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POLYMERIZATION SHRINKAGE STRAIN VERSUS C-FACTOR AND
PLACEMENT METHOD FOR DENTAL COMPOSITES
by
Shashank Godbole
DR. JOHN O. BURGESS, CHAIR
DR. JACK E. LEMONS
DR. DENIZ CAKIR-USTUN
DR. LANCE C. RAMP
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
2013
ii
POLYMERIZATION SHRINKAGE STRAIN VERSUS C-FACTOR AND
PLACEMENT METHOD FOR DENTAL COMPOSITES
Shashank Godbole
MASTER OF SCIENCE IN CLINICAL DENTISTRY
ABSTRACT
Polymerization shrinkage is an important material property when restoring teeth with
dental composite resins since all resins shrink during polymerization creating shrinkage
stress and strain around the restoration. These undesirable stresses produce failure at the
composite-tooth interface if the forces created during polymerization shrinkage exceed
the dentin bond. This may lead to marginal gaps, staining, post-operative sensitivity and
ultimately secondary caries, and crack development within the body of a tooth. To
develop new composite resins and place more durable restorations it is critical to be able
to measure polymerization strain (deformation) produced during composite
polymerization.
Objective: To measure and compare the polymerization shrinkage and shrinkage strain
of various bulk fill and incrementally placed composite resins using a novel and recently
developed model with different C-factor configurations fifteen minutes after curing.
Methods: Teflon molds were used to prepare composite blocks using Z100 composite
resin (3M ESPE), which served as a tooth analogue. To fabricate the block, Z100 was
packed into the mold in 2mm increments and light cured after the placement of each
increment. The base of the Teflon mold contained an extension which corresponded to
the cavity in the tooth analogue. For emulating a Class I cavity the dimensions of the
extension (on the base) was 2mmx4mmx4mm, while for a Class II cavity it was
iii
2mmx7mmx4mm. The Z100 composite blocks were then embedded using self-cure
acrylic in brass rings using specially fabricated Teflon molds and stored at 37˚C for 24
hours.
The cavity surface was treated with 50μ alumina particles (50μ, 45 psi, 15 sec) and
Rocatec Soft (3M ESPE) 30 μ silica-modified alumina particles (30μ, 45 psi, 15 sec).
RelyX Silane (3M ESPE) then Adper Single Bond Plus (3M ESPE) bonding agent was
applied and cured.
Strain gages (Vishay SR-4 bondable foil strain gage – 062AK) were attached to the outer
surface of the Z100 composite block using a cyanoacrylate based adhesive (M-Bond 200,
Vishay Intertechnology, Inc., Malvern, PA). The end wires of the strain gage were
connected to a Model 5100 Analog to Digital (AD) Scanner (Vishay Intertechnology,
Inc., Malvern, PA). The composite resin to be tested was placed in the cavity and cured
with Bluephase 20i (Power: 1010 mW/cm2, Ivoclar Vivadent). Strain units were recorded
on a computer using StrainSmart software (Vishay Intertechnology, Inc., Malvern, PA).
Residual strain was graphically plotted as a function of time. The strain values were
recorded for 15 minutes to record real-time progress of shrinkage strain.
Results: Significant difference between Class I and Class II residual shrinkage strain was
observed for all the materials.
Conclusion: All composite resins tested had residual strain which stresses the marginal
interface in restorations and may contribute to increased marginal breakdown. Tooth-
restoration complexes are in a pre-stressed state even before occlusal loading results in
iv
further coronal deformation. Shrinkage strain is an important characteristic of a
composite resin and plays a vital role in the success of a restoration.
Keywords: Dental Composites, Polymerization Shrinkage, Shrinkage Stress, Shrinkage
Strain, Strain Gage, C-Factor.
v
ACKNOWLEDGMENTS
Dr. John Burgess – He has been a wonderful mentor and guide and the one who
introduced me to the world of research. His inspiration, support and encouragement
during my time at UAB have been invaluable. I truly admire his dedication and
professionalism and wish to emulate the same throughout my own career. He has been
open and receptive to my ideas and suggestions related to the thesis and I thank him for
all his support.
Dr. Jack Lemons – He has been a true role model and has helped me understand
many concepts related to my thesis. He is always approachable and I have enjoyed all my
interactions and discussions with him, both formal and casual. His simplistic approach
towards clearing and understanding concepts is something I admire immensely.
Dr. Deniz Cakir-Ustun – She has been a wonderful teacher for the last two years
and has been a great help every step of the way. I appreciate her inputs in my thesis and
all her help for the other laboratory projects.
Dr. Amjad Javed – I thank him for helping me with all the formalities related to
my thesis. His guidance and co-ordination with the graduate school procedures has made
the entire process very smooth and hassle free.
Dr. Lance Ramp – He has been a great advisor with the statistical analysis for my
thesis. He helped me understand and comprehend it really well and I thank him for it.
Preston Beck – He is the heart of the Department of Biomaterials and is the one
who really keeps it functional and running. He is the most helpful person I have met till
vi
date and I thank him for all his help and support throughout the past two years. He has
been a very good mentor, teacher, instructor and friend.
UAB machine shop – I would like to thank them for their help in fabricating and
customizing the molds used in the study which made the study better and the procedures
precise and easy.
Fellow residents – I would like to thank all the Biomaterials residents (both past
and present) for their help and support. I would especially like to thank Gowri for
introducing me to the thesis topic and for all her guidance.
Family – I would not have been able to do this without my parents Dr. Anil
Godbole and Mrs. Sulekha Godbole, my wife Ketki, my brother Aditya, my sister-in-law
Sanasi and my cute little nephew Tanav. I thank them for being a constant source of
inspiration and for all their support. They have always believed in me and I dedicate all
my work and achievements to them.
vii
TABLE OF CONTENTS
Page
ABSTRACT ........................................................................................................................ ii
ACKNOWLEDGMENTS ...................................................................................................v
LIST OF FIGURES ........................................................................................................... ix
INTRODUCTION ...............................................................................................................1
COMPOSITION AND STRUCTURE OF DENTAL COMPOSITES ...............................3
CLASSIFICATION OF DENTAL COMPOSITES ..........................................................12
POLYMERIZATION SHRINKAGE OF DENTAL COMPOSITES ...............................16
POLYMERIZATION SHRINKAGE STRESS AND STRAIN ........................................22
FACTORS AFFECTING POLYMERIZATION SHRINKAGE, SHRINKAGE STRESS
AND STRAIN ...................................................................................................................26
METHODS ADVOCATED TO MEASURE VOLUMETRIC SHRINKAGE,
SHRINKAGE STRESS AND STRAIN ............................................................................32
AIM OF THE STUDY.......................................................................................................35
MATERIALS AND METHODS ......................................................................................35
RESULTS .........................................................................................................................60
STATISTICAL ANALYSIS ............................................................................................61
DISCUSSION AND CONCLUSION ..............................................................................62
viii
LIST OF REFERENCES ...................................................................................................64
ix
LIST OF FIGURES
Figure Page
1 Composite Resin Restoration ...................................................................................1
2 Structural elements of a Dental Composite .............................................................3
3 Formation of a Bis-GMA molecule .........................................................................5
4 Bis-GMA and UDMA molecules compared............................................................5
5 Initiator system.........................................................................................................7
6 Silane - Organic functional part and Inorganic functional part .............................10
7 Silane coupling the matrix with the fillers .............................................................11
8 Relationship between volumetric shrinkage, elastic modulus and stress ..............19
9 Adhesive failure at the interface producing marginal gaps ...................................20
10 Teflon molds used. Body and base ........................................................................37
11 Body and base assembly ........................................................................................37
12 Base molds. Class I and Class II ............................................................................38
13 Packing Z100 into the mold in increments ............................................................39
14 Light curing of Z100 increment .............................................................................39
15 The recovered Z100 block .....................................................................................40
16 Z100 block with Class I cavity (occlusal view) .....................................................40
17 Z100 block containing Class I cavity.....................................................................41
18 Z100 block with Class II cavity (occlusal view) ...................................................42
x
19 Z100 block with Class II (proximal view) .............................................................42
20 Teflon mold with central extension used for mounting Z100 blocks ....................43
21 Mounting mold with Z100 block held in place ......................................................43
22 Brass ring used for mounting .................................................................................44
23 Mounting assembly ................................................................................................45
24 Teflon mold inverted onto brass ring .....................................................................45
25 Z100 block with Class I cavity mounted in set acrylic resin .................................46
26 Z100 block with Class II cavity mounted in set acrylic resin ................................46
27 System used for abrasion of internal cavity walls .................................................47
28 Rely X silane ..........................................................................................................48
29 Adper Single Bond Plus adhesive ..........................................................................48
30 M-Bond 200 and surface preparatory agents from Vishay ....................................50
31 One strain gage used for Class I, two used for Class II .........................................50
32 Strain gage with end wires soldered ......................................................................51
33 End wires of strain gage .........................................................................................51
34 Pin to which end wires of the strain gage are soldered ..........................................52
35 End wires of strain gage soldered ..........................................................................52
36 StrainSmart software on the computer ...................................................................53
37 Teflon mold used to pack precise 2mm increments...............................................54
38 Specimen being light cured ....................................................................................54
39 Graph showing recorded residual strain ................................................................55
40 Strain recorded, Y-axis ..........................................................................................56
41 Time elapsed, X-axis .............................................................................................56
xi
42 Expansion recorded and cessation of curing ..........................................................57
43 Graph showing recorded strain (incrementally-filled composite) .........................58
44 ACUVOL to measure free volumetric shrinkage ..................................................59
1
INTRODUCTION
Dental composites are used in dentistry to restore the tooth structure lost due to
dental caries. Today, they have surpassed dental amalgam as the restorative material of
choice and are a standard of care 1-3
. Dental composites are more esthetic than dental
amalgams blending well with the adjacent tooth structure to give a tooth like appearance.
