Several reports have compared the duration of action of levobupivacaine and ropivacaine as local anesthetic. The most widely used method for sensory function evaluation is though means of a pinprick test. The investigators want to use the novel method of Quantitative Sensory Testing to better evaluate the anesthetic activity. This method used hot and cold sensation to assess small nerve fiber function. The investigators will assess the duration of action of different drugs when used in an inter scalene nerve block in patients scheduled for shoulder surgery.