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Viscoelastic behavior of dental restorative composites during setting
Dauvillier, B.S.
Publication date
2002
Document Version
Final published version
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Citation for published version (APA):
Dauvillier, B. S. (2002). Viscoelastic behavior of dental restorative composites during setting.
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VlSCOELASTICC BEHAVIOR OF
DENTALL RESTORATIVE COMPOSITES
Thee research in this thesis was financially supported by The Netherlands Institutee for Dental Sciences ('Interuniversitaire Onderzoekschool Tandheelkunde,, IOT') and the Academic Centre for Dentistry Amsterdamm (ACTA).
ISBN:: 90-6464-748-8
Lay-outt by: Facilitaire Dienst, V&F, ACTA Amsterdam Printedd by: Ponsen & Looijen bv, Wageningen Coverr photo by: Peter Muller, dentist, Utrecht
Copyright:: © BS Dauvillier, 2002 (b.dauvillier@wanadoo.nl)
AHAH rights reserved. No part of this publication may be reproduced, stored in a retrievalretrieval system, or transmitted in any form or by any means, mechanically, by photocopy,photocopy, by recording or otherwise, without permission by the author.
Visco-elastischh gedrag van tandheelkundig restauratie
composietenn tijdens de verharding
(mett een samenvatting in het nederlands)
ACADEMISCHH PROEFSCHRIFT
terr verkrijging van de graad van doctor aann de Universiteit van Amsterdam opp gezag van de Rector Magnificus
prof.. mr. P.F. van der Heijden
tenn overstaan van een door het college voor promoties ingestelde commissie,, in het openbaar te verdedigen in de Aula der Universiteit
o pp donderdag 21 maart 2002, te 10:00 uur door r
Bibianaa Sibbeltje D a u v i l l i e r
Promotor r
Prof.. dr. A J . Feilzer (Faculteit der Tandheelkunde)
Co-promotor r
Dr.. M.P. Aarnts (Corus group, IJmuiden, The Netherlands)
Promotiecommissie e
Prof.. dr. D.C. Watts (University of Manchester)
Prof.. dr. T.M.G.J. van Eijden (Universiteit van Amsterdam) Prof.. dr. ir. M. Naeije (Universiteit van Amsterdam)
Dr.. ir. M.M.A. Vrijhoef (3M-Europe)
Faculteit t
Tandheelkunde e
Paranymfen n
Drs.. P. Bolhuis (ACTA) Drs.. A.Dozic(ACTA)
Thiss work was performed at the department of Dental Materials Science of the Academicc Centre for Dentistry Amsterdam (ACTA), The Netherlands.
C H A P T E RR 1 General i n t r o d u c t i o n 9 C H A P T E RR 2 D e v e l o p m e n t s in s h r i n k a g e control of 15 a d h e s i v ee r e s t o r a t i v e s C H A P T E RR 3 E x p e r i m e n t a l c o n s i d e r a t i o n s 35 C H A P T E RR 4 M o d e l i n g of axial stress-strain d a t a 65 C H A P T E R SS M o d e l i n g of viscoelastic b e h a v i o r of 85
chemicallyy activated resin c o m p o s i t e s
C H A P T E RR 6 Influence of T E G D M A / B I S G M A ratio in 101 e x p e r i m e n t a ll resin c o m p o s i t e s on selected mechanicall p r o p e r t i e s C H A P T E RR 7 M o d e l i n g of the viscoelastic b e h a v i o r of 227 d e n t a ll light-activated resin c o m p o s i t e s d u r i n gg setting C H A P T E RR 8 Low s h r i n k a g e composite. 143 Partt I: M o d e l i n g of viscoelastic b e h a v i o r d u r i n gg setting C H A P T E RR 9 Low s h r i n k a g e c o m p o s i t e . 163 Partt II: Influence of C-factor and t e m p e r a t u r e on
selectedd mechanical p r o p e r t i e s d u r i n g setting
S u m m a r y ,, c o n c l u s i o n s , a n d r e c o m m e n d a t i o n s 277 S a m e n v a t t i n g ,, c o n c l u s i e s e n a a n b e v e l i n g e n 187
D a n k w o o r dd 299
Appendix x
A:Analyticall solution of linear differential i e q u a t i o nn associated to linear viscoelastic m o d e l s
GENERALL INTRODUCTION
Georgee Washington, the first president of the United States of America,, used to wear an artificial denture made of ivory and bovine teethh (Fig. 1.1). Too vain to appear in public without it, but unable to speakk when wearing it, he resorted to work from home - the White Housee - rather than from his office in the Congress building. American presidentss have made a tradition of this behavior since.
Fortunately,, a basic change in the nature of dentistry has occurred since Georgee Washington's time, most noticeably over the past few decades. Dentall care has become available to the industrialized world and a growingg portion of the population now retain all or part of their natural teethh well into old age. Thanks to regular check-ups, disease can be discoveredd and treated in an early stage. As a rule, only relatively small portionss of a tooth have to be removed and replaced by fillings.
Figuree 1.1 One of the six artificial dentures of George Washington
(1732-1799).. The elements used were made of bovine teeth, human teeth, and ivory inn a lead base, with springs that allowed the first president of the USA to open andd close his mouth. The artificial dentures, made by Dr. John Greenwood (1790),, fitted poorly and distorted the shape of his mouth.
Thee crown of a tooth consists of dentin covered with a layer of enamel aboutt one-millimeter thick. Tooth enamel contains much more hydroxyapatitee than dentin, which gives it a hardness comparable to that off semi-precious stones. Due to our diet and the presence of certain bacteriaa in saliva, a process called caries is initiated. During this process, thee acidic products of the metabolism of the bacteria (plaque) soften the enamell and eventually the dentine by dissolving hydroxyapatite, resultingg in the formation of cavities in the teeth.
restoration n marginall loss incompletee restoration adaptation n dentine e microleakage e secondaryy caries enamell micro-cracks
F i g u r ee 1.2 Effects of shrinkage stresses in restoration.
