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USE OF DIGITAL IMAGE CORRELATION TECHNIQUE TO MEASURE
DENTAL COMPOSITE SHRINKAGE
1
Shu-Fen Chuang1, Terry Yuan-Fang Chen2, Chih-Han Chang3
Institute of Oral Medicine, 2 Department of Mechanical Engineering,
3
Institute of Biomedical Engineering
National Cheng Kung University
Tainan, Taiwan 70403
[email protected]
ABSTRACT
Although resin-based composites have become widely used in dental restoration, distortion and microfracture of the bonded
tooth caused by polymerization shrinkage is criticized as their inherent drawback. Digital image correlation (DIC) is a method
based on the comparison of two characterized images to measure the displacement of a sublet. The objective of the present
study was to examine the polymerization shrinkage of a composite material and the deflection of surrounded tooth. The
polymerization shrinkage of dental composites in a simulated cavity on steel block was compared by DIC and finite element
method. Using the DIC method, the deformation on the top surface and the boundaries of the composite restoration in eight
intact extracted human molars was measured. The correlation coefficients between the pre-cured and post-cured images were
greater than 99% in all comparisons. The greatest deformation was found on the free surfaces, and the least on the gingival
wall. The 10 min post-cured image presented the greatest amount of deformation, which indicated that the deformation
continued after light irradiation. In addition, the displacement of specific location can be identified and the overall profile of
polymerization shrinkage can be described.
Introduction
The dental resin-based composites (RBC) offer benefits over the old materials such as the tooth-alike shades and excellent
translucency, comparable strength and longevity, as well as the set-on-commend capability. Although they have become the
most widely used in dental restoration, the polymerization shrinkage is criticized as their inherent defect. When the resin
composites is irradiated with a blue light of 470 nm wavelength, the containing photo-initiator generates free radical then
attacks the resin matrix to initiate the polymerization chain reaction [1]. Current composites present 0.2% to 2% in linear
shrinkages, and the volumetric shrinkage is in the range of 2 to 6% [2,3].
In dental composite restorations, the composite is bonded to tooth structure at most cavity walls. Under the conditions,
polymer shrinkage results in internal stresses and strains with the surrounding constraint [4]. The internal stress distribution in
composite restoration is dependent on its geometry and boundary condition [5]. Clinically, shrinkage leads to post-restorative
consequences such as distortion of restorations, the destruction of resin-tooth bond, the bonded tooth deflection and even
microfracture in a weakened tooth [6-7].
The measurement of composite polymerization kinetics is valuable in providing the fundamental information to prevent the
clinical consequences. Previous measurement of polymerization shrinkage employed dilatometry or infrared linometer [8,9]. In
these studies, the laboratory data for evaluation of composite materials is provided, and the obtained results were limited to
the amount of free shrinkage only. Sakaguchi et al. compared various methods, including the mercury dilatometry, strain
gauge, and thermo-mechanical analyzer for the measurement of polymerization shrinkage [10]. Different method generated a
great difference in measured shrinkage which was significantly affected by different constraints in the specimen preparation.
Moreover, various degrees of polymerization conversion (DOC) in the composite material due to inhomogeneous irradiation
received at different depth also generated heterogeneous shrinkage in local regions, and consequently contraction stress
distribution is affected [11].
Digital image correlation (DIC) is a method based on the comparison of two characterized images. Two similar speckled
images, one before deformation (reference image) and the other one after (deformed image), were acquired at different states
by a CCD camera. Based on the assumption of pattern matching that a pixel within an image can be identified by a unique
intensity pattern of the pixels subset, the intensity pattern of the subset undergoes a displacement that corresponds to the
in-plane displacement of the corresponding material element (or volume) [12,13]. With a sequence of digitizing the images,
analyzing light intensity and gray-value distribution, the correlation between subset is established and the displacement of a
specific point can be determined. Some interpolation functions, statistical correlation functions are utilized to obtain sub-pixel
accuracy. This method has been widely used to measure strain fields in engineering applications. In medical applications, use
of DIC to measure the displacement and strain field of intervertebral disc on the MRI images was reported [14]. With a full-field
measurement, DIC measurements can constitute an opportunity to bridge the connections between experiments and
simulations allowing for comparison of direct displacement and strain.
