close

Вход

Забыли?

вход по аккаунту

?

Comparison of Mechanical and Color Properties of Lithium Disilicate (IPS e.max® CAD) Obtained by Microwave and Conventional Oven Crystallization

код для вставкиСкачать
Comparison of Mechanical and Color Properties of Lithium Disilicate (IPS e.max® CAD)
Obtained by Microwave and Conventional Oven Crystallization
By
Kamolphob Phasuk, DDS
June 21, 2012
A thesis submitted to the Graduate School of
State University of New York at Buffalo
in partial fulfillment of the requirements for the degree of
Master of Science
In
Oral Sciences
i
UMI Number: 1519946
All rights reserved
INFORMATION TO ALL USERS
The quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscript
and there are missing pages, these will be noted. Also, if material had to be removed,
a note will indicate the deletion.
UMI 1519946
Published by ProQuest LLC (2012). Copyright in the Dissertation held by the Author.
Microform Edition © ProQuest LLC.
All rights reserved. This work is protected against
unauthorized copying under Title 17, United States Code
ProQuest LLC.
789 East Eisenhower Parkway
P.O. Box 1346
Ann Arbor, MI 48106 - 1346
Acknowledgements
I would like to express my sincere appreciation and thanks to the following individuals
for their time and efforts to make this research project possible.
Dr. Edward A. Monaco Jr., my research mentor and program director of Advanced
Education Program in Prosthodontics, School of Dental Medicine, The State University
of New York at Buffalo, who devoted so much of his valuable time for me. He always
instructed, supported and encouraged me through this research project as well as
enlightening me in the discipline of prosthodontics. I am very grateful for being in this
program.
I want to thank Dr. Hyeong-Il Kim for his support and understanding. His kind, patience
advise, has guided me through this research. Without his assistance, I would not have
completed this research program.
I would like to thank Dr. Jennifer Kuracina, my research committee member and clinic
mentor for her advice and support.
I would like to thank Dr. Thomas Hill for his recommendations and technical support. I
could not finish this research without him. I have learned a lot from him.
I would like to acknowledge the guidance and support of my faculty members, Dr. Jane
Brewer, Dr. Gary Alexander, Dr. Marshal Fagin and Dr. Christopher Pusateri.
ii
I would like to extend my thanks to Ms. Karen Collura and to all my class colleagues for
their support and friendship.
Last but not least, I would like to dedicate this work to my family for their love and
never-ending support. Without them I would not have achieved anything.
iii
Table of Contents
Page
Acknowledgements
ii
List of Tables
v
List of Figures
vi
Abstract
8
Introduction
11
Hypothesis
18
Materials and Methods
19
Results
21
Discussion
24
Conclusion
26
References
27
iv
List of Tables
Page
Table 1 Color data in CIELAB system
21
Table 2 Means and Standard Deviation of Biaxial Strength
22
Table 3 Means and Standard Deviation of Hardness
23
Table 4 ANOVA Results, Biaxial strength and Hardness
23
v
List of Figures
Page
Figure 1 Diagram of electromagnetic wave spectrum
13
Figure 2 Overview of the manufacturing procedures
29
Figure 3 Overview of treatment procedures
29
Figure 4 Conventional ceramic oven (Programat CS)
30
Figure 5 Microwave ceramic oven
30
Figure 6 Thermal control
31
Figure 7 Silicon carbide susceptor
31
Figure 8 Sample crystalized in microwave oven
32
Figure 9 Sample crystalized in conventional oven
32
Figure 10 Scanning Electron Microscope images of pre-crystallized sample 33
Figure 11 SEM: crystalized 2 minutes in microwave oven
34
Figure 12 SEM: crystalized 3.5 minutes in microwave oven
35
Figure 13 SEM:crystalized in conventional ceramic oven
36
Figure 14 Chart of mean biaxial strength
37
Figure 15 Chart of mean Vicker’s Hardness
38
vi
(Blank)
vii
ABSTRACT
The CAD/CAM restorative technique has become increasingly popular in the
dental profession. Lithium disilicate (emax CAD) is frequently employed as a material of
choice due to excellent material and esthetic properties. One of the final steps in the
fabrication process of this restoration is the crystallization process which requires a
heating phase to convert partially crystallized lithium metasilicate to lithium disilicate
crystals (LS2). Conventional ceramic ovens transfer heat to the ceramic by conduction
heating from the outside-in. Microwave processing ovens represent a system that
combines radiating conduction heating elements with microwave heating, resulting in
volumetric heating.
