Composite resins in the 21st century

Adhesives Symposium

Composite resins in the 21st century3. Willems* / P. Lambrechts* / M. Braem** / G. Vanherle*

Human enamel and dentin should be used as the physiologic standards with which tocompare composite resins, espeeially in the posterior region. The intrinsic surface rough-ness of composite resins must be equal to or lower than the surface roughness of humanenamel on enamel-to-enamel occlusa! cantad areas (Ra = 0.64 ¡un). Roughness deter-mines the biologic strength of composite resins. The nanoindentation hardness value ofthe filler particles {2.91 to 8,84 GPa) must not be higher than that of the hydroxyapa-tite crystals of human enatnel (3.39 GPa). Composite resins intended for posterior useshould have a Young's modulus at ¡east equal to, and preferably higher than, that ofdentin (18.500 MPa). The compressive strength of enamel (384 MPa) and dentin(297 MPa) and ihe fracture strength of a natural tooth (molar = 305 MPa; premolar= 248 MPa) offer excellent mechanical standards to select the optimal strength forposterior composite resins. The in vivo occlusal contact area wear rate of composite res-ins must be comparable to the attritional enamel wear rate (about 39 ¡imiy) in molars.Differential wear between enamel and cotnposite resin on the same tooth is a new crite-rion for visualizing and quantifying the wear resistanee of eomposite resins in a biologictvay. Posterior resins must have a radiographie opacity that is slightly in excess of thatof human enamel (198% Al). Based on these standard criteria, it can be concluded thatin the 21st century the ultrafine compact-filled composite resins may be the materials ofchoice for restoring posterior cavities. (Quintessence Int 1993;24:64i-65S.)

Introduction

The increased need for esthetic tooth-colored restor-ative materials has brought a confusing variety of:omposite resms onto the dental market. However,because it is natural tooth tissues that are being re-placed, it should be detennined whether these restor-

Departmeiit of Operative Dentistry and Dental Materials, Calh-oiic University of Lcuven, U.Z. St. Rafaël, Kapucijnenvoer 7,B-3000 Leuven, Belgium.Dental Propedeutics, University Center .A.ntwerp, Groenenbor-gcrtaan t71, B-2020 Antwerp, Belgiuni.

ative materials are acceptable substitutes for humannatural tooth tissues. The ongoing search for a bio-logically acceptable material that not only has phys-icomechanical properties similar to those of naturaltooth tissues, but also is economically and manipu-latively equivalent to amalgam, has dramatically in-creased the number of dental composite resins avail-able. The confusion and inadequate training of dentalpractitioners has caused misuse and abuse of com-posite resins (Fig 1).

To keep track of recently developed materials atidto categorize others, several classifications have beenproposed. The American Dental Association de-scribed two categories of direct filling resins in spec-ification No. 27.' More elaborate rankings are basedon the specific filler-size distribution and amount ofincorporated filler^^ as well as on filler appearanceand composifion.' Eiller content and size have been

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Fig la Misuse and abuse ot composite resin in the an-terior region. Composite resin can cause interproximai andcervical disasters it placement is careless.

Fig tb Poor use of composite resin materials in a ve-neering procedure. The interproximai areas are inacces-sible and orai hygiene is impossible.

shown to directly determine the physical and me-chanical properties of composite resin materials,^' ofwhich the dynamic Young's modulus,'"" surfacehardness,'- and intrinsic surface roughness" seem tobe the most clinically relevant for their mechanicalperformance.

In addition, adequate radiopacity is a property thatis of prime importance for radiographie diagnosis invivo. Particularly in the posterior region, bitewing ra-diographs are systematically taken to diagnose caries.Simultaneously, voids, marginal contours, and over-hangs need to be assessed because of their detrimentalperiodontal effects.'''"'*

Each property investigated should be comparedwith some kind of standard, a measure serving as abasis to which other products should conform. Ac-ceptance standards for posterior composite resins arebeing sought by dental materials groups.'^ It is evidentthat human enamel and dentin, the two major com-ponents of human natural tooth tissues, should beused as physiologic standards with which to comparethe different composite resins. A material designed torestore lost tooth tissues should indeed have proper-ties that are the same as or comparable to propertiesof the tooth substance it must replace. Composite res-ins must have an optimal combination of physical andmechanical properties to meet this criterion.

The aim of this investigation was to realize a clas-sification of most available dental composite resins tomake predictions for the composite resins ofthe 21stcentury. The present material characterization is

based on inherent properties that are being proposedas standard criteria for the selection of potential pos-terior composite resins. These criteria are intrinsic sur-face roughness. Young's modulus of elasticity, Vick-ers hardness number, hardness and Young's modulusof filler particles by nanoindentation, mean particlesize, particle size distribution, and radiographie opac-ity. The compressive strength of enamel and dentinand the fracture strength of a tooth are also directiveparameters for the selection of posterior compositeresins.

