In reply to Ads Co.: All modern screws from the big manufacturers are much of muchness, you won't go wrong with Grivel, BD, Charlet or DMM. All make very good screws. Having a flip-out handle of some type is nice but for use on a glacier like you need it for it's not the end of the world if you don't.
22 cms might be good, but to be honest the difference in their holding power over 17 cm screws isn't that great.