Itt will be clear that the materials used for the restoration of cavities shouldd be able to be brought in the appropriate anatomical shape, and thatt they should retain that shape throughout the life of the tooth. The usee of metals such as gold alloys and amalgam has for many decades providedd satisfactory results with respect to the preservation of tooth anatomy.. Nowadays, conventional glass ionomer cements, compomers, andd resin composites have gained a permanent position on the dental markett as direct restorative material. Their superior esthetics and consecutivee preparation requirements (less destructive than amalgam) havee been instrumental in this commercial success.
Thee ideal restoration has a perfect seal with the remaining tooth structure,, since otherwise bacteria and the toxins they produce can invadee and populate in the gap formed, resulting in pulp irritation and evenn secondary caries (Fig. 1.2). This perfect seal must be obtained duringg setting (solidification process) and then maintained during thermall and mechanical cycling for the lifetime of the restoration or thee patient. Unfortunately, the present generation of esthetic restorative
materiall does not yet guarantee a tight seal. Most of this shortcoming is relatedd to bulk shrinkage of the restorative material during the setting process.. Due to the adhesion to rigid tooth tissue, this shrinkage is constrained,, and this, in combination with the increasing stiffness of the restorativee material, inevitably leads to the development of mechanical stressess in and around the restoration.
Thesee stresses are a major problem, since they have a negative influence onn the durability of the restoration. While loss of adhesion can occur at anyy time, the most likely moment is when the magnitude of the shrinkagee stress exceeds the strength of the developing restoration-toothh bond. Since bulk shrinkage takes place largely within 15 minutes off setting [1], adhesive failure starts early, occasionally even before the patientt has left the dentist's chair [2]. Although the restoration will probablyy not fall out of the preparation, it has to be replaced to prevent adversee biological reactions. Even if adhesion survives the mechanical stresses,, there may be cusp movement, postoperative sensitivity, cohesivee fracture, or tooth fracture.
Thuss far the literature has given considerable attention to factors such as bulkk shrinkage, restoration configuration, water absorption, porosity, andd the effect of the kinetics of the setting mechanism on shrinkage stresss [3]. The outcome of these studies has resulted in time-consuming restorativee techniques for the practitioner (preparation design, special fillingg techniques, linings, variable light intensity, etc.) designed to obtainn restorations with a tight seal and low ultimate internal stresses. Andd yet there is still no clear understanding of why the adhesive bond failss in some situations but not in others.
Too gain more insight into the problem of shrinkage stresses, research has focusedd on the viscoelastic behavior of dental restorative materials duringg setting. This mechanical behavior during setting - when the materiall passes from a liquid to a solid state - is an important factor in thee relation between shrinkage of the restorative material and stress developmentt in the restoration. Once the viscous flow and solid property off the restorative material during setting can be quantified, then the researchh into shrinkage stress development can be enriched with numericall analyses and simulation techniques.
Finitee Element Analysis (FEA) is widely used to calculate material stressess [4]. In a finite analysis, a complex structure - such as a tooth - is subdividedd into a number of small, simply shaped elements, for which individuall stresses can be more easily calculated than for the structure ass a whole. By solving the stresses of all the small elements
simultane-ously,, the total stress of the whole structure can be approximated. The firstt attempt to study the stress build-up caused by the shrinkage of the restorativee composite by means of FEA appears promising [5]. However, forr proper simulative shrinkage stress studies, a reliable viscoelastic modell is required, one whose material parameters for the setting of thee restorative material are known.
Aimm o f t h i s r e s e a r c h project
Thee aim of this research project was to use modeling to obtain more informationn on the viscoelastic behavior of resin composites during thee setting process. Part of the research focused on finding a mechanical modell capable of predicting the viscoelastic behavior of dental compositess during setting. With a suitable model, the viscoelastic parameterss viscosity (n) an inverse related measure of viscous flow -andd elastic modulus (E) - a measure of stiffness - can be quantified on the basiss of experimental data. Although different classes of bulk restorative materialss exist, this research project focused on two-paste and light-activatedd resin composites.
Thee other part of the research dealt with the use of mechanical models to studyy the effect of resin formulation, the initiator system, configuration (C-factor),, and temperature on the mechanical behavior of composites duringg and after setting. For this purpose, conventional (dimethacrylate) andd experimental (oxirane-based) composites were studied. The quan-tificationn of the viscoelastic parameters will lead to a better under-standingg of the relation between bulk shrinkage and stress development withinn the composite.
S c o p ee o f t h i s t h e s i s
Thiss thesis represents the findings of the research project on modeling thee viscoelastic behavior of dental composites during setting. It deals firstt with the improvements in the quality of stress-strain data obtained byy the dynamic testing method. Next, the modeling of resin composites iss described, starting with chemically activated (two-paste) composites.. And finally, the effect of dimethacrylate composition in thee resin and the use of low-shrinking oxiranes as resin, are examined. Characterizationn studies such as infrared spectroscopy, wear, and scan-ningg electron microscopy, were performed to provide additional infor-mationn pertinent to the discussion of the effect of resin formulation.
Thee results described in the present thesis are of importance for future numericall analysis. They can serve as input for FEA studies, which makee it possible to simulate the stress distribution in and around the restoredd tooth. Practitioners will then have a better understanding of howw and where stress develops in composite restorations, and can then refinee their techniques to minimize the deleterious effects of shrinkage forces.. Although this investigation focused on dental resin composites,, the numerical investigation described here is also applicable too other materials used in dentistry and medicine.
Chapterr 2 reviews the underlying causes of shrinkage in polymeric
restorativee materials, and the various factors that are of influence. Some factorss affecting stress development are beyond the control of the cliniciann (e.g., the formation of the composite); however, the methods usedd for placement and light-curing are aspects which he can control directly.. This review stresses the importance of knowing the relation betweenn these manipulative factors and the development of shrinkage stresses.. Special attention is given to the polymerization reaction, the compositee structure, and its relation to viscoelasticity.
