• The lateral (posterior) tubercle of the talus has a separate center of ossification, which appears from ages 7 to 13 years.
• When this fails to fuse with the body of the talus, it is called os trigonum
• It lies lateral to the groove for the flexor hallucis longus (FHL) tendon
• A cartilage connection may or may not attach the os trigonum to the talus.
• The os trigonum has been reported to be present in 1.7% to 7% of normal, asymptomatic feet
Clinical features:
• gradual onset of pain, especially in the anterior aspect of the retrocalcaneal space
• Pain is recreated by forced plantar flexion of the ankle
• pain may be elicited by direct pressure over the posterior lip of the talus
Imaging:
• May be visible on plain radiographs.
• Stress views with the ankle in plantar flexion can show the posterior impingement.
• Three-phase bone scanning may show increased radioactivity in the case of a symptomatic nonunion.
• However, not all os trigonum with positive bone scans are symptomatic.
• CT scan: can provide detailed visualization, especially of a fibrous union or nonunion.
• MRI: may show edema within the os trigonum fragment, as well as fluid around it.
Treatment:
• Nonoperative treatment consists of NSAIDs, activity modification, and occasionally immobilization.
• Surgical treatment may be indicated if nonsurgical management fails.
• Excision can be performed arthroscopically.
• The os trigonum is visualized through the anterolateral portal, and working portal is the posterolateral portal.
• Excision is performed using arthroscopic banana knives, curettes, and graspers.
• Care should be taken to avoid injury to the FHL tendon and the posteromedial neurovascular structures.
• Cure rates with surgery are high
