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Compiled by: Matthew Penn Chanin, M.D.
History:
Ankle pain with "grinding" sensation.
Exam:
Steeply plantar flexed position. Short axis oblique FSE T1 and T2 weighted images, sagittal and coronal FSE-IR and T1 weighted images, and coronal FSE-IR.
Findings:
Mass within tarsal tunnel, parallel and posterior to the neurovascular bundle with skeletal muscle signal intensity, origininating on the posterior fascial covering of the flexor hallucis muscle and inserting on the plantar flexor musculature, specifically the quadratus plantae medial aspect.
Diagnosis:
Accessory Flexor Digitorum Longus.
Discussion:
Accessory muscles about the ankle are a common finding. The most common anomalous muscle is the peroneous quartus, occurring in 10% to 26%. This is followed by the accessory flexor digitorum longus or flexor digitorum accessorius longus (FDAL), occurring in 2% to 8%; the accessory soleus is seen in 1% to 6%.
The medial accessory muscles include the FDAL, accessory soleus, tibiocalcaneus, peroneocalcaneus, and flexor hallucis longus. The FDAL arises from the lower leg with variable reported origins including the flexor hallucis lungus (FHL), flexor retinaculum, soleus, tibia, fibula and the fascia covering the deep compartment. The FDAL has a deep-seated location within the tarsal tunnel and frequently accompanies the neurovascular bundle, bounded by the flexor retinaculum. This location distinguishes it from the accessory soleus which is located outside of the tarsal tunnel. FDAL insertion on the plantar flexor muscles, specifically the quadratus plantae and flexor digitorum longus, distinguishes it from the accessory soleus, tibiocalcaneus and peroneocalcaneus muscles. The latter three muscles typically insert on the calcaneus.
The clinical presentation of a closed compartment syndrome rather than nerve compression or tarsal tunnel syndrome also helps to distinguish the accessory soleus from all the above muscles.
The rarely described accessory flexor hallucis longus is an extension of the muscle belly into the FHL sheath, with or without destinct separate tendons, origins or insertions. A single case report indicates this accessory muscle may present uniquely as ankle locking or "trigger ankle", which is treated surgically.
Lateral accessory or accessory peroneal muscles include the peroneus quartus, peroneus digiti minimi, peroneus digiti minimi quinti, and peroneus accessories. This group of accessory muscles is characterized by variation of origin and insertion. The most common variation of these is the peroneus quartus. The laterally located muscular anomalies may present with pain, instability, and subluxation.
· Accessory Peroneal Muscles (lateral accessory muscles) - confusing terminology prompted some authors to view all lateral compartment muscles as peroneal variants and to define a peroneus quartus muscle as "a muscle arising from the lower leg and inserting onto the calcaneus". Three main subcategories were proposed by Hecker based on their insertions - into the calcaneus (peroneocalcaneus externum), the cuboid (peroneocuboideus), or the peroneus longus tendon (peroneoperoneolongus).
· Peroneus Quartus
· Definition: arising in the lower leg and inserting on the calcaneus
· Synonym: peroneocalcaneus externum, peroneus calcaneus
· Incidence: 10-26%, the most common variation of the accessory peroneal muscles.
· Presentation: pain, instability, predisposition for subluxation (but usually asymptomatic)
· Origin
· Peroneal muscle (longus or brevis)
· Distal lateral portion of the fibula
· Insertion
· Phalanges or metatarsal bone of the fifth ray
· Calcaneus, specifically the retrotrochlear eminence (RE), not the peroneal tubercle as incorrectly reported by Sobel in 1990.
· Notes
· Associated with an hypertrophic RE
· Previously incorrectly reported to accompany or cause an hypertrophic peroneal tubercle.
· Peroneus digiti minimi (quinti)
· Synonym: Peroneus digiti minimi quinti
· Incidence: 15.5-36%, reported as high as 59.7% in cadaver studies
· Presentation: asymptomatic
· Origin: peroneus brevis, small slip of the peroneus brevis tendon
· Insertion: dorsal aponeurosis of the fifth digit, with a small separate branch attached to the fifth metatarsal base (5).
· Peroneoperoneolongus
· Synonym: Peroneus accessorius
· Origin: Fibula between the longus and brevis
· Insertion: tendon of the longus in the sole of the foot
· Medial accessory Muscles
· Flexor digitorium accessorius longus (FDAL)
· Incidence: 2-8%
· Presentation: tarsal tunnel syndrome
· Origin:
· FHL
· Flexor retinaculum
· soleus
· tibia
· fibula
· fascia covering the deep compartment
· Insertion: Variable, see above
· Notes: Most common of the medial accessory muscles
· Accessory Soleus
· Synonyms: soleus accessorius
· Incidence: 1-6%
· Presentation: painful or nonpainful mass, posteromedial ankle. Compartment syndrome.
· Origin
· Anterior surface of the soleus
· Fibula and soleal line of tibia
· Insertion
· Along Achilles tendon
· Muscle or fleshy insertion on superior surface of calcaneus
· Muscle or fleshy insertion on the medial surface of calcaneus
· Tendinous insertion on the medial calcaneus
· Notes:
· Outside the tarsal tunnel
· Fasciotomy performed in symptomatic patients
· Accessory Flexor Hallucis Longus
· Incidence - rare
· Presentation
· "Trigger ankle"
· Origin & Insertion
· Variable
· May present as an abnormally long muscle slip or supernumerary tendon extending into the FHL sheath
· Peroneocalcaneus
· Synonym: peroneocalcaneus internus
· Incidence: 1%
· Presentation: posterior impingement resulting in ankle pain
· Origin: inner aspect distal fibula
· Insertion: small tubercle in the calcaneus, just distal to the sustentaculum tali
· Notes:
· descends in the tarsal tunnel
· may displace the FHL medially and thus indirectly encroach on the neurovascular bundle
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