Сraig. Dental Materials

Glass Ionomer and Hybrid Ionomer Cements

Barry TI, Clinton DJ, Wilson AD: The structure of a glass-ionomer cement and its relationship to the setting process, J Dent Res 58:1072, 1777.

Berry EA 111: The clinical uses of glass ionomer cements (vol. 4, Chap. 20A). In Hardin JF,

Clark's clinical dentistry, Philadelphia, 1773, J.B. Lippincott.

Berry EA 111, Powers JM: Bond strength of glass ionomers to coronal and radicular dentin, Oper Dent 17:122, 1774.

Council on Dental Materials, Instruments, and Equipment: Biocompatibility and postoperative sensitivity, J A m Dent Assoc

116:767, 1988.

Crisp S, Kent BE, Lewis BG et al: Glass ionomer cement formulations. 11. The synthesis of novel polycarboxylic acids, J Dent Res 591055, 1980.

Crisp S, Lewis BG, Wilson AD: Characterization of glass-ionomer cements. I. Long term hardness and compressive strength, J Dent 4:162, 1976.

Crisp S, Lewis BG, Wilson AD: Characterization of glass-ionomer cements. 5. The effect of the tartaric acid concentration in the liq-

uid component, J Dent 7:304, 1779.

Crisp S, Lewis BG, Wilson AD: Characterization of glass-ionomer cements. 6. A study of erosion and water absorption in both neutral and acidic media, J Dent 8:68, 1780.

Crisp S, Pringuer MA, Wardleworth D et al: Reactions in glass ionomer cements. 11. An infrared spectroscopic study, J Dent Res 53:1414, 1774.

Crisp S, Wilson AD: Reactions in glass ionomer cements. I. Decomposition of the powder,

J Dent Res 53:1408, 1774.

Crisp S, Wilson AD: Reactions in glass ionomer cements. 111. The precipitation reaction,

J Dent Res 551420, 1774.

Finger W: Evaluation of glass ionomer luting cements, Scand J Dent Res 71:143, 1783.

Fitzgerald M, Heys RJ, Heys DR et al: An evaluation of a glass ionomer luting agent: bacterial leakage, J A m Dent Assoc 1l4:783, 1787.

Forss H: Release of fluoride and other elements from light-cured glass ionomers in neutral and acidic conditions, J Dent Res

72:1257, 1793.

Forsten L: Fluoride release from a glass ionomer cement, Scand J Dent Res 85:503, 1777.

Fried1 K-H, Powers JM, Hiller K-A: Influence of different factors on bond strength of hybrid ionomers, Oper Dent 20:74, 1795.

Hunt PR, editor: The Next Generation; Proceedings of the 2nd International Symposium on Glass Ionomers. Philadelphia, PA, 1774.

Johnson GH, Herbert AH, Powers JM: Changes in properties of glass-ionomer luting cements with time, Oper Dent 13:171, 1788.

Kawahara H, Imanishi Y, Oshima H: Biological evaluation on glass ionomer cement, J Dent Res 58:1080, 1777.

Maldonado A, Swartz ML, Phillips RW: An in vitro study of certain properties of a glass ionomer cement, J A m Dent Assoc

96:785, 1978.

Mitra SB, Li MY, Culler SR: Setting reaction of Vitrebond light cure glass-ionomer liner/ base, 130. In Watts DC, Setcos JC: Proceedings of the Conference on Setting Mechanisms of Dental Materials, Trans

Academy Dent Mater 5(2):175, 1772.

Negm MM, Beech DR, Grant AA: An evaluation of mechanical and adhesive properties of polycarboxylate and glass ionomer cements, J Oral Rehabil7:161, 1782.

Nicholson JW: The setting of glasspolyalkenoate ("glass-ionomer") cements, 113. In Watts DC, Setcos JC: Proceedings of the Conference on Setting Mechanisms of Dental Materials, Trans Acad Dent Mater

501, 1792.

Oilo G: Bond strength of new ionomer cements to dentin, Scand J Dent Res 87:344, 1981.

Prosser HJ, Richards CP, Wilson AD: NMR spectroscopy of dental materials. 11. The role of tartaric acid in glass-ionomer cements,

Biomed Mater Res J l6:43l, 1782.

