Imaging description

Posterior impingement occurs between the distal tibia and calcaneus posteriorly. A lateral ankle radiograph may or may not show os trigonum, a triangular-shaped ossicle, posterior to the talus (Figure 60.1). Although this entity is often called “os trigonum syndrome,” impingement associated with os trigonum is considered as a subset of posterior impingement. Sources of impingement can be other bony structures or soft tissues such as flexor hallucis longus (FHL) and posterior intermalleolar ligament. Prominent lateral tubercle, a lateral border of the groove for FHL, is called Stieda’s process (Figure 60.2). Other bony variations associated with posterior impingement include a prominent superior calcaneal tuberosity and downward sloping posterior tibial plafond. MRI may show localized bone marrow edema (Figure 60.3) and evidence of inflammation in surrounding soft tissues including FHL, posterior talofibular ligament, and intermalleolar ligament.

Importance

Os trigonum is a common accessory bone seen in 13–25% of adults. When present, it is bilateral in 50% of patients. Posterior impingement is often associated with os trigonum. However, most individuals including athletes with os trigonum are asymptomatic. Other common etiologies causing posterior ankle pain (see below) should be excluded. Diagnostic injection of local anesthetic under fluoroscopy or ultrasound may be helpful for elusive cases.

Typical clinical scenario

Patients with posterior impingement syndrome may manifest posterior ankle pain with forceful plantar flexion. This syndrome often affects ballet dancers and soccer players, who are involved in plantar hyperflexion of the ankle. Os trigonum and posterior process are the most common causes of posterior ankle impingement. On physical examination, tenderness is noted behind the peroneal tendons in the back of the lateral malleolus. Reproducible pain by forced passive plantar flexion of the ankle is called the plantar flexion sign. The patients initially receive conservative treatments including modification of activities, anti-inflammatory drugs, and physical therapy. Excisional surgery may be indicated when conservative therapy has failed.