The advantages of heating ceramics with a microwave oven include greater energy
efficiency, faster sample heating, more uniform heating, and improved material
mechanical and optical properties.
Purpose
This study compares mechanical and color properties of lithium disilicate crystallized in
a microwave furnace with that fired in a conventional furnace.
Material and Method
Pre-crystallized lithium disilicate blocks were milled into 12 mm diameter by 1.4
mm thick discs (ISO:6872:1995(E) ). Specimens (N=10 discs) were crystallized in a
conventional furnace for 7 minutes (manufacturer’s recommendation) and a microwave
furnace for three different test periods of time 2 minutes, 3.5 minutes and 5 minutes.
Samples were polished then etched with 0.5% hydrofluoric acid for 1 minute and
observed under scanning electron microscope.
A spectrophotometer was used to evaluate the color properties. Biaxial strength
tests and Vicker’s hardness test were employed to evaluate the mechanical properties.
One-Way ANOVA and Tukey HSD tests were used for statistical analyses.
8
Result
Scanning electron microscope observation of samples crystallized in a microwave oven
for 3.5 and 5 minutes showed similar crystal size and shape compared with
conventionally crystallized samples.
CIELAB values were calculated from spectrophotometer observations. ΔE values
determined between conventional oven crystallization samples and samples crystalized
for 2 minutes, 3.5 minutes and 5 minutes in a microwave oven were 12.70, 1.19 and 1.33
respectively.
There was no statistically significant difference in biaxial strength among lithium
disilicate samples (F3,36=.535, P=.661) for the microwave or conventional crystallization.
A significant difference in surface hardness (Vicker’s hardness) was found (F3,36= 3.917,
P= .016). Follow up tests showed a significant difference in surface hardness between 2
minutes crystallization in microwave oven samples (5719 +147Mpa) and conventional
oven crystallization samples (5882 +144Mpa) (p= .38) as well as with 3.5 minutes(5887
+ 95 MPa.)
(p= .31) and 5 minutes (5873 + 119 MPa) ( p= .054) crystallization in
microwave samples.
Conclusion
Lithium disilicate specimens crystallized in a microwave oven for 3.5 minutes and 5
minutes have similar micro-crystal morphology compared with conventionally
crystallized samples under scanning electron microscope observation.
Lithium disilicate specimens, crystalized for 3.5 and 5 minutes in a microwave oven,
produced a minimal ΔE value when compared to lithium disilicate specimens that were
9
crystalized in a conventional furnace. The ΔE values obtained are in clinically
acceptable range.
Lithium Disilicate specimens crystallized in a microwave furnace for 3.5 minutes and 5
minutes had no statistically significant difference in biaxial strength and hardness when
compared to the specimens that were crystallized in a conventional furnace for 7 minutes.
From this study, it was determined that microwave crystallized lithium disilicate ceramic
has equivalent mechanical and color properties compared to conventional crystallized
ceramic material but required about half of the crystallization processing time.
10
INTRODUCTION
Lithium disilicate ceramic material
Lithium disilicate ceramic material was introduced by Ivoclar Vivadent Inc. in the
1990s. This material contains lithium disilicate (Li2Si2O5) as a major crystalline phase.
This lithium disilicate material was originally known as Empress II. Its predecessor,
Empress I, had a low flextural strength (130 MPa) due to ita high leucite content.
To optimize the functional options of this ceramic materials Ivoclar Vivadent Inc.
introduced IPS e.max lithium disilicate glass ceramic. IPS e.max lithium disilicate is a
material that gives favorable esthetics and reasonable strength to enable conventional or
adhesive cementation. Lithium disilicate has a crystal structure that offers excellent
strength and durability as well as outstanding optical properties. IPS e.max lithium
disilicate can be traditionally pressed or milled via CAD/CAM technology.1,2
Due to its strength and versatility the material can be used for the following applications:
anterior, posterior crowns, inlays, onlays, veneers, thin veneers, telescopic crowns and
1
implant restorations.
