Shrinkage and Hardness of Dental Composites2009
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Q U I N T E S S E N C E I N T E R N AT I O N A L
Shrinkage and hardness of dental composites
acquired with different curing light sources
Stephen S. Clifford, DDS1/Karla Roman-Alicea, DMD2/
Daranee Tantbirojn, DDS, MS, PhD3/Antheunis Versluis, PhD4
Objectives: Curing light sources propel the photopolymerization process. The effect of
3 curing units on polymerization shrinkage and depth of cure was investigated. Method
and Materials: The curing lights were a conventional and a soft-start quartz-tungstenhalogen (QTH) light source and a light-emitting diode (LED) source. The soft-start QTH
and LED intensity outputs were 9% and 17% less than the conventional QTH source,
respectively. For a 40-second light cure, the light energy was 32% and 14% lower, respectively. The light sources were applied to 4 restorative composites (microfilled, 2 hybrids,
and nanofilled). For each light unit–composite combination, the development of postgel
shrinkage during polymerization was measured with strain gauges (n = 15), and the
Knoop hardness was tested at 0.5-mm-depth increments to assess degree of cure 15 minutes after polymerization (n = 5). The results were statistically analyzed with 2-way ANOVA
at .05 significance level, followed by pairwise comparisons. Results: Both factors, light
source and composite, significantly affected postgel shrinkage and hardness (P < .05).
The conventional QTH unit generally produced the highest shrinkage and hardness (at
composite surface and 2-mm depth). The soft-start QTH unit generated the least shrinkage but achieved the lowest depth of cure. The resulting values for the LED unit were
mostly in between the results of the other 2 units. Conclusion: Curing lights should
provide sufficient light energy to thoroughly cure composite restorations, which might
be achieved without compromising shrinkage stresses if initial intensity is reduced.
(Quintessence Int 2009;40:203–214)
Key words: composite, cure, curing light, hardness, light energy, light intensity, shrinkage,
soft start
Optimized physical properties and minimized
residual shrinkage stresses for light-activated
composites are particularly important in
restorative dentistry. Optimal physical properties are achieved through adequate polymerization, usually referred to as the degree of
cure, and directly affect the physical properties
1
Summer Research Fellow, School of Dentistry, University of
Minnesota, Minneapolis, Minnesota, USA.
2
Summer Research Fellow, School of Dentistry, University of
Puerto Rico Medical Sciences, San Juan, Puerto Rico.
3
Assistant Professor, Department of Restorative Sciences, School of
Dentistry, University of Minnesota, Minneapolis, Minnesota, USA.
4
Research Assistant Professor, Department of Restorative
Sciences, School of Dentistry, University of Minnesota,
Minneapolis, Minnesota, USA.
Correspondence: Dr Antheunis Versluis, Minnesota Dental
Research Center for Biomaterials and Biomechanics, School of
Dentistry, University of Minnesota, 16-212 Moos Tower, 515
Delaware Street SE, Minneapolis, MN 55455. Fax: (612) 626-1484.
Email: [email protected]
and thus clinical performance of composite
restorations.1 Although a high degree of cure
is desirable, it inherently results in more extensive polymerization shrinkage, which may
generate residual shrinkage stresses. Stresses
created by polymerization shrinkage during
composite setting can result in leakage at the
tooth-restoration interface or, where the bonding is adequate, deformation of the tooth/
restoration complex.2 Such effects are clearly
unfavorable because of the possibility of secondary caries, cuspal fracture, or postoperative sensitivity.
How physical properties develop during
light curing depends in part on the characteristics of the curing light. High light intensity
(also referred to as power density) provides
faster conversion, but may also produce
higher postgel shrinkage (and thus the potential for higher shrinkage stresses) during
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Ta b l e 1
Three light-curing units used in this study
Type of
light-curing unit
Quartz-tungsten-halogen
Light-emitting diode
Soft-start quartz–tungsten-halogen
Model
(3M ESPE)
Light intensity*
(mW/cm2)
Elipar 2500
Elipar Free Light
Elipar TriLight
634
529
579
Light energy†
(mJ/cm2)
25,440
21,810
17,216
*Mean value at output plateau (see Fig 3) (determined with a Cure Rite Model 8000, EFOS radiometer).
†Mean value calculated from area under the light intensity–time curves (see Fig 3).
polymerization.3–7 Curing at low light intensities reduces the rate of polymerization and
residual shrinkage stresses by allowing more
flow, and thus stress relaxation, before the
composite solidifies. Low-intensity curing,
however, may not achieve the desired level of
polymerization and therefore requires additional light curing at high intensities or light
exposure over a longer period of time.8,9
Various light sources are used in dental
practices.10 Quartz-tungsten-halogen (QTH)
units have been a common source of blue
light for curing restorative composites. The
halogen bulb emits full-spectrum light that is
filtered to a 380- to 520-nm blue wavelength
range, which covers the absorption peak
(468 nm) of camphorquinone, the photoinitiator used in most dental composites.1,11
Because only a small part of the spectral
bulb output is relevant for activating the photoinitiator, the efficiency of a QTH unit is low.
Part of the light energy is released as heat.12,13
More recently, light-emitting diode (LED) curing units have become commercially available that feature narrow spectral ranges that
are highly efficient.14,15 The spectral range
emitted by dental LED units is between 440
and 490 nm, specifically targeting camphorquinone’s maximum absorption.
Given the availability of various curing light
design options with a manifold of restorative
composite compositions,16 the challenge for
clinical practitioners is to maintain optimal
physical properties through thorough polymerization while minimizing residual shrinkage
stress if possible. To gain a better understanding of the interaction between curing
light design and various composites, we
studied the effect on depth of cure and postgel shrinkage of 3 representative types of
light units (conventional QTH, soft-start QTH,
and LED) with comparable light output on 4
light-activated restorative composites. The
hypothesis was that the type of curing light
affects shrinkage stress and degree of cure
differently. Depth of cure was evaluated
using Knoop microhardness, and postgel
shrinkage was measured using a straingauge technique.
METHOD AND MATERIALS
Three Elipar light sources (3M ESPE) with
comparable light output were investigated:
2500 (conventional QTH), TriLight (soft-start
QTH), and Free Light (LED). Details and light
output are listed in Table 1. The intensities
were recorded as a function of time for the
calculation of the applied light energy for a
40-second cure (light intensity multiplied by
time, also referred to as energy density)
using a customized radiometer (Cure Rite,
Model 8000, EFOS), which was connected
to a computer that recorded the light intensity readings. The 3 curing lights were applied
to 4 commercially available light-activated
restorative composites (A110, Supreme,
Z100, Z250, 3M ESPE) (Table 2).
Shrinkage measurements
The strain-gauge method17 (Fig 1) was used to
measure the development of postgel shrinkage for the different curing unit and composite
combinations. Shrinkage strains at the bottom
of the composite samples were measured in
2 perpendicular directions using a biaxial
stacked strain gauge (CEA-06-032WT-120,
Measurements Group). Uncured composite
was placed on the strain gauge. The sample
area attached to the strain-gauge backing
was approximately 9 mm2, while the actual
gauge area was 0.656 mm2. This ensured that
sample boundary artifacts would not affect
the measurement area. The light intensity that
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Ta b l e 2
Description of light-activated restorative composites used in this study
Product (3M ESPE)
Microfilled
Filtek A110
Anterior Restorative
Nanofilled
Filtek Supreme
Universal Restorative
Hybrid
(continuum-filled)
Hybrid
(continuum-filled)
Z100 Restorative
Filtek Z250
Universal Restorative
Description of fillers
Colloidal silica with an average particle size
of 0.04 µm (particle size distribution of
0.01–0.09 µm). The filler loading is 40%
by volume.
Nanosilica filler, particle size 20 nm and
zirconia/silica nanoclusters with primary
particles sizes 5–20 nm. The cluster particle
size range 0.6–1.4 µm. The filler loading is
59.5% by volume.
Zirconia/silica filler with a particle size range
0.01–3.5 µm. The filler loading is 66% by volume.
