J O U R N A L OF ESTHETIC DENTISTRY

Resin Ionomer Restorative Materials: The New Generation

J O H N B U R G E S S B A R R Y N O R L I N G

J A M E S S U M M I T T

Glass-ionomer restoratives have changed significantly since Wilson and Kent described the material as “a new translu- cent cement for dentistry.”‘ Since glass-ionomer restorative materials bond to dentin and enamel, provide long-term fluoride release, are biocompatible, tooth colored, and allow little microleakage at the restorative material-tooth interface? it is surprising that they are not used universally. Perhaps their low usage is due to the limitations of conventional glass-ionomer restorative materials. Since early materials set slowly, finishing was delayed for 24 hours after placement, requiring a separate clinical appointment.’ In addition, the finished restorations were often too opaque, and manipula- tion of these early materials was trying. The liquids of later materials were modified to contain tartaric acid,’ which pro- duced a material capable of being finished within 7 to 15 minutes.i Copolymers of itaconic and polyacrylic acid were later developed that improved the shelf life by preventing hydrogen bonding and premature crosslinking of the acid. The addition of more translucent glass materials to the pow- der enhanced the esthetics of finished restorations and fur- ther improved the material. However, even with these improvements, a lifelike appearance was difficult to obtain with glass-ionomer restorations.

CONVENTIONAL GLASS-IONOMER MATERIALS Conventional glass-ionomer restorations are technically demanding. The difficulty in manipulating these materials is related to their sensitivity to moisture imbibition during the early setting reaction and desiccation as the materials begin to harden.’ A dry field must be maintained for at least 5 minutes, since premature contact with water produces a restoration with increased solubility and poor optical charac- teristics. Because glass ionomers dehydrate, causing the restoration to crack and become more opaque, they must be protected from desiccation, usually by applying a varnish or unfilled resin to the restorative surface during and after fin- ishing. Ifa restoration is finished dry, crazing occurs.

Even after finishing, conventional glass ionomers must be protected from desiccation. The effectiveness of varnishes and unfilled resins coating glass-ionomer restorations to pre-

tium released from tritium-labeled glass-ionomer cemenr coated with varnishes, bonding resins, or proprietary prepa- rations. The light-cured resins were the most effective in reducing water loss from the glass-ionomer cements.

In general, glass ionomen consist of a powder com- posed of silica, alumina, calcium fluoride as flux, cryolite, sodium fluoride, and/or aluminum phosphate. These raw materials are heated to 1100 to 1500°C. The resulting fluoroalurninosilicate glass is ground and used as the powder in glass-ionomer restorative materials. The liquid consists of a copolymer of acrylic and itaconic acid or copolymers of maleic or tricarboxylic acid. The liquid may be freeze dried and incorporated into the powder of the glass ionomer. In this case the liquid is a dilute solution of water and tartaric acid. Tartaric acid, a chelating agent, is added to all glass- ionomer formulations.

complex and incompletely understood, but some under- standing of the reaction is necessary to properly manipulate the The setting reaction of conventional glass ionomers will be discussed briefly for comparison to resin ionomers. When the powder and liquid are mixed together, an acid-base reaction occurs. The polyacid liquid reacts with the glass ionomer powder, the base, and the COOH attached to the polyacid ionizes to COO- and H+. The H+ attacks the glass particles liberating Ca‘. , Al’. and releasing fluoride. A layer of silica gel is slowly formed on the surface of the unreacted powder with the progressive loss of almost all metallic ions. As calcium and aluminum ions difise into the liquid, a crosslinked metallic salt is formed (polyacry- late). When the polyacrylare metallic salt begins to precipi- tate, gelation begins and proceeds until the cement is hard. The calcium and aluminum ions crosslink with two or

vent loss of water was examined.‘ Earl et al measured the tri-

The setting reaction of Conventional glass ionomers is

three coo- ions located on the polyacid to form a gel. As the crosslinking increases, especially through aluminum ions, and. as the gel is hydrated, the polyacrylate salt Precipi- tates. The setting reaction then proceeds at a reduced rare producing increased translucency and strength.

tion. Fluoride, Ca’., Al’,, and the forming calcium polyacry- late are all soluble.’ Premature water contact leaches some of these components and produces a weak, opaque cement.

