progress in dimethacrylate-based dental composite technology and curing efficiency

8/10/2019 Progress in Dimethacrylate-based Dental Composite Technology and Curing Efficiency

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d ent a l ma te r ia l s 2 9 ( 2 0 1 3 ) 139156

Available online at www.sciencedirect.com

journal homepage: www.int l .e lsevierheal th.com/ journals/dema

Review

Progress in dimethacrylate-based dental composite

technology and curing efficiency

Julian G. Leprince a,b,d,, William M. Palinc, Mohammed A. Hadis c,Jacques Devaux b,d, Gaetane Leloupa,b,d

a School of Dentistry and Stomatology, Universit catholique de Louvain, Brussels, Belgium

b Institute of Condensed Matter and Nanoscience, Bio- and Soft- Matter, Universit catholique de Louvain, Louvain-la-Neuve, Belgiumc Biomaterials Unit, University of Birmingham, College of Medical and Dental Sciences, School of Dentistry, St Chads Queensway,

Birmingham B4 6NN, UKd CRIBIO (Center for Research and Engineering on Biomaterials), Brussels, Belgium

a r t i c l e i n f o

Article history:

Received 27 September 2012

Received in revised form

3 November 2012

Accepted 4 November 2012

Keywords:

Composite materials

Resins

Dimethacrylates

Polymers

Photoinitiators

Operative dentistry

Photopolymerization efficiency

Curing lights

Irradiation modes

Curing time

Irradiance

a b s t r a c t

Objectives. This work aims to review the key factors affecting the polymerization efficiency

of light-activated resin-based composites. The different properties and methods used to

evaluate polymerization efficiency will also be critically appraised with focus on the devel-

opments in dental photopolymer technology and how recent advances have attempted to

improve the shortcomings of contemporary resin composites.

Methods. Apart from the classical literature on the subject, the review focused in particularon papers published since 2009. The literature research was performed in Scopus with the

terms dental resin OR dimethacrylate. The list was screened and all papers relevant to the

objectives of this work were included.

Results. Thoughnew monomer technologieshavebeen developed andsome of them already

introduced to the dental market, dimethacrylate-based composites still currently represent

the vast majority of commercially available materials for direct restoration. The photopoly-

merizationof resin-based composites has been the subject of numerous publications, which

have highlighted the major impact of the setting process on material properties and quality

ofthe final restoration. Many factors affect the polymerization efficiency, be they intrinsic;

photoinitiator type and concentration, viscosity (co-monomer composition and ratio, filler

content) and optical properties, or extrinsic; light type and spectrum, irradiation parame-

ters (radiant energy, time and irradiance), curing modes, temperature and light guide tip

positioning.

Corresponding author at: Universit catholique de Louvain, Ecole de mdecine dentaire et de stomatologie, Centre de recherche CRIBIO,Avenue Hippocrate, 10/5721, B-1200 Brussels, Belgium. Tel.: +32 10 47 30 88; fax: +32 10 45 15 93.

E-mail address:[email protected] (J.G. Leprince).0109-5641/$ see front matter 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.dental.2012.11.005
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d en ta l mat er ia ls 2 9 ( 2 0 1 3 ) 139156 141

drawbacks of resin composites and improve material per-

formances. A primary disadvantage related to their use is

polymerization shrinkage and the associated stress transmit-

ted to the adhesive bond and the remaining tooth structure.

Their clinical consequences include crack formation in dentin

and enamel, post-operative sensitivity, marginal discoloration

and secondary caries [6]. Further drawbacks of resin-based

composites include inferior mechanical properties comparedto tooth structures [7], in line with clinical data reporting bulk

fracture as the main reason for retreatment [3], as well as sec-

ondary caries andtooth fractures [35]. Thebiocompatibility of

dimethacrylate resin and photoinitiatorchemistry extensively

described by [8] are further concerns, as it may affect the del-

icate balance between healing and chronic inflammation in

pulp tissues [9].

Advances in material formulation may improve the

shortcomings of resin composites and in this regard, recent

reviews [1012] have summarized significant developments,

including improved filler morphology, progress with existing

dimethacrylate chemistry and novel monomer technologies.

It appears from those works that, although some non-dimethacrylate monomers materials based on ring-opening

epoxy chemistry (FiltekTM Silorane, 3M ESPE Dental Products,

Seefeld, Germany) have already been introduced to the dental

market, contemporary dimethacrylate-based composites

still represent the vast majority of commercially available

materials for direct restoration.

As stated by [10], the development and implementation

of resin composites rely on a comprehensive understanding

of each component . . .. However, the successful placement of

composite restorations depends also on the method of light

curing as well as theeffect of thecomponentson theefficiency

of the photopolymerization process. Therefore, the objective

of the present work is to review the different factors affectingthe photopolymerization efficiency of dimethacrylate-based

dental resins (Fig. 1), be they intrinsic; co-monomer com-

position and ratio, filler content, photoinitiator type and

concentration, or extrinsic; light spectrum, irradiation pro-

tocols, temperature and light guide tip positioning, with

particular focus on papers published since 2009. It is impor-

tant for researchers, and also particularly for clinicians, to

first comprehend, and second, to optimize the photopoly-

merization process of resin-based composites in order to

improve material properties. To better realize the impact

of these different factors, a fundamental description of the

photopolymerization reaction will be presented with a critical

appraisal of the different properties and methods used toevaluate the polymerization efficiency. Recent advances that

improve the photopolymerization process will be reviewed

as well.

2. Dimethacrylate photopolymerizationreaction characteristics

The photopolymerization process of dimethacrylate-based

dental resins is a reaction triggered by free radicals, which

are generated by irradiation of a light-sensitive initiator and

open the double bond of methacrylate groups (C C), gen-

erating a chain reaction. It is classically described in three

steps: initiation, propagation and termination, schematically

represented in Fig. 2. Conventionally, the extent of polymer-

ization is quantified by comparing the amount of remaining

double bonds in the polymer structure to the initial amount.

