Сraig. Dental Materials

Time (min)

Fig. 21-13Viscosity of six fluid resins as a function of time at 10 rpm.

(From Vermilyea SG, Powers JM, Koran A: J Dent Res 57:227, 1978.)

LIGHT-CURED DENTURE PLASTIC

Light curing of a denture is a novel method compared with other processing methods. After the try-in of the waxed-up trial denture is completed, a roll of light-activated acrylic is placed over the occlusal surfaces of the teeth to form a template having three reference areas on the master cast (Fig. 21-14, A). The template is cured in the light chamber for 10 minutes, then the teeth are removed from the trial denture. Removal is simple because the wax softens under the heat of the high-intensity light bulbs. The teeth, the attached template, and the cast are placed in boiling water to remove all traces of wax (Fig. 21-14, B).

After coating the master cast with a release agent, a sheet of the light-activated denture base material is adapted to the cast and trimmed to the boxing edge (Fig. 21-14, C). The base is then polymerized in the light chamber.

A strip of the light-activated acrylic is placed on the underside of the teeth after they have

been coated with a bonding agent. The teeth are then repositioned in the original position on the denture base using the template (Fig. 21-14, Dl. The teeth are held in position by polymerization in the light chamber.

The anatomical portion of the denture is completed using more of the base material to sculpt the surface and develop the final shape of the denture (Fig. 21-14, E). After contouring, final polymerization is accomplished in the light chamber and the denture is removed from the cast and finished in a conventional manner.

FACTORS INVOLVED I N DENTURE RETENTION

Accuracy of fit has been cited as an important factor in the retention of dentures. Other factors are (1) capillary forces involving the liquid film between the oral tissues and the denture base;

(2) surface forces controlling the wetting of the plastic denture base by the saliva; (3) the thickness of the saliva film between the denture and the oral tissues; (4) the surface tension of the saliva; (5) the viscosity of the saliva; and (6) atmospheric pressure. High surface tension, area, and wetting increase retention, as does a thin film of saliva (see Chapter 2). A technical discussion of all factors involved in the retention of dentures is not within the scope of this text.

EFFECT O F AUXILIARY MATERIALS

ON DENTURE PLASTICS

A number of materials are used in making a denture. These materials may affect the final properties and function of the denture. Examples of such materials are (1) dental plaster and stone,

(2) impression materials, (3) wax, (4) mold separators, (5) artificial teeth, (6) characterization materials, (7) metal inserts, (8) repair and reline materials, and (9) denture cleansers.

Plaster and Stone The strength of the plaster or stone used to invest the wax denture is of concern, because a weak investment resulting from a thin mix or incomplete mixing will not adequately support the artificial teeth during packing of the denture mold. As a result of teeth

shifting, the finished dentures may have faulty occlusion. Another problem involved in the use of a stone mold is that the thermal coefficient of expansion, or contraction, is different from the thermal coefficients of the teeth and the plastic used to form the denture. After the polymerization reaction, the plastic and investment cool to room temperature and attempt to contract ac-

cording to their individual thermal coefficients. The stone cast with a different thermal coefficient creates residual stresses in the acrylic as it cools. These stresses may be released after the processed denture is deflasked and deformation or crazing may result. One study suggests that using a high expansion dental stone in the final impression will compensate for the polymerization

shrinkage of the denture base resin and provide greater accuracy in the denture base.

Impression Materials When alginate materials are used to make an impression, it is important that the cast be poured as soon as possible. If dimensional changes occur in the impression, these inaccuracies will be reflected in the final fit of the denture base. When zinc oxide-eugenol impression materials are used, residual eugenol in the cast will function as an inhibitor in the polymerization of the plastic.

Waxes Baseplate wax that is not removed from the crown portion of the teeth before the denture is flasked will cause problems. During the removal of the wax in boiling water, the wax on the coronal area of the teeth is also removed and will result in shifting of the teeth, poor articulation, or broken teeth when the denture is packed. A more common problem is the presence of a residual wax film on the gingival portions of the artificial teeth after the boilout of the wax, which prevents the adherence of the denture base to the teeth. This may be avoided by the addition of detergent to the water used in the boilout procedure, followed by rinsing with clear boiling water.

Mold Separators For many years tinfoil was the most acceptable separating medium. Tinfoil, however, is difficult to apply and as a result a number of tinfoil substitutes have been developed. Materials such as aqueous solutions of sodium silicate, calcium oleate, or sodium or ammonium alginate have been used. The common alginate separators contain about 2% sodium alginate in water with small amounts of glycerin, alcohol, sodium phosphate, and preservatives. Care must be exercised to avoid coating plastic teeth with these release agents because this will interfere with the bond between the denture base and the teeth.

