BACKGROUND: The os trigonum is an accessory bone that is not fully fused with the talus during secondary ossification, and is one of the risk factors of posterior malleolus impact syndrome. The purpose of this study was to classify the os trigonum and to guide the diagnosis and treatment of related clinical diseases.

MATERIAL AND METHODS: Ankle computed tomography (CT) scans of 586 Chinese patients between October 2014 and October 2018 were reviewed. CT images of 1011 ankle joints were reconstructed to evaluate the classification of the os trigonum and the measurement of anatomical parameters.

RESULTS: The incidences of os trigonum in 3 groups were determined as type I (1.9%), type II (10.5%), and type III (14.7%). The macro-axis of type II (0.89±0.31) cm was significantly larger than with type I (0.65±0.24 cm) and type III (0.74±0.23 cm) (p<0.05).The minor axis of similar of type I (0.41±0.23 cm) was significantly shorter than that of type II (0.58±0.32 cm) and type III (0.55±0.16 cm) (p<0.05).The distance from os trigonum to calcaneal tubercle was significantly different than that of type I (1.33±0.52 cm), type II (1.67±0.55 cm), and type III (1.84±0.45 cm) (p<0.05).

CONCLUSIONS: This study showed that os trigonum has a high incidence. Type I was the least common, the volume of type II was larger, and type III was more common. The anatomical parameters of each type may improve treatment of related diseases and the further development of ankle arthroscopic surgery.

Keywords: Ankle, Models, Anatomic, Talus, Adolescent, Aged, 80 and over, Ankle Joint, Imaging, Three-Dimensional, Tomography, X-Ray Computed