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POLYMERIZATION SHRINKAGE STRAIN PRODUCED BY LOW-SHRINKAGE
COMPOSITE RESINS
by
Gowri Natarajarathinam
Dr. JOHN O. BURGESS, COMMITTEE CHAIR
Dr. JACK E. LEMONS
Dr. MARK S. LITAKER
Dr. AMJAD JAVED
A THESIS
Submitted to the graduate faculty of The University of Alabama at Birmingham,
in partial fulfillment of the requirements for the degree of
Master of Science
BIRMINGHAM, ALABAMA
2010
Copyright by
Gowri Natarajarathinam
2010
iii
POLYMERIZATION SHRINKAGE STRAIN PRODUCED BY LOW-SHRINKAGE
COMPOSITE RESINS
Gowri Natarajarathinam
MASTER OF SCIENCE IN CLINICAL DENTISTRY
ABSTRACT
Polymerization shrinkage is one of the primary concerns of a dental clinician when
placing direct resin-based posterior composite restorations. Polymerization shrinkage
stress has the potential to initiate failure of the composite-tooth interface if the forces of
polymerization contraction exceed dentin bond strength. Such gaps may cause post-
operative sensitivity, micro-leakage, and secondary caries.
Objective: To measure and compare polymerization shrinkage and shrinkage strain of
four low shrinkage composite resins and compare with a conventional BisGMA based
composite resin.
Method: Plastic molds were used to fabricate composite blocks. Z100 (3M ESPE) was
packed with a Teflon base with to correspond to the cavity. Fabricated composite blocks
were stored in de-ionized water (37˚C for 24 hours) and mounted in self-cure clear
acrylic in PVC rings for mounting purposes. Strain gauges were then attached to the
specimen on one side and end wires on the other side and connected to a Model 5100
Analog to Digital (AD) Scanner. OptiBond Solo Plus (Kerr) bonding agent was applied
in the prepared cavity according to manufacturer’s instructions. Composite resin was
placed in the prepared cavity and light cured (3M ESPE Elipar S10). The strain on the
tooth surface was recorded during the whole procedure and 15 minutes after curing.
iv
Results: Z100 was excluded from analysis due to micro fractures that occurred within the
specimens.
Conclusion: Means were compared among the materials using one-way ANOVA.
Tukey’s test was used for pairwise comparison of group means. The F-test in comparing
the maximum strain (p-value < 0.0001) and the minimum strain (p-value < 0.0003)
proved that the means of the different study groups differ. The analysis demonstrated
that Filtek LS had a significantly lower minimum strain than GC Kalore and N’Durance.
SureFil SDR had significantly lower minimum strain than N’Durance.
Keywords: Composites, Low-shrinkage, Polymerization Shrinkage, Shrinkage Stresses,
Shrinkage Strain
v
DEDICATION
This thesis is dedicated to my beloved parents Dr. A. Natarajarathinam and Amutha
Natarajarathinam who have supported me all the way since the beginning of my studies,
my sisters Malini and Anusha and my brother-in-law Dr. Sudarsan Rangan who have
been a great source of motivation and inspiration.
vi
ACKNOWLEDGMENTS
Dr. John Burgess – He introduced research to me, believed in me and inspired in
many ways. He supported my ideas and made it possible to design a model for my thesis
project. His guidance showed me a new perspective of restorative materials. I greatly
appreciate his encouragement and advice throughout the project. He is the reason for
everything I have done in the Department of Biomaterials.
Dr. Jack Lemons – He was a great mentor for my thesis and many journal clubs in
my program. He explained many new concepts of my thesis project with great clarity. His
input made the model a more precise one.
Dr. Mark Litaker – He helped me frame the experimental design and the statistical
analysis of my thesis project. He was also a great support in all my journal club
discussions throughout the program.
Dr. Amjad Javed – He helped me coordinate all the processes and procedures
between the graduate school and dental school to complete my masters program in a
timely manner.
Preston Beck – He was a great support for all my projects especially my thesis
project. He introduced strain gauges to me and walked me through many procedures of
my thesis project. He was a great support throughout the project.
Deniz Cakir – She trained me in many in-vitro projects and was a constant
support in all my endeavors in the biomaterials lab.
vii
Ian and Courtney – This program was a terrific experience only because of my
esteemed classmates Courtney and Ian. They were by my side every minute for the past
two years providing a fun platform to learn serious concepts.
Family – I could not have completed my masters degree without the love and
support from my family. My sisters Malini and Anusha and my brother-in-law Dr.
Sudarsan were a great support throughout my time at school. My parents Dr. A.
Natarajarathinam and Amutha Natarajarathinam believed in me and let me pursue what I
wanted to. Only with their support and encouragement I was able to complete this task.
Finally I thank God for the wisdom and perseverance that he has been bestowed
upon me during this research project, and indeed, throughout my life.
viii
TABLE OF CONTENTS
Page
ABSTRACT ....................................................................................................................... iii
DEDICATION .....................................................................................................................v
ACKNOWLEDGMENTS ................................................................................................. vi
LIST OF FIGURES .............................................................................................................x
INTRODUCTION ...............................................................................................................1
CHARACETRISTICS OF COMPOSITES .........................................................................2
CLASSIFICATION OF COMPOSITES .............................................................................5
POLYMERIZATION SHRINKAGE OF COMPOSITES ..................................................8
FACTORS THAT AFFECT COMPOSITE RESIN SHRINKAGE ..................................12
ORIGIN OF STRESS IN POLYMERIZING DENTAL COMPOSITES .........................15
METHODS TO CONTROL THE EFFECTS OF SHRINKAGE STRESS ......................18
MATERIAL APPROACH TO REDUCE SHRINKAGE STRESS ..................................23
POLYMERIZATION SHRINKAGE STRAIN .................................................................30
METHODS ADVOCATED TO MEASURE SHRINKAGE STRAIN ............................33
AIM OF THE STUDY.......................................................................................................35
MATERIALS AND METHODS .......................................................................................36
RESULTS ..........................................................................................................................52
POST EXPERIMENT ANALYSIS ...................................................................................54
ix
STATISTICAL ANALYSIS .............................................................................................64
DISCUSSION AND CONCLUSION ...............................................................................65
LIST OF REFERENCES ...................................................................................................66
x
LIST OF FIGURES
Figure Page
1 Composite Resin Restoration ...................................................................................1
2 Structure of Bis-GMA, TEGDMA ..........................................................................2
3 Polymerized Bis-GMA Monomer............................................................................3
4 Macro-filled Composite ...........................................................................................5
5 Micro-filled Composite ............................................................................................6
6 Adhesion vs. Bond Strength ....................................................................................9
7 Problems Associated with Polymerization Shrinkage ...........................................10
8 Composition of Resin Matrix.................................................................................13
9 Degree of conversion and Stress ............................................................................14
10 C Factor ..................................................................................................................15
11 White Line .............................................................................................................17
12 Siloxane and Oxirane .............................................................................................24
13 Silorane Polymerization .........................................................................................25
14 Polymerization Modulator in SureFil SDR............................................................26
15 GC Kalore Polymerization .....................................................................................29
16 GC Kalore Monomer DX-511 ...............................................................................29
17 GC Kalore Filler Particle .......................................................................................30
18 Plastic Molds ..........................................................................................................36
xi
19 Teflon Extension ....................................................................................................36
20 Mold with the extension fit-in................................................................................37
21 Applying Vaseline .................................................................................................37
