Abstract The entrapment of the tibialis posterior tendon causing a block to reduction of lateral subtalar dislocation is recognized. The mechanism which allows tendon excursion is less clearly understood. We present a case in which extensive tearing of the musculotendinous junction of the tibialis posterior muscle occurred during tendon displacement and entrapment in lateral subtalar dislocation. In view of this previously poorly recognized finding the two theories regarding entrapment are discussed. Recognition of the cause of the tendon lengthening with an intact flexor retinaculum allows appropriate early treatment.