HYBRID “RESIN-IONOMER” MATERIALS Hybrid glass-ionomer materials or resin ionomers combine glass-ionomer and composite-resin technologies.’” ” Although it is difficult to define a “true” glass ionomer, a practical definition is a material with polyacid and an acid- base chemical cure sufficient to produce a clinically set restoration without light cure. Resin ionomers can be classi- fied according to their curing reactions on a continuum, with conventional glass ionomers at one end and composite resins at the other. The setting reaction of hybrid materials, or resin ionomers, uses multiple curing mechanisms.’o I’ ’’ In general, in these materials, the powder is fluoroaluminosili- a t e glass, and the liquid is hydroxyethyl methacrylate (HEMA), water and polyacrylic acid (or an analog), with or without pendent methacrylic groups.

The first setting reaction is typical of conventional glass ionomers and is an acid-base reaction. Protons attack the fluoroaluminosilicate glass of the powder liberating calcium and aluminum ions. These ions combine with the polymer to form an ionically crosslinked network. The photoinitiated setting reaction occurs through methacrylate groups grafted onto the polyacrylic acid chain and methacrylate groups of the HEMA and begins when the mix of powder and liquid is exposed to light. In other words, two separate setting reac- tions occur: one common to conventional glass ionomers and the other to photoinitiated resin composites. The pho- toactivation may affect the material’s final properties depending upon the strength of the glass-ionomer cure. With certain materials, a third curing mechanism occurs, consisting of a chemically initiated free radical methacrylate cure of the polymer system and HEMA. Commercially available materials produce varying degrees of polymeriza- tion by each setting mechanism (Table 1).

Glass ionomers are sensitive to early water contamina-

THE GLASS-IONOMER - HYBRID RESIN IONOMER - RESIN-COMPOSITE CONTINUUM These curing mechanisms allow materials to be classified a5 part of a continuum. At one extreme, the materials differ lit- tle from traditional glass ionomers and cure principally by the chemical acid-base setting reaction. At the other, the materials are very similar to light-cured resin composites and cure almost exclusively through light-initiated free radical

TABLE 1. Glass-lonomer Materials Classified According to Setting Mechanism -

Setting Mechanism

Polymerization /nitration Acid-Base Visible Light Chemical

Material Manufacturer

Ketac-Fil ESPOPremier w Fuji11 GC w Variglass L D. Caulk

Geristore Den-Mat m Fuji II LC GC w w I V~remer 3M w I

Photac-FiI ESPffPremier .weak

polymerization. In the middle of the con~i i i i iun i . ihc inirial phoroiniation ofthe resin is combind with the r \ p i L . i I

chemical cure that occurs with convcnrion.il gl.i\\-iononirr materials (Table 2).

Properties of the various materi.il\ v x y ni.irkcJIy according to how they are placed on rhc cont inuuni . Materials that lie on the composite-resin end of rhr conrinu- um have increased thermal expansion .ind d c c r c r d tluoridc release, while materials thar lie on thr gla\\-iononrcr end have low thermal expansion and high fluoride rclc.i\c. Some very important clinical characteristick ot rhc nurcri.Ih Jre dictated by their location on the conrintiuni. For esmple. those near the composite-resin end exhihir lirtlc o f r h c inher- ent glass-ionomer adhesion to dentin and require denrind bonding agents to obtain meaningful .idhrsion. whik rhos near the glass-ionomer end use only condirioning Jgcnts thn are rinsed off prior to placing the mixed n1rtrri.d. \.Lriglaa requires light curing after mixing to produce .I uuhle mate& al and relies primarily on the light-iniri.itcd frcc radical &- merization mechanism. Geristore and I’horrc-ti1 h.iw moderate dual-curing capabilities. However. thew dual-cu- capabilities are achieved in different W:I!.L I’hot.lr.-Fil p l y - merizes using a traditional acid-base gla\+iononier curing mechanism and a photoinitiated setring rc.icrion through

TABLE 2. The Glass-lonomer - Composite-Resin Conhum - - - -~ -

. .