The IPS e.max lithium disilicate is composed of quartz, lithium dioxide, phosphor
oxide, alumina, potassium oxide, and other components. These powders are combined to
produce a glass melt. Once the correct viscosity is achieved, similar to that of honey, the
melted glass is poured into a separate steel mould. The material is left to cool in the mold
until it reaches a temperature where no additional deformations occur. This process
produces minimal pores or other internal defects due to the glass flow process and
provides for easy quality control due to the translucent nature of the glass. The blocks or
ingots are produced in separate batches depending on the shade and size of the materials.
Overall, this composition yields a highly thermal shock-resistant glass ceramic, due to the
2
low thermal expansion that results when it is manufactured.
11
IPS e.max lithium disilicate CAD
IPS e.max CAD is a lithium disilicate glass-ceramic block for the CAD/CAM
processing technique. It is fabricated using a process which results in a homogenous
material. The block can be milled in a CAD/CAM unit in its pre-crystallized (“blue”)
state. The typical color of the pre-crystallized IPS e.max CAD blocks ranges from blue to
bluish-grey.
2
The shade is a result of the composition and the microstructure of the
glass-ceramic. The strength of the material is approximately 130 to 150 MPa, which is
comparable to other glass-ceramic blocks currently available on the market. After the
blocks are milled, the restoration is crystallized in a conventional ceramic furnace.
Crystallization program is completed after approximate 20 to 25 minutes. Crystallization
occurs at a temperature of 840°C (1544°F) for 7 minutes. This causes a transformation of
the microstructure during which lithium disilicate crystals grow in a controlled manner.
2
This process gives the definitive restoration a fine grain glass ceramic with 70% crystal
volume incorporated in a glass matrix. Expressed in the form of a chemical reaction,
lithium disilicate is created from the glassy phase of lithium metasilicate and Silicon
dioxide. ( Li2SiO3+SiO2 = Li2Si2O5) The 0.2% shrinkage is compensated in the computer
design software and taken into account during milling.2,3 The main physical parameters,
such as the 360 MPa biaxial strength, Vickers Hardness of 5800 MPa, and the
corresponding optical properties, are achieved through this transformation of the
microstructure.2
12
Microwave Heating.
Microwaves are electromagnetic waves with a frequency range from 300 MHz
(Megahertz) to 300 GHz (Gigahertz). The corresponding wavelength ranges from one
meter to sub-millimeter.7 Microwaves form part of the continuous electromagnetic
spectrum extending from the low frequency alternating currents to cosmic rays. The
lower range of the microwave region borders radio frequencies while the upper end is
next to the infra-red frequencies. 7
Fig1. Diagram of electromagnetic wave length and frequency
Microwave energy was primarily invented for communications, defense electronics
and cooking. Following the significant success of microwaves used to process food,
research expanded using microwave technology to process many different materials such
as rubber, polymers and ceramics. This technology is still in the develop mental stage and
continued research is needed to provide new approaches to improve manufacturing
efficiency and physical properties of materials.
Processing of materials by microwave technology mainly depends on coupling
microwave to atoms or atomic groups and not to water molecules as compared to the
processing of food products.7 In food processing, microwave ovens generate electric
fields and excite water molecules within the food product.
13
Water molecules have the unique capability of being “dipole”. Dipole means that
molecules organize their electrons in such a way as to make one side of the molecules
positively charged and the other side negatively charged. Because of this, when food is
placed in the microwave generated field, water molecules inside the food are activated.
The water molecules rotate to align themselves to minimize the force on their positive
and negative pole. This rotating movement causes the molecules to bump into each other
and generate kinetic energy which transforms into heat. Even though ceramic materials
do not have water molecules, microwave energy is able to transfer energy to ceramic
material by their dielectric property. Dielectric property is a thermophysical
characterization of material. It is a behavior of material molecules under a specific
electromagnetic frequency and temperature.7,17 At a prescribed temperature and
electromagnetic frequency, similar to the water, the interaction of microwaves with a
dielectric ceramic material results in translation of dipole molecular movement and
subsequent rotation energy produces volumetric heating. 6
Microwave Hybrid Heating(MHH)
MHH solves many problems associated with direct microwave heating by utilizing
susceptors. In Microwave Hybrid Heating (MHH), samples are heated by another source
along with microwave heating. This additional heating source is called a susceptor such
as silicon carbide (SiC),silicon dioxide (SiO2), aluminum oxide (Al2O3) and zirconium
oxide (ZrO2). The susceptors have complex properties. They allow microwave to
penetrate into the material (permittivity), absorb and store energy that is induced by
microwave (loss factor) as well as convert absorbed energy in to heat (loss tangent). The
samples and susceptors
are placed together in the microwave chamber. When the
microwave ceramic processing process is initiated, microwaves radiate to the susceptors
and polarize molecules of susceptors. The susceptors absorb microwave radiation and
14
convert this energy into heat. The heat from the susceptors radiate to the surface of the
ceramic sample.4 As the temperature increases, the ceramic sample becomes more
susceptible to microwave energy. Heat is produced inside the sample and combines with
the heat from susceptors. This hybrid heating method promotes effective and rapid
heating. The special advantage of this type of heating can be applied to the majority of
ceramic materials that are transparent to microwaves at room temperature.7
Additional Dental Applications.
