Background: The os trigonum is known as one of the main causes of posterior ankle impingement. In the literature, a wide variation of occurrence has been reported.
Methods: All foot and/or ankle computed tomography (CT) scans made between January 2012 and December 2013 were reviewed. CT images were assessed, blinded for patient characteristics, for the presence of an os trigonum, size of the os trigonum, and type of os trigonum. In addition, the shape of the lateral tubercle of the posterior talar process was assessed.
Results: A total of 628 patients (1256 ankles) were included. In 32.5% of the patients of the cohort, an os trigonum was present. In 14.3% of these patients, it was present bilaterally. In a subgroup of patients without posterior ankle impingement the prevalence was 30.3%. Of the nonaffected ankles, an os trigonum was present in 23.7%. Patients with posterior ankle impingement were more likely to have an os trigonum (adjusted odds ratio [OR], 1.86). Afro-Caribbean/Surinamese/Central African origin was associated with a lower rate of occurrence of os trigonum (adjusted OR 0.43). In the ankles without an os trigonum, an enlarged lateral tubercle of the posterior talar process was found in 34.9% and 36.5% of the ankles.
Conclusion: This study showed that os trigonum is a common accessory bone. With a prevalence of 30.3% in a population of patients with CT imaging of both ankles and 23.7% of the nonaffected ankles, the os trigonum is more common than previously reported. Patients with posterior ankle impingement complaints had a higher prevalence of an os trigonum. In one-third of the patients without an os trigonum, there was an enlarged lateral tubercle of the posterior talar process.
Level of evidence: Level III, retrospective comparative study.
Keywords: anatomy; ankle; os trigonum; posterior ankle impingement.