Factors involved in the development
of polymerization shrinkage stress in
resin-composites: A systematic review
Roberto R. Bragaa,*, Rafael Y. Ballestera, Jack L. Ferracaneb
aDepartment of Dental Materials, School of Dentistry, University of Sa ˜o Paulo, Av. Prof. Lineu Prestes,
2227, Sa ˜o Paulo, SP 05508-900, Brazil
bDivision of Biomaterials and Biomechanics, Department of Restorative Dentistry, Oregon Health and
Science University, Portland, OR, USA
Received 6 January 2005; accepted 7 April 2005
materials may have a negative impact on the clinical performance of bonded
restorations. The purpose of this systematic review is to discuss the primary factors
involved with polymerization shrinkage stress development.
Data. According to the current literature, polymerization stress of resin composites
is determined by their volumetric shrinkage, viscoelastic behavior and by restrictions
imposed to polymerization shrinkage. Therefore, the material’s composition, its
degree of conversion and reaction kinetics become aspects of interest, together with
the confinement and compliance of the cavity preparation.
Sources. Information provided in this review was based on original scientific
research published in Dental, Chemistry and Biomaterials journals. Textbooks on
Chemistry and Dental Materials were also referenced for basic concepts.
Conclusions. Shrinkage stress development must be considered a multi-factorial
phenomenon. Therefore, accessingthe specific contribution of volumetric shrinkage,
viscoelastic behavior, reaction kinetics and local conditions on stress magnitude
seems impractical. Some of the restorative techniques aiming at stress reduction
have limited applicability, because their efficiency varies depending upon the
materials employed. Due to an intense research activity over the years, the
understanding of this matter has increased remarkably, leading to the development
of new restorative techniques and materials that may help minimize this problem.
Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Objectives. Polymerization shrinkage stress of resin-composite
Dental Materials (2005) 21, 962–970
0109-5641/$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
*Corresponding author. Tel.: C55 11 3091 7840x224; fax: C55
11 3091 7840x201.
E-mail address: email@example.com (R.R. Braga).
In the last three decades, adhesive dentistry has
evolved remarkably, greatly due to the develop-
ment, in the late 1950s, of BisGMA-based compo-
sites . The incorporation of new monomers (e.g.
UEDMA, BisEMA), new initiation systems and filler
technologies have significantly improved the physi-
cal properties of these materials, expanding their
use as direct and indirect restoratives. However,
even considering the intense research on bonding
mechanisms between composites and the dental
substrate, clinical failure due to the disruption of
the bonded interface remains a frequent occur-
rence . Such interfacial defects may develop as a
consequence of long-term thermal and mechanical
stresses, or during the restorative procedure itself,
due to stresses generated by composite polymeriz-
ation shrinkage . In fact, a direct relationship
between polymerization shrinkage stress and mar-
ginal integrity has been demonstrated, in vitro, in
class V restorations [4–6] and in teeth restored with
bonded porcelain inlays .
Contraction stress in composite restorations is
the result of polymerization shrinkage taking place
under confinement, due to bonding to cavity walls.
The material’s viscoelastic behavior, characterized
by its flow capacity at early stages of the curing
reaction and by the elastic modulus acquired
during polymerization, has also been identified as
another important factor in contraction stress
As both volumetric shrinkage and viscoelastic
properties are influenced by the same variables,
accessing their specific role on stress development
is a difficult task. For example, composites with
relatively high inorganic filler content present
lower shrinkage values but higher stiffness (i.e.
lower strain capacity), compared to materials
with lower inorganic content [9,10]. On the other
hand, increasing degree of conversion of the
polymer matrix increases volumetric shrinkage
and elastic modulus simultaneously [11,12]. The
complexity of this issue is heightened by the fact
that stress development is affected by reaction
kinetics. As the composite’s plastic deformation (or
viscous flow) is a time-dependent event, slower
curing rates may provide extended periods where
the material is able to yield to contraction forces
before acquiring higher elastic modulus . In
fact, reducing polymerization rates in composites
has been shown to lower stress levels significantly
The influence of confinement conditions imposed
on the composite sample and the compliance of the
bonding substrate have also been a subject of
intense debate in the literature. A direct relation-
ship between contraction stress and confinement of
the composite sample was found when a rigid, non-
compliant testing system was employed .