Zirconia/silica filler. Particle size distribution
is 0.01–3.5 µm with an average particle size
of 0.6 µm. The filler loading is 60% by volume.
Shade
Lot no.
A2D
3BA
A2 Body
3BF
A2
HE
A2
3XC
Curing light
Curing light
Composite
sample
Composite
sample
Cover
Light cell
d
Data
output
Strain gauge
Mold
s
Glass slide
Fig 1 Experimental design for measuring postgel
shrinkage. Shrinkage strain is acquired while a composite sample is light cured on the strain gauge; the
light cell records exact light-curing start and duration.
Fig 2 Experimental design for measuring depth of cure. A composite
sample in a mold is covered and light cured from 1 direction through
a glass slide. After curing, hardness is measured at the surfaces that
were covered by the glass slide (s is surface hardness measurement
location at 0-mm depth) and cover plate (d is hardness measurement
location for depth of cure).
reaches the composite from a light source
diminishes with increasing distance from the
light curing tip.18 In this shrinkage experiment, the distance of the curing light guide
was standardized at 2 mm above the sample.
Samples were light cured for 40 seconds. A
light-sensitive photocell was placed next to
the composite sample. The output of the photocell was recorded with the strain outputs to
register the exact start and duration of the
light cure. The shrinkage strain was recorded
for 10 minutes after initial light activation. The
relationship between shrinkage strain and
time was obtained by averaging the 2 perpendicular strain components. The sample size
for each light source and composite combination was 15.
Postgel shrinkage values at 40 seconds
and 10 minutes were used for statistical
analysis. Two-way analysis of variance
(ANOVA) at a significance level of .05 was
performed to determine if there was any difference in shrinkage as a result of light
sources, composites, or composite*light
source interaction.
Hardness measurements
Microhardness as a function of depth was
measured (Fig 2) to evaluate the distribution of
degree of cure within the cured composite.19
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700
Light intensity (mW/cm2)
600
500
400
300
200
QTH
100
LED
Soft-start QTH
0
0
10
20
Time (s)
30
40
Fig 3 Light intensity output during 40-second light cure for the 3 light
sources used in this study: Elipar 2500 (QTH), Elipar Free Light (LED), and
Elipar TriLight (soft-start QTH). The surface area under the curves represents the light energy (reported in Table 1).
Although not a direct measurement for the
degree of polymerization such as Fourier
Transform Infrared Spectroscopy (FTIR)
Fourier Transform Infrared Spectroscopy20 or
Raman21 techniques, a good correlation has
been shown between the development of
degree of cure and Knoop hardness.20,22,23
This correlation is specific for each resin, and
as such, the microhardness cannot be used
as an absolute number for the degree of cure
across different resins.22 The sample was prepared by packing an uncured composite into
a rectangular slot (2 mm 2 mm 8 mm) of
a plaster mold (green Die-Keen, Heraeus
Kulzer). The top surface was covered with a
brass plate, and the side was covered with a
160-µm-thick clear glass slide (cover slip).
The composite was light-cured from the
side through the glass slide for 40 seconds.
The curing tip was placed directly onto the
glass slide, the purpose of which was to create
a flat surface without oxygen inhibition so
that the surface hardness could be measured. Knoop microhardness tests were performed 15 minutes after curing, using a
Micromet 2004 (Buehler) at 25-g load.
Indentations were placed at 0.5-mm increments, starting 0.5 mm from the light-cured
edge until the composite was too soft to
measure. In addition, the hardness at 0-mm
depth was measured from the composite
surface cured against the glass slide. The
sample size for each light source and composite was 5. Only the hardness values at the
composite surface (0 mm) and 2-mm depth
were used for statistical analysis.
Two-way ANOVA at a significance level of
.05 was performed to determine if there was
any difference in hardness as a result of light
sources, composites, or composite*light
source interaction.
RESULTS
Each light source had its characteristic irradiation pattern, as shown in Fig 3, where the
light intensity was recorded as a function of
time. The mean intensities of the QTH and
LED units were 634 and 529 mW/cm2,
respectively (n = 3). The light intensity output
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A110
Supreme
500
0
100
200
300
400
500
600
–500
QTH
–1,000
LED
–1,500
Soft-start QTH
–2,000
–2,500
–3,000
Postgel shrinkage (microstrain)
Postgel shrinkage (microstrain)
500
–3,500
0
100
200
300
400
500
600
–500
QTH
–1,000
LED
–1,500
Soft-start QTH
–2,000
–2,500
–3,000
–3,500
Time (s)
Time (s)
Z100
Z250
500
500
0
100
200
300
400
500
600
–500
QTH
–1,000
LED
–1,500
Soft-start QTH
–2,000
–2,500
–3,000
–3,500
Postgel shrinkage (microstrain)
Postgel shrinkage (microstrain)
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100
200
300
400
–500
500
600
QTH
–1,000
LED
Soft-start QTH
–1,500
–2,000
–2,500
–3,000
–3,500
Time (s)
Time (s)
Fig 4 Development of shrinkage strain (postgel shrinkage) during polymerization for 10 minutes after the start of light cure.
Curves are the mean of each light source–composite combination (n = 15), where positive values indicate expansion and negative values contraction.
tended to be highest in the first few seconds,
after which it leveled off. The output of the
QTH units oscillated (± 10% at approximately
0.3 Hz), while that of the LED unit was stable.
The intensity of the soft-start QTH unit, in the
ramp-curing mode, increased exponentially
in the first 15 seconds, after which it reached
a plateau of 579 mW/cm2 (n = 4). The calculated light energy (intensity time) for a 40second cure of each light source is shown in
Table 1.
Shrinkage results
Shrinkage strain (or postgel shrinkage)
development in each composite, cured with
different light sources, was recorded for 10
minutes. Mean curves were created by calculating the mean strain-time curves for each
light unit–composite combination (Fig 4).
During the initial few seconds after the start
of the light cure, the strain values became
positive, indicating thermal expansion, which
is caused by the temperature rise due to the
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Ta b l e 3
Postgel shrinkage (mean ± SD microstrains; n = 15) at 40 seconds
and 10 minutes after initial curing
Light-curing unit
Supreme
Z100
A110
Postgel shrinkage at 40 seconds
QTH
1,784 ± 71a
LED
1,620 ± 166e
Soft-start QTH
1,153 ± 131h
Postgel shrinkage at 10 minutes
QTH
2,754 ± 81k
LED
2,485 ± 220o
Soft-start QTH
2,014 ± 175s
Z250
1,496 ± 117b
1,328 ± 56f
1,005 ± 74i
2,242 ± 163c
2,042 ± 77g
1,384 ± 327j
1,208 ± 121d
1,290 ± 86f
900 ± 69i
2,489 ± 151l
2,263 ± 78p
2,130 ± 176s,t
3,045 ± 168m
2,744 ± 85q
2,196 ± 347t
1,938 ± 137n
2,010 ± 108r
1,662 ± 106u
Vertical lines connect results within each composite that are not significantly different. Same letter denotes mean values within
each light unit that are not significantly different. (Two-way ANOVA, pairwise comparisons; P > .0167).
A110
Supreme
Z100
Z250
3,500
40 s
Shrinkage strain (10–6)
3,000
10 min
2,500
2,000
1,500
1,000
500
0
QTH LED
Soft- QTH LED
start
QTH
Soft- QTH LED
start
QTH
Soft- QTH LED
start
QTH
Softstart
QTH
Fig 5 Postgel shrinkage (mean and SD microstrains; n = 15) at 40 seconds
and 10 minutes after the initial curing. Lowercase letters group values for
composites that were not significantly different for the same curing light,
while capital letters group values for curing lights that were not significantly different for the same composite (2-way ANOVA, pairwise comparisons;
P = .0167).
exothermic reaction and the heat induced by
the light source. After the initial expansion,
the strain value became negative, indicating
that the polymerization shrinkage contribution had overtaken the thermal expansion
effects. The soft-start curing unit generated
the slowest development in shrinkage strain
during the first 40 seconds. When the curing
light was turned off, the thermal strain contribution from the curing light was taken away.
