Imaging description

Peroneocalcaneus internus (PCI) muscle is a rare accessory muscle of the posterior ankle. MRI shows an accessory muscle posterolateral to the flexor hallucis longus (FHL) tendon behind the ankle. On the axial images, the PCI is located deep to the flexor retinaculum and superficial relative to the posterior tibial neurovascular bundle behind the talus (Figure 67.1A). The PCI descends posterior and lateral to the FHL (Figure 67.1B) and inserts into the tubercle on the medial aspect of the calcaneus below the sustentaculum tali, which may be better delineated on the coronal oblique images.

Importance

Peroneocalcaneus internus muscle has been reported in one out of 100 asymptomatic volunteers. This condition is thought to be asymptomatic for most of the cases. However, a case associated with posterior impingement has been reported. With increasing popularity of endoscopic surgery, a possible risk of neurovascular injury during the procedure has been posed in patients with PCI. Because the FHL is used as the landmark for a medial boundary in the posteromedial portal, the surgeon may misidentify the PCI as the FHL and incorrectly direct the instruments toward the neurovascular bundles (Figure 67.2).

Typical clinical scenario

The diagnosis is clinically made based on anterior ankle pain with limited and painful dorsiflexion. Soft tissue swelling and a palpable spur over the anterior ankle may be detected on physical examination. Conservative treatment is usually successful for most patients with anterior impingement. Surgical or arthroscopic resection of spurs and soft tissue lesions is effective for patients with no tibiotalar articular disease.