The os peroneum (OP) is a small sesamoid bone located inside the peroneus longus tendon (PLT), close to the cuboid. The OP can be the cause of pain and can be associated with lesions of the PLT. OP involvement in PLT disorders is frequently misdiagnosed by radiologists. Painful os peroneum syndrome (POPS) refers to a variety of conditions presenting with pain localized on the lateral aspect of the cuboid area. The syndrome can be observed as a consequence of local acute trauma such as ankle sprains or chronic overuse. Because of its intra-tendinous location, in tears of the peroneus longus tendon, the OP can show changes in its morphology or position, depending on the location of the tendon's tear. Based on the level of the PLT tears, we propose a classification in three subtypes: tears localized proximal to the os peroneum (type I), at its level (type II) or distal to it (type III). These tears present with different changes on OP morphology or location. The aim of this article is to review the normal anatomy, imaging appearance and differential diagnosis of disorders of the OP as well as post-treatment imaging. Teaching points • PLT tears can be classified in three subtypes according to OP location. • POPS is characterized by pain on the lateral aspect of the cuboid. • OP involvement in PLT disorders is frequently misdiagnosed by radiologists.

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