MRI Web Clinic — November 2008

Accessory Muscles of the Ankle
John F. Carroll, M.D.

Clinical History: A 43 y/o female presents with Achilles region pain. An (1a) axial T1-weighted image is provided. What are the findings? What is the diagnosis?

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Figure 1:

An (1a) axial T1-weighted image

Findings

 

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Figure 2:

Within the distal lower leg, a triangular shaped accessory muscle (red) abuts the adjacent neurovascular bundle (yellow) and lies posterior to the flexor hallucis longus muscle (FHL). The soleus (S) and flexor digitorum longus (FDL) muscles are also indicated.

 

Diagnosis

Flexor Digitorum Accessorius Longus

Introduction

Multiple accessory, supernumerary, and anomalous muscles have been described in the radiologic, surgical, and anatomic literature. Accessory muscles of the ankle are typically asymptomatic, but can cause pain, compressive neuropathy, compartment syndrome, or rigid hindfoot deformities, and can also mimic soft tissue tumors.1-8

Magnetic resonance imaging (MRI) is the modality of choice in diagnosing accessory muscles, delineating their relationship to adjacent structures, and differentiating them from soft tissue tumors. Accessory muscles are isointense to skeletal muscle on all pulse sequences, and can insert by fleshy muscular or tendinous insertions. Accessory muscles around the ankle include: the flexor digitorum accessorius longus, the peroneocalcaneus internus, the accessory soleus, and the accessory peroneal muscles.

Discussion

Flexor Digitorum Accessorius Longus

The flexor digitorum accessorius longus (FDAL) is an anomalous muscle with a reported prevalence of 2%-8% in cadavaric studies.1,2,9,10-14 The FDAL can originate from many posterior compartment structures, including the flexor retinaculum, the tibia, the fibula, the flexor hallucis longus, and the soleus. The FDAL courses through the tarsal tunnel, where it remains muscular until just prior to exiting (4a,5a). It lies deep to the deep aponeurosis and flexor retinaculum, differentiating it from the accessory soleus muscle.

 

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Figure 3:

A 3D illustration of the flexor digitorum accessorius longus (FDAL) demonstrates its course, which is deep to the deep aponeurosis (DA) and flexor retinaculm (FR) before inserting on the the quadratus plantae (QP). It lies along the posterior margin of the flexor hallucis longus muscle and tendon (FHL). The flexor digitorum longus (FDL) and posterior tibial tendon (PTT) are also labeled.

 

The FDAL is intimately related to the neurovascular bundle and may abut, compress, or impinge upon the posterior tibial and/or lateral plantar nerves (6a). Because of its close relationship to the flexor hallucis longus tendon, the FDAL has also been associated with flexor hallucis longus tenosynovitis. The FDAL either inserts onto the flexor digitorum longus (FDL) tendon prior to the FDL splitting into its four tendon slips, or onto the quadratus plantae muscle. The FDAL is isointense to normal skeletal muscle on all pulse sequences. Functionally, the FDAL is thought to assist in toe flexion.

 

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Figure 4:

Sequential axial T1-weighted MR images of a different patient (52 y/o female with chronic pain and swelling). The FDAL (red) courses posterior to the flexor hallucis longus (FHL), abuts the neurovascular bundle (yellow), lies deep to the deep aponeurosis and flexor retinaculum (black arrowheads), and inserts (red arrowhead) onto the quadratus plantae muscle (QP).

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Figure 5:

The FDAL (arrows) remains fleshy until just prior to exiting the tarsal tunnel. The muscle is seen posterior to the flexor hallucis longus tendon (FHL).

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Figure 6:

Axial T1-weighted MR image in a different patient (23 y/o male college golfer with tarsal tunnel syndrome). Deep to the flexor retinaculum, this patient's FDAL muscle (arrows) extends posterior to and compresses the neurovascular bundle (yellow).

