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The Effect of Translucency and Background
Variations on the Color Difference of
CAD/CAM Lithium Disilicate Glass Ceramic
Abdulaziz Al Ben Ali, DMD
“Thesis submitted in partial fulfillment of the requirements for the degree of
Master of Science”
“Tufts University School of Dental Medicine”
Post-Graduate Prosthodontics Division
ii
Principal Advisor:
Hiroshi Hirayama, DDS, DMD, MSc, FACP
Thesis Committee Members:
Kiho Kang, DDS, DMD, MSc, FACP
Roya Zandparsa, DDS, DMD, MSc
Matthew Finkelman, PhD
iii
ABSTRACT Statement of the problem: Replication of natural tooth color is one of the most difficult
challenges in prosthetic dentistry, especially in single tooth restorations in the anterior region and
when the adjacent tooth is discolored or non-vital.
Purpose of the study: The purpose of this study is to compare the effect of translucency and
background variations on the color difference of different shades of CAD/CAM lithium disilicate
glass ceramic.
Materials and methods: A sample size of n=10 was suggested according to a pilot study.
Lithium disilicate glass ceramic cylinders (12 mm in diameter and 13 mm in length) in three
different ceramic shades (BL1, A2 and C3) were fabricated by CAD/CAM technology.
Specimen disks were cut into sections of 1.2 mm thickness and 12 mm diameter. Four different
combinations of translucency and background color were considered among the different shades:
group 1 (HT ND1), group 2 (HT ND4), group 3 (LT ND1), and group 4 (LT ND4). Each
specimen was placed against different backgrounds (ND1, ND4). A spectrophotometer was used
to measure the color difference (ΔE) and ΔLab. Non-parametric tests (Kruskal-Wallis tests) were
used to evaluate the color differences among the tested groups, and follow-up tests (Mann-
Whitney U tests) were used with Bonferroni correction. Furthermore, for each ceramic shade,
high translucency groups were compared to low translucency groups to determine the influence
of translucency on color differences using a non-parametric test (Mann-Whitney U test).
Results: All tested groups displayed statistical significance (P< 0.001). Significant differences
were present among the tested groups of the same shade (P< 0.001). Additionally, the data
revealed that there were significant differences between the high translucency and the low
translucency groups (p < 0.001).
iv
Conclusion: Within the limitations of this study, it is suggested that the translucency and
background color significantly influenced the lithium disilicate glass ceramic color among the
BL1, A2, and C3 shades. Changing the underlying color from lighter (ND1) to darker (ND4)
resulted in increased color differences (ΔE).
v
DEDICATION
To my beloved wife and closest friend, Reem Al-Dakheel, who inspired me to follow my
dreams, believed in me and knew I would succeed.
To my parents, Wedad Al-Mukhaizeem and Mubarak Al-Ben Ali, for their unconditional
support, love and their gift of life.
To my dear family and friends who always encouraged me and supported me.
To everyone in my life who motivated me to move forward. I owe you all an unlimited debt of
appreciation.
vi
ACKNOWLEDGMENTS
First, I am grateful to Allah, The Almighty God, for all the rewards he grants me and for
giving me strength throughout my life. Second, to my home country, Kuwait, and the
Government of the State of Kuwait for supporting me through out my education. Without their
support I would not have been able to achieve my goals.
I owe my deepest gratitude to my principal advisor, Dr. Hiroshi Hirayama, for all the help and
support he offered me to complete this project and for the time contributed. It is his great
knowledge and guidance that helped me accomplish my project.
I would like to express my appreciation to Dr. Matthew Finkelman for his help with the
statistical analysis section and his way of simplifying and delivering the information, which
helped finalize this project.
I would also like to extend my gratefulness to Dr. Ki-Ho Kang for always helping me to think
outside the box. It is his great ideas and critical way of thinking that always inspired me to reach
higher levels.
I am thankful as well to Dr. Roya Zandparsa for her great support, guidance and encouragement
throughout the project. I would also like to thank Mrs. Susan Brown for her hard work in
coordinating me, the committee, and the companies.
Last but not least, a special thanks to Ivoclar Vivadent and Olympus for kindly providing me
with the materials that I used in this study.
vii
Table of Contents ABSTRACT .................................................................................................................................. iii DEDICATION ............................................................................................................................... v ACKNOWLEDGMENTS ........................................................................................................... vi Index of Tables: .......................................................................................................................... viii Table of Figures: .......................................................................................................................... ix
Introduction: ................................................................................................................................. 2 Background: .................................................................................................................................. 3
Color in dentistry: .................................................................................................................................. 3 Ceramics in dentistry: ............................................................................................................................ 3 All-ceramic restorations: ........................................................................................................................ 4 IPS e.max lithium disilicate glass ceramic: .......................................................................................... 5 Effect of abutment color: ....................................................................................................................... 6 Color determination: .............................................................................................................................. 8
Visual color determination: .................................................................................................................. 8 Instrumental color determination: ........................................................................................................ 9
Color measurement systems: ............................................................................................................... 12 Munsell color system: ........................................................................................................................ 12 CIE color system: ............................................................................................................................... 13
Aim: .............................................................................................................................................. 15
Hypotheses: .................................................................................................................................. 15 Clinical Implication: ................................................................................................................... 15
Materials and methods: .............................................................................................................. 16 Sample size: ........................................................................................................................................... 16 Specimen fabrication: ........................................................................................................................... 16 Finishing: ............................................................................................................................................... 17 Crystallization: ...................................................................................................................................... 18 Glaze Firing: ......................................................................................................................................... 18 Colored background: ........................................................................................................................... 18 Spectrophotometric measurement: ..................................................................................................... 19 Statistical analysis: ............................................................................................................................... 20
Results: ......................................................................................................................................... 21 Discussion: ................................................................................................................................... 24
Limitation of the study: ........................................................................................................................ 27 Future study: ......................................................................................................................................... 27
Conclusions: ................................................................................................................................. 29
Bibliography: ............................................................................................................................... 30
viii
Index of Tables: TABLE 1-‐ CRYSTALLIZATION/GLAZING CYCLE OF LITHIUM DISILICATE GLASS CERAMIC FURNACE
(PROGRAMAT P700). ...................................................................................................................................................................... 30 TABLE 2-‐ LAB VALUES OF REFERENCE GROUPS OF BL1 GROUP, A2 GROUP, AND C3 GROUP. .............................. 30 TABLE 3-‐ LAB VALUES AND ΔE VALUE OF BL1 HT ND4 (MEAN AND STANDARD DEVIATION). ........................... 30 TABLE 4-‐ LAB VALUES AND ΔE VALUE OF BL1 LT ND1 (MEAN AND STANDARD DEVIATION). ............................ 31 TABLE 5-‐LAB VALUES AND ΔE VALUE OF BL1 LT ND4 (MEAN AND STANDARD DEVIATION). ............................. 31 TABLE 6-‐ LAB VALUES AND ΔE VALUE OF A2 HT ND4 (MEAN AND STANDARD DEVIATION). .............................. 31 TABLE 7-‐ LAB VALUES AND ΔE VALUE OF A2 LT ND1 (MEAN AND STANDARD DEVIATION). ............................... 32 TABLE 8-‐ LAB VALUES AND ΔE VALUE OF A2 LT ND4 (MEAN AND STANDARD DEVIATION). ............................... 32 TABLE 9-‐ LAB VALUES AND ΔE VALUE OF C3 HT ND4 (MEAN AND STANDARD DEVIATION). .............................. 33 TABLE 10-‐ LAB VALUES AND ΔE VALUE OF C3 LT ND1 (MEAN AND STANDARD DEVIATION). ............................ 33 TABLE 11-‐ LAB VALUES AND ΔE VALUE OF C3 LT ND4 (MEAN AND STANDARD DEVIATION). ............................ 33 TABLE 12-‐ KRUSKAL-‐WALLIS TEST RESULTS AND DESCRIPTIVE ANALYSIS FOR BL1 TESTED GROUP (STD.
DEV.=STANDARD DEVIATION + IQR= INTERQUARTILE RANGE) ............................................................................. 34 TABLE 13-‐ KRUSKAL-‐WALLIS TEST RESULTS AND DESCRIPTIVE ANALYSIS FOR A2 TESTED GROUP (STD.
DEV.=STANDARD DEVIATION + IQR= INTERQUARTILE RANGE). ............................................................................ 34 TABLE 14-‐ KRUSKAL-‐WALLIS TEST RESULTS AND DESCRIPTIVE ANALYSIS FOR C3 TESTED GROUP (STD.