The goal in restorative dentistry should be to makehighly wear-resistant occluding surfaces that do notcause wear on the opposing structures. The mass ofdata generated about the various competing prodtictsis very confusing, and ranking the different materialsaccording to their laboratory results does not neces-sarily refiect their ciinical performance.-" Therefore,long-term quantitative in vivo wear measurements arestill essential to determine the clinical performance ofnew posterior composite resin materials. Traditionalcomposite resins, for example, have absolutely shownto be not wear resistant, and they suffer from me-chanical and chemical degradation (Fig 2). If poste-rior composite resins are misused or abused, resto-rative disasters can be expected (Fig 3). Therefore, aclear indication, classification, and training for den-tists are required.Thedurability and behavior of com-posite resins iu posterior teeth have been chnicallyevaluated using US Public Health Servicecriteria.^' -" In this study, the 3-year wear results of

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Fig 2a Occlusal wear ot tradiiional composite resinstwo maxillary premolars after 7 years.

Fig 2b In the long run, margins are exposed, marginaileakage occurs, and recurrent caries becomes evident.

-ig 3a Occlusai view of unrestricted use of posterior com-aosite resih in the premolar-molar region, interproximalsplinting causes discomfort and marginal leakage becomesavid ent.

Fig 3b Buccal view of the iatrogenic interproximal dam-age.

Ive posterior composite resins on occlusal contactireas (OCA) and on contact-free oeelusal areasCFOA) were determined quantitatively witb an ac-:urate tbree-dimensional measuring tecbnique."

Method and materials

'^article size distribution and mean particle size

^ computer-con trolled particle size analyzer (Coulter-S Series 100, Coulter Electronics), with laser-dif-raction technology (laser ligbt of 750 nm), was usedD determine particle size distribution and mean par-cle size of all 89 investigated composite resins listed1 Table 1.

Young's modulus and perceniage volumetric fillercontent

For some of tbe test materials, the dynamic Young'smodulus was determined nondestrnetively with theGrindosonic instrument (Leinmens Electronics) byBraem'" and Braem et al." The present study used tbesame specimen preparation technique and measttringequipment to detennine tbe Young's modulus formore recent materials. Braem"' has proposed a phén-oménologie model for dental composite resins, givenin equation 1:

E = 3103.33 (1)

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644 Quintessence International Voiume 24, Number 9/1993

Adhesives Symposium

vhere E is the Young's modulus and X is tlic pcr-:entage volumetric filler content. Inorganic filler vol-ime percentages were derived from equation 1 for the•xperimentally obtained Young's moduli.

'niriiisic surface roughness

With a computerized roughness tester (Form Talysurf10, Rank Taylor Hobson), the average roughness (Ra)jf composite retiin surfaces was measured after aoothbrush-dentifrice abrasion test, extensively de-;cribed in the literature.'- The average surface rough-less at enamel-to-enamei occlusal contact areas on•eplicas was determined with the same technique to;et up a standard for comparison.

Table 2 Composite resins used in wear testing

Product

Exp.LFMarathonP-30P-30 APCP-50 APC

Type

UMCFCCUCCUCCUCC

Vol%

5568707511

AV

1.07.93.53.51.5

n

10131388

UMC = ultrafine midway-filled composiLe resin: FCC = fine com-paet-filled composite resin; UCC = ultrafine com pa et-fi lied com-posite resin: Vol% = inorganic filler volume percent; AV = averagesize of filler particles; n = number of restorations plaeed.

Vickers liardiie.s.'i

\ Vickers diamond pyramid indenter test (Durimetnicrohardness tester, Leitz), was used for determininghe microhardness of composite rebins. The procedure:overed the requirements of the standard test methodor microhardness of materials."''

Radiograpiiic opacity

rhe radiographie opaeity of 55 anterior and posterior:omposite resins was evaluated using a 99.5% pureilumimim step-wedge as a reference according to theechnique of Hein et al.-' The data were comparedvith the values for enamel and dentin.

Compres.sive .slrenglh and fracture strength

rhe compressive strength values for all materials wereaken from the literature and from manufacturers'nformation. The compressive strength values forinamel and dentin are from Craig,'' and the fracturetrength values of premolars and molars are from

Vear

Mfty-two Class II restorations were placed in man-ibular and maxillary first and second permanent mo-irs; extremely large restorations were avoided.-' Fiveifferent types of light-curing composite resins werelaced. The resins are listed in Table 2 together with

the numbers of restorations placed. The materials in-vestigated were an ultrafine midway-filled compositeresin, three ultrafine eompact-filled composite resins,and one fine compact-filled composite resin.'"-"

The wear of composite resin surfaces at the occlusalcontact areas and contact-free occlusal areas wasquantitatively measured with a three-dimensionalmeasuring technique after 6 months, 1, 2, and 3years.-* The OCA and CFOA wear values were thenaveraged at each recall session to calculate the generalwear of each composite resin.