 

Accessory Soleus

The accessory soleus muscle was originally described by Cruvelhier in 1843, and it is thought to represent a splitting of the soleus anlage early in development.15-18 The accessory soleus has a reported prevalence of 0.7% to 5.5% in cadavaric studies.1,2,19,20 It commonly presents in the 2nd or 3rd decades of life, and has a 2:1 male to female ratio. The accessory soleus originates from the anterior surface of the soleus muscle or from the tibia and fibula, and is invested in its own fascia, distinguishing it from the normal soleus. It descends anterior or anteromedial to the Achilles tendon, and superficial to the flexor retinaculum. There are five types of insertions: a tendinous insertion onto the upper calcaneus (8a), a muscular insertion onto the Achilles tendon, a muscular insertion upon the upper surface of the calcaneus, a tendinous insertion upon the superior calcaneus, and a tendinous insertion upon the medial calcaneus (9a,9b).2 The accessory soleus is supplied by the posterior tibial artery and innervated by the posterior tibial nerve.

 

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Figure 7:

3D rendering demonstrating the accessory soleus muscle (AS) located superificial to the deep aponeurosis (DA) and flexor retinaculum (FR). The soleus (S) and Achilles tendon (A) are also indicated.

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Figure 8:

Accessory soleus with a tendinous insertion. Sequential axial T1-weighted MR images of a 50 y/o female with ankle pain. Although the accessory soleus muscle (arrows) courses superficial to the flexor retinaculum (black arrowhead), and resides outside the tarsal tunnel, it still causes extrinsic compression upon the underlying neurovascular bundle (yellow). This accessory soleus muscle has a tendinous insertion (red arrow head) on the medial calcaneus. (Medial talar dome osteochondral injury (asterisk) is noted).

 

9a
9b
Figure 9:

Accessory soleus with a fleshy insertion. Axial (9a) and sagittal (9b) T1-weighted MR images of a 43 y/o female with ankle pain. An accessory soleus (arrows) with a fleshy insertion on the medial calcaneus (red arrowhead) is apparent.

 

Presenting signs and symptoms have included painless mass, painful mass, localized compartment syndrome, and hindfoot and clubfoot deformities.1,2,5,6,15,21 It has been suggested that pain may related to increased intrafascial pressure, exercise induced claudication secondary to inadequate blood supply, or compression of the posterior tibial nerve. Although the accessory soleus resides outside the tarsal tunnel, it has been implicated in tarsal tunnel syndrome, likely related to extrinsic compression. Successful surgical treatments for the symptomatic accessory soleus have included fasciotomy, muscle debulking, tendon release, and accessory muscle excision.15,22

Peroneus Quartus

Multiple accessory peroneal muscles have been described throughout the literature, including peroneus tertius, peroneus accessorius, peroneocalcaneus externum, peroneus digiti minimi, and peroneus quartus (PQ) muscles. There is much confusion in the literature, as there are multiple overlapping classifications and a vast array of descriptive terminology regarding the accessory peroneal muscles. Occasionally the term peroneus quartus has been used to refer to several or even all of the accessory peroneal muscles.2,22,24 The reported prevalence is highly variable because of the vast array of classification systems. The peroneus tertius has a reported prevalence of 83% to 95%, the peroneus digiti minimi has a reported prevalence of 15.5% to 34%, and the PQ has a reported prevalence of 10% to 26%.1,2,25-29 The PQ has a male predominance, is unique to humans, and is often bilateral. Classically, the peronealcalcaneal variant of peroneus quartus is the most common, originating from the peroneus brevis and inserting on the retrotrochlear eminence of the calcaneus (11a,11b). Other origins include the peroneus longus and the posterior surface of the fibula. Other insertions include the peroneal tubercle of the calcaneus, inferior peroneal retinaculum, cuboid (peroneocuboideus), and peroneus longus (peroneoperoneolongus).

 

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Figure 10:

The peronealcalcaneal variant of the the peroneus quartus. The peroneus quartus (PQ) arises from the peroneus brevis muscle (PB) and courses medial and posterior to the peroneus longus (PL) and peroneus brevis (PB) muscles and tendons before inserting on the retroctrochlear eminence of the calcaneus (asterisk).

 

The PQ courses medial and posterior to the other peroneal tendons, where it acts predominately as a foot pronator. Though frequently asymptomatic, the PQ has been associated with pain, swelling, ankle instability, subluxations, mechanical attrition, longitudinal tears and tenosynovitis of the peroneal tendons. Symptomatic relief has been reported with surgical excision.