By definition, composites are materials composed of an organic polymer matrix,
inorganic filler particles, and a silane coupling agent 4. In the early 1960’s Dr. Rafael
Bowen introduced the first effective resin-based dental composite restorative material.
This consisted of a newly developed “BisGMA” monomer which was reinforced with
silica particle fillers 5.
At the beginning of the 1960’s silicate cements and unfilled methyl methacrylate
resins were used for direct esthetic restorations. Both restorative materials had significant
esthetic, physical and mechanical drawbacks. Silicate cements were brittle and sensitive
to desiccation 6, while unfilled methyl methacrylate resins exhibited high polymerization
shrinkage and poor wear resistance 7 which led to a number of clinical problems like
marginal leakage, marginal staining, and post-operative sensitivity. Bowen’s resin helped
Fig 1 - Composite Resin Restoration
2
eliminate most of these drawbacks as it was stronger than the silicate cements and shrank
lesser than methyl methacrylate, and was considered a good replacement.
In the last 50 years dental composites have evolved steadily and the contemporary
composites have improved from the dental composite introduced by Bowen. Composite
modifications to the matrix systems have led to those improvements 8. Of late, there has
been a great emphasis on developing and introducing low-shrinkage resin matrix systems
9. Filler sizes have decreased enhancing the polish retention, stain resistance and
mechanical properties of the composite resins 10
. Another significant improvement is
improved curing systems and polymerization protocols 11, 12
. These improvements have
produced increased physical, mechanical and esthetic properties of composites and
increased the longevity of resin composite restorations. Dental composites continue to
evolve and improve.
3
COMPOSITION AND STRUCTURE OF DENTAL COMPOSITES
Dental composites typically consist of three different component systems or
phases. They are the organic polymer matrix, inorganic filler particles and a coupling
agent. The organic phase (matrix) constitutes the bulk of the composite, the inorganic
filler particles (dispersed phase) is blended in the organic matrix, and an organosilane
(coupling agent) is used to bond the filler particles to the organic resin 13
.
In addition to these basic phases dental composites also contain an initiator-
accelerator system to convert the resin composite from a soft, plastic material into a hard,
rigid restoration. Other additives like inhibitors, UV absorbers extend the shelf life and
storage capacity of the resins, while pigments match the color of the composite to the
tooth structure.
Fig 2 - Structural elements of a Dental Composite
4
Each phases of the dental composite affects its overall material properties 14.
The
color stability and shrinkage of a composite is dependent upon its matrix 15, 16
, whereas
strength, stiffness, wear and coefficient of thermal expansion are influenced by the filler
and coupling agent 17, 18
. Properties such as polymerization shrinkage and water sorption
depend on both the matrix and filler 19-21
. The ultimate properties exhibited by the dental
composites, as well as their clinical performance are closely related to their composition
and structure.
Organic Phase – Polymer Resin Matrix
The organic phase is the backbone of a dental composite and can be composed of
a variety of resin chemistries. A dental composite typically consists of a system of mono-,
di- or tri-functional monomers. The most popular and widely used among these are the
dimethacrylates - BisGMA and UDMA.
Bis-GMA (2,2-bis[4-(2-hydroxy-3-methacrylyloxypropoxy)phenyl]propane) was
synthesized and named by Bowen in 1956. This monomer is produced by combining one
molecule of an epoxy (bisphenol A) and two molecules of a methacrylate (glycidyl
methacrylate). To date, Bis-GMA remains the most widely used monomer in
contemporary dental composites 22
.
5
Fig 3 – Formation of a Bis-GMA molecule
UDMA (l,6-bis(methacrylyloxy-2-etlioxycarbonylamino)-2,4,4-trimethylhexan)
or urethane dimethacrylate is the second major class of dimethacrylates used in dental
composites. The UDMA monomer replaces the stiff aromatic center of Bis-GMA with a
more flexible methylene diisocyanate linkage.
Fig 4 - Bis-GMA and UDMA molecules compared
6
The Bis-GMA (MW= 513 g mol−1) and UDMA (MW = 471 g mol−
1) molecules
have reactive carbon double bonds (C=C) on each end that can take part in free-radical
polymerization reactions, which converts the individual monomers into a highly cross-
linked polymer. Most composites contain a primary high-molecular weight monomer (as
discussed above, usually a dimethacrylate). They may be present individually (Bis-GMA
or UDMA by itself) or as a combination of two or more monomers (Bis-GMA and
UDMA) 22
.
Diluents. A secondary diluent monomer is incorporated to reduce the viscosity
which improves the clinical handling of the composite. These diluents are low molecular
weight dimethacrylates such as ethylene glycol dimethacrylate (EGDMA), triethylene
glycol dimethacrylate (TEGDMA) 23
. Diluents reduce the mean molecular weight of the
composite resin but increase the volumetric shrinkage percentage of the composite 23
.
Therefore, these additions are limited.
The other organic constituents of the resin matrix are: a free radical
polymerization initiation or a photo-initiator system, a co-initiator or accelerator system,
and a polymerization inhibitor or stabilization system to increase the shelf life of a dental
composite 8. Composites also contain UV-stabilizers, and small amounts of
supplementary components (tints, pigments, etc.) that vary according the manufacturer.
Dental composites are formulated so that free-radical polymerization reaction (addition
polymerization reaction) may proceed with light activation (light cure), chemical
activation (self-cure) or a combination of both self and light activation (dual cure).
7
Photo-initiators and accelerators. In light activated composites, the free radical
polymerization in initiated by an alpha diketone such as camphoroquinone present in
limited amounts (0.2-1.0%). Camphoroquinone absorbs light in the wavelength range of
400-500 nm, with 468 nm being the peak absorptive wavelength 25
. Camphoroquinone is
used in conjunction with a tertiary aliphatic amine reducing agent, which helps to
accelerate the reaction by production of free radicals 22
.
