A major problem in designing maxillary fixed implant prostheses is that the usual angulation of the anterior maxillary ridge is such that it is often difficult to place implants totally within the confines of the prosthesis. To get around this problem, it is not unusual to make a substructure bar with a superstructure prosthesis retained by palatal screws. Screws are not standardized in terms of size and very often cannot be interchanged.
Technicians usually use slotted screws.
These work just fine on the lab bench, but in the mouth they are a nightmare.