How are subperiosteal dental implants different than other implants?

The major difference in the subperiosteal implant and root form or plate implants is that the subperiosteal implant is placed under the tissue of the lower jaw laying on the bone as opposed to the root form or plate implants which are place into the bone. Usually subperiosteal implants are recommended when either the amount or quality of bone in the lower jaw will not allow the placement of the other type of implants. In most cases the subperiosteal implant will be restored with a removable appliance which will snap onto fittings that extend above the tissue from the implant.

Subperiosteal implants are implants that rest on top of the jaw bone instead of inside the bone. These were done many years ago before the current "root form" implants came into vogue. 

I placed several of these and they worked well for a while. Unfortunately they almost always developed problems and had to be removed. They are not used much these days because root form implants have been so successful.