1. Influence of photoactivation method and
mold for restoration on the Knoop hardness
of resin composite restorations
William Cunha Brandt, Lais Regiane
Silva-Concilio, Ana Christina Claro
Neves, Eduardo Jose Carvalho de SouzaJunior, et al.
Lasers in Medical Science
ISSN 0268-8921
Lasers Med Sci
DOI 10.1007/s10103-012-1184-2
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Lasers Med Sci
DOI 10.1007/s10103-012-1184-2
ORIGINAL ARTICLE
Influence of photoactivation method and mold for restoration
on the Knoop hardness of resin composite restorations
William Cunha Brandt & Lais Regiane Silva-Concilio &
Ana Christina Claro Neves &
Eduardo Jose Carvalho de Souza-Junior &
Mario Alexandre Coelho Sinhoreti
Received: 25 October 2011 / Accepted: 6 August 2012
# Springer-Verlag London Ltd 2012
Abstract The aim of this study was to evaluate in vitro the
Knoop hardness in the top and bottom of composite photo
activated by different methods when different mold materials
were used. Z250 (3M ESPE) and XL2500 halogen unit (3M
ESPE) were used. For hardness test, conical restorations were
made in extracted bovine incisors (tooth mold) and also metal
mold (approximately 2 mm top diameter × 1.5 mm bottom
diameter × 2 mm in height). Different photoactivation methods
were tested: high-intensity continuous (HIC), low-intensity
continuous (LIC), soft-start, or pulse-delay (PD), with constant
radiant exposure. Knoop readings were performed on top and
bottom restoration surfaces. Data were submitted to two-way
ANOVA and Tukey’s test (p00.05). On the top, regardless of
the mold used, no significant difference in the Knoop hardness
(Knoop hardness number, in kilograms–force per square millimeter) was observed between the photoactivation methods.
On the bottom surface, the photoactivation method HIC shows
higher means of hardness than LIC when tooth and metal were
used. Significant differences of hardness on the top and in the
bottom were detected between tooth and metal. The
W. C. Brandt (*)
Department of Dentistry, Implantology Area,
University of Santo Amaro,
Rua Prof. Eneas de Siqueira Neto, 340,
04829-300 São Paulo, SP, Brazil
e-mail: williamcbrandt@yahoo.com.br
L. R. Silva-Concilio : A. C. C. Neves
Department of Prosthodontics, Dentistry School,
University of Taubate,
Rua Expedicionário Ernesto Pereira, 110,
12020-330 Taubate, São Paulo, Brazil
E. J. C. de Souza-Junior : M. A. C. Sinhoreti
Dental Materials Division, Department of Restorative Dentistry,
Piracicaba Dental School, University of Campinas,
Piracicaba, Brazil
photoactivation method LIC and the material mold can interfere in the hardness values of composite restorations.
Keywords Dental materials . Composite resins . Surface
properties . Hardness test . Dental restoration
Introduction
Although light-cured resin composites have become the material of choice in directly restoring anterior and posterior
teeth, these materials undergo significant volumetric shrinkage when polymerized [1]. Placement and bonding of composites in preparations induce development of mechanical stress
inside the material as well as at the bonded interface [2]. Stress
is also transmitted via bonded interfaces to tooth structures [3,
4]. The rapid conversion rate in light-cured composites quickly induces an increase in composite stiffness, causing high
shrinkage stresses at the bonded interface [3]. This stress may
disrupt bonding between the composite and the preparation
walls or even cause cohesive failure of either the restorative
material or the surrounding tooth tissue [3–5]. This stress
development is the main cause of marginal failure and subsequent leakage in resin composite restorations [6].
In order to attenuate the stress generation during the polymerization process, different solutions have been recommended, such as modified filler particle interfaces, cavity lining
with flowable composite, and employment of non-shrinking
resins. There is also a possibility to decrease the polymerization reaction rates and consecutive mechanical stress transferred. This is achieved by decreasing an irradiance of a light
source or by using different photoactivation protocols, such as
low-intensity continuous (LIC), soft-start (SS), and pulsedelay (PD) [1–6]. The main goal of these methods is to
increase the time for the composite to flow during the earlier

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stages of the polymerization and to enable a certain degree of
polymer chain relaxation before reaching the rubbery stage [3,
7]. Indeed, previous investigators have described improved
marginal adaptation and increased bond strength [3, 7, 8] in
comparison to the standard high-intensity continuous (HIC)
method. However, a recent study has reported that the degree
of conversion (DC) for these methods might be lower when
compared to the conventional method [9].
