[ common peroneal nerve compression by a synovial cyst]

The synovial cyst compressing the fibular common nerve is a little known entity at the origin of diagnosis mistakes. The authors recall about a new case the clinical symptomatology: pain of the knee followed by steppage. The palpation can recover a swelling at the level of the fibular neck. The electromyogram permits to localize the nervous lesion comfortably . Our observation puts in evidence the interest of the magnetic resonance imaging. The surgical intervention must search for a possible communication of the cyst with the proximal tibio-fibular joint. The neurological prognosis is generally good in case of recent deficit