22 Z100 1st Increment .................................................................................................38
23 Z100 Final Increment .............................................................................................38
24 Light Cure after each increment ............................................................................38
25 After light curing....................................................................................................39
26 Composite block out of the mold ...........................................................................39
27 Finished composite block ......................................................................................40
28 PVC Ring ...............................................................................................................40
29 Acrylic Resin Mixed ..............................................................................................41
30 Composite block placed in acrylic resin ................................................................41
31 Composite block in set acrylic resin ......................................................................42
32 Strain Gauge...........................................................................................................42
33 End Wire ................................................................................................................43
34 End wire connected to strain gauge .......................................................................43
35 Completed Strain Gauge ........................................................................................44
36 Completed Specimen .............................................................................................44
37 Metal Foil Strain Gauge .........................................................................................45
38 Normal Strain gauge ..............................................................................................45
39 Strain Gauge under Tension and Compression ......................................................46
40 Strain Smart Software ............................................................................................47
41 Optibond Solo Plus Bonding Agent .......................................................................47
xii
42 3M ESPE Curing Light ..........................................................................................48
43 Mounted Specimen ................................................................................................48
44 Teflon Mold to limit 2mm .....................................................................................49
45 Mold placed after 2mm increment .........................................................................49
46 Light cured after each increment ...........................................................................50
47 Strain Smart Software ............................................................................................50
48 Strain recorded while curing ..................................................................................51
49 SureFil SDR Graphs ..............................................................................................52
50 Filtek LS Graphs ....................................................................................................52
51 GC Kalore Graphs..................................................................................................53
52 N’Durance Graphs .................................................................................................53
53 Z100 Graphs...........................................................................................................54
54 Bottom margin of Z100 restoration 200X .............................................................55
55 Bottom margin of Z100 restoration 80X ...............................................................55
56 Bottom margin of Z100 restoration 40X ...............................................................56
57 Fractures within Z100 material ..............................................................................56
58 Fractures within Z100 material ..............................................................................57
59 Z100 Shearing at the bottom margin .....................................................................57
60 Z100 fracture at the 2mm increment line ...............................................................58
61 SureFil SDR Meniscus at the free margin .............................................................58
62 Surefil SDR - Side margin of the restoration .........................................................59
63 Surefil SDR - Side margin of the restoration .........................................................59
64 Surefil SDR - Bottom margin of the restoration ....................................................60
xiii
65 Surefil SDR Meniscus in the free margin of the restoration ..................................60
66 Surefil SDR Meniscus in the free margin of the restoration ..................................61
67 N’Durance - Side margin of the restoration ...........................................................61
68 N’Durance - Side margin of the restoration ...........................................................62
69 N’Durance - Bottom margin of the restoration ......................................................62
70 Defective vs. Normal Strain Gauge .......................................................................63
1
INTRODUCTION
The development of direct esthetic materials began in 1871 with silicate cements,
followed by the introduction of unfilled polymethyl methacrylate resins in 1937. Both
materials had significant limitations in physical and mechanical properties and esthetic
properties (wear resistance, surface roughness, polymerization shrinkage etc) which
limited their clinical success. Development of modern dental composites started in early
1960s when Bowen et al began reinforcing epoxy and related polymeric resins with filler
particles. On this humble beginning, composite resin materials with various
improvements in physical, mechanical and esthetic properties have developed. Composite
resins are currently the most frequently used tooth colored materials1 , having completely
replaced silicate cements and unfilled resins. Continued improvements in composite
restoratives, have led to the widespread clinical acceptance of these materials by dental
practitioners1. Coupled with an increasing demand by patients for esthetic restoration of
tooth structure the use of polymer based composite resins has steady growth 2.
Dimethacrylate based composites have been used in dentistry for over 30 years since
Bowen developed the Bis-GMA monomer and polymer to improve the physical
properties of acrylic resins. Acrylic resin monomers allow the formation of only linear
Fig 1 – Composite Resin Restoration
2
chain type polymers3. The acrylic resin monomers had increased shrinkage and reduced
wear and chemical stability. These properties lead to various clinical problems such as
color change of the restoration, marginal leakage and sensitivity due to shrinkage4. Early
chemically cured composites were two paste systems requiring a base paste to be mixed
with the catalyst, leading to problems with proportions, mixing process and color
stability5. In 1970, composite materials polymerized by electromagnetic light radiation
were introduced, replacing chemical systems which required mixing. At first, an
ultraviolet light source (365 nm) was used to provide the light energy required to
polymerize the composite resin, but its shallow polymerization and iatrogenic side-effects
led to its replacement with visible light (427-491 nm) curing units6. Composite
development continues to improve and evolve into more effective materials.
CHARACTERISTICS OF COMPOSITES
Fig 2-Structure of Bis-GMA, TEGDMA
3
Composite is a compound of two or more distinctly different materials attached to one
another with properties superior to those of individual components. The physical,
mechanical and esthetic properties and the clinical behavior of composites depend on
their structure. Basically, dental composites are composed of three chemically different
materials: the organic phase which forms the matrix of the composite, disperse phase
which is the filler particles blended in the organic matrix, and an organosilane or
coupling agent to bond the filler to the organic resin. The coupling agent is a macro
molecular structure with silane groups at one end (which forms an ion bond to SiO2) and
methacrylate groups at the other (which produces a covalent bond with the resin). The
organic matrix of composite resins is made up, in essence, of
a) System of mono-, di- or tri-functional monomers
b) Free radical polymerization initiation system, which in light cure composite resins
is an alpha diketone like camphoroquinone which is used in combination with a
tertiary aliphatic amine reducing agent and in chemically-curable ones is a
Fig 3- Polymerized Bis-GMA Monomer
4
peroxide-compound, benzoyl peroxide, used in combination with an aromatic
tertiary amine7
c) Acceleration system (dimethylaminoethyl methacrylate or DMAEM, ethyl-4-
dimethylaminobenzoate or EDMAB, or N,N-cyanoethyl-methylaniline or
CEMA), which acts on the initiator, allowing curing to take place in a clinically
acceptable time7
d) Stabilizer or inhibitor system such as hydroquinone monomethyl ether to
maximize the product’s storage life prior to curing and its chemical stability7
e) Absorbers of ultra-violet wavelengths below 350 nm, such as 2-hydroxy-4-
methoxybenzophenone, to provide color stability and eliminate the effects of UV
light on the amine compounds in the initiator system that can cause discoloration
in the medium to long term7.
