Prompted by clinical failures of percutaneous radiofrequency neurotomy in the treatment of back pain and neck pain, we performed a study to determine the shape and size of lesions made by radiofrequency electrodes. Experimental lesions were made in egg white and fresh meat at temperatures recommended in clinical practice. The cardinal finding was that lesions do not extend distal to the tip of the electrode. They only extend radially around the electrode tip in the shape of an oblate spheroid, with a maximal effective radius of only 2 mm. Consequently, if electrodes are directed perpendicularly onto a nerve, the nerve may not be encompassed by the lesion generated. Some of the clinical failures of percutaneous medial branch neurotomy ("facet rhizotomy") may be due to this phenomenon. We suggest modified techniques for medial branch neurotomy in which the electrodes are introduced parallel to the target nerve whereupon it is more readily encompassed by the radial spread of the lesion.