Thee choice of a mechanical model requires a thorough knowledge of the viscoelasticc properties of the resin composites. Dynamic tests are necessaryy in order to determine the major characteristics of the compos-ites.. Moreover, these tests must provide reliable stress-strain data, whichh are important for the modeling of the viscoelastic behavior of dentall composites during setting. Chapter 3 describes the experimental detailss of the present research, including the preparation of specimens, thee dynamic test system, and test protocols. On the basis of the recorded stress-strainn data, the limitations of the dynamic test system for shrinkingg dental restoratives are discussed.
Chapterr 4 introduces the modeling of axial stress-strain data. It is
intendedd for readers who are not familiar with mechanical models and havee no experience in data modeling. A number of possible models for thee description of the viscoelastic behavior of dental composites are presented,, and a modeling procedure capable of calculating material parameterss from a set of experimental data is described step-by-step. A schemaa of the validated procedure for parameter identification is given, andd the influence of noise on the identification procedure is discussed.
Chapterss 5,7, and 8 deal with the modeling of the viscoelastic behavior
off two-paste dimethacrylate composites, light-activated dimethacrylate composites,, and light-activated oxirane composites respectively. On thee basis of the modeling results, a suitable mechanical model has been
selected,, w h i c h can p r e d i c t the viscoelastic b e h a v i o r of the c o m p o s i t e s d u r i n gg s e t t i n g .
Thee effect of bisGMA-TEGDMA on the mechanical p r o p e r t i e s of two-p a s t ee c o m two-p o s i t e s is described in Chatwo-pter 6. Stwo-pecial attention is given to t h ee q u e s t i o n of w h e t h e r flowable c o m p o s i t e s u n d e r g o a p r o l o n g e d viscouss flow state, w h i c h m a y ultimately lead to less shrinkage stress in t h ee m a t e r i a l . T h e s e t t i n g p r o c e s s of s e v e r a l e x p e r i m e n t a l b i s G M A -TEGDMAA composites has been monitored and characterized by dynamic tests,, d i l a t o m e t r y , infrared spectroscopy, a n d m a t h e m a t i c a l m o d e l i n g . Inn a d d i t i o n , t h e tensile strength of t h e c o m p o s i t e s after one h o u r of s e t t i n g ,, a n d t h e w e a r p r o c e s s o v e r a p e r i o d of o n e y e a r w e r e e v a l u a t e d . .
Chapterr 9 d e s c r i b e s a p r e l i m i n a r y s t u d y focusing on the p o t e n t i a l of a
l o w - s h r i n k a g ee c o m p o s i t e for use in restorative dentistry. The shrinkage s t r a i n ,, stiffness d e v e l o p m e n t , a n d t e n s i l e s t r e n g t h at different c o n f i g u r a t i o n ss (C-factor) of an e x p e r i m e n t a l oxirane c o m p o s i t e h a v e beenn m e a s u r e d and analyzed at room t e m p e r a t u r e and oral temperature. Lastly,, the s u m m a r y describes w h a t h a s been a c c o m p l i s h e d with this p r o j e c tt a n d a n u m b e r of c o n c l u s i o n s are g i v e n . In a d d i t i o n , s u g g e s t i o n ss for f u t u r e w o r k related to the viscoelastic b e h a v i o r a n d s h r i n k a g ee s t r e s s relief of dental r e s t o r a t i v e s are s u g g e s t e d .
R e f e r e n c e s s
1.. Hegdahl T, Gjerdet NR (1977): Contraction stresses of composite resin fillingg materials, Acta Odontol Scand 35:191-195.
2.. Davidson CL, De Gee AJ, Feilzer AJ (1984): The competition between the composite-dentinn bond strength and the polymerization contraction stress ƒƒ Dent Res 63:1396-1399.
3.. See chapter 2 of this thesis.
4.. Zienkiewicz OC, Taylor RL: The finite element method, Fourth edition, London:: McGraw-Hill Book Company (UK) limited (1991).
5.. Hübsch PF, Middleton J, Knox J (2000): A finite element analysis of the stresss at the restoration-tooth interface, comparing inlays and bulk fill-ings,, Biomater 21:1015-1019.
DEVELOPMENTSS IN SHRINKAGE CONTROL OF
ADHESIVEE RESTORATIVES
Basedd on the article:
Dauvillierr BS, Aarnts MP, Feilzer AJ (2000): Developments in shrinkage controll of adhesive restoratives, J Esthet Dent 12:291 -299.
Abstract t
Thiss chapter reviews material properties and application techniques impor-tantt in minimizing effects of polymerization shrinkage during the setting reaction off direct restorative resin composites used in adhesive dentistry. Since it was recognizedd that shrinkage, which takes place during the setting reaction of restorativee composites, may cause severe problems in adhesive dentistry, considerablee effort has been put into reducing the negative effects. The most importantt problem is the debonding of the restoration-tooth interface, resulting inn increased microleakage and, ultimately, in secondary caries. Despite all efforts,, there is still no material or general application method that guarantees aa leak-proof and durable restoration. It is of the utmost importance that dental practitionerss know how to deal with the problems related to resin composite shrinkage,, so that they can choose the material and procedure most likely to producee a leak-proof and durable restoration, maximizing the potential for clinicall success.
I n t r o d u c t i o n n
Directt restorative resin composites have gained a permanent position onn the dental market. Their superior esthetics and consecutive preparationn requirements (less destructive than amalgam) have been instrumentall in this commercial success. The ideal restoration has a tightt seal with remaining tooth structure, since otherwise, bacteria and toxinss produced by bacteria can invade and grow in the gap formed, resultingg in pulp irritation and even secondary caries (Fig. 1.2) [1, 2]. This perfectt adaptation must be obtained during setting and then main-tainedd during thermal and mechanical cycling for the lifetime of the restorationn or the patient. Currently, no commercially available resin compositee guarantees an intact seal. Because the resin has no anti-microbiologicall activity, it is important that a restoration must be placed inn such a way that the best possible marginal seal is obtained.