Sterilization and disinfection
After World War II microwave energy was widely known as a successful way to
process food for domestic use. There are several institutions performing research to
maximize microwave applications including microwave sterilization and material
processing. Recently, a group of researchers turned a common microwave into a plasma
generating equipment sterilizer. By placing a vacuum vessel into the microwave they
were able to generate oxygen and ozone rich plasma, that is highly oxidizing and
disinfecting. This invention encouraged surgeons, dentists and veterinarians to move
away from steam-based sterilization methods and to take up a microwave aided plasmabased approach. It has been shown to be 100% effective against the most difficult
pathogens.10
Microwave ovens can be used to clean dentures. Articles have been published
supporting the use of household microwave ovens to disinfect dentures. Microwave
radiated dentures for 2 minutes demonstrated equal performance reducing the number of
Methicillin-resistant Staphylococcus aureus (MRSA) as compared to soaking dentures in 2%
chlorhexidine gluconate for 10 minutes.11 In addition, microwaves can effectively
disinfect dentures infected with candidiasis.13 Microwave disinfection of dentures does
not have an adverse effect on bond strength between denture teeth and the denture base.12
15
Importantly, only denture acrylic complete dentures are recommended for microwave
oven disinfection.
Microwave assisted processing dental material
Polymer
Microwave assisted polymer synthesis is a commonly employed industrial process.
Microwaves can initiate a polymerizing reaction with many different kinds of monomer.
With the microwave assisted polymerization, the degree of heating, polymerization speed
and degree of polymerization can be well controlled.14
In dental material applications, especially in restorative and prosthodontics,
microwave processing has been researched and applied to fabricate restorations,
including fabrication of dentures. It was found that acrylic resin cured by microwave
energy is more resistant to mechanical failure in terms of hardness and strength than
conventionally cured acrylic.14 In addition, the results show superior shear bond strength
between denture teeth and denture base. The microwave processing technique can safely
be applied to the production of denture bases, as specified by ADA specification No.12.15
Ceramic Restoration
Ceramics meet all criteria for processing with microwave energy. By using
microwave energy to process ceramic restorations, quality and physical properties can be
fully controlled.9 The main benefits of using microwave energy to process dental ceramic
restorations are cost saving (time and energy), uniform internal heating, precise and
selective heating.
For dental ceramic application, the potential advantages of utilizing microwave
technology in the processing of dental ceramic have been known for many years.
However, serious research attention has been directed in this area only in the past decade.
Many studies have been developed in the field of processing dental restorations with
16
microwave energy. When dental ceramic materials are processed in microwave ovens
not only is external heat generated, but also internal heat which occurs inside the
restoration material. As a result of internal and volumetric external heating, materials can
be heated up very rapidly regardless of size and shape.
Numerous studies have investigated different aspects of fabricating dental ceramic
restoration. Del Regno and Saha performed extensive research in microwave processing
of dental ceramics. They observed that using microwaves is predicted to achieve the same
results and provide improvements in all aspects of dental ceramics ranging from
mechanical properties, esthetics and bonding capabilities of porcelain to the underlying
metal substructure.9
There are several studies that support this observation. For example, Pan et al.
studied the bonding of a ceramic and Ni-Cr metal substructure. They found that the
adherence between ceramic and the substructure in samples that were prepared under
microwave heating increased two to three times compared to the conventional heat
process.18 A study from Prasad et al. showed that the microwave glazed porcelain
exhibited better results than conventional glazed porcelain. Microwave processing has
demonstrated favorable outcomes when used to sinter zirconia.17 Recent studies have
shown that the average microstructure of microwave sintered samples had fewer voids
and uniform grains when compared to conventional sintering. They concluded that
microwave sintering resulted in rapid and more reliable processing of complex dental
ceramics with better mechanical and microstructure properties.14,17
The effect of microwave crystalizing on lithium disilicate ceramic material is not
known. Therefore, the purpose of this study is to compare mechanical (biaxial strength
and Vickers hardness) and color properties of lithium disilicate samples crystallized in
microwave oven and conventional ovens.