This ratio, expressed in %, is termed the degree of conversion

(DC). Generally, DC values vary for a wide range of resin com-

posite types, ca. 3577% [8]. Dimethacrylate monomers have

the potential to bind to four other molecules or structures,

whereas mono-methacrylates can only bind to the two adja-cent molecules within a linear polymer chain. As a result,

and in contrast to forming unconnected long linear chains

associated with thermoplastic polymers, a highly crosslinked

network is formed, with covalent bonds between polymer

chains. The crosslinked structure has major consequences

on the material properties as well as on the kinetics of the

polymerization reaction. Following photopolymerization, the

degree of conversion and crosslinking density increaserapidly.

An infinite network forms, resulting in a rapid increase of

the system viscosity and in the first change of state, from a

viscous liquid to an elastic gel, called gelation [10]. At the gel

point, the mobility restriction mostly affects radicals, located

on large molecules (growing polymer chains), whereas smallmonomer molecules can still diffuse easily. Consequently,

bimolecular termination (Fig. 2) decreases dramatically while

new growth centers are still created by initiation. Hence,

the free radical concentration increases, which results in a

rapid increase of the rate of polymerization (Rp, fraction of

double bond converted per second, representing the speed

of the reaction) called autoacceleration [13]. As the reaction

proceeds, the viscosity becomes so high that it limits dif-

fusion even for monomer molecules, resulting in significant

decrease of Rp, or autodeceleration. This corresponds to the

second change of state, from rubbery to glass, or vitrifica-

tion [14], which can be visualized by plottingRp against DC

(Fig. 3). Vitrification prevents any further extensive reaction,and explains why DC cannot reach 100%, even with optimum

irradiation conditions. Though DC is known to slightly evolve

up to 24h after irradiation, probably due to free volume relax-

ation [15], the major part of the polymerization occurs during

irradiation.

Another major consequence of vitrification is the entrap-

ment of free radicals in the polymer network, which is

sometimes referredto as monomolecular termination in polymer-

related publications [16,17]. However, such explanation may

not be wholly appropriate, as trapped free radicals do not

necessarily terminate since they still exist within the

network for several weeks after vitrification [18,19]. If the

mobility of the system increases (swelling by solvents, tem-perature increase, etc.) or through reaction-diffusion [13,19],

radicals remain capable of reacting with remaining dou-

ble bonds and continuing polymerization. Hence, a more

accurate terminology may be defined as radical immobi-

lization or radical entrapment. Besides free radicals, other

active species remain trapped in the polymer network due

to vitrification, i.e. pendant double bonds, free monomers

and photoinitiators. The remaining active species can influ-

ence the biological and/or mechanical properties of the

material. Importantly, the characteristics of the photopoly-

merization process are significantly modified by the radical

concentration, the double bond concentration, the degree of

crosslinking or the system viscosity [13], which depend on
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142 d en ta l mat er ia ls 2 9 ( 2 0 1 3 ) 139156

Fig. 1 Schematic representation of the different properties used to evaluate photopolymerization efficiency, and of various

extrinsic and intrinsic factors by which it is affected. Gray arrows indicate the influence of one factor on another or on the

curing efficiency. The black arrow symbolizes the fact that the curing efficiency is not only governed by extrinsic

parameters, but by intrinsic parameters as well, since differences in inherent material properties have a major influence on

the way extrinsic factors affect the success of photopolymerization.

numerous intrinsic and/or extrinsic factors described fur-

ther.

3. Evaluation ofphotopolymerizationefficiency

3.1. Degree ofconversion

DC is a very important material feature as it is significantly

correlated to several other important material characteristics,

such as mechanical properties [20,21], volumetric shrinkage

[22], wear resistance [23] and monomer elution [24]. There-

fore, DC is frequently measured to evaluate polymerization

efficiency and the most common method is by spectro-

scopic techniques that infer the quantity of remaining double

bonds, either mid-infrared Fourier transform (FT) [25] or

Raman spectroscopy [26]. FT mid-IR is based on the reflec-

tion of infrared radiation and has been used for many years

to measure DC by comparing the height of the 1640cm1

peak corresponding to CH CH2 (vinyl) stretching vibration

before and after curing[25]. Another peak corresponding to

aromatic rings at 1608cm1 is used as a reference as it does

not vary with polymerization. Raman spectroscopy is based

on the dispersion of the light by the polymer, using similar

measurement peaks as mid-IR spectroscopy [26]. Microscope

attachments for mid-IR techniques and the focused beam

used for micro-Raman spectroscopy enable the measurement

of DC at specific points by mapping the surface of a sample

under high magnification, which is useful when considering

polymerization in depth. A disadvantage of mid-IR techniquesremains high absorption in this wavelength range, which

may decrease signal-to-noise ratio and increase variability of

results. While it is generally assumed that vinyl groups on

silane are neglectible compared to the amount in the resin, it

has also been reported that vinyl C C absorption of the silane

layer at the fillermatrix interface may complicate polymer

conversion calculations [27]. More recently, near-infrared FT

spectroscopy (FT NIR), which is based on transmission, was

shown to be an efficient and a more reliable method to mea-

sure DC in real time, providing accurate measurement ofRp.

FT NIR is based on transmission, and monitors the decrease

in the vinyl peak centered at 6164cm1 [28] and less absorp-

tion allows analysis of thick specimens. DC has also been


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Fig. 2 Schematic representation of the 3 steps of photopolymerization reaction CQ: camphorquinone, A: tertiary amine.

indicates a radical species. (1), (2), (3) and (n) represent the theoretical steps of linear monomer addition. n and m: large

amount of monomer units. A trapped radical is illustrated in the termination section and segmental mobility is represented

by a twisting gray arrow. A double gray arrow indicates radicals about to react together (bimolecular termination) during

initiation, a free radical is generated through the activation of a photosensitive molecule or photoinitiator by a photon of

specific wavelength. The newly formed free radical quickly reacts with a nearby monomer, opening its C C bond. Then

follows a chain reaction, the propagation, by which the polymer grows by repetition of this C C opening by the radical and

addition of a large number of monomer units one after the other. The polymer can either grow linearly by reaction with

monomers (reactions 1, 2, 3, n), leaving unreacted or pendant double bonds, or grow tri-dimensionally by reacting with

another polymer chain, creating cross-links. The addition of supplementary units continues until free radicals react with

each other to form a stable covalent link through bimolecular termination reaction.

indirectly evaluated by microhardness measurements (Vick-

ers, Knoop) as a good linear correlation has generally been

observed between DC and microhardness values [27,29,30].

