Posterior Intermalleolar Ligament

The posterior intermalleolar ligament, also known as the marsupial meniscus, is a small ligament in the posterior ankle. It is present in about 60%-80% of the population on autopsy, but can only be seen in about 20% of patients on routine MRI.

The posterior intermalleolar ligament has been erroneously considered as a synonym for the tibial slip of the posterior talofibular ligament. The tibial slip is a band of fibers connecting the posterior talofibular ligament to the medial malleolus, while the posterior intermalleolar ligament converges laterally to a bundle distinct from the posterior talofibular ligament.

Laterally, the fibers of the posterior intermalleolar ligament (red) are attached to the superior margin of the malleolar fossa of the fibula. The medial attachment site is much more variable, and includes the lateral border of the medial malleolar sulcus, the medial border of the medial malleolar sulcus through the septum between flexor digitorum longus and the tibialis posterior tendons, the posterior tibial cortex, the joint membrane covering the posterior process of the talus, and the floor of the flexor hallucis longus tunnel.

On MRI, the normal posterior intermalleolar ligament is a thick string or two or more fine parallel stripes on coronal images. It appears as a linear structure on axial images. On sagittal images, the ligament appears as scattered dots medially and as a thin flat or nodular structure laterally.

The posterior intermalleolar ligament may extend anteriorly, especially along its lateral extent, and give an appearance similar to that of a meniscus. This has been termed the marsupial meniscus due to its similarity to a structure found in marsupials. In marsupials, the talus has a lateral extension that serves as the receptive surface for the fibula, which is a significant weight-bearing component of the joint. A meniscus can be seen in these animals between the fibula and talus.

Entrapment and tearing of the posterior intermalleolar ligament can lead to posterior impingement syndrome, a phenomenon first described in ballet dancers with otherwise structurally normal ankles. The etiology is thought to be repetitive plantar flexion, causing intra-articular extension of the ligament and subsequent fraying and tearing.

MRI findings of posterior impingement syndrome due to posterior intermalleolar ligament pathology include a prominent posterior intermalleolar ligament as indicated by its presence in three different imaging planes and with a caliber comparable to other posterior ankle ligaments seen in the same imaging plane.

References

• Fiorella D, Helms CA, Nunley JA 2nd. The MR imaging features of the posterior intermalleolar ligament in patients with posterior impingement syndrome of the ankle. Skeletal Radiol. 1999 Oct;28(10):573-6.
• Lewis OJ. The joints of the evolving foot. Part I. The ankle joint. J Anat. 1980 May;130(Pt 3):527-43.
• Oh CS, Won HS, Hur MS, Chung IH, Kim S, Suh JS, Sung KS. Anatomic variations and MRI of the intermalleolar ligament. AJR Am J Roentgenol. 2006 Apr;186(4):943-7.
• Rosenberg ZS, Cheung YY, Beltran J, Sheskier S, Leong M, Jahss M. Posterior intermalleolar ligament of the ankle: normal anatomy and MR imaging features. AJR Am J Roentgenol. 1995 Aug;165(2):387-90.