DEV.=STANDARD DEVIATION + IQR= INTERQUARTILE RANGE). ............................................................................ 34 TABLE 15-‐ POST-‐HOC TEST (MANN-‐WHITNEY U TEST) BL1 GROUPS ............................................................................... 34 TABLE 16-‐ POST-‐HOC TEST (MANN-‐WHITNEY U TEST) A2 GROUPS ................................................................................. 35 TABLE 17-‐ POST-‐HOC TEST (MANN-‐WHITNEY U TEST) C3 GROUPS. ................................................................................ 35 TABLE 18-‐ DESCRIPTIVE ANALYSIS FOR BL1 TESTED GROUP (STD. DEV.=STANDARD DEVIATION + IQR=
INTERQUARTILE RANGE) DEPENDENT VARIABLE ΔL ΔA ΔB. ..................................................................................... 35 TABLE 19-‐ DESCRIPTIVE ANALYSIS FOR A2 TESTED GROUP (STD. DEV.=STANDARD DEVIATION + IQR=
INTERQUARTILE RANGE) DEPENDENT VARIABLE ΔL ΔA ΔB. ..................................................................................... 36 TABLE 20 -‐DESCRIPTIVE ANALYSES FOR C3 TESTED GROUP (STD. DEV.=STANDARD DEVIATION + IQR=
INTERQUARTILE RANGE) DEPENDENT VARIABLE ΔL ΔA ΔB. ..................................................................................... 36 TABLE 21-‐ MANN-‐WHITNEY U TEST RESULTS FOR HT AND LT OF BL1 GROUPS. DEPENDENT VARIABLE= ΔE
.................................................................................................................................................................................................................. 36 TABLE 22-‐ MANN-‐WHITNEY U TEST RESULTS FOR HT AND LT OF A2 GROUPS. DEPENDENT VARIABLE= ΔE
.................................................................................................................................................................................................................. 37 TABLE 23-‐ MANN-‐WHITNEY U TEST RESULTS FOR HT AND LT OF C3 GROUPS. DEPENDENT VARIABLE= ΔE
.................................................................................................................................................................................................................. 37
ix
Table of Figures: FIGURE 1-‐ LEFT: PARTIALLY CRYSTALLIZED (HT) CAD/CAM LITHIUM DISILICATE GLASS CERAMIC.
RIGHT: FULLY CRYSTALLIZED (HT) CAD/CAM LITHIUM DISILICATE GLASS CERAMIC. ............................... 38 FIGURE 2-‐ LEFT: PARTIALLY CRYSTALLIZED (LT) CAD/CAM LITHIUM DISILICATE GLASS CERAMIC.
RIGHT: FULLY CRYSTALLIZED (LT) CAD/CAM LITHIUM DISILICATE GLASS CERAMIC. ................................ 38 FIGURE 3-‐ MUNSELL COLOR SYSTEM. ............................................................................................................................................... 38 FIGURE 4-‐ CIE LAB L: LIGHTNESS, A: RED AND GREEN, AND B: YELLOW AND BLUE. ................................................ 38 FIGURE 5-‐ E4D CAD/CAM MACHINE AND PRE-‐CRYSTALLIZED LITHIUM DISILICATE GLASS CERAMIC. .......... 39 FIGURE 6-‐ ISOMET 1000 MACHINE USED TO CUT THE CYLINDER INTO DISKS WITH 1.2 MM THICKNESS. ... 39 FIGURE 7-‐ BL1 SHADE GROUPS. ........................................................................................................................................................... 39 FIGURE 8-‐ A2 SHADE GROUPS. .............................................................................................................................................................. 40 FIGURE 9-‐ C3 SHADE GROUPS. .............................................................................................................................................................. 40 FIGURE 10-‐ LEFT: PRE-‐CRYSTALLIZED A2 LT LITHIUM DISILICATE GLASS CERAMIC SPECIMEN.
MIDDLE: PROGRAMAT P700 FURNACE. RIGHT: CRYSTALIZED A2 LT LITHIUM DISILICATE GLASS CERAMIC SPECIMEN. ....................................................................................................................................................... 41
FIGURE 11-‐ IPS E.MAX CERAM GLAZE PASTE. ............................................................................................................................... 41 FIGURE 12-‐ DIGITAL CALIPER (DENTAGUAGE 1) CONFIRMING THE SPECIMEN THICKNESS 1.2MM. ............... 41 FIGURE 13-‐ ND1 AND ND4 SHADE IPS NATURAL DIE MATERIAL. ....................................................................................... 41 FIGURE 14 LEFT: ND4 BACKGROUND WITH SPECIMEN HOLDER. RIGHT: SPECIMEN, ND1 AND
SPECIMEN HOLDER IN THE DARK BOX. ................................................................................................................................ 42 FIGURE 15-‐ SPECTROPHOMETER (CRYSTALEYE) WITH CUSTOM-‐POSITIONING JIG AND DARK BOX. .............. 42 FIGURE 16-‐ LEFT: ΔE* AND LAB* OF THE TARGET (SPECIMEN) AND THE REFERENCE (CONTROL GROUP) OF
GROUP 7 (A2 LT ND1). RIGHT: 2 MM IN DIAMETER POSITIONED OVER THE MIDDLE REGION OF THE SPECIMEN. .......................................................................................................................................................................... 42
FIGURE 17-‐ BOXPLOT ILLUSTRATION OF THE RESULTS OF BL1 GROUPS ΔE*. ............................................................. 43 FIGURE 18-‐ BOXPLOT ILLUSTRATION OF THE RESULTS OF A2 GROUPS ΔE*. ................................................................ 43 FIGURE 19-‐ BOXPLOT ILLUSTRATION OF THE RESULTS OF C3 GROUPS ΔE*. ................................................................ 43 FIGURE 20-‐ MANN-‐WHITNEY U TEST RESULTS FOR HT AND LT OF BL1 GROUPS. DEPENDENT VARIABLE=
ΔE ............................................................................................................................................................................................................. 44 FIGURE 21-‐ MANN-‐WHITNEY U TEST RESULTS FOR HT AND LT OF A2 GROUPS. DEPENDENT VARIABLE= ΔE
.................................................................................................................................................................................................................. 44 FIGURE 22-‐ MANN-‐WHITNEY U TEST RESULTS FOR HT AND LT OF C3 GROUPS. DEPENDENT VARIABLE= ΔE
.................................................................................................................................................................................................................. 44
1
The Effect of Translucency and Background
Variations on the Color Difference of
CAD/CAM Lithium Disilicate Glass Ceramic
2
Introduction:
All-ceramic materials were introduced as metal-free restorations. Ceramics improve
esthetics by allowing light transmission through the restoration and the underlying tooth
structure1-3. One commonly used all-ceramic system is the lithium disilicate glass ceramic, in
which the alumino-silicate glass has lithium disilicate crystals. Lithium disilicate crystals are
needle-like in shape and include about two-thirds of the glass ceramic volume, presenting
outstanding esthetics, high strength, and the ability to be cemented or adhesively bonded4. In
fact, due to the fairly low refractive index of the lithium disilicate crystals, this material can be
very translucent4-6. However, the material can be fabricated in either a pressable or machinable
manner with CAD/CAM technology 7,8.
Although the high translucency of lithium disilicate glass ceramic is advantageous, it might be
challenging in cases of restoring non-vital or discolored teeth. Yet, the influence of different
translucencies on the resulting color has not been tested.
The purpose of this study is to compare the effect of translucency and background
variations on the color difference of different shades of CAD/CAM lithium disilicate glass
ceramic.
3
Background:
Color in dentistry:
In dentistry, the combination effects of extrinsic and intrinsic colors circumscribe the
tooth color. The extrinsic color is correlated with the absorption of materials onto the surface of
the enamel and dentin, whereas the intrinsic tooth color is connected with the light absorption
and scattering properties of the enamel9-11. Lemire described tooth color as a result of the
reflection of light from the tooth surface combined with light redirected from the dentin that has
undergone some internal refractions and reflections12,13. Moreover, Munsell explained that color
has three dimensions: hue, chroma and value14.
Translucency is one of the vital properties that should also be taken into consideration to
fabricate an esthetic restoration. The latter is defined as the amount to which light is diffused
rather than reflected or absorbed15. High translucency enamel overlaying the dentin results in
greater translucency at both the incisal third and the proximal surfaces of a tooth16. The middle
third of a tooth has a large amount of yellowish dentin, which influences the color of the
covering enamel16. However, the translucent blue-grey enamel often alters the dentin so that the
ending color is a combination of yellow, orange, blue and grey16-21.
