Electrodes Internalization
If the stimulation trial is satisfactory, achieving at least 50% pain relief without intolerable side effects such as unpleasant paresthesias, the patient is prepared for electrode internalization. The final system selection (IPG vs RF receiver), which depends on optimal stimulation parameters and patient preferences, can be made before internalization. The internalization procedure is usually performed under general anesthesia, as IPG/receiver insertion and especially wire tunneling are very painful and poorly tolerated by awake patients. It is possible to implant the generator/receiver in the superior glu-teal area, but most surgeons prefer to place it in the anterior abdominal wall. Posterior placement allows the patient to remain in the prone position for internalization, whereas anterior placement is usually performed with the patient in the lateral decubitus position, which permits access to both lumbar and anterior abdominal regions. For SCS systems that focus on the cervical or upper thoracic spinal cord (as in cases of arm pain or intractable angina), the IPG or receiver may be implanted in the subclavicular area, as is done with pacemakers or deep brain stimulators. When placing RF receivers, one must consider the patient’s convenience in applying the antenna and avoid areas that are covered with body hair or that may subject the antenna to excessive sweating. It is also a good idea in all cases to discuss the insertion site with the patient before the surgery and to avoid the belt line, which may irritate subcutaneously placed hardware.
The internalization procedure starts with cutting the temporary extension wire with sterile instruments at the skin level after the entire area has been disinfected and sterilized. Both the lumbar and the anterior abdominal areas are prepped and draped in standard fashion after the patient is placed in the lateral decubitus position (with the operative side up) and padded/ secured with a deflated ”bean bag.” The original incision is then reopened, taking care to avoid cutting the electrode lying above the fascia. The connection of the electrode and the temporary extension wire is exposed, the silicone sleeve is removed, all screws are unscrewed, and the remainder of the temporary extension wire is discarded. A second incision is then made in the anterior abdominal area, taking care to ensure that the IPG/receiver will not overlay the bony structures (ribs, iliac crest). A pocket for the IPG or receiver is made above the fascia, but not too far under the skin, because the thick layer of tissue above the IPG/receiver may interfere with programming or RF coupling. Generally, RF receivers should be placed within 1 cm of the skin’s surface to maintain a good connection with the external antenna.
Once the pocket is prepared, a permanent set of extension wires is passed subcutaneously using a special passer that usually comes sterilely prepacked with an appropriate extension wire kit. In most cases, it is necessary to move the passer in a posterior-to-anterior direction, so the extension wire can be brought from the abdominal incision to the lumbar one. The connection between the extension wire and the electrode is secured with all four screws, an additional silicone sleeve, and several nonabsorbable sutures. The electrode is coiled under the connection site so it will be better protected from accidental damage if the patient requires electrode revision. Anteriorly, the extension wire is connected to the IPG/receiver, and the connection is secured with built-in screws. The IPG or receiver needs to be secured to the underlying fascia with one or more nonabsorbable sutures. Excess extension wire is coiled behind the IPG/receiver, not only for cosmetic reasons but also to avoid interference with programming or RF coupling. It also allows easy identification of the IPG when it needs to be revised/replaced (ie, when the battery is exhausted).
After the implantation is complete, both incisions are irrigated with antibiotic solution and closed in a standard layer-by-layer fashion. Sterile dressings are placed over the incisions and changed during the postoperative period as necessary. Sutures are removed after all incisions are healed.
Postimplantation Programming
The programming of the SCS system may be performed soon after the patient’s awakening from the general anesthesia. Obviously, stimulation parameters similar to those that were tested during the trial may be used. Some electrode configurations, however, are technically impossible during the stimulation trial. The best example is "monopolar” stimulation, when one of the electrode contacts is active and the IPG case serves as a reference. This mode allows for a lower stimulation voltage compared with the bipolar configurations and should be tried first when a completely internalized system is used.
Stimulation settings need to be checked regularly during the postoperative period. It is often necessary to adjust not only the amplitude of the stimulation, but also the frequency and the pulse width and even the electrode configuration, months or years after system implantation. This may be explained in part by the dynamic character of pain syndromes and by the development of tolerance to the stimulation or possible movement of the electrode in the epidural space.
Patient education is extremely important and should be started before surgical implantation. Patients with fully implanted SCS systems must understand how to use the handheld programmer or the magnet to turn the stimulator on and off and to change certain stimulation parameters. Patients with RF-coupled systems should be even more carefully educated about the power source/programming module because it allows more freedom in adjusting the stimulation parameters. In all cases, patients need to clearly understand the signs of system malfunction and should be able to contact the neurosurgeon (or other implanting physician) or their clinical personnel at any time. Spinal cord stimulation is unlikely to cause life-threatening complications (except perhaps a systemic infection); however, these patients must be closely followed, both to check the SCS status and functionality and to monitor response to SCS therapy.
