The dura is the sac that encloses the spinal fluid and nerves of the spine. In the cervical spine and thoracic spine, the spinal cord also is contained within the dura and the spinal sac. The SCS device is very small and fits inside the epidural space. It is composed of electrodes (leads) and wires and works directly on the spinal cord to stop pain signals.

A common problem with these units is hardware failure. The delicate wires can get broken allowing the electrodes to move away from their intended site. It's difficult to design a device of this type that can hold up under the stress and strain of spinal motion.

If there's breakage, the unit stops working and no longer delivers electrical impulses to the spinal cord. If the leads stay intact but the unit shifts location, it doesn't have to move far to stop working.

Sometimes the X-ray doesn't show a change in the location of the SCS device but pain is not relieved. In these cases the doctor may say there is "poor pain coverage." For any of these problems, a second revision operation may be needed to repair or replace the device.