Imaging description

Accessory soleus can be detected on the lateral radiograph of the ankle, which shows soft tissue opacity in the pre-Achilles fat pad (Kager’s triangle) (Figure 68.1). MR imaging reveals an accessory muscle between the Achilles tendon and flexor hallucis longus muscle (Figure 68.2). Normally the space is filled with fat. Accessory muscle originates from the anterior (deep) surface of the soleus from the tibia and fibula. It inserts into the Achilles tendon, superior surface of the calcaneus, or medial aspect of the calcaneus.

Importance

Accessory soleus muscle has a prevalence of 0.7–5.5% according to cadaveric studies. Real incidence of the symptomatic cases is debated. However, accessory soleus is commonly seen by musculoskeletal radiologists on radiography, CT, and MRI. Presenting symptoms include painless or painful swelling or mass on the posteromedial aspect of the ankle. Development of compartment syndrome, inadequate blood supply from the posterior tibial artery, and compression of the adjacent posterior tibial nerve have been postulated as the mechanisms of the symptomatic accessory soleus. High association with Achilles tendinopathy has been reported in patients with accessory soleus.

Typical clinical scenario

Accessory soleus is usually asymptomatic until the second decade of life. Increased muscle volume and physical activity seem to be related to the presentation of the symptoms. It is more common in males than females (2:1). A painful swelling of the posteromedial ankle is a common presenting symptom. The pain is typically exertional. Soft tissue mass is another presenting history. The patients with painful swelling are managed with avoidance of pain-producing activities and physeal therapy. Fasciotomy and excision may be indicated when conservative management has failed.