The monomer system is the backbone of the composite resin system. Bis-GMA continues
to be the most-used monomer for manufacturing present-day composites7. (Dimers,
EMA, urethane methacrylate etc). Alone or in conjunction with urethane dimethacrylate,
Bis-GMA constitutes most of standard composite resin compositions. It has been
accepted that lower the mean molecular weight of the monomer or monomer
combination, greater the percentage of shrinkage8. As this monomer (bisGMA) has a high
viscosity , to facilitate the manufacturing process and clinical handling it is diluted with
low-viscosity monomers (low molecular weight) which are considered viscosity
controllers, such as bisphenol A dimethacrylate (Bis-DMA), ethylene glycol
5
dimethacrylate (EGDMA), triethylene glycol dimethacrylate (TEGDMA), methyl
methacrylate (MMA) or urethane dimethacrylate (UDMA)8
CLASSIFICATION OF COMPOSITES
Composite resins have been classified in different ways depending on monomer used,
(bis-GMA, Urethane, Orimers, Silorane), curing mechanism (Light, chemical or dual
cured), filler particle size (Microfilled, Macrofilled and Hybrid) and viscosity (flowable,
condensable) allowing dentists to identify and use them more easily.
A very popular classification which is still valid is that of Lutz and Phillips, based on
filler particle size which classifies composite resins as macro filled composites (particles
from 0.1 to 100 μ), micro filled composites (0.04 μparticles) and hybrid composites
(fillers of different sizes)9 A more detailed classification by Willems et al is based on a
number of parameters such as Young’s modulus, the percentage (by volume) of inorganic
filler, the size of the main particles, surface roughness and compressive strength10
Macro-filled Composites:
Fig 4-Macro-filled composite
6
Crystalline quartz, barium glass, strontium glass, borosilicate, prepolymerized resin
particles are the filler particles used in composite resins11. In macro-filled resins, the filler
particles were was ground into a fine powder containing particles 1 to 50 microns (µM)
in diameter. These composites are 70% to 80% glass by weight, 60% to 65% by volume.
The advantages of macro-filled composites are that they have good wear resistance and
increased compressive strength. Unfortunately, macrofill composites have some
undesirable qualities. The size of the filler particles makes polishing of macrofilled
compsoite resins difficult and poor polish retention. As a result of these limitations filler
sizes were reduced to improve wear and wear and enhancing properties, primarily
polishability12 .
Micro-filled Composites:
Microfillers smaller than 1 micron (µm) are used to fabricate microfilled composites. A
smaller particle has a relatively greater surface area in relationship to its volume than a
larger one. The high surface area allows only a small percent of filler loading and a
higher amount of monomer which increases shrinkage, wear and staining. When first
Fig 5-Micro-filled composite
7
formulated, microfilled composites were filled to a maximum of 38% by weight, 25% by
volume. Even though the particles are small, and thus are better retained in the plastic
matrix, the low density of glass particles resulted in poor mechanical properties,
including lower flexural, yield and tensile strengths9 .
Hybrid Composites: Hybrids contain a range of particle sizes. They do not retain a
high polish for long, due to the tendency of the largest particles to pop out of the surface,
but they retain their easy working characteristics due to the high percentage of larger
particle sizes. They are also much more resistant to wear than the macrofilled composite
resin because of the range of size of the particles and because of the presence of the
submicron particles, which are more difficult to dislodge than the larger particles. The
organic filler was difficult to silanate making it harder to bond the filler in to composite
resin. Limitations of these materials include intermediate polish and gloss retention, but
had better wear and mechanical properties. Also, they can be filled to a much higher
density with glass particles than those composites containing only micro sized particles.
The larger particles are necessary to keep the consistency of the paste from becoming too
stiff, while the relatively small percentage of sub micron size particles takes up the space
between the larger particles improving filler load13. Because of the high particle density,
hybrids were the first composites that were promoted for use in posterior restorations13.
Heliomolar (Ivoclar Vivadent) is advocated and still used successfully as a posterior
microfilled composite resin.
8
Nanotechnology has led to the development of a new composite resin characterized by
containing nanoparticles measuring approximately 25 nm and nanoaggregates of
approximately 75 nm, which are made up of zirconium/silica or nanosilica particles14.
The aggregates are treated with silane so that they bind to the resin14. The distribution of
the filler (aggregates and nanoparticles) gives a high load, up to 79.5%14. As the particle
size is smaller, resins made with this type of particle give the restoration a better gloss
reducing the material’s loss of polish retention over time. This technology has sufficient
mechanical properties for its use in the anterior and posterior restorations15. It has also
been mentioned that lesser amount of filler load due to the clusters produces less
shrinkage15. A new monomer developed by 3 M also contributes to the lower shrinkage
1.9%, creates less cusp wall deflection16 and reduces the presence of micro fissures in the
enamel edges, which are responsible for marginal leakage16, color changes, bacterial
penetration and possible post-operative sensitivity16. The drawback is that since the
particles are so small they do not have the same interactions with light, so they are
combined with larger-sized particles clusters of nanoparticles, with an average diameter
within visible light wavelengths (i.e. around 1μm), to improve optical, fracture toughness
and wear .
POLYMERIZATION SHRINKAGE OF COMPOSITE RESINS
Polymerization shrinkage is a primary concern when placing direct resin-based posterior
composite restoration. Studies measuring the shrinkage of composite resins have reported
about 2% to 5% polymerization shrinkage by volume in highly filled composite resin
9
restorative material. The polymerization shrinkage produced in a given composite resin is
related to the shade, opacity, and composition of the composite resin, exposure times of
the curing light used any incompatibility between a photo-initiator system and the
spectral output of the curing light, cavity preparation geometry, and composite layer
thickness17.
Fig 6- Adhesion vs Bond Strength
10
Volumetric shrinkage accompanying polymerization in dental composites complicates
placement and use of composite resins. Bulk shrinkage in vinyl addition polymerizations
is an unavoidable result of the formation of new covalent bonds that bring monomer units
closer together and reduce their mobility as part of an extended polymeric structure. On
an atomic scale, molecular vibration shows with increasing cross linking and therefore
the new polymerized structure shrinks. The greatest limitation in the use of composite
resin as a posterior restorative material seems to be shrinkage during polymerization,
which leads to poor marginal seal, marginal staining, and recurrent caries. Polymerization
converts monomer into a polymer network by activating photo initiators exposed to a
blue light, with an emission spectrum between 400 nm and 500 nm18 . Molecules placed
Fig 7-Problems associated with polymerization shrinkage
11
equidistantly by a van der Waals’ force change their position through covalent bonding18.
Through this process, the intermolecular distance is reduced, because of the formation of
a macromolecular network from discrete monomer species involving conversion of
intermolecular distances of 0.3–0.4 nm into primary, covalent bonds with lengths of
about 0.15 nm18.
The polymerization of the resin matrix produces a gelation in which the restorative
material is transformed from a viscous-plastic phase with flow into a rigid elastic phase
19. Shrinkage that occurs in a cavity before the gel point is reached, while the monomer–
polymer is still flowable, can be partially compensated by movement of molecules of the
resin composite from the free or unbound surfaces of the restoration. This mode of
compensation is not possible after gelation and, consequently after the polymer has
polymerized to the point that it stretches from one side of the cavity preparation to the
other, stresses built up in the composite which produces strain at the composite resin
tooth interface. The amount of contraction stress has been determined to be dependent on
the extent of the reaction19 , rate of modulus development the stiffness of the composite
and its ability to flow.
