The Medial Malleolar Fleck Sign: A Case Report Highlighting Retinacular Origin Rather Than Deltoid Ligament Avulsion

The medial malleolar fleck sign (MMFS) is a radiographic indicator of medial ankle instability, conventionally interpreted as an avulsion of the deltoid ligament from its tibial insertion. However, its precise anatomical origin remains unclear, with limited intraoperative confirmation reported in the literature. We present a case of a 51-year-old woman with a left ankle inversion injury, resulting in a Weber B fibular fracture, posterior malleolar fracture, and a cortical fragment consistent with MMFS, accompanied by medial clear space widening. Intraoperatively, the deltoid ligament was ruptured but remained attached to its tibial origin. Notably, the MMFS fragment was avulsed from the tibial insertion of the flexor retinaculum (FR) and displaced anteriorly, with fibrous tissue extending forward, a finding highly suggestive of attachment from the extensor retinaculum (ER). The fragment was anatomically reduced and fixed, and the deltoid ligament was repaired. The patient regained full, pain-free function at one year. Given previous anatomical studies describing fascial continuity between the FR and ER, this case suggests a possible alternative mechanism of composite retinacular avulsion. These findings underscore that MMFS does not invariably indicate deltoid ligament injury, highlighting the importance of careful intraoperative assessment for accurate diagnosis and surgical planning.