A 35 years old male, presenting with pain in the right ankle.
The patient, a 35 years old male, reported that nearly 5 years ago he had suffered a right ankle strain during an outdoor sporting activity. Since then he has been complaining of an intermittent exercise induced, rather subtle, right ankle pain. During this 5 years period he was evaluated several times and was treated with physical therapy, rest, guarded weight bearing and semisolid casts, all without benefit.
He was referred to our department for an MRI examination of the ankle.
The examination consisted of T1- weighted spin echo (SE), T1- weighted spin echo (SE) with gadolinium enhancement (SE + GD) and T2 weighted turbo spin echo (TSE) sequences, performed in the axial, coronal and sagittal planes.
The study revealed the presence of an anomalous muscular mass at the medial aspect of the right ankle. The finding was consistent with the presence of a flexor digitorum accessory longus muscle.
The (FDAL) flexor digitorum accessory longus muscle (accessory of the accessory of Turner, second accessory of Humphrey, long accessory of the long flexors) [1] is a supernumerary muscle with a reported incidence of 2%-8%. It is the most common accessory muscle of the medial aspect of the ankle joint, and the second most common accessory muscle of the ankle joint in general, after the peroneus quartus [1, 2].
It is regarded as a regressive remnant, assisting the quadratus plantae and the flexor digitorum longus muscles, in the plantar flexion of the toes [1, 2].
The FDAL may originate from the tibia, the fibula, the transverse intramuscular septum, the deep fascia of the leg, and even the calcaneus. The muscle after its origin descends posteromedially to the flexor hallucis longus muscle, underneath the flexor retinaculum and posterolateraly to the neurovascular bundle. It enters the tarsal tunnel, where it may present a fleshy portion. Finally, the FDAL inserts at the lateral head of the quadratus plantae muscle and at the undivided portion of the tendon of the flexor digitorum longus muscle [1-3].
A dual head variant of the muscle has been described. The FDAL is distinguished from the other accessory muscles of medial aspect of the ankle (accessory soleus, tibiocalcaneus, peroneocalcaneus which inserts to the calcaneus), whereas its deep location in the tarsal tunnel and its tight relationship to the flexor retinaculum, distinguishes it from the accessory soleus particularly. Although generally asymptomatic, the presence of the FDAL could be associated with tarsal tunnel syndrome (TTS) [1].
The clinical entity of tarsal tunnel syndrome is defined as a compression neuropathy of the tibial nerve, in the tarsal tunnel. Clinical findings include pain in the longitudinal arch; intermittent dysesthesias, paresthesias or anesthesia in the planta pedis; burning pain during night that radiates distally, into the hall of the foot and proximally along the medial or posterior aspect of the leg [4].
Numerous conditions have been reported to predispose or have a causal relationship to TTS. These include: direct injury or repetitive trauma, systemic diseases, space-occupying lesions of the tarsal tunnel and the presence of regional anatomic deformities or accessory muscles.
The close proximity of the FDAL muscle belly to the neurovascular bundle in the tarsal tunnel can cause compression of the posterior tibial nerve resulting in TTS, with prior trauma or strenuous activities as triggering factors.
Conservative treatment may be successful, particularly in the early stages of TTS. In cases that conservative treatment has failed, excision of the accessory muscle to relieve nerve compression has been reported, with variable results [3, 4].
Tarsal tunnel syndrome caused by flexor digitorum accessory longus muscle.
Based on the patient’s MRI of the right ankle, an abnormal muscular structure is observed near the medial malleolus. It is located posteromedially relative to the normal flexor tendon group, closely associated with the posterior tibial nerve and adjacent tendons. The signal of this abnormal muscle is similar to that of skeletal muscle. It has a slender shape, descending through the tarsal tunnel and positioned adjacent to or connected with the Flexor Digitorum Longus tendon and the Quadratus Plantae muscle. No significant osseous abnormality or fracture sign is detected.
This additional muscle shown on the images is consistent with the typical radiological features of the Flexor Digitorum Accessorius Longus (FDAL). Within the tarsal tunnel, its muscular belly may be closely apposed or mildly enlarged, potentially exerting pressure on the posterior tibial nerve and vascular bundle.
The cause is the presence and abnormal path of this muscle within the tarsal tunnel, potentially compressing the nerve and resulting in characteristic symptoms such as foot or arch pain, numbness, and nocturnal burning pain.
For example, synovial cysts or tendon sheath cysts may also compress the nerve, but they can be differentiated from a muscular structure on imaging.
Accessory muscles in the medial ankle region can also cause symptoms, but the imaging characteristics here mainly point to FDAL.
Considering the patient’s age (35 years), symptoms (ankle pain), and imaging features (an accessory muscle pathway within the medial tarsal tunnel, adjacent to the posterior tibial nerve), the most likely diagnosis is:
Flexor Digitorum Accessorius Longus (FDAL) Leading to Tarsal Tunnel Syndrome (TTS)
If further confirmation is required, nerve conduction studies (electrophysiological tests) or ultrasound evaluations of the soft tissue blood supply and dynamic nerve compression may be considered.
For Tarsal Tunnel Syndrome caused by FDAL, treatment can be divided into conservative and surgical approaches.
If conservative therapy does not achieve marked improvement and the patient’s symptoms significantly affect daily life, surgical exploration and excision of the abnormal accessory muscle or tarsal tunnel decompression may be considered.
Throughout the rehabilitation process, follow the FITT-VP Principle (Frequency, Intensity, Time, Type, Volume, Progression) by increasing exercise volume in an individualized manner and scheduling regular follow-up assessments of the ankle.
This report is based on the provided medical history and imaging data for reference purposes. It does not substitute for a comprehensive, in-person evaluation and treatment. Patients should still combine clinical examination results, laboratory findings, and other tests under the guidance of professionals for a full assessment and management. If you have any concerns or if symptoms worsen, please seek prompt medical attention.
Tarsal tunnel syndrome caused by flexor digitorum accessory longus muscle.