Polymerization shrinkage stress may produce failure of the composite-tooth interface
(adhesive failure) if the forces of polymerization contraction exceed the bond strength
between the composite and the dentin. Such gaps between the composite resin and cavity
walls may cause post-operative sensitivity, micro-leakage, and secondary caries20 .
12
Another problem is that the shrinkage causes deformation of cusp tips which causes
cracks in the tooth.
Composite polymerization always involves a degree of shrinkage, depending on the
organic matrix. Consequently, to reduce this negative effect, a great variety of monomers
have been developed and tested, including Spiro ortho carbonates (SOCs), epoxy-polyol
system combinations, which show 40%-50% less shrinkage in vitro than traditional
systems, the siloxane-oxirane based resins patented by 3M-ESPE21 or the use of high
molecular weight molecules such as multiethylene glycol dimethacrylate. Ormocers
(modified composites with organic and inorganic fillers) have also demonstrated their
ability to reduce curing shrinkage to a minimal extent 22
FACTORS THAT AFFECT COMPOSITE RESIN SHRINKAGE
Polymerization shrinkage, stress, and modulus development are dependent on filler
volume fraction, composition of resin matrix, degree of conversion of resin matrix,
elastic modulus, and C-Factor.
Filler Volume Fraction: Composite resins consist of polymer matrix and filler material.
With current composite resins, shrinkage is a direct function of the volume fraction of
polymer matrix in the composite. The more monomer entities unite into polymer chains
and form networks, the higher the composite contraction. On the other hand, the space
occupied by filler particles does not participate in the curing contraction. Therefore, the
13
presence of high filler levels is fundamental to reduce shrinkage of the composite during
polymerization23. Filler content directly influences the mechanical properties and wear
resistance of a composite resin24. Because of its effect on elastic modulus and volumetric
shrinkage the amount of filler contained in a resin-based composite is a major factor in
the development of polymerization contraction stress25. Rate of modulus and flow of
composites are also important factors that determine shrinkage of the resin.
Composition of Resin Matrix:
Bis-GMA, Urethane dimethacrylate (UEDMA) and triethylene glycol dimethacrylate
(TEGDMA) are the most commonly used components of dental composites. Bis-GMA is
a high viscosity monomer which imparts its viscosity to composite resin to thin this
monomer low molecular weight monomers like TEGDMA are mixed with Bis-GMA.
Diluents reduce resin viscosity allowing high filler levels with and clinically usable
consistencies. Unfortunately, this also increases the polymerization shrinkage.
Fig 8- Composition of resin matrix
14
Degree of Conversion of the Resin Matrix:
Each monomer has a different degree of shrinkage as a result of a different degree of
conversion and monomer molecular weight. Polymerization shrinkage increases when the
molecular weight decreases. Among the most commonly utilized monomers, UDMA and
TEGDMA exhibit a much higher degree of monomer to polymer conversion than Bis-
GMA and Bis-EMA26 .
Elastic Modulus: In vitro studies have shown the interfacial stress at the margin of the
restoration during the setting shrinkage of a resin composite is positively correlated with
the stiffness rate of the setting material known as elastic modulus or Young’s modulus27 .
Therefore, at a given shrinkage value, the most rigid material (the material showing the
highest elastic modulus) will cause the highest stress. Obviously, the elastic modulus also
increases as the polymerization reaction proceeds28.
Fig 9-Degree of conversion and stress
15
C- Factor:
Cavity configuration factor is defined as the ratio between bonded and unbonded surfaces
of the composite specimen. Shrinkage stress is low for configuration factors (C-Factors)
under 1.0, and rises very quickly before leveling off at C-Factors over 3.O29. Depending
upon cavity geometry and the clinical application technique, C-Factors over 2.0 are
frequently encountered with direct restorations. An increased C-value leads to a
decreased flow capacity which causes a higher rate of shrinkage stress development. The
experimental model developed for this study had a C factor of 5 which will enable to
measure the highest possible shrinkage strain.
ORIGIN OF STRESS IN POLYMERIZING DENTAL COMPOSITE
One of the main issues associated with shrinkage in dental composite restoratives is the
mechanical stress due to shrinkage developed during polymerization. The phenomenon of
internal force development in contracting materials was first described by Bowen in
196730. It is well established that the magnitude of contraction depends on factors related
Fig 10 – C Factor
16
to the geometry of cavity preparation as well as on the compositional and curing
characteristics of the composites.
Composite resin polymerizes by free radical polymerization generated when a photo-
initiator, such as camphoroquinone, absorbs light energy (photons) emitted from the
curing light and initiates polymerization by reacting with a photoreducer, a tertiary amine
forming free radicals and initiating cross-linking31. Camphoroquinone has a maximum
absorption at 468 nm and can be polymerized with LED curing lights 31 . Some composite
resins use other photoinitiators, such as 1-phenyl-1, 2-propanedione (with a peak
absorption of 410 nm), bisacylphosphine oxide, or triacylphosphine oxide (with peak
absorptions of 320 nm to 390 nm), which fall outside the curing range of most LED
curing lights31. These photoinitiators are used to reduce the strong yellow color produced
by camphoroquinone. The exothermic reaction created when the monomer converts to the
polymer produces a volume reduction in the polymer with a resulting decrease in
molecular vibration and intermolecular distances32.
As the polymer is formed, the resin matrix changes from a paste or pregel state to a
viscous solid and the composite resin contracts by about 1.5% to 5%. The gel point is the
point at which the resin changes from a viscous paste to an elastic solid. When the gel
point is reached, stress is transmitted from the composite resin to the surrounding tooth
structures. When composite resin is a paste, or pregel state, no stress is conducted to
surrounding tooth structure. As curing begins, the material flows from unbound surfaces
17
to accommodate for shrinkage. As the composite resin becomes more rigid because of the
increasing modulus of the composite, flow stops and the bonded composite resin
transmits shrinkage stresses generated to the surrounding tooth. This point is called the
gel point and the stress generated may exceed the adhesive bond or the cohesive strength
of the tooth or the composite, producing a marginal defect. When composite resin is
bonded on all tooth surfaces, shrinkage must be compensated by strain (flow) of the
composite, tooth, or adhesive33. If this stress is greater than the cohesive strength of the
composite, damage occurs within the composite. If the stress exceeds the tensile strength
of enamel, the enamel fractures. If the adhesive was placed improperly, it leads to
adhesive failures. These failures can be seen as a white line that appears during
restoration finishing because the finishing debris collects in the defect and changes the
index of refraction of light. Cracks or fractures are seen in teeth with bucco-lingually
wide restorations 34 because the cavity walls are primarily enamel, which is brittle and
too thin to withstand the forces generated by polymerization shrinkage.
