I've had spinal fusion procedures done at the C5-C6 and C6-C7 levels in my neck. The surgeries were done about a year and a half apart by 2 different neurosurgeons at 2 different hospitals because my first doctor retired. When they did the C5-C6 fusion, they used a piece of bone from a cadaver, along with a plate and screws.
When the second surgery was done, they did the same thing, and they removed the plate and screws from the first surgery because the bones were fused together, so they were no longer necessary.
I would assume the holes would fill in with new bone as time goes on, they didn't say that anything else was necessary. It's been 7 years now with no new issues so far.