Spinal instrumentation is popular nowadays. After applying the Harrington distraction rod, there might be a severe, tragic complication, paraplegia. Recently SSEP is widely used to detect the early change of the spinal cord dysfunction. The force to correct the spinal deformity is usually distraction. The distraction force will stretch the spinal dura. But the relation of neural deficit between distraction force and duration is not clearly determined yet. Also the anatomical level of neural deficit is not known. The purpose of this study is to find that the distraction force of 200gm on the spinal dura will change the SSEP by the stimulation of the posterior tibial nerve and if there is change of SSEP, what portion of nerve stimulation pathway is responsible for. Results are as followings. 1) The distraction force of 200gm makes change of SSEP wave. 2) P2 wave is most susceptible, N1 wave is less susceptible and P1 is relatively contant for one hour dural traction. 3) After release of traction, the wave is partially reversible but not completely for next one hour recovery period. 4) While the wave of SSEP is changed by the dural distraction force, the nerve conduction between the nerve root just outside of the spinal dura and calf shows constant latency. That means the anatomical level responsible for the change of SSEP wave is proximal to the spinal dura, not the peripheral nerve distal to the spinal dura.