17
HYPOTHESES
1. There is no difference in biaxial strength between lithium disilicate samples
crystallized in microwave ovens and samples crystallized in conventional ovens.
2. There is no difference in surface hardness between lithium disilicate samples
crystallized in microwave ovens and samples crystallized in conventional ovens.
3. There is no difference in color between lithium disilicate samples crystallized in
microwave ovens and samples crystallized in conventional oven.
18
Materials and Methods
Pre-crystallized lithium disilicate blocks (Emax CAD, A2 LT, Ivoclar Vivadent,
Amherst, USA.) Lot # N 39036 were milled to disc form (14 mm. in diameter, 1.2 mm. in
thickness) according to International Standard for Dental Ceramic (ISO 6872) [8] . The
discs were polished with 420-grit, 120-grit diamond polishing disc (Metlab Corp.,
Niagara Falls, USA) and 5µm. aluminum oxide powder under copious water in order to
standardize the surface roughness.
A total of forty samples were divided into four groups of 10 samples each, for the
crystallization test. The control group samples were crystallized in a conventional
ceramic oven (Programat®CS, Ivoclar Vivadent, Amherst, USA.) at a temperature of
840°C (1544°F) for 7 minutes according to the manufacturer’s recommendation.1,2 The
test groups consist of 3 subgroups. They were crystallized in a microwave oven (Therm
wave, Research Microwave system, Alfred, New York) for 2 minutes, 3.5 minutes and 5
minutes.
Microstructure was evaluated with scanning electron microscope (SEM) operated
at 20 kV. (Hitachi S 4000). Represented samples from each group were randomly
selected. Samples were etched with 0.5 % Hydrofluoric acid for 1 minute. Samples were
carbon coated prior to SEM. SEM images showing the microstructure of each sample
were taken.
Color was measured using a spectrophotometer in conjunction with EasyMatch
HQ software (UltraScan PRO, Hunter Associates Laboratory, Inc). Color was expressed
using CIELAB color scale. Color differences were calculated by the following
equation22:
ΔE = [(L2-L1)2+(a2-a1)2+(b2-b1)2]1/2
19
Biaxial flexural strength tests were performed. All samples were fractured to
determine the fractural strength. A three point bend test was performed using an Instron
Universal testing machine (machine #5293X) with cross head speed of 1.0 mm/min.
Flexural strength (S) was calculated by this following equation:
S = -0.2387 P (X-Y)/d2
Where
S is the maximum centre tensile stress (MPa)
P is the total load causing fracture (N):
X = (1+ν)ln(r2/r3)2+[(1-ν)/2](r2/r3)2
Y = (1+ν)[1+ln(r1/r3)2]+(1-ν)(r1/r3)2
in which ν is Poisson’s ratio = 0.25, r1 is the radius of support circle (mm.), r2 is the radius of loaded
area (mm.), r3 is the radius of specimen (mm.)
d is the specimen thickness at fracture origin (mm.)
Hardness tests (Vickers) were performed in Hardness Tester (Matsuzawa, Japan).
Samples ware embedded in auto-polymerizing acrylic resin (Fast Cure Acrylic, Metlab
Co.,NY,USA) blocks and were loaded with 1000 g for 15 seconds. Hardness was
calculated by this following equation:
HV = 2Fsin (136/2)/d2
where F is loading force, d = arithmetic means of the two diagonals, d1 and d2 in mm.
One way analyses of variance (ANOVA) were used to statistically determine differences
in biaxial strength and surface hardness by crystallization process of lithium disilicate.
Follow-up multiple comparison tests (Turkey) were used to determine specific mean
differences when ANOVA result were statistically significant. A significance level of .05
was used for all tests.
20
Results
Microstructure
Representative samples from each group were assessed subjectively under scanning
electron microscope. Lithium disilicate crystals are elliptical shapes dispersed in a glass
matrix1,2. The crystal morphology of representative lithium disilicate sample crystallized
for 3.5 minutes in a microwave oven showed similar crystal morphology to samples
crystallized in a microwave oven for 5 minutes.