As a result, the strain curve shows a drop at
the 40-second time interval. Contraction
strain continued to develop at a decreasing
rate after the curing light was turned off,
practically leveling off at 10 minutes.
Mean strain values and standard deviations at 40 seconds and 10 minutes were
compiled (Table 3). Two-way ANOVA indicated that curing lights, types of composite, and
the composite*light interaction significantly
affected the postgel shrinkage (P < .05).
Vertical lines in Table 3 connect mean values
within each composite that were not significantly different (pairwise comparisons,
P > .05 / 3 = .0167). Figure 5 shows the same
data in graphical form. All composites cured
with the soft-start QTH unit had significantly
less postgel shrinkage at 40 seconds. This
trend was maintained after 10 minutes,
except for Supreme, for which the difference
between the soft-start QTH and LED units
was not significant. The conventional QTH
light source created the highest strain values
in 3 of 4 composites evaluated.
Table 3 and Fig 5 also show the differences between composites cured with the
same light source. Z100 had the highest postgel shrinkage, followed by A110, Supreme,
and Z250. At 10 minutes, these values were
significantly different when the composites
were cured with the conventional QTH and
LED light sources. The differences among
Z100, A110, and Supreme were less when the
soft-start unit was used. Z250 consistently
showed the lowest postgel shrinkage values.
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A110
Supreme
80
80
QTH
QTH
70
70
LED
60
Knoop hardness
Knoop hardness
LED
Soft-start QTH
50
40
30
20
60
Soft-start QTH
50
40
30
20
10
10
0
0
0
0.5
1
1.5
2
2.5
3
3.5
0
0.5
1
Depth (mm)
1.5
2
2.5
3
3.5
Depth (mm)
Z100
Z250
80
80
QTH
QTH
70
70
LED
60
Soft-start QTH
Knoop hardness
Knoop hardness
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40
30
20
10
LED
60
Soft-start QTH
50
40
30
20
10
0
0
0
0.5
1
1.5
2
2.5
3
Depth (mm)
3.5
0
0.5
1
1.5
2
2.5
3
3.5
Depth (mm)
Fig 6 Knoop microhardness (mean and SD) of 4 composites cured by various light sources, measured 15 minutes after light curing (n = 5).
Hardness results
Knoop hardness profiles were determined at
various depths, as an indication of the
achieved degree of polymerization in the
cured composite (Fig 6). For all light sources
and composites, hardness values decreased
with increasing depth. In general, the hybrid
composite Z100 had the highest hardness
values, while the lowest values were found
for the anterior microfilled composite A110.
The differences between curing lights were
the largest for the nanofilled composite
Supreme and hybrid composite Z250.
The hardness values at the surface
(0 mm) and 2-mm depth were used for the
statistical analysis (Table 4). Vertical lines
connect mean values within each composite that are not significantly different (pairwise comparisons, P > .0167). Figure 7
shows the same data in graphical form. The
surface hardness values of 2 composites,
A110 with the lowest hardness values and
Z100 with the highest hardness values, were
not significantly affected by the different light
sources. The conventional QTH unit generally produced the highest hardness values,
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Ta b l e 4
Microhardness (mean ± SD) at composite surface (0 mm) and at 2-mm
depth measured after 15 minutes’ postcuring
Light-curing unit
Supreme
Z100
A110
Microhardness of
composite surface (0 mm)
QTH
LED
Soft-start QTH
Microhardness at 2-mm depth
QTH
LED
Soft-start QTH
Z250
29.9 ± 3.3
28.7 ± 6.3
33.8 ± 4.6
55.0 ± 2.7
36.5 ± 2.3
44.1 ± 7.1
61.9 ± 4.7
58.9 ± 3.4
64.3 ± 8.9
52.7 ± 1.4
50.2 ± 2.5
36.3 ± 5.8
14.8 ± 6.8
10.0 ± 6.1
9.4 ± 3.6
28.4 ± 3.0
18.2 ± 3.4
17.8 ± 2.3
23.0 ± 6.6
28.1 ± 7.3
18.2 ± 3.6
38.4 ± 4.0
27.2 ± 3.8
12.5 ± 4.7
Vertical lines and bracket connect results within each composite that are not significantly different (2-way ANOVA, pairwise comparisons; P > .0167).
A110
Supreme
Z100
Z250
75
0 mm
2 mm
Knoop hardness
60
45
30
15
0
QTH LED
Soft- QTH
start
QTH
LED Soft- QTH LED
start
QTH
Soft- QTH LED
start
QTH
Softstart
QTH
Fig 7 Knoop microhardness (mean and SD) at composite surface (0 mm)
and at 2-mm depth measured after 15-minute postcuring. Capital letters
group values for different curing lights that were not significantly different
for the same composite (2-way ANOVA, pairwise comparisons; P > .0167).
especially in Supreme, where the difference
was significant. The soft-start QTH unit produced a significantly lower hardness value
in Z250. At 2-mm depth, the hardness was
generally less than half of the surface value.
Two-way ANOVA showed that curing lights,
types of composite, and composite*light
interaction significantly affected the hardness (P < .05).
DISCUSSION
Clinicians, researchers, and dental industries
likewise perceive polymerization shrinkage,
which threatens the adhesive bond and
restoration longevity, as one of the most challenging properties of restorative composites.1 Apart from improvements in the
resin matrix chemistry, curing light philosophy and clinical techniques have brought
about some reduction in polymerization
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shrinkage stress development.4 Reduction in
shrinkage, however, should not compromise
other properties, especially the degree of
cure. Although poorly cured composite has
lower shrinkage, it will not attain its optimal
mechanical and biocompatibility properties.
Therefore, this study, in its assessment of
different curing lights, not only measured
polymerization shrinkage but also evaluated
hardness as a function of depth to assess the
degree of cure. Curing light source technology
develops rapidly. The units used in this study
have already been superseded by improved
models at publication time.15 However, the
objective of this study was to explore general
principles of curing light characteristics that
remain relevant irrespective of particular
curing light models. This was accomplished
by choosing different light source types with
similar outputs (see Table 1).
How do the curing lights differ
in output?
Light intensity is an important factor for the
performance of a curing light, which can be
easily determined using one of the many
available radiometers. Although the exact
interpretation of radiometer readings may be
less than straightforward, they are generally
considered acceptable for measuring curing
light output.24,25 In the current study, 2 evaluated light-curing units emitted radiation from
a QTH lamp, which was filtered to a blue light
spectrum with a wavelength between 380
and 520 nm. The LED curing source used
junctions of doped semiconductors to generate blue light mainly in the wavelength range
of 440 to 490 nm. According to the manufacturer, the optimal match for the camphorquinone photoinitiator (468 nm) ensures
that polymerization performance is similar to
that of a QTH unit, even though an LED unit
may record a lower light intensity on the
CureRite radiometer (EFOS).
It is important to note that besides the differences in light spectra between curing
sources, each radiometer may also have its
own filter. The radiometer used in this study
contained a selective filter between 400 and
500 nm, according to its manufacturer.
Therefore, a higher curing light intensity
measured by a particular light meter does
not necessarily indicate a better light curing
source, because the filter may measure a
wider spectrum. Furthermore, different curing lights may have different thermal outputs
due to infrared radiation. It is well-known that
thermal effects can affect the rate of polymerization reactions. In this study the tested commercial curing lights and radiometer readings
were taken on face value.
Do the curing lights decrease
shrinkage stress?