However, some disagree with this relationship as a general

rule [31] because other factors than DC affect the micro-

hardness measurement, notably the degree of crosslinking.

In any case, microhardness measurement does not provide

any quantitative information on the actual change in reactive

groups.

3.2. Degree ofcrosslinking

Besides DC, the degree of crosslinking is another important

determinant of mechanical properties [32] as well as struc-

tural stability, since the lesscrosslinked the material,the more

it swells and degrades in solvents [3335]. To our knowledge,

there is currently no method to directly measure the degree

of crosslinking. Indirectly, as increased crosslinking reduces

molecular mobility, it can be highlighted by an increase of


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Fig. 3 Rp as a function of DC during a 70s real time

measurement by FT-NIRS, for 70/30 mass%

bis-GMA/TEGDMA resins, either unfilled (a) or filled (b) with

75mass% fillers. Gray and black curves correspond

respectively to MAPO-based and CQ based resins.

Irradiation modes are indicated on the chart (time andirradiance). By plotting DC againstRp, the point in

conversion where Rp approaches its maximum reveals the

onset of severe diffusion limitation for propagation and

approximately corresponds to vitrification. It appears

clearly that the change of state is influenced by fillers,

irradiation protocol and photoinitiator. It can also be

observed that fillers reduce the maximum DC, and that

MAPO-based resins reach significantly higher DC, even at

such short irradiation time as 3 s (at 3000mW/cm2). CQ:

camphorquinone, MAPO: monoacylphosphine oxide, Rp:

Rate of polymerization, DC: degree of conversion.

glass transition temperature (Tg), which can be measured

either by dynamic mechanical analysis or differential scan-

ning calorimetry. Tgrepresents the temperature at which the

polymer vitrifies during the polymerization process (e.g. about

47.5 C for a 50/50 mass% bis-GMA-TEGDMA mixture [36]), and

above whicha polymer wouldreturn from a glassyto a rubbery

state. Although Tg provides an indication of the crosslink-

ing density, it also depends on DC and other factors such as

monomer viscosity. It is therefore very difficult to distinguish

between the effects of the different factors that contribute

to crosslink density. Another indirect evaluation method pro-

posed forcrosslinking density is theevaluation of thematerial

softening in ethanol, i.e. the comparison of the surface hard-

ness before and after ethanol storage [33,37].

3.3. Mechanical properties

From an engineering perspective, the flexural strength of the

set material should be maximized, and its elastic modu-

lus should remain similar to surrounding tissues in order toavoid inadequate stress-transfer on loading. Elastic modulus

and flexural strength are generally measured by three-point

flexural bending tests on 25 mm2 mm2mm samples,

as specified by ISO 4049. However, its reliability in assess-

ment of photopolymerization efficiency is questionable given

that specimen preparation requires an overlapping curing

regime to accommodate manufacture of the rectangular-

shaped geometry [38]. To allow for consistent irradiation

especially where the effect of polymerization characteris-

tics on static mechanical properties are studied, single-shot

curing protocols are employed as alternative tests with spec-

imen geometries ensuring full sample coverage by the light

tip, which include bi-axial flexure (discs) [39,40] and minithree-point flexure (bars) [41]. Although more clinically rel-

evant geometries and uniform curing regimes are realized,

the former test does not provide appropriate data for elastic

modulus calculation and the latter may exhibit loading stress

patterns that are not comparable with those of the standard

ISO specification, which may result in non-comparable data.

Another alternative would be the use of nanoscale measure-

ment methods to evaluate mechanical properties locally in

samples cured with single shot curing protocols. Such meth-

ods have already been proved useful in the characterization of

dental resin composites, notably nano-dynamic mechanical

test [42,43], atomic force microscopy [30] or nanoindenta-

tion [44]. Finally, the use of a rheometer seems promising forthe measurement of the initial viscoelastic modulus change

of composites during curing, especially with respect to the

buildup of shrinkage stress [45].

3.4. Shrinkage and shrinkage stress

Since the inception of dental resin-basedcomposites there has

beena five-decade foray of research investigating the effects of

filler content, morphology and resin chemistry on shrinkage

[1,10]. As volumetric shrinkage has significant correlation to

DC [22], both properties should always be measured in parallel

when considering strategies to reduce shrinkage or shrink-

age stress. This is rarely the case, but necessary to make surethat any reduction in shrinkage and/or stress is not due to an

inferior DC [46]

Although an important consideration, shrinkage prop-

erties per se are not wholly responsible for interfacial gap

formation and the clinical implication of lower shrinkage

values for a particular resin composite may not necessar-

ily be beneficial [6,47]. The generation of shrinkage stress

throughout and following cure is not an intrinsic property of

the material but a complex multi-factorial process affected by

shrinkage, elastic modulus, polymerization rate, vitrification,

and restrictions imposed to the composite, hence cavity

configuration factor and compliance of bonded surfaces

[6,46,48,49], whether regarding teeth in vivo or the test system


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for in vitro stress measurements. Altering compliance of the

testing system will generate different stress values for the

same material/conditions, which may account for the lack of

comparable stress data in previous studies [50]. Interestingly,

when comparing polymerization stress with four different

mechanical testing systems, a good agreement was observed

for experimental composites, but very few similarities for

commercial composites [51], since not only their filler contentdiffers, but also potentially several other components as will

be addressed further.

