Teaching Case

A 38-year-old man presented to the emergency department with right ankle pain after an ankle injury. Because of high clinical suspicion of a fracture, a CT-scan was performed The patient did not attend any of the follow-up clinics and was lost to follow-up.

Coronal (Figure 1) and axial (Figure 2) CT images show a flake of bone in the soft tissues on the medial side of the medial malleolus representing an avulsion fracture of the insertion of the medial flexor retinaculum of the ankle. In this case, the avulsion fracture of the medial flexor retinaculum of the ankle was occult on X-ray. In addition, the CT-scan showed a fracture of the medial side of the talus.

Background The medial flexor retinaculum extends from the medial malleolus to the posterosuperior aspect of the calcaneus and forms the medial and posterior boundary of the tarsal tunnel [1,2,3]. The function of a retinaculum is to maintain approximation of tendons to the underlying bone [1,3]. The tarsal tunnel contains from anterior to posterior, the posterior tibial tendon, the flexor digitorum longus tendon, the posterior tibial artery, posterior tibial vein, and posterior tibial nerve, and the flexor hallucis longus tendon [4]. A focal density in the soft tissues can represent a foreign body, an ossification or a calcification [5].

Clinical Perspective Periosteal avulsion of the medial flexor retinaculum of the ankle is rare [2,6]. Injuries of the medial flexor retinaculum usually occur at the insertion on the medial malleolus [2]. Mechanisms of injury include forced ankle eversion and forced dorsiflexion combined with a contracted tibialis posterior tendon or acute injury of the ankle with fracture [2,3,6]. An important complication of an injury to the medial flexor retinaculum is anterior dislocation of the posterior tibial tendon [2,6].

Imaging Perspective An avulsion fracture of the medial flexor retinaculum of the ankle can be occult on X-ray [6]. CT is the best modality to visualize the bony flake. MRI with the ankle in different scan positions (i.e. imaging in motion) as dislocation of the posterior tibial tendon can be missed in static imaging. Ultrasound including dynamic manoeuvres can also be used to diagnose dislocation of the posterior tibial tendon [4,6,7].

Outcome The treatment of an avulsion fracture of the medial flexor retinaculum is conservative [2]. Unless posterior tibial tendon dislocation is present as well, in which case surgery is recommended [2,6].

Take Home Message / Teaching Points

When you notice a bony flake on the medial side of the ankle on X-ray or CT think of an avulsion fracture of the medial flexor retinaculum of the ankle.

When an avulsion fracture of the medial flexor retinaculum of the ankle is present, remember to check for dislocation of the posterior tibial tendon.