Cyamella, an accessory sesamoid ossicle of the popliteus in the region of the proximal myotendinous junction, is an extremely unusual finding, with a paucity of current published literature describing its MR appearance. In this case study, we describe MRI findings in a 44-year-old female presenting with pain after a stretching workout. Bone edema in the cyamella, edema in the myotendinous portion of the popliteus, and pericapsular edema at the posterolateral corner were identified. While commonly seen in certain mammalian species, cyamella is rare in humans. The upright walking posture of humans requires posterolateral corner structures to play a greater role in structural support and weight-bearing. We hypothesize that the cyamella, while intrinsic to certain mammalian species, may interfere with these biomechanical functions in upright walking posture, thereby predisposing to isolated injury of the posterolateral corner in humans.