Stimulating catheters have been introduced to reduce the incidence of secondary failure after continuous peripheral nerve blocks, but they effectiveness over traditional nonstimulating catheters is still controversial. Furthermore no volume-response study has compared the success rates of the two techniques for continuous lumbar plexus block. The aim of this prospective, randomized, blinded study is to detect if stimulating catheters decrease the minimal effective volume (MEAV) of 1.5% mepivacaine required for successful lumbar plexus block (LPB) in 50% of patients compared with conventional non-stimulating catheters.