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?
Flexor Digitorum Accessorius Longus
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.
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.
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.
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.
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
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).
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.
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.
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
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.
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