Ryan MD, Powers JM, Johnson GH: Properties of glass ionomer luting cements, Mich Dent Assoc ~ 6 7 : 1 71985,.

Shalabi HS, Asmussen E, Jorgensen KD: Increased bonding of a glass-ionomer cement to dentin by means of FeCl,, Scand J Dent Res 89:348, 1981.

Wilson AD: Resin-modified glass-ionomer cements, Int J Prosthodont 3:425, 1990.

Wilson AD, Crisp S, McLean JW: Experimental luting agents based on the glass ionomer cements, Br Dent J 142:117, 1977.

Wilson AD, Crisp S, Prosser HJ et al: Aluminosilicate glasses for polyelectrolyte cements,

I&EC Prod Res & Develop 19263, 1980.

Resin Cements

Balderamos LP, O'Keefe KL, Powers JM: Color accuracy of resin cements and try-in paste,

Int J Prosthodont 10:111, 1997. Blackman R, Barghi N, Duke E: Influence of

ceramic thickness on the polymerization of light-cured resin cement, J Prosthet Dent 63:295, 1990.

Blalock KA, Powers JM: Retention capacity of the bracket bases of new esthetic orthodontic brackets, A m J Orthod Dentofac Orthop

107:596, 1995.

Buzzitta VAM, Hallgren SE, Powers JM: Bond strength of orthodontic direct-bonding cement-bracket systems as studied in vitro, A m J Orthod 81:87, 1982.

Chan KC, Boyer DB: Curing light-activated composite cement through porcelain, J Dent Res 68:476, 1989.

Cochran D, O'Keefe KL, Turner DT et al: Bond strength of orthodontic composite cement to treated porcelain, A m J Orthod Dentofac Orthop 111:297, 1997.

DeSchepper EJ, Tate WH, Powers JM: Bond strength of resin cements to microfilled composites, A m J Dent 6:235, 1993.

Dickinson PT, Powers JM: Evaluation of fourteen direct-bonding orthodontic bases, A m J Orthod 78:630, 1980.

Evans LB, Powers JM: Factors affecting in vitro bond strength of no-mix orthodontic cements, A m J Orthod 87:508, 1985.

Farah JW, Powers JM, editors: Esthetic resin cements, Dent Advis 17(3):1, 2000.

Faust JB, Grego GN, Fan PL et al: Penetration coefficient, tensile strength, and bond strength of thirteen direct bonding orthodontic cements, A m J Orthod 73512, 1978.

Jordan RE, Suzuki M, Sills PS et al: Temporary fixed partial dentures fabricated by means of the acid-etch resin technique: a report of

86 cases followed for up to three years, J A m Dent Assoc 96994, 1978.

Livaditis GJ, Thompson VP: Etched castings: an improved retentive mechanism for resinbonded retainers, J Prosthet Dent

47:52, 1982.

Mennemeyer VA, Neuman P, Powers JM: Bonding of hybrid ionomers and resin cements to modified orthodontic band materials, A m

J Orthod Dentofac Orthop 115:143, 1999. Miller BH, Nakajima H, Powers JM et al: Bond

strength between cements and metals used for endodontic posts, Dent Mater

14312, 1998.

Noie F, O'Keefe KL, Powers JM: Color stability of resin cements after accelerated aging, Int J Prosthodont 8 : j1, 1995.

O'Keefe KL, Miller BH, Powers JM: In vitro tensile bond strength of adhesive cements to new post materials, Int J Prosthodont

13:47, 2000.

O'Keefe K, Powers JM: Light-cured resin cements for cementation of esthetic restorations, J Esthet Dent 2:129, 1990.

O'Keefe K, Powers JM, McGuckin RS et al:

In vitro bond strength of silica-coated metal posts in roots of teeth, Int J Prosthodont 5:373, 1992.

Powers JM: Adhesive Resin Cements, Shigaku 79:1140, 1991.

Powers JM, Kim H-B, Turner DS: Orthodontic adhesives and bond strength testing, Semin Oflhod 3:147, 1997.

de Pulido LG, Powers JM: Bond strength of orthodontic direct-bonding cement-plastic bracket systems in vitro, A m J Orthod 83:124, 1983.

