The trialing physician must first identify the areas of pain and correlating intensities, perhaps even marking them (or asking the patient to mark them) on the skin. Marking has great utility. Marking not only makes it easy to identify the nerve or nerves responsible for sensation to the area, but also clearly communicates to the physician the areas that are important to the patient—a process so seemingly simple that many gloss over the step. Missing essential areas likely contributes to significant failure of the modality.

Once the areas are clear to the physician, a decision-making process occurs, with consideration given to dense or overlapping paresthesia and its potential benefits, ease of deployment, stability of the permanent system, safety to the patient, and the ability to create a montage of paresthesia using a single implantable pulse generator (IPG). If the painful area is small (the size of a business card or less), and especially if it is superficial, a single PNfS lead may serve the area well. If the area is larger, then cross-talking (transmitting current from one lead to a different lead at a distance) one or more leads may be appropriate, as cross-talking may increase the area of paresthesia. The practice of cross-talk additionally introduces the concept that the depolarized nerve may not actually be within the direct vicinity of the electrode array. If a single nerve wholly serves the area, then consideration must be given to direct stimulation of that nerve, if possible. Often, as the nerve courses proximally it becomes mixed and deeper, and thus is more difficult to access percutaneously, being guarded by sensitive structures, muscle, or bone. The more distal the electrode array from the anchoring point, the more likely lead migration becomes. Appendicular placement may further confound the system, as nerves may be deep and encircled by dynamic muscles, and leads may have to cross joint lines. System stability is important not only in the long term but also moment to moment, because with movement of a limb and thus of stimulation relative to a nerve’s braided fascicles, its often-mixed nature results in momentarily variable (and thus tenuous) results.

If the area of pain includes axial pain, or if it may be advantageous to stimulate at multiple locations along the path of pain transmission, inclusion of a spinal lead may be constructive. The spinal lead may function independently, thus generating overlapping paresthesia, or in hybrid fashion, cross-talking to the peripheral lead, be it a PNfS or direct peripheral nerve lead. Table 26.3 identifies several hybrid montages for specific pain patterns. Although this nascent technique does not yet have significant support in the literature, many seasoned implanters will testify that hybrid stimulation presents yet a different feeling and pattern of paresthesia than parallel but overlapping stimulation—a flow of paresthesia that often crosses areas of the body not stimulated by either lead alone.

The placement of a lead for PNfS use appears easy, and often it is, but several important pitfalls exist. Obviously, pain outside the field of paresthesia will never be reduced. Aim to create lines of current that connect through the area of pain, using the length of the array to advantage. More electrodes do not necessarily equate with better relief; at present, quadripolar arrays are usually the best compromise between redundancy and the judicious use of multiple leads. Though not studied, it is our opinion that an octapolar array rarely confers added benefit, except when possibly needed to stimulate fine or extremely important nearby nerves, as in stimulation of occipital or facial nerves. As previously mentioned, small areas may be well treated with one lead, but larger areas may need multiple leads in cross-talk fashion. Particularly hypersensitive areas or areas of anesthesia dolorosa do not respond well to leads in the immediate vicinity, but instead respond better to leads placed to bracket the area with energy cross-talked through the particularly sensitive areas.

Depth, too, must be well thought out; in PNfS, the target is the terminal sensory nerve fibers that exist deep to the basal layers of the skin within the deep dermis. Passing the needle (and thus the lead and electrode array) within the dermis is painful and may subject the lead to bacteria that live within the papillae and sweat glands. Placement of the lead too deep within the adipose of the subcutis drives up the electrical resistance of the system, and placing it deeper yet to adipose results in painful recruitment of muscle fiber. The best position is just deep to the dermis at the dermal junction with the fat. After attention to sterile preparation, a constrained wheal of local anesthetic prepares the skin for a small stab wound to permit needle entry. The needle is then directed parallel, with palpation, deep to the dermis, resulting in easy advancement with minimal resistance; though not pleasant, the patient’s pain is generally manageable. Knowing the correct layer is important, and so lifting the needle will cause the skin to tent over it; horizontal depression of the needle, when it is within the correct layer, produces minimal inflection. Once deployed, the lead body is barely detectable. In some newer devices, the use of a nerve stimulator can help identify the exact target prior to deposition of the lead. Stimulation testing ensues if only limited and short-acting anesthesia is used, and comfortable programs are saved into the external pulse generator. Sometimes painful paresthesia is generated, even in the appropriate layer. In this instance, increasing the amplitude often changes the quality to pleasant. High amplitude, small paresthetic distribution, or pain may all possibly be ameliorated with slight withdrawal of the lead to stimulate appropriate fibers. Lead ligation to the skin, radiographic documentation, and wound dressing conclude the procedure. The patient is then taken to recovery for a short observation period and possible electrode programming, prior to discharge for an outpatient trial.

Once the trial has been completed with acceptable pain relief, the patient is offered a permanent implant. The physician should carefully evaluate any cosmetic or structural issues that may affect lead choice, device choice, pocket placement, and incision location. The patient is returned to the operating theater and the percutaneous lead is replaced based on pain mapping, fluoroscopy, and review of the previous films with landmarks and/or ultrasound. Often, a curve is added to the deployment needle to allow approximation of many of the curved planes of the body. As lead erosion is a concern not only at the anchor but also at the most distal tip of the lead, it is recommended to turn the bevel of the needle deep and advance the terminal few millimeters of the array deeper within the adipose. No studies have evaluated this technique, but it seems to lessen the incidence of distal tip erosion. Once the lead is in good position, an incision is made sufficiently deep to allow multilayer closure, possibly to the fascia at the area of the lead proximal to the electrodes, so that the lead can be secured without affecting the electrical fields.

Anchoring methods vary, but the aim is universal: a well-secured lead with minimal risk of erosion, migration, or lead fracture. Conventional SCS anchors used for PNfS are frequently too bulky and increase erosive risk. Using nonabsorbable sutures in a drain stitch fashion without a formal anchor is more compact, but it introduces variation, and the implanter needs to be sensitive to lead security without increasing the likelihood of lead fracture. If an anchor is chosen, it is imperative for the implanter to close the tissue in two or three planes to protect the anchor from erosion. Once the lead is secured, a pocket is made in the appropriate location. Pocket proximity to the lead array is encouraged. As with SCS, the greater the distance between the pocket and the lead array, the greater the likelihood of lead torsion, strain, fracture, and migration. If proximity is difficult, especially when the construct crosses joint lines, the use of multiple buried lead loops is recommended to permit freedom of movement without lead compromise. The smaller size of the IPGs now being produced has increased options for the pocket location. External pulse generators in development may even further reduce this concern. As with any pocket, the implanting doctor should consider the bony margins, skin condition, and body habitus before selecting an appropriate location. Programming of the device will stabilize over 6 weeks, with many patients receiving improved stimulation as fibrosis develops around the lead.

The limits of terminal nerve or field stimulation—smaller paresthetic area, difficult or multiple areas to stimulate, or simply ineffectual paresthesia—may lead to consideration of direct nerve stimulation. The recent adoption of ultrasound, especially when coupled with the following stimulating technique, has allowed the surgeon to place coaxial leads near or next to peripheral nerves with ease and increased safety. When placing more than one lead, a second and reference electrode can be placed under the skin, away from the intended neural target. With the needle stylet removed, the lead can be placed within the needle lumen with the distal (and active) electrode just outside the beveled tip. With either sensory (i.e., 50 Hz) or motor (i.e., 2 Hz) stimulation parameters, the needle and lead are directed to the target.