@suev I think the doctor might be talking about replacing a disc with a fusion in the lower back (lumbar spine). A bad disc is removed, and they have to put something in it's place and can refer to that as a "spacer". I had a cervical fusion, and my spacer is a milled piece of bone. Sometimes they put in a cage that can be metal or made of a plastic material. These spacers can have a place to add bone cells to seed bone growth for the fusion. Bone cells are removed during surgery such as bone spurs or bone overgrowth, or what they remove to prepare the surface of the vertebrae for the fusion.