Ceramics in dentistry:
In the 18th century, ceramics were first used in dentistry for porcelain dentures21,22. Due
to the brittle nature of this material, it was not until 1962 that the Weinstein brothers invented the
clinically reliable porcelain fused to metal (PFM) crown. This was accomplished by adding
leucite to porcelain preparations, which raised the coefficient of thermal expansion to permit
their merging to certain gold alloys to fabricate full crowns and fixed partial dentures5,23. In
4
1965, McLean introduced the aluminous porcelain all-ceramic crown, and since then, it has been
improved and employed by both clinicians and technicians16. Currently, there are five methods to
produce all-ceramic crowns: cast and creaming, slip casting, condensation and sintering,
pressing, and computer-aided design / computer-aided manufacturing (CAD / CAM) milling of
pre-crystallized blocks or ceramic blocks. The increased requirement for esthetic restorations has
resulted in the demand for all-ceramic crowns2. The spectrum of dental ceramics is divided into
three fields: predominantly glassy materials, particle-filled glasses, and polycrystalline ceramics.
Highly esthetic dental ceramics are predominantly glassy, while crystalline ceramics are
considered to be higher strength ceramics5.
All-ceramic restorations:
In 1903, the first jacket crown was invented. Since that time, all-ceramic restorative
materials have been under continuous development17,18. To meet esthetic requirements, several
all-ceramic systems have been established. The method of fabrication varies among the systems.
Some systems contain cores, veneered with core material5. The core enhances the strength of the
restoration and may range from opaque to semi-translucent4,16.
Layering the porcelain veneer over ceramic frameworks can achieve inherent beauty and
beneficial light scattering5,24. The veneering material usually has fluorescent properties that
resemble those of the natural teeth25. Usually, veneering porcelains involves a leucite, aluminum
oxide, or glass and crystalline phase of fluoroapatite26. Veneering a zirconium oxide core with
glass, aluminum oxide, or lithium disilicate allows dental technicians to tailor the form and
esthetics of these restorations in terms27. The chipping of the veneering porcelain and/or the
fracture of the coping are the most commonly encountered main clinical complication that results
the in failure of all-ceramic restorations26,28-34.
5
IPS e.max lithium disilicate glass ceramic:
Several advancements have taken place in combination with lithium disilicate (Li2Si2O5)
glass ceramics35-37. The principal crystal phase is fabricated in the different base glass of the
SiO2–Li2O–K2O–ZnO–P2O5–Al2O3–La2O3 system through heterogeneous nucleation and
crystallization36,38.
In development, an interconnecting microstructure with a crystal content of 460-vol% is
established. The developing product was named lithium disilicate (IPS Empress 2, Ivoclar
Vivadent AG). Additionally, for use in the anterior region, it was introduced to the global market
in 199938-40. A substantial enhancement over IPS Empress 2 was introduced in the SiO2–Li2O–
K2O–Al2O3–ZrO2 system and developed in the production of lithium disilicate glass ceramic
(IPS e.max, Ivoclar Vivadent AG)41,42.
Thorough analysis of the crack propagation in this glass ceramic revealed that the high
fracture toughness, which is 2.3 MPa 0.5 mm, measured by the SEVNB method as a KIC value,
was initiated by crack divergence in the vicinity of the disilicate crystals23. A considerable
amount of energy is lost from the propagating crack while deviating23. This yields an increased
amount of flexural strength (up to 440 MPa) and toughness23.
This material provides prime esthetics, yet has the strength for conventional or adhesive
cementation. In addition to outstanding optical properties, lithium disilicate glass ceramic has a
needle-like crystal configuration that offers excellent durability and strength4,7,23. Furthermore, it
can be conventionally pressed or contemporarily processed through CAD/CAM technology23.
This lithium metasilicate glass ceramic exhibits a single blue color6,23. The blue glass ceramic
undergoes a heat treatment at 850 °C after it has been milled and successfully tried in the
patient’s mouth23. Throughout thermal treatment, it is converted into lithium disilicate glass
6
ceramic23. Among the tested materials, pressed lithium disilicate glass ceramic demonstrated the
highest light transmission rate42.
Two levels of translucency may be acquired based on the pre-crystallization treatment of
the CAD/CAM ceramic blocks. There is a small number and bigger size of lithium metasilicate
crystals in the pre-crystallized state of the high translucency (HT) material (Fig.1)42, whereas the
low translucency (LT) material contains a larger number of smaller crystals (Fig.2)42. Covered
lithium disilicate crystals (1.5 × 0.8 µm) in a glassy matrix are observed in the HT ceramic
following complete crystallization heat treatment at 850 °C for 10 minutes (Fig.1)42. Highly
soluble lithium phosphate spherical crystals look like spherical pores42. The fully crystallized LT
ceramic has a large number of small (0.8 × 0.2 µm) interconnected lithium disilicate crystals
along with spherical pores, referred to as lithium phosphate crystals (Fig. 2)42.
Effect of abutment color:
When ceramics have higher translucency, more light is able to transmit and scatter. This
means that the underlying abutment has a major impact on the final color43.
However, in clinical conditions, all-ceramic crowns are frequently used in cases such as a
non-vital tooth that has been endodontically treated or a multichromatic abutment44. Thus, it is
crucial to consider the color of the crown as well as the color of the abutment tooth involved45.
The combination of the thickness of the ceramic, the underlying abutment color, and the color of
the cement articulates the optical behavior of ceramic restorations43.
The color of the restoration may be influenced by several aspects, including the ceramic firing
temperature46-48, the number of ceramic firing cycles 46,49,50, surface glaze 51, opaque ceramic
thickness 52-54, ceramic thickness43,49,52,53,55-57, manufacturer50,58-61, metal surface treatment62, and
type of substructure43,62-67.
7
Previous studies showed that the underlying tooth structure is a principal factor affecting
the look of final ceramic restorations 43,44,57,65,68,69. The color underneath the crown may lead to
shadowing and discoloration of the restoration if an all-ceramic restoration was restored on a
dark underlying tooth structure, for example, a tooth treated with a root canal treated tooth43,44.
Chaiyabutr et al studied the collective outcome of the tooth abutment color, cement color,
and ceramic thickness in the subsequent optical color of a CAD/CAM lithium disilicate glass
ceramic crown44. Their findings suggested that the ΔE values of a CAD/CAM lithium disilicate
glass ceramic crown were significantly affected by the tooth abutment color (P < 0.001), cement
color (P < 0.001), and ceramic thickness (P < 0.001). In addition, significant differences existed
between these three variables (P < 0.001). For example, a dark-colored abutment tooth presented
the greatest ΔE values compared to the other variables. Additionally, there was a significant
decrease in ΔE values when the ceramic thickness was increased (P < 0.01). Furthermore, when
the crowns were cemented using the opaque cement, the ΔE values slightly decreased44.
In a study that assessed the impact of the abutment material on the color of IPS Empress
2 ceramic copings with different thicknesses, they used different degrees of thickness (1, 1.2, 1.4,
1.6, 1.8, or 2 mm) against three different abutments, which were composite, a gold alloy, or a
silver palladium alloy43. Using a colorimeter, color was evaluated according to the CIE LAB
system, and ΔE was calculated43. They found that the abutment material and ceramic thickness
significantly affected the ΔE values43.
Nakamura et al stated that the abutment impacts the ceramic color when the thickness of
ceramic is less than 1.6 mm43, while other studies suggest that the thickness of the ceramic
should be at least 2.0 mm to moderate the effect of the abutment tooth on the overall
color1,44,57,70,71. Furthermore, Heffernan et al studied the effect of core and core-veneering
8
ceramic thickness on the resultant translucency71,72. According to Lee et al, based on the type of
all-ceramic core material, the layered color of various all-ceramic and veneer combinations was
different, even though the thickness of the layered specimen was set to 1.5 mm24.
Kwiatkowski et al and Ahmad et al discussed that there is an influence of the underlying
tooth structure on the increased light transmission of all-ceramic restorations, whether normal,
discolored, or treated with a post-and-core or a buildup73-75. Moreover, with the recent
technological developments in ceramic, zirconium and fiber-reinforced posts, there are new
possibilities for restorative solutions73,74.
