I am considering the tens unit being surgically implanted. I know this is minimally invasive surgery. What is the recovery time and restrictions following the surgery. Do they ever have to go back in the unit to do anything to the battery? Are these effective for chronic pain following a back surgery 8 years ago for collapsed vertebrae and titanium spacer with implanted at L4/L5. I have nerve damage and chronic pain in the back, hip, leg and foot.