When comparing these to the
conventionally crystallized sample they were subjectively observed as almost identical.
The representative sample crystallized for 2 minutes in a microwave oven showed
characteristics of incomplete crystallization. (Fig.9).
Color
CIELAB and delta E (ΔE) values are presented in Table 1. ΔE between conventional
oven crystallization samples and 2 minutes, 3.5 minutes and 5 minutes crystallization in
microwave ovens were 12.70, 1.19 and 1.33.
Mean L*/SD
Mean a*/SD
Mean b*/SD
Conventional
crystallization
(Control)
73.77+ 0.78
1.42+0.44
20.1+0.66
2 minutes
microwave
crystallization
79.06+13.8
1.83+1.2
8.56+6.27
12.70
3.5 minutes
microwave
crystallization
74.17+0.61
1.56+0.11
18.98+0.48
1.19
74.16+0.61
1.41+0.11
18.83+0.68
1.33
5 minutes
microwave
crystallization
Table 1 color data in CIELAB system
21
ΔE
Biaxial strength
Mean fractural strength values are presented in Table 2. One way ANOVA (table 4)
indicates no statistically significant difference in fractural strength among different
crystallization processes.(F3,36=.535p=.661).
Conventional
crystallization
(Control)
2 minutes
microwave
crystallization
3.5 minutes
microwave
crystallization
5 minutes
microwave
crystallization
Mean Biaxial
Strength
(MPa)
374.6
344.60
375.44
388.40
SD
68.62
69.89
68.25
87.6
Table 2 Means and Standard Deviation of biaxial strength (N=10 per group)
Hardness
Mean hardness values are presented in Table 3. One way ANOVA indicated a significant
difference in hardness by crystallization process (F3,36= 3.917, P= 0.016). Follow up tests
showed significant differences in surface hardness between 2 minutes crystallization in
microwave oven samples and control (p= 0.38) and 3.5 minutes (p= 0.031). There was a
trend toward lower hardness for 2 minutes versus 5 minutes (p= 0.054).
22
Conventional
crystallization
(Control)
2 minutes
microwave
crystallization
3.5 minutes
microwave
crystallization
5 minutes
microwave
crystallization
Mean
Hardness
(HV)
599.76
583.2
600.25
598.85
SD
14.76
15.08
9.77
12.28
Table 3 Means and Standard Deviation of Hardness (N=10 per group)
Biaxial
Strength
Hardness
source
Sum of
Squares
df
Mean
Square
F
P value
Between
10317.900
3
3439.300
0.535
0.661
Within
231325.600
36
6425.711
Total
241643.500
39
Between
2032.217
3
677.406
3.917
0.016*
within
6226.294
36
172.953
Total
8258.511
39
* p < 0.05
Table 4 ANOVA Results Biaxial strength and Hardness
23
DISCUSSION
Conventionally milled lithium disilicate restorations require crystallization at a
temperature of 840°C (1544°F) for 7 minutes. This process causes a transformation of
the microstructure during which lithium disilicate crystals grow. The crystallization
process gives the definitive restoration a fine-grain glass ceramic with 70% crystal
volume incorporated in a glass matrix. Important mechanical and optical properties of
lithium disilicate restorations are controlled by crystalization of lithium disilicate filler
particles.20
Lithium disilicate crystals crystallized in the microwave oven for more than 3.5
minutes showed identical crystal size and morphology to the conventionally crystallized
sample at the same temperature. In the dental material science when similar
microstructure of material was observed, we could expect similar material properties.20
The observations from SEM suggest that the microwave processing lithium disilicate
produces comparable outcome with the conventional process. The represented sample of
2 minutes crystallized in a microwave oven showed partial crystallization. The crystal
structure can be seen as an incomplete elliptical shaped crystal. The incomplete crystal
samples are expected to have inferior performance. (Fig 9)
In this study, spectrophotometer was used to evaluate color. The value of color
measurement was observed in CIELAB color system. CIELAB(CIE L*a*b*) is the color
space specified by the International Commission on Illumination. It describes all the
color visible to the human eye. The three co-ordinates of CIELAB represent the lightness
of the color (L* = 0 yields black and L* = 100 indicates diffuse white; specula white may
be higher), its position between red/magenta and green (a*, negative values indicate
24
green while positive values indicate magenta) and its position between yellow and blue
(b*, negative values indicate blue and positive values indicate yellow). 19
The color difference (ΔE) between conventional crystallized samples for 3.5
minutes and 5 minute crystallized in a microwave oven were 1.19 and 1.33 respectively.
