Сraig. Dental Materials

11.Dimensional stability over temperature and humidity ranges normally found in clinical and laboratory procedures for

a period long enough to permit the pro-

duction of a cast or die

12. Compatibility with cast and die materials

13.Accuracy in clinical use

14.Readily disinfected without loss of accuracy

15.No release of gas during the setting of the impression or cast and die materials

TYPES OF IMPRESSION

The alginate hydrocolloid, agar hydrocolloid, and synthetic elastomeric impression materials are the most widely used today, and the properties of these are examined first. The zinc oxideeugenol materials, gypsum, and compound im-

pression materials are discussed later in this chapter for use as bite registration materials.

Dental alginate impression materials change from the sol phase to the gel phase because of a chemical reaction. Once gelation is completed, the material cannot be reliquefied to a sol. These hydrocolloids are called irrevenible to distinguish them from the agar reversible hydrocolloids described later. Alginate- impressions are ukieljused to form stud!. casts used to plm treatment, monitor changes. and create cronms. bridges, and removable prostheses.

Alginate impression products have acceptable elastic properties and compare well with agar materials. Preparation for use requires only the mixing of measured quantities of powder and

water. The resulting paste flows well and registers acceptable anatomical detail. Gypsum casts or models are made by pouring dental plasters, stone, or investment into the impression; no separating medium is necessary. The powder is supplied in bulk containers along with suitable measures for dispensing the correct quantities of powder and water. The powder is also available in small sealed packets containing a quantity suitable for one impression and ready for mixing with a measured quantity of water. These methods of packaging, together with the measuring devices supplied by the manufacturer, are shown in Fig. 12-3.

COMPOSITION AND CHEMISTRY

Potassium and sodium salts of alginic acid have properties that make them suitable for compounding a dental impression material. Alginic acid, which is prepared from a marine plant, is a high-molecular weight block copolymer of anhydro-p-D-mannuronic acid and anhydro-P-D-guluronic acid, as shown in the top part of the formula for alginate on p. 334. The properties of alginate raw material depend largely on the degree of polymerization and the ratio of guluronan and mannuronan blocks in the polymeric molecules. The mannuronan regions are stretched and flat, whereas the guluronan regions contribute less flexibility. Also, mainly guluronan blocks bind with ~ a +Therefore,~. alginates rich in guluronan form strong, brittle gels, whereas those rich in mannuronan form weaker and more elastic gels.

Solutions of these soluble salts, when reacted with a calcium salt, produce an insoluble elastic gel commonly called calcium alginate;the structures are shown below. Upon mixing with water, the alginate impression material first forms a sol. Following the chemical reaction described on p. 334, a gel is formed to create the set impression material. The gel-forming ability of alginates is mainly related to the proportion of L-guluronan blocks. The concept of sols and gels is presented in the discussion of colloids in Chapter 2.

The nature of this chemical reaction is shown on p. 335 for the sodium salt. The equally common potassium salt reacts similarly. In an alginate impression compound, the calcium sulfate dihydrate, soluble alginate, and sodium phosphate are included in the powder. When water is added to the powder, compounds disassociate as shown. Calcium ions from the calcium sulfate dihydrate react preferentially with phosphate ions from the sodium phosphate and pyrophosphate to form insoluble calcium phosphate. Calcium phosphate is formed rather than calcium alginate because it has a lower solubility; thus the sodium phosphate is called a retarder and provides working time for the mixed alginate.

After the phosphate ions are depleted, the calcium ions react with the soluble alginate to form the insoluble calcium alginate, which together with water forms the irreversible calcium alginate gel. The calcium alginate is insoluble in water, and its formation causes the mixed material to gel. This reaction is irreversible; it is not possible to convert the calcium alginate to a sol after it has set.

To meet the critical requirements of a dental impression material, this reaction must be controlled to attain the desirable properties of consistency, working time, setting time, strength, elastic quality, and smooth, hard surfaces on gypsum casts. These requirements are achieved by adding agents to control the rate of the reaction, develop strength and elasticity in the gel, and counteract the delaying effect of alginate on the setting of gypsum products. The use of suitable fillers in correct quantities produces a consistency that is suitable for various clinical uses.