Studies evaluating different curing strategies generally concentrate on the conversion of double bonds. The DC is an
important factor because it determines the final properties of
composites and can be analyzed indirectly by the hardness test
[10, 11]. Regardless of the method as a composite restoration is
activated, this should have appropriate properties. However,
these properties should not be evaluated only on the surface of
the composite, but in the bottom. Because an inadequate
polymerization of the composite in the bottom interfere in the
properties of the composite restoration causing it to fail [3].
Moreover, many kinds of mold materials (tooth, metal,
and silicone) are used to evaluated the hardness of dental
composites in vitro. The use of different molds can cause
differences in the spread of light within the composite
inserted into the mold, which can cause different results of
DC and, consequently, in hardness. Then, the results
obtained in these studies can be extrapolated for use on teeth
and applied in the clinical dentistry.
Therefore, the aim of this study was to investigate in vitro
the influence of different light-curing methods and mold
material used to evaluated the Knoop hardness on the top
and bottom of resin composite. The first hypothesis of this
study was that different photoactivation methods could produce the same hardness values when the same radiant exposure is used and the second hypothesis was that the mold
could influence the hardness of dental composites.
Materials and methods
Restorative procedures
Forty bovine incisors and 40 metal mold were obtained.
After extraction, the bovine incisors were cleaned and stored
in 0.5 % chloramine-T solution at 4 °C, for a week. After
removal of root portions, buccal faces were wet-ground with
400-, 600-, and 1,200-grit SiC abrasive papers to obtain a
Table 1 Description of the
photoactivation methods
flat surface on the enamel. For both molds to be restored
(bovine incisors and metal mold), standardized conical cavities (approximately 2 mm top diameter × 1.5 mm bottom
diameter × 2 mm in height) were then prepared, using #3131
diamond burs (KG Sorensen, Barueri, Sao Pãulo, Brazil) at
high speed, under air–water cooling. A custom-made preparation device allowed standardization of the cavity dimensions. The burs were replaced after every three preparations.
In order to expose the bottom surface of the cavities in the
bovine incisors, the lingual faces were ground following the
same procedure described for flattening the buccal aspects.
The specimens were placed onto a glass slab and the
restorative procedures were carried out using the resin composite Filtek Z250 (3M ESPE, St. Paul, MN, USA, shade
A2), which was bulk inserted into the cavity from its wider
side. Different photoactivation procedures, as described in
Table 1, were tested. For each method, ten specimens were
prepared. Photoactivation was performed only at the top of
the molds.
Prior to the curing procedures, the output power of the
halogen curing unit XL2500 (3M ESPE, St. Paul, MN,
USA) was measured with a calibrated power meter (Ophir
Optronics, Danvers, MA, USA) and the diameter of the light
guide tip with a digital caliper (Mitutoyo, Tokyo, Japan).
Light irradiance (in milliwatts per square centimeter) was
computed as the ratio of the output power and the area of the
tip. Different curing times were used in order to maintain a
total radiant exposure of approximately 18.5 J/cm2 for all
samples. Irradiance at high light intensity (935 mW/cm2)
was carried out with the light guide tip positioned directly
onto the restoration, which had been previously covered
with a polyester strip. To produce an output of 150 mW/
cm2, a standard black acrylic cylinder separator was used to
allow positioning the light guide tip 1.2 cm away from the
restoration surface, and the irradiance was confirmed with
the power meter.
Irradiance measurements were also performed through of
the metal mold or tooth mold with the restoration. The
purpose of this measurement was to obtain the amount of
light (irradiance, in milliwatts per square centimeter) which
could cross the resin composite inserted in the molds used
(metal mold or tooth mold) after the restoration.
Spectral distributions were obtained by using a calibrated
spectrometer (USB2000, Ocean Optics, Dunedin, FL,
USA). The irradiance and the spectral distribution data were
Photoactivation method
Exposure protocol
High-intensity continuous (HIC)
Low-intensity continuous (LIC)
Soft-start (SS)
Pulse-delay (PD)
935
150
150
150
mW/cm2
mW/cm2
mW/cm2
mW/cm2
for
for
for
for
20 s
125 s
10 s + 935 mW/cm2 for 18 s
5 s + 3mim without light + 935 mW/cm2 for 18 s

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integrated using the Origin 8.0 software (OriginLab Northampton, MA, USA). Spectral distributions measurements
were also performed through of the metal mold or tooth
mold with the restoration. The purpose of this measurement
was to obtain the emission spectrum of light which could
cross the resin composite inserted in the mold used (metal
mold or tooth mold) after the restoration. Irradiance measures between wavelength 450 and 490 nm were performed
to evaluated the amount of light available in the region for
better absorption of light by the photoinitiator contained in
the composite resin: camphorquinone [12]. All these measures were carried out as HIC photoactivation method.
