An accessory peroneocalcaneus internus muscle with MRI and US correlation.

The peroneocalcaneus internus (PCI) is a rare accessory muscle of the medial ankle with typical MRI anatomic features allowing differentiation from the more common accessory flexor digitorum longus muscle. To our knowledge, there have been no previously published sonographic images of the peroneocalcaneus internus. A PCI is typically an incidental, asymptomatic finding, but knowledge of the entity may avoid confusion when initially encountered by diagnostic ultrasound. We review the anatomic imaging features which allow for a confident imaging diagnosis and the clinical relevance of this anomaly.