Eggs are comprised of approximately 70 to 80 percent water. With the older, slow freeze process, ice crystals would often form during the thaw, and would damage the egg. With vitrification and use of a cyroprotectant ice crystals do not have time to form.
For decades, fertility clinics utilized a slow freeze technique for freezing of eggs and embryos. This method was successful with embryos, but frozen eggs showed only a 50% survival rate. It was clear that a more efficient and successful technique was needed. Today, vitrification allows for 90% or greater survival for both embryos and eggs.