However, other authors have found that these two
variables were inversely related when using a less
rigid, more compliant set-up [18,19].
From a clinical standpoint, it is important to
determine how these laboratory results can be
applied to the restorative procedure. The purpose
of this systematic review is to discuss the various
factors that influence the development of contrac-
tion stresses in dental composites, i.e. volumetric
shrinkage, viscoelastic properties, and extent of
cavity constraint, as well as their influence on
different aspects of the restorative procedure.
When monomers in proximity react to establish a
covalent bond, the distance between the two
groups of atoms is reduced and there is a reduction
in free volume, both of which translate into
volumetric shrinkage. The magnitude of volumetric
shrinkage experienced by a composite is determined
by its filler volume fraction and the composition and
degree of conversion of the resin matrix.
Shrinkage values reported for BisGMA (5.2%) and
TEGDMA (12.5%) are substantially higher than those
displayed by typical composites, which range
between 2 and 3% [9,20,21]. This difference is due
to the fact that in hybrid composites, approxi-
mately 60% of the volume is occupied by filler
particles. Microfilled composites, though their
inorganic content is typically about 40 vol%, have
shrinkage values similar to hybrids, due to the
presence of pre-polymerized composite particles,
sometimes referred to as ‘organic fillers’, which
render them similar to hybrid composites in terms
of the actual volume fraction of polymerizing resin.
Low-viscosity (flowable) composites present volu-
metric shrinkages up to 5%, in large part due to their
reduced inorganic content, which is typically below
50 vol% . The shrinkage values reported should
be considered approximate, because they are
completely dependent upon the extent of the
polymerization reaction. This makes the compari-
son of shrinkage data from different studies a
tenuous proposition, as it is not typical for authors
to measure degree of conversion along with
shrinkage, except in rare cases [15,16].
The volumetric shrinkage of composites has been
shown to be proportional to its degree of conversion
Polymerization shrinkage stress—a review 963
[12,16]. In the photoactivated materials, degree
of conversion is determined by the product of
light irradiance and exposure time (radiant
exposure, J/cm2) . As curing rate is pro-
portional to the square root of the light intensity
applied to the composite , it has been proposed
that the method by which light energy is delivered
to the composite is capable of delaying the
acquisition of elastic modulus, allowing polymeric
chains to re-arrange and microscopically and
macroscopically accommodate the reduction in
volume by plastic deformation . It is the so-
called ‘rigid contraction’ (sometimes previously
referred to as ‘post-gel contraction’) that is the
fraction of the total volumetric shrinkage respon-
sible for stress development [25,26]. In the
photoactivated composites, the fast reaction rate
virtually eliminates the time allowed for viscous
flow, and it is estimated that the polymer matrix
becomes ‘rigid’ within seconds after a relatively
low degree of conversion . As a result, stress
begins to build-up almost immediately after pol-
ymerization is triggered , and nearly all of the
shrinkage occurs after the polymer matrix has
reached a significant level of rigidity, the magni-
tude of which continues to increase with time. Self-
cured composites, on the other hand, develop
lower contraction stress values than light-cured
materials, in part due to their slower reaction rate,
but also because the self-initiated reaction gen-
erates a smaller number of free-radicals than
photoactivation, often resulting in lower degrees
of conversion [7,13].
the time allowed for viscous flow (and, conse-
quently, for non-rigid shrinkage) several photoacti-
vation methods have been proposed as alternatives
for continuous high-intensity irradiation [28,29].
These curing routines, generically referred to as
‘soft start’, use reduced light irradiance during the
first few seconds of light activation, switching to
high irradiance for the remaining curing time in
order to provide the material with sufficient radiant
exposure. The efficacy of these curing methods has
been demonstrated by several studies reporting
significant reductions in shrinkage strain when
compared to continuous high-intensity photoactiva-
tion [12,22,30,31]. However, significant reductions
in reaction rate do not necessarily correspond to
significant reductions in contraction stress [14,16].
