Snapping low-lying articularis genu muscle belly: a rare and unexpected cause of superolateral knee pain and knee snapping.

We report a rare case of a low-lying anomalous muscle belly, the articularis genu muscle, as a cause of snapping and pain. The initial MR examination was reported as normal. Dynamic ultrasound evaluation allowed identification of the low-lying muscle as the etiology of the snapping and pain. Ultrasound-guided anesthetic injection confirmed the diagnosis with relief of symptoms and allowed subsequent further treatment with botulinum toxin. To our knowledge, the articularis genu muscle has not been previously described as a cause of knee snapping.