When the inherent polymerization shrinkage is initiated by sufficient interfacial adhesion
between the developing polymer and a non-freely compliant substrate, as is the case with
Fig 11 – White Line
18
chemically bonded dental restorations, stress is conveyed to the substrate. Thus, in spite
of surface treatments that provide improved adhesion of composites to dentin, as well as
multi-step composite layering techniques, reliable adhesion without marginal gap
formation has not been demonstrated. Therefore, a substantial amount of research and
product development effort in dental materials has been directed toward understanding
and potentially reducing the development of shrinkage strain and stress in composites as
a means to further improve these materials.
METHODS TO CONTROL THE EFFECTS OF SHRINKAGE STRESS
Several articles have suggested that modifying curing light output may reduce
polymerization shrinkage and improve marginal integrity35. Three modes are available. A
continuous cure is used when the output is constant for a specified period of time. The
step- or ramp-cure begins at low intensity and switches to higher intensity36. The pulse-
delay cure is a discontinuous curing procedure. With this technique, increments of
composite resin are inserted and cured using the continuous cure. The final occlusal
increment receives a brief low-intensity cure and, after a delay during which the resin is
finished, the material is then fully polymerized to gain final mechanical properties.
Although it has been suggested that the curing mode and composite resin placement
technique may affect the marginal integrity of a composite resin restoration, in vitro
results to date have been mixed, with some investigators showing improved resistance to
leakage while others have been unable to demonstrate efficacy with these techniques36.
19
Originally, a vertical placement incremental technique was proposed to reduce the total
shrinkage in a composite resin restoration37. Since then, many variations of the
incremental placement technique have been advocated. A gingivo-occlusal layering
(horizontal)37 and wedge-shape layering (oblique) method is to place and polymerize
wedge-shaped composite increments from the occlusal surface38; the successive cusp
build-up technique is to apply the first composite increment to a single dentin surface
without contacting the opposing cavity walls, and to build up the restoration by placing a
series of wedge-shaped composite increments; each cusp is built up separately. One early
publication39 advocated a three-sided light-curing technique and incremental placement
of the composite to decrease the polymerization stress generated in the tooth structure
during composite resin curing. In this technique, a transparent matrix is used and a light-
reflecting wedge is placed interproximally at the gingival margin of the preparation.
Light-curing progressed by curing through the wedge to cure the initial gingival
increment of composite, then the buccal, and then the facial increment was placed and
cured. Even though this technique has been used by many clinicians, Losche reported that
little light reaches the center of the preparation. The three-sided curing technique’s
success is not due to the three-sided method, but to decreased light transmission and poor
composite resin polymerization. This brings into question all techniques where
increments greater than 2 mm are used or the composite resin is cured through the tooth.
In the centripetal build-up technique, developed for class II cavity restorations, an initial
vertical composite increment is applied on the cervical margin against the metal matrix.
Cavity filling is then completed by horizontally layering. This technique allows
20
transformation of class II cavities into class I cavities40. These techniques did not have
any significant effect in reducing polymerization shrinkage.
Bulk placement and curing have been recommended to reduce stress at the cavosurface
margins41. Using transenamel polymerization, advocated by Belvedere, the adhesive, a
flowable composite, and a composite resin are placed into the preparation in bulk and
then polymerized by curing through the tooth from the buccal and lingual.
Polymerization is completed by curing from the occlusal. This method of curing
composite resin was tested by measuring the leakage in Class I restorations. In two
separate studies42 , composite resin was used to restore premolars using four different
placement and curing techniques. No difference in leakage was found from the bulk-
filling technique vs any of the incremental curing techniques, even the pulse-delay curing
technique. In these studies, the composite resin was placed incrementally using different
types of increments—horizontal, diagonal, and with a slit in the center of the composite
that was filled with a final incremental of composite. There was no statistical difference
in leakage among any group and the bulk fill had the same leakage as the other placement
methods. After the microleakage was measured, the hardness of the sectioned teeth was
measured, beginning from the occlusal and proceeding toward the pulp42. The hardness of
the bulk-filled restorations was significantly less than the incrementally cured
restorations, which again demonstrates the limited depth of cure of composite resin.
21
Composite placed incrementally ensures more complete curing. Incompletely cured
composite resins may release components into the oral cavity that may be detrimental43 .
The reason for the reduced shrinkage with the bulk-curing technique is obvious—uncured
composite resin does not shrink as much as completely cured resin. A significant factor in
the reduction of curing effectiveness with a bulk-filling technique is that, as previously
discussed, light attenuates while penetrating through the tooth structure. As light passes
through the tooth structure, it drops dramatically from 500 mW/cm2 to 80 mW/cm2 when
curing through 2.5 mm of tooth43.
Goracci and co-workers44 in 1992 slowly polymerized composite resin over a 4-minute
period, while controlling the output of the curing unit with a rheostat. They showed fewer
gaps and marginal defects with this technique. The slow polymerization technique was
verified, but required so much time to polymerize composite resin that it was clinically
ineffective. They did show that slow polymerization methods have merit. Highlight by
3M ESPE (St. Paul, MN) was the first curing light with a step- or soft-cure, but was
discontinued and replaced with the 3M ESPE Elipar® TriLight, which has an exponential
output mode in which the output slides from low to high. In two studies45, no significant
difference could be found between using this technique and bulk-curing or other
placement techniques for that matter. Another technique is the pulse-delay, or the pulse-
cure technique. This requires placing increments of composite resin and curing for 20
seconds. The final enamel replacement increment is cured with a brief burst of energy for
2 to 3 seconds. A 3-minute delay is then allowed to enable the composite time to flow
22
and shrink while the restoration is finished and polished. After finishing, the restoration is
cured at high intensity to totally polymerize the material. Mechanical properties are
maintained when these techniques are applied. All the above studies concluded that the
different curing mechanisms did not have any effect on the shrinkage of the composite
resins.
Many investigators have compared incremental curing with bulk polymerization of
composite resin. Eakle and Ito46 compared four incremental insertion methods and noted
that diagonal insertion was best. Crim and Chapman47 reported that incremental
placement of composite resin was no more effective than bulk placement in reducing
leakage. Coli and Brannstrom48 reported that in composite resin restorations with bulk
insertion, the number of restorations with gaps was similar to a two-stage insertion.
Versluis and colleagues49 reported that incremental filling techniques reduce cusp
movement in teeth with a well-established bond. This brief and incomplete survey of the
investigations evaluating the bulk and incremental insertion of composite resin reveals
that neither method consistently produces superior results. Some report less leakage with
the incremental technique; others less with bulk placement.
In several studies50, no clear benefit to the soft-curing or the pulse-delay technique could
be seen. The effectiveness of the soft-cure or ramp-cure techniques in decreasing leakage
and stress at the margins of Class II restorations is not clear and it has not been
reproduced in clinical trials. Either the evaluation methods in clinical trials were not
23
refined enough to detect these differences or the amount of shrinkage is compensated by
other factors (water sorption or compliance of the tooth). Composites are able to
compensate for volumetric shrinkage by flow before the resin reaches a solid state,
although this compensation is limited29; in this study the authors found only 20% of the
shrinkage was completed at the maximum flow. Therefore 80% shrinkage occurs when
the material cannot flow which leads to shrinkage stresses. To accomplish improved
marginal integrity, the composite resin must flow during its change from a viscous paste
to an elastic solid to accommodate the resin shrinkage and to yield sealed margins. A
composite resin that does not shrink is necessary to consistently improve marginal
integrity Continued work in this area is essential.
