After obtaining written informed consents, the patients scheduled to undergo posterior lumbar arthrodesis on degenerative spine,.will be randomly allocated to receive a continuous wound infusion of either 0.2% ropivacaine (ropivacaine group A) or 0.9% saline (control group B) when they will arrive in the operating room. The surgeon will perform a standardized posterior median incision at the level of lumbar intervertebral instability. Before putting retractors, he will infiltrate all surgical strata and the paraspinal muscles all long the wound bilaterally with a solution of ropivacaine 0.5% 20 mL (group A) or with a solution of 0,9% NaCl 20 ml (group B). At the end of surgery, a multi-holed wound catheter will be placed by the surgeon, under direct visualization, in the paravertebral space ; between the muscle fascia and subcutaneous tissues all along the wound, and fixed at the skin by a stitch. The patients will be thereafter randomly assigned to receive through the catheter either 0.2% ropivacaine (study group) (5-ml bolus followed by an infusion of 8 ml/h during 48 h) or the same protocol with 0.9% NaCl (control group), thanks to a prefilled elastomeric pump (400ml), set to deliver a 8-ml/h connected with the catheter. In addition, all patients will receive patient-controlled intravenous morphine analgesia. After the induction, the anaesthesist will install a peripheral venous catheter for each patient in order to make blood samples easier (eight peroperative and two postoperative blood samples for each patient, with the aim of ropivacaine pharmacokinetic study).