634 Cha~ter20 CEMENTS

Rabchinsky DS, Powers JM: Color stability and stain resistance of direct-bonding orthodontic cements, A m J Orthod 76:170, 1979.

Rochette AL: Attachment of a splint to enamel of lower anterior teeth, J Prosthet Dent 30:418, 1973.

Siomka LV, Powers JM: In vitro bond strength of treated direct-bonding metal bases, A m J Orthod 88:133, 1985.

Tate WH, DeSchepper EJ, Powers JM: Bond strength of resin cements to a hybrid composite, Am J Dent 6:195, 1993.

Wright WL, Powers JM: In vitro tensile bond strength of reconditioned brackets, A m

J Orthod 87:247, 1985.

Cavity Varnishes, Liners and Bases

Bryant RW, Wing G: A simulated clinical appraisal of base materials for amalgam restorations, Aust Dent J 21:322, 1976.

Chong WF, Swartz ML, Phillips RW: Displacement of cement bases by amalgam condensation, J A m Dent

Assoc 74:97, 1967.

Costa CAS, Mesas AN, Hebling J: Pulp response to direct capping with an adhesive system, A m J Dent 1341, 2000.

Farah JW, Hood JAA, Craig RG: Effects of cement bases on the stresses in amalgam restorations, J Dent Res 54:10, 1975.

Farah JW, Powers JM, Dennison JB et al: Effects of cement bases on the stresses and deflections in composite restorations, J Dent Res 55:115, 1976.

Fisher FJ: The effect of three proprietary lining materials on micro-organisms in carious dentin, Br Dent J 143:231, 1977.

Gordon SM: Gum copal solution for cavity lining and varnish, J A m Dent Assoc

23:2374, 1936.

Gourley JM, Rose DE: Comparison of three cavity base materials under amalgam restorations, J Can Dent Assoc 38:406, 1972.

Hilton TJ: Cavity sealers, liners, and bases: Current philosophies and indications for use, Oper Dent 21:134, 1996.

Leinfelder KF: Changing restorative traditions: The use of bases and liners, J A m Dent Assoc 125:65, 1994.

McComb D: Comparison of physical properties of commercial calcium hydroxide lining cements, J A m Dent Assoc 107:610, 1983.

Plant GC, Wilson HJ: Early strengths of lining materials, Br Dent J 129:269, 1970.

Soremark R, Hedin M, Rojmyr R: Studies on incorporation of fluoride in a cavity liner (varnish), Odontol Rev 20:189, 1969.

Swam ML, Phillips RW, Norman RD et al: Role of cavity varnishes and bases in the penetration of cement constituents through tooth structure, J Prosthet Dent 16:963, 1966.

Poly(methy1 methacrylate) polymers were introduced as denture base materials in 1937. Previously, materials such as vulcanite, nitrocellulose, phenol formaldehyde, vinyl plastics, and porcelain were used for denture bases. The acrylic resins were so well received by the dental profession that by 1946,98%of all denture bases were constructed from methyl methacrylate polymers or copolymers. Other polymers developed since that time include vinyl acrylic, polystyrene, epoxy, nylon, vinyl styrene, polycarbonate, polysulfone-unsaturated polyester, polyurethane, polyvinylacetate-ethylene, hydrophilic polyacrylate, silicones, light-activated urethane dimethacrylate, rubber-reinforced acrylics,

and butadiene-reinforced acrylic.

Acrylic polymers have a wide variety of applications in prosthetic dentistry as artificial teeth, denture repair materials, facings in crown and bridge restorations, impression trays, record bases, temporary crowns, and obturators for cleft palates.

rials that meet these requirements. The vast majority of dentures made today are fabricated from heat-cured poly(methy1 methacrylate) and rubber reinforced poly(methy1 methacrylate). Fractures of dentures still occur, but are usually associated with carelessness or unreasonable use by the patient. Considering functional stresses, the oral environment, and expected service life, denture base materials perform remarkably well.

The following list indicates the requirements for a clinically acceptable denture base material.