MATERIAL APPROACH TO REDUCE POLYMERIZATION SHRINKAGE
STRESS
Material science has been focused to reduce the polymerization shrinkage by numerous
methods discussed above which clearly did not solve the problem. Materials science
developed new materials with less shrinkage but in the past we tried techniques to control
the shrinkage and reduce shrinkage effects. Now materials science has produced new
materials like silorane, ormocer, SDR etc. In the past decade, a promising approach to
solve this problem emerged which was the introduction of low shrinkage composite resin
materials. Most researchers have developed low shrinkage composite material using a
monomer system other than bis-GMA. Some of the low shrinkage composite resins that
were used in this study are described below:
24
FILTEK LS:
Fig 12 – Siloxane and Oxirane
25
A new monomer system called silorane, is obtained from the reaction of oxirane and
siloxane molecules22. Siloranes are a totally new class of compounds for the use in
dentistry. The name silorane derives from its chemical building blocks siloxanes and
oxiranes. The combination of the two chemical building blocks of siloxanes and oxiranes
provides the biocompatible, hydrophobic and low-shrinking silorane base of Filtek LS
Low Shrink Posterior Restorative. The polymerization process of Filtek LS restorative
occurs via a cationic ring-opening reaction which results in a lower polymerization
contraction, compared to the methacrylate-based resins which polymerize via a radical
addition reaction of their double bonds. The low shrinkage and stress mechanism in this
system is achieved by opening the oxirane ring during the polymerization process. As
silorane-based composite polymerizes, "ring-opening" monomers connect by opening,
flattening and extending toward each other. The result is significantly less volumetric
Fig 13 – Silorane Polymerization
26
shrinkage (1%) compared to methacrylate-based composites which has linear monomers
that connect by actually shifting closer together in a linear response while curing which
results in volume loss.
Filtek LS restorative is filled with a combination of fine quartz particles and radiopaque
yttrium fluoride. From the filler side, Filtek LS restorative is to be classified as a
microhybrid composite. The quartz surface is modified with a silane layer which was
specifically matched to the silorane technology in order to provide the proper interface of
the filler to the resin for long-term, excellent mechanical properties.
Filtek LS also requires a dedicated adhesive bonding resin, LS Bond (3M ESPE) to
achieve similar bond strengths to enamel and dentin. Filtek LS restorative, along with 3M
ESPE LS System Adhesive Self-Etch Primer and Bond, work together to alleviate most
of clinical problems associated with polymerization shrinkage.
SUREFIL SDR:
Fig 14 – Polymerization modulator in SureFil SDR
27
SureFil SDR Flow (DENTSPLY Caulk) is a low stress composite material indicated as a
bulk-fill base beneath posterior composite restorations and can be bulk filled in layers up
to 4 mm in depth. This material (Stress Decreasing Resin) was engineered to create a new
resin system that would allow internal reduction of the stress from polymerization
shrinkage. To control stress, the overall modulus development should be regulated while
maintaining the polymerization rate and conversion. In the SDR material, a
polymerization modulator was chemically embedded in the polymerizable resin
backbone. This modulator interacts with the camphroquinone to slow modulus
development, allowing stress reduction without reducing the polymerization rate or
conversion. This material provides significantly lower stress while maintaining the high
degree of conversion. The modulating effect allows extended polymerization without
sudden increase in cross-link density. Thus the extended curing phase maximizes the
overall degree of conversion and minimizes the polymerization stress. During light-cured
polymerization, resin monomers are in chaotic movement as they move towards one
another to form the polymer matrix. But in SDR Technology, the polymerization
modulator controls the chaos and allows a more orderly, well-paced formation of the
polymer network, absorbing stress as the curing continues without marginal breakdown,
post-op sensitivity, the formation of white lines, or cracks that are believed to occur with
high-stress materials. High translucency of this material allows significant light
transmission to bulk polymerize the material. This material is not suitable for occlusal
restoration due to its low wear resistance. Although this is not a low shrinkage material, it
is a low-stress producing material.
28
N”DURANCE:
N'Durance (Septodont) is a unique innovative mix of dimethacrylate monomers –
monomer based on Dimer Acid, DDCDMA (Dimer Dicarbamate Dimethacrylate),
EBPADMA (Ethoxylate Dimethacrylate) and UDMA (Urethane Dimethacrylate). The
unique resin based matrix contains approximately 19 wt % of ethoxylated BisGMA,
UDMA and the new dicarbamatedimethacrylate dimer acid. N'Durance composite
combines nano technology and dimer chemistry (nano-dimer conversion technology) that
result in natural, durable anterior and posterior restorations. N’Durance has the latest
nano-filler technology and consists of an optimized combination of Ytterbium fluoride
nano particles (40nm) and nano clusters (2-8 µm), barium glass (0.5 µm), and silica
(0.01µm). This distinctive blend of monomers comes from the Polymer-Induced Phase
Separation (PIPS) which significantly reduces the shrinkage effect (2.6%). Since this
composite is a blend of methacrylate, the conventional bonding agent can be used to
work with this composite resin.
29
GC KALORE:
KALORE (GC America) has a new monomer technology licensed from DuPont. The
three components factor into KALORE's unique design is the new monomer technology
licensed exclusively to KALORE, GC America’s newly-developed HDR (High Density
Radiopaque) prepolymerized filler and the proprietary interface between the filler and the
matrix which is a critical factor in the success and longevity of this composite.
DX-511 is the new monomer system used in KALORE along with), UDMA (Urethane
Dimethacrylate), Dimethacrylate. DX-511 Monomer has a long rigid core helps reduce
polymerization shrinkage, a flexible side arms help increase monomer reactivity. The
Fig 16 – GC Kalore Monomer DX-511
Fig 15 – GC Kalore Polymerization
30
resin has a high molecular weight and low number of C=C double bonds help reduce
polymerization shrinkage. Also, the monomer is compatible with current adhesive and
composite products.
The filler phase in KALORE contains Prepolymerized filler (with Lanthanoid Fluoride)
30 – 35% by weight, prepolymerized fluoroaluminosilicate glass 20 – 30% by weight,
St/BiLa filler, Strontium/Barium glass 20 – 33% by weight, silicon dioxide (Nanofiller)
1–5% by weight.
POLYMERIZATION SHRINKAGE STRAIN
Shrinkage stress that develops during volumetric polymerization contraction is
accompanied by a modulus development. The shrinkage stresses are transferred to the
surrounding tooth structure which can deform the tooth. The resulting deformation may
result in post-operative sensitivity and may open pre-existing enamel microcracks.