The data were subjected to an analysis of variance(ANOVA) that used the general linear-model proce-dure of the SAS statistieal software package (SASInstitute). An ANOVA procedure {P < .01) and aSchefte's test {P < .01) for the dependent variablewere perfomied to detennine significant differencesbetween restorative materials at eaeh recall session. Atrend analysis was possible despite the small numherof restorations: the careful preseleetion of teeth andpatients and the high-precision measuring techniquecompensated for the small size of the study.^'

Nanoindeniation hardness of filier partieles

The Nano Indenter (Nano Instruments) was used tomake indentation depths of less than 1 |jm into thefiller particles of 13 posterior composite resins, threedental computer-aided designing/maehining porce-lains, sintered porcelain, and amalgam." These valueswere compared with the nanoindentation hardness ofhuman enamel. The recently developed nanoinden-tation technique was used for this purpose. Such

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Table 3 Densified composite resins: Midway-filled

Product

Ultrafine

Command Ultrafine*Herculite condensableHercuhte XRCharismaConquest DFCBiogloss*Brilliant*Brilliant Direct InlayBrilliant Lux*Brilliant DentinLumifor*PekafillPost Cotnp II LCPrisma APH

Fine

Ful-Fil compulesFul-Fil*Pertac HybridGem-CCIGem-Lite IPrisma-Fil CompulesPrisma-Fil*Superlux Molar

MPS

1.10.91.00.70.83.42.82.22.13.62.01.72.73.1

6.06.04.27.69.38.47.35.5

Mo

0.90.80.8—

—3.93.12.01.8

10.01.81.22.83.5

8.18.18.1

11.212.412.410.04.3

Ra

0.210.120.12—

—0.34—

0.200.190.110.21——

0.29

0.390.50

„

0.940.460.680.60—

Y-Mod

14.80314.87116.04214.06017.51115.19016.58617.17614.45115.82113.20815.51416.07913.644

13.84214.46515.06218.487

—

13.36214.25113.984

Vol%^

52.552.655.250.758.153.356.357.551.754.748.654.055.349.7

50.251.753.159.9—

49.051.250.6

Voiy«"

49.958.057.059.468.551.953.956.549.855.254.855.270.0—

52.352.861.061.864.451.052,060.0

CS

344414397417483350328342330377——

345383

372—

450——.353—

343

HV

876574819573

—85708484849777

8797

126117144

798390

RO

——227—

———

223232231217173231269

255————

253272

—

MPS = mean particle size (jim); Mo = mode of the particle size distribution (fj.ni); Ra = intrinsic surface rougtiness ([im); Y-Mod =Young's modulus of elasticity (MPa); Vol%* = inorganic filler volume percentage, as calculated according to Braem'"; Vol%^ = inorganicfiller volume percentage, as obtained from the manufacturers; CS = compressive strength (MPa); HV = Vickers hardness (kg/mm'); RO =radiographie opacity, taken from Willems et al. *' Young's modulus data taken from Braem.'°

depth-sensing instruments, unlike conventional mi-croindentation techniques, do not require visualiza-tion of the indentation left after the test. Indeed, loadand depth of indentation arc monitored on-line dur-ing a loading-unloading sequence, and nanohardnessis then calculated from the load-displacement curves,taking into account the geometry of the indenter. Thedata were subjected to ANOVA. and a Scheffe's testfor the independent variable was conducted to deter-mine significant differences between the dental res-torative materials.

Results

The results of the different investigated properties arelisted in Tables 3 through 8. Because the amount ofgenerated data was quite extensive, the 89 compositeresins were divided into five categories: densified com-posite resins, microfine composite resins, miscella-neous eomposite resins, traditional composite resins,and ftber-reinforced composite resins. This ranking isbased on transttion areas irt criteria such as Young'smodulus, inorganic filler volume percent, mean par-ticle size, intrinsic surface roughness, surface hard-ness, and compressive strength. This new classifica-

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''able 4 Densified composite resins: Compact-filled

'roduct

Jhrafine

Adaptic IIP-30*P-30 APCP-50 APCPalfique Lite PosteriorValuxZIOO

'^ine

Bis-Fil 1Bis-Fil PClearfil Photo PosteriorClearfil RayClearfil Ray PosteriorEstilux Hybrid VSEstilux Posterior CVSGraftMarathonOcclusin*OpaluxP-10 (Paste A-B)*Photo Clearfil AVisio Molar Radiopaque