Fig 5 - Initiator system
Some light activated resins use 1-phenyl-1,2-propanedione (PPD) as the photo-
initiator. PPD has its peak absorption wavelength at 410 nm and must be cured with a
halogen lamp for best results 26
. In the chemically activated composites, benzoyl peroxide
is the initiator most commonly used. It is used in conjunction with an aromatic tertiary
amine 22
.
Once activated, the initiator systems stimulate the co-initiators or accelerator
systems to produce free radicals (which are molecules with unpaired electrons) and
propagate the polymerization process. Routinely used co-initiators are DMAEMA (2-
(dimethylamino)ethyl methacrylate) or dimethylaminoethyl methacrylate, EDMAB
8
(ethyl-4-(dimethylamino) benzoate) and CEMA (N,N-cyanoethyl-methylaniline). The
addition of co-initiators reduces the curing time to clinically acceptable levels 22
.
Inhibitors. Inhibitor or stabilization systems are added to the composite to prevent
the premature polymerization of the resin matrix. This increases the shelf life of the
composite and makes it chemically stable 22
. Some of the commonly used inhibitors are
BHT (2,4,6 – tritertiarybutyl phenol) and hydroquinone monomethyl ether in
concentrations of 0.1% or less.
Compounds such as 2-hydroxy-4- methoxybenzophenone are added to absorb the
UV light wavelengths (350 nm and below). This helps to negate the deleterious effects of
UV light on the amine compounds in the initiator system. If absent, the UV light may
produce discoloration of the composite over a period of time. UV-stabilizers improve
color stability of dental composites 22
.
Inorganic Phase – Filler Particles
The inorganic or the dispersed phase within dental composites consists of fine
particulate fillers composed of several inorganic materials such as quartz, silica glass
(containing barium or strontium), colloidal silica or ceramics 27
. These filler particles can
be fabricated using two different methods, namely mechanical grinding and chemical
separation 4.
Filler particles act as reinforcing agents (only if linked to the monomer) and have
a significant impact on the physical and mechanical properties of the composite 28
. The
incorporation of fillers into the resin matrix:
9
a) Reduces the total amount of volumetric shrinkage a composite undergoes during
polymerization. Incorporating large amounts of fillers in a composite decreases the
volume of resin matrix which undergoes polymerization shrinkage 29
. However it
increases the viscosity of the composite and therefore diluents are added to thin them.
b) Fillers improve hardness, wear resistance, compressive strength, and reduce the
coefficient of thermal expansion 30
.
c) They impact the optical and esthetic properties by moderating color, translucency and
radiopacity 31
.
The dispersed glass or silica fillers are bound to the matrix of the composite
material via the coupling agent to form the resin-based restorative material 13
.
Coupling Agent – Connector
Since the organic phase (matrix) of dental composites is hydrophobic in nature
and the inorganic phase (filler) is hydrophilic, they lack chemical affinity. Thus a
coupling agent is required to bond the two phases together 32
. This bond is achieved by
using a di-functional silane coupling agent.
Manufacturers treat the surface of the fillers with a coupling agent before
incorporating them into polymeric matrix. This process is known as filler silanization.
Silanization enhances the wettability of the filler particles with the resin matrix to
produce a stable composite mix 33
.
Chemically, the coupling agent (silane) is di-functional organic silicon
macromolecule. It consists of an organic functional part that establishes a bond with the
10
resin matrix, and an inorganic functional part which bonds to the fillers. The most
common coupling agent (silane) is MPS (3-methacryloxypropyltrimethoxysilane) 27
.
Fig 6 – Silane - Organic functional part (green) and Inorganic functional part (red)
The methacrylate group on the organic end of the silane (shaded green in the
figure above) forms a covalent bond with the resin matrix. The methoxy group on the
inorganic end (shaded red) undergoes a condensation reaction with the hydroxyl (-OH)
groups on the filler particles to form an ionic bond 34
. Thus, the interfacial adhesion
between the inorganic particles and the organic matrix is improved, which improves
mechanical properties of the composite resin.
11
Fig 7 - Silane (green) coupling the matrix (top) with the fillers (bottom)
12
CLASSIFICATION OF DENTAL COMPOSITES
Many classification systems have been used and proposed till date for dental
composites. The most commonly used and widely accepted system for classifying
composites is based on the size of the filler particles 4. Based on the size of the filler
particles alone, more than one classification system have emerged over the years as the
composites have evolved. We will take a look at these systems later in this section.
Apart from classifying dental composited based on fillers, they can also be
classified based on the type of resin matrix, for example, Bis-GMA containing, UDMA
containing, silorane based, ormocer based, dimer acid-based dimethacrylates, etc 9. As
mentioned earlier, they can also be classified based on the curing mechanism into light-
cured, chemical-cured and dual cured resins.
Composites can be classified according to their viscosity into flowable (least
viscous), packable (most viscous) and universal (intermediate in viscosity) 59
. Flowable
composites are placed into cavities using a syringe and adapt well to the cavity floor and
walls. They have lower filler content and exhibit the highest amount of polymerization
shrinkage (by volume) 40
. Packable composites which are packed into a cavity using a
hand instrument have higher filler content and lower shrinkage 40
. Universal composites
can be placed using either a syringe, or an instrument, depending on its viscosity.
Lutz and Phillips (1983) classified composite resin by their filler size into macro-
filled (larger than 0.1 microns, or 0.1 – 100 microns), micro-filled (colloidal silica
particles around 0.04 microns), and hybrid composites (containing both macro- and
13
micro-filler particles of varying sizes) 4. Lutz and Phillips sub-classified the micro-filled
(around 0.04 microns) into homogeneous micro-filled and heterogeneous micro-filled.
The homogeneous group contained silica fillers only, while the heterogeneous group
contained a mixture of silica filler and highly-filled, pre-polymerized resin fillers
(PPRF)4.
Macro-filled composites. The fillers for this type of composite were manufactured
by mechanical grinding of large quartz or strontium or barium glasses into irregularly
sized and shaped particles ranging from 0.1 to 100 microns in diameter 4. Traditional
macro-filled composites contained approximately 75-85 filler weight%, or 60-65 filler
volume%. Macro-filled composites were strong, but were difficult to polish and
roughened with time due to their large filler particles.
To solve the problems related to polish and surface roughness, filler sizes were
reduced to 0.04 microns (i.e. micro-filled composites) 60, 61
.
Micro-filled composites. These were introduced in the late 1970’s. The fillers for
this type of composite were manufactured using pyrogenic and precipitation methods to
produce colloidal silica particles with a mean filler size of 0.04 microns in size 62
. Micro-
filled composites contained 35-65 filler weight%, or 25-35 filler volume% 62
. The reason
for limiting the filler fraction loads in micro-filled composites was the size and shape of
the colloidal silica particles. A small, spherical filler particle has a very high surface area
relative to its volume. When mixed in resin these fillers agglomerate and restrict filler
loading. Traditional micro-filled composites polished well and had good esthetics, but
had increased polymerization shrinkage, poor wear and lower mechanical properties than
composites with larger fillers 4.
14
To eliminate the limitations of macro- and micro-filled composites, and attain a
balance between mechanical properties and esthetics, hybrid composites were formulated
and introduced in the early 1980’s.
Hybrid composites. Hybrids were a mixture of mechanically ground quartz or
glass (10 to 50 micron diameter) filler particles and micro-fill colloidal silica (around
0.04 micron diameter). A high filler load percentage is achieved in hybrids as the small
spaces in between the larger macro-fill particles can be occupied by the smaller micro-fill
particles 13
. The higher filler load reduces polymerization shrinkage and improve
handling characteristics of hybrids. Hybrid composites have good wear and mechanical
properties due to the size range of the filler particles 117
. The polish and gloss retention of
hybrids is intermediate as the larger filler particles tend to dislodge from top surface over
a period of time 117
.
