An accessory muscle is seen anterior to the Achilles tendon. The tendon of the accessory soleus inserts slightly anterior and medial to the Achilles tendon.
Clinically, accessory soleus may present as a soft tissue swelling or chronic pain. Such clinical presentations were commonly described in case reports and case series published in 1960s-1980s. In the days prior to MRI, radiography revealed soft tissue density in Kager’s triangle. CT appearance was described in later reports, but it was not always possible to confidently diagnose abnormal soft tissue density as muscle even on CT. Other causes of soft tissue swelling had to be included in differential diagnoses list based on radiographic and CT appearance. Since other soft tissue masses could not be ruled, the only options were surgical exploration or serial follow up. Today, diagnosis is straightforward on MRI, as accessory soleus demonstrates signal characteristics identical to other muscles.
Discussion of classification systems generally includes discussion of muscle types and discussion of attachment sites. Low-lying musculotendinous junction of soleus and a separate true accessory muscle are the two categories of muscle variations. In case of a separate muscle, accessory soleus may arise from soleus proper or from tibia and fibula. The insertions types are:
- Distal Achilles tendon
- Superior calcaneus by a “fleshy muscular fascicle”
- Superior calcaneus by a tendon
- Medial calcaneus by a “fleshy muscular fascicle”
- Medial calcaneus by a long tendon
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