3.5. Depth ofcure

In order to ensure optimal material properties throughout the

depth of a restoration, it is generally advocated to build them

up using layering techniques, which can have the additional

advantage of reducing cuspal deflection due to shrinkage

stress [52,53]. The maximum thickness of each composite

layer is often referred to as depth of cure. It is generally

acknowledged that curing depth depends on the material aswell as the irradiation conditions (light type, light spectrum,

irradiance, time, etc.). Depth of cure is usually assessed by

scraping tests (ISO 4049) and penetrometer techniques [54]

or by comparing the DC [55] or microhardness [56,57] at var-

ious depths to the surface values. However, recent data has

highlighted the insufficiency or inappropriateness of these

methods, which may overestimate depth of cure. They are

indeed unable to highlight the change of state of the resin

matrix (glassy to rubbery) occurring at a certain depth, which

correspondsto a major difference in resin modulus [30]. This is

in line with previous work, showing that despite similar con-

version in depth, increased resin softening can be observed

after ethanol storage due to lower crosslinking [35]. Thesekey factors of cure depth measurements should be consid-

ered, especially whenevaluating the newso-calledbulk cure

composites [57].

3.6. Trappedfree radicals

The study of trapped free radicals is particularly relevant

regarding dimethacrylate-based resins as their polymeriza-

tion leads to highly crosslinked networks, where radical

entrapment becomes the dominant termination pathway [17].

Owing to thisfeature, freeradicals can be detecteddays, weeks

or even months after initial photoirradiation, depending on

the storage conditions, by means of electron paramagneticresonance (EPR) spectroscopy [18]. The height of specific

EPR peaks can be used to determine the relative concentra-

tion of trapped radicals [19]. A strong relationship between

autoacceleration and radical entrapment was established [58],

which highlights the importance of considering the amount of

trappedradicals whenstudying the impact of photoirradiation

protocols. The measurement of this parameter was used to

better understand the impact of clinically relevant irradiation

parameters [19]. EPR imaging was also recently demonstrated

to be a reliable tool to study photoactive dimethacrylate-based

dental resins [59], and provided complementary informa-

tion to other techniques in the evaluation of depth of cure

[30].

3.7. Biocompatibility

Unlike the other properties described above, biocompatibil-

ity is not a direct way to assess the photopolymerization

efficiency. However, it is an important prerequisite of all

bio-materials, and should be considered, since a light-cured

resin composite can affect the surrounding biological tis-

sues in at least two ways: through the elution of bioactivespecies and by the temperature rise during the polymer-

ization. First, regarding elution, an efficient polymerization

process should prevent or at least limit leaching of poten-

tially harmful components from the restoration material,

which in case of dimethacrylate-based composites, concerns

three main species: unconverted dimethacrylate monomers

or oligomers, free radicals and photoinitiator molecules. The

release of freemonomersfrom resin-based materials, which is

inversely correlated to DC [24], can induce undesirable biolog-

ical responses, such as tooth pulp damage, mucosal irritation,

contact dermatitis and allergic reactions [60]. Low molecu-

lar weight monomers such as TEGDMA and HEMA were also

recently shown to be responsible for disturbances in the dif-ferentiation potential of dental stem cells[61]. Unpolymerized

monomers also seem to promote bacterial colonization at

the composite surface [62]. The release of hydroxyl radicals

from dental resins has also been recently reported, possi-

bly due to the reaction between oxygen and the unconverted

methacrylate groups [63]. Reactive oxygen species, either

released directly or created in situ by unconverted monomers,

are responsible for oxidative stress and potentially cell struc-

ture damage [64]. Finally, as photoinitiator molecules (CQ

and amines) leach from the material [65], there exists some

concern related to the biological consequences, such as the

tertiary amine co-initiators, which may exhibit significant

cytotoxicity [64]. Hence, it is necessary to modify materialsand optimize their photopolymerization conditions in order

to reduce the amount of leachable species. Similarly, it seems

reasonable to limit the temperature rise in the pulp cham-

ber as much as possible during photopolymerization, in order

to avoid damage to pulp tissues. Pulpal temperature rise is

related to both the exotherm resulting from the composite

polymerization andthe irradiation by thecuring light.The lat-

ter is responsible for the majority of the increase in heat, and

depends on time and on irradiance, which can be particularly

high in modern light-curing units [66,67]. Although manufac-

turers suggest that high irradiance and reduced curing time

will not result in excessive temperature rise or damage to the

pulp, the effect of thermal shock of such curing protocols ondental soft-tissue is not fully elucidated. Hence, any improve-

ment in the material and/or photopolymerization process

should be validated in terms of pulp thermal safety [67].

4. Factors affecting photopolymerizationefficiency

4.1. Intrinsicfactors

4.1.1. Photoinitiator systems

In dental composites, the most commonly used photoinitiator

system is a combination of camphorquinone (CQ) and an


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electron donor, or co-initiator, which are generally differ-

ent types of tertiary amines [68,69]. A key factor affecting

photopolymerization efficiency of dimethacrylate resins is

photoinitiator concentration. An increase in DC and hardness

was observed with increased photoinitiator concentration

[70,71], probably due to the increase of the maximum rate

of polymerization [71]. Consequently, a shrinkage stress of

higher magnitude may have been expected, although notobserved, whereby shrinkage stress was significantly influ-

enced by conversion, but not by the rate of polymerization

[71]. When CQ/amine concentrations are increased beyond a

certain optimum value, a reduction of DC and hardness was

observed, probably due to an excessive absorption of light

in the superficial regions, resulting in less light transmission

to the deeper layers, hence in suboptimal polymerization

[70]. The type of co-initiator [72] as well as their ratio to the

photoinitiator will influence the resulting polymerization

quality: any increase above a certain optimal co-initiator level

will not translate into an increase in polymerization efficiency

[73] but it may affect clinically important parameters such

as material yellowing[74] or biocompatibility, as amines mayexhibit significant cytotoxicity [64]. To date, no specific value

exists for the optimal ratio of photoinitiator to co-initiator

as large differences exist within the variants of resin-based

composite components (i.e. monomer types and ratios, fillers

types, morphology and ratio, pigment content, etc.). Hence,

the concentration of both CQ and amine and their ratio need

to be optimized for different properties. The use of ternary

photoinitiator systems such as the addition of iodonium salts

to CQ/amine, results in a substantial increase in polymeriza-

tion rate, and superior DC, degree of crosslinking, mechanical

properties and color stability [7578].

