An impression is a negative reproduction of a given area of the oral cavity. The area reproduced may be composed of either hard or soft tissues or both. The material must be inserted into the mouth while it is too soft to retain its shape. A rigid base is needed to carry it to the mouth and hold it against the tissues until it hardens. For this purpose, a variety of trays, called stock trays, are available. These are shaped to fit over the average maxillary and mandibular arches. Some can be trimmed and bent to the requirements of the individual patient. Trays may also be fabricated for each individual patient.
REQUIREMENTS FOR IMPRESSION MATERIAL
a. General. An impression material must meet a wide range of requirements in order to provide an accurate impression of the different tissues.
Impression materials are of three types: the rigid type, the thermo- plastic type, and the elastic type. Thermoplastic materials soften when warmed and harden when cooled with no change in chemical makeup. Elastic materials expand and contract with no change in structure or shape. All three types have advantages and disadvantages. The dental officer determines which material best meets the requirements of each particular case. He frequently will use two or more materials to make a single impression.
a. General. The agar-type hydrocolloids are thermoplastic, elastic materials. They are called reversible hydrocolloids because they are softened by heating, hardened by cooling, and used repeatedly. In the hardened state, they are flexible and elastic.
Agar-type hydrocolloids become fluid at temperatures much higher than the temperature at which they gel. The temperature range varies slightly from one manufacturer to another. The tube containing the material used in making impressions is placed in boiling water to soften the material. The tube then is stored in a water bath at a temperature of 145o to 155o F (63o to 68o C) until it is needed. Before using the tube, it is "tempered" by cooling it to about 115o F (46o C) so that it will be of a consistency to remain in the impression tray, set quickly, and not be uncomfortable to the patient. Since most brands gel at 97o F (36o C) (slightly below mouth temperature), a water-cooled tray is used. Water at about 70o F (21o C) is circulated through the tray for about 5 minutes to gel the impression.
a. Water Content. The water content of agar-type hydrocolloid impression materials is most important for dimensional stability.
Figure 3-1. Instruments and materials for agar hydrocolloid impressions
(2) Imbibition. If the impression is placed in water, it will expand (take up water). This process is called imbibition. Unfortunately, the expansion caused by imbibition will not restore an impression to its original dimensions.
Some agar-type hydrocolloid impression materials retard the setting of gypsum products. The surface of a cast in contact with the gel may harden very slowly or not at all. The cast will also absorb water from the gel. As a result, the surface will be soft and rough. The hardness of the cast can be improved by "fixing" the impression with a hardening solution. For this purpose, a 2 percent solution of potassium sulfate is recommended. The impression is immersed in the solution for 5 to 10 minutes. Immersion for longer time may affect the dimensional stability of the hydrocolloid gel. The stone cast or die should be left in the impression for at least 30 minutes--preferably for 60 minutes--before the impression is separated from the cast. This is recommended because the setting time of the gypsum product in contact with the impression material will be lengthened even though a proper hardening solution is used.
a. General. The alginate-type hydrocolloids are an elastic type impression material. An alginate is a salt of alginic acid (an extract from seaweed). Alginate-type hydrocolloids gel by chemical action. Once the gelation process begins, it is irreversible.
Water and powder measuring cups are provided by the manufacturer in each can of alginate-type material. Lines on the water measurer correspond to the number of scoops of powder used. The ratio of use is 1 to 1 or, for example, three scoops of powder to three units of water. The actual amount will vary, depending on the size of the impression tray. Prior to mixing, it is necessary to tumble the container in order to fluff the powder.
MIXING AND SETTING TIME
The water-powder ratio recommended by the manufacturer must be used. Too little or too much water will weaken the gel. Mixing must be timed. Undermixing may prevent the chemical reaction from occurring evenly, and overmixing may break up the gel. Either can decrease the strength of the material by as much as 50 percent. The strength of alginate-type hydrocolloids increases for several minutes after the initial gelation. Consequently, the impression must not be removed from the mouth for at least 2 or 3 minutes after gelation has occurred.
Alginate-type hydrocolloid impression materials are influenced by syneresis, imbibition, strain, and stress in the same way as the agar-type materials. Hence, for the most accurate results, the impression should be fixed and the cast poured soon after the impression is removed from the mouth. If the impression must be stored for a short period of time, it should be placed in a humidor in which the relative humidity is 100 percent.
Alginate-type hydrocolloid impression materials affect gypsum products in the same manner as the agar-type materials affect them. Some alginates do not require the use of hardening solutions because the manufacturer has incorporated these materials in the powder. However, the hardness of the surfaces of the cast can always be improved by "fixing" the impression with a hardening solution.
SYNTHETIC RUBBER BASE IMPRESSION MATERIALS--CHARACTERISTICS
a. General. Synthetic rubber base impression materials are flexible, rubber-like, and sufficiently elastic to return to their original shape after slight distortion. They are used for making impressions of areas containing undercuts, especially for crowns, inlays, and removable and fixed partial dentures. Figure 3-2 shows typical instruments and materials setup for rubber base impression materials.
Figure 3-2. Setup for rubber base impression material
Equal lengths (usually 6 inches each) of both the rubber base impression material and the chemical reactor (a catalyst) should be laid out side by side onto a polymer paper or parchment pad. The base material has a tendency to spread once it is dispensed. For that reason, it is better to put the rubber base impression material on the pad first in order to give it time to spread. The chemical reactor should not touch the base material until everything is ready to mix.
The polysulfide base and its chemical reactor are both furnished as pastes that are packaged in separate tubes. The silicone base is also a paste that is packaged in tubes. Its chemical reactor may be either a paste or a bottled liquid. With both types, the proportions and the method of spatulation recommended by the manufacturer must be followed exactly. The bases and the chemical reactors are of different colors. They must be spatulated together until no streaks remain. Undermixing will prevent material from polymerizing evenly and overmixing will increase both the set and the strain pattern, but especially the set. Increased relative humidity and temperature tend to shorten both setting and mixing time, particularly for the polysulfide type. The chemical reactor is necessary to polymerize the material. It is often called an accelerator because the setting time may be shortened by increasing the amount used. It does not change the structure of the materials as does the accelerator used with gypsum products. Varying the amount of chemical reactor is the only method of changing the setting time of the silicone type.
During polymerization, synthetic rubber base impression materials undergo some shrinkage that continues for some time after the impression is made, particularly with the silicone type. However, these materials are not subject to syneresis and imbibition because they are hydrophobic (water-hating). They have more dimensional stability than do the hydrocolloids. They react to strain and stress in the same way the hydrocolloids do but do not change in volume. The cast must be poured within 30 minutes after an impression is made. This is necessary because the material continues to polymerize, and bubbles trapped in the material are apt to collapse causing the cast to be faulty. No separating material is required between the rubber base impression materials and the cast or die material.
a. General. Modeling plastic (modeling composition) is an impression material (thermoplastic type) which can be softened by heat into a soft plastic mass and then hardened by cooling with either a stream of cold water or a blast of air. Modeling plastic is used primarily to make impressions of the edentulous arches (the tooth ridges without teeth).
a. General. Impression paste is a thermoplastic-type impression material. It is usually supplied as two separate units, a base and a hardener. The principal ingredients are zinc oxide and eugenol. When the base and the hardener are mixed together in specific proportions, they form a paste. No separator is required when pouring the cast in an impression taken with this material.