Theree are, however, many side effects that frustrate the goal of a perfectlyy sealed restoration. Most of these effects are related to poly-merizationn shrinkage of the restoration during the setting process. Commerciallyy available composites still undergo a volumetric shrinkage off 2 to 9 % [3-6]. Therefore, a major portion of this chapter is devoted to whatt the practitioner can do to minimize the negative effects of poly-merizationn shrinkage.
R e s i nn c o m p o s i t e s
Dentall restorative composites comprise a blend of hard, inorganic particless bound together by a soft, resin phase. The resin contains: (i) a monomerr system, (ii) an initiator system for free radical polymerization, (iii)) inhibitor for maximizing the storage stability of the unpolymerized composite,, and (iv) color pigments for maximizing the chemical stability off the polymerized composite. The inorganic filler consists of particulates suchh as glass, quartz, zirconia, a n d / o r fused silica. The coupling agent, usuallyy an organo-silane, bonds chemically the reinforcing filler to the resinn phase.
Thee resin phase of a dental composite is a polymeric matrix. The major partt of the resin, the monomer system, consists of a mixture of a high molecularr monomer with a less viscous (usually low molecular) monomerr as viscosity controller. The process by which these two types off monomers are joined together into a polymer network is called polymerization.. As monomers used in dental composites are liquids,
Radicall addition polymerization of bisGMA 1)) Initiation
Two-pasteTwo-paste initiation system:
o nn <CH2)2OH C ^ O ^ ) - ^^ H C ^ K ^ f ™ Electron transfer, @ . + c 0 ; + ^ ^ ^ (pastee 1) Benzoyll peroxide "(CH2)2i i (pastee 2) DHEPT T
SingleSingle paste initiation system: H3CyC H3 3 Camphorquinone e 5000 n m . intersystem m crossing g initiatorr radical H3CyCH3 3 (CH^OH H inactivee radical II singlet ^ o o /triplet t short-livedd exciplex o o H 2C V j ^ O ' ' C H , , , C H , ,
II
H J C ^ C H J J Electron/protonn Htransfer r 3C v-CH3 DMAEMA A triplet t short-livedd exciplex*<$-^°-*<$-^°-, C H2 2 ~CH, ,
inactivee radical initiatorr radical
InitiationInitiation of monomer: (initiator radical) I
2)) Propagation I f C H ,, OH W C Hy OH CH/H bisGMA A
II
IIIKMIUUIUUEIIIHU.ill ^ ^ HH CH3 QTH WC H3W QTH C H3V J __H>CH3 J y WC H3 -,CH H Ul-II 0 | H , O CH Ö J f11 C H 3 OH CH3 OH C l / C H j OH CH3 OH CH3 H i1
00 CH o I | H ^ o ^ O ^ + ^ O ^ ^ O ^ S S MM C H 3 OH CH3 OH C H 7 > I | Shrinkage e II 4- bisGMA X. + i bisGMA — I 4- bisGMA X+3 3
3)) Temination
Combinationn of polymer radicals: |4-bisGMA). + l I 4 - bisGMA 44- bisGMA j - 1
Combinationn with initiator radical: I-j-bisGMAJ- + - i I-(-bisGMA j - 1
Disproportionation: :
l 4 - b i s G M A - C — C || + l4-hisGMA—C—Cl *- 1-f-bisGMA — C = c ) + I 4-bisGMA—C—c)
ii n Ix \ Ix \ ix \ | ix
HH H H H
Figuree 2.1 Reaction scheme of free radical polymerization of bisGMA [9, 10].
Thee initiation reaction for chemically activated (two paste) composites [9] and light-activatedd (single paste) composites [11] is included in detailin this scheme.
c c c c „o o <N N CL L CO O
polymerizationn of the resin system converts the soft composite to a solidd composite. Therefore, polymerization of dental composites is oftenn denoted as the setting or curing process.
Commerciallyy available dental composites are assigned to two groups accordingg to the high molecular monomer type:
Conventional Bowen (bisGMA1) composite; Urethane dimethacrylate (UEDMA2) composite.
qjj BisGMA finds widespread use in current commercially available dental |jj composites (Z100, 3M; Silux Plus, 3M; Clearfil F2, Kuraray). The high ** viscous monomer is mixed with low molecular dimethacrylate Joo monomers, such as EGDMA and TEGDMA, to achieve a viscosity
suitablee for incorporating fillers. UEDMA has been used alone (Isocap, Vivadent;; Isopast, Vivadent; Isomolar, Vivadent) or in combination «« with other monomers, e.g., bisGMA and TEGDMA (Heliomolar, aa Vivadent; Estic Microfill Composite, Kulzer; Estilux Microfill, Kulzer;
Durafilll Light-curing Composite, Kulzer)
Thee start of the setting process of resin composites requires activation of thee initiator system. The initiator system consists of two compounds, whichh generates free radicals for the polymerization reaction of the monomerr system at ambient temperature (Fig. 2.1). Based on the initiator system,, direct restorative dental composites are supplied as a single pastee or as a pair of pastes. For the two - paste or chemically activated composites,, one paste contains an accelerator, usually N,N-bis(2-hydroxyethyl)-p-toluidinee (DHEPT), and the other an initiator, commonlyy benzoyl peroxide (BPO) [12]. When the two pastes are mixed, thee amine reacts with the peroxide to form free radicals.
Thee single paste or light-activated composites remain the standard for clinicall use, whereas chemically activated composites are proposed for somee specific applications, such as core build-ups or Class II restorations. Singlee paste composites employ photosensitized compounds. Traditionally,, l,7,7-trimethylbicyclo(2,2,l)heptane-2,3-dione (camphor-quinone)) and N,N-dimethylaminoethyl methacryiate (DMAEMA) have beenn the standard in dental composites [14]. When this initiator system iss exposed to visible light in the spectral light region of 400-500 nm, the diketonee (initiator) reacts with the amine (co-initiator) to form an exci-plex,, which breaks down to yield free radicals (Fig. 2.1). The kinetics of thee light induced polymerization reaction, and the factors affecting the polymerizationn process of light activated composites are described in detaill by Watts [15] and Stansbury [16] respectively.