 

11a
11b
Figure 11:

Peroneus quartus (peronealcalcaneal variant). Axial (11a), and sagittal (11b) T1-weighted MR images show a fleshy accessory peroneus quartus muscle (arrows) coursing posterior the peroneal longus (PL) and peroneus brevis (PB) tendons and inserting onto the retrotrochlear eminence of the calcaneus (asterisk).

 

Peroneocalcaneus Internus

The peroneocalcaneus internus (PCI) muscle is a rare muscle located deep to the flexor retinaculum in the posterior compartment of the lower leg (J). It was originally described in 1872 by Macalister.30 It has a prevalence of 1%.1,31 The PCI muscle originates along the inner part of the lower third of the fibula. It is bordered medially by the flexor hallucis longus muscle and tendon, where there is muscular interdigitation. The PCI is bordered anteriorly by the tibia, interosseous ligament, and tibiotalar joint. Posteriorly, the PCI is bordered by the soleus, and laterally by the fascia separating the PCI from the peroneal muscles. The PCI tendon passes inferior to the sustentaculum tali, along with the the flexor hallucis longus tendon, and the PCI tendon inserts onto a small tubercle on the medial calcaneus below the sustentaculum tali.

 

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Figure 12:

The peroneocalcaneus internus (PCI) courses lateral to the flexor hallucis longus muscle and tendon (FHL) before inserting below the sustentaculum tali (asterisk). The posterior tibial (PTT) and flexor digitorum longus (FDL) tendons are also labeled.

 

The PCI is typically asymptomatic, but it can displace the flexor hallucis longus muscle medially, indirectly compressing the neurovascular bundle. The tendons of the PCI and flexor hallucis longus course along side of one another and can cause mechanical attrition or tenosynovitis. Symptomatic relief has been reported with steroid injection and surgical excision.33

 

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Figure 13:

Peroneocalcaneus internus (PCI). Sequential axial T2-weighted MR images in a 73 y/o female patient with heel pain, numbness, and a clinical diagnosis of tarsal tunnel syndrome. At the distal tibia, the PCI muscle (red) interdigitates with the flexor hallucis longus muscle (blue). Distally, the PCI tendon (red arrow) is seen lateral to the flexor hallucis longus tendon (blue arrow). Both tendons are highlighted by tenosynovial fluid (asterisk) posterior to the talus and sustentaculum tali. The neurovascular bundle is seen medial to these tendons (yellow outline). The PCI tendon inserts on the medial calcaneus below the sustentaculum tali (red arrow head).

 

Conclusion

Accessory muscles around the ankle are frequently asymptomatic, but can be associated with pain, a mass, compressive neuropathy, compartment syndrome, or rigid hindfoot deformities. Knowledge of these accessory muscles, their specific location and their characteristic MRI appearance (with isointensity to skeletal muscle on all pulse sequences) assists in avoiding wrong diagnoses, guiding treatment, and directing surgical options.

References

1 Cheung YY, Rosenberg ZS. MR imaging of accessory muscles around the ankle. MRI Clinics of North America 2001; 9(3):465-473.

2 Sookur PA, Naraghi AM, Bleakney RR, Jalan R, Chan O, White LM. Accessory muscles: anatomy, symptoms, and radiologic evaluation. RadioGraphics 2008; 28:481-499.

3 Moorman CT, Monto RR, Bassett FH. So-Called trigger ankle due to an aberrant flexor hallucis longus muscle in a tennis player. A case report. Journal of Bone and Joint Surgery 1992; 74:294-295.

4 DosRemedios ET, Jolly GP. The accessory soleus and recurrent tarsal tunnel syndrome: case report of a new surgical approach. Journal of Foot and Ankle Surgery 2000 May-June; 39(3):194-197.

5 Danielsson LG, el-Haddad I, Sabri T. Clubfoot with supernumerary soleus muscle. Report of 2 cases. Acta Orthopaedica Scandinavica 1990: Aug; 61(4):371-373.

6 Bonnell J, Cruess RL. Anomalous insertion of the soleus muscle as a cause of fixed equinus deformity: a case report. Journal of Bone and Joint Surgery Am. 1969; 51:999-1000.