In the late 1980’s and into the 1990’s, the filler size in conventional composites
was reduced by further grinding of the macro-fill particles. This led to another
classification system being introduced by Bayne et al in 1994 63
. Bayne classified
composites based on the size of the largest filler particle it contained. This system
included macro-fills (10 - 100 microns), midi-fills (1 – 10 microns), mini-fills (0.1 – 1
micron) and micro-fills (0.01 – 0.1 micron). The mini-fills (which are partially filled with
0.04 micron colloidal silica) are also commonly referred to as micro-hybrids (universal
composites) 60
. The term nano-fill was also introduced around the same time for
composites containing fillers in the nanometer range.
Nanotechnology. Nano-composites contain filler particles in the nanometer
scale27
. Nanoparticles, about 20-25 nanometers in diameter are made from a colloidal
15
mixture of silica and zirconium. Nano-aggregates amounting to 75 nanometers are also
produced 64
.
Ferracane 60
, in 2011, added ‘nano-hybrids’ to the classification system for
composites containing nano-particles and PPRF’s (pre-polymerized resin fillers).
16
POLYMERIZATION SHRINKAGE OF DENTAL COMPOSITES
One of the main concerns when placing direct composite resin restorations in
posterior teeth is the associated polymerization shrinkage produced during curing. A resin
composite undergoing polymerization exhibits volumetric shrinkage as its individual
monomer units form covalent bonds with neighboring mers to form polymers. These
bonds reduce the distance between them producing shrinkage during polymerization24
.
Most commercially available dental composites undergo free polymerization shrinkage
from 1% to 6% by volume 35, 36
.
A brief step-wise overview of the polymerization process in light activated
composites will be described here to better elucidate the phenomenon of polymerization
shrinkage.
When a light cured composite resin is packed into a cavity and anatomically
molded, it is irradiated with blue light using a curing gun. The blue light has an emission
spectrum from 400 nm to 500 nm 24
. As discussed previously, most dental composite
formulations employ the camphoroquinone (CQ) - tertiary aliphatic amine as the photo-
initiator - accelerator system 35
. CQ absorbs light in the wavelength range of 400-500 nm,
with 468 nm being the peak absorptive wavelength 25
.
Blue light activates CQ and transforms it chemically into an excited triplet state
which reacts with the tertiary amine to produce free radicals 12
. This reaction also
stimulates the accelerators to produce larger amounts of free radicals which begin the
polymerization process. The generated free radicals react with the monomer molecules in
17
the resin, forming active centers for polymerization. This step is the initiation of the
polymerization reaction and is directly dependent on the amount of free radicals 12
.
Following this, propagation of the polymerization process continues by sequential
addition of the monomers to the active centers to produce long, cross-linking polymer
chains. This reaction brings the individual monomer molecules closer together to form
covalent bonds. Prior to the polymerization reaction, the individual monomer molecules
are placed/grouped equidistantly by van der Waals’ forces 24
.
The average distance between the molecules held by van der Waals’ forces (i.e.
before polymerization) is 0.3 nm – 0.4 nm (3 Å – 4 Å), while the intermolecular distance
between covalently bonded molecules (i.e. after polymerization) is about 0.15 nm (1.5 Å)
24, 38, 39. During formation of these covalent bonds between monomers a reduction in the
intermolecular distance results which produces volumetric polymerization shrinkage. On
an atomic scale, molecular vibration decreases with increasing cross linking and therefore
the new polymerized structure shrinks.
The magnitude of volumetric shrinkage a composite exhibits depends on the filler
volume fraction and the degree of conversion of the resin matrix. Generally, flowable
composites (low filler content) produce higher volumetric shrinkage than traditional
packable composites (higher filler content) 40
. Volumetric shrinkage is directly
proportional to the degree of conversion (i.e. C=C double bond conversion into C-C
single bonds) 41
.
Other factors which influence shrinkage are the composition of the resin matrix,
intensity of the curing light 42
, exposure time 43
, mode of curing, compatibility between
18
the spectral output of the curing light and the photo-initiator system, thickness of the
composite resin, shade and opacity of the composite 44
.
Composite flow and the origin of polymerization shrinkage stress. As mentioned
above, all resin composites have volumetric shrinkage during the polymerization.
During the initial stages of the polymerization process, the composite mass is able
to undergo plastic deformation. In other words, there is some amount of flow exhibited
by the composite which can compensate for the some of the shrinkage. It is believed that
in this early plastic stage, only chain formation occurs and cross-linking is not complete,
which may allow the molecules to move into new positions (thus, flow). This is called the
pre-gelation phase.
After this, a gel-point (or a range of gelation) is reached when the resin matrix
changes from a pre-gel or viscous-plastic paste into a rigid-elastic material. The gel point
is defined as the moment at which the material can no longer provide viscous flow to
keep up with the curing contraction 45
. Within a filled cavity, the gel-point is a point at
which the polymer chains stretch from one side of the cavity preparation to the other.
Solidification or post-gel contraction follows, during which the material becomes
increasingly stiffer as it loses its potential for plastic deformation. During post-gelation,
the elastic modulus or Young’s modulus of the material increases dynamically 46
.
In a clinical situation where there is an intermediate adhesive/bonding agent layer
between the tooth and the composite resin, the ability for plastic deformation of the
composite is hindered at the interface. When composite is placed and polymerized in a
19
constrained cavity 47
internal tensile stresses (polymerization shrinkage stress) develop
within the composite.
Fig 8 - Relationship between volumetric shrinkage, elastic modulus and shrinkage stress
Prior to reaching the gel point, a part of the developing stresses are compensated
by flow of the composite from the free or the unbonded surface 46
. This mode of
compensation is not possible after gelation and, subsequently, large stresses are created in
the composite which produces strain the tooth-composite interface 48, 49
. The amount of
shrinkage stress development is dependent on the rate of modulus development, ability of
the material to flow, and the extent of the polymerization reaction 47
.
Clinical relevance. The latter part (rigid) of the contraction process has been
strongly linked to the production of shrinkage stresses and clinically visible marginal
defects 19,46,49,50
. If the magnitude of forces exerted due to polymerization shrinkage stress
20
exceeds the bond strength of the adhesive, we may see adhesive failure at the tooth-
restoration interface and marginal gap formation 49,50
.
Fig 9 - Adhesive failure at the interface producing marginal gaps (above)
Such a failure may progress to marginal staining, post-operative sensitivity 51, 65
,
micro-leakage 66
, and secondary caries 67
.
If however, the dentin bond is greater than the shrinkage stress and the enamel or
the composite does not fracture there is no adhesive failure and the stresses are
transferred to the adjacent substrate. This shrinkage of the composite produces strain on
the restored tooth 45
, and produces cuspal deflection or polymerization shrinkage strain 52-
57. The coronal deformation which is produced may produce enamel micro-cracks.
Enamel micro-fracture can occur immediately after polymerization 58
. Therefore, the
21
tooth-restoration complex is in a pre-stressed state (immediately after restoration), even
prior its exposure to occlusal stress loading which produces further coronal deformation
48.
22
POLYMERIZATION SHRINKAGE STRESS AND STRAIN
Dr. Rafael Bowen, the inventor of the modern dental composite, first described
the development of internal stresses within contracting dental composites in 1967 68
.
Bowen attributed the development of internal tensile stresses within composites to the
confinement of the resin polymerization due to the presence of an intermediate bonding
agent. Later, several authors (Davidson and de Gee (1984), Feilzer et al (1987), Zanchi et
al (2006) and Alomari et al (2007)) have studied the subject and have postulated
mechanisms, factors and outcomes for this shrinkage.
