As I already told you all in a recent post bagel injections, usually in ones forehead, are a saline injection. But I had not found much information on them beforehand. (original bagel implant post can be found here
These injections are very popular so far in the mod world of Japan. Also, the injection is only temporary lasting about 24 hours so for a more permanent bagel a person would have to get it done again and again. This could be annoying but good for someone who is not sure they want a permanent implant. A person could probably get a silicone implant in this shape.
There is a risk of infection and irritation, headache, discoloration of the skin where the injection is, and stretching of the skin.