MPS

3.22.83.02.13.62.41.0

8.2—

7.58.08.28.89.66.77.98.68.9

5.1-3.67.97.5

Mo

3.97.32.51.25.92.30.8

10.0—

10.010.012.412.412.48.1

10.012.412.4

10.0-3.910.010.0

Ra

0.480.670.710.48—

0.270.27

0.80—

0.531.180.650.901.481.261.070.990.791.021.171.08

Y-Mod

21.87623.38523.15525.00726.29319.72821.030

19.99120.21925.34327.38426.43522.59424.50821.71920.34323.77422.33625.11725.22526.754

Vol%^

65.667.867.570.171.862.164.3

62.663.070.573.172.066.769.465.463.268.466.370.270.472.4

Vol%fl

66.569.969.077.073.0—

71.0

74.074.071.069.071.068.468.4—

68.069.0—

69.169.068.4

CS

380340394395421430448

345339408350408345345332299348111390350400

HV

106107157159143107120

102114159—174130133128100125111174166186

RO

238228217277—

<40 '—

195195189.—174133126164245322153—65

220

ee Table Í for an explanalion of the abbrevYoune's modulus data taken from Braem.'

ion is presented visually in Figs 4 and 5 and will beurther reviewed. Table 9 lists the results of the in-'estigated properties of human enamel and dentin.fhese values can serve as a basis for comparison forhe investigated dental materials.

"^article size distribution and mean particle size

"he mean particle sizes for the studied composite res-ns are given in Tables 3 through 8 together with thenode of filler distribution. This is the particle size/hose frequency of occurrence is greatest."

Young's modulus and percentage volumetric fillercontent

The Young's modulus of elasticity values refiect thestiffness of the different composite resin construc-tions. For the volumetric filler content, the theoreticalvalues are listed next to the volumetric filler percent-ages compiled from the literature and manufacturer'sinformation.'"

Intrinsic surface roughness

The intrinsic surface roughness of the composite res-ins can be compared to the average roughness of

iuintessence International Voiume 24, f^umber 9/t993 647

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648 Quintessence International Volume 24, Number 9/1993

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Densified composite resins

Miscellaneous composite resins

V ' * ' í, ' '

ppl Sintered agglomerates Spherical ppf

Homogeneous Heterogeneous

Microtine composite resins

Traditional composite resins Fiber-reinforced composite resins

Fig 4 Categorization of composite resins.

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-able 8

roduet

.estolux

Fiber-reinforced

SP-4

composite resin

MPS Mo

11.2 15.3

Ra

1.19

Y-Mod

23.839

Vol%'^

68.5

VolVû"

—

CS

—

HV

125

RO

163

i

:e Table 6 for an explanation of the abbreviations.

ig 5 Three-dimensional graphepresenting the data obtained on'oung's modulus, surlaee rough-less, and Vickers hardness. Com-ilete data sets were available for¡1 eomposite resins. (MIC) Micro-ne eomposite resins; (UMF) ui-rafine midway-filled compositeeslns; (FWF¡ fine midway-filledlomposite resins; (UCF) ultrafineompact-filled composite resins;FCF) tine compact-filled compositeesins; (MiS) misceiianeous com-)osite resins; (TRA) traditionalcomposite resins; (FRC) fiber-rein-orced composite resin.

Classification of oomposite resins

ii MICUMFFtulFUCF

ÍÍ . FCF4. MIS• :TRAa: FRC

t9733 12717

Young's moaulus (MPa)

'^able 9 Mean Vickers hardness values (VIIN) ra-iiograpbic opacity (RO) and compressive strengthCS) of human enamel and dentin

inamel)entin

VHN

Mean*

40860

n— 22 indentations.

SD

334

RO

Mean

198107

SD

10.7.

61

CS (MPa)

384297

namel-to-enamel occlusal contacts, 0.64 tm. Someomposite resins showed a significantly rougher sur-ace than that of enamel contact areas.

'ickers hardness

"he Vickers hardness values of the composite resins

can be compared to the Vickers hardness numbers ofenamel (408 kg/mm') and dentin (60 kg/mm^).

Radiographie opacity

The radiographie opacity values^^ of the resins shouldbe cotnpared to those of enamel (198) and dentin(107).

Compressive strength and fracture strength

The compressive .strength values can be compared tothe compressive strength values of cnarnci and dentin(Table 9), as well as fracture strength values of pre-molars and molars.

Wear

The average wear and standard deviations on OCAsand on CFOAs were measured and are shown in

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242(107)

50 100 150 200

Vertical OCA wear (jim)

Fig 6 Mean (SD¡ wear at (OCA|occlusai contact areas of fiveposterior composite resin mate-riais. Marathon differed significant-ly (P < .01] from P-30 APC, P-50APC, and P-30 at each recail, asindicated with an asterisk. A verti-cal bar indicates other significantdifferences (P < .Ot) betweenExp.LF and the ultrafine compact-filied composite resins.

