accessory soleus muscle

Pseudotumoural

soft tissue lesions of the foot and ankle: a pictorial review. Accessory muscle. Sagittal T1-WI. An accessory soleus (black asterisk) is seen anterior-inferior to the soleus muscle (white asterisk), arising from the tibia (origin not shown) and with a tendinous insertion onto the superior surface of the calcaneus anteromedial to the Achilles tendon (arrow). This is just one of the five types of accessory soleus that have been described on the basis of insertion characteristics

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The accessory soleus muscle is an anatomical variant characterized by an additional distinct muscle encountered along a normal soleus muscle. It is uncommon with a prevalence of ~3% (range 0.7-5.5%).

Summary

origin: fibula, soleal line of the tibia, or the anterior surface of the soleus muscle
insertion: calcaneus, at either the upper surface or the medial cortex
- into the Achilles tendon distally
- fleshy insertion into the upper surface of the calcaneus
- fleshy insertion into the medial cortex of the calcaneus
- tendinous insertion into the upper calcaneal surface
- tendinous insertion into the medial surface of the calcaneus
arterial supply: posterior tibial artery
innervation: tibial nerve

Gross anatomy

The accessory soleus muscle arises from the deep surface of the soleus or from the fibula and descends anteriorly to the calcaneal (Achilles) tendon. Its insertion may occur into the calcaneal tendon, the upper surface of the calcaneus, or in the medial aspect of the calcaneus.

Clinical presentation

Patients with an accessory soleus muscle can be asymptomatic and thus it will be detected incidentally during imaging performed for an unrelated reason . If symptomatic, the usual presentation is a soft mass in the posteromedial distal third of leg . This mass increases in size with activity, particularly plantar flexion . Pain is another common symptom (2/3 of patients) and it worsens with jumping and running . These symptoms are likely due to chronic compartment syndrome as the accessory soleus muscle increases in size during physical activity. Neuropathy from compression of the posterior tibial nerve has also been reported .

Radiographic features

Plain radiograph

The Kager fat pad is obliterated by a well-defined area of increased soft-tissue attenuation.

MRI

MRI is considered the most specific and sensitive test for diagnosis. The accessory soleus muscle appears as an oval or fusiform, well-defined image with its own fascia . It additionally has normal muscle signal intensity on both T1 and T2 .