Numerous photoinitiators have been considered as

alternatives curing systems in dental composites, either toimprove esthetic quality by reducing CQ and its yellowing

effect or because of significantly increased molar absorptiv-

ity and thus, improved polymerization efficiency. Examples

of alternative photoinitiators include phenylpropanedione

(PPD), mono- or bis-acylphosphine oxides (MAPO and BAPO,

respectively), benzoyl germanium or else benzil [7985].

Some of these photoinitiators have already been identified

in commercially available products [67,86,87]. The alterna-

tive photoinitiators reviewed here all exhibit higher molar

absorptivity (also called molar absorption coefficient or molar

extinction coefficient , L mol1 cm1) compared with CQ

(Table 1), which means that the probability for CQ to absorb

light at the peak of its absorption range is much lowerthan for the other photoinitiators. Some also have a sig-

nificantly higher quantum yield conversion (Table 1), which

correspondsto the quantum yield of theinitiation step (Fig. 2).

In other words, it is the ratio of the number of converted

photoinitiators, i.e. those that have lost their absorption prop-

erties, to the number of absorbed photons [88]. The value

of 0.07 measured for CQ combined with dimethylaminoethyl

methacrylate, expresses that the absorption of 14 photons

is necessary for the conversion of one CQ molecule [88]. By

contrast, much higher initiation quantum yield values were

reported for MAPO (Table 1). Furthermore, each converted CQ

molecule only generates one free radical that will actually ini-

tiate polymerization. On the contrary, other photoinitiators

are able to generate several active radicals per molecule, e.g.

two for MAPO, and four for BAPO [80,89]. This explains the

lower polymerization quantum yield of CQ (Table 1), which

is the amount of monomer (or vinyl double bonds) polyme-

rized per absorbed photon [80]. All these features justify the

superiority of other systems, notably phosphine oxides, over

CQ systems in terms of polymerization efficiency [84], but this

also depends on the light source and irradiation parameters,since conventional LED curing units are not compatible with

photoinitiators that exhibit peak absorptions at shorter wave-

lengths (Section 4.2.1). However, the use of MAPO instead of

CQ leads to higher DC, in shorter irradiation times, even as

short as 3 s, and with a conventional halogen curinglamp [67].

This higher efficiency of MAPO has not been confirmed for

other properties yet, but preliminary results show a signifi-

cant increase in elastic modulus, similar or improved flexural

strength, andquite surprisinglygiven the reactionspeed,com-

parable levels of shrinkage stress (unpublished data). The

higher DC might also lead to reduced elution, which must also

be confirmed. The use of Type I photoinitiators such as phos-

phine oxides has other significant advantages, in that theydo not require the use of amines, which avoids optimizing

photoinitiator and co-initiator ratio, but may alsoimprovebio-

compatibility [64] and color stability [90], although there exists

little evidence regarding the biocompatibility of the alterna-

tive photoinitiators. Removal of tertiary amine co-initiators

from light- or dual-cured resin-based materials within acidic

and aqueous conditions would also prevent premature reac-

tions or hindrance of effective polymerization in one-step

self-adhesive systems, or contact of dual-cured materialswith

the oxygen inhibitedlayerof simplifiedself-etching adhesives.

Despite many advantages, phosphine oxides have been

reported to lead to lower depth of cure [83,91,92]. However, in

these studies, the light emission spectra were more adaptedto CQ than to MAPO (Section 4.2.1). Combinations of CQ and

MAPO were also investigated, resulting in higher DC and poly-

merization quantum yield compared to CQ alone, but lower

than the values obtained with MAPO alone [91,93]. The latter

is probably due to an energy transfer from a more efficient

initiator (MAPO) to a less efficient one (CQ) [93]. Recently, an

innovative approachhas beenproposed to overcome the depth

of cure limitation of classical photoinitiator systems, by the

use of phosphors. Thelatter produce blue light insidethe com-

posite itself under the stimulation by near-infrared light [94].

However, the up-conversion of NIR to blue light suffers signif-

icant intensity loss so either unacceptable high laser power is

required, or conversion is limited.In summary, it is clear that some photoinitiator systems

have intrinsic advantages over the classical CQ system. How-

ever, this should be considered in relation to extrinsic factors

such as light spectrum, irradiance and irradiation time. This

is particularly well exemplified by observations concerning

commercial resin composites containing alternative photoini-

tiators. Whilst recent work reports that single diode blue

LED lights achieve similar degrees of polymerization than

broadband (multiple diode) LED (Section 4.2.1) and halogen

lights [95], the samples were light-cured for 40s, which is

far longer than the manufacturers recommendation for the

same light and material combination, i.e. usually 10s. A fur-

ther study investigating a similar material/light-curing unit


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Table 1 Characteristics of the photoinitiators used in resin-based dental composites (CQ: camphorquinone, MAPO: monoacylphosoxide, PPD: phenylpropanedione, B: benzil, BTG: benzoyltrimethylgermane, DBTG: dibenzoyldiethylgermane).

Photoinitiator UVvis absorption Quantumyields(mol/einstein)

Quatum yield efficiency(photons:convertedphotoinitiator molecule)

Numinitiradi

Absorptionrange/max(nm)

Molar extinctioncoefficient at max(Lmol1 cm1)

CQ 400550/470

28

6.61103 0.07 (14:1) 140

46

MAPO 300430/381 520

31.64103 0.35 (3:1)a 2660

BAPO 300440/370 300 9.62103 0.10 (9:1)a 4

PPD 300480/393 37 9.46103 0.10 (9.1)a 2

B 300460/385 50 b b 2

BTG


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combination reportedlarge differences in compositehardness

at shorter exposure time (10 and20 s) between single and mul-

tiple diode LED lights, especially in depth. Besides, increased

pulpal temperature rise went along with spectrum mismatch

and excessive irradiation [67]. Besides extrinsic factors, other

key material properties such as viscosity may significantly

affect curing efficiency of a given photoinitiator compared to

another.