Color determination:
Visual color determination:
In clinical dentistry, the most frequently applied method is visual determination. The
human eye is capable of spotting the minute color variations between two objects and is essential
for color communication76. By using the shade tabs available in the market, the best matching
shade is communicated to the dental laboratory through verbal, graphic and photographic means
77-79. The VITA Classic shade guide system (Vident, Brea, CA) is commonly applied78,79. It
classifies the shades into letter groups according to the tooth hue: A (Orange), B (Yellow), C
(Yellow / Gray), and D (Yellow, Orange or Brown)77-79. Then, to describe chroma and value,
numbers are added to the letters77-79. For example, 1 is less chromatic and higher in value, while
4 is more chromatic and lower in value. Further rational shade guide systems, such as Vitapan
3D-Master, have been introduced to simplify or decrease subjectivity in the process of shade
selection77-79. This shade guide system is based on the sequential determination of the value, the
chroma, and finally, the hue77-79. The human eye is more sensitive to changes in value than subtle
changes in hue15. Usually, the restoration is clinically acceptable when value and chroma are
9
correct, even if the hue is not precisely matched15. Yet, the visual determination of shade has
been discovered to be unpredictable and subjective. In general, differences in color perception
are present between 25 different subjects: patients are usually more pleased with the shade match
of their PFM restorations than their dentists, especially if the practitioners are prosthodontists80.
Inconsistencies may occur due to one or more of the following: general variables (external light
conditions and background), physiologic variables (such as color blindness), and personal
features (experience, nutrition, fatigue of the human eye, medications, emotions, and age)15,81,82.
Nevertheless, there are limited standardized verbal means to communicate visually assessed
color characteristics83.
Moreover, color matching with shade tabs may be difficult84 for two reasons: because it
is hard to control parameters during the creation of shade tabs, and because tooth shades can vary
compared to the numbers of the shade tabs85,86.
Instrumental color determination:
Instrumental measurements serve as a more objective way to quantify color and they may
enable more precise and uniform color communication. We applied computer analysis to
photographic images to apply the RGB (red, green, blue) color model. The development of high-
tech colorimeters and spectrophotometers has augmented their use in dental color
communication and in dental research9,87-89.
Colorimeter:
The colorimeter represents an easier way of measuring tooth color. Hence, it has been
widely used in the dental field16. This instrument is designed in a way that directly measures the
color, similar to how the human eye identifies it16. Accuracy comparable to spectrophotometers
can be recorded15,16, but only limited data points may be stored15,16. Colorimetric readings have
10
been compared to spectrophotometer measurements and are believed to be accurate and reliable
for color difference readings77,88,90-93. Colorimetric measurements have also been compared to
human observations, but the results were inconclusive88,90,94. In most of the studies where a
positioning jig has been used, the measurements have been consistent and repeatable93. Although
colorimeters have a numerous advantages, they also exhibit disadvantages. Color may be prone
to errors because these instruments are fabricated to measure flat surfaces. Further, small
aperture colorimeters are disposed to substantial edge-loss effect77. Additionally, the results can
be severely affected by wet and dry conditions77. These systematic errors can adversely affect the
instrument accuracy, and subsequently, the intra-instrument repeatability93.
Spectrophotometer:
The spectrophotometer measures one wavelength at a time from the reflection or the
transmittance of an object. It can measure the visible spectra of both extracted and vital
teeth76,95,96. Very accurate and extensive data can be collected by the spectrophotometer and
translated into a formatted spectral curve15,76,97. Dual spectrometer devices can measure the
entire spectral range simultaneously76,96. Traditionally, spectrophotometers were complex to use
and it was difficult to obtain in vivo tooth color measurements with them90. Recently, however,
easy-use chair-side spectrophotometers have been developed. These spectrophotometers are very
precise and allow the reading of teeth translucency and reflectivity through the use of more
filters. Usually, these machines come with advanced software systems15. A recent study
concluded that the mean ΔE* value for crowns fabricated using the spectrophotometric technique
was significantly lower than the values for the crowns made by the conventional method98.
A number of studies70,76,96,99 that compared the use of different spectrophotometers to the
use of visual shade guides for determination of tooth shade revealed that spectrophotometric
11
methods bring about higher accuracy and better color match. A recent study100 evaluating visual
shade selection versus a spectrophometer concluded that spectrophotometric shade analysis
seemed to be more reproducible than visual shade determination that resulted in darker
recordings. In another study, a spectrophotometer was compared to three different shade guides;
the former provided more accurate results than visual selection101.
A study by Judeh et al. found that visual shade selection lowered the precision of shade
selection to approximately 31%; this is consistent with previous studies. It is linked to
inconsistencies inherent in color perception by individual person96. Moreover, two studies that
tested dental devices including color shade guides reported a reduction in shade match precision
to nearly 48%94,96,102.
Kim-Pusateri et al. examined the reliability and accuracy of four dental color-matching
devices. The reliability was defined as the consistency of the device in matching the same
specimen while the accuracy was defined as the ability of the device to provide a correct match
for a given specimen103. Three commercial shade guides, Vitapan classical (vident, Bera, CA),
Vitapan 3-D Master (Vident, Brea, CA), and Chromascop (Ivoclar, Vivadent, Schaan,
Liechtenstien) were used103. Ten shade guides from each system and four dental shade-matching
devices, ShadeVision (X-Rite America, Inc, Grand Rapids, MI), VITA Easyshade (Vident, Brea,
CA), ShadeScan (Cynovad, Montreal, Canda), and SpectoShade (MHT Optic Research AG,
Niederhasli, Switzerland) were tested103.
For the reliability evaluation, each shade tab from one Chromascop shade guide (20
shade tabs), one Vitapan Classical shade guide (16 shade tabs), and one Vitapan 3DMaster shade
guide (26 shade tabs) was measured ten separate times by each shade matching instrument103.
Whereas for the accuracy study, each shade tab from thirty shade guides was measured by each
12
shade-matching instrument103. Most device comparisons had comparable, high reliabilities (over
96%), showing predictable shade values from frequent measurements103. However, noticeable
variability in the devices’ accuracy (67-93%) was observed in most of the device comparisons103.
Odaira et al clinically assessed the spectrophotometer (Crystaleye, Olympus, Tokyo,
Japan)104. They moved the spectrophotometer to modify the position of the tooth to be
captured104. Then, they automatically transmitted the captured images and analyzed them by the
Crystaleye Application Master Software104. To assess the accuracy, Color Analyzing
Spectrophotometer-Iwate Medical University School of Dentistry Type1 (CAS-ID1) and
MultiSpectral Camera system (MSC-2000; Olympus, Tokyo, Japan) were used104. The reliability
of color measurement tests from three color measuring devices showed no significant difference
for the Lab values among the three devices104. The ∆E was between 0.1 and 0.9 and the mean ∆E
was 0.6 +/- 0.3 in the results of the accuracy of repeated color measurements assessment104. The
effect of exterior lighting on color measurements showed the ∆E between the two conditions was
0.9. Performing a t-test, no significant difference was found in the Lab values between the two
conditions104. While evaluating the effect of the examiner on the color measurement, there was
no significant difference in the Lab values between the five examiners. Evaluation of the
reproducibility of tooth color using the Crystaleye Spectrophotometer indicated the mean ∆E
between the target teeth and the fabricated crown as 1.2 +/- 0.4104.
Color measurement systems:
Munsell color system:
In terms of hue, value, and chroma, color may be defined according to the Munsell Color
Space14. Hue represents the color tone and it is the characteristic that allows one to differentiate
among different groups of color14. Value specifies the relative lightness or darkness of a color,
13
ranging from pure black to pure white14. Chroma is the level of color saturation; it defines the
color strength, intensity or vividness14 (Fig. 3). Television and computer monitors, examples of
emissive media, emit wavelengths that are a mixture of red, green and blue, thereby creating
color. These three colors are considered additive primary colors because almost all of the colors
in the visible spectrum can be produced through their combination. This media is called the RGB
color model14. However, reflective and transmissive media, like photographs or printed
materials, apply a different color model, named CMY (Cyan, Magenta, Yellow)15. In this color
system, the primary colors are those produced by the absorption of one of the RGB wavelengths
and the transmission or reflection of the others15. Cyan is produced when red is absorbed and
blue and green are transmitted or reflected; magenta is produced when green is absorbed and
blue and red are transmitted or reflected; yellow is produced when blue is absorbed and green
and red are transmitted or reflected15. Thus, cyan light is an equal mixture of blue and green,
magenta is an equal mixture of blue and red, and yellow is an equal mixture of green and red15.
