A Unique Case Of An Isolated Partially Ruptured Accessory Soleus Muscle

By Jason Miller, DPM, FACFAS, Benjamin Marder, DPM, AACFAS and Austin Mishko, BS

Anatomic variations in the musculature of the human body are overall rather common. These variations could include the absence of a normally present muscle, the presence of a normally absent muscle (accessory or supernumerary muscle) or abnormalities in regard to the muscle’s normal anatomic course.¹ In this blog, we present a unique case involving a partially torn medial muscle belly of the accessory soleus muscle secondary to disruption of deep fascia following strain of the plantaris tendon. 

The incidence of accessory muscles in the lower extremity varies greatly. A recent meta-analysis by Yammine and Erić indicated that the peroneus tertius muscle of the anterior compartment is present in about 93.5 percent of individuals.² While the peroneus tertius is very common, other accessory muscles like the accessory soleus have a prevalence rate of 0.7 to 10 percent.³ Cruveilhier first described the accessory soleus muscle in the literature in 1843 and it is a benign finding in many individuals.^4,5 

When symptomatic, the accessory soleus muscle may present clinically as a tense and painful mass near the ankle. One may mistake this for a tumor, tarsal tunnel syndrome from compression of the tibial nerve in the lower medial leg, and exercise-related pain during sports, sometimes even causing compartment syndrome.^4,6 On plain radiographs, the accessory soleus may present as a partial disruption of Kager’s triangle, anterior to the Achilles tendon. 

On magnetic resonance imaging (MRI), the variability of this accessory muscle is well documented. It is split into five types based on its insertion as follows:

Type 1: Achilles tendon, one to two cm superior to the calcaneus; 

Type 2: Superior aspect of the calcaneus, anteromedially to the Achilles tendon as a direct muscular insertion;

Type 3: Superior aspect of the calcaneus, anteromedially to the Achilles tendon as a short tendon;

Type 4: Medial aspect of the calcaneus as a direct muscular insertion;

Type 5: Medial aspect of the calcaneus as a tendinous insertion.3,7

When A Patient Presents After A Possible Acute Achilles Injury

A 57-year-old male, employed as a custodian, presented to our office with new-onset left ankle pain. The patient stated the pain began one week prior when he “squatted down” while at work. He claimed he felt a “pop” and immediate sharp pain. He then went to an urgent care clinic. Providers at the urgent care clinic obtained plain film radiographs and subsequently had the patient utilize a controlled ankle motion (CAM) boot. The providers instructed the patient to take ibuprofen and remain weightbearing as tolerated in the CAM boot until his appointment at our office. The patient stated that he felt these measures had helped. 

Immediately after the injury, the patient said there was swelling and bruising about his left lower leg. Upon his initial presentation to our office in October 2019 there was still ecchymosis, edema and mild erythema to the lower leg as well as pain on palpation about the Achilles tendon. The pain extended laterally to the areas of the calcaneofibular and anterior talofibular ligaments. The patient’s vascular status was normal. However, he exhibited ankle equinus as well as an abnormal delve at the medial aspect of the midsubstance of the Achilles tendon. Although painful on palpation the distal Achilles tendon, the patient noted much less pain while weightbearing and had returned to work without the use of his CAM boot the day prior to presentation to our office. 

We reviewed the patient’s X-rays and determined that there were no apparent soft-tissue or bony abnormalities. We then recommended an MRI to assess the integrity of the Achilles tendon. We transitioned the patient into an ankle-foot orthosis (AFO) gauntlet brace and scheduled follow-up in three weeks.

At his next appointment in early November 2019, the patient noted complete pain resolution while weightbearing with his AFO brace. He stated that he only noticed pain at the Achilles tendon “first thing in the morning” but it resolved with ambulation. On the MRI report, the radiologist noted no abnormalities to the Achilles tendon nor any inflammation of the paratenon or other surrounding soft tissues. Upon examining the MRI images ourselves, we discovered that the patient’s soleal muscle belly extended abnormally distal in the leg. The soleus was muscular, not tendinous, almost to its insertion on the calcaneus, a finding that is most consistent with a type 1 accessory soleus muscle.

Upon further clinical examination and correlation with the MRI, that “delve” felt at the previous appointment was actually the myotendinous junction of the distally-based accessory soleus muscle. We instructed the patient to remain weightbearing as tolerated in his AFO brace and to follow up only as needed.

Understanding The Various Presentations And Diagnosis Of The Soleus Accessorius 

The incidence of a accessory soleus muscle is rare, ranging anywhere from 0.7 to 10 percent.³ According to Sarrafian, the accessory soleus muscle may originate from the fibula and the posterior aspect of the tibia, but other sources cite origins from the ventral aspect of the normal soleus muscle belly or other flexor tendons.^4,8,9 Its insertion may be into the Achilles tendon or the superior or medial aspects of the calcaneus, and can vary between a tendinous or direct muscular insertion.⁴ The muscle is bilateral in about 15 percent of individuals, is more common in men than women and is interestingly more common in athletes than sedentary individuals.^9,10

Plain film radiographs may show obliteration of Kager’s triangle on the lateral view but they are of little use overall for the diagnosis of an accessory soleus muscle.⁷ Computed tomography (CT) scans and ultrasound imaging may also be useful, note MRI is the most specific imaging technique for identifying this structure.9 Interestingly, the accessory soleus muscle in our patient was not mentioned in the original MRI report. This is probably due to a combination of the rarity of this congenital abnormality as well as the radiologist not being able to correlate the image clinically to the patient’s symptoms. 

In Conclusion

In our case, the patient’s subjective description of his injury, feeling a “pop” in his posterior leg that led to immediate pain, swelling, and bruising gave us a clue that he was indeed suffering from an acute injury. In addition to the objective findings of pain on palpation and a distally-based accessory soleus muscle belly, the radiographic findings of an accessory soleus led us to diagnose a partially ruptured accessory soleus muscle belly, which healed in the four weeks from the time of injury to the time of the patient’s MRI. In 2018, Lintingre and colleagues presented a similar case of a ruptured accessory soleus muscle, but this was a complete rupture with myotendinous retraction.¹¹

To the best of our knowledge, our case is the first published case of a partially ruptured accessory soleus muscle. This 57-year-old male presented with a partially ruptured accessory soleus muscle, which caused pain, edema and ecchymosis. After a successful course of rest, offloading and ibuprofen, the patient reports no further complications with this condition.

Dr. Miller is the Director of the Pennsylvania Intensive Lower Extremity Fellowship at Premier Orthopaedics in Malvern, Pa.

Dr. Marder is a Fellow with the Pennsylvania Intensive Lower Extremity Fellowship in Malvern, Pa.

Mr. Mishko is a fourth-year podiatric medical student at Temple University School of Podiatric Medicine in Philadelphia, PA

References

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