We report a unique case of sesamoiditis in an extremely rare accessory genicular ossicle. Common to lower primates, the cyamella or popliteus tendon sesamoid bone is usually absent in humans. A 19-year-old male sustained a twisting injury to the right knee and presented with mechanical symptoms of knee pseudo-locking. A plain radiograph of the knee illustrated the presence of a cyamella. Magnetic resonance imaging demonstrated marked bone marrow oedema of this sesamoid structure and in the adjacent popliteus tendon. To our knowledge, documented sesamoiditis of this osseous structure on radiological imaging has not been published in English literature and this case highlights the imaging features of this uncommon entity.