Age / Sex : 40 / M

Chief complaint: Incidentally detected finding on ankle MR

What is your impression?

Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer).

Courtesy : Byeong Seong Kang, M.D., Department of Radiology, University of Ulsan College of Medicine, Ulsan University Hospital, Ulsan, Korea

Discussion

Answer: Flexor digitorum accessorius longus (FDAL)

Findings:

- Axial T1-weighted MR images of the right ankle joint show a fleshy FDAL posteromedial to the flexor hallucis longus (FHL) and deep to the flexor retinaculum.

- Sagittal proton density-weighted MR images show that FDAL inserts into the quadratus plantae muscle.

Differential Diagnosis: None

Diagnosis: Flexor digitorum accessorius longus

Discussion:

FDAL is one of the most common accessory muscles of the ankle, with a reported prevalence ranging from 6% to 8%. It is also most common in men. The origin of the FDAL is variable; it can arise from any structure in the posterior compartment, such as the FHL, fascia of the deep posterior compartment, and medial and lateral margins of the fibula. The distal tendon inserts into the quadratus plantae muscle or the flexor digitorum longus (FDL) tendon.

On axial MR images, FDAL is seen deep to the deep aponeurosis, within the common compartment that houses both the FHL and the posterior tibial neurovascular bundle. This feature differentiates the FDAL from the accessory soleus. FDAL descends with the posterior tibial neurovascular bundle in the tarsal tunnel and can remain fleshy in the tight fibro-osseous tunnel. Its alternative insertion to the FDL before its division can sometimes be difficult to show because the small FDAL distal tendinous slip may be obscured by the surrounding muscles and soft tissues. The presence of FDAL within the tight fibro-osseous tarsal tunnel increases the risk of compression or impingement of the posterior tibial neurovascular bundle.

References:

1. Cheung Y. Normal variants: Accessory muscles about the ankle. Magn Reson Imaging Clin N Am. 2017;25:11-26.


2. Sookur PA, Naraghi AM, Bleakney RR, Jalan R, Chan O, White LM. Accessory muscles: anatomy, symptoms, and radiologic evaluation. Radiographics. 2008;28:481-99.