The clinical concern about polymerization
shrinkage is not so much the physical contraction but rather the development of residual stresses. In other words, “shrinkage” is
not the same as “shrinkage stress.” How
much shrinkage stress is generated
depends on many factors,26 such as
mechanical properties of the composite and
its substrates, cavity and substrate geometry,
bonding conditions, and of course polymerization shrinkage. However, not all polymerization shrinkage is relevant for the resulting
shrinkage stress.27 Only the so-called postgel
shrinkage (ie, the shrinkage after a composite
has become too rigid to relax stresses
through flow) is relevant for residual shrinkage stresses. This postgel shrinkage can be
measured using a strain-gauge technique,
which excludes shrinkage that is not able to
generate stresses.7
Because polymerization shrinkage is the
result of the dimensional changes that take
place within the resin when its components
react and cross-link to form a polymer
network,1 it is not surprising that different
shrinkage values would be measured for the
different composites.16 Two-way ANOVA confirmed that difference in shrinkage strain of
each composite, averaged across the curing
lights, was highly significant (P < .0001).
Z100 and Z250, both hybrid composites,
had the highest and lowest shrinkage values
in the present study. The relatively high polymerization shrinkage of Z100 results from
the amount of a low molecular weight component, triethylene glycol dimethacrylate
(TEGDMA). The shrinkage properties were
improved for Z250 by replacing TEGDMA
with higher molecular weight resins. The
anterior composite, A110, also resulted in
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relatively high polymerization shrinkage
because of the low molecular weight component and low filler loading. The filler loading
of A110 is 40% by volume, compared to 60%
by volume filler loading for Z250. According
to the manufacturer, the nanofilled composite Supreme has the same resin system but a
slightly lower filler loading than Z250. The
reaction of nanofillers with curing light may
also have an effect on the resulting shrinkage
strain.
It was to be expected that different composites would result in different shrinkage
values. The results of this study show that the
amount of postgel shrinkage, and thus
potential shrinkage stress, also vary for different curing lights. Statistical analysis indicated
that the differences between composites
depended on the light source, and vice
versa. The conventional QTH unit, which
recorded the highest light intensity and total
energy, created the highest strain values in 3
out of 4 composites (see Table 3 and Fig 5).
Except in the hybrid composite Z250, the
LED unit was associated with lower shrinkage strain than the conventional QTH light.
The soft-start QTH unit generated the least
amount of shrinkage strain. This outcome
seems to support the concept of slow-start
or ramped curing, which allows more time
for flow and stress relaxation before composite becomes solid.28–30 The slower strain
development of the soft-start unit can
be seen in the initial segment of postgel
shrinkage curves in Fig 4. The disparity in
shrinkage between curing units was more
profound at 40 seconds than after 10 minutes (see Table 3 and Fig 5). The clinical significance of this observation is that shrinkage
stress is most critical in the initial phase of
curing when the bonding between composite and cavity wall is not yet well-developed.
Do the curing lights cure
the composites?
A lower shrinkage strain value can be the
result of a better light-cure technique, where
postgel shrinkage is delayed, or it can be the
result of incomplete polymerization.31
Clinically, a high level of polymerization is
essential to attain the required physical properties and biocompatibility.1 Hardness is
often used to assess the achieved degree of
cure,32 which is justified based on its proven
correlation with degree of cure.20,22,23 It
should be reemphasized, however, that this
correlation between hardness and degree of
cure only applies within the same composite
group. Using hardness values to compare
the degree of cure between different composites is invalid.
The results show that the highest hardness values were at the composite surface,
while they decreased with increasing depth
from the exposed surface (Fig 6). The hardness values at the composite surface (0 mm)
were not significantly different for A110 and
Z100 between the 3 light sources (see Table
4 and Fig 7), despite the differences in lightintensity outputs. The hardness values may
have reached a saturated level at the surface,
indicating a complete cure. At 2-mm depth,
however, the achieved degree of cure was
consistently lower (28% to 73% of the surface value), and the differences between the
3 curing lights became significant. With the
exception of Z100, composites cured with
the conventional QTH unit achieved the highest hardness (and thus degree of cure) and
the soft-start QTH unit the lowest. This suggests that adequate surface hardness may
not ensure sufficient subsurface polymerization
of a restoration.
What is the relation between
shrinkage stress and degree of cure?
It is persuasive to speculate that there is a
correlation between shrinkage and hardness.
As discussed before, hardness correlates
with degree of cure. Because a well-cured
composite must have a higher density (and
thus total shrinkage) than an under-cured
composite, it seems intuitive that hardness
and shrinkage stress should also correlate.
However, shrinkage stress can vary even
when the composite has attained the same
degree of cure.7 Shrinkage stress is apparently not directly related to the degree of
cure. Consequently, evaluating curing-light
units based on only degree of cure indicators
(such as hardness, density, and total shrinkage) do not adequately assess all polymerization effects that are important for clinical
assessments. Curing lights should thus also
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be tested for the development of residual
shrinkage stress effects.
Shrinkage stress has been shown to be
highly affected by the intensity of the initial
light exposure.4,33 High intensity values during the first seconds of polymerization resulted
in higher postgel shrinkage strains, and thus
potentially higher shrinkage stresses.7 The
soft-start curing technique was proposed to
lower the initial intensity to reduce the development of shrinkage stresses.28–30 Because
it has also been shown that degree of cure
depends on the applied total light energy
(light intensity multiplied by the exposure
time),34 soft-start modes should not have to
compromise degree of cure if sufficient energy
is ensured, either by an increased final light
intensity or increased exposure time.8,9
The literature reports mixed results for the
effectiveness of soft-start curing. Besides references that suggest positive effects for softstart light curing, others have reported that it
had no effect or even worsened marginal
adaptation or microleakage.35–37 Although
these interfacial qualities are often associated
with shrinkage stresses, there are other factors
that more directly determine the quality of an
adhesive bond. The present study found that
the lowest postgel shrinkage was achieved
with the soft-start QTH unit. However, hardness measurements indicated that the composites did not attain the same degree of cure
with the soft-start QTH unit as with the other
curing sources. The lower hardness likely
resulted because the total light energy was
reduced by about 25% due to the exponential
soft-start profile (see Fig 3). Increasing the final
light intensity level or extending the total exposure time beyond 40 seconds may eliminate
the differences between the acquired degree
of cure of the soft-start QTH light and the other
2 curing units.
stress are both crucial for clinical performance of composites, and since degree of
cure and shrinkage stress do not have a
direct correlation, both variables were
assessed through the combination of hardness and postgel shrinkage measurements.
It was found that both the curing unit and the
type of composite significantly affected the
postgel shrinkage and hardness. The softstart QTH curing unit reduced postgel
shrinkage (and thus potential shrinkage
stress) in most of the composites tested but
produced optimal hardness only at the surface of 2 composites. The conventional QTH
unit (highest light intensity output) usually
provided favorable hardness, but this was
associated with high postgel shrinkage. The
tested LED unit, with its intermediate light
intensity output, achieved intermediate values for both shrinkage strain and hardness.
The ideal conditions for a high degree of
cure and a low postgel shrinkage were not
easy to obtain together. This likely requires a
soft-start light cure followed by a higher intensity or extended exposure time.
ACKNOWLEDGMENTS
This research was supported by the Minnesota Dental
Research Center for Biomaterials and Biomechanics and
by NIH grant 5T35DE07098. The authors thank Dr James
S. Hodges for his statistical advice.
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34. Suh BI. Controlling and understanding the polymer-
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19. Watts DC, Amer O, Combe EC. Characteristics of vis-
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20. Rueggeberg FA, Craig RG. Correlation of parameters
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Determination of the degree of cure of dental
37. Lindberg A, van Dijken JWV, Hörstedt P. In vivo inter-
resins using Raman and FT-Raman spectroscopy.
facial adaptation of class II resin composite restora-
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Shrinkage and hardness of dental composites
acquired with different curing light sources
Stephen S. Clifford, DDS1/Karla Roman-Alicea, DMD2/
Daranee Tantbirojn, DDS, MS, PhD3/Antheunis Versluis, PhD4
Objectives: Curing light sources propel the photopolymerization process. The effect of
3 curing units on polymerization shrinkage and depth of cure was investigated. Method
and Materials: The curing lights were a conventional and a soft-start quartz-tungstenhalogen (QTH) light source and a light-emitting diode (LED) source. The soft-start QTH
and LED intensity outputs were 9% and 17% less than the conventional QTH source,
respectively. For a 40-second light cure, the light energy was 32% and 14% lower, respectively. The light sources were applied to 4 restorative composites (microfilled, 2 hybrids,
and nanofilled). For each light unit–composite combination, the development of postgel
shrinkage during polymerization was measured with strain gauges (n = 15), and the
Knoop hardness was tested at 0.5-mm-depth increments to assess degree of cure 15 minutes after polymerization (n = 5). The results were statistically analyzed with 2-way ANOVA
at .05 significance level, followed by pairwise comparisons. Results: Both factors, light
source and composite, significantly affected postgel shrinkage and hardness (P < .05).