'bisGMAA = 2,2-bis[4-(2-hydroxy-3-methacrylyloxypropoxy)phenyl]propane [7] 2
Adequatee shelf life of dental composites is essential. To inhibit prematuree polymerization of the monomer system under ordinary storagee conditions, an inhibitor is added to the matrix phase. The most widelyy used inhibitors in dental composites are butylated hydroxy-toluenee and monomethyl ether of hydroquinone [17]. Finally, color pigmentss are incorporated in the matrix phase, to give the hardened compositee its desired color and shade.
Fillerr is mixed in the matrix phase to reinforce the restorative material andd to reduce volumetric shrinkage. Dental composites are classified on basiss of the particle size and size distribution: (i) traditional
macro-en. . R R I I , - S I --I --I O-CH, , Hydrolysis s VAO O R R I I _-- S i - OH I I HO O Condensation n TPM:: R-Si-(OCH3)3 CH, , I I R:: H,C = C— C -II I O O 00 — C H , — C H , — C H , V/////////////, V/////////////, Fillerr surface
Condensationn with surface silanol
. S i -I -I Oligomers s O O I I _ - S i --I --I .0~n .0~n Chemisorption n onn filler surface
VAO O R R I I -- S i - O H I I 0 - H H Fillerr surface
Figuree 2.2 Deposition of silane coupling agent (TPM =
y-methacryloxypropyl-trimethoxysilane)) on a filler surface [13]. Hydrolyzed TPM oligomer chemisorb onn the filler surface and condense with surface silanol and neighbour oligomers. Duringg the setting reaction of the composite, the vinyl (-C=C-) group in the R-groupp of TPM undergoes a radical polymerization reaction with the growing dimethacrylatee polymer in the resin (Fig. 2.1).
filledd (mean diameter 5-30 ^m), (ii) microfilled (mean diameter 0.04
jim),jim), and (iii) hybrid composites. Hybrid composites combine the
characteristicss of macrofilled and microfilled composites [18].
couplingg agent, to enhance wetability of the filler surface by the monomerr and to promote an adhesive bond between filler and polymer matrix.. A common silane coupling agent for dental glass particles is y-methacryloxypropyltrimethoxysilanee (MPS) [19]. The silane coupling layerr is obtained either by dry-blending the filler particles and the couplingg agent, or by deposition the coupling agent from solution on the §>> filler surface (Fig. 2.2).
<gg Shrinkage
0) )
'35 5
4) )
CN N
Ass mentioned previously, the matrix of most contemporary compositess consists of methacrylate-based monomers [20]. Volume 1§§ reduction during setting results from closer packing of monomer .££ molecules in the polymerized resin matrix, as depicted in Figure 2.2, and -22 the better packing efficiency of the polymer network [21, 22]. Thus, «JJ dimensional stability of the restoration is poor in the early stages of Q.. setting, whereas the density of the material increases. To prevent "55 shrinkage, it is important to minimize the density difference between the
«« polymerized and the unpolymerized composite.
Q Q
MM Upon polymerization, unfilled resins containing mainly bisGMA and ™™ TEGDMA undergo a volumetric shrinkage of approximately 7 to 14 %
[3],, However, the presence of filler particles considerably reduces that shrinkagee [23, 24]. As discussed in the next paragraph, an increase in the ^^ percentage of filler loading is also accompanied by a significant draw-err back. The present generation of flowable chemically and light-activated resinn composites undergo a free volumetric shrinkage of 4 to 9 %. For non-flowablee or condensable composites, this value ranges from 2 to 5 vol%,, with most values near 3.5 vol% [4-6, 23].
Severall variables are known to influence polymerization shrinkage. Onee variable is the size of the monomer molecule undergoing polymerization.. The larger the molecule before polymerization, the lowerr the polymerization shrinkage for a given volume of monomer. [22, 25-27].. Another variable is the volume fraction of the inorganic filler, includingg prepolymerized resin powder, within the composite. High fillerr loading results in lower polymerization shrinkage [23]. This rela-tionn holds true until the point where a relatively high level of filler resultss in a clay-like paste, owing to increased viscosity. At high filler loading,, the proportion of diluents (small monomers) in the resin system mustt increase to ensure acceptable handling properties. However, this additionn may negate the effect of the high filler loading on polymer-izationn shrinkage. Moreover, composite with high filler loading results
inn a high degree of stiffness, which ultimately causes high shrinkage stress.. Finally, the nature of the resin undergoing polymerization plays ann important role in shrinkage. Several research groups are currently attemptingg to develop new resins that undergo less polymerization shrinkagee [28, 29]. Commercial development of these resins may be manyy years away, as the process of gaining acceptance by the Food andd Drug Administration (FDA) is time-consuming and expensive. However,, if such resins ultimately are developed, they will largely eliminatee the clinical consequences of polymerization shrinkage and willl allow simple bulk placement of the material.
Stress s
Itt should now be clear that shrinkage of resin composites, which up too now has been regarded as inevitable, must be controlled and directed towardd the preparation walls, to prevent gap formation. However, as a resultt of adhesion to preparation walls, volumetric shrinkage is constrained.. This constraint, in combination with an increasing modulus off elasticity, inevitably leads to development of stress. Although loss of adhesionn from the tooth structure can occur at any time, the most likely momentt is when the magnitude of shrinkage stress exceeds the strength off the developing restoration-tooth bond.
Inn principle, a shrinking material pulls away from the weakest bond. In dentall practice, the weakest bond is generally the free, unbonded surface off the restoration, provided that good adhesion between the restoration andd the tooth is achieved. Adhesion to dentin is usually enhanced by the usee of etching techniques, conditioners, bonding systems, and other meanss [30]. Although of crucial importance, the subject of bonding systemss is beyond the scope of this review, and in the remainder of the chapterr an optimal adhesion between the tooth and restoration is assumed. .
AA large portion of shrinkage occurs in the early stage of the setting reaction:: after about 15 minutes for chemically activated materials, and afterr about 60 seconds for light-activated materials (Fig. 2.3). Thus, problemss associated with adhesion loss often start during this early stagee of setting, occasionally even before the patient has left the dentist's chair. .
