Imagine someone has a broken bone and after removing any slivers there is a void between the two bone fragments. In order to assure that the bone grows back correctly, a piece is inserted between the two ends. The piece is not attached to the bone parts, which are held together through a plate. Rather it fills up the space in-between the parts to alleviate pressure on the plate and screws. I am looking for a word to describe this piece.
My first thought was wedge but the object in question is more cylinder-shaped than wedge-shaped. I went on to think of words ending on filler, but I have so far found no satisfactory word. Which word best describes the piece in question?