A measurement system to identify the shrinkage direction and amount is necessary in evaluating the restorative materials and
techniques. The objective of the present study was to examine the polymerization shrinkage of a composite material and the
displacement of surrounded tooth, by means of DIC technique. Test materials and methods are reported and the results are
discussed.
Materials and Methods
To measure the polymerization shrinkage, an image acquisition system is assembled with an optical microscope (Zoom
microscope ML-Z07545D, Moritex Inc., Japan), a high-resolution CCD (MTV-12V1E, Mintron Co., Taiwan), a SCSI interfaced
image acquisition card and a personal computer (Fig. 1). The CCD converts the spatial light intensity into digital signal and
output into the personal computer through the image acquisition card. A custom program developed at National Cheng Kung
University was used to execute the computation. The errors of the measured displacements as obtained in previous works is
less than 2 % when the displacement is larger than 10 m [15].
The composite shrinks in 3 dimensions, while the z-direction displacement may alter the light intensity on a warped surface. To
dealing with this case, a preliminary experiment was set up to verify the practicability of DIC program. A stainless steel block
with a rectangular slot of 5 mm (W) x 3 mm (D) x 20 mm (L) on the top surface was used as a simulated cavity. The surfaces
Figure 1. A DIC image acquisition system.
of the slots were sandblasted to create the surface retentive feature. Dental composite Z250 (3M/ESPE, St. Paul, MN, USA)
was placed into the slot and surface flushed with the steel cube. In order to create a characteristic pattern on the specimen
surface, one side of the specimen is sprayed by TiO2 powder (ProCad contrast medium, Ivoclar Vivadent) for a white
background and black powder deposition on the white background (Fig. 2a). The reference image was captured by the CCD
camera. Following fully irradiation of composite under quartz-tungsten halogen lamp, the deformed image was captured and
the two images were compared with the DIC analyzing program.
A corresponding 3D finite element model is constituted as a stainless steel cube with composite filled in a slot (Fig 2a). The
mat
er
i
alpr
oper
t
i
es ofst
eelar
e 21000 GPa ofYoung’
s modul
usand 0.
3 ofPoi
sson’
s ratio. The material properties of
composite in different increments shown in Fig 2b were obtained from experimental values. The boundary condition is
assumed as a bonded condition between composite and steel and fixation around the borders of the steel cube. As the
composite shrank, the deformation was assumed to happen merely in the exposed surfaces. Deformation on the top surface
was analyzed with ANSYS Workbench (ANSYS, Inc., Canonsburg, PA) to compare with the results in DIC method.
Eight intact extracted human molars were used for the composite polymerization in real teeth. The teeth were mounted in
acrylic resin to embed the roots with their long axes perpendicular to the bottom. Cavities were prepared using a dental high
speed handpiece (Kavo Dental GmbH) and diamond burs (Shofu #411, Shofu Inc., Japan). A transverse cavity, with 4 mm
deep and 2 mm wide, was prepared on the occlusal surface. The cavity surfaces were etched with phosphoric acid and treated
with resin adhesive. Dental composite Z250 was placed into the cavity in a bulk. Subsequently the mounted tooth block was
secured on a holding device. One proximal surface was sprinkled with white powder (TiO2) black powder as the pattern. After
the image of unpolymerized restoration was acquired by the CCD Camera, a halogen light cured the composite for 40 sec from
the top. The polymerized composite restoration was serially photographed per minute for 10 min after polymerization to
compare the effect of composite shrinkage and stress relaxation. The reference and deformed images were digitized and
analyzed with DIC program. Bicubic-spline function was applied to fit the experimental images for interpolation. The proposed
algorithm executed fine search of the interested point in which was characterized by a 51 x 51 pixel area. Points to be
observed were located on the free surface and boundaries of composite fillings.
(a)
(b)
Young’
s modulus in different
composite increments
Depth(mm)
< 0.5
0.5-1
1-1.5
1.5-2
2-2.5
2.5-3
a()
Young’
smodul
us(
GPa)
17
16.735
16.520
15.834
14.520
12.641
Figure 2 (a) A stainless steel block used to simulate a composite restoration. (b) The corresponding finite element model for
comparisons with the shrinkage measured with the DIC method.
