The os trigonum is a small oval accessory bone found in less than 10% of the population9 (Fig. 70-4). In 1955, McDougall high-
lighted the painful conditions associated with this accessory bone.10 The ossicle represents an un-united secondary ossification center of the talus. Therefore, its presence alone is not an indication of pathology. However, if an ossicle is present in one foot, it is commonly found in both. The ossicle is located adjacent to the posterolateral talus.
Due to its periarticular position, the bone is vulnerable to impingement with ankle plantarflexion. Injury may result from either a single traumatic injury or from repetitive trauma. Furthermore, injury may involve the ossicle or its fibrous connection (synchondrosis) with the talus.
The os trigonum syndrome (OTS), therefore, is a painful condition of the posterior triangle of the ankle. The syndrome most commonly involves athletes that perform with the ankle positioned in extreme plantarflexion. Ballet dancers, particularly those who perform en pointe, appear to be the most commonly afflicted athletes. Such athletes may also develop tenosynovitis of the flexor hallucis longus tendon. This latter entity may masquerade as OTS; the two may coexist. Therefore, surgical decompression of the long flexor tendon may be necessary at the time of surgical ablation of the os.11
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