4.1.2. Viscosity, monomers andfillers

There is now compelling evidence that initial resin viscosity is

an important parameter in the reaction kinetics and final DC

of dimethacrylate polymers, as it affects the mobility of each

monomer, hence their reactivity [14]. Two main factors can

affect local viscosity of the monomer system: first, monomer

composition, and second, filler content.

Variations of monomer molecular structure (di- or poly-

methacrylates, molecular weight, molecule stiffness, etc.) and

proportions can significantly affect polymerization efficiency.

As regards the most common bis-GMA/TEGDMA mixture, it

appears that the high reactivity is mainly related due tobis-GMA, which therefore largely controls the polymeriza-

tion mechanisms and kinetics [14,96]. For pure bis-GMA, the

maximum polymerization rate occurs at less than 5% con-

version due to the very high viscosity, and the final DC is

limited to about 30%. By contrast, for pure TEGDMA, which

is far less viscous, the maximum rate is observed around

22% conversion, with a final DC of over 60%, while the differ-

ent co-monomer mixtures show intermediate values between

these two extremes [96]. Based on these observations, it can

be expected that any new dimethacrylate monomer affect-

ing the viscosity differently can also potentially influence

the polymerization efficiency. For example, ethoxylated ver-

sions of bis-GMA(bis-EMA)and hyperbranchedpolymers(highmolecular weight tree-like molecules), which are used to

reduce polymerization shrinkage, have a relatively low viscos-

ity despite their high molecular weight [9799].

Fillers have also a major impact on polymerization effi-

ciency. The impact of fillers within a resin is a decrease in

the maximum DC [27,83,100] (Fig. 3). Even at constant filler

volume (56.7%) for a given resin formulation and differences

in filler size and geometry resulted in significant differences

in DC, from 48 to 61% [101]. Since DC was measured at the

upper composite surface differences in light scattering as a

consequence of filler morphology was unlikely [101]. More-

over, changes in DC may be affected by differences in local

monomer mobility, modulated by variations of the fillerresincontact area, whichcan lead to local changesin viscosityof the

resin surrounding the fillers [102], and hence favor early vitri-

fication. This is in line with the report of trapped free radical

concentrations nearly 3 times higher in the organic matrix of

a filled composite than in a corresponding unfilled resin [18].

Another important aspect of fillers is that the lower the

filler content, the lower the shrinkage and shrinkage stress,

which has been demonstrated in experimental composites

[51]. However, this is not verified in commercial composites,

probably due to the broader range of shrinkage and elastic

modulus values [51], as not only is filler content different,

but also filler type, size and distribution; photoinitiator sys-

tem, monomer type and co-monomer ratio may differ as well.

However, recent research has suggested that the resin matrix

has significantly more influence on polymerization shrink-

age stress than filler content, and underlined the importance

of reducing stress by modifying the resin chemistry with-

out sacrificing filler content and DC [44]. There has been

interest in reducing resin shrinkage and stress generation by

using higher molecular weight resins. For example, hydro-

genated dimer acids admixed withbis-GMAand UDMAexhibithigher conversion without significantly increasing shrinkage

compared with conventional dimethacrylate resins [47,103].

Also, the incorporation of reactive organic nanogel prepoly-

mers into conventional dimethacrylate matrices have been

reported to delay vitrification and modulus development by

altering polymerization reaction rates to reduce stress without

the expected concomitant decrease in polymer conversion or

modulus associated with conventional methacrylate systems

[104]. The use of chain-transfer agents such as hybrid thiol-

ene/methacrylate systems can delay the mobility restriction

through the creation of shorter polymer chains, which leads to

an increase of theDC at whichvitrification takes place without

concomitant increase in shrinkage stress [105,106]. Photo-inducedbond rearrangement using allyl sulfide functionalities

[107] or else trithiocarbonate [108] are other promising strate-

gies to reduce shrinkage stress. Finally, although this review

focuses on dimethacrylates, it is worth mentioning that

non-dimethacrylate chemistries (ring-opening mechanisms)

exhibiting reduced shrinkage have also been developed [109].

However, it is not clear whether they result in lower stress

and better marginal adaptation, as contradictory results were

found: low shrinkage stress was observed for these materi-

als in an experimental setting with low C-factor (0.2) [110],

and a high stress with higher C-factor (3) [111]; similarly, pos-

itive [112] and negative [113] impact of these materials were

reported regarding marginal adaptation.

4.1.3. Opticalproperties

The optical properties of a resin composite and their pho-

topolymerization reaction are interdependent and material

constituents affect the way light is transmitted through it,

and therebyinfluence the polymerization quality, especially in

depth. Several factors can limit light transmission through the

composite. First, light reflection occurs at the surface [54,114].

Second, light can be absorbed, either by pigments [31], which

explains the lower depth of cure observed for darker and

more opaque shades [115], or by photoinitiators [81,88]. In that

regard, the higher the molar absorptivity, the lower the depth

of cure [83]. Filler particles can also hinder light transmissionby scattering, which is dependent on the particle size and on

the incident wavelength of the curing light, with increased

scattering by filler particles dimensions approaching half of

that wavelength [116118]. For example, increase in silica par-

ticle size was shown to result in reduced microhardness and

rate of polymerization in depth [119]. Fillers also hamper light

transmission by refraction at the resinfiller interface, due

to a mismatch between their refractive index [120,121]. The

latter describes how light propagates through a medium; as

light passes through the resinfiller interface, it is accompa-

nied by a change in velocity and a directional change. This

change depends on the medium, and is therefore affected by

the monomer composition. For instance, resins formulated


11/18


with a high percentage of diluent monomer showed the great-

est change in refractive index (and polymerization shrinkage)

[122]. In the same way, a lack of filler silane coating in com-

posites cured under constrained conditions can generate gaps

at the resinfiller interface, resulting in more light attenuation

[123].

