Removable partial dentures are either tooth supported or tooth and gum supported.
When entirely tooth supported, the space has been replaced or filled in, but the supporting teeth remain the same. In other words, the load has been increased on the remaining teeth.
In the case of a partial denture that is both tooth and gum supported the areas where teeth are missing have been filled in with gum supported dentures. This means that the gum tissue and bone under these dentures will gradually shrink, and the partial denture will have to be remade or relined periodically.
If these areas are not relined, a space develops under the denture. It is not bearing its fair share of the chewing load, and the remaining natural teeth are overworked. The teeth are overloaded and under these conditions, the remaining teeth will undergo accelerated bone loss.