The composition of a typical alginate impression material and the function of its ingredients are shown in Table 12-1. Manufacturers adjust the concentration of sodium phosphate to produce regularand fast-set alginates. They also adjust the concentration of filler to control the flexibility of the set impression material from soft-set to hard-set. Although alginate impressions are usually made in a tray, injection types are much more fluid after mixing and more

Function

To dissolve in water and react with calcium ions

To react with potassium alginate to form an insoluble calcium alginate gel

To counteract the inhibiting effect of the hydrocolloid on the setting of gypsum, giving a highquality surface to the die

To react preferentially with calcium ions to provide working time before gelation

To control the consistency of the mixed alginate and the flexibility of the set impression

Organic glycols

Wintergreen, peppermint, anise

Pigments

Disinfectants (e.g., quaternary ammonium salts and chlorhexidine)

1

1

/

Small

Trace

Trace

1-2

I

1

1

To make the powder dustless

To produce a pleasant taste

To provide color

To help in the disinfection of viable organisms

flexible after setting. The alginate powder is finely divided, and considerable dust may be involved during dispensing. The dimensions of 10% to 15% of the siliceous dust particles are similar to asbestos fibers that produce fibrogenesis and carcinogenesis; therefore inhalation of the dust should be avoided. Coating the powder with a glycol results in a dustless alginate, and no detectable levels of dust have been measured at the operator level for the dustless products. Alginates containing disinfectants reduce the viable organisms by up to 90%; however, additional disinfection by solutions or sprays should be carried out.

PROPORTIONING AND MIXING

The proportioning of the powder and water before mixing is critical to obtaining consistent results. Changes in the watedpowder ratio will alter the consistency and setting times of the mixed material and also the strength and quality of the impression. Usually the manufacturers provide suitable containers for proportioning the powder and water by volume, and these are sufficiently accurate for clinical use.

The mixing time for regular alginate is 1 minute; the time should be carefully measured, because both undermixing and overmixing are detrimental to the strength of the set impression. Fast-set alginates should be mixed with water for 45 seconds. The powder and water are best mixed in a rubber bowl with an alginate spatula

or a spatula of the type used for mixing plaster and stone.

Automatic mixing systems have been developed for paste/paste alginates. These systems consist of a mixing unit that mixes an aqueous base paste and an organic initiator paste in a 4:l ratio. The base paste is believed to contain sodium alginate and polyacrylic acid as a viscosity modifier, the initiator paste contains calcium sulphate-hemihydrate and sodium phosphate. The mixer uses a dynamic mixing principal and is available in Japan.

PROPERTIES

Some typical properties of a tray-type alginate impression material are listed in Table 12-2, along with comparable values for agar impression material, which are discussed in the next major section.

Working Time The fast-set materials have working times of 1.25to 2 minutes, whereas time of the regular-set materials is usually 3 minutes, but may be as long as 4.5 minutes. With a mixing time of 45 seconds for the fast-set types, 30 to 75 seconds of working time remain before the impression needs to be completely seated. For the regular-set materials, a mixing time of 60 seconds leaves 2 to 3.5 minutes of working time for materials that set at 3.5 to 5 minutes. In both cases, the mixed alginate must be loaded into the tray and the impression made promptly.

Setting Time Setting times range from 1 to j minutes. The ANSI/ADA Specification No. 18 (IS0 1563) requires that it be at least that value listed by the manufacturer and at least 15 seconds longer than the stated working time. Lengthening the setting time is better accomplished by reducing the temperature of the water used with the mix than by reducing the proportion of powder. Reducing the ratio of powder to water reduces the strength and accuracy of the alginate. Selecting an alginate with a different setting time should also be considered rather than changing the watedpowder ratio.

The setting reaction is a typical chemical reaction, and the rate can be approximately doubled by a temperature increase of 10" C. However, using water that is cooler than 18" C or warmer than 24" C is not advisable. The clinical setting time is detected by a loss of surface tackiness. If possible, the impression should be left in place 2 to 3 minutes after the loss of tackiness, because the tear strength and resistance to perinanent deformation increase significantly during this period.

Color-changing alginates provide a visual indication of working time and setting time. The mechanism of the color change is a pH-related change of a dye. One such alginate changes its color from light pink to white.

Permanent Deformation A typical alginate impression is compressed about 10% in areas of undercuts during removal. The actual magnitude depends on the extent of the undercuts and the space between the tray and the teeth. The ANSI/ADA Specification requires that the recovery from deformation be more than 95% (or a permanent deformation of less than 5%) when the material is compressed 20% for 5 seconds at the time it would normally be removed from the mouth. As indicated in Table 12-2, a typical value for recovery from deformation is 98.2%. The corresponding permanent deformation is 1.8%.

The permanent deformation, indicated as percent compression set, is a function of percent compression, time under compression, and time after removal of the compressive load, as illus-

trated in Fig. 12-4. Note that permanent deforination is a time-dependent property. Lower permanent deformation (higher accuracy) occurs

(1)when the percent compression is lower,

(2)when the impression is under compression a shorter time, and (3) when the recovery time is longer, up to about 8 minutes after the release of the load. Clinically these factors translate into requirements for a reasonable bulk of alginate between the tray and the teeth and a rapid or snap removal of the impression. The usual procedures followed to produce a gypsum model provide adequate time for any recovery that might occur.

Flexibility The ANSI/ADA Specification permits a range of 5% to 20% at a stress of 1000 g/cm2, and most alginates have a typical value of 14%. However, some of the hard-set

-

30% 10 sec.

30% 5 sec.