Optical characteristics will also change along with the

polymerization of the composite [122]. Upon irradiation,the photoinitiator will decompose/photobleach, and thereby

increase light transmission, as observed for CQ-based sys-

tems [124]. For the latter, the photobleaching rate is much

slower than the polymerization rate andonly 20%of theinitial

amount of CQ is consumed before the polymerization reac-

tion is over [125]. Hence, photoinitiator concentrations should

be optimized to avoid composite yellowing and cure depth

reduction due to excess photoinitiator molecules. MAPO pho-

tobleaching was reported to be somewhat counterbalanced

by the production of colored oxidative by-products during

the polymerization, possibly associated with high polymer-

ization rate and reaction temperature [124]. These colored

by-products can reduce light transmission in depth and affectesthetic quality, though MAPO-based composites still seem

to have overall lower color values than CQ-based ones [84].

Polymerization shrinkage occurring during light cure influ-

ences light transmission as well, as it reduces the optical

path length with the outcome of increased light transmis-

sionthroughthe sample accordingto BeerLambertslaw [126].

Finally, as the resin polymerizes, its refractive index increases

due to the rapid buildup of crosslink density and viscosity;

if it approaches that of the filler, interfacial filler/resin scat-

tering is minimal and light transmission is increased [120].

Hence, optimizing refractive index matching increases light

transmission.

4.2. Extrinsicfactors

Within the literature, there exists sufficient evidence to sug-

gest that the impact of light-curing on the polymerization

process cannot be fully understood independently, but should

be considered in relation with inherent material properties,

which can have a major influence on the efficiency and suc-

cess of photopolymerization.

4.2.1. Light curing units and emission spectrum

It can reasonably be stated that, based on technical develop-

ments, the latest generations of LED curing lights are capable

of competing with other curing light technologies, and evento outperform them in some aspects (cordless, lightweight,

high irradiance, long service life, etc.) [127]. However, the

importance of the light delivery through the curing tip on

polymerization should not be overlooked. A rapid decrease in

irradiance as distance increases has been highlighted for so-

called, turbo light guides [87], and well-collimated straight

light guides should therefore be given priority. There are also

concerns regarding heterogeneity of cure over the surface,

eitherdue to the use of an arrayof severaldiodes(withor with-

out light guide) or to the way light is transmitted through the

tip [87,128,129]. Theuniformity of thelight intensity should be

improved, for example by using additional optical elements

(mixing tube and diffusing screen), which despite causing a

Fig. 4 Comparison of commonly used photoinitiators and

commonly used halogen and LED lights and 3rd generation

(or Polywave). The right axis represents the absolute

irradiance of the light sources, i.e. XL: XL2500 (halogen;

410<


12/18


regarding polymerization, but also to avoid excessive pulpal

temperature rise, since any photon that is absorbed either

by photoinitiators, pigmentsor reactionby-products, anddoes

not initiate polymerization is transformed into heat [67,124].

Finally, while inferior depthof cure was observed forMAPO- vs

CQ-based composites (at similar filler content and equimolar

photoinitiator concentration) [83], the effective irradiance of

MAPO absorption range was much lower than for CQ, whichmight account for the lower depth of cure.

Once spectral correspondence between light and mate-

rial is optimized, a further question is then which irradiation

parameters should be used: the level of irradiance, and irradi-

ation time, or total energy.

4.2.2. Radiant exposure, irradiance and irradiation time

Radiant exposure (J/cm2) sometimes incorrectly, termed

energy density [132] is the total amount of energy deliv-

ered to a resin composite surface during the entire irradiation

procedure. It is the product of light irradiance (mW/cm2) and

irradiation time (s). A certain number of works support the

fact that radiant exposure is the main determining factorof the material properties [133137]. Based on these obser-

vations, the principle known as exposure reciprocity law

states that comparable material properties can be achieved

as long as radiant exposure is kept constant, no matter how

it is obtained, by different combinations of irradiances and

times. This encourages a tendency among dentists and man-

ufacturers to use or suggest a high-power illumination to

reduce curing time. However, the acceptance of this law, as

a general rule, is contradicted by several other works. First,

the increase in mechanical properties saturates above a cer-

tain radiant exposure, and the threshold at which saturation

occurs is property- and material-specific [138]. Second, it was

suggested that despite DC, flexural strength, flexural modulusand depth of cureincreasing at higher radiant exposure, differ-

ent combinations of time and irradiance can leadto significant

differences in the material properties (Fig. 5) within a certain

radiant exposure [41]. This is explained by the fact that there

is probably a loss in radical growth centers by bimolecular ter-

mination during the early stages of polymerization wherein

system mobility is high. This loss (or early termination) is

greater for high irradiance protocols, since at low conversion,

termination is proportional to the squared concentration of

free radicals [19,139].

As highlighted earlier, polymerization efficiency is inti-

mately linked to intrinsic factors (Fig. 1). Therefore, it makes

sense that they also affect the applicability of the law, as itwas recently demonstrated [83]. While large differences in

DC (4472%) were observed between different irradiance/time

combinations (constant radiant exposure) in a 50/50wt%

bis-GMA/TEGDMA unfilled resin, the differences were small

(5158%) when the same resins were filled. Similar observa-

tions have been reportedin unfilled resinsif theresin viscosity

is increased, where reciprocity holds quite well for60/40 wt%

bis-GMA/TEGDMA ratios, but large differences in DC were

reported for less viscous resins [140]. In this regard, though

low molecular weight monomers such as TEGDMA are gener-

ally assumed to reach higher DC [96], it is generally based on

polymerization at low irradiance (0.42.9 mW/cm2) and long

irradiation time (8min). However, this is not representative

of the clinical conditions in which these materials are used,

and it is not verified in all irradiation conditions. For instance,

DC of a 70/30 wt% bis-GMA/TEGDMA mixture is higher than

with 40/60 wt% bis-GMA/TEGDMA (70 vs 55%, respectively)

when cured at 50mW/cm2 during 5s [140]. Regarding pho-

toinitiators, it was previously reported that no significant

difference in DC was observed between CQ-based and PPD-

based [141] or MAPO-based resins [142]. However, these worksare generally based on a single light irradiance, and changes

of irradiation time/irradiance combinations can affect the

results greatly, as illustrated in Fig. 5. While exposure reci-

procity law for DC is generally upheld for MAPO-based resins,

even at low viscosity, it is not the case at all for CQ- and

PPD-based resins. Importantly, even in the cases when reci-

procity law is upheld for one property (DC in general), it is

not necessarily the case for others key material properties

[41,83,138].