The conventional QTH unit generally produced the highest shrinkage and hardness (at
composite surface and 2-mm depth). The soft-start QTH unit generated the least shrinkage but achieved the lowest depth of cure. The resulting values for the LED unit were
mostly in between the results of the other 2 units. Conclusion: Curing lights should
provide sufficient light energy to thoroughly cure composite restorations, which might
be achieved without compromising shrinkage stresses if initial intensity is reduced.
(Quintessence Int 2009;40:203–214)
Key words: composite, cure, curing light, hardness, light energy, light intensity, shrinkage,
soft start
Optimized physical properties and minimized
residual shrinkage stresses for light-activated
composites are particularly important in
restorative dentistry. Optimal physical properties are achieved through adequate polymerization, usually referred to as the degree of
cure, and directly affect the physical properties
1
Summer Research Fellow, School of Dentistry, University of
Minnesota, Minneapolis, Minnesota, USA.
2
Summer Research Fellow, School of Dentistry, University of
Puerto Rico Medical Sciences, San Juan, Puerto Rico.
3
Assistant Professor, Department of Restorative Sciences, School of
Dentistry, University of Minnesota, Minneapolis, Minnesota, USA.
4
Research Assistant Professor, Department of Restorative
Sciences, School of Dentistry, University of Minnesota,
Minneapolis, Minnesota, USA.
Correspondence: Dr Antheunis Versluis, Minnesota Dental
Research Center for Biomaterials and Biomechanics, School of
Dentistry, University of Minnesota, 16-212 Moos Tower, 515
Delaware Street SE, Minneapolis, MN 55455. Fax: (612) 626-1484.
Email: [email protected]
and thus clinical performance of composite
restorations.1 Although a high degree of cure
is desirable, it inherently results in more extensive polymerization shrinkage, which may
generate residual shrinkage stresses. Stresses
created by polymerization shrinkage during
composite setting can result in leakage at the
tooth-restoration interface or, where the bonding is adequate, deformation of the tooth/
restoration complex.2 Such effects are clearly
unfavorable because of the possibility of secondary caries, cuspal fracture, or postoperative sensitivity.
How physical properties develop during
light curing depends in part on the characteristics of the curing light. High light intensity
(also referred to as power density) provides
faster conversion, but may also produce
higher postgel shrinkage (and thus the potential for higher shrinkage stresses) during
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Ta b l e 1
Three light-curing units used in this study
Type of
light-curing unit
Quartz-tungsten-halogen
Light-emitting diode
Soft-start quartz–tungsten-halogen
Model
(3M ESPE)
Light intensity*
(mW/cm2)
Elipar 2500
Elipar Free Light
Elipar TriLight
634
529
579
Light energy†
(mJ/cm2)
25,440
21,810
17,216
*Mean value at output plateau (see Fig 3) (determined with a Cure Rite Model 8000, EFOS radiometer).
†Mean value calculated from area under the light intensity–time curves (see Fig 3).
polymerization.3–7 Curing at low light intensities reduces the rate of polymerization and
residual shrinkage stresses by allowing more
flow, and thus stress relaxation, before the
composite solidifies. Low-intensity curing,
however, may not achieve the desired level of
polymerization and therefore requires additional light curing at high intensities or light
exposure over a longer period of time.8,9
Various light sources are used in dental
practices.10 Quartz-tungsten-halogen (QTH)
units have been a common source of blue
light for curing restorative composites. The
halogen bulb emits full-spectrum light that is
filtered to a 380- to 520-nm blue wavelength
range, which covers the absorption peak
(468 nm) of camphorquinone, the photoinitiator used in most dental composites.1,11
Because only a small part of the spectral
bulb output is relevant for activating the photoinitiator, the efficiency of a QTH unit is low.
Part of the light energy is released as heat.12,13
More recently, light-emitting diode (LED) curing units have become commercially available that feature narrow spectral ranges that
are highly efficient.14,15 The spectral range
emitted by dental LED units is between 440
and 490 nm, specifically targeting camphorquinone’s maximum absorption.
Given the availability of various curing light
design options with a manifold of restorative
composite compositions,16 the challenge for
clinical practitioners is to maintain optimal
physical properties through thorough polymerization while minimizing residual shrinkage
stress if possible. To gain a better understanding of the interaction between curing
light design and various composites, we
studied the effect on depth of cure and postgel shrinkage of 3 representative types of
light units (conventional QTH, soft-start QTH,
and LED) with comparable light output on 4
light-activated restorative composites. The
hypothesis was that the type of curing light
affects shrinkage stress and degree of cure
differently. Depth of cure was evaluated
using Knoop microhardness, and postgel
shrinkage was measured using a straingauge technique.
METHOD AND MATERIALS
Three Elipar light sources (3M ESPE) with
comparable light output were investigated:
2500 (conventional QTH), TriLight (soft-start
QTH), and Free Light (LED). Details and light
output are listed in Table 1. The intensities
were recorded as a function of time for the
calculation of the applied light energy for a
40-second cure (light intensity multiplied by
time, also referred to as energy density)
using a customized radiometer (Cure Rite,
Model 8000, EFOS), which was connected
to a computer that recorded the light intensity readings. The 3 curing lights were applied
to 4 commercially available light-activated
restorative composites (A110, Supreme,
Z100, Z250, 3M ESPE) (Table 2).
Shrinkage measurements
The strain-gauge method17 (Fig 1) was used to
measure the development of postgel shrinkage for the different curing unit and composite
combinations. Shrinkage strains at the bottom
of the composite samples were measured in
2 perpendicular directions using a biaxial
stacked strain gauge (CEA-06-032WT-120,
Measurements Group). Uncured composite
was placed on the strain gauge. The sample
area attached to the strain-gauge backing
was approximately 9 mm2, while the actual
gauge area was 0.656 mm2. This ensured that
sample boundary artifacts would not affect
the measurement area. The light intensity that
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Ta b l e 2
Description of light-activated restorative composites used in this study
Product (3M ESPE)
Microfilled
Filtek A110
Anterior Restorative
Nanofilled
Filtek Supreme
Universal Restorative
Hybrid
(continuum-filled)
Hybrid
(continuum-filled)
Z100 Restorative
Filtek Z250
Universal Restorative
Description of fillers
Colloidal silica with an average particle size
of 0.04 µm (particle size distribution of
0.01–0.09 µm). The filler loading is 40%
by volume.
Nanosilica filler, particle size 20 nm and
zirconia/silica nanoclusters with primary
particles sizes 5–20 nm. The cluster particle
size range 0.6–1.4 µm. The filler loading is
59.5% by volume.
Zirconia/silica filler with a particle size range
0.01–3.5 µm. The filler loading is 66% by volume.
Zirconia/silica filler. Particle size distribution
is 0.01–3.5 µm with an average particle size
of 0.6 µm. The filler loading is 60% by volume.
Shade
Lot no.
A2D
3BA
A2 Body
3BF
A2
HE
A2
3XC
Curing light
Curing light
Composite
sample
Composite
sample
Cover
Light cell
d
Data
output
Strain gauge
Mold
s
Glass slide
Fig 1 Experimental design for measuring postgel
shrinkage. Shrinkage strain is acquired while a composite sample is light cured on the strain gauge; the
light cell records exact light-curing start and duration.
