Impression Materials in Prosthetic Dentistry

Abstract

The impression materials used in prosthetic dentistry are classified as being rigid, thermoplastic or elastic materials. Of the rigid materials, plaster of paris is used as a secondary impression material for plaster wash impressions or for full plaster impressions in modeling plastic, resin, or base shellac trays. The metallic oxide pastes are similarly used as a secondary impression material but almost universally as a full correction rather than as a wash correction. Both of these materials are free flowing and are capable of giving a pressure-free impression. Other secondary impression materials are the mouth-temperature waxes and the mercaptan base materials. The former is classified as a thermoplastic material and may be used as a full correction material or for correcting or refining the peripheries of the impression. All mouth-temperature waxes continue to flow as long as they are in the mouth, and will equalize tissue loading if given sufficient space and time in which to flow. They displace soft tissues and are used with this fact in mind. Mouth-temperature waxes are used as a secondary impression material in both complete and partial denture impression techniques. A second, but older, thermoplastic material is modeling plastic, more commonly known as modeling compound. This material is still the preferred material for full denture primary impressions because of the fact that it is correctible and can be muscle-molded. It is rarely used today as a final impression material, because of the fact that it displaces tissue yet does not continue to flow at mouth temperature. Thus, there is no rebound of tissue which has been overdisplaced as with the mouth temperature waxes. Of the elastic materials, alginate hydrocolloid is used primarily as an impression material for study casts, partial dentures and immediate dentures. It has been misused as a primary impression material for complete dentures owing to the indiscriminate use of ill fitting stock trays which cannot be shaped or trimmed. An impression tray made from an alginate primary can, however, be suitably corrected if the dentist will take the time to do so. Agar hydrocolloid is not considered as an impression material for complete dentures. It is used primarily in indirect crown, bridge and inlay techniques. Its use for partial denture impressions is not considered to be particularly advantageous over alginate, since results with the latter, when properly handled, are equal to the agar material. Mercaptan base material is used as a secondary impression material for complete dentures in much the same manner as metallic oxides. It is used also as an immediate denture material because of its elastic nature. It is less fluid, but satisfactory results have been reported. It does seem to record peripheral tissue detail more faithfully, as the setting characteristics permit more muscle-molding than many of the metallic oxide pastes. Like the alginate, its convenience has led to some careless usage, but when used with care it seems capable of recording tissue detail faithfully. It is too early to evaluate the silicone materials as impression materials for complete dentures. Their use has been limited by their cost, their delicate nature when cured, and the fact that other less costly and less delicate materials are giving satisfactory results. One may expect to see continued improvements in these newer products which may make them even more suitable as prosthetic impression materials than the materials now in use. They undoubtedly offer much promise as impression materials for immediate and partial dentures because of their greater stability after curing. Improvements in texture and flow may eventually make them adaptable to all prosthetic impression procedures where secondary impressions in individual trays are used.