Acute effects of hip and knee positions on motor-evoked potentials of the sciatic nerve in total hip arthroplasty.

In 23 patients, motor-evoked potentials (MEP) of the sciatic nerve were elicited during total hip arthroplasty by using a stimulating electrode at the level of the acetabulum and recording from the middle portion of the tibialis anterior. The distal motor latencies were determined before dislocation (control), during dislocation, and after reduction with the trial prosthesis. While the hip was dislocated, recording was performed at varying angles of the hip and knee joints. During dislocation, the distal motor latencies were significantly increased in all positions except at hip flexion of 60 degrees and internal rotation of 60 degrees with the knee joint in maximum flexion. No significant correlations were found between the latency increase and limb lengthening. No patient had sciatic nerve palsy or causalgia after operation.