If you are missing all your teeth you need a denture. An overdenture is simply a denture that is made to be over either natural teeth or dental implants. When an overdenture is made to fit over natural teeth, it usually rests passively on those teeth. This provides a little extra stability, and does help protect the bone from shrinking over the years. But when it is made over dental implants, it usually has some type of attachment to make it even more secure. On the right is a picture of the most basic implant overdenture. There are two snap fixtures that hold it down fairly securely, but make it so that it is also removable. Another type would be one that attaches to the implants with screws.