I L J V

Conventional Resin-lonomer F -Releasing Glass lonorners Resins

BURGESS ET A L

HEMA and methacrylate groups grafted onto polyacrylic acid and methatrylic groups of the HEMA. Geristore poly- merizes primarily through light-initiated free radical poly- merization, which is supplemented with free radical methacrylate cure of the resin. Multiple-curing mechanisms occur with Vitremer and Fuji I I LC, which produce a poly- merized material with similar mechanical properties whether exposed to a light cure or not. These tri-curing capabilities provide a significant advantage when the materials are poly- merized with a visible light-curing unit that has less than optimum output, when greater thicknesses of the material are used, or when some of the material lies in the shadow of tooth structure o r of metallic restorative materials. It should be noted that Vitrcrrier and Fuji 11 1I: bond poorly to dentin if no light cure is applied, even though rhese materi- als achieve more than 90°/tr of their ultimate mechanical properties without light exposure. It is strongly recommend- ed that a layering technique be used with all resin-ionomer restorative materials and t h a t an initial thin layer covering the dentin surface have a separare light cure.

As Figure I demonstrates, Fuji 11 LC and Vitremer have strong glass-ionomer curing mechanisms that produce mate- rials with excellent mechanical properties without light exposure. Geristore and Photac-Fil have intermediate com- pressive strengths when no visible light cure is applied, and Variglass requires photoactivation to produce a suitable cure (Fig. 2). Resin-ionomer restorative materials have lower compressive modulus than conventional glass ionomers whether a light cure is applied or not."

Geristore can be classified as a low-fluoride-releasing dual-cured composite resin. The material is radiopaque and is 50% filled with barium fluorosilicate glass averaging 3 to 4 microns in size."

blend of two glasses: strontium and barium boron alu- According to the manufacturer,"' Variglass powder is a

minum silicate glass. These glasses provide radiopacity and fluoride release for Variglass. The liquid is polyacrylic acid, water, PENTA, and VLC active monomers. PENTA is a polymerizable phosphoric acid ester. The material can be mixed in varying powder-liquid ratios allowing one material to be used for bases, restorations, and cores. Variglass is used with the Probond primer to improve dentin bonding. Variglass is supplied as a capsule or in bulk for hand mixing in five tooth-colored shades and a blue core shade.

Vitremer, one of the true resin ionomers, has three dis- tinct curing mechanisms." This material is available only as a hand-mixed powder and liquid. According to the manufac- turer, the powder contains fluoroaluminosilicate glass, pig- ments, and the catalyst system producing the dark cure of the resin Component of the resin ionomer. The liquid con- tains the Vitrebond copolymer, HEMA, water, and photo- initiators. A light-cured primer is supplied with this system and has the same constituents as the liquid, with ethanol substituted for the water. The first two curing mechanisms involve light cure of the resin and chemical cure of the ionomer; the third curing mechanism involves chemical cur- ing of the resin component with microencapsulated potassi- um persulfate - ascorbic acid redox catalyst."

Another of the true resin ionomers is Photac-Fd, sup- plied only in capsule^.'^ The system has a 25% polyacrylic acid conditioner, a powder, and a liquid. The liquid is com- posed of monomers, oligomers, camphoroquinone and water. The powder consists of sodium calcium aluminum sil- icofluoride glass, the copolymer-of maleic and acrylic acid and an activator. Photac-Fil has a slower dark cure than Vitremer or Fuji I1 LC, and without light to activate the polymerization reaction, the material achieves only 67% of its light-cured compressive strength. An additional limita- tion of the material is its lack of radiopacity.