CIE color system:
With the advanced technologies, data logging machines have been developed to precisely
define color, most frequently by the CIE L*a*b* color space system (Commission Internationale
d’Eclairage, International Commission on Illumination). In 1976, this color system was
introduced. It agrees with the acknowledged theory of color sensitivity based on three separate
color receptors of red, green and blue105. In the CIE Lab color space, the color space is uniform
with evenly perceived color differences. There are three axes, L*, a*, b* in this three-
dimensional color space. The main advantage of this system is that differences in color can be
stated in units that can be linked to clinical significance and visual perception. The L* color
coordinate ranges from 0 to 100 and it characterizes lightness; the a* color coordinate ranges
14
from -90 to 70 and it characterizes greenness on the positive axis and redness on the negative;
the b* color coordinate ranges from -80 to 100 and it characterizes yellowness (positive b*) and
blueness (negative b*)16,70,106 (Fig. 4). The differences within the specimens are measured in
ΔL*, Δa* and Δb*, and their combination is described by ΔE*, determined by different
equations: ΔE* = [(ΔL*)2+ (Δa*)2+(Δb*)2]1/215.
15
Aim:
To evaluate the effect of translucency and background variations on the color difference
of CAD/CAM lithium disilicate glass ceramic among different shades.
Hypotheses: 1. High translucency CAD/CAM lithium disilicate glass ceramic will exhibit more color
difference than low translucency CAD/CAM lithium disilicate glass ceramic of the same
shade.
2. Natural die 4 shade group will have more color difference than natural die 1 shade group.
Clinical Implication: The knowledge learned from this study may assist the practitioner in predicting the final color of
lithium disilicate glass ceramic restoration in the case of a multi-colored abutment by
determining how translucency and background color will affects the overall color of CAD/CAM
lithium disilicate glass ceramic.
16
Materials and methods:
Variables:
Translucency and background color were the two variables were tested in the study.
Translucency was divided into High Translucency (HT) or Low Translucency (LT) while
background color was divided into Natural Die (ND, Ivoclar Vivadent, Schaan, Liechtenstein)
(ND1 and ND4) among different ceramic shades (BL1, A2, or C3 shade).
Sample size:
Power calculations were performed using nQuery Advisor (version 7.0). Assuming a
variance of means of 0.535 for translucency, a variance of means of 1.215 for background, and a
standard deviation of 1.1 (values obtained from pilot study) a sample size of n=10 per group was
adequate to obtain a type I error rate of ∝= 0.05, a power of 98% for translucency, and a power
of 99% for background.
Specimen fabrication:
The specimens were designed by using a software on CAD/CAM (E4D, D4D
Technologies, LLC, Richardson, TX) to make a digital mold with 12 mm in diameter and 13 mm
length lithium disilicate glass ceramic cylinder (IPS e.max CAD, Ivocler Vivadent, Schaan,
Liechtenstein) in three different shades BL1, A2 and C3. Cylinders were fabricated with lithium
disilicate glass ceramic blocks and they were milled using CAD/CAM E4D according to
manufacturer’s recommendations (Fig. 5). The lithium disilicate glass ceramic cylinders were cut
in the pre-crystallized stage using a low speed (275 rpm) diamond disc (Isomet 1000, Buehler,
17
Lake Bluff, IL). The specimen disks’ thicknesses were standardized to 1.2 mm (Fig. 6), which
represents the thickness needed for middle area1.
Each ceramic shade was divided into two main groups according to translucency and two
subgroups according to background color. A total of twelve groups were derived as follows:
BL1 Shade (Fig.7) High Translucent group (HT): Group 1: HT ND1 shade (reference). Group 2: HT ND4 shade. Low Translucent group (LT): Group 3: LT ND1 shade. Group 4: LT ND4 shade.
A2 Shade (Fig.8) High Translucent group (HT): Group 5: HT ND1 shade (reference). Group 6: HT ND4 shade. Low Translucent group (LT): Group 7: LT ND1 shade. Group 8: LT ND4 shade. C3 Shade (Fig.9) High Translucent group (HT): Group 9: HT ND1 shade (reference). Group 10: HT ND4 shade. Low Translucent group (LT): Group 11: LT ND1 shade. Group 12: LT ND4 shade.
Finishing:
A green stone (Dura-Green Stone TC2, SHP, Shofu, Kyoto, Japan) was used to smooth
out the roughness around all specimens’ margins caused by the cutter machine. The thickness of
specimens was checked using a digital caliper (Dentaguage 1, Erskine Dental, Marina Del Rey,
CA).
18
Crystallization:
Crystallization was carried out in a ceramic furnace (Programat P700 furnace, Ivoclar
Vivadent, Schaan, Liechtenstein) following the sequence described in Table 1 (Fig. 10).
Glaze Firing:
While glaze firing can be performed simultaneously with the crystallization procedure, in
this study, it was conducted in a separate step. This ensured the completion of the crystallization
step in accordance with the manufacturer’s recommendation. The glazing material (IPS e.max
Ceram Glaze Paste) (Fig. 11) was applied on a layer of approximately 0.01 mm on one side of
each specimen using a medium flat brush (no.G4, Ivoclar Vivadent, Schaan, Liechtenstein). The
glaze firing was conducted on a honeycombed firing tray using Programat P700 furnace in the
same cycle as described in (Table.1); a digital caliper was used to ensure the specimens’
thickness had not deviated from 1.2 mm (Fig. 12).
Colored background:
For the fabrication of the ND1 and ND4 background, a pink baseplate wax (Kerr
Manufacturing Co, Romulus, MI) was used (14 mm X 27 mm X 3 mm); the dimensions were
adjusted according to the dimension of the specimen holder in the dark box. Light cured urethane
dimethacrylate (TRIAD™ Colorless, Visible Light Cure Material; Dentsply International Inc.,
York, PA) was used to fabricate a mold. ND1 and ND4 backgrounds were injected into the mold
and cured in a halogen spectral range between 400 and 500 nm (TRIAD™ 2000; Dentsply
International Inc., York, PA) to fabricate ND1 and ND4 backgrounds107 (Fig. 13,14).
19
Spectrophotometric measurement:
The specimens were measured by a spectrophotometer (Crystaleye, Olympus, Tokyo,
Japan). Each specimen, along with the tested background, was placed in a specimen holder
inside a black box which served to eliminate the impact of external light. Using an auto-
polymerizing resin (Ivolen, Ivoclar Vivadent, Schaan Liechtenstein), a custom-positioning jig
was made to achieve measurement repeatability and accuracy. The distance between the
specimen surface and the tip of the spectrophotometer was approximately 4 mm and the angle
was 45 degrees, in accordance with the manufacturer’s recommendations (Fig. 15). Prior to color
measurements, all specimens were cleaned and visually checked for dust.
The specimen to be measured was placed in the center of the display screen. As the disks
were combined, a droplet of distilled water was positioned between them (refraction index close
to 1.7)52. This was performed to enhance the optical contact during the spectrophotometric
measurement which served to minimize the loss of light through the margins of the specimens
(known as edge-loss)70. Over the middle region, we located a standardization area of 2 mm in
diameter. This was determined by the fabrication of a custom-made template that fits the
computer screen. During all of the measurements, the custom-made template was used (Fig. 16).
In addition, before each measurement, the spectrophotometer was recalibrated with a calibration
plate. HT ND1 group was chosen to be the reference group among different shades44. The
reference group specimens were measured as mentioned previously. The means of Lab* were
calculated (Table.2) and the data were then used to calculate the ΔE and ΔLab.
Five measurements per specimen over both background colors (ND1 and ND4) were
recorded, in CIE (CIE Lab*) coordinates, to minimize the effect of any possible misreadings.
The capture time was 0.2 seconds and the reflectance values were from 400-700 nm with 1-nm
20
intervals for each pixel. The data were obtained in CIE Lab color system by calculating ∆E*
through the specimens over ND1 and ND4 backgrounds. Color difference between the
specimens was calculated using the equation: ∆E* = [(∆L*) 2+(∆a*) 2+(∆b*) 2]1/215 (Tables.3-
11).
Statistical analysis:
Descriptive statistics (mean, standard deviations, median, interquartile range) were
computed for each of the nine groups and each outcome. Data analyses were conducted in IBM-
SPSS version 20. Levene’s test showed significant difference among the tested group, in other
words, there was no equality of variance and one of the ANOVA assumptions was violated.