One hypothesis that might explain this discrepancy
is the above-mentioned fact that dimethacrylate
composites develop elastic modulus at very low
conversions . Therefore, even at relatively low
composite ceases to allow significant plastic
deformation is reached rapidly. Therefore, it is
likely that only impractical applications of low light
power for prolonged periods may provide the slow
reaction rates necessary to significantly reduce
composites contraction stresses in the clinical
Reducing contraction stress by changing polym-
erization rate has limitations. First, its efficacy
varies according to composite formulation. A study
evaluating the effect of low curing rates on
contraction stress development of three commer-
cial materials found stress reductions between 19
and 30% . A photoelastic study observed stress
reductions between 3 and 7% in five composites
. Another potential problem with this method is
that it may produce alterations in the nature of the
polymer network formed. Some studies have
provided evidence that suggests that the quality
of the polymer network formed is independent of
the mode of light energy application, and is solely
dependent upon the overall degree of conversion
[33,34]. However, other researchers have observed
that materials cured by non-continuous photoacti-
vation were more susceptible to ethanol degra-
dation than those photoactivated using continuous
high-intensity irradiation, even though their
degrees of conversion were equivalent [35–37].
The authors have suggested that the use of low
irradiances generates a small number of free
radicals, resulting in longer polymeric chains with
low cross-linking density, evidenced by a more
severe softening effect of the ethanol. Unpublished
observations suggest that this negative effect may
not be the result of low initial irradiances per se,
but the association of slow curing with a relatively
low overall energy application.
The concentration of diluent in the resin matrix
also affects shrinkage. A recent study verified that
higher TEGDMA/BisGMA ratios in experimental
composites resulted in higher contraction stress
values due to increased volumetric shrinkage, as a
result of enhanced conversion . Because they
typically have lower molecular weight than the host
monomers, ‘diluent’ monomers increase the den-
sity of polymerizable carbon double bonds, which
may lead to more shrinkage. Furthermore, the
mobility in the reaction environment is increased
due to the lower viscosity and Tgof the diluent,
allowing a more efficient conversion . It is
interesting to note that the reduction in composite
viscosity and stiffness resulting from high diluent
concentrations was not able to compensate the
effect of increased shrinkage on stress development
. This result suggests that conversion and its
resultant volumetric shrinkage are the most
R.R. Braga et al.964
important factors affecting the development of
contraction stress in dental composites.
Several studies have found that for regular or
packable composites, contraction stress is directly
proportional to filler content, regardless of differ-
ences in matrix composition [6,41,42]. Those
findings suggest that within a relatively narrow
range of shrinkage values, viscoelastic character-
istics are determinant factors in stress develop-
ment. However, stress values produced by
composites having lower filler content to produce
lower viscosity, and consequently lower stiffness,
are similar to those displayed by other materials
possessing higher filler contents and greater stiff-
ness . This indicates that when a wide range of
shrinkage values is considered, volumetric shrink-
age prevails over viscoelastic properties in deter-
mining contraction stress.
Resin composites are solids with complex viscoe-
lastic behavior. When submitted to an instan-
taneous load that generates stress below the
elastic limit, these materials undergo elastic
deformation which is promptly recovered at load
removal. If the load is applied for a certain length of
time, these materials begin to present viscoelastic
deformation, characterized by a combination of
elastic deformation that is recovered after load
removal and viscous (permanent) deformation. The
response to load application is dependent on the
composite’s filler content , matrix chemistry,
and degree of conversion . Strain capacity is
inversely related to inorganic filler content .
Considering the relatively narrow range of volu-
metric shrinkage values displayed by hybrids and
microfilled composites with regular consistency,
contraction stress onset and its final value are
directly affected by the composite’s inorganic
content [6,42]. Microfilled composites usually
develop lower stress values compared to hybrids
due to their lower elastic modulus, higher strain
capacity and similar volumetric shrinkage .
Likewise, materials with high degrees of conversion
undergo less deformation, because enhanced poly-
mer chain entanglement and high cross-linking
density hinders chain movement in the polymer
Initially, some studies tried to characterize
contraction stress development using the linear
elastic model [24,26]. According to this model, the
increment in stress for a certain time interval would
be proportional to the product of the increase in
volumetric shrinkage by the increment in the
material’s elastic modulus, according to Hooke’s
law. However, the stress values calculated by this
method were far greater than those verified in
mechanical tests . The difference between
calculated and observed stress is explained by the
occurrence of viscous flow, mostly occurring prior
to the acquisition of significant elastic modulus.
