A 21-year-old male professional football player presented with a two-month history of progressive, posterior right ankle pain. This was made worse by sprinting, by striking a ball and with any activities requiring ankle plantarflexion. On examination he was found to have a positive posterior thrust test and an otherwise normal examination. His left ankle was clinically normal.
A lateral x-ray image of the right ankle shows a large well corticated ossicle lying adjacent to the posterior talus. This can also be seen on the oblique image. The AP and Mortise views are normal. A similar, and larger, ossicle can be seen on the clinically normal left side.
This player was successfully managed with a steroid injection into the posterior ankle, followed by a short (less than two week) period of rehabilitation. On his return to training, his ankle was taped to avoid terminal plantarflexion. He has continued to play for more than one year with no recurrence of his symptoms.
An os trigonum represents a failure of fusion of the lateral tubercle of the posterior process. It is estimated to be present in approximately 7% of adults and is bilateral in about in 2% of the population. In the majority of cases, it is discovered as an incidental finding. As this case illustrates, an os trigonum can predispose a player to developing posterior impingement syndrome. This condition is caused by activities, like kicking a football, that involve repetitive ankle plantar flexion leading to soft tissue compression, synovitis and capsulitis. This condition causes posterior ankle pain that is provoked by ankle plantarflexion. While this case did not require any further imaging, MR imaging can be helpful in some cases to exclude other causes of pain (for example a talar dome injury) and to confirm the diagnosis by demonstrating increased signal either within, or adjacent to, the os trigonum. While this problem can often be treated with time, rehabilitation and anti-inflammatory modalities, excision of the os can be considered in refractory cases.
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