A spinal cord stimulator (SCS) is an implanted medical device used to manage chronic pain by delivering low-voltage electrical current to the spinal cord. This current interrupts pain signals before they reach the brain, often replacing the sensation of pain with a mild tingling feeling. The system consists of thin wires called electrodes or leads, which deliver the current, and a battery-powered Internal Pulse Generator (IPG). Precise positioning of both components is determined by the patient’s anatomy and the distribution of their pain.
Precise Anatomical Placement of the Electrodes
The electrodes are positioned directly over the spinal cord within the epidural space. This space is located between the dura mater, which is the protective membrane surrounding the spinal cord, and the vertebral bone wall. The epidural space provides the location for the leads to transmit their electrical pulses.
Once inside the epidural space, the electrodes are carefully advanced to lie over the dorsal columns of the spinal cord. The dorsal columns are tracts of nerve fibers that carry sensory information, and by stimulating this area, the SCS can interfere with the transmission of pain signals. Achieving optimal placement requires careful alignment, often as close to the midline of the spine as possible, to ensure uniform electrical coverage.
The specific vertebral level for lead placement is determined by pain mapping, which correlates the patient’s painful areas with corresponding spinal nerve segments. For instance, pain in the lower extremities and lower back typically requires electrodes to be positioned in the mid-to-low thoracic spine, often around the T8 to T10 vertebral level. Conversely, chronic pain in the arms or neck requires a more superior placement in the cervical spine.
The entry site for the placement needle is often in the lumbar spine, such as the L1-L2 interspace, to access the epidural space. From this entry point, the leads are then advanced upward to the required thoracic or cervical level.
Location of the Internal Pulse Generator (IPG)
The Internal Pulse Generator (IPG) acts as the power source for the entire system. Unlike the electrodes, the IPG is implanted subcutaneously, resting just beneath the skin and fatty tissue. This placement protects the device while allowing for relatively easy access for charging or maintenance.
The IPG is typically positioned in a comfortable, discreet location that minimizes interference with the patient’s daily movements. Common sites include the abdomen (lower flank), the upper buttock (gluteal region), or sometimes the upper chest. The surgeon and patient collaboratively choose the exact placement site, taking into account the patient’s body type, clothing preferences, and whether the device will need frequent external charging.
An extension lead connects the electrodes in the epidural space to the IPG. This wire is tunneled under the skin from the spine to the chosen pocket site. This tunneling ensures the IPG is placed far from the lead incision site, which reduces irritation and improves comfort.
The Staged Implantation Procedure
The process of placing a spinal cord stimulator is divided into two distinct phases, beginning with a temporary assessment to ensure effectiveness. This initial phase, known as the trial period, involves a minimally invasive procedure to temporarily place the leads. During the trial, thin electrodes are inserted through a specialized needle into the epidural space using a percutaneous placement technique.
These temporary leads are connected to an external generator that the patient wears on a belt outside the body for approximately one week. The purpose of this trial is to evaluate whether the stimulation provides a significant reduction in chronic pain, often requiring at least a 50% decrease for the trial to be considered successful. If the trial is successful, the temporary leads are easily removed, and the second phase is scheduled.
The permanent implant procedure is a surgical operation that involves securing the final components of the system. For the permanent leads, the surgeon may use the percutaneous technique or opt for a more stable paddle-style lead. The paddle-style lead requires a small surgical opening in the bone, called a laminotomy or laminectomy, for direct placement.
Once the permanent leads are positioned and anchored to prevent movement, the surgeon creates a separate subcutaneous pocket for the IPG, typically in the flank or buttock. The leads are then tunneled beneath the skin and connected to the IPG, completing the placement of the entire internal system.