Hardness assessment
After light-curing procedures, the specimens were dry-stored
at 37 °C for 24 h in light-proof containers. Thereafter, both the
top and bottom surfaces were wet-polished with 1,200-grit
SiC paper to obtain a planar surface. Knoop hardness measurements were taken on both surfaces using an indenter
(HMV-2, Shimadzu, Tokyo, Japan), under a load of 490 N
(equivalent to 50 gf) for 15 s. Five readings were performed
for each surface. The Knoop hardness number (KHN, in
kilogram–force per square millimeter) for each surface was
recorded as the average of the five indentations. Data were
submitted to two-way ANOVA (photoactivation method vs.
mold material) followed by Tukey’s test (p00.05).
Results
Tables 2 and 3 show the means hardness Knoop on the top
and bottom when different photoactivation methods and
different mold for restoration were used. For top hardness,
irrespective of the light-curing method, no significant differences were detected, both for the mold in tooth and in metal.
On the other hand, significant differences were detected
between tooth and metal when HIC and LIC were used for
the photoactivation. The means of hardness Knoop were
higher in the metal that in tooth (p<0.05).
In the bottom surface, the photoactivation methods HIC,
SS, and PD show higher means of hardness Knoop that LIC
when tooth was used. When metal was used, HIC
shows higher means of hardness Knoop in the bottom
than LIC (p<0.05), and SS and PD show intermediated
means without statistical difference. Significant differences were detected between tooth and metal when SS
and PD were used for the photoactivation. The means
of hardness Knoop were higher in the tooth that in
metal (p<0.05).
Emission spectra of the light-curing unit (LCU) with
and without material mold used in this study are shown
in Fig. 1. The irradiance of the halogen curing unit
XL2500 (3M ESPE) measured with the calibrated power
meter was 935 mW/cm2 with emission peak at 485 nm
and irradiance between the wavelength 450 and 490 nm
of 521 mW/cm2.
After restoration of the molds with composite resin, when
the bovine incisors was used as mold (tooth mold), the
irradiance of 230 mW/cm2 with emission peak at 488 nm
across the tooth mold restored with resin composite. Between the wavelength 450 and 490 nm, only the irradiance
of 129 mW/cm2 across the tooth mold restored with resin
composite. When metal mold was used, only the irradiance
of 110 mW/cm2 with emission peak at 487 nm across the
metal mold restored with resin composite, and between the
wavelength 450 and 490 nm, only 63 mW/cm2 across the
metal mold restored with resin composite.
Discussion
As one of the main disadvantages of dental composite
remains in its polymerization shrinkage [1, 4, 5]. Therefore,
techniques that reduce the tension caused by the contraction
of polymerization are used. Thus, photoactivation methods
such as SS and PD are used for this purpose [3, 7–9]. The
most usual light unit used for composite resin polymerization is the conventional halogen light. The halogen bulb
consists of a filament, which is heated and which emits
white light with unwanted wavelengths being filtered out;
a polychromatic spectrum of blue light is therefore produced
(covering the area from 400 to 500 nm of the visible spectrum). There are, however, some side effects, such as temperature increase during light-curing [13, 14].
However, doubts about the properties of the polymer
formed when these photoactivation methods are used remain [10]. Knoop hardness test is widely used to evaluated
the properties of dental composites, especially with regard
to the DC [10].
Table 2 Means (standard deviations) for top hardness (KHN, in kilograms–force per square millimeter)
High-intensity continuous
Tooth
Metal
Low-intensity continuous
61.0 (5.0) B, a
65.2 (2.7) A, a
57.5 (6.4) B, a
63.9 (4.6) A, a
Soft-start
Pulse-delay
59.7 (3.6) A, a
61.9 (2.1) A, a
58.5 (1.9) A, a
61.9 (1.4) A, a
Means followed by distinct capital letters in the same column, and small letters in the same line, are significantly different at p<0.05

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Table 3 Means (standard deviations) for bottom hardness (KHN, in kilograms–force per square millimeter)
High-intensity continuous
Tooth
Metal
Low-intensity continuous
58.6 (3.3) A, a
55.9 (3.0) A, a
53.1 (3.3) A, b
51.2 (4.2) A, b
Soft-start
Pulse-delay
57.3 (3.4) A, a
54.5 (2.3) B, a, b
58.4 (2.8) A, a
52.4 (2.1) B, a, b
Means followed by distinct capital letters in the same column, and small letters in the same line, are significantly different at p<0.05
Therefore, in the present investigation, hardness was
assessed to estimate DC. The present findings show that,
for top hardness, no significant differences were observed
among the curing methods. This corroborates with the assumption that similar DC can be obtained by different
activation strategies, as long as the total radiant exposure
is kept constant [15]. However, when assessing the bottom
hardness, LIC yielded significantly lower values when compared to all of the other methods when the mold used was
the tooth and also showed lower values compared with HIC
when the mold was the metal, which is probably a result of a
lower DC. Consequently, the first hypothesis was rejected.