VOLUME 6 . N U M B E R 5 209

JOURNAL OF ESTHETIC DENTISTRY

PROPERTIES OF AND USES FOR RESIN-IONOMER HYBRIDS In addition to the properties possessed by conventional glass ionomers (bonding to tooth structure and metal, fluoride release, biocompatibility, and thermal insulation), the hybrid resin-ionomer materials have significantly greater early com- pressive and diametral tensile strengths, are more resistant to moisture and desiccation, and are easier to finish.

SURFACE FINISH. In general, resin ionomers are smoother when finished than conventional glass ionomers. In a study done in our laboratory to evaluate the effectiveness of several finishing systems to polish glass ionomers, disks of Variglass, Fuji I1 LC, and Ketac-Fil were made in split molds. The materials were covered with a polyethylene strip and allowed to polymerize. The roughness average (Ra) was measured with a Profilometer (Surfanalyzer 4000, Federal Products, Providence, RI) after the matrix strip was removed. The specimens were finished using several finishing instruments; Two Striper Diamonds (ESPE\Premier, Norristown, PA), Sof-La disks, (3M, St. Paul, MN), the Enhance system (LD Caulk, Milford, DE), and a series of three carbide burs (Brasseler, Augusta, GA). AU materials had the lowest Ra val- ues, in other words were the smoothest, when they were allowed to polymerize against the matrix strip. Sof-Lex disk and the Enhance system produced the smoothest surface on all materials compared to the diamonds and burs. All materi- als were finished using a water spray. Although resin- ionomer restorations are more water stable than conventional glass ionomers, coating the resin ionomers with an unfilled resin fills small defects, but it also decreases fluoride release and may inhibit later fluoride uptake by the resin ionomer. Therefore, the application of unfilled resin to the surface is recommended only when low caries-risk patients are being treated.

If a defect in a resin-ionomer restoration is encountered during finishing, additional material may be added, and the bond between the existing and new materials will be quite strong. The material may be repaired after the patient returns by surfacing the restoration with a sandpaper disk, etching with phosphoric acid for 20 seconds, applying an unfilled resin, placing the freshly mixed resin ionomer onto the prepared surface, and light curing. Since composite resin bonds to resin ionomers, repair may also be accomplished using composite resin.

EFFECT OF VARYING POWDER~LIQUID RATIO. Since some clinicians have recommended varying the powder/- liquid ratio to improve handling and mixing properties of resin ionomers, a study was begun to examine the flexural strengths and bond strengths of resin ionomers at various powderliquid ratios. The flexural strength ofVitreme= and

Fuji 2 LC were measured at the following powderlliquid rarios: 1.5,2.0, 2.5, 3.0, 3.5, and 4.0 mg to 1 mg of liquid. The flexural strength of Fuji I1 LC was lowest at 1.5/1, greatest at 3.5/1. (Standard powdedliquid ratio is 3.011 for Fuji II LC). The flexural strength ofVitremer was higher at 3.011 (Standard powder1liquid ratio is 3.511 of Vitremer). Bond strength of the two materials at the same powder/- liquid ratios were highest for Vitremer at 3.0. For Fuji 11 Lc, the bond strength remained constant over a wide range of powder/liquid ratios, being approximately constant from 2.0 to 3.5/1. We are now evaluating the abrasion resistance of the two materials at the same powder/liquid ratios.