Because of this, within each ceramic shade all groups were compared using a non-parametric test
(Kruskal-Wallis test). A post-hoc (Mann-Whitney U) test was used with the Bonferroni
correction to the α=0.05/3 to detect any significant difference between the groups within the
same ceramic shade. Furthermore, in each ceramic shade high translucency groups were
compared to low translucency groups to determine the influence of translucency on color
difference. Because the histograms showed the data were not normally distributed, a non-
parametric test (Mann-Whitney U test) was used among each ceramic shades.
21
Results:
A summary of the descriptive statistical analysis of the results showing the mean color
difference (ΔE) and the standard deviation for all the groups is shown in (Tables.12-14). For
illustration, box plots of the values for ΔE of the tested groups are shown in (Fig.17-18). The
means and standard deviations calculated for the BL1 shade were described as following (Mean
± Std. Dev.) N: group 2 (HT ND4) specimens is (11.30 ± 0.29) N; group 3 (LT ND1) specimens
is (4.36 ± 0.18) N; group 4 (LT ND4) specimens is (4.93 ± 0.14). While A2 shade N; group 6
(HT ND4) specimens is (8.74 ± 0.23) N; group 7 (LT ND1) specimens is (1.23 ± 0.13) N; group
8 (LT ND4) specimens is (4.63 ± 0.13). Finally, C3 shade were described as following N; group
10 (HT ND4) specimens is (6.74 ± 0.17) N; group 11 (LT ND1) specimens is (1.26 ± 0.14) N;
group 12 (LT ND4) specimens is (4.66 ± 0.25).
After computed of the descriptive analysis, a non-parametric test (Kruskal-Wallis test) was
carried out for each ceramic shade. The test revealed a significant difference between the groups
for each shade (p< 0.001). Statistical significance was set at alpha = 0.05. Looking at the box
plot in (Fig. 17-18) and (Tables 12-14), the data demonstrate a statistical significance difference
among the groups (P-value < 0.001).
Three follow-up tests (Mann-Whitney U tests) were conducted to detect any significant
differences between the groups within the same shade. The Bonferroni correction was considered
in this case, and the α of the test was adjusted to be equal to 0.05/3 = 0.017. The statistical results
of this test are summarized in Tables 15-17 and illustrated in a box plot (Fig. 17-18). The
22
outcome presented a statistically significant (p < 0.001) difference between the groups within
the same shade.
The Lab* values for descriptive statistics, with means and standard deviation, are reported for
each group and for each of the coordinates. The results showing the mean ΔL* values and the
standard deviation for all the groups are shown in Tables 18-20. The means and standard
deviations calculated for the BL1 shade were described as the following (Mean ± Std. Dev.) N:
group 2 (HT ND4) specimens is (7.49 ± 0.18) N; group 3 (LT ND1) specimens is (2.1 ± 0.17) N;
group 4 (LT ND4) specimens is (3.42 ± 0.15). While A2 shade N; group 6 (HT ND4) specimens
is (4.74 ± 0.33) N; group 7 (LT ND1) specimens is (0.39 ± 0.16) N; group 8 (LT ND4)
specimens is (2.25 ± 0.62). Finally, C3 shade were described as following N; group 10 (HT
ND4) specimens is (4.83 ± 0.23) N; group 11 (LT ND1) specimens is (0.84 ± 0.27) N; group 12
(LT ND4) specimens is (3.62 ± 0.16).
The mean Δa* values and the standard deviation for all the groups are shown in Tables 18-20.
The means and standard deviations calculated for the BL1 shade were described as follows
(Mean ± Std. Dev.) N: group 2 (HT ND4) specimens is (3.2 ± 0.25) N; group 3 (LT ND1)
specimens is (0.33 ± 0.06) N; group 4 (LT ND4) specimens is (2.48 ± 0.14). While A2 shade N;
group 6 (HT ND4) specimens is (4.97 ± 0.17) N; group 7 (LT ND1) specimens is (0.85 ± 0.06)
N; group 8 (LT ND4) specimens is (3.64 ± 0.04). Finally, those of C3 shade were described as
the following N; group 10 (HT ND4) specimens is (4.09 ± 0.18) N; group 11 (LT ND1)
specimens is (0.29 ± 0.14) N; group 12 (LT ND4) specimens is (2.88 ± 0.26).
23
The results showing the mean Δb* values and the standard deviation for all the groups are shown
in Tables 18-20. The means and standard deviations calculated for the BL1 shade were described
as the following (Mean ± Std. Dev.) N: group 2 (HT ND4) specimens is (7.43 ± 0.39) N; group 3
(LT ND1) specimens is (3.75 ± 0.21) N; group 4 (LT ND4) specimens is (1.35 ± 0.18). While
A2 shade N; group 6 (HT ND4) specimens is (5.39 ± 0.22) N; group 7 (LT ND1) specimens is
(0.75 ± 0.25) N; group 8 (LT ND4) specimens is (1.57 ± 0.53). Finally, those of C3 shade were
described as the following N; group 10 (HT ND4) specimens is (2.17 ± 0.80) N; group 11 (LT
ND1) specimens is (0.83 ± 0.25) N; group 12 (LT ND4) specimens is (0.34 ± 0.22).
Furthermore, non-parametric tests (Mann-Whitney U tests) were carried out in each ceramic
shade group, and the data revealed that there were significant differences between the high
translucent and the low translucent groups (p < 0.001). The HT showed more ΔE than the LT
groups, and these findings were observed in all ceramic shades (Tables 21-23, Fig. 20-22).
24
Discussion:
In the past few years, lithium disilicate glass ceramic has been used more
often in clinical practice. This material provides optimum esthetics as well as
increased strength, which enables adhesive or conventional cementation6,42. Lithium
disilicate glass ceramic has a needle-like crystal structure that offers excellent
durability and strength, as well as exceptional optical properties4,7,23. Although the
variation in the translucency of the lithium disilicate glass ceramic is considered to be
an advantage, it has a negative effect on the resulting color when the underlying tooth
structure is dark.
Earlier research has demonstrated that the underlying tooth structure has a
major impact on the appearance of definitive ceramic restorations43,44,65,68,69.
However, the influence of different translucencies on the resulting color has not been
tested. Thus, the main focus of this study was to assess the effect of translucency and
background variations on the color difference of different shades of CAD/CAM
lithium disilicate glass ceramic. Translucency and background color were the two
variables that were tested in the study. Translucency was divided into high or low,
while background was divided into ND1 and ND4 among the different ceramic
shades (BL1, A2, or C3 shade). The hypothesis of this study was of two parts: first,
the high translucency CAD/CAM lithium disilicate glass ceramic will have more
color differences than the low translucency ceramic of the same shade; second, the
ND4 group will have more color differences than the ND1 group. The reason behind
the interest in different ceramic shades as conditioned variables in this study was to
define the influence of translucency variations of the same shade and consistency
among the different ceramic shades.
25
Using the CIELAB system, the color difference (ΔE) was calculated. These
coordinates were taken from a spectrophotometer’s spectral reflectance
measurements. They give a numerical description of the color position in a 3-
dimensional color space70,108. CIELAB units were equally spaced in relation to visual
sensitivity, so that the spectral measurements can be associated with subjective
observations; this is an advantage over the Munsell system (hue, chroma, and
value)70,108.
The results of this study showed that the color difference (ΔE) of a CAD/CAM
lithium disilicate glass ceramic is affected by the level of translucency and the
background color. Background findings of the study were in agreement with earlier
studies in the literature12,17,19,44. High translucent groups and darker background
groups showed increased ΔE values compared with other groups. The highest ΔE
values were noticed in group 2 (BL1 HT ND4), while the lowest ΔE values were
found in both group 7 (A2 LT ND1) and group 11 (C3 LT ND1).
In BL1 groups, the results of this study demonstrated that color difference
(ΔE) interaction was significant among the three groups (Table 15). However, the
most affected group was shown to be group 2 (BL1 HT ND4) and the lowest was
group 3 (BL1 LT ND1) (Table 12). The possible reason for high ΔE values may be
due to the combined effect of high translucency and darker background in the tested
group. Chaiyabtur et al showed a similar finding, as the color of lithium disilicate
glass ceramic was affected by many factors, including the underlying tooth color44.
Additionally, the same finding was observed in the A2 groups and the C3 groups.
However, the effect of translucency and background color was smaller when
compared with BL1 groups (Tables 13, 14). A possible explanation is that an increase
26
in the chromaticity of the A2 and C3 shades has the potential to reduce the color
effect of the underlying tooth structure.