Fig 2 Experimental design for measuring depth of cure. A composite
sample in a mold is covered and light cured from 1 direction through
a glass slide. After curing, hardness is measured at the surfaces that
were covered by the glass slide (s is surface hardness measurement
location at 0-mm depth) and cover plate (d is hardness measurement
location for depth of cure).
reaches the composite from a light source
diminishes with increasing distance from the
light curing tip.18 In this shrinkage experiment, the distance of the curing light guide
was standardized at 2 mm above the sample.
Samples were light cured for 40 seconds. A
light-sensitive photocell was placed next to
the composite sample. The output of the photocell was recorded with the strain outputs to
register the exact start and duration of the
light cure. The shrinkage strain was recorded
for 10 minutes after initial light activation. The
relationship between shrinkage strain and
time was obtained by averaging the 2 perpendicular strain components. The sample size
for each light source and composite combination was 15.
Postgel shrinkage values at 40 seconds
and 10 minutes were used for statistical
analysis. Two-way analysis of variance
(ANOVA) at a significance level of .05 was
performed to determine if there was any difference in shrinkage as a result of light
sources, composites, or composite*light
source interaction.
Hardness measurements
Microhardness as a function of depth was
measured (Fig 2) to evaluate the distribution of
degree of cure within the cured composite.19
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700
Light intensity (mW/cm2)
600
500
400
300
200
QTH
100
LED
Soft-start QTH
0
0
10
20
Time (s)
30
40
Fig 3 Light intensity output during 40-second light cure for the 3 light
sources used in this study: Elipar 2500 (QTH), Elipar Free Light (LED), and
Elipar TriLight (soft-start QTH). The surface area under the curves represents the light energy (reported in Table 1).
Although not a direct measurement for the
degree of polymerization such as Fourier
Transform Infrared Spectroscopy (FTIR)
Fourier Transform Infrared Spectroscopy20 or
Raman21 techniques, a good correlation has
been shown between the development of
degree of cure and Knoop hardness.20,22,23
This correlation is specific for each resin, and
as such, the microhardness cannot be used
as an absolute number for the degree of cure
across different resins.22 The sample was prepared by packing an uncured composite into
a rectangular slot (2 mm 2 mm 8 mm) of
a plaster mold (green Die-Keen, Heraeus
Kulzer). The top surface was covered with a
brass plate, and the side was covered with a
160-µm-thick clear glass slide (cover slip).
The composite was light-cured from the
side through the glass slide for 40 seconds.
The curing tip was placed directly onto the
glass slide, the purpose of which was to create
a flat surface without oxygen inhibition so
that the surface hardness could be measured. Knoop microhardness tests were performed 15 minutes after curing, using a
Micromet 2004 (Buehler) at 25-g load.
Indentations were placed at 0.5-mm increments, starting 0.5 mm from the light-cured
edge until the composite was too soft to
measure. In addition, the hardness at 0-mm
depth was measured from the composite
surface cured against the glass slide. The
sample size for each light source and composite was 5. Only the hardness values at the
composite surface (0 mm) and 2-mm depth
were used for statistical analysis.
Two-way ANOVA at a significance level of
.05 was performed to determine if there was
any difference in hardness as a result of light
sources, composites, or composite*light
source interaction.
RESULTS
Each light source had its characteristic irradiation pattern, as shown in Fig 3, where the
light intensity was recorded as a function of
time. The mean intensities of the QTH and
LED units were 634 and 529 mW/cm2,
respectively (n = 3). The light intensity output
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A110
Supreme
500
0
100
200
300
400
500
600
–500
QTH
–1,000
LED
–1,500
Soft-start QTH
–2,000
–2,500
–3,000
Postgel shrinkage (microstrain)
Postgel shrinkage (microstrain)
500
–3,500
0
100
200
300
400
500
600
–500
QTH
–1,000
LED
–1,500
Soft-start QTH
–2,000
–2,500
–3,000
–3,500
Time (s)
Time (s)
Z100
Z250
500
500
0
100
200
300
400
500
600
–500
QTH
–1,000
LED
–1,500
Soft-start QTH
–2,000
–2,500
–3,000
–3,500
Postgel shrinkage (microstrain)
Postgel shrinkage (microstrain)
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0
100
200
300
400
–500
500
600
QTH
–1,000
LED
Soft-start QTH
–1,500
–2,000
–2,500
–3,000
–3,500
Time (s)
Time (s)
Fig 4 Development of shrinkage strain (postgel shrinkage) during polymerization for 10 minutes after the start of light cure.
Curves are the mean of each light source–composite combination (n = 15), where positive values indicate expansion and negative values contraction.
tended to be highest in the first few seconds,
after which it leveled off. The output of the
QTH units oscillated (± 10% at approximately
0.3 Hz), while that of the LED unit was stable.
The intensity of the soft-start QTH unit, in the
ramp-curing mode, increased exponentially
in the first 15 seconds, after which it reached
a plateau of 579 mW/cm2 (n = 4). The calculated light energy (intensity time) for a 40second cure of each light source is shown in
Table 1.
Shrinkage results
Shrinkage strain (or postgel shrinkage)
development in each composite, cured with
different light sources, was recorded for 10
minutes. Mean curves were created by calculating the mean strain-time curves for each
light unit–composite combination (Fig 4).
During the initial few seconds after the start
of the light cure, the strain values became
positive, indicating thermal expansion, which
is caused by the temperature rise due to the
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Ta b l e 3
Postgel shrinkage (mean ± SD microstrains; n = 15) at 40 seconds
and 10 minutes after initial curing
Light-curing unit
Supreme
Z100
A110
Postgel shrinkage at 40 seconds
QTH
1,784 ± 71a
LED
1,620 ± 166e
Soft-start QTH
1,153 ± 131h
Postgel shrinkage at 10 minutes
QTH
2,754 ± 81k
LED
2,485 ± 220o
Soft-start QTH
2,014 ± 175s
Z250
1,496 ± 117b
1,328 ± 56f
1,005 ± 74i
2,242 ± 163c
2,042 ± 77g
1,384 ± 327j
1,208 ± 121d
1,290 ± 86f
900 ± 69i
2,489 ± 151l
2,263 ± 78p
2,130 ± 176s,t
3,045 ± 168m
2,744 ± 85q
2,196 ± 347t
1,938 ± 137n
2,010 ± 108r
1,662 ± 106u
Vertical lines connect results within each composite that are not significantly different. Same letter denotes mean values within
each light unit that are not significantly different. (Two-way ANOVA, pairwise comparisons; P > .0167).
A110
Supreme
Z100
Z250
3,500
40 s
Shrinkage strain (10–6)
3,000
10 min
2,500
2,000
1,500
1,000
500
0
QTH LED
Soft- QTH LED
start
QTH
Soft- QTH LED
start
QTH
Soft- QTH LED
start
QTH
Softstart
QTH
Fig 5 Postgel shrinkage (mean and SD microstrains; n = 15) at 40 seconds
and 10 minutes after the initial curing. Lowercase letters group values for
composites that were not significantly different for the same curing light,
while capital letters group values for curing lights that were not significantly different for the same composite (2-way ANOVA, pairwise comparisons;
P = .0167).
exothermic reaction and the heat induced by
the light source. After the initial expansion,
the strain value became negative, indicating
that the polymerization shrinkage contribution had overtaken the thermal expansion
effects. The soft-start curing unit generated
the slowest development in shrinkage strain
during the first 40 seconds. When the curing
light was turned off, the thermal strain contribution from the curing light was taken away.
As a result, the strain curve shows a drop at
the 40-second time interval. Contraction
strain continued to develop at a decreasing
rate after the curing light was turned off,
practically leveling off at 10 minutes.
Mean strain values and standard deviations at 40 seconds and 10 minutes were
compiled (Table 3). Two-way ANOVA indicated that curing lights, types of composite, and
the composite*light interaction significantly
affected the postgel shrinkage (P < .05).