CharacterizationMaterials A variety of materials are used for the characterization of dentures. Included in this group of materials are dyed synthetic fibers, pigments, dyes, and clear

plastic powder. These materials have the effect of improving the esthetics of the dentures and have no substantial effects on the strength or other properties of dental plastics. For example, the addition of dyed acrylic fibers to simulate the blood vessels of the oral mucosa does not alter the water sorption or strength of the dental plastic significantly. There is a justified concern over the toxicity of some of the pigments used to color denture base materials.

Denture Cleansers Denture cleansers and cleaning methods may scratch and wear dentures. Denture acrylic has been tested for wear using various con~merciallyavailable denture cleaning pastes, an experimental paste, soap and water, and water against a reciprocating soft toothbrush. The results shown in Fig. 21-15 are dramatic. The use of water, or soap and water, produced little or no wear compared with the commercial denture cleansers and toothpaste. Daily brushing with a very soft brush (similar to the toothbrushes used in periodontal therapy) is very effective in keeping dentures clean and will not abrade the denture or teeth appreciably if abrasive cleansers are not used. Most immersion

Fig. 21-15 Wear of denture base acrylic in various media, as illustrated by a histogram showing loss of thickness after 60,000 stokes.

(Adapted from Heath JR, Davenport JC, Jones PA: J Oral Re- hab;/ 10:159, 1983.)

denture cleansers are effective in the removal of mucin, stains, and loosely attached food debris. Some immersion cleaners have been demonstrated to be effective sanitizing agents. A water solution containing a hypochlorite and a glassy phosphate (Calgon) is an effective denture cleanser that does not cause discoloration of the dental plastic or the metal retainer pins in porcelain artificial teeth when used occasionally. A solution consisting of 1 tsp of a hypochlorite, such as Clorox, and 2 tsp of Calgon in half a glass of water has been recommended for occasional overnight immersion of plastic dentures. This cleanser is not recommended for use on prostheses containing cobalt-chromium or nickelchromium alloys because chlorine solutions tend to darken these metals.

REPAIR MATERIALS

An important application of plastics in prosthetic dentistry is in the repair of broken dentures. Repair materials are usually acrylic plastics of the powder-liquid type, similar to those used for denture bases, and are usually either heataccelerated or chemically accelerated. Lightactivated acrylic has also been shown to be a fast and effective repair material. The material of choice will depend on the following factors:

(1)length of time required for making the repair,

(2)transverse strength obtainable with the repair material, and (3) degree to which dimensional accuracy is maintained during repair.

One technique for making a repair requires holding, or luting, the broken pieces together with sticky wax, pouring a stone model on the inside of the denture, and investing the model and denture in a flask. Then the wax is removed, the fracture line is opened with a bur to allow for a reasonable amount of repair plastic, and the ground surfaces are painted with monomer or a 4:l mixture of monomer and polymer. Acrylic dough is packed into the area being repaired, and the material is cured, cooled, deflasked, and finished.

If a heat-accelerated acrylic is used, the denture should be completely flasked, and curing of the repair material preferably should be carried

out at temperatures no greater than 74" to 77" C for 8 hours or longer. This procedure minimizes the dimensional change of the denture base.

In a similar manner, the repair may be done without flasking, by use of a chemically accelerated acrylic. After a cast is poured, the fracture line is opened and the acrylic is painted into the defect. The denture is then placed in a pressure vessel under air pressure until polymerization is complete. Repairs using light-activated acrylic are done in a similar manner. After the acrylic dough is packed into the defect, curing is done in the light chamber. A bonding agent is used to increase the strength of the repair.

The use of chemically accelerated and lightactivated acrylic resins for repairs does not require flasking. The procedure is rapid and may be done while the patient waits. The dimensional accuracy is maintained because not enough heat is present during polymerization to cause warpage from the release of stresses. The chemically accelerated acrylic resins, however, have the disadvantage of lower transverse strength in the repaired area than a heat-cured acrylic. In general, the transverse strength of a heat-cured repair is about 80% of that of the original plastic, and the transverse strength of a chemically cured repair is approximately 60% of the original material. Light-activated acrylic is affected by water storage, resulting in lower transverse strength than the self-cured repair resin. Regardless of the type of plastic used for repair, the shape of the edges in the repair area should be tapered and all corners rounded to avoid areas of high stress concentration. These ground edges are painted with monomer to soften the plastic to obtain a chemical bond between the repair material and the denture.