Based on our current knowledge and literature search, a standard reference of
∆Lab* was not available to facilitate color matching. However, a further analysis of
the Lab* values was performed, with the evaluation of L* values, a* values, and b*
values among tested groups, to achieve a better understanding of the color
differences109.
In this experiment, the ΔL* (lightness) value was greater when HT with an
ND4 background was used among the different ceramic shades (Tables 18-29). This
may be due to the optical properties of the material itself. The ceramic is an optical
mixture of a glass matrix and the lithium disilicate crystalline phase, which allows the
light to pass through a material and reflects the color of the background3,68. The Δa*
and Δb* values, which represent the hue and chroma of color, differed for specimens
against the ND1 compared to the ND4 background, proposing that yellow and red
coloration shows through the ND4 background,43 and this difference was noticed the
most in the BL1 group (Table 18). Furthermore, the reason could be the nature of the
BL1 shade – it possesses less chroma than the other shades. The a* and b* values had
no consistency within the tested groups, and previous studies presented the same
finding110,111. Thus, further studies need to be conducted for a better evaluation.
According to the human perception of color, the color difference (∆E) is
noticed. Fifty percent of people visually notice color difference when ∆E is greater
than 1 ∆E unit58,83. However, under uncontrolled clinical conditions, minor
differences in color may be undetected, because average color differences below 3.7
are believed to be a match in the oral environment88,112. According to Douglas and
Brewer, thresholds for acceptability are at ∆E=1.7 in control environment113. In the
27
current study, by evaluating ∆E values of LT ND1 groups in A2 and C3 groups, ∆E
was less than 1.7. This indicates that the translucency had a smaller effect than the
ND1 background in the A2 and C3 groups in clinical conditions.
The study data demonstrated that there were significant differences between
the high translucency and the low translucency groups, and this finding was observed
in the three ceramic shades. The HT groups showed more ΔE than the LT groups,
suggesting that the microstructure of HT groups presented bigger lithium disilicate
crystals compared to LT groups. Thus, the light transmission rate is high, which
allows more light to pass through and reflect the background color (Fig. 20-22).
Limitations of the study:
Using one all-ceramic system with one thickness was one of the limitations of this
study. Furthermore, three ceramic shades and two background colors were used in the
study. Thus, more ceramic shades and background color variations need to be
considered. Based on our literature search, only a few studies addressed the optical
properties of lithium disilicate glass ceramic. Additionally, the literature was lacking
in studies that evaluated the effect of translucency. Thus, a comparison of the results
with previous research could not be performed. In addition, based on current
knowledge, a standard Lab* for all shades is not available.
Future study:
Using different backgrounds and utilizing different ceramic fabrication
techniques (i.e., the pressed technique) may provide us with more knowledge and
insight into the influence of translucency and background colors on ceramics. In
addition, measuring the color difference of low translucency A and C shade groups in
different thicknesses could provide valuable information. Future studies should also
consider the tooth form, as the light reflection will differ between the flat surface and
28
the convex or concave surfaces. The testing different thicknesses and different
opacities is also required. Furthermore, comparing the optical properties of
CAD/CAM lithium disilicate glass ceramic to different all-ceramic systems would be
beneficial in expanding our knowledge in this field.
29
Conclusions: Within the limitations of this study, the following conclusions can be drawn:
1. The translucency and background color significantly influenced lithium disilicate glass
ceramic color difference among BL1, A2, and C3 ceramic shades.
2. Changing the underlying color from lighter (ND1) to darker (ND4) resulted in increased
color difference (ΔE).
30
Table 1- Crystallization/Glazing cycle of lithium disilicate glass ceramic furnace (Programat P700).
Firing temperature (F)
Holding time (min)
Vacuum 1 Stage 1 (F) Stage 2 (F)
Vacuum 2 Stage 1 (F) Stage 2 (F)
Long-term cooling (F)
840 7:00 550 1022
820 1508
700
Stand-by temperature (F)
Closing time (min)
Heating rate (F/min)
Firing temperature (F)
Holding Temperature (min)
Heating rate (F/min)
403 6:00 90 820 0:10 30 Table 2- Lab values of reference groups of BL1 group, A2 group, and C3 group.
Reference Group L a b BL1 HT ND1 81.2 (.03) -0.4 (.08) 5.2 (.16) A2 HT ND1 74.3 (.07) 0.1 (.02) 17.1 (.14) C3 HT ND1 69.4 (.1) 1.4 (.3) 21.9(.1)
Table 3- Lab values and ΔE value of BL1 HT ND4 (mean and standard deviation).
BL1 HT ND4 L a b ΔE 1 73.74 3.84 12.94 11.59 2 73.96 3.7 12.43 11.05 3 73.83 3.67 12.02 10.86 4 73.92 3.59 12.11 10.83 5 73.75 3.6 12.98 11.51 6 73.87 3.7 12.82 11.37 7 73.29 3.24 12.21 11.21 8 73.81 3.71 12.94 11.49 9 73.74 3.8 12.92 11.56 10 73.74 3.84 12.94 11.59 Mean (SD) 73.76 (0.18) 3.66 (0.17) 12.63 (0.39) 11.30 (0.29)
31
Table 4- Lab values and ΔE value of BL1 LT ND1 (mean and standard deviation).
BL1 LT ND1 L a b ΔE 1 79.1 -0.69 8.84 4.21 2 79.28 -0.82 8.57 4.53 3 79.39 -0.82 9.3 4.51 4 79.22 -0.71 9.07 4.37 5 79.21 -0.72 9.01 4.32 6 79.18 -0.66 8.89 4.23 7 79.29 -0.79 8.83 4.13 8 78.81 -0.79 9.11 4.61 9 78.89 -0.63 9.14 4.59 10 79.15 -0.69 8.75 4.12 Mean (SD) 79.15 (0.17) -0.73 (0.06) 8.95 (0.21) 4.36 (0.18) Table 5-Lab values and ΔE value of BL1 LT ND4 (mean and standard deviation).
BL1 LT ND4 L a b ΔE 1 77.88 3.14 6.3 5.02 2 77.82 3.04 6.25 4.98 3 77.97 2.71 6.66 4.75 4 77.88 2.73 6.63 4.82 5 77.94 2.87 6.85 4.94 6 78.09 2.75 6.63 4.69 7 77.75 2.94 6.5 5.01 8 77.55 3 6.43 5.18 9 77.74 2.83 6.66 4.99 10 77.74 2.8 6.6 4.96 Mean (SD) 77.83 (0.15) 2.88 (0.14) 6.55 (0.18) 4.93 (0.14) Table 6- Lab values and ΔE value of A2 HT ND4 (mean and standard deviation).
A2 HT ND4 L a B ΔE 1 69.09 5.3 22.38 8.99 2 69.07 5.26 22.45 9.02 3 69.74 5.43 22.71 8.91 4 69.13 5.16 22.52 8.97 5 69.71 5.31 22.63 8.81 6 69.81 5.23 22.13 8.4 7 69.81 5.06 22.54 8.55 8 69.88 5.04 22.57 8.52 9 69.7 4.83 22.99 8.77 10 69.76 5.05 22.38 8.47 Mean (SD) 69.57 (0.33) 5.16 (0.17) 22.53 (0.22) 8.74 (0.23)
32
Table 7- Lab values and ΔE value of A2 LT ND1 (mean and standard deviation).
A2 LT ND1 L a b ΔE 1 74.97 0.98 17.67 1.16 2 74.91 0.97 17.55 1.07 3 74.88 1.07 17.63 1.16 4 74.08 0.96 17.9 1.11 5 74.8 1.12 17.79 1.24 6 74.02 1.08 17.84 1.17 7 74.04 1.04 18.18 1.37 8 74.05 1.11 18.21 1.43 9 74 1 18.3 1.45 10 74.06 1.09 17.9 1.2 Mean (SD) 74.38 (0.44) 1.04 (0.06) 17.89 (0.25) 1.23 (0.13)
Table 8- Lab values and ΔE value of A2 LT ND4 (mean and standard deviation).