Vertical lines in Table 3 connect mean values
within each composite that were not significantly different (pairwise comparisons,
P > .05 / 3 = .0167). Figure 5 shows the same
data in graphical form. All composites cured
with the soft-start QTH unit had significantly
less postgel shrinkage at 40 seconds. This
trend was maintained after 10 minutes,
except for Supreme, for which the difference
between the soft-start QTH and LED units
was not significant. The conventional QTH
light source created the highest strain values
in 3 of 4 composites evaluated.
Table 3 and Fig 5 also show the differences between composites cured with the
same light source. Z100 had the highest postgel shrinkage, followed by A110, Supreme,
and Z250. At 10 minutes, these values were
significantly different when the composites
were cured with the conventional QTH and
LED light sources. The differences among
Z100, A110, and Supreme were less when the
soft-start unit was used. Z250 consistently
showed the lowest postgel shrinkage values.
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A110
Supreme
80
80
QTH
QTH
70
70
LED
60
Knoop hardness
Knoop hardness
LED
Soft-start QTH
50
40
30
20
60
Soft-start QTH
50
40
30
20
10
10
0
0
0
0.5
1
1.5
2
2.5
3
3.5
0
0.5
1
Depth (mm)
1.5
2
2.5
3
3.5
Depth (mm)
Z100
Z250
80
80
QTH
QTH
70
70
LED
60
Soft-start QTH
Knoop hardness
Knoop hardness
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50
40
30
20
10
LED
60
Soft-start QTH
50
40
30
20
10
0
0
0
0.5
1
1.5
2
2.5
3
Depth (mm)
3.5
0
0.5
1
1.5
2
2.5
3
3.5
Depth (mm)
Fig 6 Knoop microhardness (mean and SD) of 4 composites cured by various light sources, measured 15 minutes after light curing (n = 5).
Hardness results
Knoop hardness profiles were determined at
various depths, as an indication of the
achieved degree of polymerization in the
cured composite (Fig 6). For all light sources
and composites, hardness values decreased
with increasing depth. In general, the hybrid
composite Z100 had the highest hardness
values, while the lowest values were found
for the anterior microfilled composite A110.
The differences between curing lights were
the largest for the nanofilled composite
Supreme and hybrid composite Z250.
The hardness values at the surface
(0 mm) and 2-mm depth were used for the
statistical analysis (Table 4). Vertical lines
connect mean values within each composite that are not significantly different (pairwise comparisons, P > .0167). Figure 7
shows the same data in graphical form. The
surface hardness values of 2 composites,
A110 with the lowest hardness values and
Z100 with the highest hardness values, were
not significantly affected by the different light
sources. The conventional QTH unit generally produced the highest hardness values,
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Ta b l e 4
Microhardness (mean ± SD) at composite surface (0 mm) and at 2-mm
depth measured after 15 minutes’ postcuring
Light-curing unit
Supreme
Z100
A110
Microhardness of
composite surface (0 mm)
QTH
LED
Soft-start QTH
Microhardness at 2-mm depth
QTH
LED
Soft-start QTH
Z250
29.9 ± 3.3
28.7 ± 6.3
33.8 ± 4.6
55.0 ± 2.7
36.5 ± 2.3
44.1 ± 7.1
61.9 ± 4.7
58.9 ± 3.4
64.3 ± 8.9
52.7 ± 1.4
50.2 ± 2.5
36.3 ± 5.8
14.8 ± 6.8
10.0 ± 6.1
9.4 ± 3.6
28.4 ± 3.0
18.2 ± 3.4
17.8 ± 2.3
23.0 ± 6.6
28.1 ± 7.3
18.2 ± 3.6
38.4 ± 4.0
27.2 ± 3.8
12.5 ± 4.7
Vertical lines and bracket connect results within each composite that are not significantly different (2-way ANOVA, pairwise comparisons; P > .0167).
A110
Supreme
Z100
Z250
75
0 mm
2 mm
Knoop hardness
60
45
30
15
0
QTH LED
Soft- QTH
start
QTH
LED Soft- QTH LED
start
QTH
Soft- QTH LED
start
QTH
Softstart
QTH
Fig 7 Knoop microhardness (mean and SD) at composite surface (0 mm)
and at 2-mm depth measured after 15-minute postcuring. Capital letters
group values for different curing lights that were not significantly different
for the same composite (2-way ANOVA, pairwise comparisons; P > .0167).
especially in Supreme, where the difference
was significant. The soft-start QTH unit produced a significantly lower hardness value
in Z250. At 2-mm depth, the hardness was
generally less than half of the surface value.
Two-way ANOVA showed that curing lights,
types of composite, and composite*light
interaction significantly affected the hardness (P < .05).
DISCUSSION
Clinicians, researchers, and dental industries
likewise perceive polymerization shrinkage,
which threatens the adhesive bond and
restoration longevity, as one of the most challenging properties of restorative composites.1 Apart from improvements in the
resin matrix chemistry, curing light philosophy and clinical techniques have brought
about some reduction in polymerization
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shrinkage stress development.4 Reduction in
shrinkage, however, should not compromise
other properties, especially the degree of
cure. Although poorly cured composite has
lower shrinkage, it will not attain its optimal
mechanical and biocompatibility properties.
Therefore, this study, in its assessment of
different curing lights, not only measured
polymerization shrinkage but also evaluated
hardness as a function of depth to assess the
degree of cure. Curing light source technology
develops rapidly. The units used in this study
have already been superseded by improved
models at publication time.15 However, the
objective of this study was to explore general
principles of curing light characteristics that
remain relevant irrespective of particular
curing light models. This was accomplished
by choosing different light source types with
similar outputs (see Table 1).
How do the curing lights differ
in output?
Light intensity is an important factor for the
performance of a curing light, which can be
easily determined using one of the many
available radiometers. Although the exact
interpretation of radiometer readings may be
less than straightforward, they are generally
considered acceptable for measuring curing
light output.24,25 In the current study, 2 evaluated light-curing units emitted radiation from
a QTH lamp, which was filtered to a blue light
spectrum with a wavelength between 380
and 520 nm. The LED curing source used
junctions of doped semiconductors to generate blue light mainly in the wavelength range
of 440 to 490 nm. According to the manufacturer, the optimal match for the camphorquinone photoinitiator (468 nm) ensures
that polymerization performance is similar to
that of a QTH unit, even though an LED unit
may record a lower light intensity on the
CureRite radiometer (EFOS).
It is important to note that besides the differences in light spectra between curing
sources, each radiometer may also have its
own filter. The radiometer used in this study
contained a selective filter between 400 and
500 nm, according to its manufacturer.
Therefore, a higher curing light intensity
measured by a particular light meter does
not necessarily indicate a better light curing
source, because the filter may measure a
wider spectrum. Furthermore, different curing lights may have different thermal outputs
due to infrared radiation. It is well-known that
thermal effects can affect the rate of polymerization reactions. In this study the tested commercial curing lights and radiometer readings
were taken on face value.
Do the curing lights decrease
shrinkage stress?
The clinical concern about polymerization
shrinkage is not so much the physical contraction but rather the development of residual stresses. In other words, “shrinkage” is
not the same as “shrinkage stress.” How
much shrinkage stress is generated
depends on many factors,26 such as
mechanical properties of the composite and
its substrates, cavity and substrate geometry,
bonding conditions, and of course polymerization shrinkage. However, not all polymerization shrinkage is relevant for the resulting
shrinkage stress.27 Only the so-called postgel
shrinkage (ie, the shrinkage after a composite
has become too rigid to relax stresses
through flow) is relevant for residual shrinkage stresses. This postgel shrinkage can be
measured using a strain-gauge technique,
which excludes shrinkage that is not able to
generate stresses.7
Because polymerization shrinkage is the
result of the dimensional changes that take
place within the resin when its components
react and cross-link to form a polymer
network,1 it is not surprising that different
shrinkage values would be measured for the
different composites.16 Two-way ANOVA confirmed that difference in shrinkage strain of
each composite, averaged across the curing
lights, was highly significant (P < .0001).
