Clinical History
A 16-year-old female professional volleyball player reported the development of a hard-elastic, slightly painful swelling in the posteromedial aspect of the left ankle.
Imaging Findings
In the past, she had sprained her ankle on four occasions while playing volleyball; however, the pain in the mass was exacerbated by exercise. A comparative ultrasound examination of her ankles identified the
presence of muscle-like tissue with no apparent echo-structural alterations in correspondence to Kager’s fat triangle on the left. A subsequent MR examination, performed with equipment 1,5 T
with coil dedicated, of the ankle confirmed the presence of a muscular mass that inserted autonomously into the calcaneus—D type—and whose signal intensity, morphology and topography
corresponded to an accessory soleus muscle.
Discussion
The accessory soleus is not so rare especially now that MRI of the extremities is commonly performed,is a condition which may clinically present as a tender swelling in the soft tissues in the space between the medial tibial malleolus and Achilles tendon. The anatomic frequency in the population varies widely,the reported range being 0.7%-6%;the clinical incidence is low, because this condition is often asymptomatic. Etiopathogenesis is likely related to fragmentation of the embryonic soleus muscle during growth. The swelling usually develops during the second and third decade of life and, in conditions of functional overload or hypertrophy in persons who practise sport, it may be painful on palpation. Accessory soleus muscle is a well-documented condition—often discovered incidentally during surgery—that has come to be detected more frequently as a result of the widespread use of imaging methods capable of differentiating it from other space-occupying lesions of the ankle. The first detailed description of this supernumerary muscle was provided by Le Double in 1897. Not always independently vascularised and innervated, the accessory soleus may manifest clinically as a soft tissue mass bulging medially between the medial tibial malleolus and Achilles tendon.It may be unilateral or bilateral, and not always symmetrical in volume. Its origin, within the soleus and deep in the flexor muscles, is relatively constant. Five types of insertion have been described: Type A: via a fleshy body that inserts on the front of Achilles tendon on the posterior aspect of the calcaneus. Type B: with an insertion along the distal portion of Achilles tendon. Type C: with a separate tendon that inserts behind Achilles tendon on the superior aspect of the calcaneus. Type D: via a fleshy body that inserts on the medial aspect of the calcaneus. Type E: with a tendon on the medial aspect of the calcaneus. It may sometimes cause pain on exercise, related to ischaemia,due to muscle compression that leads to a functional overload syndrome in patients engaged in specific sporting activities(soccer,tennis,long-distance running, basketball, athletics, judo, skiing,volleyball and parachuting).The differential diagnosis includes other painful soft tissue“masses”in the posteromedial region of the ankle, such as lipomas, haemangiomas, synoviomas and sarcomas.Ultrasonography allows accurate evaluation of swellings in the Achilles tendon region;it can also demonstrate increased soft tissue volume,with echogenicity similar to muscle (and not displaying echostructural alterations within)as compared to the corresponding contralateral region. Magnetic Resonance imaging is better able to document this anatomical variant, thanks to its multiplanar capabilities that allow improved topographical definition, recognition of the muscle structure and fat planes, and demonstration of the site of insertion onto the calcaneus.Knowledge of this anatomical variant is fundamental for proposing an appropriate therapeutic approach that will reduce the pain and improve relaxation of the hindfoot during running. Treatment depends on the clinical presentation,the volume of the supernumerary muscle and the site of insertion of the distal tendon (conservative treatment,changes to the training schedule,a simple aponeurotic incision allows expansion of the accessory,excision of the supernumerary muscle.
Differential Diagnosis List
accessory soleus muscle D-type
Final Diagnosis
accessory soleus muscle D-type