Early in the polymerization reaction, composites
present a predominantly viscous behavior and,
gradually, they become predominantly elastic
. The idea that viscous flow accommodates a
significant fraction of the total volumetric shrink-
age is supported by studies that measured contrac-
tion stress development as a function of degree of
conversion. These studies agree that stress build-up
occurs at a faster rate at high conversions [16,45]. A
possible hypothesis to explain such findings is that
early in the reaction, chain growth and primary
cyclization occur preferentially to cross-linking.
The absence of covalent bonds between polymer
chains would allow plastic deformation and stress
development would be relatively slow. Beyond a
certain conversion level, cross-linking would pre-
vail, and small increments in conversion would lead
to significant increase in stress, due to the high
stiffness of the polymer network. This rationale is
supported by studies showing that the glass
transition temperature of composites, considered
an indicative of cross-linking density, also increases
at a faster rate at high conversion levels [27,46].
Nevertheless, even after acquiring measurable
elastic properties, the composite material is able to
flow . Two studies evaluated contraction stress
in relation to shrinkage strain development. One
study found that a self-cured composite had
developed less than 10% of its maximum stress
when shrinkage strain had reached 50% of its final
value . In the second study, there was a two-
second delay between detectable shrinkage strain
and contraction stress offset in a photoactivated
As mentioned before, changes in composite
structure during the early stages of conversion are
characterized mechanically by an increase in
viscosity and elastic modulus. The ratio between
these two parameters is denoted by the ‘stress
relaxation time’, and represents the time necessary
for the stress to decline to 1/e (w37%) of the initial
value . As viscosity is a time-dependent
property, the material should present relatively
low viscosity in order to be able to yield to
contraction forces and postpone stress build-up.
After the material begins to acquire elastic
modulus, a low stress relaxation time is preferable
to allow for more rapid plastic deformation to
Polymerization shrinkage stress—a review965
relieve stresses. Therefore, the influence of
reaction kinetics on stress development becomes
evident. Fast curing rates do not allow enough time
for viscous flow. Moreover, elastic modulus acqui-
sition in composites occurs rapidly , which
further shortens the time available for stress
relaxation. Thus, rate of conversion is a significant
factor affecting the generation of contraction
stress in dental composites, though it is not possible
to determine the magnitude to which it influences
stress in relation to the other significant factors like
volumetric shrinkage and elastic modulus, because
they are all interrelated.
Restrictions imposed on composite
When composite shrinkage is restricted by adhesion
to the cavity walls, two variables must be
considered. First, the level of confinement imposed
on the material, which is estimated as the
percentage of composite surface that is bonded to
the substrate in relation to the total surface area,
and second, the compliance of the bonding
substrate. The substrate’s compliance, is charac-
terized by the stiffness and mobility of its walls. For
example, the compliance of a mesio-occlusal-distal
preparation is related to the elasticity of the dentin
and enamel substrate and to the possibility of
cuspal deflection. The effect of confinement and
compliance of the bonding substrate on stress
values and on the integrity of the bonded interface
has been the object of intense debate in the
The majority of the information regarding
contraction stress development has been obtained
through mechanical testing. The most frequently
used set-up consists of placing the composite
between two flat surfaces (usually metal or glass)
connected to the opposing fixtures within a
universal testing machine. In order to create a
highly rigid (i.e. near-zero compliance) confine-
ment condition for the curing composite, the
approximation of the opposite bonding substrates
due to the material’s axial shrinkage is constantly
monitored by a feedback system and counteracted
by the displacement of the cross-head in the
opposite direction, keeping the specimen height
constant [3,17,42]. Therefore, the load cell regis-
ters the force necessary to keep the initial height of
the specimen, and stress is calculated by dividing
the force by the cross-section area of the bonding
substrate. Some authors choose not use a feedback
system to keep the specimen height constant
[18,19,41]. In this case, the composite is able to
shrink in its long axis without restriction, other than
that represented by the force needed to deform the
load cell and other components of the testing
apparatus that may resist the shrinkage. Recently,a
novel stress measuring device based on the
cantilever beam theory has been introduced, with
the main feature of allowing variations in the
system compliance, in order to mimic the com-
pliance o the tooth structure .
