The 'custom trays' also known as 'special trays' are fabricated so that the final impression of the patient’s edentulous mouth can be made accurately using selective pressure technique with proper border extensions. The selective pressure technique helps in making the impression in such a way that when the newly fabricated dentures are in function in the mouth of the patient, they exert pressure only on those areas of the mouth that can withstand the masticatory load. The selective pressure technique of impression making spares the non-pressure bearing areas of the edentulous jaw and thus eliminates the possibility of trauma resulting in ulceration and pain in the oral mucosa.
The special tray is made on the primary cast that has been prepared of plaster of Paris by pouring the primary impression made in alginate or impression compound. The impression taken in impression compound records the mucosa in a compressed state whereas the impression taken with alginate records the mucosa in a non-compressed state. From these two opposite states of mucosal records, a dentist proceeds to prepare a special tray that is used to make an impression with pressure on the mucosa in selected areas. We will learn here as to how to make a special tray on a cast that has been made from impression compound.