Selective dorsal rhizotomy (SDR) or selective posterior rhizotomy involves surgically cutting a portion of the sensory nerves of a limb or limbs with spasticity, a form of muscle tightness. Rhizotomy is another name for cutting a nerve root or the portion of a peripheral nerve that lies within the spinal canal. Dorsal or posterior refers to the portion of the nerve root that contains the sensory fibers, as opposed to the ventral or anterior root which contains the motor fibers or those fibers that tell a muscle to move. Selective indicates that the cutting is incomplete and the portion of sensory nerve fibers that are cut is directed by some form of “selection” process. There are many different selection criteria which have been used to determine which sensory nerve fibers to cut. In general, the selection process in North America involves electrically stimulating bundles of sensory fibers and monitoring the pattern of muscle contraction that occurs in the body in response. A specific pattern of muscle contraction is expected, depending on which fiber bundle is stimulated. When this pattern occurs, the response is labeled as “normal” and when muscles other than those which were expected to contract are seen to contract, the response is labeled as abnormal. Lesioning is typically isolated to those sensory nerve fibers whose stimulation causes “abnormal” patterns of muscle contraction.