SHEAR BOND STRENGTH. The shear bond strength typi- cally reported for conventional glass-ionomer cements is low, varying from 3 to 5 MPa.z”22 The failure mechanism of these materials is primarily cohesive within the materid, so that

the bond strength measure is actually a measure of the cohe- sive strength of the material. In general, the mean shear bond strength of resin-ionomer restorative materials is greater than the bond strength of conventional glass ionomers; however, this is primarily due to the increased cohesive strength of the materials and not increased adhe- sion to tooth structure. Typically, reported shear bond strength values for resin ionomers vary from 0.7 to 12 MPa depending on the material evaluatedzs28 (Fig. 3). The condi- tioner or primer supplied with a resin ionomer must be used when the materials are placed. The shear bond strength of glass ionomer is decreased when no light cure is applied and when no primer is used. The bond strength of Fuji I1 LC is shown in Figure 4 as an example. In addition to the necessiq of using a primer or conditioner, it appears that bond strengths of Photac-Fil and Variglass are decreased when bonding to deep dentin compared to superficial dentin?’

Figure 3. Shear bond strength of glass ionomer.

BURGESS ET AL

Although glass-ionomer restorative materials are hydrophilic, isolation must be used to prevent contaminat- ing the dentin surface with salvia. When the conditioned dentin surface is contaminated with saliva, the contaminated dentin should be rinsed and reconditioned, and the glass ionomer should be applied to reconditioned dentin (Fig. 5).29

COEFFICIENT OF THERMAL EXPANSION. The coefficient of thermal expansion has been cited as a significant reason for the clinical effectiveness of conventional glass-ionomer restorations. Even though the shear bond strength of glass- ionomer restorative materials does not approach that of fourth-generation dentin-bonding agents, glass-ionomer restorations placed in Class 5 preparations are very durable. Recently the thermal expansion of some glass ionomers and resin ionomers has been measured.)0, In our laboratory, cylindrical specimens were fabricated, stored for 2 months in deionized water, and placed into a Theta Dilatometer and the linear expansion measured as temperature changed from 0 to 60°C.30 The results are presented in Table 3.

Even though resin ionomers have higher coefficients of thermal expansion compared to tooth structure, their thermal expansion compares favorably to tooth structure. In general, higher thermal expansion is seen as a material moves along the continuum from a glass ionomer to a composite resin.

FLUORIDE ACTION. Many in vitro and in vivo studies have examined the fluoride release from glass ionomers'"* and the effects of this release on plaque and These investigations have reported that fluoride is released from glass-ionomer materials. The fluoride release is influ- enced by the specimen shape, method of mixing, medium, and glass-ionomer material. Fluoride release is initially very high and quickly decreases to a low long-term release.

Figure 4. Shear bond strength of glass ionomets. * Using conditioner or primer and light cure ** No conditioner or primer with light cure *** With conditioner but no light cure

Figure 5 . Shear bond strength of glass ionomer.

Fluoride release from glass ionomer has been collected in whole saliva in vivo. The fluoride has been demonstrated in bacteria and has been shown to inhibit bacteria. Although it is assumed that the fluoride released from glass-ionomer restorations will shih the demineralization - reminerializa- tion curve of tooth enamel to the reminerialization side, to date there is little clinical support for this assumption. Tyas reported the recurrent caries rate around class 5 composite resin and glass-ionomer restorations 5 years after placement, and found no significant difference in recurrent caries rates between the two restorative materials."

In our laboratory the fluoride release of materials on the glass-ionomer continuum has been measured (Fig. 6). The mean fluoride release data for Photac-Fil and Vitremer are significantly higher than the release from both Fuji I1 products. The fluoride release for Geristore and Variglass are significantly lower than the other glass ionomen. Manufacturers' recommend finishing resin ionomen and placing a varnish or unfilled resin over the finished restora- tion. Unfilled resin placed over glass-ionomer restorative materials decreases fluoride release (Figs. 7 and 8). Even though coating glass ionomers with unfilled resin decreases

TABLE 3. Thennal Expantion st 2 Months (PPmpC) ~- ~ ~ .-

Mateficll ThennrlExpanrion

FUJI 2 1 8 (0 6) Ketac-hl 4 9 (28) PhotacRI 5 6 (09) Vitrerner 134 ( I 6) FUJI II LC 16 4 (2 1) Variglass 36 7 (04) Geristore 42 9 (5 1) FluoroCore 442 (32)

( I Standard Deviation

V O L U M E 6. N I ' M R F R 5 211

fluoride release, the light-cured primer used with Vitremer does not inhibit fluoride release. Using this primer, therefore, would not inhibit the passage of fluoride to the cut deniin surface. Resin ionomers release fluoride over extended periods of time. Initially the release rate is high, but the fluoride released decreases in about 7 days to a low sustained rate.