Shrinkage induced enamel microfracture reportedly occurs immediately after
polymerization. Therefore, the tooth-restoration complex is in a pre-stressed state even
before occlusal stresses result in further coronal deformation19. Therefore shrinkage strain
Fig 17 – GC Kalore Filler particle
31
is an important characteristic of a composite resin and plays a vital role in the success of
a restoration success.
It is important to understand the various stages of polymerization in order to correlate
how shrinkage stress and strain progress as a consequence of the reaction process. A wide
range of studies have been conducted to elucidate the reaction kinetics of the network
polymers51. As the extent of polymerization advances, it passes through several
interrelated physical and kinetic landmarks, including the gel point, auto-acceleration
leading to a rate maximum, vitrification and residual unsaturation in the final polymer.
All these features can be expected to impact the development of polymerization
shrinkage strain.
A gel-point is a hypothetical turning point in the mechanical response during the
polymerization process, where the composite behavior changes from elastic stage to
plastic stage. It is the appearance of an insoluble polymer fraction which involves a
continuous network structure that can span the entire macroscopic specimen dimension.
Many dental materials studies31 have rightly pointed out that shrinkage strain that occurs
prior to gelation does not contribute to stress since this involves viscous but unrestricted
flow52. However, there may be some confusion as to when the gel point is reached and
how this feature can potentially be manipulated to reduce shrinkage strain. The gel point
is conversion dependant and this critical extent of polymerization is not expected to be
dramatically altered by slowing the polymerization process through use of ramped,
32
stepped or pulse photo-curing modes. Once gelation occurs at very low conversion,
viscoelastic flow is still possible, but the timescale is increased relative to flow in the pre-
gel regime53 .
As modulus continues to increase along with conversion, the glass transition temperature
of the developing polymer reaches the effective cure temperature54 . Glass transition
temperature is a parameter (Tg), which describes the temperature at which amorphous
polymers undergo a second-order phase transition from a rubbery stage to from a
crystalline solid. As the temperature of a polymer drops below Tg, it behaves in an
increasingly brittle manner. As the temperature rises above the Tg, the polymer becomes
more rubber-like. At this time, the composite resin reaches the vitrification stage where
the material transitions from a rubbery to a glassy polymer. Unlike the gel point, which
has a well-defined onset, vitrification is a more gradual process due to the heterogeneity
of the polymer network at this latter stage of conversion. The residual unsaturation
present in fully cured dental polymers is a mixture of pendant reactive groups and free
monomer. Loss of the unreacted base monomer that constitutes the majority portion of
free monomer left behind in dental composites is partly responsible to a gradual decline
in polymer mechanical properties over extended aqueous storage intervals. Water
sorption and leaching also causes unreacted monomer release. Therefore, when
considering polymer stability, higher conversion appears desirable even though this also
leads to greater shrinkage and stress development55. There have been several studies that
evaluated the rate of shrinkage stress or strain development based on the reaction
33
kinetics. Regardless of the means used to manipulate the reaction kinetics, higher reaction
rates lead to higher rates of shrinkage and stress development56. It is already evident that
rapid development of polymerization shrinkage stress and strain may compromise the
competitive formation of an adequate adhesive bond between the composite and the
tooth57.
METHODS ADVOCATED TO MEASURE SHRINKAGE STRAIN
Understanding polymerization shrinkage strain is of vital importance because it may
expose clinically relevant implications for restorative procedures and so, there have been
few attempts to measure shrinkage strain. Dental researchers have measured shrinkage
strain using Stress-Strain-Analyzer testing machine, stress-strain-gauges19 or the method
of Watts and Cash. The simulation of the shrinkage behavior with a finite element
analysis (FEA) is an alternative approach to collect more insight into the clinical
situation, but is limited by some necessary assumptions for the FEA51. In vitro
experiments, using extracted teeth, based on dye penetration and quantitative marginal
gap analysis seem to be the most valid approaches to evaluate composite dentin bonding
agent and methods to minimize the consequences of curing contraction. However, since
the introduction of the hydrophilic dentin bonding agents the dye penetration technique is
of limited use because these hydrophilic dentin bonding agents are stained by the dye
themselves and it is very hard to differentiate the true gaps from the stained dentin
bonding layers. However, none of these measurements match the clinical situation
because most setups are an idealization and simplification of the true conditions.
34
Therefore, an experimental model in association with real clinical situations is mandatory
to assess the polymerization shrinkage strain of dental composite material.
In this model, composite resin blocks were used as a substrate in which different
materials were filled and shrinkage strain was recorded. Initially, natural teeth were used
to measure strain but later decided not to use them for various reasons. There were too
many variations in natural teeth shape, size and thickness of enamel in different areas of
the tooth. As shrinkage strain greatly depends on the geometry of the testing area, the
shape and size of the teeth was a major factor to be considered. Also, to attach strain
gauge, the enamel has to be etched which changes the structure of the tooth surface. To
avoid these issues, a material that is similar to tooth and can be made in standard size and
shape has to be used to get better results. Z100 (3M ESPE) was the material of choice in
making the substrate for testing material due to its resemblance in expansion closer to
tooth.
This study assessed polymerization shrinkage strain of various low shrinkage composite
resins as an attempt to verify the contribution of this property to the success of a
restoration.
35
Objective/Specific aims/Experimental Plan
To measure and compare polymerization shrinkage and shrinkage strain of 4 lower
shrinkage composite resins and one high strain producing resin different groups of
composite resins based on different monomer system immediately after curing
Hypothesis to be tested:
There is no difference in the polymerization shrinkage strain produced by low shrinkage
composite resins compared to the conventional composite resin.
Materials and Method: The experimental materials were selected based on different
chemistries and each of them compared with a positive control the standard methacrylate
monomer system.
Experimental Design:
Group 1: Z100– 3M ESPE (Control)
Group 2: Kalore – GC America
Group 3: N’Durance – Septodont
Group 4: SureFil SDR – Caulk Dentsply
Group 5: Filtek LS – 3M ESPE
Replication – 10 for each material
36
Method:
Clear acrylic molds (7mmx7mmx10mm) were used to prepare composite blocks. Z100
(3M ESPE) composite resin was used to prepare these blocks.
Fig 18 – Plastic Molds
Fig 19 – Teflon Extension
37
Composite resin was packed in to the mold in 2mm increments with a Teflon base with
2mmx4mmx4mm extension which would correspond to the cavity preparation on the
composite block
Fig 20 – Mold with the extension fit-in
Fig 21 – Applying vaseline
38
Fig 22 – Z100 1st Increment
Fig 23 – Z100 Final Increment
Fig 24 – Light cured after each increment
39
Fig 25 – After Light Curing
Fig 26 – Composite block out of the mold
40
These blocks were then mounted in self-cure clear acrylic in PVC rings which will aid in
mounting the specimens.
Fig 27 – Finished composite block
Fig 28 – PVC ring
41
Fig 29 – Acrylic Resin Mixed
Fig 30 – Composite block placed in acrylic resin
42
Strain gauges (Vishay SR-4 bondable foil strain gauge – 062AK) were attached to the
mounted composite block using M-Bond 200 (Vishay) adhesive which is a cyanoacrylate
based adhesive.