As discussed before, dental composites shrink significantly during
polymerization. As the composite mass begins to shrink, it initially flows (undergoes
plastic deformation) to accommodate for the shrinkage and relieve some of the
developing stresses. In a clinical situation, plastic deformation occurs only at the free or
unbonded surface of the restoration. On the internal cavity walls and the cavity floor, the
flow is curbed due to the intermediate adhesive layer and shrinkage stresses are generated
as a result. Thus, the greater the number (or surface area) of free composite surfaces, the
greater the flow and stress relaxation, and vice versa.
As polymerization continues the composite modulus increases, flow stops and the
material starts becoming rigid. At the gel-point the polymerizing material can no longer
undergo plastic deformation to compensate for the shrinkage as the composite changes
from a viscous paste (pre-gel) into an elastic solid. Prior to the gel point, the developing
internal stresses are not transmitted to the surrounding tooth structure, but after the gel
23
point is reached, stresses are transmitted to the tooth. Post-gelation the resin becomes
increasingly stiffer and is associated with a substantial increase in the elastic modulus of
the material. It is in this post-gel or rigid contraction phase, that we see an increase in
shrinkage stress.
Thus, the materials ability to undergo plastic deformation (viscous flow) in the
initial stages of polymerization, and its rate and extent of modulus development are
important factors in polymerization shrinkage stress development 49
. The configuration
factor (C-factor) 69
also has a significant impact on the composite deformation. The C-
factor is the ratio between bonded to the unbounded cavity preparation surfaces. A higher
number of bonded surfaces limit the plastic deformation and increases stress
development, whereas a higher number of unbounded (free) surfaces enable greater
plastic deformation (flow) to relieve the developing stresses 69
. Other factors such as the
degree of conversion of the resin 41, 73
, composition of the composite, amount of material
used 74
also influence the stress development. These factors will be discussed in detail in
the subsequent chapters.
As a consequence of shrinkage stress development and its transmission to the
surrounding tooth, the tooth-restoration interface is in a pre-stressed situation even before
functional occlusal loading 48
. What ensues is dependent on the magnitudes of the
contraction stresses that have developed. Many researchers (Feilzer et al (1987); Watts
and Cash (1991); Zanchi et al (2006) and Alomari et al (2007)) have measured the stress
values using different methods, polymerization methods and under varying configuration
factors, reporting them to vary from 5 MPa to 17 MPa 69-72
.
24
If the developed stress exceeds the bond strength of the intermediate adhesive
(which may occur in the event such as an incorrect adhesive application technique) we
may see marginal gap formation due to adhesive failure 49, 50
. Marginal staining (white
line around the restoration) is often seen when composite restorations are finished and
polished. When debris generated from the finishing process collects and is packed in the
marginal gaps the refractive index of this debris is different from that of the adjacent
tooth structure and the finished restoration surface and gives the appearance of a white
line. Micro-leakage, post-operative sensitivity, secondary caries are all possible sequels
of the adhesive failure at the margin. In fact, a couple of in vitro studies have been able to
exhibit a direct relationship between marginal integrity and shrinkage stress in Class V
restorations 78,79
. It must also be noted that to reduce or control these undesired effects,
the adhesive bond strength must not only withstand the initial shrinkage stress during
placement of the restoration, but must also endure the subsequent thermal and mechanical
stresses during the life of the restoration 49
.
Cohesive failure within the body of the composite will occur if the generated
stresses are lower than the bond strength of the intermediate adhesive, but greater than
the cohesive strength of the composite producing micro-cracks or internal damage in the
composite 75
. If the developing stresses are less than the bond created by the adhesive
(bonding layer) as well as the cohesive forces (within the composite), they are transmitted
to the surrounding somewhat compliant tooth structure producing tooth deformation. This
phenomenon brings about some degree of dimensional change within the remaining
enamel and dentin. This deformation/change is called as ‘polymerization shrinkage
strain’.
25
If the magnitude of the transmitted stresses are less than the tensile strengths of
the enamel and dentin we may see cuspal deflection 52-57, 76
. These forces may also be
enough to open up the pre-existing defects in the tooth such as enamel lamellas or cracks
49 and also lead to post-operative sensitivity. The enamel or the cusps may even crack or
fracture if the transmitted stress exceeds the tensile strength of enamel 80
. Versluis
demonstrated that large, wide cavities with thin enamel walls fracture when restored with
composites generating moderate amounts of shrinkage stress 77
demonstrating that
stresses less than the adhesive bond strength can cause significant damage to the thin and
brittle residual tooth structure 77
.
Shrinkage strain is a significant inherent feature of the composite resin and plays
a vital role in the clinical success of a restoration. Since such a strain developing from
polymerization shrinkage produces significant problems with composite resins
developments continue to reduce polymerization shrinkage.
26
FACTORS AFFECTING POLYMERIZATION SHRINKAGE, SHRINKAGE
STRESS AND STRAIN
The process of free volumetric polymerization shrinkage as well as the factors
directly influencing it is incompletely understood. However the roles played by the filler
volume fractions in the composite, the composition and formulation of the resin matrix
and the degree of conversion of the resin in free volumetric shrinkage are well
understood. But the phenomenon of stress (and strain) development during
polymerization is very complex and is dependent on a multitude of factors and the
interplay between them. Apart from the factors directly affecting free volumetric
shrinkage, many other properties of the composite such as its ability to flow or undergo
plastic deformation while undergoing polymerization, the rate of the polymerization
reaction, the rate and magnitude of elastic modulus development, the configuration factor
(C-factor) as well as the curing mode and placement techniques influence the generation
of stresses.
Filler volume fraction: The volume of fillers in a given resin has an inverse
relation to volumetric shrinkage. When a composite undergoes polymerization, the
monomer molecules undergo an addition polymerization reaction and are converted into
polymers by covalent bond formation. Thus the atomic vibration and intermolecular
distances between the individual monomers is reduced resulting in volumetric shrinkage.
Only the resin matrix phase of the composite shrinks. Hence, if the filler volume fraction
in a composite is higher polymerization shrinkage decreases 40,81
.
27
Hybrid composites typically have filler fractions around 60 vol% and shrink
between 1 and 3 vol%. Homogeneous micro-filled flowable composites contain filler
fractions below 50 vol% and produce approximately 5 vol % shrinkage due to their
reduced inorganic content 40
. Micro-filled composites develop lower stress values
compared to hybrids since their modulus of elasticity is lower. Heterogeneous micro-
filled composites (which contain a combination of micro-fill particles and ‘organic’ pre-
polymerized resin fillers) exhibit shrinkage values similar to hybrids since the pre-
polymerized organic fillers increase their total filler volume. Highly filled packable
composites have similar shrinkage values due to their higher filler loads 79, 82, 83
.
Composition and formulation of the resin matrix: Most traditional composites
contain dimethacrylates (Bis-GMA and/or UDMA) and some kind of a diluent like
TEGDMA. Bis-GMA is a high molecular weight monomer and is very viscous by itself.
Unfilled Bis-GMA undergoes shrinkage close to 5.2 vol% when polymerized. To
improve its handling properties, a low molecular weight diluent like TEGDMA
(shrinkage value = 12.5 vol%) is added to Bis-GMA resin. But this addition dramatically
increases the overall polymerization shrinkage value 40
.
Higher ratios of TEGDMA/Bis-GMA result in greater polymerization shrinkage,
and higher stress production 84
. Recent advances in resin chemistry have led to the
development of novel resin systems like silorane 85,86
and high molecular weight dimer-
acid based monomers which have attempted to reduce shrinkage values and the resulting
shrinkage stress. Silorane systems have a ring-opening polymerization reaction show
volumetric shrinkage as low as 1 vol%.
28
Degree of conversion of the resin matrix: The degree of conversion (DC) of a
composite resin is the number of (C=C) double bonds converted to (C-C) single bonds
during polymerization to form long chain polymers. The volumetric shrinkage of a
composite shows a direct relation to its degree of conversion 41, 87
. The DC varies from
monomer to monomer, based on its molecular weight and initial concentration of (C=C)
double bonds. TEGDMA shows a higher degree of conversion than Bis-GMA 88
.