The use of chemically accelerated repair material without flasking at room temperature and pressure may result in a repair area containing porosity. Polymerization in the center of the repair proceeds rapidly, but polymerization at the surface is slowed by the inhibiting effect of the oxygen. When the repair is done in water, under air pressure of 0.2 MPa and a temperature of 30" C, the porosity is greatly reduced. The residual monomer content of specimens pro-

cessed either in air or under pressure is approximately 3%.

The reported net dimensional change across the molar region of a maxillary denture for a heat-cured repair conducted at 73.5" C is -0.3%, whereas for a chemically cured repair it is +0.2%. It also has been reported that a denture repaired with chemically accelerated plastic will fit the model better. This may be the result of less warpage or because the denture is slightly oversized. Studies have been done on the contour of denture bases before they were broken and after repair. The results indicate that better reproduction of dimensions results when chemically accelerated repair materials are used to repair dentures originally prepared from heat-cured or self-cured acrylic plastics. Superior results also are obtained when a heat-curing acrylic repair material is used to repair a denture prepared from a chemically accelerated acrylic plastic. Less satisfactory reproduction of dimensions is obtained when a heat-curing repair material is used to repair a heat-cured denture. If the cause of breakage is not corrected, such as faulty occlusion, fit, or both, repairs usually will fail regardless of which repair material is used.

ANSI/ADA Specification No. 13 for Denture Self-curingRepair Resins The requirements of the chemically accelerated repair plastics or the self-curing repair resins are listed in ANSI/ADA Specification No. 13. The specification includes self-curing powder-liquid plastics, which may be either pink or clear. These materials must satisfy the requirements for denture base plastics, with two exceptions. First, the plasticity test shall be conducted as in the denture base specification, except the test should be started 3 minutes rather than 5 minutes after the proper plasticity is reached. This change allows for the faster setting of the repair materials.

Second, the transverse deflection between a 1500and 2500-g load shall be not more than 1.5 mm, and the deflection between a 1500and 4000-g load shall be not less than 1 mm and not more than 4.5 mm. This transverse deflection requirement is difficult to compare with the test for chemically accelerated denture base plastics,

because the loads used are different and therefore the deflection limits are not the same. It does appear, however, that the deflection limits for the repair materials are somewhat more lenient.

In general, the color stability test is difficult for chemically accelerated repair materials to pass.

RELINING AND REBASING DENTURES

The fit of a denture may be satisfactory when it is first delivered to the patient. However, because of changes in the contour of the soft tissues and resorption of underlying bone, the denture may gradually lose retention. If the occlusion and vertical dimension of the dentures have not been greatly altered, the retention may be regained by either relining or rebasing.

Relining Relining is a process in which a film of plastic is added to the inside of the denture to obtain an improved fit with the denture-bearing mucosa. This is accomplished by (1) making an impression of the denturebearing mucosa using the denture as a tray, reflasking the denture, removing the impression material, and packing and curing the new liner; or (2) making a chairside reline where the reline material is used to make the impression.

Two different types of reline materials may be used-one permanent, the other temporary. The former may be either a heat-accelerated, chemically accelerated, or light-activated acrylic. The relining process is carried out either by a flasking procedure or curing in a light chamber, depending on the type of material. Permanent reline materials may be identical to those from which permanent denture bases are made.

The problems involved in relining a denture are much the same as those encountered in repairing a denture: (1) a good chemical bond is desired between the reline plastic and the denture plastic;(2) satisfactory strength of the relined denture is necessary; (3) no warpage or dimensional change should result in the denture because of the relining procedure; and (4) relining should take as short a time as possible for patient convenience. For heat-polymerized reline materials the area to be relined is softened by mono-

mer and the acrylic is packed soon after reaching the doughy consistency to achieve a chemical bond between the two materials. The polymerization should occur at temperatures between 74" to 77" C to prevent warpage. Chemically accelerated reline materials have an advantage because lower peak temperatures during polymerization may minimize warpage.

The light-activated material is used directly in the denture to record the tissue surface. A bonding agent is used before placing the material in the denture. Polymerization is achieved in a light chamber. The light-activated acrylic allows the entire reline procedure to be completed in 30 to 45 minutes.

Temporary relining plastics are used directly in the mouth, and a chemically accelerated curing system is used. These materials are considered temporary because they are porous and stain easily or foul with microorganisms. In addition, most of these products are not color stable. Temporary reline materials polymerize at mouth temperature and the reaction is exothermic, with peak polymerization temperatures of 59" to 79" C and peak temperature times of 6 to 11 minutes. The lower temperatures generally correspond to longer times. Peak temperatures of 79" C can cause tissue injury. To avoid burning of the tissues, the denture is usually taken from the mouth after a few minutes, chilled in cool water, and returned to the mouth. In addition to the exothermic heat, direct contact of monomer on the oral tissues may elicit a burning sensation. Temporary reline plastics, however, do not cause any clinically significant warpage of the denture.

