A rare accessory muscle in the hand--the flexor digitorum superficialis indicis

Accessory muscles are easily overlooked during imaging evaluation. Although usually discovered incidentally, they are occasionally symptomatic. With increasing utilization of cross-sectional imaging, the radiologist should be prepared to readily identify these anomalous muscles. It is particularly important to distinguish these anatomical variants from soft-tissue tumors prior to invasive intervention, reserving biopsy and surgery for children who are symptomatic. This report discusses a case of a flexor digitorum superficialis indicis muscle, an extremely rare but well-described accessory muscle, presenting as a painful mass in a 15-year-old girl. The report includes the clinical presentation, radiologic findings, and the significance to management.