Increasing degree of conversion also simultaneously increases the modulus of elasticity
of the polymer matrix 89
. In commercially available dental composites, the average range
for final degree of conversion is between 55 to 75 % 88, 92
.
High degrees of conversion can lead to greater stress generation compared to low
degrees of conversion 90, 91
. Unfortunately poor mechanical properties may result from
lower degrees of conversion. Very early in the polymerization process the absence of
cross-linking and covalent bond formation allows for viscous flow of the composite
which limits stress build-up. As the polymerization proceeds, cross-linking occurs and
stress develops. Beyond a certain level, a slight increase in the degree of conversion will
lead to high amounts of modulus development, increased stiffness, greater cross linking,
and very high stresses. One publication states that degree of conversion is one of the most
crucial parameters which dictate polymerization shrinkage and stress development 47
.
Modulus of elasticity: The modulus of elasticity or the Young’s modulus is the
stiffness of the material. It increases with increasing filler volume fraction 94
and also
with the degree of conversion 89
. Stress development has a direct relation with the
magnitude and the rate of modulus development 93
.
29
C-Factor: The term C-factor is short for configuration factor and is defined as the
ratio of bonded to unbonded surfaces of the resin composite restoration. The term was
first introduced in 1987 by Feilzer et al 69
who described the relationship between stress
development and different cavity confinement conditions. A typical Class I cavity had a
C-factor of 5 as there are 5 bonded surfaces against 1 unbonded surface (C-factor = 5/1).
On the same principle, the C-factor for Class II cavities (MOD) is 1, for Class II (MO or
DO only) is 2, for Class III is 1, for Class IV is 0.5 and that for Class V is 5. According to
Feilzer et al, a cavity preparation with higher C-factors (i.e. high number of bonder
surfaces), produce increased shrinkage stress in the composite. A C-factor of 5 will allow
for viscous flow and stress relief only at one surface which leads to increased stress.
Conversely, a higher number of unbonded surfaces in a restoration will lead to lower
stress generation as there will be greater surface area which will be capable of undergoing
plastic deformation to relieve the generated stresses. Stress generated is lowest for C-
factors below 1 and rises steeply for C-factor 3 and higher. It is the maximum for a C-
factor of 5.
Even within cavities exhibiting the same C-factors (for example 2 different Class
I preparations) there will a difference in the generated stress depending on the cavity
design. For the same volume of restorative material, a shallow and wide Class I will show
lower stress generation as opposed to a deep and narrow one 95
. In clinical situations C-
factors and stress development will be influenced by the extent of caries removal,
geometrical complexity of the cavity preparation, size of the tooth and amount of
remaining tooth structure, type of tooth and location of tooth in the arch (anterior or
posterior), composite used, temperature, curing light, photo-initiator, etc.
30
The experimental model used in the current study for mechanically testing cuspal
deflection due to shrinkage stress (i.e. the shrinkage strain) studies the effect of a Class I
(C-factor of 5) and a MOD Class II (C-factor of 1) cavities.
Placement method: Traditionally composite resins are placed in 2mm increments
or less, and light cured after each increment. This placement method is advocated to
ensure adequate depth of cure for each composite resin increment. Many placement
techniques have been proposed till date, which include the vertical placement technique,
the oblique (wedge-shaped) layer placement, horizontal (gingivo-occlusal) layer
placement, successive cusp build-up technique, etc. 111
. In addition to the placement
techniques, many associated light-cure modes supplementing the placement techniques
have been advocated. In spite of the many claims of reduction in shrinkage stress by each
individual technique, the in vitro results comparing them have been mixed and no clear
ideal incremental placement method has been provided. Advocates of the incremental
placement technique say that a reduction in the shrinkage stress and strain can be
achieved by placing composites by this method, though the results supporting these
studies are mixed. One major drawback of placing composites in increments can be the
creation of voids between layers due to improper packing of the composites. Placing
composites in increments increases chair-side time. Recently, dental composites designed
to be placed in large increments up to 4mm have been marketed. These composites have
higher concentrations of photo-initiators and increased translucency to produce this
increased depth of cure. The bulk-fill composites reduce placement time and void
formation in the restoration. Bulk-filling and curing of composites have been
recommended to reduce stress at the cavo-surface margin according to one study 112
.
31
Many studies have been conducted so far to compare the two placement
techniques (i.e. incremental vs bulk-fill) and the results have been mixed. A study by
Eakle and Ito 113
suggested that placing composites in diagonal (wedge-shaped)
increments produced the least amount of strain and micro-leakage. Another study by Coli
et al suggested that the marginal integrity of composites placed in bulk is similar to that
of those placed in increments 114
. Versluis et al report that incremental placement of
composites produces less cuspal deflection compared to bulk-placement 115
. Another
study by Crim concluded that there was no difference in marginal integrity and leakage
when composite resin is placed in increments or with bulk placement 116
. These studies
do not reveal a clear-cut solution as to which placement method is better than the other.
Hence additional research in this area is needed.
32
METHODS ADVOCATED TO MEASURE VOLUMETRIC SHRINKAGE,
SHRINKAGE STRESS AND STRAIN
A plethora of shrinkage measurement methodologies have been proposed and
used to measure polymerization shrinkage and its effects.
Free volumetric shrinkage measurement. The mercury dilatometer 96
was one of
the first methods to be used to measure volumetric shrinkage, and many investigators
used this testing method for a number of years. In this the polymerization shrinkage of
the resin was calculated by evaluating the volume change in a calibrated mercury column
surrounding the resin. Watts and Cash in 1991 used the bonded-disc method to calculate
volumetric shrinkage. They placed a disc-shaped sample of the resin to be tested inside a
brass ring having a square outline, both of which were placed and bonded between two
glass slides. A rigid glass slide was used at the bottom, while a flexible glass coverslip
was used on the top. Shrinkage measurements were made based on the linear deflection
of a linear variable differential transformer (LVDT) probe whose tip was positioned in
contact with the top flexible cover slip 70
. Strain gages ware used by Sakaguchi et al 48
and by several other authors after some adaptation 97-99
. Apart from these, the linometer
100, laser speckle contrast analysis
101,102 and other mathematical models
103,104 have been
used to measure volumetric shrinkage. More recently X-ray micro tomography has also
been used to measure shrinkage 105
.
Shrinkage stress and strain measurement. Measurement of shrinkage stress and
strain is vitally important because it helps in materials development and yields clinical
33
outcomes. Researchers have used photo elastic analysis 106
, the tensometer 49
, and strain
gage methods 48
, universal testing machines modified with extensometers (Instron), ring
slitting method 107
, crack propagation method 109
and finite element analysis 110
. Among
these the tensometer and the modified Instron are the most widely used. These systems
afford versatility and a wide range of application as the apparatus can be modified easily
to vary the C-factor. Another method was developed by Watts and Cash in 2003 called
‘The Bioman’ and is based on the cantilever load cell deflection principle.
Unfortunately, all these methods have differing working principles and their
outcomes are based on specific instrument compliance. This makes the stress values
highly variable and comparative analysis difficult. Also, specimen geometry in many of
these systems (tensometer, bioman, universal testing machine) is cylindrical or disc-
shaped which is not an accurate representation of clinical conditions as they cannot be
modified to duplicate different C-factors which makes their application rather limited.
The ultimate objective of in vitro research and testing is to provide reliable and
valid data using testing conditions which simulate clinical conditions. Properties of the
test specimens and its compliance should be similar to natural tooth; the model should be
standardized and easily reproduced.
