HSS Ultrasound Case 77

Clinical History 25 year old female with intermittent posterior ankle pain.

MRI showed the presence of an os trigonum, but without bone marrow edema.

Ultrasound-guided anesthetic injection of the os trigonum was requested to assess whether it was a symptom generator.

Ultrasound of the Month – Case 77

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Diagnosis: Painful os trigonum Discussion Os trigonum is a fragment of bone of the lateral tubercle of the posterior process of the talus that has failed to fuse with the remainder of the talus. The incidence is said to be 7% of the population.

Although it may be an incidental finding, it can be the source of pain in individuals who do activities requiring repetitive plantarflexion of the ankle, such as ballet dancers or soccer players.

A painful os trigonum is often called “os trigonum syndrome�, and it is one of a few conditions that can result in posterior ankle impingement.

Radiological studies play a pivotal role in evaluating patients with clinical symptoms of posterior ankle impingement.

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Discussion MRI can identify an os trigonum as well as the extent of soft tissue involvement such as synovial scarring and FHL tenosynovitis.

On MRI, bone marrow edema (high signal intensity on inversion recovery sequences) in the os trigonum and posterior process of talus are suggestive of a symptomatic os trigonum, but it is not 100% sensitive.

Ultrasound can also identify an os trigonum, but more importantly, it can be used to guide needle placement for a targeted injection of anesthetic with or without cortisone.

If the patient feels symptomatic relief after injection of anesthetic around the os, it can increase the diagnostic confidence of the foot and ankle specialist that the os trigonum is the symptom generator.

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