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%) 1.
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Summary
origin: fibula, soleal line of the tibia, or the anterior surface of the soleus muscle
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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 Achilles 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 2. If symptomatic, the usual presentation is a soft mass in the posteromedial distal third of leg 3. This mass increases in size with activity, particularly plantar flexion 3-5. Pain is another common symptom (two-thirds of patients) and it worsens with jumping and running 3-5. 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 6.
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 5,6. It additionally has normal muscle signal intensity on both T1 and T2 5,6.
Related pathology
associated exertional pain can occur, especially in athletes 7