Voidss or microcracks in the restoration are formed during polymeriza-tion,, when local stress exceeds polymer network strength. These voids andd microcracks, as well as poor interfacial adhesion between filler
andd matrix, can induce cohesive restoration fractures [31]. 10--</> > 4* * !0 0 </> > <u u k. . 01 1 i i </> > « « re re 0. .
s s
M M O O < < l/> > 0) ) D) ) re re JC C c c c c (0 0 C C E E a a .o o "5 5 (U U Q Q csi i Q. . TO TO -c c O O W W 22 — TT 60s VV 15 min OO 60 minCuringg light off—*.
0.400 0.80
Shrinkagee strain (%)
1.20 0
Figuree 2.3 Relation between axial shrinkage stress (y-axis) and axial shrinkage
strainn (x-axis) of a chemically activated resin composite (Silar, 3M) and an analogouss light-activated resin composite (Silux Plus, 3M) during setting at room temperaturee for 1 hour. The chemically activated composite (C=0.5) was mixed 1:11 w/w and the light-activated composite (C=1.0) was exposed for 40 seconds withh a light unit (Elipar Highlight, standard mode, ESPE) at the distance of 4 mm. Thee light intensity at the light exit tip was 600 mW/cm2 (radiometer, model 100, Demetron).. Note temperature effect after light exposure and difference in onsett shrinkage strain.
Stresss r e l i e f
Twoo factors have a major impact on the ultimate stress level of the restoration:: the chemical and physical properties of a material, and the wayy a material is handled during its application. The material properties aree largely determined by the manufacturer, although a practitioner cann influence those properties to some extent. For example, a dentistt can alterr the ratio of a two-paste system or use special curing lights and light-curingg procedures that affect polymerization rate and degree of conver-sionn [32-35]. Obviously, this manipulation will also influence the final materiall properties. The choice of a specific material, application method, orr type of restoration can also have an impact on the ultimate quality of thee restoration. This statement implies that practitioners must have considerablee material expertise if they are to make a well-informed
decisionn in favor of a particular material or application method.
Chemicall and physical p r o p e r t i e s
Forr minimal impact on the integrity of a restored tooth, stress developmentt must be minimized. One possible solution would be a reductionn in the amount of polymerization shrinkage. Changing the chemicall and physical properties of composite materials in such a way thatt shrinkage stress is no longer a problem is primarily the concern of thosee developing new resin composites. The development of non-shrinkingg materials (shrinkage lower than 0.4 % of volume) might be a solution,, but unfortunately there is no nonshrinking material on the dentall market that can compete on all levels with conventional compositess [29, 36-38]. Moreover, the solution of one problem might very welll create a new one (e.g., water sorption after setting might frustrate thee high expectations of a nonshrinking material).
Anotherr approach to reduce shrinkage stress is to modify the resin compositionn so that the polymerization rate is lowered without influencingg the degree of conversion. A slow polymerization rate may be expectedd to increase the ability of the material to flow without damaging itss internal structure. In a restorative material with increased flow capacity,, the volume change attributable to shrinkage is compensated by materiall flow from the unbonded, outer surface, ultimately resulting in lowerr stress. Resin composites can be chemically modified to reduce the polymerizationn rate in various ways. Use of less reactive resins is one possibility,, but this method may have a negative effect on degree of conversion,, resulting in more residual, unreacted monomer remaining inn the polymerized composite [39]. Addition of retardative agents requiress a careful choice of biocompatible chemicals [40]. Reducing the amountt of initiator system components requires no other chemicals thann those already used in current systems 141-42]. However, a balance mustt be found between a low reaction rate, on the one hand, and adequatee conversion of the monomers, on the other hand. In all probability,, the best way to obtain a lower polymerization rate in light-activatedd resin composites, together with a sufficiently high conversion, iss by will developing new initiator systems.
Floww / viscoelastic behavior
Thee solutions to stress reduction previously mentioned are mainly of interestt to researchers and manufacturers of composites. However, the
«J J
o o la, ,
% %
dentistt has to deal with a wide variety of commercial products. Although materialss differ in monomer composition, concentration of initiating system,, filler type, size, loading, and coating, resin composites can be dividedd into two general groups on the basis of the initiation system: light-activatedd and chemically activated composites. Light-activated resinn composites are popular among dentists because they can be "cured onn command". However, it has been demonstrated that, under the same conditions,, light-activated composites generate higher polymerization shrinkagee stress and more exothermic heat than the analogous chemicallyy activated composites [43-44]. Dental literature has given considerablee attention to a variety of methods designed specifically for light-activatedd resin composites, to reduce internal stresses in the restoredd tooth [45]. c c Q. . <u u <^J J CI I CO O -C C
m$ m$
Growingg polymer Filler MatrixPre-gel l Gell point Post-gel l
Predominant t viscouss behavior
Viscouss flow in balance withh elastic behavior
Predominant t elasticc behavior
Figuree 2.4 Setting of dimethacrylate (resin) phase in dental composites. The
restorativee composite is transformed from a viscoelastic liquid, in which the viscouss flow predominates over the elastic behavior, into a viscoelastic solid, wheree the elastic behavior predominates over the viscous flow behavior.
Owingg to the presence of the polymer matrix, dental resin composites exhibitt viscous as well as elastic characteristics. The combination of thesee effects is called viscoelasticy. The molecular origin of the viscous floww property is the sliding of polymer-polymer segments or filler-polymerr segment along one another. For elasticity, it is the elongation/compressionn of the filler and polymer segment. Elastic deformationn of a polymer segment occurs by rotations along the polymer backbone.. In this situation, the polymer has high local mobility, while thee overall mobility of the chain is blocked (Fig. 2.5). The energy spent to
producee elastic deformation is recoverable, whereas that for viscous floww is permanent.
Figuree 2.5 The elastic deformation of a polymer subchain within the specimen
(brokenn line), (a) indicates an unstretched specimen, and (b) the same spec-imenn after elongation in the z direction . Assuming that no volume changee accompanies the deformation, the x and y dimensions of the volume elementt (solid line) as well as the specimen are decreased by 1/al/2 compared too the original dimensions (subscript 0) [21].