Strength and durability Satisfactory thermal properties

Processing accuracy and dimensional stability

Chemical stability (unprocessed as well as processed material)

Insolubility in and low sorption of oral fluids

Absence of taste and odor Biocompatible

.Natural appearance Color stability

Adhesion to plastics, metals, and porcelain

Ease of fabrication and repair Moderate cost

There are many commercially available mate-

PHYSICAL FORM AND COMPOSITION

Denture base plastics are commonly supplied in a powder-liquid or a gel form. The powder-liquid type may contain the materials listed in Table 21-1.

Powder ~ o scommercialt materials contain poly(methy1 methacrylate), modified with small amounts of ethyl, butyl, or other alkyl methacrylates to produce a polymer somewhat more resistant to fracture by impact. The powder also contains an initiator such as benzoyl peroxide (see formula below) or diisobutylazonitrile to initiate the polymerization of the monomer liquid after being added to the powder.

(Courtesy The LD Caulk Co, Milford, Del, 1959.)

The peroxide initiator may be added to the polymer or be present as a residual from the polymerization reaction and is present in amounts from 0.5%to 1.5%.

Pure polymers, such as poly(methy1 methacrylate), are clear and are adaptable to a wide range of pigmentation. The pigments used to obtain the various tissuelike shades are compounds such as mercuric sulfide, cadmium sulfide, cadmium selenide, ferric oxide, or carbon black, although the use of cadmium salts is suspect because of demonstrated toxicity. These pigments may be locked into the polymer beads by addition during the commercial polymerization, or mechanically mixed with the polymer beads after polymerization, as shown in Fig. 21-1. Generally, the latter method is used, and the uneven distribution of the pigment in the final denture provides a mottled, natural appearance. Dyes are occasionally used, but generally are not as satisfactory because they tend to be leached out of the plastic by oral fluids, thereby gradually lightening the shade. In addition to coloring agents, zinc or titanium oxides are used as opacifiers, with titanium dioxide being most effective. Dyed synthetic fibers made from nylon or acrylic are usually added to simulate the minute blood vessels of oral mucosa.

Plasticizers such as dibutyl phthalate may be incorporated in the powder or the monomer. Inorganic particles such as glass fibers and beads or zirconium silicate have been added to plastics. The particles are usually treated with a coupling agent such as an unsaturated triethoxysilane to improve the wetting and bonding of the inorganic particles and the plastic. Studies have reported on the addition of whiskers of alumina, silicon carbide, boron nitride, and carbon fibers to dental plastics. Adding glass fibers and alumina (sapphire) whiskers increases the stiffness, decreases the thermal coefficient of expansion, and increases thermal diffusivity. Polyethylenewoven yarn and polyaramid fabric have also been used to reinforce acrylic polymers.

Most denture bases are transparent on x-ray examination. Pieces of fractured dentures or temporary acrylic crowns have been aspirated by patients during traumatic injury and are difficult if not impossible to locate. A few denture base materials contain heavy metal compounds of elements such as barium or radiopaque glass fillers added to improve the radiopacity. It is necessary to add up to 20% by weight of these compounds to give sufficient radiopacity, and this results in a reduction in the strength of the material and a change in the appearance of the

denture. A 3-year clinical study of a commercial radiopaque polymer, however, showed the dentures performed well and remained radiopaque. Other additives that provide radiopacity include bismuth or uranyl salts at concentrations of 10% to 15% and zirconyl dimethacrylate at 35%. Recently a new radiopaque terpolymer has been synthesized containing [2'3'5'-triodobenzyoyll- ethyl methacrylate, methyl methacrylate and 2-hydroxyethyl methacrylate. This methacrylate terpolymer may find use as a radiopaque denture base material. It has been shown that esthetically pleasing radiopaque plastics can be made that do not demonstrate cytotoxicity or mutagenicity and have reasonable properties for prosthetic applications. In the future, efforts must be made to improve the handling properties, transverse deflection, and water sorption of radiopaque denture base materials.

Liquid The liquid component of the powder-liquid type acrylic resin is methyl methacrylate, but it may be modified by the addition of other monomers. Because these monomers may be polymerized by heat, light, or traces of oxygen, inhibitors are added to give the liquid adequate shelf life. The inhibitor most commonly used to prevent premature polymerization is hydroquinone, shown below, which may be present in concentrations of 0.003% to 0.1%.

