Dentures are considered the third set of teeth a person might need in a lifetime.
Dentures are not as strong as our teeth, simply because they are only supported by the gums.
The main problem with dentures is retention.
Retention is resisting movement under normal function.
For retention, dentures depending on adhesive forces to the gums with the help of a thin film of saliva. In simpler words, the dentures depend on suction forces to stay in place.
In upper dentures, this is usually enough to be comfortable. However with lower dentures it is another story.
The tongue keeps moving the denture, and being U shaped means less surface area for the denture to stick to.
An important use of dental implants is to overcome this shortcoming of the Dentures.
In a procedure called overdenture, two implants are placed in the jaw bone, these implants use another piece called Locator abutment that stabilizes the denture in place.