm. Diagnosis may be difficult because many other soft tissue tumours (lipomas, ganglions, giant cell tumours and hamartomas) may present similarly. Magnetic resonance imaging helps to define the extent and characteristics of this anomalous muscle belly and to distinguish it from a soft tissue sarcoma, whereas plain radiographs are of little value. Three types of flexor digitorum superficialis muscle anomalies have been described, and treatment consists of subtotal or total surgical debulking of the mass if symptoms persist or if the diagnosis is in question. Most patients have complete resolution and full recovery. To date, 20 cases have been reported in the literature, usually involving the right small finger. In the present paper, the case of an anomalous flexor digitorum superficialis muscle in a 17-year-old male patient's left index finger is reported. Symptoms were relieved following surgical debulking and hand-based occupational therapy.