ANSI/ADA Specification No. 17 for Denture Base Temporary Relining Resin

ANSI/ADA Specification No. 17 for temporary relining resin is for hard-setting, self-curing plastics of the powder-liquid type. This specification contains requirements for the consistency, temperature rise, hardening time, ease of polishing, translucency, porosity, hardness, water sorption and solubility, and color stability. The peak temperature reached during processing should not be more than 75" C, and the time of hardening should be between 6 and 15 minutes. The pro-

cessed plastic should have a smooth glossy surface when polished by usual methods and should be translucent and free of large numbers or sizes of bubbles. The Knoop hardness shall be not less than 10 kg/mm2.The water solubility and sorption shall be less than 0.07 and 0.7 mg/cm2, respectively, when stored for 24 hours at 37' C. The requirements for hardness and solubility in water therefore are less demanding for temporary relining materials than for denture base plastics. Temporary relining plastics, however, must pass the color stability test for denture plastics, which requires that the specimen show no more than a slight color change after an exposure of 24 hours. This requirement may be difficult to satisfy, because many products show noticeable color changes when exposed to ultraviolet light.

Rebasing Rebasing refers to a technique in which the dimensional relations of the teeth are maintained and the entire denture base is replaced. A cast is prepared from an impression made in the denture. After flasking, all of the old plastic is removed except around the teeth. The denture is then processed according to standard procedures with new acrylic. The end result is a new denture base retaining the original teeth in the same position as in the original denture.

Tissue Conditioners Tissue conditioners are soft elastomers used to treat an irritated mucosa supporting a denture. They are mixed at chairside, placed in the denture, and seated in the patient's mouth. These materials will conform to the anatomy of the residual ridge, gel in that position, and continue to flow slowly after application. They are used only for short-term applications and should be replaced every 3 days. Inhibition of the growth of oral bacterial flora is associated with some materials, and this should promote healing of inflamed tissues.

Tissue conditioners are composed of a powder containing poly(ethy1 methacrylate) and a liquid containing an aromatic ester-ethyl alcohol (up to 30%) mixture. Tissue conditioners are very soft elastomers with a hardness of from 13 to 49 Shore A hardness units 24 hours after mixing. They will show a weight loss of from 4.9% to

668 Chapter 21 PROSTHETICAPPLICATIONSOFPOLYMERS

9.3% after 24 hours as a result of the loss of alcohol. These materials deform easily, and with a stress of 200 g'cm2 applied 15 minutes after the set of the material, the compression will range from 60% to 83% of the original length. When the stress is removed, there will be a recovery of from 22% to 48%. When unstrained samples are left in a humidifier for 24 hours, they will tend to "slump" or shorten under their own weight. Within a few days, tissue conditioners become stiffer as a result of the loss of alcohol.

Tissue conditioners are formulated to have specific viscoelastic properties. The viscosities of a number of tissue conditioners are listed in Table 21-11. The values were obtained 2 hours after mixing and under prolonged static loads. Under cyclic loading, which parallels cyclic masticatory forces, the materials demonstrate elastic behavior, particularly when the frequency is more than 1 Hz.

The properties that make tissue conditioners effective are (1) viscous behavior, which allows adaptation to the irritated denture-bearing mucosa over a period of several days; and (2) viscoelastic and elastic behavior, which cushions

Liquid/

Adapted from Braden M:J Dent Res 49:496, 1970.

the cyclic forces of mastication and bruxism. The viscoelastic properties are influenced by the molecular weight of the polymer powders and the powerAiquid ratio.

It has been suggested that the initial flow depends on the time of loading, volume of the material, and load applied when seating the denture. A rheologic study has shown that different products must be seated in the mouth at different times from the start of mixing because they may differ both in viscosity or gelation time. The gelation time is related to the molecular weight and particle size of the polymer powder, the ethanol content, and the plasticizer used.

An evaluation of the viscoelastic properties of tissue conditioners consisting of poly(ethy1 methacrylate) powders and butyl phthalate/butyl glycolate-ethanol liquids demonstrates that the elastic modulus, relaxation time, and instantaneous modulus increase as a function of time. The rate of increase in the coefficient of viscosity is greater than the elastic modulus in all cases. This confirms that compliance measurements and the Maxwell model are the most useful when describing the viscoelastic properties of tissue conditioners.

Viscoelastic finite element analysis has been used to evaluate the stress concentrations caused by simulated dentures with soft liners during function. The results indicate that the viscous flow of soft liners and tissue are affected by the load and the duration of the load placed on the denture. This affects the stress distribution in the supporting tissues.