Resin ionomers are usel l as restoratives for older and high caries-risk patients since they possesses long-term fluo- ride release. These materials are excellent for restoring the geriatric dentition and may reduce caries in salvia deficient patients. Radiation, as well as some medications and syn- dromes, reduce salivary flow producing characteristic carious lesions: cervical caries, incisal caries in anterior teeth, and rampant recurrent caries around existing restorations. Resin- ionomer restorative materials may be used to treat these patients. It has been reported that toothpaste containing fluoride will replenish the fluoride contained in glass ionomer.'" In addition, topical neutral fluoride solutions may be used to recharge fluoride depleted from glass- ionomer restorations1' (Fig. 7); therefore, high caries-risk individuals should have frequent neutral fluoride applica- tions to replenish lost fluoride. Attention should be paid to the p H of the topical fluoride. Acidulated phosphate fluo- ride solutions and other acidified fluoride preparations should be avoided since these solutions have been shown to alter the surface of conventional glass-ionomer restorative materials." To examine the surface-altering effects of topical fluoride solutions on conventional and resin ionomers, disks ofvitremer, Fuji 11, and Fuji I1 LC were fabricated in split Teflon molds. After polymerization the specimens were fin- ished with a series of abrasive disks and a profilometer was used to obtain an Ra value between two points drawn on the disks. Three days after fabrication, the disks were coated with one of three topical fluoride solutions. The p H of each solution was measured usinga p H meter (TabLe 4).

Figure 6. Mean cumulative fluoride release (ar 6 weeks).

DAY1 DAY2 DAY3 DAY7 DAY14 DAY21 0 1 3

*fUJI 2 A FUJI 2 K 'BMOIAC f l l V VIIREMER 5 GERISIORE YARIGUSS

Figure 7 . Fluoride release from uncoated specimens.

All topical fluoride solutions were applied following the manufacturers' directions. After the application, the fluoride was rinsed from the glass-ionomer disks with tap water. The disks were dried and profrlometric tracings were again obtained between the two points. The surface of all glass ionomers was rougher after the topical fluoride solutions were applied. Acid phosphate solutions produced rhe great- est increase in surface roughness, while the neutral sodium fluoride produced the least. Resin ionomers were more resii tant to surface degradation than the conventional glass ionomer Fuji 11. This supplies additional evidence that the polysalt matrix found in conventional glass ionomers is less resistant to acid attack than the polyHEMA or methacrylate matrix found in resin ionomers.

CORE MATERIALS. Resin-ionomer restorative materials are designed to be used as Class 3 and 5 restoratives and as fom- dations for crowns. Materials with strong dual- or tri-curing capabilities are recommended for this use. Since resin ionomers are, in part, water-based restorative materials, the

R U R C . F S \ t I' A l

dimensional stability and fracture resistance of foundations made with these materials was investigated. Several classes of materials are used to replace missing coronal tooth structure prior to crown preparation. Traditionally, amalgam and cast posts and cores have been the restorative materials of choice. However, in recent years, composite resin and glass ionomer have gained popularity as core materials. The dimensional stability and microleakage of foundations made from an amalgam alloy (Tytin), a composite resin (FluoroCore), a cermet glass ionomer (Ketac-Silver), and a resin-ionomer glass ionomer (Vitremer) were omp pa red.^' The occlusal sur- face of 75 freshly extracted molars was reduced to a flat sur- face. Four Minim pins were placed in each tooth. The core materials were mixed and inserted into copper bands placed over the teeth. Each tooth and foundation was prepared on a lathe with a deep chamfer margin at the base of the core. Rexillium 111 crowns were fabricated. The marginal opening of the crowns at predetermined points on the buccal and lin- gual surface was measured before and after water immersion to determine the dimensional stability of the foundation material. Amalgam was statistically more dimensionally sta- ble than all other materials. Composite resin was statistically less stable than all other core materials. The clinical signifi- cance of this is that if the provisional restoration covering a large core is lost, using a composite-resin core material may result in a casting with an unacceptable fir. Dimensional sta- bility of materials also allows them to be classified on the glass-ionomer - composite-resin continuum. Composite resin has significantly less dimensional stability than conven- tional glass ionomers; resin ionomers are intermediate.

