A 43-year-old white female patient was examined in the orthopaedic department because of persistent ankle pain and numbness, while standing or walking. The physical examination showed restricted movement and moderate tenderness of the medial ankle. There was neither swelling nor erythema. There was no history of previous trauma.
X-rays of the left ankle were performed, followed by MR imaging with 1.5 Tesla scanner using standard sequences.
The AP and oblique radiographs of the ankle showed no abnormalities (not shown).
The axial T2-w TSE MR images (Fig. 1) show the flexor digitorum accessorius longus (FDAL) muscle coursing posteromedial to the flexor hallucis longus and abutting the neurovascular bundle. The accessory muscle originates from the flexor hallucis longus muscle. The coronal T1-w MR image (Fig. 2) shows the flexor digitorum accessorius longus muscle medial to the calcaneal bone.
A. The FDAL muscle is seen in about 7% of the general population, more often in men and in about 12% of patients with a clinical diagnosis of tarsal tunnel syndrome [1, 2]. Its origin varies and includes the tibia, fibula or any structure in the posterior compartment [3]. The FDAL tendon has a course within the tarsal tunnel in close proximity to the posterior tibial artery and tibial nerve [1]. Distally, the tendon inserts either onto the quadratus plantae muscle or the flexor digitorum longus tendon [1, 2]. The close relationship of the FDAL with the posterior tibial nerve may explain its association with the clinically diagnosed tarsal tunnel syndrome [4].
B. The usual clinical presentation is ankle pain or/and numbness. Mass effect could be the pathophysiologic mechanism. Radiologists have to include in the list of differential diagnosis those entities that cause symptoms compatible with tarsal tunnel syndrome. In this respect, a careful clinical examination and access to the history of the patient are important before performing imaging.
C. The FDAL is depicted on axial MR images, located in the tarsal tunnel deep to the neurovascular bundle [3]. Its signal intensity follows that of the surrounding muscles in all pulse sequences. Coronal images (Fig. 2), are important in assessing the lack of calcaneal insertion which is seen in other accessory muscles within the tarsal tunnel, such as peroneocalcaneus internus and tibiocalcaneus internus [3].
D. MRI is the imaging method of choice in the differential diagnosis of the entities that cause tarsal tunnel syndrome, and is able to suggest the presence of an FDAL. Therefore, MRI contributes to early diagnosis and accurate treatment planning of this rare entity.
E. An accessory muscle may be the cause of tarsal tunnel syndrome. The delayed onset of symptoms of a congenital lesion may be related to modified activities, i.e. sports or profession-related. MRI is the gold standard for diagnosis and treatment planning.
Flexor digitorum accessorius longus muscle
In the provided ankle MRI images, an additional tendon/muscle belly structure can be observed near the medial malleolus (suspected to be an accessory muscle of the flexor hallucis or flexor digitorum longus, commonly referred to as “Flexor Digitorum Accessorius Longus, FDAL”). This structure is in close proximity to the posterior neurovascular bundle, located on the posteromedial side of the talus. Its signal intensity is similar to that of the surrounding muscle tissue, with no obvious swelling or significant effusion. There is no apparent bony damage or fracture in the tibia, talus, and other key bones. The overall soft tissue shows no significant edema, and surrounding ligaments appear relatively intact.
Considering the patient’s chronic ankle pain and numbness, along with the clear imaging findings showing an accessory muscle (FDAL) adjacent to the posterior neurovascular bundle, the most likely diagnosis is “Tarsal Tunnel Syndrome caused by the accessory muscle.”
Treatment Strategy:
1. Conservative Management:
Rehabilitation Exercises and Exercise Prescription (FITT-VP Principle):
This report provides a reference analysis based on existing imaging and clinical information and cannot replace in-person consultation or the diagnosis and treatment recommendations of professional medical institutions. Patients should follow the guidance of professional physicians to further refine their diagnosis and treatment plan.
Flexor digitorum accessorius longus muscle
