label: Os trigonum
label_lc: os trigonum
description: Os trigonum (OT) is one of the most common accessory ossicles in the foot and ankle region. In clinical practice, OT is a considerable factor that may predispose patients to posterior ankle impingement syndrome (PAIS). Lateral ankle x-ray with foot in plantarflexion reveals an accessory ossicle at the posterior aspect of the talus, between the posterior distal tibia and calcaneal tuberosity.