When a chemical accelerator rather than heat is used to speed up the peroxide decomposition and enable the polymerization of the monomer at room temperature, an accelerator is included in the liquid. These accelerators are tertiary amines, sulfinic acids, or the more stable salts of sulfinic acid. Commonly used amines are &N-dimethyl-para-toluidine, and 4N-dihydroxyethyl-para-toluidine.

These are referred to as self-curing, coldcuring, or autopolymerizing resins. The pour type of denture resin is included in this category.

Plasticizers are sometimes added to produce a softer, more resilient polymer. They are generally relatively low-molecular weight esters, such as dibutyl phthalate.

Plasticizer molecules do not enter the polymerization reaction but do interfere with the interaction between polymer molecules. This makes the plasticized polymer softer than the pure polymer. One disadvantage in using plasticizers is that they gradually leach out of the plastic into oral fluids, resulting in hardening of the denture base. A polymer also may be plasticized by the addition of some higher ester such as butyl or octyl methacrylate to methyl methacrylate. The esters polymerize and form a more flexible plastic. This type of internal plasticizing does not leach out in the oral fluids, and the material remains flexible.

If a cross-linked polymer is desired, organic compounds such as glycol dimethacrylate are added to the monomer.

Cross-linking compounds are characterized by reactive -CR=CH- groups at opposite ends of the molecules and serve to link long polymer molecules together. Using cross-linking agents provides greater resistance to minute surface cracking, termed crazing, and may decrease solubility and water sorption. Cross-linking materials may be present in amounts of 2% to 14%, but have little effect on the tensile strength, transverse properties, or hardness of acrylic plastics, although recovery from an indentation by a metal ball such as a Rockwell Superficial Hardness indenter is somewhat improved.

Gel Types It also is possible to supply denture base plastics such as vinyl acrylics in a gel form. These gels have, in general, the same components as the powder-liquid type, except the liquid and powder have been mixed to form a gel and shaped into a thick sheet. Chemical accelerators cannot be used in a gel, because the initiator, accelerator, and monomer would be in intimate contact. The storage temperature of a gel and the amount of inhibitor present have a pronounced effect on the shelf life of the material. When stored in a refrigerator, the shelf life is about 2 years.

OTHER DENTURE MATERIALS

Several modified poly(methy1 methacrylate) materials have been used for denture base applications. These include pour type denture resins, hydrophilic polyacrylates, high-impact strength resins, rapid heat-polymerized acrylics, and lightactivated materials.

Pour Type Denture Resin The chemical composition of the pour type denture resins is similar to poly(methy1 methacrylate) materials that are polymerized at room temperature. The principal difference is in the size of the polymer powder or beads. The pour type denture resins, commonly referred to asJuid resins, have much smaller powder particles; when mixed with monomer, the resulting slurry is very fluid. The mix is quickly poured into an agar-hydrocolloid or modified plaster mold and allowed to polymerize under pressure at 0.14 MPa. Centrifugal casting and injection molding are techniques used to inject the slurry into the mold.

High-Impact Strength Materials Denture base materials that have greater impact strength have been introduced. These polymers are reinforced with butadiene-styrene rubber. The rubber particles are grafted to methyl methacrylate to bond to the acrylic matrix. These materials are supplied in a powderliquid form and are processed in the same way as other heat-accelerated methyl methacrylate materials.

Rapid Heat-Polymerized Resins Dentists and technicians are always looking for ways to d o things quicker and better. The rapid heatpolymerized materials were introduced with that in mind. These are hybrid acrylics that are polymerized in boiling water immediately after being packed into a denture flask. The initiator is formulated from both chemicaland heat-activated initiators to allow rapid polymerization without the porosity one might expect. After placing the denture in boiling water, the water is brought back to a full boil for 20 minutes. After bench cooling to room temperature, the denture is deflasked, trimmed, and polished in the conventional manner.