101(72)

t5l(lO9)

50 100 150 200

Vertical OCA wear (um)

Fig 7 Mean (SD) wear at (CFOAjcontact-free occiusai areas of fiveposterior composite resin mate-rials. Exp.LF diftered significantly(P < .01) from all other materiais,as indicated with an asterisk.

Figs 6 and 7, respectively. The ultrafuic compact-filledcomposite resitis P-30, P-30 APC, and P-50 APCshowed OCA wear rates ranging frotn 110 to 149 j mafter 3 years of clinical service. The ultrafine mîdway-filled composite resin Exp.LF and the fine compact-filled composite resin Marathon had high OCA wearrates of 215 jim and 242 [im, respectively, after 3 yearsof in vivo function. Exp.LF also showed an excep-tionally high CFOA wear value of 151 pm after 3years. P-30, P-30 APC, and P-50 APC had approxi-

mately the same OCA wear value, and all three ofthem showed significant differences in OCA wearcompared with Marathon, whereas Exp.LF showedno difference with Marathon during the 3 years ofthis study. At the 6-month recall, Exp.LF differedsignificantly from P-30 APC, at 1 year from P-30,P-30 APC, and P-50 APC, and at 2 and 3 years fromboth P-30 APC and P-50 APC. The CFOA wear wasapproximately the same for all products, except forExp.LF, which showed a significantly higher material

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loss at eaeb recall session compared with the otherfour resins.

Nanoitidentation hardness of filler particles

Nanoindentation hardness values ranged from 0.16GPa for prepolymerized resin filler up to 8.84 GPafor quartz particles (Table 10). Of the dental materialstested, only five materials contained filler particleswith a nanobardness not statistically different fromthat of enamel. Tbe predominant filler particles in allother materials, except for amalgam and prepoly-merized resin filters, were found to be statisticallyharder."

Discnssion

Particle size distribution and mean particle size

Tbe criteria used in this study to describe tbe com-mercially available composite resins were carefully se-lected. Eirst. tbe inorganic filler fraction in volumepercent and the mean particle size are criteria onwhich most classifications are based.-"'•"' One advan-tage of the present study is that tbe mathematicalcalculation used to determine tbe particle size distri-bution was done using the same algorithm inberentto tbe particle sizing apparatus, so the acquired in-dividual data ean accurately be compared witb oneanotber.

You?-ig's modulus and percentage volumetric filler

content

An elaborate categorization, bowever, should bebased on additional criteria to obtain a more accuratedescription of tbe material. Young's modulus of elas-ticity is a very sensitive parameter for evaluating andranking particle-reinforced composite resins.'"" In-deed, a material with a low modulus will deform morennder masticatory stresses, particularly in posteriorregions, resulting in catastrophic failures.''-" Tbe mostappropriate modulus of elasticity for a composite res-in would be one comparable to that of dentin (18.500MPa) and preferably bigber. ** Tbe spread witbin theso-marketed '"posterior composite resin" group is verylarge, but it seems tbat tbe 21,000 MPa tbat is ob-tained witb bigbly filled materials gives the compositeresins enough rigidity to be minimally deformed underphysiologic occlusal forces. Resins witb a low mod-tilus must be banned because of the sneaking dangerof material fatigue.

Table 10 Nanoindentation hardness

Substrates GPa

Visiomolar RadioqaqueVita Mark IIVita Mark IVita VMK 6NEstilux Posterior CVSP-50MarathonP-10Restolux SP-4Bis-Fil PClearfil Ray PosteriorAdaptic IIPost Conip IIENAMEL HydroxyapatiteEnl Eil CompulesDicor MGCP-30AmalgamDENTIN IntertubularPrepolymerized Resin Filler

6.555.505,235.225.084.924.424.304.284.204.043.953.393.183.142.911.490.500.16

Additionally, the exponential dependence ofYoung's modulus of elasticity on tbe volumetric fillerfraction offers an opportunity to check the volumetricfiller percentages obtained from manufacturers.

Intrinsic surface roughness

Anotber criterion to consider is tbe intrinsic surfaceroughness of a material. This surface condition isachieved after extensive toothbrush-dentifrice abra-sion of the composite resin and is an inberent char-acteristic of the material tested that determines itschnical bebavior. such as surfaee gloss, staining, andfriction. An enamel surface roughness value at enam-el-to-enamel occlusal contact areas of 0.64 + 0.25 jimis considered to be the standard witb which to com-pare the roughness values of composite resins." Be-cause resins with a high roughness value can causecatastrophic wear on tbe opposing enamel, the intrin-sic roughness of posterior composite resins must beequal to or lower tban tbe intrinsic enamel roughnessat occlusal contact areas. Apart from roughness, of

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course, the hardness of the filler particles is of tre-mendous importance.