Newly fabricated cast crowns were cemented on the cores with zinc phosphate cement and the specimens ther- macycled for 6000 cycles between 6" and 60°C. The samples were placed in methylene blue dye, sectioned, and examined for microleakage. Amalgam and Vitremer without a dentinal primer had significantly greater leakage than the FluoroCore, Vitremer with primer, and Ketac-Silver cores. Resin-ionomer, glass-ionomer, and composite-resin founda- tion materials should be used with primers andlor dentin bonding agents.

The fracture resistance of foundation materials were evaluated."""Teeth were sectioned at the CEJ, four pins

TABLE 4 Topical Fluoride Solutions, pH and Classification.

Topical

F h d e Classlflcatlon pH Manufacturer

Gel Kam Stannous Fluoride 4 42 Colgate. Canton M A PreviDent Neutral Sodium Fluorlde 6 54 Colgate. Canton. MA Mlnl1 Gel Acid Phosphate Fluoride 3 95 Oral-B. Redwood. CA

- ~-

inserted, matrices placed, and one of eight materials inserted into the bands: Tyrin amalgam, Ketac-Silver, Fluorocore, Variglass, Fuji 2 LC, Vitremer, and Ti-Core (Essential Dental Systems, Hackensack, NJ).

ture that positioned it at a 45-degree angle. A 1-mm bevel was prepared on the line angle at the junction of the occlusal and the facial surfaces of the amalgam. Each specimen was tested in an Instron Testing Machine and loaded in compres- sion to failure. The mean load data was recorded and is shown in Figure 1 1. The analysis divided the fracture resis- tance data into four different groups. Ketac-Silver founda- tions were the weakest and statistically different from all other groups. Tytin Foundations were the second weakest core and statistically separate from the other groups. There was no difference between the composite resin cores Fluorocore, Ti-Core, and Variglass, or the resin ionomer Fuji 2 LC. Vitremer separated into two statistically different groups; those without pins were weaker than those with pins. However, the Vitremer foundations were significantly stronger than all other groups.

Seven days later, each specimen was inserted into a fix-

CONCLUSION We have attempted to demonstrate the broad range of prop- erties encompassed by the resin ionomers. These materials are not just another category of material somewhat resem- bling both glass ionomers and composite resins. Instead, they are a spectrum spanning the gulf between the two tradi- tional materials. Depending upon the application, a more resin-like material may be the better choice: for example, when restoring areas exposed to occlusal stress. Regardless of their differences, the resin ionomers share one common strengrh: esthetic translucency far superior to their glass- ionomer progenitors. This improvement in translucency and fluoride release makes them strong contenders for material of choice for the restoration of Class 3 and Class 5 lesions.

I ;

D A Y 1 D A Y 2 DAY 3 DAY4 DAYS DAY6 DAY7 DAY8 DAY14 DAY21 DAY28 1 " . A I v full 2 fUll2 LC FMlACfII VlRtMtR G~KISlOKt VARIGLdSI

Figure 9. Fluoride release from specimens depleted of tluoride and placed into sodium tluoride.

IOURNAL OF ESTHETIC DENTISTRY

Figure 11. Fracture resistance of pin-retained foundations. Figure 10. Marginal openings of castings after water immersion of the cores.

*No pin retention

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