Electrical stimulation for bone growth has been used for almost 50 years now. Most of this has been with bone fractures. The use of electric current to stimulate bone after a spinal fusion is a relatively new area of research.
There doesn’t appear to be any strict guidelines about device removal. Generally, these implants are only taken out if the patient has pain or discomfort. Of course, the device is removed if there is a malfunction or other problem with the unit.
Electrical implants do interfere with some imaging studies such as magnetic resonance imaging (MRI). The unit might be removed if there is a great need for MRI and no other imaging study will give the needed information.