Vickers hardness

The relative importance of a Vickers hardness lest liesin the fact that it throws light on the mechanical prop-erties of the materials investigated. This is true be-cause of the relation that exists between hardness andother physical properties." The Vickers hardness val-ues presented in this study were obtained after com-plete postpolymerization of the samples; this pro-dneed somewhat higher hardness values. ^

Radiographic opacity

According to Tveit and Espelid,-'" the optimal radi-opacity required for clinical use of composite resinsin Class II preparations should be slightly greater thanthat of enamel. Of the presently tested materials,30.9% satisfy this criterion. Several resins presentlyavailable for posterior use display a radiopacitygreater than that of enamel. However, a relativelylarge number of the resins intended for posterior usewere found to lack the radiopacity necessary for thisapplication.

In the literature, relative radiopacity values of ap-proximately 198 to 220 %A1 are given for enam-g] 27.'W.4i \Y¡j|j composite resins, this level of radiopac-ity is attained through the incorporation of elementswith a high atomic number into the inorganic fillerphase. Presently, barium is one of the most commonlyused elements for enhancing the radiopacity of com-posite resins. It has a higher radiopacity than otherelements used for this purpose, such as strontium,zirconium, zinc, ytterbium, and lanthanum. Theamount of included glasses with incorporated highatomic number atoms is restricted, because this affectsother properties of the resin, such as translucency. Ifthere is a mismatch in the refractive Index betweenfiller and resin, the composite resin will be clinicallyopaque. Incorporating large percentages of radio-paque fillers can also lead to chemical disintegrationby hydrolysis of the silane bond between filler andresin'' '" and can cause loss of dimensional stability inthe form of substantial localized wear in occlusal con-tact areas.

Compressive strength arid fracture strength

Finally, compressive strength values obtained from

the literature are given. The compressive •itr -iigth, inparticular, indicates the ability of a material to with-stand vertical stresses; this ability is vital in areas ofhigh stress. The compressive strength of enamel (384MPa), dentin (297 MPa), and the fracture strength ofa natural tooth {molar = 305 MPa; premolar = 248MPa) offer an excellent mechanical standard to selectthe optimal strength for posterior composite resins.

Composite classification

All composite resins were ranked in a classificationcontaining five categories (see Figs 4 and 5).

Composite resins intended for posterior use shouldhave a Young's modulus of elasticity at least equal tothat of denlin'^ and preferably higher. Dentin has aYoung's modulus of elasticity of 18.500 MPa.'- Thiscorresponds to an imaginary volume percentage of60%, according to equation 1.'" Therefore, the groupof densified composite resins was subdivided into twoclasses, midway-filled {see Table 3) and compact-filled(see Table 4), according to the amount of inorganicfiller content (< 60 vol% and > 60 vol%, respec-tively). Each class was further divided into two suh-classes, as a function oi the mean particle size (MPS)of the composite resins: ultrafine (MPS < 3 \m\) atidfine {MPS > 3 [am).''"* Indeed, according to Lein-felder,* the wear rate of composite resins can be sub-stantially reduced by decreasing the filler particle sizeand by increasing the inorganic filler fraction. Higherfiller levels result in increased stiffness, hardness, andcompressive strength.'''

The heterogeneous microfine composite resins {seeTable 5} and the miscellaneous composite resins (seeTable 6) were subdivided according to the type ofprepolymerized or agglomerated fillers used. Tables 7and 8 give the results for the traditional compositeresins and the fiber-reinforced composite resins, re-spectively. In addition, the intrinsic surface roughnessof resins should be less than or equal to that of en-amel-to-enamel occlusal contact areas, ie, 0.64 + 0.25|im.'' Table 9 also gives the obtained radiopacity valueand the average Vickers hardness of human enameland dentin that correlates with literature data.'-''''""Restorative materials can be compared with thesestandards.

The ultrafine compact-filled composite resins arevery promising materials for posterior use when all ofthe above is taken into account. They have a Young'smodulus of elasticity that is higher than that of den-tin,'"* and they have a high amount of inorganic par-

654 Quintessence Irternationai Volume 24, Number 9/1993

Adhesives Symposium

tieles. They also display good Vickers hardneses val-ues compared to dentin and relatively high compres-sive strength values; these suggest that they are ableto support occlusal stresses. Furthermore, their sur-face roughness ranges from 0.48 to 0.71 jxm, valuesvery similar to that of enamel." The ultrafine com-pact-fiUed composite resins are thns the materials ofchoice for restoring posterior cavities.

