Many patients undergoing surgery on their hand receive a form of anesthetic called "axillary brachial plexus block". This involves the injection of local anesthetic (freezing medication) around the nerves that supply the upper limb, which are located in the arm pit. The technique currently used by physicians performing this procedure is far from perfect, and in up to 1 of every 5 patients it does not work well enough to allow surgery, and a different form of anesthetic may need to be used.