7 Dunn AW. Anomalous muscles simulating soft-tissue tumors in the lower extremities: report of three cases. Journal of Bone and Joint Surgery Am. 1965; 47:1397-1400.

8 Wu KK. Accessory soleus muscle simulating a soft tissue tumor of the posteromedial ankle region. Journal of Foot Surgery 1991 Sept-Oct; 30(5):470-471.

9 Cheung YY, Rosenberg ZS, Colon E, Jahss M. MR imaging of flexor digitorum accessorius longus. Skeletal Radiology 1999;28:130-137.

10 Driver J. The morphology of the long accessorius muscle. Anat Rec. 1914; 8:341.

11 Lewis O. The comparative morphology of m. flexor accessorius and the associated long flexor tendons. J. Anat. 1962; 96:321.

12 Nathan H, Gloobe H, Yosipovitch Z. Flexor digitorum accessorius longus. Clinical Orthopaedics 1975;158.

13 Peterson DA, Stinson W, Lairmore JR. The long accessory flexor muscle: an anatomical study. Foot and Ankle International 1995;16:637.

14 Woods J. Variations in human mycology observed during winter session of 1867-1868. Proc R Soc Lond 1867-1868;16:438.

15 Brodie JT, Dormans JP, Gregg JR, Davidson RS. Accessory soleus muscle. A report of 4 cases and review of literature. Clin Orthop 1997;337:180-186.

16 Gordon SL, Matheson DW. The accessory soleus. Clin Orthop 1973;97:129-132.

17 Assoun J, railhac JJ, Richardi G, Fajadet P, Fourcade D, Sans N. CT and MR of accessory soleus muscle. J Comput Assist Tomgr 1995;19(2):333-335

18 Leswick DA, Chow V, Stoneham GW. Answer to case of the month: #94 accessory soleus muscle. Canadian Association of Radiologists 2003;53(5)313-315.

19 Peterson DA, Stinson W, Carter J. Bilateral accessory soleus. A report on four patients with partial fasciectomy. Foot and Ankle. 1993;14:284.

20 Wu KK. Accessory soleus muscle simulating a soft tissue tumor of the posteromedial ankle region. Journal of Foot Surgery 1991:30:470.

21 Trosko JJ. Accessory soleus: a clinical perspective and report of three cases. J Foot Surg 1986;25:296.

22 Featherstone T. MRI diagnosis of accessory soleus muscle strain. Br J Sports Med 1995;29:277-278.

23 Aammit J, Singh D. The peroneus quartus muscle: anatomy and clinical relevance. J Bone Joint Surg Br 2003; 85:1134-1137.

24 Sobel M, Levy ME, Bohne WH. Congenital variations of the peroneus quartus muscle: an anatomic study. Foot Ankle 1990;11:81-89.

25 Witvrouw E, Borre KV, Willems TM, Huysmans J, Broos E, De Clercq D. The significance of the peroneus tertius muscle in ankle injuries: a prospective study. Am J Sports Med 2006;34:1159-1163.

26 Sarrafian S. Myology: anatomy of the foot and ankle, Vol 2. Philadelphia, JB Lippincott, 1993, 218-226.

27 Cheung YY, Rosenberg ZS, Ramsinghani R, Beltran J, Jahss M. Peroneus quartus muscle: MR imaging features. Radiology 1997; 202:745-750.

28 Hecker P. Study of the peroneus on the tarsus. Anat Rec. 1923; 26:79-82.

29 Sobel M, Levy ME, Bohne WH. Congenital variations of the peroneus quartus muscle: an anatomic study. Foot and Ankle 1990;11:81-89.

30 Macalister A. Additional observations on muscular anomalies in human anatomy. Trans R Irish Adad 1872;25:125-130.

31 Mallado JM, Rosenberg ZS, Beltran J, Colon. The peroneocalcaneus internus muscle: MR imaging features. AJR 1997;169:585-588.

32 Perkins J. An anomalous muscle of the leg. Peroneocalcaneus internus. Anat Rec 1914;8:21.

33 Best A, Giza E, Linklater J, Fracs M. Posterior Impingement of the ankle caused by anomalous muscles: a report of four cases. JBJS 2005;87:2075-2079.

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