This amount of difference is within clinical acceptable range (ΔE <3.5).21 On the other
hand, samples crystallized for 2 minutes in a microwave oven group show large color
differences when compared with conventionally crystallized samples (ΔE=12.70).
These findings could result from the difference of lithium disilicate crystal
formation. For the samples that were crystallized for 2 minutes in microwave oven,
incomplete crystal formation resulted. They have low b* (blue) which is the color of precrystallized sample. The incomplete crystallized sample also showed high value (high
L*). This can be explained by incomplete lithium disilicate crystals being dispersed
randomly in the glass matrix.
This non uniform crystal formation and orientation,
blocked light transmission, resulting in opaque bluish like coloration.
Strength in ceramics can be achieved when appropriate fillers are added and
uniformly dispersed throughout the glass, a phenomenon termed ‘dispersion
strengthening’.
The first successful strengthened ceramic substructure was made of
feldspathic glass filled with particles of aluminum oxide (app. 55 mass%).21 With this
concept, when appropriate lithium disilicate crystals were developed, a moderate biaxial
strength and hardness of lithium disilicate samples were achieved. With the hybrid
microwave heating process, heat was conducted to samples inside microwave ovens
rapidly and volumetrically. Lithium disilicate crystals crystallized for 3.5 minutes in the
microwave oven were similar when compared with lithium disilicate crystalized from
conventional crystallized. Beside similarities in crystal structure, the results from this
study show no significant differences in biaxial strength and surface hardness between
conventional crystallization groups and 3.5 minutes and 5 minutes crystallized samples in
microwave oven
25
Conclusions
Based on this study, it was determined that lithium disilicate ceramic samples
crystallized 3.5 minute in the microwave oven have identical microstructure, equivalent
mechanical properties, and color compared to conventional crystalized samples. This
crystallization technique required half the heating time when compared with conventional
crystallization process.
26
References
1. Ivoclar Vivadent, Inc. IPS emax scientific report, February 2011
2. Burke., IPS e.max Press and IPS e.max CAD Two stage of the art glass ceramics. Report, Research
and development Ivoclar vivadent. July 2006
3. Cerec InLab software information, Sirona USA
4. Mohan, Microwave Sintering of Dental Ceramic for oral rehabilitation. A thesis submitted to the
faculty of Alfred University, NY , September 2003
5. Prasad S, Monaco EA Jr, Kim H, Davis EL, Brewer JD. Comparison of porcelain surface and flexural
strength obtained by microwave and conventional oven glazing.J Prosthet Dent 2009 Jan;101(1):20-8
6. Sutton WH, Microwave Processing of Ceramics. American Ceramic Society Bulletin. 1989;
68(2)376—386.
7. Committee on Microwave processing Material: Microwave processing of materials. National
Academy Press. Washington, D.C. 1994
8. International Standard for Dentalceramic (ISO) : 6872
9. Kashi A, Saha S. Microwave sintering of dental Materials. Journal of Dental Technology. 2005; 22(5)
28-30
10.Altieri KT, Sanita PV, Machado AL, Giampaolo ET, Pavarina AC, Vergani CE. Effectiveness of two
disinfectant solutions and microwave irradiation in disinfecting complete dentures contaminated with
methicillin-resistant Staphylococus aureus. J Am Dent Assoc. 2012 Mar; 143(3): 270-277
11.Ribeiro CR, Giampaolo ET, Machado AL, Vergani CE, Pavarina AC. Effect of microwave
disinfection on the bond strength of denture teeth to acrylic resins. Int J Adhesion and Adhesives.
2008; 28(6): 296-301
27
12.
Banting DW, Hill SA. Microwave disinfection of dentures for the treatment of oral candidiasis.
Spec Care Dentist. 2001;21(1):4-8.
13.
Wiesbrock F, Hoogenboom R. Microwave-Assisted Polymer Synthesis: State-of-the-Art and
Future Perspectives. Macromolecular Rapid communications. 25(20) 2004: 1739-1764.
14. Ilbay SG, Guvener S. Processing denture using microwave technique, Journal of Oral
Rehabilitation. 1994; 21(1):103-109
15.