Light-Activated Denture Base Resins

This denture base material consists of a urethane dimethacrylate matrix with an acrylic copolymer, microfine silica fillers, and a photoinitiator system. It is supplied in premixed sheets having a claylike consistency. The denture base material is adapted to the cast while it is still pliable. The denture base can be polymerized in a light chamber without teeth and used as a record base. The teeth are processed to the base with additional material and the anatomy is sculptured while the material is still plastic. The acrylic is polymerized in a light chamber with blue light of 400 to 500 nm. The denture rotates in the chamber to provide uniform exposure to the light source. Various formulations of light activated acrylic are used for many prosthetic applications.

ANSIIADA SPECIFICATION NO. 12

(IS0 1567)FOR DENTURE BASE RESINS

The scope, requirements, and procedures for evaluating denture base plastics are listed in ANSVADA Specification No. 12. The specification includes acrylic, vinyl, and styrene polymers, or mixtures of any of these polymers, as well as copolymers. Denture base resins may be heatcuring or self-curing.

The specification lists a number of general requirements for the non-processed materials. The liquid should be as clear as water and free of extraneous material, and the powder, plastic

cake, or precured blank should be free of impurities such as dirt and lint. The specification further states that (1) a satisfactory denture shall result when the manufacturer's instructions are followed; (2) the denture base should be nonporous and free from surface defects; (3 ) the cured plastic should take a high gloss when polished; (4) the processed denture should not be toxic to a normal, healthy person; (5) the color should be as specified; (6) the plastic should be translucent; and (7) the cured plastic should not show any bubbles or voids.

The specific requirements are that (1) within 5 minutes after reaching the proper consistency, indicated by clean separation from the walls of a glass mixing jar, the material shall have adequate flow properties so it will intrude to a depth of at least 0.5 mm into a 0.75-mm diameter hole when a load of 5000 g is placed on a plate 5 mm thick and 50 mm2 in area (this test is modified for pour type of plastics); (2) the water sorption shall not be more than 0.8 mg/cm2 after immersion for 7 days at 37' C; (3) the solubility shall not be more than 0.04 mg/cm2 after the water sorption

specimen is dried to constant weight; (4) the plastic shall show no more than a slight color change when exposed 24 hours to a specified ultraviolet lamp test; and (5) the transverse deflection shall be within the limits listed in the discussion on transverse deflection.

Properties for various types of plastics are shown in Table 21-2. Although the values are typical for each group, they can vary considerably for different commercial products.

STRENGTH PROPERTIES

Conventional heat-accelerated acrylic resins are still the predominant denture base materials in use. These materials are typically low in strength, soft and fairly flexible, brittle on impact, and fairly resistant to fatigue failure. The properties of poly(methy1 methacrylate) and polyvinyl acrylic are shown in Table 21-3. Several properties of newer denture base materials are seen in Table 21-4.

TensileandCompressiveStrength Table 21-3 reveals small differences between poly- (methyl methacrylate) and polyvinyl acrylic. The two plastics have adequate tensile and compressive strength for complete or partial denture applications. Fractures in these materials are usually caused by accidental dropping of a denture or by faulty fabrication. Fractures may also be caused by flexure fatigue from cyclic stresses of low magnitude in service.

Elongation Elongation, in combination with the ultimate strength, is an indication of the toughness of the plastic. The larger the area under the stress-strain curve, the tougher the material is. Materials having a combination of reasonable tensile strength and elongation will be tough materials, and those with low elongation will be brittle. Examples of tough materials are polyvinylchloride or polyethylene, whereas poly(methy1 methacrylate) is more brittle.

Values for percent elongation of polyvinyl acrylics are considerably higher than for poly-

(methyl methacrylate) and, as expected, the polyvinyl acrylics are tougher and permit larger deformation before fracture.

Elastic Modulus The higher strain or elastic deformation for a given stress for polyvinyl acrylic is reflected in the lower value for elastic modulus of 2.8 GPa. This value may be cornpared with that of 3.8 GPa for poly(methy1 methacrylates). A denture constructed of polyvinyl acrylic will deform elastically to a greater extent under the forces of mastication than a comparable poly(methy1 methacrylate) denture. The modulus of elasticity for several newer denture base materials is seen in Table 21-4. When cornpared with metals used as denture bases, the elastic moduli of all plastics are quite low.

Proportional Limit There is some question as to whether dental plastics possess a true proportional limit, because they may be permanently deformed at low stresses, and, as a result, the proportional limit obtained is a function of