Dissecting the accessory soleus muscle: a literature review, cadaveric study, and imaging study

The accessory soleus muscle (ASM) has been an unusual anatomical variant since its first recordings in Guy's Hospital Reports of the early nineteenth century. Individuals with an ASM may present with symptoms of pain and/or swelling and were often misdiagnosed as soft-tissue tumors such as hemangioma, sarcoma, or lipoma. The aim of our study was threefold: (1) to review the cadaveric and clinical literature to determine the reported prevalence of ASM; (2) to conduct a cadaveric study investigating the prevalence and attachment sites of the ASM; (3) to conduct a retrospective analysis of magnetic resonance imaging (MRI) of patients presenting with ankle symptoms to determine prevalence and attachment sites of the ASM. Our findings demonstrated that the prevalence of the muscle (3%) was as stated in the literature (0.7-5.5%), but with males more likely to possess unilateral ASM and females more likely to possess bilateral ASM. Three common attachment types were reported in the literature: (i) a distal attachment to the medial aspect of the calcaneus by a separate tendon (26.1% of ASM subjects), (ii) a distal tendinous attachment to the calcaneal tendon (3.5%), and (iii) a distal fleshy attachment to the medial surface of the calcaneus (4.3%), with the remaining 66.1% of ASM subjects from previous studies with unidentified attachment types. Our cadaveric specimens were found to possess each attachment type, whereas imaging patients all possessed distal attachments to the medial calcaneus via a separate tendon. Furthermore, a rare cadaveric specimen with two distal attachments was also found. We believe it is important to recognize the prevalence of this condition and be aware of its morphology in order to understand its clinical presentation, accurately diagnose the condition, and pursue effective forms of management.