Os Trigonum

Between 8 and 13 years of age, an ossification center forms posterior to the talus. Normally, this ossification center fuses with the talus within 1 year. In approximately 7% of the population, it remains separate and is referred to as the os trigonum. The os trigonum is corticated and articulates with the lateral talar tubercle through a synchondrosis.

Os trigonum syndrome results from repetitive microtrauma or acute forced plantar flexion of the foot. The chondro-osseous border of the synchondrosis may be injured either as a chronic stress fracture or, less frequently, as an acute fracture.

An os trigonum is usually round or oval with well-defined corticated margins, whereas a fracture of the lateral tubercle typically has irregular serrated margins between the fragment and the posterior talus.

Lateral radiographs obtained with the foot in plantar flexion may show the os trigonum impinged between the posterior tibia malleolus and the calcaneal tuberosity. Our case shows  stippled opacity with post talar margin clear with no serrated edges, it is possibly developing os Trigonum with well maintained fat planes. Case by Dr MGK Murthy, Sr Consultant Radiologist