I

\

\

\//

\ -- / - /

. 20% 10 sec.

I

(Adapted from Wilson HJ: Br Dent J 121:466, 1966.)

Fig. 12-5 Sketch of tear strength specimen with load applied in the directions of the arrows; the specimen tears at the V-notch.

materials have values from 5% to 8%. A reasonable amount of flexibility is required for ease of removal of the impression.

Strength The compressive and tear strengths of alginates are listed in Table 12-2. Both properties are time dependent, with higher values obtained at higher rates of loading. Compressive strengths range from 5000 to 9000 g/ cm2. The ANSVADA Specification requires that certified products have a compressive strength of at least 3570 g/cm2. Tear strengths vary from 380 to 700 g/cm, and this property is probably more important than the compressive strength. The tear strength is a measure of the force/ thickness ratio needed to initiate and continue tearing and is often determined on a specimen of the shape shown in Fig. 12-5. Tearing occurs in the thin sections of the impression, and the probability of tearing decreases with increasing rates of removal. The effect of loading rate on the tear strength of several alginates is shown in Fig. 12-6. Values for tray materials range from 3.8 to 4.8 N/cm at 2 cm/min to 6 to 7 N/cm at 50 cm/ min. The lower tear strength at corresponding rates for the syringe materials reflects the decreased alginate in the syringe material.

Compatibilitywith Gypsum The selection of an alginate-gypsum combination that produces good surface quality and detail is highly important. The surface quality and ability of alginate-gypsum combinations to reproduce fine V-shaped grooves are shown in Fig. 12-7, A and B. A Type I11 model plaster was poured against an alginate in Fig. 12-7, A, and Type IV dental stone was poured against the same alginate in Fig. 12-7, B. The finest groove was 0.025 mm wide in each instance. The combination in Fig.

Loading rate (cmlmin)

Fig. 12-6 Tear strength of alginate impression materials as a function of rate of loading; materials A, B, and Care designed to be used in a tray; D is a syringe material.

(Adapted from MacPherson GW, Craig RG, Peyton FA: J Dent Res 46:717, 1967.)

12-7, B, was not as compatible as the one in Fig. 12-7, A, with respect to either surface quality or detail. For purposes of comparison, in Fig. 12-7, C, the same Type IV dental stone used in Fig. 12-7, B, was poured against polysulfide impression.

The impression must be rinsed well in cold water to remove saliva and any blood, and then disinfected. Next, all free surface water should be removed before preparing a gypsum model. Saliva and blood interfere with the setting of gypsum, and if free water accumulates, it tends to collect in the deeper parts of the impression and dilute the model material, yielding a soft, chalky surface. The excess surface water has been removed when the reflective surface becomes dull. If the alginate impression is stored for 30 minutes or more before preparing the model, it should be rinsed with cool water to remove any exudate on the surface caused by syneresis of the alginate gel; exudate will retard the setting of the gypsum. Thereafter it should be wrapped loosely in a

moist paper towel and sealed in a plastic bag to avoid moisture loss.

The set gypsum model should not remain in contact with the alginate impression for periods of several hours because contact of the slightly soluble calcium sulfate dihydrate with the alginate gel containing a great deal of water is detrimental to the surface quality of the model.

Dimensional Stability Alginate impressions lose water by evaporation and shrink on standing in air. Impressions left on the bench for as short a time as 30 minutes may become inaccurate enough to require remaking the impression. Even if the impression stored for more than 30 minutes in air is immersed in water, it is not feasible to determine when the correct amount of water has been absorbed, and in any case the previous dimensions would not be reproduced. For maximum accuracy, the model material should be poured into the alginate impression as soon as possible. If for some reason the models cannot be prepared directly, the impressions

should be stored in 100% relative humidity in a plastic bag or wrapped in a damp (but not wringing-wet) paper towel.

Storage of alginate impressions in 100% relative humidity is satisfactory for some materials for periods up to 2 hours, as indicated in Fig. 12-8 for material A . Materials C and D should not be stored in 100% relative humidity, even for short periods. However, some cases requiring less accuracy, such as study of orthodontic models, properly stored alginate impressions are sent to a laboratory where the model is prepared.

Disinfection Disinfection of impressions is a concern with respect to viral diseases such as hepatitis B, acquired immunodeficiency syndrome, and herpes simplex, because the viruses may be transferred to gypsum models and presents a risk to dental laboratory and operating personnel.

All alginate colloid impressions should be disinfected before pouring with gypsum to form a cast. The most common form of disinfection is by

This material forms a colloid with water, which liquefies between 71" and 100"C and sets to a gel again between 30" and 50" C, varying with the concentration of the agar.

A typical composition and the functions of the various ingredients are listed in Table 12-3. The

material described is a tray type and is considerably stiffer at the time of making the impression than a syringe type. The agar content is reduced in the syringe type of material, so it is much more fluid at the time of injection than is the tray material at the time of insertion.