Retracted os peroneum with partial integrity of the peroneus longus tendon.

Painful os peroneum syndrome encompasses a spectrum of disorders associated with lateral foot and ankle pain. In the setting of an os peroneum fracture or diastasis of a partitioned os peroneum, marked displacement of the proximal fragment on radiographs is often used as an imaging surrogate for detection of a complete peroneus longus tendon tear. We present a case of a displaced proximal fragment of the os peroneum above the level of the ankle joint on radiographs and MRI associated with incomplete tear of the peroneus longus tendon. We hypothesize that such an injury pattern results from an anatomic prerequisite where the os peroneum occupies a portion of the cross-sectional diameter of the tendon. We suggest that the retracted proximal moiety of the sesamoid bone is the result of elastic recoil of delaminated fibers of the peroneus longus directly inserting on the os, whereas eccentric bundles of the tendon draping over the os remain in continuity. Although treatment implications are debatable, the case questions the assumption of a complete peroneus longus tear based on a retracted os peroneum on radiography and highlights the role of MRI in providing a full description.