Atypical anomalous peroneal (fibularis) tendon: Potential pitfall in musculoskeletal sonography

The first sonographic description of an anomalous peroneal tendon variant is presented. The sonologist or sonographer may then erroneously conclude that the tendon had inserted into the peroneus brevis and misdiagnose a peroneus brevis split tear. Multiple other peroneal tendon variations have been described arising from the peroneus brevis, peroneus longus, and/or interosseous membrane and having one or more insertions into the retrotrochlear eminence, peroneal tubercle, peroneus brevis, peroneus longus, inferior peroneal retinaculum, cuboid, fifth metatarsal, and/or fifth digit. Whether considered an atypical peroneus quartus or perhaps a bifid peroneus longus, the variant observed in this report has important clinical implications. When 3 tendinous structures are encountered in the retromalleolar groove, the differential diagnosis is typically limited to a split peroneus brevis tear with 2 hemitendons, or a peroneus quartus.