Granted that knao, with its basic meaning of scraping is the source word for knestis, the cheese grater, and that it's possible that the grater was the model for the spine, from a language perspective,I am puzzled why a-knestis for spine or backbone. What sense would alpha-privative make? Is it too far-fetched to think this alpha is not privative but alpha-combining as in a Gk word like a-koites, having the same place to lie down, sharing the same bed = spouse? So, having the same appearance as a cheese-grater = spine. I don't know but it's worth throwing out this kind of speculation which sounds awfully much like a popular etymology. But back to it: how to account for a-knestis?