Postoperative pain is a common complication after lumbar spine surgery due to inherent tissue damage during surgical procedures. Many analgesic options have been explored. Opioid analgesics carry the risk of respiratory depression. Nonsteroidal anti-inflammatory drugs (NSAIDs) are popular and widely used as first-line treatment for acute pain following spinal surgery, but can provide inadequate analgesia. High doses of NSAIDs have also been linked to non-unions in spinal fusion surgery. Patient-controlled analgesia or epidural injection analgesia are usually used. One of US guided regional anesthesia methods is the ultrasound-guided caudal block that was first described by Klocke and colleagues in 2003. One of the more recent techniques that has been described recently is quadratus lumborum plane block(QLB),that is posterior abdominal wall fascial plane block first described by Blanco in 2007.In this study we will compare between analgesic effect of ultrasound guided caudal block to that of ultrasound guided quadratus lumborum plane block as well as safety of both modalities in adult patients undergoing lumbar spine fixation surgeries.