Duringg the early stage of polymerization, monomers are mainly convertedd into polymeric chains. After a certain degree of conversion has beenn attained, the predominant reaction is the cross-linking of the poly-mericc chains, resulting in a strong polymeric network [46]. Although duringg the chain-growing period material viscosity rapidly increases, the polymericc chains can still slide along one another to relieve stress (Fig. 2.4).. When the cross-linking reaction becomes predominant, there is lesss ability of individual polymer chains to slide. At this stage, usually denotedd as the post-gel phase, the polymeric chains reach sufficient moduluss of elasticity to develop a strong, rigid viscoelastic material. Any furtherr composite shrinkage will generate mechanical stress in the restoration.. When adhesion survives the stress, microcracks or, in severe cases,, voids can be generated in the viscoelastic material.
Configuration n
I I
2 2 o o </> >s s
Qi i c c £ £ o. . o o1 1
Q Q <N N a. . CO O -c cReductionn of the negative effects of shrinkage stress can be controlled byy practitioners; it involves the design of the preparation and the methodss used to apply a restoration. The relation between the shape of aa preparation and shrinkage stress development in composites has been demonstratedd by Feilzer et al. [47]. In this context, the shape of the preparationn is often described by means of the configuration factor (C-factor). . [E E IV V CLASS S
EE Iv.
ÖÖ(M M
J J J J
Silar r C=5.0 00.22 0.5 1.0 2.0 5.0 C-value e w w 55 10 15 20 25 30 Curee time (min)
Figuree 2.6 (a) The relation between different schematic, rectangular
restora-tions,, the corresponding configuration (C) factor values, and standard Class II, IV,, and V restorations [47]. (b) Time-axial shrinkage stress relation of a chem-icallyy activated resin composite (Silar, 3M) during setting at room temperature forr various C-factor values.
Thee C-factor denotes the ratio between the bonded and the free area of thee restoration. It should be noted that the term "bonded area" means bondedd to a rigid surface. In general, more bonded area leads to higher shrinkagee stress, since composite flow is largely restricted to the small, freee area of the material. This factor explains why the adhesive Class IV restorationn has proved to be so successful, whereas other classes, in whichh the restoration is bordered by preparation walls (i.e., a high C-factor),, often display marginal defects. The practitioner is in full control off preparation design; however, there are many other factors that influ-encee the actual shape of the preparation, including some with an unde-sirablyy high C-factor. The next section reviews several methods by whichh practitioners can reduce the impact of shrinkage stress on the qualityy of a restoration with a high C-factor.
Layers,, liners and porosities
Thee methods described in this section to reduce the effects of shrinkagee stress are all based upon a reduction of the effective C-factor. Forr preparations with a large C-factor (Class I and V), the dentist can applyy a restorative material in several layers or increments. The advantagee of this technique is twofold: (1) the C-factor for a small incre-mentt is lower than for bulk filling; and (2) small-increment light-acti-vatedd composites can be more thoroughly polymerized, since light intensityy diminishes with the fourth power of light penetration [48]. Off course,, the main disadvantage of this method is that it is a time-consumingg procedure [49-50].
Itt is thought that, when the walls of a preparation with an unfavorable
(i.e.,(i.e., high) C-factor are covered with a relatively thick layer of a low
elasticc modulus material, the bulk shrinkage of the main restoration acquiress some freedom of movement from the adhesive liner [51-52]. Thiss concept is feasible when the liner extends to the cavosurface margin.. Additionally, the elastic liner between the tooth and composite iss often less wear-resistant at the restoration surface, resulting in surface pitting,, which may provide a site for bacteria growth [53].
Thee real effect of a low modulus lining material is probably its contributionn to a more equal distribution of tensile and shear stresses overr the adhesive interface. This material could dissipate the shear peakk stress and generate no high polymerization shrinkage stress on the adhesivee layer. Thus the adhesive, which is oftenly often not properly polymerized,, owing to oxygen inhibition, is given time to polymerize beforee the high-bulk shrinkage stresses of the overlying higher filler-loadedd composite begin to act on it. Although the mechanisms are not clear,, layering and low modulus liners are now generally accepted as meanss of reducing polymerization shrinkage stress. Both methods have thee disadvantage of additional time-consuming steps during restoration. Thee literature provides no clarity with respect to the "sandwich tech-nique",, in which glass ionomer cements are used as liner [54].
Deliberatelyy admixing small air bubbles into a composite prior to use resultss in porosities in the polymerized composite. These porosities cann be considered as unbonded areas, and they lead to a lower effective C-factorr and, thus, lower shrinkage stress [43-55]. However, it should be keptt in mind that porosities can have a negative effect on other compositee properties (i.e., water sorption, Young's modulus, wear etc.). [56-58].. For this reason, the practice of deliberately inducing porosities inn a composite should be discouraged.
Light-sources s
w> >
AA recent method designed to reduce the polymerization rate of light-activatedd resin-based materials involves varying light intensity on the restoration,, either by reducing the output of the curing light or by increasingg the distance between the light exit tip and the composite «>> [32, 59, 60]. A significant problem presented by the use of low light §§ intensities is a reduced curing depth, which further declines when the
oo quality of the light source in the curing unit deteriorates with age [61-64]. ^^ A predictive model for depth-of-cure devised by Rueggeberg et al. q,, suggests that the duration of setting compensates for the lower intensity
[65].. Although present-day conventional light-activated composites weree developed for traditional procedures with a conventional halogen lightt source (40-60 s exposure with light intensity 600 mW/ cm2), many studiess report significantly lower exposure durations involving new lightt units [33, 59, 66-68]. However, a valid comparison between light ** units requires that the spot diameter, intensity, wavelength distribution, Q.. exposure duration, and distance between light exit tip and composite 55 must be specified. Failure to specify these parameters makes comparison «« between light units impossible. The physical and mechanical properties off composites are greatly influenced by the extent to which the resin has beenn polymerized [34, 69, 70]. As polymerization efficiency and a lower polymerizationn rate may be diametrically opposed to each other, a balancee must be found between low shrinkage stress, on the one hand, andd a adequate monomer conversion level, on the other [71].
