pedicle screws are inserted in spinal vertebrae in a manner to reduce the likelihood of nerve damage caused by improper screw placement . a screw opening is started in part of a skeletal region , e . g ., a pedicle of a lumbar vertebra and an electric potential of a certain magnitude is applied to the inner surface of the opening while the patient is observed for nervous reactions such as leg twitching . the opening continues to be formed while the electric potential is applied until a desired hole depth is obtained in the absence of nervous reaction to the potential . the direction in which the screw opening is being formed is changed to a direction other than the last direction , after observing patient reactions to the electric potential when the screw opening was being formed in the last direction . a system for carrying out the procedure includes a tool handle and a probe member extending from the handle with a tip for working an opening in bone tissue . an electrical stimulator in the form of a self - contained nerve stimulator unit , or circuitry provided inside the tool handle , produces an electric potential at a selected level . the potential is applied to the tip of the probe member while the tool handle is manipulated and the tip is directed toward the bone tissue .