Here are a few sites containing info about Greenie risks.. The first link I'll tell you before hand is extremely graphic and not for those with weak stomachs. It contains actual photos of a dogs intestines crammed full of greenies while it is going through surgery.
http://improvehomelife.com/greenies.html - Beware! VERY graphic
http://www.quikonnex.com/channel/item/15576 http://www.all-creatures.org/aip/nl-...1-warning.html