Fig 31 – Composite block in set acrylic resin
Fig 32 – Strain Gauge
43
.
Fig 33 – End wire
Fig 34 – End wire connected to strain gauge
44
A metal-foil strain gauge is commonly used when a very small amount of strain and
dimensional change occurs. This type works by measuring the change in electrical
resistance that occurs when an electrically conductive material deforms. This type of
strain gauge typically consists of a series of very thin parallel lines of conductive metal
applied to a thin foil. A small amount of stress in the direction of the orientation of the
parallel lines results in a multiplicatively larger strain over the effective length of the
Fig 35 – Completed Strain gauge
Fig 36 – Completed specimen
45
conductor and hence a multiplicatively larger change in resistance than would be
observed with a single straight-line conductive wire.
Fig 37 – Metal Foil Strain Gauge
Fig 38 – Normal Strain Gauge
46
The end wires of the specimen were connected to a Model 5100 Analog to Digital
(AD) Scanner (Vishay Intertechnology, Inc., Malvern, PA). The strain values were
observed using a computer with StrainSmart software (Vishay Intertechnology, Inc.,
Malvern, PA) for measuring data from the strain gauges.
Fig 39 – Strain Gauge under tension and compression
47
OptiBond Solo Plus (Kerr) bonding agent will be applied in the prepared Class I cavity
according to manufacturer’s instructions. The first increment (2mm) of composite resin
was placed in the prepared cavity.
Fig 40 – Strain Smart Software
Fig 41 – Optibond Solo Plus Bonding Agent
48
A Teflon mold with a 2mmx4mmx2mm extension was used to condense composite to a
depth of 2mm so that each increment was exactly 2mm in thickness. The composite was
light cured according to the manufacturer’s instructions. The final 2mm increment is
placed and light cured.
Fig 42 – 3M ESPE curing light
Fig 43 – Mounted Specimen
49
Fig 44 – Teflon Mold to limit 2mm
Fig 45 – Mold placed after 2mm increment
50
The strain on the tooth surface will be recorded during the whole procedure.
Fig 46 – Light Cured after each increment
Fig 47 – Strain Smart Software
51
This method will enable real-time registration of the progress of shrinkage strain,
corresponding to elastic modulus development. Strain from the two axes of the strain
gage will be averaged and plotted as a function of time.
Fig 48 – Strain recorded while curing
52
RESULTS:
The strain graphs for each study group are shown below.
Fig 49 – SureFil SDR
Fig 50 – Filtek LS
53
Fig 51 – GC Kalore
Fig 52 – N’Durance
54
POST EXPERIMENT ANALYSIS:
Z100 which is the negative control group of the study was expected to show the highest
strain. But in this experiment Z 100 did not show a significant difference from the other
groups (p>.05). Further analysis was done by making sections of the specimen and
visually analyzed in Keyance digital microscope. The images showed micro fractures
within the material (i.e. pulling away from the bonding surface), adhesive failure and
cohesive failure. It was concluded that the measurement recorded for Z100 was not the
actual strain of the material but the strain at the time of fracture.
Fig 53 – Z100
55
Fig 54 – Bottom Margin of Z100 restoration 200X
Fig 55 – Bottom Margin of Z100 restoration 80X
56
Fig 56 – Bottom Margin of Z100 restoration 40X
Fig 57 – Fractures within Z100 material
57
Fig 58 – Fractures within Z100 material
Fig 59 – Z100 Shearing at the bottom margin
58
Other study groups of the experiment were analyzed following the same procedure
SureFil SDR: SureFil SDR showed good adaptation to the composite block on the side
and bottom margins.
Fig 60 – Z100 Fracture at 2mm increment line
Fig 61 – Meniscus at the free margin
59
Fig 62 – Side margin of the restoration
Fig 63 – Side margin of the restoration
60
Fig 64 – Bottom margin of the restoration
Fig 65 – Meniscus in the free margin of the restoration
61
N’Durance: N’Durance also showed perfect seal between the composite block and the
restoration. There were no micro fractures or shearing of the material.
Fig 66 – Meniscus in the free margin of the restoration
Fig 67 – N’Durance Side margin of the restoration
62
Fig 68 – N’Durance Side margin of the restoration
Fig 69 – N’Durance bottom margin of the restoration
63
Outliers of the data:
Examination of the sample distribution identified two potential outliers. These extreme
values were found to be due to malfunctions in the measurement process, and these two
records were excluded from analysis. The two specimens that showed extreme values had
defective strain gauges attached to them.
Fig 70 – Defective vs. Normal Strain gauge
64
RESULTS:
Excluding the experimental group Z100 and the two outliers, results are displayed below.
Material
Maximum Strain
(µ strain units)
Mean ± SD
Minimum Strain
(µ strain units)
Mean ± SD
Total Strain
(µ strain units)
Mean ± SD
GC Kalore 52.1±37 -204.8±55 256.9±45
N’Durance 52.7±35 -215.3±64 268.1±57
SureFil SDR 95.3±29 -150.2±55 245.5±71
Filtek LS 116.9±29 -111.2±35 228.1±49
STATISTICAL ANALYSIS:
Means were compared among the materials using one-way ANOVA. Filtek LS showed
the least minimum residual strain followed by SureFil SDR, GC Kalore and N’Durance.
When total strain is considered highest strain was recorded for N’Durance followed by
GC Kalore, SureFil SDR, Filtek LS.
Tukey’s test was used for pairwise comparison of group means. The F-test in comparing
the maximum strain of the groups had p-value < 0.0001 and the minimum strain had p-
value < 0.0003 which proves that the means of the different study groups differ. Tukey –
Kramer test for multiple comparisons showed that Filtek LS had a significantly lower
minimum strain than GC Kalore and N’Durance. SureFil SDR had significantly lower
minimum strain than N’Durance.
65
DISCUSSION:
The model used in this experiment has many advantages. Composite blocks used in this
experiment proved close resemblance to natural tooth. (Elastic modulus of Dentin – 17-
21 GPa, Elastic modulus of Z100- 20-21 GPa). The results were consistent within each
study group. The block is simple to modify and can be adjusted according to materials
tested. This model also allows visual analysis of macro and micro level changes in the
specimen. This method of recording strain is easier than recording strain in patient’s
mouth as it does not depend on patient compliance. This method can be used as a method
to rank materials according to the shrinkage strain produced during polymerization.
Future Research: Mechanical retention within the blocks can be improved using Rocatec
system. One specimen tested with Z100 on Z100 showed the highest strain – 440 µ strain.
The margins in the preparation can be rounded to avoid bonding agent accumulation
within the cavity preparation. Recording time in this study was 15 minutes which is an
approximate time for finishing and polishing the restoration, but it is also important to
measure long term strain. Recording time can be extended for 1 hour or 24 hours
CONCLUSION:
Tooth-restoration complexes are in a pre-stressed state even before occlusal loading
results in further coronal deformation.
Shrinkage strain is an important characteristic of a composite resin and plays a vital role
in the success of a restoration.
Dental researchers are working to balance the material properties of composites.
66
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