Academy of Dental Knowledge

Design of a Custom Tray for Final Impression for Complete Denture

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 record, a dentist proceeds to prepare a special tray that is used to make an impression with selective pressure on the mucosa. We will learn here as to how to make a special tray on a cast that has been made from impression compound.

Certain areas of the edentulous jaw are covered by masticatory mucosa that can bear the masticatory load. Masticatory mucosa covers the hard palate, attached gingiva and dorsal surface of tongue. They have resiliency, their surface texture feels rubbery, and they provide a firm base for complete dentures. Histologically, they show keratinized epithelium and interdigitated interface with many rete ridges and CT papillae with a thin layer of submucosa or sometimes none.

Stress bearing area of upper and lower edentulous jaw

The areas that can bear the masticatory load are divided into two types: They are

1. Primary stress bearing areas
2. Secondary stress bearing areas

Primary supporting areas

These are the main load bearers. These areas are

Hard palate
Posterior-lateral slopes of residual ridge

Stress bearing areas of the upper edentulous jaw

Secondary stress bearing areas

These can bear less masticatory load. These areas are

Residual ridge area
Rugae area
Maxillary tuberosity area

Relief area

These are the areas that cannot bear load, and if overloaded, become sore. If neglected, lead to bone resorption. These areas are

Incisive papilla
Median palatine raphe
Torus palatinus
Sharp spiny processes
Cuspid eminence
Zygomatic process

How to make a special tray

The wax spacer or impression compound of specific thickness (2.0 mm.) is placed all over the primary cast. It is trimmed all around the border to make it 1.5 mm. shorter. The stoppers are cut at four points at the crest of the residual ridge area. The wax borders are sealed by a hot wax knife to prevent ingress of self-cure acrylic. Self-cure resin is mixed, put between wet surfaces of glass slab and presses to make 2 mm sheet, then transferee over the spacer. The excess acrylic is cut at the border areas short of 1.5 mm. A small handle in the anterior area and support at each molar area is placed. The smaller handle helps protrude tongue while border molding and the support handles help in stabilizing the tray during impression making. Stoppers placed in a custom tray prevent the tray from sinking beyond the limit into the soft tissues, so that the impression material remains of a specific thickness to record proper functional or static state. Holes are drilled in the acrylic tray in non-load bearing areas as escapeway for the impression material thereby resulting in no pressure on oral mucosa.