Result and Discussion
Figures 3a and 3b shows the deformation on the top surface of the composite restoration measured from the steel block and the
finite element model, respectively. The deformation was found greater on the middle and gradually decreased to both sides. The
displacements at the points (Fig. 3) comprising of x and y direction movements measured by DIC and FE simulation are listed in
Table 1. The consistent result found between them shows the applicability of DIC technique in measuring the dental composite
shrinkage. The y-direction displacements in all the measured points were greater than x-direction except at two attached ends. The
greatest y-direction displacement was 54 m.
The movements on the boundaries of the composites at different time were also measured in the real teeth cases. The
correlation coefficients obtained between the pre-cured and post-cured images were greater than 99% in all comparisons.
Figure 4 shows a typical result of the displacements measured on the boundaries of a dental restoration after post-cured 10
min. The greatest deformation was on the free surfaces, and the least on the gingival wall. The axial wall displacement on the
base reaches the amount of 12 m. The composite displacements at different points on the occlusal surface (Fig 4) over
curing time were plotted in Figure 5. The greatest amount of shrinkage was found 10 min after post-cured, which indicated that
the deformation continued after light irradiation. In addition, a large displacement of the axial wall was noted in the tooth model.
The measured value is comparable with the cusp deflection obtained in the other reports [7,16].
In previous study, the displacement of the composite-bonded cusps was measured by LVDT and the result was restricted merely in a
local area. Recently, ESPI was applied to measure the 3D nature of a tooth deformation after the composite was restored. However,
the Interferometry method is time-consuming and the experimental set up need precautious verification. Contrarily, using DIC
achieves the in-planer displacements but the laboratory restriction and cost is greatly reduced.
b
a
Point
1
2
3
4
5
Figure 3 (a) The displacement on the surface of composite restoration measured by DIC method. (Illustrated by 20x
magnification) (b) The x and y direction deformation measured on a simulation model.
Table 1. The amount of displacement on the top surface of the half composite restoration and FE model.
point
1
2
3
4
5
DIC
x direction
y direction
di
spl
acement(
μm) di
spl
acement(
μm)
5.62
4.98
18.35
22.90
16.22
40.95
12.20
48.31
7.09
53.07
Occlusal 1
Occlusal 2
FE simulation
x displacement
y direction
(
μm)
di
spl
acement
(
μm)
17.83
10.14
17.71
28.69
14.97
43.17
7.08
50.82
1.68
57.14
Occlusal 3
Occlusal 4
Figure 4. Illustration of contraction direction (50x) at different points on the boundary of a composite restoration 10
min after irradiation.
30
displacement
25
20
post-cured
15
1 min
10 min
10
5
0
Occlusal 1
Occlusal 2
Occlusal 3
Occlusal 4
Figure 5. The measured displacement (
μm)of different points on the occlusal surface after different curing time.
Before the present study, the determination of shrinkage was only limited in measuring the amount and directions of composite
shrinkage only. The shrinkage was measured indirectly by monitoring deflection of a thin glass coverslip in contact with the surface of
the composite specimen with a Dynamic Mechanical Analyzer or LVDT [17]. The result showed a 40 μm ex
t
r
usi
onoft
het
ops
ur
f
ac
e
after the light irradiation. The magnitude of shrinkage measured in the present study is similar to the method mentioned above but in
an intrusion direction. Furthermore, in the present study, the composites were placed in a real dental cavity instead of a Teflon or
metal mold thus providing a more practical bonded constraint. Therefore the composite restoration deformation determined by the
DIC method in our study can be more close to a clinical situation.
Conclusion
A novel application of DIC techniques to measure dental composite shrinkage has been presented. Using DIC technique, the
displacement of specific location can be identified and the overall profile of polymerization shrinkage can be described. DIC
exhibited the ability to measure the full-field deformation, which providing a bridge to connect the experimental and simulation
data. Based on the present study, DIC is considered useful in evaluating the composite shrinkage in a dental cavity. This
technique can be extensive used to investige different composite materials and also for the improvement of dental restoration
techniques.
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