The posterior lumbar plexus block approach was described approximately 30 years ago, using surface anatomical landmarks and a loss-of-resistance technique, but over the years it was almost abandoned for its low successful rate, the huge numbers of adverse effects and its difficulties to be performed. The introduction of the nerve stimulator and ultrasound guided technique in recent years increased the success rate in various peripheral nerve blocks and decreased adverse effects. Ultrasound guided LPB as well as sciatic nerve block technique in a small patient population resulting in a encouraging result, but published clinical data on optimal patient position for ultrasound-guided LPB are limited. We designed this prospective randomized study to compare the success rate, time of performance and complications of ultrasound guided posterior lumbar plexus block as well as subgluteal or sub-greater trochanter sciatic nerve block under lateral decubitus position v.s. prone position.