The patient will be placed in the lateral or sitting position. With a curve array probe or a high frequency linear probe, depending on the BMI of the patient, will be placed in a longitudinal position 2-3 cm lateral of the vertebral column. The transverse processes of the vertebrae at the (mid) level of surgery, the erector spinae muscle and the psoas muscle are identified. Depending on the depth a 5 or 8 cm 22 G ultrasound needle (pajunk) will be inserted in an in plane technique in a cephalad to caudad direction until bone contact with the top of the transverse process is reached. After slight retraction of the needle, 20 ml of ropivacaine 0,375% will be injected behind the erector spinae muscle. The same procedure will be repeated on the contralateral side.