Peroneus Tertius Tendon Tear: A Rare Cause of Lateral Ankle Pain.

The peroneus tertius (PT) muscle is a variably present muscle, uncommonly found in humans. Injury to the PT tendon is rare with virtually no cases reported in the literature. As a consequence of the rarity of this injury, there is little clinical information regarding injury or rupture of the PT muscle and tendon. We present a case of injury involving this rare anatomical variant. Magnetic resonance (MR) imaging demonstrates a short segment longitudinal split tear adjacent to the tendinous insertion of the peroneus tertius muscle. Knowledge of this rare anatomic variant and the potential for associated pathology is critical in the management of the patient. Directing the orthopedic surgeon, or podiatrist, to this finding is critical for directing intervention.