Recognition of the Flexor Digitorum Accessorius Longus

Abstract

The flexor digitorum accessorius longus muscle was observed during a cadaveric surgery course on the foot and ankle for third-year podiatric medical students. The cadaveric foot had been amputated just proximal to the ankle level so that the muscle origin could not be determined; its insertion, however, was found to be into the flexor digitorum longus tendon, just before the tendon split into its digital slips. This article reviews the literature on the muscle and its clinical implications and describes and shows the muscle as it was seen in this case.

Foot and ankle surgeons often overlook the high incidence of the flexor digitorum accessorius longus muscle. The rate of occurrence varies from 4% to 8% of limbs and up to 13% of cases studied.[1-4] It is more commonly unilateral than bilateral.[1] Awareness of its presence is important when performing tarsal tunnel surgery. Several authors[1-9] have reported that tarsal tunnel syndrome can be provoked by an accessory muscle. This syndrome is typically caused by compression of the posterior tibial nerve in the tarsal canal, which may lead to the classic “shooting”-type pain. Removal of the muscle in such cases could lead to reduction of symptoms, yet it is frequently ignored as part of the differential diagnosis of tarsal tunnel syndrome.

The literature has described the flexor digitorum accessorius longus as having one or two heads with variable origins from the tibia, posterior surface of the fibula, flexor hallucis longus, flexor digitorum longus, tibialis posterior, soleus, and fascia of the deep posterior compartment.[1-7,10-12] It descends in the tarsal tunnel deep to the laciniate ligament, medial to the neurovascular bundle and flexor hallucis longus tendon, and it has a muscular or tendinous insertion into the quadratus plantae or the flexor digitorum longus tendon in the foot. Although it has not been studied, the function of the flexor digitorum accessorius longus is probably to act synergistically with the flexor digitorum longus. Its origins and insertions make it a weak inverter of the foot.

Sammarco and Stephens[5] reported treating a patient with a 10-year history of pain in her left foot that conduction velocities showed to be tarsal tunnel syndrome. After conservative management did not relieve her symptoms, surgical exploration revealed an anomalous flexor digitorum accessorius longus muscle looping around the posterior tibial nerve before entering the tarsal tunnel. The patient’s symptoms improved significantly after excision of the muscle from the tarsal canal. Sammarco and Conti[9] later reported finding this muscle in six ankles with symptomatic tarsal tunnel syndrome. They found that surgical decompression of the nerve from the overlying muscle did not have good results compared with decompression associated with tumors, synovial cysts, and varicosities because permanent damage may have occurred from the long-standing occult compression of the nerve. Because it is not known how many asymptomatic individuals have an accessory muscle in that area, it is possible that the muscle did not cause the tarsal tunnel symptoms in all cases.[9] Ho et al[8] also reported tarsal tunnel syndrome resulting from a strained flexor digitorum accessorius longus muscle diagnosed by means of magnetic resonance imaging (MRI).

Observations

The flexor digitorum accessorius longus muscle was found during a surgery course for third-year podiatric medical students in an embalmed cadaveric specimen that had been transected 8 cm above the medial malleolus (Fig. 1 and Fig. 2). The single flattened muscle belly was shaped like a pyramid that converged into a distal apex under the flexor retinaculum extending into the tarsal tunnel. It had a circumference of 4 cm and a length of 5 cm at the site of the transection. The narrow tendon was 9.5 cm long and 2 mm wide. The muscle traveled through the posterior compartment of the leg between the flexor digitorum longus and flexor hallucis longus muscles while remaining in a separate osteofascial compartment (Fig. 3). The tendon coursed inferior and lateral to the flexor hallucis longus as it entered the tarsal tunnel. At the level of the talus, the tendon ran superficial to the neurovascular bundle and deep to the quadratus plantae and abductor hallucis muscles. On entering the porta pedis, the tendon progressed laterally and deep to the master knot of Henry. The tendon then divided into two separate slips: one inserted into the undivided aspect of the flexor digitorum longus tendon at the talonavicular joint level and one continued distally and inserted into the flexor digitorum longus tendon, just proximal to the division of its five digitations (Fig. 4).

Discussion

The flexor digitorum accessorius longus in this case had a presentation similar to that of others reported. It traversed the tarsal tunnel, beneath the laciniate ligament, to insert into the flexor digitorum longus tendon before the latter split into its four digital slips. It had a muscular belly before entering the tarsal canal and quickly became tendinous as it progressed through the tarsal canal and the porta pedis.

Tarsal tunnel syndrome is a widely recognized phenomenon in the medial ankle region that often results from compression of the posterior tibial nerve in the third compartment of the flexor retinaculum.[13] Many clinical entities have been associated with tarsal tunnel syndrome, including plantar fasciitis, heel spur syndrome, acute and chronic myofascial foot strain, neurofibromas, cysts, lipomas, ganglions, lumbosacral radiculopathy, and pain associated with peripheral neuritis, peripheral vascular disease, and drug toxicity.[14] However, more often than not, the flexor digitorum accessorius longus muscle is not included in the differential diagnosis of tarsal tunnel pain, probably because of the lack of knowledge of the muscle’s existence in the region.

Four variant muscles are found along the medial ankle, including the flexor digitorum accessorius longus. The peroneocalcaneus internus arises from the medial surface of the fibula and then enters the fourth compartment of the flexor retinaculum before inserting into the medial border of the calcaneus. The tibiocalcaneus internus arises from the medial tibial crest, running superficial to the neurovascular bundle, to insert just anterior to the Achilles tendon on the medial calcaneus.[4,5,12] The accessory soleus arises from the oblique line of the tibia, the deep surface of the soleus, and the aponeurosis of the flexor digitorum longus to run posterior to the neurovascular bundle before inserting into the medial aspect of the calcaneus.[4,5,12,15]

With the development of better imaging techniques, such as MRI, detection and information regarding surgical intervention have been improved.[15] The muscle can be readily differentiated from other variant muscles[7] in the region because it lies deep to the flexor retinaculum, has a fleshy belly close to the posterior tibial nerve, and ultimately inserts into the long flexor tendon or the quadratus plantae.[4] Ischemic pain in this region rather than nerve compression symptoms would suggest that an accessory soleus muscle is present because of its diminished blood supply.[16]

Although the exact frequency of tarsal tunnel symptoms associated with the presence of the long accessory muscle is unknown, the position of the muscle in relation to the posterior tibial nerve as it travels through the canal may explain the variance in symptoms. Cases in which the muscle runs superficial to the nerve in the canal have been reported to be more symptomatic than those in which it runs deep to the nerve.[5]

Conclusion

The flexor digitorum accessorius longus muscle has frequently been seen in lower-extremity dissection. However, foot and ankle surgeons and clinicians often do not include this muscle in the differential diagnosis of tarsal tunnel syndrome. The advent of better imaging modalities such as MRI has facilitated detection of this muscle, which should not be overlooked as a factor in tarsal tunnel syndrome.