As with any method, contraction stress measure-
ments via mechanical testing present limitations.
The main drawback refers to the fact that, though
the shrinking composite develops a tri-axial stress
state, only the stress manifested in the long axis of
the specimen is registered . Another criticism
refers to the low compliance of the testing system,
resulting from the high stiffness of the material
used as bonding substrate, associated with the axial
restriction to shrinkage. It is reasonable to assume
that the deformation of the dental substrates would
relieve part of the stress during the curing of
composite restorations . Consequently, a near-
zero compliance testing set-up would overestimate
stress values, which may range from 4 to 25 MPa,
depending on the composite tested, specimen
dimensions and photoactivation method [15,42,
52]. However, a photoelastic study found that,
considering an ideal adhesion of the composite to
the cavity walls, contraction stress can reach up to
23 MPa at the internal angles of the cavity ,
which leaves this question open. In spite of the
controversy, studies have demonstrated a good
correlation between stress values obtained using
low compliance systems and results of microleak-
age in bulk-filled composite restorations [5,6].
In order to describe confinement conditions and
correlate them with stress values, the term ‘cavity
configuration factor’ (C-factor) was created, and is
defined as the ratio between bonded and unbonded
surfaces of the composite specimen . Using a
near-zero compliance testing system, the authors
observed that higher C-factors corresponded to
higher stress values. Since composite flow is more
likely to occur from the free surfaces of the
specimen, a higher proportion of free composite
surface would represent a smaller restriction to
shrinkage, thereby reducing stress. In contrast,
when free surface is reduced, shrinkage perpen-
dicular to the long axis of the specimen is restricted
by the adhesion to the substrate and a larger
proportion of the volumetric shrinkage manifests in
the long axis of the specimen, increasing stress
values [51,54]. The influence of the confinement on
stress values was confirmed in previous studies
using a similar testing apparatus [24,52]. However,
R.R. Braga et al.966
when composite shrinkage on the long axis of the
specimen is only partially restricted, C-factor and
stress seem to be inversely related [18,55]. More-
over, the volume of shrinking composite becomes a
variable to be considered. For a certain bonded
area, taller specimens, which would have lower the
C-factor, exert higher contraction forces on the
load cell of the testing machine .
The application of the C-factor concept to
clinical practice must be done very carefully. Cavity
preparations present a much more complex geo-
metry than the specimens used in in vitro mechan-
ical testing, resulting in a very heterogeneous stress
distribution [51,53]. Photoelastic and microleakage
studies suggest that the C-factor would be appli-
cable to restorations only when comparing cavities
with similar volume [56,57]. In restorations with
different volumes, minimal correlation was found
between interfacial gaps and C-factor .
The use of incremental insertion techniques for
the purpose of reducing the effects of cavity
confinement has also been questioned by some
authors. Finite element analysis, photoelastic and
microleakage studies have found no significant
reduction in stress or enhancements in marginal
adaptation when composite is inserted in small
increments compared to insertion in bulk [59–61].
These findings suggests that either the confinement
is not a determinant factor in stress development of
dental restorations in natural teeth, or that the
reduction in C-factor obtained by incremental
insertion is not sufficient to cause significant stress
Clinically, stress magnitude can be reduced by
applying a low stiffness material between the
composite restorative and the cavity walls to
increase the compliance of the bonding substrate.
Another benefit from this procedure is that stress
distribution is more uniform along the low elastic
modulus layer . This technique, known as
‘elastic cavity wall’ is accomplished with the use
of an intermediate layer of low-viscosity composite
between the adhesive layer and the restorative
composite . In vitro evaluations of this tech-
nique, however, show inconsistent results [64,65].
Two factors interact to produce significant stress
relief with the use of a low-stiffness layer: the
thickness of the intermediate layer and its elastic
modulus . Thicker layers favor stress relief
because elongation, in absolute numbers, is pro-
portional to the material’s initial dimension . A
study using finite element analysis verified a
reduction of 10% in shear stresses at the tooth/
restoration interface in a situation where a 40-mm
layer of intermediate material with elastic modulus
of 5 GPa was included in the model, compared to
the control condition (without intermediate layer).
