LAUSR

Fig. 1. Ultrasonography of the knee of a 60-year-old woman showing a cyst displacing the sciatic nerve (arrows) backward. A cyst acutely affecting one or more branches of the sciatic nerve can include Baker’s synovial popliteal cyst resulting from the popliteal bursa [1–3] or proximal tibiofibular joint [4,5], an intraneural mucoid or ‘‘ganglion cyst’’ of controversial origin [6–8]; however, the acute affection of the branches of the sciatic nerve at the knee may rarely be caused by a cyst. Here, we report a case of acute tibial and sciatic neuropathy caused by a synovial cyst from an osteoarthritic femorotibial joint. A 60-year-old woman with no unusual medical record showed a brutal paresis of the right foot and toes without any specific triggering cause, preceded the day before by a transient pain at the calf level. Muscle deficit predominated over the fibers for the common fibular nerve (dorsiflexion of the foot and toes, eversion of the foot, 4/5) than those for the tibial nerve (plantar flexion 4+/5). Achilles reflex could not be triggered. Being overweight, the patient had pain of the right knee, but examinations were normal. Electromyography revealed blocked nerve conduction in the right sciatic nerve. Blood test, lumbar MRI, CT scanning of the pelvis and lumbar puncture gave normal results, but radiography of the knee showed advanced femorotibial arthritis, mainly medial. Ultrasonography (Fig. 1), then MRI of the knee revealed a fusiform multilobular cyst (73 23 27 mm), hyperintensive in T2-weighted images and hypointensive in T1-weighted images, not enhanced by gadolinium, contacting the osteoarthritic femorotibial joint by a thin opening running along the joint nerve branches of the sciatic nerve. The cyst pushed back the main sciatic nerve, which was thickened in hypersignals, the lower part located 6 cm above the joint interline (Fig. 2a,b). As the deficiency worsened (dorsiflexion 0/5, plantar flexion 2/5), its excision after 21 days (Fig. 3) revealed a swollen cyst pushing the sciatic nerve back and out of its course over 3 cm along the tibial nerve, which itself was pushed back, leaving the common fibular nerve untouched. The highest point of the cyst was 5 cm above the division of the sciatic nerve. The walls of the cyst with a synovial nature were resected, except when in contact with the tibial nerve because of adhesion.