In summary, even if exposure reciprocity law holds for

some materials for some properties, the important point is

that is does not for others, and it can therefore not be con-

sidered as a general rule. This is especially important givenmanufacturers reluctance to divulge (in some cases due to

proprietary components) accuratecomposition of commercial

resin composites. As highlighted by comparing the reactivity

of paste composites and their parent flowable formulations,

unknown differences in either co-monomer composition,

filler content and/or photoinitiators can result in substantially

reduced DC for high irradiance curing parameters [143]. Con-

sequently, when evaluating new monomer technologies, such

as the hyperbranched polymers, nanogels, thiol-enes men-

tioned earlier, it is difficult to strictly conclude that these

systems lead to improved properties, since in most develop-

mental studies, they are usually polymerized with a single

irradiation protocol, sometimes not clinically relevant (e.g.50mW/cm2 during 5 min [104]). Hence, it would be appro-

priate to test any composite innovation for a large range of

irradiances and irradiation times.

4.2.3. Irradiation modes

In order to reduce stress transmission at the bonded inter-

face, besides modifications of the material, soft-start curing

protocols (ramp, step or pulse-delay modes) were pro-

posed to delay the onset of polymer gelation, and thereby

contribute to shrinkage stress reduction. Certain authors

explained this effect by a lower conversion and/or infe-

rior crosslinking [36,37,144146]. However, others observed

that soft-start regimen had the potential to reduce shrink-age stress, while keeping DC and/or mechanical properties

constant [42,147149]. Despite an important number of pub-

lications on this subject, there is still no definitive answer as

whether or not soft-start modes are beneficial. Again, this is

probably due to the differences in composition of the various

resin-composites used in the different studies, which proba-

bly affect the efficiency of the soft-start curing modes and the

resulting properties.

4.2.4. Temperature

The temperature during polymerization can significantly

affect polymerization efficiency, as a rise from room tempera-

ture (22 C) to mouth temperature (35 C) results in increased


13/18


Fig. 5 Comparison of DC (measured in real-time by FT-NIR) for 50/50 bis-GMA/TEGDMA unfilled resins containing

commonly used photoinitiators, CQ, MAPO and PPD, the latter with (+) and without () amine co-initiator. They were cured

at similar radiant exposure (18J/cm2) but different times and irradiances, as indicated on the right side on the figure. As it

can be observed, MAPO-based resins display the highest DC, are less dependent upon time/irradiance combination.

CQ-based resins reached higher DC than PPD(+), both varying importantly depending on time/irradiance combination.

PPD() was almost inefficient to initiate polymerization, which means that PPD requires an amine as co-initiator, in

accordance with [85].

DC (610%), hardness and rate of polymerization [150], due

to improved monomer mobility, allowing more of the reac-

tion to occur prior to vitrification [151]. This involves first that

temperature stability is important when studying resin-based

composites, as in any experiment, in order to avoid any arti-

fact due to temperature. Second, it highlights the interest of

preheating composites, not only for practical purposes (better

handling), but also to improve conversion and reduce curingtime [152,153].

An excessive temperature rise might affect pulp vitality as

well as create damage to gingival tissues [154]. Hence, man-

ufacturers and clinicians should be aware that temperature

rise is energy-related, and therefore, the curing parameters

should be optimized to ensure improved material properties,

while avoiding pulp overheating[67,131], which is more haz-

ardous with thinner dentinal tissue coverage above the pulp

[155].

4.2.5. Light guide tippositioning

The adequate positioning of the light guide tip can signifi-cantly affect the energy received by the RBC, and thereby the

quality of its polymerization. First, the irradiance decreases

with the increase of the distance between tip and restora-

tion [156], resulting in reduced DC [157]. This distance is

partly governed by the cavity depth (roughly between 2 and

7mm), but practitioners should also make sure that the tip

is placed as close as possible to the material. Second, the

light guide tip should be placed perpendicular to the mate-

rial surface to optimize light transmission in depth, and its

position should be stabilized during the irradiation proce-

dure, since a lack of control of the tip positioning during the

procedure can result in a decrease of the energy delivered

[158].

5. Conclusion

Photopolymerization conditions have a major impact on the

final mechanical, physical and biological properties of resin

composite materials. Hence, it is clear that dentists play

a significant part in the quality of their restoration by the

appropriate choice of curing conditions. However, this reviewhighlights that the proper selection of extrinsic curing factors

is intimately connected with inherent material characteris-

tics, which are often unknown or, at best, vaguely identified

by the clinicians. In addition, even if the exact product com-

position was disclosed, all factors discussed above affect a

given material property either positively or negatively, making

it impossible to predict the resulting polymerization quality.

In light of this, it is clear that there is a critical need for better

information frommanufacturers on their productsin order for

dentists to be able to knowingly adapt and optimize the use

of resin-based composites in their daily practice. Hence, this

review calls for a significant change in thefuture regarding the

way instructions for use are provided. For each new mate-rial appearing on the market, some kind of identity card

should be required, providing essential information such as

the absorption spectrum, and the impact of various irradi-

ance/time combination on the principal material properties,

e.g. in the form of contour maps [138]. Such situation where

dentists make a decision based on very little information does

not correspond to the currently advocated trend toward a

practice of our medical discipline driven by science and evi-

dence.

Acknowledgment

JL is a Postdoctoral Researcher at F.R.S.-FNRS.


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