When the thickness was increased to 80 mm, stress
reduction was 38% . Other authors found that an
increase from 40 to 200 mm in the unfilled resin
layer thickness applied to the bonding substrate of a
mechanical testing set-up resulted in a 24 and 30%
reduction of the stress developed by a hybrid
composite, depending on the C-factor of the
composite sample . In the same study, the
application of thicker layers of unfilled resin to
cavity walls of cylindrical class V restorations
resulted in significant reduction in leakage.
Besides differences in thickness, discrepancies
among authors may happen due to differences in
the elastic modulus of the materials used as
intermediate layers. Elastic modulus of low-vis-
cosity composites varies greatly, with values
between 6.5 and 12.5 GPa being reported . A
materials (with flexural modulii between 4.1 and
8.2 GPa), one unfilled resin (2.1 GPa) and a regular-
viscosity composite (12.3 GPa) as pre-cured sub-
strate for a hybrid composite. Observed stress
reductions were between 22 and 53%, but these
were only significant with the unfilled resin and one
of the flowable composites . Interestingly, the
composite allowing significant stress reduction was
not the least rigid, suggesting an inconsistent
behavior of low-viscosity composites with flexural
modulus in the range of 4 and 6 GPa as intermediate
materials. Also, these results suggest that some
low-viscosity composites would not be indicated to
increase the compliance of the cavity walls and
reduce stress build-up.
Composite flow may also be increased by
modifications in material’s structure. Though the
alternatives described below are experimental,
some of them may serve as the basis for future
material development. The incorporation of pores
in unfilled resin was shown to reducestress between
17 and 42% . The authors suggested that the
presence of pores increase the material’s free
surface, facilitating flow. Also, the oxygen in the
pores may reduce degree of conversion, which also
contributes to reducing the stress. However, this is
an option of limited applicability, as a porous
material may have its cohesive strength impaired.
Unbonded filler particles were also tried as a way to
increase the available sites for composite flow,
without reducing its mechanical properties .
Stress reduction observed in hybrid and microfilled
composites ranged between 30 and 50%, respect-
ively. Subsequent studies have shown that the use
of unbonded nanofiller did not significantly reduce
flexural strength or fracture toughness of compo-
sites materials . In vivo, however, a tendency
Polymerization shrinkage stress—a review967
for increased wear after 1 year was observed .
High-density polyethylene (HDPE)spheres were also
tried as additives or substitutes of part of the
inorganic filler . When 20 wt% of HDPE was use,
stress reductions up to 25% were accomplished
either by reduction in volumetric shrinkage (when
added to the microfilled composite) or by reduction
in elastic modulus (when replacing part of the filler
in the hybrid composite). However, the applica-
bility of this approach in hybrid composites is
limited by the significant reduction in mechanical
An intense research activity in the last few years
brought many contributions to the knowledge on
polymerization contraction stress of resin compo-
sites. Nevertheless, several aspects regarding this
extremely complex phenomenon remain unclear.
Considering the inherent volumetric shrinkage of
BisGMA-based materials and the desire to maximize
degree of conversion and elastic modulus for
enhanced clinical performance, maximizing viscous
flow seems to be a key mechanism to reduce stress.
Therefore, further investigations on viscoelastic
behavior and reaction kinetics of these materials
are necessary. The resources currently available to
reduce contraction stress are somewhat limited.
Nevertheless, based on scientific evidence, a few
aspects of clinical interest can be observed:
1. Volumetric shrinkage should not be the only
parameter considered to predict composite
Materials with relatively low shrinkage due to
high inorganic filler content also present high
elastic modulus, which may result in increased
2. Reduced polymerization rate due to the use of
alternative photoactivation methods does not
necessarily lead to significant reductions in
contraction stress. Also, a particular curing
routine may not be efficient with composites
from different manufacturers. Stress reduction
cannot be achieved at the expense of adequate
degree of conversion.
3. Increasing the compliance of the cavity walls by
applying an intermediate low-modulus layer may
lead to significant stress relief depending on its
thickness and elastic modulus. Some low-vis-
cosity ‘flowable’ composites may be too stiff to
be successfully used for this purpose.
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