Keeping the above considerations in mind, a novel methodology has been
designed to accurately measure the shrinkage strain produced by composite resins. The
test specimens closely resemble the physical and mechanical properties of natural tooth
structure, are easily reproducible, have been designed to replicate genuine clinical
conditions and can be easily modified making it adaptable to different clinical conditions.
The fabrication and testing procedure is greatly standardized using custom made molds
34
and other mounting devices which makes it easily reproducible and helps limit any
operator induced variables.
This study evaluated polymerization shrinkage strain of the newer bulk-filled
composite resins and compared them to a conventional incrementally-filled composite
resin. This study also evaluated the effects of varying the C-factor on the shrinkage strain
produced. Finally, we also measured the free volumetric shrinkage of all the composite
resins to evaluate any co-relation with the production of shrinkage strain.
35
Objective/Specific aims/Experimental Plan:
1. To measure polymerization shrinkage strain produced by the newer bulk-fill
composite resins and compare those to a conventional incrementally filled
composite resin.
2. To measure the effect of differing C-factors on the shrinkage strain produced by a
composite resin.
3. To measure the effect of free volumetric shrinkage on the shrinkage strain
produced by the composite resin.
Hypothesis to be tested:
1. There is no difference in shrinkage strain produced by bulk fill composite resins
compared to the traditional incrementally filled composite resins.
2. Different cavity configuration factors have no effect on the shrinkage strain
produced by placement of dental composites.
3. There is no relation between free volumetric shrinkage and shrinkage strain
produced by a dental composite.
Materials and Method:
Six commercially available bulk-fill composite resins were selected for the study.
In addition one traditional incrementally placed composite was selected as the negative
control.
36
Experimental Groups:
Group 1 - SureFil SDR flow - DENTSPLY Caulk – Flowable bulk-fill composite (4mm
increment)
Group 2 - Tetric EvoCeram Bulk Fill - Ivoclar Vivadent – Packable bulk-fill composite
(4mm increment)
Group 3 - Filtek Bulk Fill – 3M ESPE - Flowable bulk-fill composite (4mm increment)
Group 4 – SonicFill – Kerr - Packable bulk-fill composite (4mm increment)
Group 5 - Venus Bulk Fill - Heraeus Kulzer - Flowable bulk-fill composite (4mm
increment)
Group 6 – Quixx - DENTSPLY Caulk - Packable bulk-fill composite (4mm increment)
Group 7 – Z100 – 3M ESPE – Packable incrementally filled composite (2mm
increments)
37
Method:
Custom fabricated Teflon molds were used to fabricate the Z100 composite resin
(3M ESPE) block (tooth simulator). The Teflon mold assembly was composed of a base
and a body (Fig 10 and 11)
Fig 10 - Teflon molds used. Body (left) and base (right)
38
Fig 11 - Body and base assembly
The body had a central area measuring (7mm X 7mm X 10mm) which was used
to contain the Z100 during the block fabrication (Fig 11 - arrow). The base extension was
two different sizes which corresponded to a class I or II preparation (Fig 12). The
extension on the base for the Class I cavity measured (4mm X 2mm X 4mm) while that
for the Class II cavity measured (7mm X 2mm X 4mm) (Fig 12).
Fig 12 - Base molds. Class I (left) and Class II (right)
Z100 composite resin was packed and cured in 2mm increments using the Elipar
S10 (3M ESPE) curing light (Fig 13 and Fig 14). A test tube holder stationed the curing
light in a standardized position (Fig 14).
39
Fig 13 - Packing Z100 into the mold in increments
Fig 14 - Light curing of Z100 increment
40
After the final increment of Z100 was light cured, the block was recovered and the
internal cavity and the outer surface inspected (Fig 15)
Fig 15 - The recovered Z100 block (in this case Class I) in the center
The Class I cavity was 4 mm long, 2mm wide and 4mm deep. The outer
dimensions of the block were 7mm long, 7 mm wide and 10 mm in height (Fig 16).
These dimensions were selected to simulate those of an actual tooth.
Fig 16 - Z100 block with Class I cavity (occlusal view)
41
Fig 17 - Z100 block containing Class I cavity
The Class II preparation (MOD) was 7mm long, 2mm wide and 4mm deep (Fig
18 and Fig 19). The outer dimensions of the block for Class II were identical to the Class
I (i.e. 7mm X 7mm X 10 mm) (Fig 18 and Fig 19).
42
Fig 18 - Z100 block with Class II cavity (occlusal view)
Fig 19 - Z100 block with Class II (proximal view)
The fabricated Z100 blocks were mounted in self-cure acrylic resin (Fig 25 and
Fig 26) contained in brass rings (Fig 22) using a custom fabricated Teflon mold (Fig 20).
43
The mold had a central extension which held the Z100 block in place while the self-cure
acrylic resin polymerized (Fig 20 and Fig 21).
Fig 20 - Teflon mold with central extension used for mounting Z100 blocks
Fig 21 - Mounting mold with Z100 block held in place
44
Fig 22 - Brass ring used for mounting
The inner rim of the Teflon mold used for mounting snugly fit the outer diameter
of the brass ring (Fig 23). The Teflon mold with the Z100 block was inverted onto the
brass ring containing uncured acrylic resin and left in place till the resin set (Fig 24).
45
Fig 23 - Mounting assembly
Fig 24 - Teflon mold inverted onto brass ring containing uncured acrylic resin - used for
standardized mounting of the Z100 block
46
Fig 25 - Z100 block with Class I cavity mounted in set acrylic resin
Fig 26 - Z100 block with Class II cavity mounted in set acrylic resin
47
After mounting, the specimens were stored in an incubator for 24 hours at 37
degrees Celsius. When the specimens were removed from the incubator the internal
surface of the cavity was treated to enhance the bonding of the composite resin to the
cavity walls using tribochemical bonding:
Initially, the internal cavity surface was abraded using 50 micron alumina
particles (Fig 27) for a period of 15 seconds (45 psi) using RONDOflex 2013
(KaVo, Bismarckring, Biberach, Germany).
Then, Rocatec Soft (3m ESPE) 30 micron silica-modified alumina particles (Fig
27) were used to treat the internal cavity surface for 15 seconds (45 psi) using
RONDOflex 2013 (KaVo, Bismarckring, Biberach, Germany).
Fig 27 - System used for abrasion of internal cavity walls
48
Then a RelyX (3M ESPE) silane coupling agent (Fig 28) was applied (2 coats) to
the floor and the walls of the cavity and dried using a hair dryer.
Fig 28 - Rely X silane
After the silane was dried Adper Single Bond Plus (3M ESPE) adhesive (Fig 29)
was applied and light cured for 20 seconds using a Bluephase 20i LED curing
light (Ivoclar Vivadent, 1100 mW per cubic centimeters).
Fig 29 - Adper Single Bond Plus adhesive
49
A strain gage is a device routinely used to record minute strains and dimensional
changes. Typically, it is composed of a metallic foil pattern supported by a flexible and
insulating backing and framework. A strain gage works on the principle of electrical
conductance of a metallic material and its dependence on the geometry of the conductor.
Strain gages measure the change in electrical resistance when the conductive materials
deforms as a result of the strain. If there happens to be a small amount of stress in the
direction of orientation of the parallel metal foil lines, it results in a multiplicatively
larger strain over the effective length of the conductor. Thus, there is a larger resistance
change than what would be recorded using a single straight line conductive wire. It
amplifies the signal (Fig 32).
Following the surface treatment of the cavity, surface contaminants were removed
from the block to ensure optimum attachment of the strain gage (Vishay SR-4 bondable
foil strain gauge – 062AK) by cleaning the outer surface of the composite block using M-
Prep neutralizer 5A (Vishay) and M-Prep conditioner A (Vishay) (Fig 30). Strain gages
were attached to the composite block using M-Bond 200 (Vishay) which is a
cyanoacrylate based adhesive system (Fig 30).