The ultrafme midway-filled composite resins couldhe very satisfactory materials for the restoration ofanterior teeth. They have a Young's modulus of elas-ticity that is high enough to resist the functionalstresses that are less important in anterior restorationswhen normal occlusion is considered.'-'"*" Their rel-atively low intrinsic roughness is due to their verysmall particle size (MPS < 3 |am) and makes themvery suitable for anterior use. They should be used inlarge Class III and Class V restorations and in ClassIV cavities.

The highly poHshable microfine composite resinsare frequently used in small Class III and Class Vrestorations beeause of their glossy appearance.^'However, they are not indicated for larger cavitiesbeeause of their low modulus of elasticity and highchipping sensitivity." No matter what type of pre-polymerate is used, they all have a very low inorganicfiller eontent. which makes them more susceptible toücclusal deformation, thus yielding high failure ratesin occlusal cavities and large Classes III, IV, and Vrestorations.^" Microfine composite resins withspherical prepolymerized fillers have been found insome experimental produets.'" Homogeneous miero-fine composite resins are not commercially availablebecause of manufacturing difficulties. Indeed, insert-ing a large amount of finely divided colloidal silica of40-nm average particle size {Aerosil, Degussa) con-siderably increases the material's viscosity and jeo-pardizes its handling eh ara cíe ris lies." It should benoted that the MPS value of 0.04 )im listed in Table5 for the microfine composite resins represents a com-monly accepted mean particle size of the finely dividedeolloidal silica. Slight variations in the size of thesesilica particles do exist, ranging from 0.007 to 0.115[im.-* However, the Coulter LS Series 100 only meas-ured the mean particle size of prepolymerized resinfillers and sintered agglomerates; this is representedby the PPF value in Table 5.

Both fine midway-filied and fine com pact-filledsubdivisions contain resins with an appreciably higherparticle size (6 to 10 tm), resulting in a decreasedpolishabihly. As stated above, bigger particles also

tend to increase the wear rate of the materials. Finemidway-filied and fine compact-filled composite res-ins are, therefore, less suitable for anterior or poste-rior use, respectively.

The traditional composite resins are older materialsand are no longer promoted. Traditional compositeresins very often contain large quartz particles thatare very hard. Exposure of these filler particles be-cause of resin matrix wear results in a higher surfaceroughness and a dull appearance chnicaliy. In addi-tion, quartz lacks the radiopacity required for pos-terior restorations.'•* Most of these resins have there-fore been replaced by products with an improved fillerconeept.

Miscellaneous composite resins are materials thatcontain a blend of prepolymerized and inorganie fill-ers. The latter are sometimes intentionally added toincrease the radiopacity of these resins. Trying to rankthese materials is very difficult because of their het-erogeneous composition. Pekalux is a typical miscel-laneous composite resin with spherieal prepolymer-ized fillers.

The last subdivision contains the fiber-re in forcedcomposite resins. Restolux SP-4 was the only productinvestigated in this group. This composite resin iscomposed of glass-ceramic fibers of a maximal lengthof approximately 3Û0 |im. Restolux SP-4 has a verysatisfactory Young's modulus of elasticity and couldperfonn well in posterior restorations. However, itsrather high intrinsic roughness and relatively hardglass-ceramie fillers" probably would generate wearvalues significantly higher than the enamel stan-dard.'"'^ This makes the material less suitable as anamalgam substitute.

Data on Young's modulus of elasticity, Vickershardness, and surface roughness were assembled for62 composite resins (see Fig 5). These 62 materials,for which data for all three properties were available,represent all subdivisions of the classification intro-duced in this paper. Three main groups can clearly bedifferentiated. These are the microfine, the midway-filled, and the compact-fil led composite resins. Thereis also a trend for a further subdivision (ultratine ver-sus fine) of the last two groups, based on their surfaceroughness, whieh reflects their mean particle size. Thetraditional composite resins are situated next to thecompact-filled composite resins. They exhibit a ratherhigh surface roughness. These materials are nowabandoned and replaced by technologically improvedproducts. Chemically curing composite resins, such asthe traditional composite resin Epolite 100 (circle

Quintessence International Volume 2d, Number 9/1993 655

Adhesives Symposium

tiiarked with asterisk in Fig 5), and the midway-filledeomposite resin Gem-CCI (cross marked with asteriskin Fig 5), clearly display higher surface roughness be-cause of the incorporation of air bubbles and voidsduring the required rnixing process. The surfaceroughness of the compact-filled cornposite resin Valux(diamond marked with asterisk in Fig 5) slightly ap-proxirnates the midway-filled composite resin range,but its Young's modulus of elasticity and Vickershardness clearly belong to the range of the compact-filled composite resins. Ideally, high Young's tnodulusand Vickers hardness values combined with a rela-tively low surface roughness value would be the char-acteristic properties of the posterior cotTiposite resinof choice. Only three composite resins approximatethis concept, namely Adaptic II. P-50 APC. and Zl 00(al! ultrafine eompaet-filled cotnposite resins—smallfilled diamonds in Fig 5).