The irradiance intensity is a critical factor for the in-depth
cure of composites, since the incident light is attenuated
with increasing distance from the irradiated surface, as a
result of absorption and scattering effects [14, 16]. Indeed,
Rueggeberg [17] reported that about only 9 % of the light
energy hitting the top surface of the composite is available at
2 mm depth. Therefore, during continuous activation at
150 mW/cm2, an intensity reaching the bottom layer around
15 mW/cm2 might be expected. Also, this low light energy
would be distributed along the incident electromagnetic
spectrum, between 390 and 520 nm. Therefore, as camphorquinone presents an optimal spectral absorbance range between 450 and 490 nm, with an absorbance peak at 470 nm
[17], the energy available between 450 and 490 nm might
have been insufficient to excite the photoinitiator to the
same level than during activation with higher light intensity,
Fig. 1 Emission spectra of the LCU with and without material mold
leading to poorer polymerization. Indeed, Watts [16] stated
that a minimum threshold of light irradiance reaching a specified depth is required to activate effective polymerization.
In this study, only 25 % of the light initially available
passed through the restoration when the tooth mold was
used. When the metal mold was used, a lower amount of
light passed through by restoration, only 12 %.
Another factor that might be considered is heat generation during the photoactivation procedures. High light intensities result in a high temperature increase within the
composite [9, 18], which can account for greater double
bond conversion, even at the bottom layer, due to increased
monomer mobility in the environment and also increased
reaction rate parameters [18, 19]. Additionally, the light
guide tip was positioned distant from the cavity surface for
producing 150 mW/cm2, in order to approximate the clinical
situation. This could also be related to less heat generation.
According to the results of this study, we can say that
different materials of mold can interfere in the hardness
values. In this study, when the tooth was used as a mold
for the composite restorations, it produced restorations with
lower hardness on the top and higher hardness values in the
bottom compared with the hardness values produced when
the metal was used as a mold. Consequently, the second
hypothesis was accepted.
This probably occurred because the amount of light emitted by the LCU when different molds were used was different on the top and bottom of the restorations, due the
difference in the material of the mold. The composite restoration made in the metal mold had higher hardness values on
the top because some light inside of the cavity was reflected
to the top again. The metal has good ability to reflect light,
and with the cavities' conical shape, it was possible, unlike
the tooth that has no capacity for reflection as good. This
was due to higher capacity for reflection of light by the
metal mold. Because of this higher capacity for reflection
of light, a larger amount of photons are reflected back to the
top surface of the sample and increases the curing.
However, the composite restorations had higher hardness
values in the bottom when the tooth mold was used. Although the tooth has no reflection of light that was as good
as the metal, the tooth allows that some light to pass through
it. Thus increasing the availability of light at the bottom of
the restoration, as opposed to metal that blocks all light.
These values are consistent with the results obtained from

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analysis of the LCU, for more light passes through the tooth
mold (230 mW/cm2) of the metal mold (110 mW/cm2). In
addition, the presence of the restoration and mold affected
the emission spectrum of light from the LCU because differences existed in peak light output. These results are more
evident when the amount of light available in the region for
better absorption of light by the photoinitiator contained in
the composite resin (camphorquinone) is compared. Between the wavelength 450 and 490 nm, the irradiance of
129 mW/cm2 across the tooth mold restored with resin
composite. When metal mold was used, only 63 mW/cm2
across the metal mold restored with resin composite, decreasing the amount of photons available, especially when
the metal mold was used; thus, decreasing the cure, and
consequently, the Knoop hardness in the bottom surface of
restorations made in the metal mold.
Much research in dentistry compare in vitro the properties of photoactivated composites into molds made of different
materials, such as metals, teeth, and silicones. Then, the results
obtained in these studies are extrapolated for use on teeth. But
during the photopolymerization, the light must travel a path
within the composite, and consequently, into the mold which
is inserted in the composite. As the path traveled by light
during the photopolymerization may be different when molds
made of different materials are used, results with different
properties of the composites can be obtained. Therefore, dentists should be careful in the interpretation of these results
before applying them in the clinical dentistry.
Conclusion
Considering the limitations of this study, the hypotheses were
partially accepted:
The photoactivation method LIC produced lower Knoop
hardness values in the bottom of the resin composite
restoration made in the tooth mold and metal mold.
Different mold materials for restorations, such as tooth
or metal, produced resin composite restorations with
differences in the Knoop hardness values.
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