50
Fig 30 - M-Bond 200 and surface preparatory agents from Vishay
For the Class I specimen only one strain gage was attached to the block, while
two gages were attached on opposite sides of the block for the Class II specimen (Fig 31).
Fig 31 - One strain gage used for Class I (left), two used for Class II (right)
Note: The specimens above have undergone testing with a meniscus formation for the
Class II specimen (arrow in the figure).
51
Fig 32 - Strain gage with end wires soldered
Fig 33 - End wires of strain gage
52
Fig 34 - Pin to which end wires of the strain gage are soldered
Fig 35 - End wires of strain gage soldered before connecting to scanner
53
The end wires (Fig 33) of the strain gage were connected to a Model 5100 Analog
to Digital (AD) Scanner (Vishay Intertechnology, Inc., Malvern, PA). The scanner was
connected to a computer (Fig 36) and the strain values were interpreted using the
StrainSmart software (Vishay Intertechnology, Inc., Malvern, PA).
Fig 36 - StrainSmart software on the computer
After tribochemical bonding a four mm increment of bulk fill composites was
placed in the cavity and light cured using the Bluephase 20i LED curing light (Ivoclar
Vivadent, 1100 mW per cubic centimeters) following the manufacturer’s directions. For
traditional composites (Z100) the cavity was filled using two 2mm increments of the
composite and light cured. This 2mm increment was placed with the aid of another
Teflon mold (Fig 37) designed to place a precise 2mm increment in the cavity. After the
first increment was light cured, the final 2mm increment was placed in the cavity and
light cured (Fig 38). At all times the position of the curing light was standardized using a
54
table-top laboratory clasp arm and its tip held directly above the sample being tested (Fig
38).
Fig 37 - Teflon mold used to pack precise 2mm increments
Fig 38 - Specimen being light cured
55
The polymerization process and shrinkage strain was recorded in real-time using
the StrainSmart software for 15 minutes. A graph was recorded which produced residual
shrinkage strain for each sample. A representative graph for bulk filled composites placed
in one single increment is shown in the image below (Fig 39).
Fig 39 - Graph showing recorded residual strain (bulk-filled composites, one increment)
The graph can be interpreted as follows:
1. Strain is represented on the Y-axis in micro-strain units (Fig 40). A reading above
0 represents expansion, while a reading below 0 represents contraction.
56
Fig 40 - Strain recorded, Y-axis (box)
2. Time is the X-axis in seconds (Fig 41). Residual strain values at 900 seconds ware
recorded for each sample. 900 seconds (15 minutes) was decided based on the
average time required to place, finish and polish a restoration in an actual patient.
Fig 41 - Time elapsed, X-axis (box)
57
3. After curing was initiated, the graph shows an upward (positive) deflection which
denotes expansion (Fig 42 - arrow). This expansion is due to the heat of the curing
light and the exothermic polymerization reaction of the curing composite. The
highest point of the upward deflection (peak) is the moment at which the curing
light was turned off (Fig 42 – circle).
Fig 42 - Expansion recorded (arrow) and cessation of curing (circle)
4. After turning the curing light off, the graph starts to fall down which indicates
contraction of the apparatus due to the shrinkage of the composite which pulls the
walls of the tooth inwards (i.e. shrinkage strain). After a certain time (about 300
seconds) the contraction process slows down and the strain levels off.
58
5. The continuous effect of polymerization shrinkage on the tooth was recorded on
the graph. The micro-stain value at 900 seconds was recorded as the residual
strain.
For bulk-fill composites, one single increment (4mm) was placed and cured. For the
incremental composite (Z100), two increments (2mm each) were placed and each
increment was cured. This resulted in a graph having 2 distinct peaks (one for each
increment). Apart from this, all the other features of the recorded graphs were similar.
A graph for Z100 is shown below (Fig 43).
Fig 43 - Graph showing recorded strain (incrementally-filled composite, two increments)
Free volumetric shrinkage was calculated using the ACUVOL™ (Bisco, Inc,
Schaumburg, IL). This instrument uses a video imaging technique allowing comparison
of volumes of composite before and after cure (shrinkage) (Fig 44).
59
Fig 44 - ACUVOL to measure free volumetric shrinkage
60
RESULTS:
Material
Residual
shrinkage strain
for Class I
(μ strain units)
Mean ± SD
Residual
shrinkage strain
for Class II
(μ strain units)
Mean ± SD
Volumetric shrinkage
%
Mean ± SD
SureFil SDR flow -182.5±45.3 -60.7±26.6 4.40±0.25
Tetric EvoCeram Bulk
Fill -174.2±33.5 -41.7±22.3 2.37±0.09
Filtek Bulk Fill -156.4±43.5 -64.2±14.6 4.40±0.13
SonicFill -192.3±65.1 -41±16.5 1.78±0.29
Venus Bulk Fill -222±92.1 -77.7±14.3 6.70±0.17
Quixx -229.8±74.6 -34.2±8.4 1.8±0.22
Z100 -251.5±70.21 -68.7±30.1 3.03±0.08
61
STATISTICAL ANALYSIS:
Means for the residual shrinkage strain produced by Class I cavity were analyzed
using ANOVA and Tukey’s post-hoc t-test for pairwise comparison of group means.
Statistical analysis showed a significant difference in Class I residual shrinkage strain
values between Filtek Bulk Fill and Z100 (p = 0.022).
Means for residual shrinkage strain produced by Class II cavity were analyzed
using ANOVA. No significant difference was observed for the residual shrinkage strain
produced (p > 0.05).
To check for the effect of differing cavity configurations on the residual shrinkage
strain produced, an unpaired t-test was used to analyze the data. Significant difference
between Class I and Class II residual shrinkage strain was observed for all the materials
(p < 0.05).
Free volumetric shrinkage was compared with the residual shrinkage strain
produced by both Class I and Class II cavities using a correlation co-efficient analysis.
Statistical analysis showed no correlation between free volumetric shrinkage and Class I
strain (r = 0.043), while a negative correlation was observed between free volumetric
shrinkage and Class II strain (r = -0.87).
62
DISCUSSION:
Differences observed in the shrinkage strain for Class I and Class II cavities can
be attributed to the differences in C-factor. A lower C-factor for Class II cavities affords
greater flow of the material which results in higher stress relief. Compliance of the
composite rein model might play a role in the difference observed.
The model has many advantages:
It can be used to rank order materials as it gives predictable results for preparation with
different C factors both Class I and II cavities. It adds to the relative value of in vitro
studies related to the practice of restorative dentistry. The properties of the substrate used
to mimic tooth are close to natural teeth (Modulus of Z100 = 20-21 MPa; Modulus of
Dentin = 17-21 MPa). The standardized protocol makes the procedure replicable by any
researcher. The model allows modifications; hence different clinical scenarios can be
replicated to broaden the scope of the study. Shrinkage strain and stress are important
properties of composites and accurate testing methods are necessary to predict their
clinical outcome. The UAB model shows good promise in doing so.
Residual shrinkage strain for bulk-fill composites was measured to be in the normal range
of traditional incrementally placed composites.
Significant difference in Class I and Class II strain owing to compliance & C-factor
further validates the model.
Future modifications:
Test the same composites using bulk and incremental techniques.
63
Modify model to change the thickness of the walls to test the effects of
compliance.
Bilayered model to mimic enamel and dentin.
CONCLUSIONS:
A negative correlation was observed between free volumetric shrinkage and Class
II residual shrinkage strain (r = -0.87)
A significant difference in Class I residual shrinkage strain was observed between
Filtek Bulk Fill and Z100 (p = 0.022)
A significant difference in Class I & Class II residual shrinkage strain was
observed for all the 7 materials (p < 0.05)
Residual shrinkage strain is an important characteristic of a composite resin which plays
a critical role in the overall success of a restoration. Research to balance the properties of
dental composites continues in an effort to improve the quality and longevity of dental
restorations.
64
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