Wear

P-30, P-30 APC, and P-50 APC showed OCA wearrates that did not differ significantly from one an-other. These ultrafine compact-filled composite resinsintended for posterior use are actually extremely wearresistant at CFOAs and have an OCA wear rate near-ly similar to the enamel wear rate in rnolars.'^ Theenamel-like wear resistance is important, because re-storative materials should sirnulate the properties ofenamel and dentin. as is the case with the rnodulus ofelasticity,"* ' ' hardness,'- surface roughness,'^ com-pressive strength,'- thermal expansion coefficient,'-•'*radiopacity,"-^^ eolor match.'"^ and wear.'*"** A dis-crepancy between these properties will endanger therestoration's longevity."^

The fine compact-filled Marathon showed an un-acceptable wear rate of 242 |am after 3 years. Thistype of material should not, therefore, be used forposterior restorations.

The ultrafine midway-filled Exp.LF showed verylow attrition and abrasion resistance. Actually, itseemed that the entire restoration was gradually beingwashed away during clinical service. Differences be-tween OCA and CFOA wear were not pronounced.Most of the wear was due to abrasion of the com-posite resin as the food bolus was forced across thesurface. The composite resin Exp.LF is also unsuit-able for rehabilitation of posterior lesions because itcannot guarantee long-term occlusal support; this lossof support rnay cause loss of vertical dtmension, thuscausing parafunction.

All restorations were placed in mandil'^uiar andmaxillary first and second rnolars. This is tmportantfor the interpretation of the measured wear rates be-cause restorations in molars have been shown to wearat a higher rate than those in premolars. *^ ' It shouldalso be noted that 3 years of clinical service may notbe sufficient to adequately estimate the wear resist-anee of posterior cornposite resin. However, obvioustrends in the behavior of the materials studied can bederived frorn the 3-year results.

Finally, it is necessary to stress that the large stan-dard deviations in the results were due to the multi-factorial aspect of the wear process and biologic var-iation. Tooth wear is a very cotiiplex phenomenonthat depends on several extrinsic and intrinsic fac-tors.''-^* Obviously, all of these factors cause a largebiologic spread between individuals. These variancesare translated into large standard deviations, whicharc inherent to in vivo studies.

Nanoindentation hardness of filler particles

The nanoindentation hardness of the hydroxyapatitecrystallites is 3.39 GPa. This value is used as a max-imal acceptable hardness standard for composite resinfiller particles. The filler hardness tnust be less thanor equal to that of hydroxyapatite. Most glasses, likebarium glass and zinc glass, have a hardness value ofabout 3 to 4 GPa, comparable to that of enamel. Butsome manufacturers use a great deal of quartz, sihconnitride, or zirconium oxide, any of which is much toohard and can darnage the opposing enamel. The newergeneration of posterior composite resins has smallerand softer subtnicron glass particles and is, therefore,less hkely to act as a destructive abrasive.

Summary

Recently developed posterior composite resins canhave a promising future if an extremely technique-sensitive restorative procedure is meticulously fol-lowed and if the limited indications are recognizedand respected." Proper case selection and standard-ized procedures are itnperative for the long-term suc-cess of posterior composite resin restorations. Ultra-fine compact-filled cornposite resins with OCA wearrates ranging frotn 110 to 149 ^m after 3 years arethe materials of choice for posterior composite resinrestoration.

656 Quintessence International Volume 24, Number 9/1993

Adhesives Symposium

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Phitlips RW. Skinner's Science of Dental Materials, ed 8. Phil-adelphia: Saunders, 1982:217-247,Norman RD, Wright JS, Rydberg RJ, Felkner LL. A 5-yearstudy comparing a posterior composite resin and an amalgam.J Prosthet Dent ]990;64:523-529,Willems G. Lambrechts P. Braem M, Vanherle G. Three-yearfollow-up of five posterior composites: In vivo wear. J Dentt993;21:74-.78.Willems G, Lambrecllts P, Braem M, Vanherle G. Three-yearfollow-up of five posterior composites: SEM study of differ-ential wear. J Dent i993;2l:79-86Braem M, Lambrechts P, Van Doren VE, Vanherle G, In vivoevaluation of four posterior composites: Quantitative wearmeasurements and clinical behavior. Dent Mater 1986;2:106-113.Willems G, Celis JP, Lambrechts P. Braem M, Vanherle G,Hardness and Young's modulus of Tiller particles of dental res-torative materials compared with human enamel. J Biomed Ma-ter RKS 1993;27:747-755.

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658

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