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Within Normal Limits

Normal Variant vs. Fracture

Os Peroneum

Hasan, Usama DO; Patel, Himanshu MD; Patel, Shivani MD

doi: 10.1097/01.EEM.0000512781.09023.5a
Within Normal Limits

Dr. Hasanis a third-year radiology resident at Westchester Medical Center in Valhalla, NY, whereDr. Himanshu Patelis the director of musculoskeletal radiology at Westchester Medical Center in Valhalla, NY, and an assistant professor of radiology at New York Medical College. Dr. Shivani Patelis an emergency physician at Stamford (CT) Hospital. Read their past columns athttp://bit.ly/EMN-Limits.

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A 74-year-old man presented with a complaint of pain along the lateral aspect of his left foot. Radiographs demonstrated two osseous densities at the lateral plantar aspect, with an 8 mm distraction between them, consistent with an acute fracture of the os peroneum. An MRI of the left foot showed a full thickness tear and complete disruption of the peroneus longus tendon, causing the distraction between the fracture fragments.

The os peroneum is a sesamoid bone located adjacent to the lateral margin of the cuboid within the distal peroneus longus tendon, and is present in approximately 26 percent of the population. (AJR Am J Roentgenol 2001;177[1]:257.) This accessory ossicle can be bipartite or multipartite. It is important to make the distinction between a bipartite os peroneum and an acute fracture. It has been shown that an os peroneum fracture fragment separation of 6 mm or more, like the one in our case, is associated with a full-thickness tear of the peroneus longus tendon. (Radiology 2005;237[1]:235.) Delayed diagnosis can result in unnecessary prolonged pain, ankle instability, and peroneal compartment syndrome.

Acute fractures of the os peroneum occur with sudden inversion of the ankle due to contraction of the peroneus longus tendon, leading the os peroneum to come into forced contact with the cuboid, resulting in fracture and possible disruption of the tendon. Accurate documentation of clinical history and mechanism of the trauma may assist in raising suspicion for the injury.

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Sesamoid bone fractures should be considered when interpreting radiographs in the setting of trauma; they are commonly overlooked due to their subtlety. Distraction of the osseous fragments by more than 6 mm in particular should prompt further workup.

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