A 15 year old boy presents two fractures of the left inferior member (femur and tibia), after a motor-cycle accident. An osteosynthesis by plate and screws was performed, in emergency. In immediate post-operative days, a radicular paresia of the left inferior member appear especially on exterior popliteus sciatic nerve. The electromyogram confirm the diagnosis. However, these is nor pain nor fracture of the sacro-lumbar spine. The consolidation of femur and tibia fractures is normal. The neurologic recuperation is slow but progressive. A year after the accident, because the recuperation is not complete, an MRI is performed. An arachnoidal cyst surrounding the L5 and S1 roots is diagnosed. The surgery is decided. The ablation of the cyst is performed in the same operative time that the ablation of plate. The histology of the paries of the cyst confirm, by hemosiderimic deposits, the traumatic etiology. The arachnoïdal cyst is rare, particularly in the childhood. It is always difficult to know the relation between the cyst and the symptom. It is also impossible to be certain of traumatic etiology: traumatic cyst or traumatised cyst? The surgical indication is lawful by the existence of clinical signs, because 50 per cent of cases are improved or recovered.