c c
Q Q
<N N
Waterr Hygroscopic expansion
Thee hygroscopic properties of a composite, although difficult to determine,, can influence ultimate shrinkage stress [72-78]. Hygroscopic expansionn (swelling) due to water sorption from saliva may, after setting,, substantially relieve shrinkage stress [79]. Unfortunately, swellingg is much more marked for restorations with a low C-factor, in whichh shrinkage stress is not as great a problem. In the case of high C-factorr restorations, the surface of the restoration, which is exposed to the orall cavity, will initially gain in volume. This gain produces a gradient fromm the outer surface to the bulk of the restoration, thus adding additionall stress. Finally, owing to the slow process of water sorption fromm saliva, stress relief may come too late, after fractures have already formed.. Although water sorption in generally recognized as a stress-relievingg mechanism, there are few quantitative data available to assess itss true impact. After a prolonged period of swelling, nonshrinking compositess may encounter major problems related to expansion stress in
somee types of restorations (e.g., mesio-occlusodistal (MOD) restorations).
C o n c l u s i o n n
Inn the p a s t 10 years, a great deal of effort h a s b e e n m a d e t o w a r d the d e v e l o p m e n tt of n o n s h r i n k i n g and even e x p a n d i n g composite materials forr d e n t a l a p p l i c a t i o n s . H o w e v e r , at p r e s e n t , t h e d e n t a l p r a c t i t i o n e r stilll has to deal w i t h shrinking resin composites a n d the a c c o m p a n y i n g p r o b l e m s .. Because t h e r e is as yet n o easy, g e n e r a l s o l u t i o n to t h e s e p r o b l e m s ,, a p r o p e r u n d e r s t a n d i n g of t h e m e c h a n i s m s c a u s i n g t h e problemss and the m e t h o d s that can be used to reduce their impact on the q u a l i t yy of a r e s t o r a t i o n is of c r u c i a l i m p o r t a n c e . T h e i n f o r m a t i o n p r e s e n t e dd is i n t e n d e d to h e l p the p r a c t i o n e r o b t a i n m a x i m u m benefit fromm the selection a n d a p p l i c a t i o n of resin c o m p o s i t e s .
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V) ) 4) ) "O O C C C C O O E E a a 'S» » Q Q <N N CL L O O
EXPERIMENTALL CONSIDERATIONS
Basedd on the articles:
Dauvillierr BS, Feilzer AJ, De Gee AJ, Davidson C L (2000): Visco-elastic para-meterss of dental restorative materials during setting, J Dent Res 79:818-823. Dauvillierr BS, Hübsch PF, Aarnts MP, Feilzer AJ (2001): Modeling of viscoelastic
behaviorr of dental chemically activated resin composites during curing,
JJ Biomed Mater Res (Appl Biomater) 58:16-26.
Dauvillierr BS, Aarnts MP, Feilzer AJ (2002): Modeling of the viscoelastic behaviorr of dental light-activated resin composites during curing, Denf /Wafer
(accepted). .
Abstract t
Thiss chapter describes the results of the process of optimizing an automated universall testing machine by which reliable stress-strain data can be obtained onn the mechanical behavior of dental restorative materials during setting. The contentss will be of interest to those who are not familiar with mechanical testing,, or who are not aware of the pitfalls involved in the dynamic testing of smalll amounts of setting materials. The test system displays high versatility, and iss capable of performing various static and dynamic tests related to axial tensionn and compression. The deformation signal feedback loop permits the crossheadd to accurately perform sinusoidal deformations on the shrinking specimenn on a submicrometer level. The electronics used to process the signalss excludes the risk of electronically based phase shifts in stress-strain measurementt when submicrometer deformations are applied at frequencies < 11 Hz. The use of a light sensor device proved capable of detecting the initiation andd duration of the light irradiation process for cure-on-demand materials. Preliminaryy experiments on commercially available chemically activated and light-activatedd resin composites indicate that the universal testing machine is highlyy useful for detailed static and dynamic studies of the mechanical behavior off dental restorative materials during setting. The sophisticated mechanical experimentss provide a sound basis for characterizing the mechanical properties off dental restorative behavior during setting by means of modeling.
I n t r o d u c t i o n n
Appropriatee modeling of linear viscoelasticity of dental resin compositess d u r i n g setting requires a good understanding of the mechanicall properties of the materials involved. Experimental tests are necessaryy to find out the important characteristics of the composites, and hencee provide the data for the modeling investigation of linear viscoelasticc behavior of dental composites during setting.
Inn the next section, several mechanical test methods for measuring the mechanicall behavior of materials were screened for use as test method inn this research project. The choice of test method is made on requirementss that must be met by the method when dealing with shrinkingg dental resin composites. Various tests were performed on commerciallyy available dental resin composites to characterize the mechanicall behavior of setting composites, and to make an inventory of thee possibilities and limitations of the chosen, and in this project further improved,, dynamical test system. Details of the equipment, experimental proceduress and materials are given in the remainder of this chapter. Unlesss stated otherwise, the mechanical tests referred to in other chapterss are performed as described in this chapter.
C h o i c ee m e c h a n i c a l test m e t h o d
Differentt mechanical test m e t h o d s and testing instruments for measuringg the mechanical behavior of materials have been standardized andd are described in the publications of the American Society for Testing andd Materials [1]. Besides the ASTM standard tests, also general referencee books have been published on testing polymers and viscoelasticc materials [2-4]. For selecting the test method for measuring thee mechanical behavior of dental resin composites during setting, it is necessaryy to make an inventory of requirements that must be met by the method. .
Requirementss mechanical m e t h o d
Thee most important requirement of all is that the method must producee reliable experimental data of dental resin composites. To be genuinelyy useful, the method must generate data (i) with high acquisitionn rate to ensure proper characterization of the material propertiess by means of modeling, (ii) without the influence of the
