Active adult patient presented with localized swelling in the lateral aspect of the knee. Clinically, it was below the femorotibial joint line. Ultrasound findings favor a cyst. Its neck extends to the proximal tibiofibular joint (better appreciated with low dynamic range image, which makes black more black). Cyst elevates common peroneal nerve; however, there is no extension in or around the common peroneal nerve. There was no clinical sign of common peroneal nerve palsy.

Similar findings on follow-up MRI (imaging not available for upload).