Soft or Resilient Denture Liners Soft or plasticized acrylic, vinyl polymers, and copolymers, as well as natural and silicone rubber products, have been used as denture liners. As mentioned previously, these soft liners are suggested for use in patients with irritation of the denture-bearing mucosa, areas of severe undercuts, or congenital or acquired defects of the palate. Some products reportedly have an inhibitory effect on the growth of Candida albicans whereas others appear to support the growth of microorganisms. Desirable properties are

(1) high bond strength to the denture base, (2) dimensional stability of the liner during and after processing, (3) permanent softness or resilience, ( 4 )low water sorption, ( 5 ) color stability, (6) ease of processing, and (7) biocompatibility. Soft liners may be mouth cured or processed in the laboratory.

Mouth-Cured Soft Liners Conventional mouth-cured soft liners are used for periods to improve the comfort and fit of an old denture until it can be remade or permanently relined. After several weeks they may begin to foul and debond from the denture. They are mixed chairside, placed in the denture, and seated in the patient's mouth until polymerized, which generally takes a few minutes. Three types of materials are generally used:

1. Powder-poly(ethy1 methacrylate) and peroxide initiator; liquid-aromatic esters, ethanol, and tertiary amines

2.Powder-poly(ethy1 methacrylate), plasticizers such as ethyl glycolate, and a perox-

ide initiator; liquid-methyl methacrylate and tertiary amines

3. Addition silicone elastomers

Biocompatibility of these materials is of interest because it has been demonstrated both in

vitro and in vivo that both tissue conditioners and chairside soft liners leach out significant amounts of alcohol and phthalate esters.

Processed Soft Liners Processed soft liners are used with denture patients who experience chronic soreness with their dentures because of heavy bruxism or poor health. The cushioning effect of soft liners has been evaluated using an impact test with an accelerometer. Of four soft liners tested, all were found to reduce impact forces compared with denture base resin. Although these materials f~~lfilla need, they will generally not last long in use. A year is considered good service. These materials tend to pull away from the denture base or become porous and foul smelling. They are processed in the laboratory in a manner similar to processing of a denture base. Several additionsilicone elastomers can be processed either intraorally or in the laboratory. Finishing of soft denture liners is often difficult because the materials are soft. Several types of processed soft liners are available, including plasticized acrylics, plasticized vinyl acrylics, heatand room temperature-cured silicones, hydrophilic acrylates, and polyphosphazine. The compositions of various commercial products of plasticized acrylic and silicones are shown in Tables 21-12 and 21-13.

Adapted from Wright RS:J Dent 9:210, 1981.

Cross-LinkingAgent

Triethoxy silanol

Ethyl polysilicate

Tetraethoxy silane

Methyltriacetoxy silane

Acryloxyalkyl silane

Several physical properties of four categories of laboratory processed soft liners, determined 24 hours after processing, are presented in Table 21-14. The properties of these materials show a wide range of values. This variability is difficult to interpret because there is no ANSVADA specification for laboratory processed soft liners. For example, a plasticized acrylic may have very high tear resistance and a high Shore A hardness value. Because softer materials are generally considered kinder to the tissues, the gain in tear resistance is offset by hardness. Conversely, a silicone soft liner may have a low Shore A hardness value and low tear resistance. Many of these materials tend to increase in hardness with time, and their physical and mechanical properties are also affected by prolonged storage in water or in the oral environment.

The water solubility and sorption of soft liners is complex. When placed in water, plasticizers and other components may leach out over extended periods while water is absorbed until equilibrium is reached. Absorbed water can have a detrimental effect on the adhesion of soft liners

to acrylic denture bases, particularly if the rate of diffusion is rapid. At 1 week, water sorption will be from 0.2 to 5.6 mg/cm2 and solubility from 0.03to 0.40mg/cm2for various commercial products. An ideal processed soft liner would have no soluble components and low water sorption.

Processed soft liners are intended to be used for extended periods. In some patients it has been observed that a yeast, Candida albicans, and other microorganisms may grow on and within the liner, resulting in a rough and hardened surface. Antimicrobial agents have been proposed to eliminate this problem. Color changes have also been demonstrated for some liners subjected to accelerated aging.

The bond strengths for the materials listed in Table 21-14 are seen in Table 21-15. The materials were tested in a two-phase tensile test. Two test conditions were used; the soft liners were processed against both polymerized and unpolymerized acrylic resin. Again the results were variable. For both test conditions the polyphosphazine material demonstrated higher bond strength. It also was surprising that for three of