Z100 and Z250, both hybrid composites,
had the highest and lowest shrinkage values
in the present study. The relatively high polymerization shrinkage of Z100 results from
the amount of a low molecular weight component, triethylene glycol dimethacrylate
(TEGDMA). The shrinkage properties were
improved for Z250 by replacing TEGDMA
with higher molecular weight resins. The
anterior composite, A110, also resulted in
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relatively high polymerization shrinkage
because of the low molecular weight component and low filler loading. The filler loading
of A110 is 40% by volume, compared to 60%
by volume filler loading for Z250. According
to the manufacturer, the nanofilled composite Supreme has the same resin system but a
slightly lower filler loading than Z250. The
reaction of nanofillers with curing light may
also have an effect on the resulting shrinkage
strain.
It was to be expected that different composites would result in different shrinkage
values. The results of this study show that the
amount of postgel shrinkage, and thus
potential shrinkage stress, also vary for different curing lights. Statistical analysis indicated
that the differences between composites
depended on the light source, and vice
versa. The conventional QTH unit, which
recorded the highest light intensity and total
energy, created the highest strain values in 3
out of 4 composites (see Table 3 and Fig 5).
Except in the hybrid composite Z250, the
LED unit was associated with lower shrinkage strain than the conventional QTH light.
The soft-start QTH unit generated the least
amount of shrinkage strain. This outcome
seems to support the concept of slow-start
or ramped curing, which allows more time
for flow and stress relaxation before composite becomes solid.28–30 The slower strain
development of the soft-start unit can
be seen in the initial segment of postgel
shrinkage curves in Fig 4. The disparity in
shrinkage between curing units was more
profound at 40 seconds than after 10 minutes (see Table 3 and Fig 5). The clinical significance of this observation is that shrinkage
stress is most critical in the initial phase of
curing when the bonding between composite and cavity wall is not yet well-developed.
Do the curing lights cure
the composites?
A lower shrinkage strain value can be the
result of a better light-cure technique, where
postgel shrinkage is delayed, or it can be the
result of incomplete polymerization.31
Clinically, a high level of polymerization is
essential to attain the required physical properties and biocompatibility.1 Hardness is
often used to assess the achieved degree of
cure,32 which is justified based on its proven
correlation with degree of cure.20,22,23 It
should be reemphasized, however, that this
correlation between hardness and degree of
cure only applies within the same composite
group. Using hardness values to compare
the degree of cure between different composites is invalid.
The results show that the highest hardness values were at the composite surface,
while they decreased with increasing depth
from the exposed surface (Fig 6). The hardness values at the composite surface (0 mm)
were not significantly different for A110 and
Z100 between the 3 light sources (see Table
4 and Fig 7), despite the differences in lightintensity outputs. The hardness values may
have reached a saturated level at the surface,
indicating a complete cure. At 2-mm depth,
however, the achieved degree of cure was
consistently lower (28% to 73% of the surface value), and the differences between the
3 curing lights became significant. With the
exception of Z100, composites cured with
the conventional QTH unit achieved the highest hardness (and thus degree of cure) and
the soft-start QTH unit the lowest. This suggests that adequate surface hardness may
not ensure sufficient subsurface polymerization
of a restoration.
What is the relation between
shrinkage stress and degree of cure?
It is persuasive to speculate that there is a
correlation between shrinkage and hardness.
As discussed before, hardness correlates
with degree of cure. Because a well-cured
composite must have a higher density (and
thus total shrinkage) than an under-cured
composite, it seems intuitive that hardness
and shrinkage stress should also correlate.
However, shrinkage stress can vary even
when the composite has attained the same
degree of cure.7 Shrinkage stress is apparently not directly related to the degree of
cure. Consequently, evaluating curing-light
units based on only degree of cure indicators
(such as hardness, density, and total shrinkage) do not adequately assess all polymerization effects that are important for clinical
assessments. Curing lights should thus also
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be tested for the development of residual
shrinkage stress effects.
Shrinkage stress has been shown to be
highly affected by the intensity of the initial
light exposure.4,33 High intensity values during the first seconds of polymerization resulted
in higher postgel shrinkage strains, and thus
potentially higher shrinkage stresses.7 The
soft-start curing technique was proposed to
lower the initial intensity to reduce the development of shrinkage stresses.28–30 Because
it has also been shown that degree of cure
depends on the applied total light energy
(light intensity multiplied by the exposure
time),34 soft-start modes should not have to
compromise degree of cure if sufficient energy
is ensured, either by an increased final light
intensity or increased exposure time.8,9
The literature reports mixed results for the
effectiveness of soft-start curing. Besides references that suggest positive effects for softstart light curing, others have reported that it
had no effect or even worsened marginal
adaptation or microleakage.35–37 Although
these interfacial qualities are often associated
with shrinkage stresses, there are other factors
that more directly determine the quality of an
adhesive bond. The present study found that
the lowest postgel shrinkage was achieved
with the soft-start QTH unit. However, hardness measurements indicated that the composites did not attain the same degree of cure
with the soft-start QTH unit as with the other
curing sources. The lower hardness likely
resulted because the total light energy was
reduced by about 25% due to the exponential
soft-start profile (see Fig 3). Increasing the final
light intensity level or extending the total exposure time beyond 40 seconds may eliminate
the differences between the acquired degree
of cure of the soft-start QTH light and the other
2 curing units.
stress are both crucial for clinical performance of composites, and since degree of
cure and shrinkage stress do not have a
direct correlation, both variables were
assessed through the combination of hardness and postgel shrinkage measurements.
It was found that both the curing unit and the
type of composite significantly affected the
postgel shrinkage and hardness. The softstart QTH curing unit reduced postgel
shrinkage (and thus potential shrinkage
stress) in most of the composites tested but
produced optimal hardness only at the surface of 2 composites. The conventional QTH
unit (highest light intensity output) usually
provided favorable hardness, but this was
associated with high postgel shrinkage. The
tested LED unit, with its intermediate light
intensity output, achieved intermediate values for both shrinkage strain and hardness.
The ideal conditions for a high degree of
cure and a low postgel shrinkage were not
easy to obtain together. This likely requires a
soft-start light cure followed by a higher intensity or extended exposure time.
ACKNOWLEDGMENTS
This research was supported by the Minnesota Dental
Research Center for Biomaterials and Biomechanics and
by NIH grant 5T35DE07098. The authors thank Dr James
S. Hodges for his statistical advice.
REFERENCES
1. Ferracane JL. Current trends in dental composites.
Crit Rev Oral Biol Med 1995;6:302–318.
2. Tantbirojn D, Versluis A, Pintado MR, DeLong R,
Douglas WH. Tooth deformation patterns in molars
after composite restoration. Dent Mater 2004;20:
535–542.
3. Bouschlicher MR, Vargas MA, Boyer DB. Effect of
composite type, light intensity, configuration factor
CONCLUSION
and laser polymerization on polymerization contraction forces. Am J Dent 1997;10:88–96.
This study’s objective was to evaluate the
effect of different light characteristics found
in 3 commercially available curing lights on
the curing results of restorative composites.
Because degree of cure and shrinkage
4. Watts DC, al Hindi A. Intrinsic ‘soft-start’ polymerisation shrinkage-kinetics in an acrylate-based resincomposite. Dent Mater 1999;15:39–45.
5. Emami N, Söderholm KJM, Berglund LA. Effect of
light power density variations on bulk curing properties of dental composites. J Dent 2003;31:189–196.
COPYRIGHT © 2008 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO
PART OF THIS ARTICLE MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER
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22. Ferracane JL. Correlation between hardness and
different modes of LED and halogen light curing
degree of conversion during the setting reaction of
units. Oper Dent 2004;29:317–324.
unfilled dental restorative resins. Dent Mater 1985;
7. Versluis A, Tantbirojn D, Douglas WH. Distribution of
1:11–14.
transient properties during polymerization of a
23. Santos GB, Medeiros IS, Fellows CE, Muench A, Braga
light-initiated restorative composite. Dent Mater
RR. Composite depth of cure obtained with QTH
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VOLUME 40
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NUMBER 3
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MARCH 2009