American dental association. ADA No. 12. denture base polymer (resin)
16.
Encyclopedia of Britannica, Online.
17.
Borrell A, Salvador MD, Penaranda-Foix FL, Catala-Civera JM. Microwave Sintering of zirconia
materials: mechanical and microstructural Properties. Applied ceramic material.
DOI: 10.1111/j.1744-7402.2011.02741.x
18.
Pan, E.G., Ravaev, A.A. The enhancement of metal ceramic adhesion bond under sintering in
microwave fields. Advanced engineering materials. 6(1-2),61-64, 2004
19.
Kelley, R. Ceramic material in dentistry:historical evaluation and current practice. Australian
Dental Journal; Vol 56 (supp1) June 2011
20.
McLean JW, Hughes TH. The reinforcement of dental porcelain with ceramic oxides. Br Dent J
1965;119:251–267
21.
O’Brien, W.J.;Boenke, K.M.;Groh, Coverage error of two shade guides. Int J Prosthodon. Jan-Feb
4(1);45-50, 1991
28
Appendix
max CAD
.max CAD “blue block” uses a two-stage
crystallization process. The two-stage crystallization uses
Appendix
led double nucleation process where lithium meta-silicate crystals are precipitated during the first
ure 2). The resulting glass ceramic demonstrates
processing properties for milling and tends to be
color” in this state depending on
unt of added colorant. In a second heat
step preformed after the milling process
rred, the meta-silicate phase is completely
and the lithium disilicate crystallizes. This
tment occurs at approximately 840-850ºC in
in furnace. This process gives the definitive
on a fine-grain glass ceramic with 70% crystal
Fig. 2 Overview of manufacture procedure
Figure 2
ncorporated a glass matrix.
Shade Determination and
preparation
max Press
.max Press material is produced similarly to the IPS e.max CAD as far as the formation of the initial
Scanning and Milling
ots, as they are composed of different powders
melted and cooled to room temperature to pross ingots. Following the glass formation, the
Cryatallization
e then nucleated and crystallized in one heat
t to produce the final ingots (Figure 3). These
e then pressed at approximately 920ºC for 5-15
Cementation
to form a 70% crystalline lithium disilicate
on.
Figure 3
Fig 3. Overview of Treatment procedure
ostructure
30
max CAD
rocessing, the IPS e.max CAD material has two crystal types and two microstructures that provide
e properties during each phase of its use. The intermediate lithium meta-silicate crystal structure,
allows the material to be easily milled without excessive bur wear. It is strong enough to be milled
high tolerances and
integrity. In this state,
erial will have a deeper
en the final restoration
e chroma (Figure 4).
s ceramic in the “blue”
ntains approximately
lume lithium
cate crystals with an
mate crystal size of 0.5
own in Figure 5.
Figure 5
Figure 4
3
Fig. 2 Overview of manufacturing process2
CAD/CAM RESTORATIONS
Shade determination and preparation
Scanning, design and milling
Crystallization
Cementation
Wednesday, August 1, 12
Fig 3. Overview of Treatment procedure
29
Fig 4. Conventional ceramic oven (Programat CS, Ivoclar Vivadent)
Fig 5 Microwave ceramic oven
30
Fig 6 Thermal control
Fig 7 Silicon Carbide susceptors individually and in position inside a microwave chamber
31
Pre-crystalized (Blue stage)
2 mins microwave crystalized
3.5 mins microwave crystalized
5 mins microwave crystalized
Fig 8 Samples crystalized in microwave oven
Fig 9 Sample crystalized in conventional oven
32
Fig 10 Scanning Electron Microscope images of pre-crystallized sample.
33
Fig. 11 Scanning electron microscope image of sample crystalized in microwave oven for 2minutes..
34
Fig. 12 Scanning electron microscope image of sample crystalized in microwave oven for 3.5 minutes..
35
Fig. 13 Scanning electron microscope image of sample crystalized in conventional ceramic oven.
36
Biaxial Strength (MPa)
Fig. 14 Chart show mean biaxial strength (MPa)
37
Hardness (MPa)
Fig 15. Chart show mean Vicker’s Hardness (MPa)
38
Документ
Категория
Без категории
Просмотров
0
Размер файла
5 144 Кб
Теги
sdewsdweddes
1/--страниц
Пожаловаться на содержимое документа