A2 LT ND4 L a b ΔE 1 72.76 3.86 19.36 4.56 2 72.76 3.82 19.45 4.58 3 72.81 3.82 19.18 4.42 4 71.57 3.82 18.33 4.7 5 71.52 3.91 18.27 4.79 6 71.58 3.84 18.38 4.73 7 71.55 3.85 18.44 4.76 8 71.64 3.86 18.21 4.66 9 71.56 3.88 18.18 4.74 10 72.79 3.73 19.31 4.42 Mean (SD) 72.05 (0.62) 3.83 (0.04) 18.71 (0.53) 4.63 (0.13)
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Table 9- Lab values and ΔE value of C3 HT ND4 (mean and standard deviation).
C3 HT ND4 L a b ΔE 1 64.82 5.55 24.72 6.79 2 64.65 5.77 24.81 7.07 3 64.23 5.56 23.21 6.76 4 64.54 5.48 23.47 6.53 5 64.43 5.42 23.86 6.68 6 64.16 5.08 22.51 6.43 7 64.77 5.66 24.49 6.79 8 64.71 5.61 24.53 6.82 9 64.74 5.43 24.8 6.8 10 64.75 5.55 24.65 6.8 Mean (SD) 64.58 (0.23) 5.511 (0.18) 24.10 (0.8) 6.74 (0.17) Table 10- Lab values and ΔE value of C3 LT ND1 (mean and standard deviation).
C3 LT ND1 L a b ΔE 1 68.43 0.99 21.73 1.08 2 68.15 1.18 22.78 1.54 3 68.31 1.47 22.83 1.42 4 68.27 1.07 22.67 1.4 5 68.81 0.97 22.84 1.17 6 68.58 1.28 22.7 1.13 7 68.84 1.04 22.9 1.18 8 68.54 1.3 22.78 1.22 9 68.97 1.06 23.1 1.3 10 68.8 1.02 22.91 1.22 Mean (SD) 68.57 (0.27) 1.13 (0.16) 22.72 (0.36) 1.26 (0.14) Table 11- Lab values and ΔE value of C3 LT ND4 (mean and standard deviation).
C3 LT ND4 L a b ΔE 1 65.75 4.55 22.56 4.86 2 65.74 4.58 22.4 4.88 3 65.82 4.61 22.56 4.85 4 65.81 4.57 22.5 4.83 5 65.99 4.08 21.99 4.34 6 65.94 4.03 22.27 4.37 7 65.59 4.34 21.98 4.82 8 65.77 4.07 21.69 4.52 9 65.95 3.94 22.32 4.31 10 65.46 4.32 21.82 4.91 Mean (SD) 65.78 (0.16) 4.30 (0.26) 22.2 (0.31) 4.66 (0.25)
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Table 12- Kruskal-Wallis Test results and descriptive analysis for BL1 tested group (Std. Dev.=Standard Deviation + IQR= Interquartile Range)
Tested Group
ΔE
BL1 Mean Std. Dev. Median IQR P-value (Sig.)
HT ND4 11.30 0.29 11.43 0.56 < 0.001 LT ND1 4.36 0.18 4.34 0.36 < 0.001 LT ND4 4.93 0.14 4.97 0.21 < 0.001 Table 13- Kruskal-Wallis Test results and descriptive analysis for A2 tested group (Std. Dev.=Standard Deviation + IQR= Interquartile Range).
Tested Group
ΔE
A2 Mean Std. Dev. Median IQR P-value (Sig.)
HT ND4 8.74 0.23 8.79 0.47 < 0.001 LT ND1 1.23 0.13 1.18 0.24 < 0.001 LT ND4 4.63 0.13 4.68 0.22 < 0.001 Table 14- Kruskal-Wallis Test results and descriptive analysis for C3 tested group (Std. Dev.=Standard Deviation + IQR= Interquartile Range).
Tested Group
ΔE
C3 Mean Std. Dev. Median IQR P-value (Sig.)
HT ND4 6.74 0.17 6.79 0.16 < 0.001 LT ND1 1.26 0.14 1.22 0.24 < 0.001 LT ND4 4.66 0.25 4.82 0.50 < 0.001
Table 15- Post-hoc test (Mann-Whitney U Test) BL1 groups
BL1 Group Mean Rank (n=10)
P-Value (Sig.)
HT ND4 15.50 < 0.001 LT ND1 5.50 < 0.001 LT ND4 15.50 < 0.001 LT ND1 5.50 < 0.001 HT ND4 15.50 < 0.001 LT ND4 5.50 < 0.001
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Table 16- Post-hoc test (Mann-Whitney U Test) A2 groups
A2 Group Mean Rank (n=10)
P-Value (Sig.)
HT ND4 15.50 < 0.001 LT ND1 5.50 < 0.001 LT ND4 15.50 < 0.001 LT ND1 5.50 < 0.001 HT ND4 15.50 < 0.001 LT ND4 5.50 < 0.001
Table 17- Post-hoc test (Mann-Whitney U Test) C3 groups.
C3 Group Mean Rank (n=10)
P-Value (Sig.)
HT ND4 15.50 < 0.001 LT ND1 5.50 < 0.001 LT ND4 15.50 < 0.001 LT ND1 5.50 < 0.001 HT ND4 15.50 < 0.001 LT ND4 5.50 < 0.001
Table 18- Descriptive analysis for BL1 tested group (Std. Dev.=Standard Deviation + IQR= Interquartile Range) Dependent variable ΔL Δa Δb.
BL1 Group Mean Std. Dev. Median IQR ΔL
HT ND4 7.49 0.18 7.48 0.14 LT ND1 2.10 0.17 2.06 0.24 LT ND4 3.42 0.15 3.42 0.21
Δa HT ND4 3.20 0.25 3.28 0.31 LT ND1 0.33 0.06 .31 0.12 LT ND4 2.48 0.14 2.45 0.27
Δb HT ND4 7.43 0.39 7.67 0.76 LT ND1 3.75 0.21 3.75 0.31 LT ND4 1.35 0.18 1.41 0.26
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Table 19- Descriptive analysis for A2 tested group (Std. Dev.=Standard Deviation + IQR= Interquartile Range) Dependent variable ΔL Δa Δb.
A2 Group Mean Std. Dev. Median IQR ΔL
HT ND4 4.74 0.33 4.58 0.69 LT ND1 .39 0.16 0.30 0.32 LT ND4 2.25 0.62 2.70 1.21
Δa HT ND4 4.97 0.17 5.0 0.26 LT ND1 0.85 0.06 0.86 0.12 LT ND4 3.64 0.04 3.65 0.04
Δb HT ND4 5.39 0.22 5.39 0.27 LT ND1 0.75 0.25 0.73 0.53 LT ND4 1.57 0.53 1.27 1.07
Table 20 -Descriptive analyses for C3 tested group (Std. Dev.=Standard Deviation + IQR= Interquartile Range) Dependent variable ΔL Δa Δb.
C3 Group Mean Std. Dev. Median IQR ΔL
HT ND4 4.83 0.23 4.73 0.38 LT ND1 0.84 0.27 0.85 0.52 LT ND4 3.62 0.16 3.62 0.24
Δa HT ND4 4.09 0.18 4.13 0.20 LT ND1 0.29 0.14 0.35 0.27 LT ND4 2.88 0.26 2.91 0.51
Δb HT ND4 2.17 0.80 2.58 1.34 LT ND1 0.83 0.25 0.87 0.21 LT ND4 0.34 0.22 0.36 0.49
Table 21- Mann-Whitney U test results for HT and LT of BL1 groups. Dependent variable= ΔE
Tested Group
ΔE
BL1 Mean Rank (n=20)
P-Value (Sig.)
HT 15.50 < 0.001
LT 5.50 < 0.001
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Table 22- Mann-Whitney U test results for HT and LT of A2 groups. Dependent variable= ΔE
Tested Group
ΔE
A2 Mean Rank (n=20)
P-Value (Sig.)
HT 15.50 < 0.001
LT 5.50 < 0.001
Table 23- Mann-Whitney U test results for HT and LT of C3 groups. Dependent variable= ΔE
Tested Group
ΔE
C3 Mean Rank (n=20)
P-Value (Sig.)
HT 15.50 < 0.001
LT 5.50 < 0.001
38
Figure 1- Left: Partially crystallized (HT) CAD/CAM lithium disilicate glass ceramic. Right: Fully crystallized (HT) CAD/CAM lithium disilicate glass ceramic.
Figure 2- Left: Partially crystallized (LT) CAD/CAM lithium disilicate glass ceramic. Right: Fully crystallized (LT) CAD/CAM lithium disilicate glass ceramic.
Figure 3- Munsell color system.
Figure 4- CIE LAB L: Lightness, a: Red and Green, and b: Yellow and Blue.
39
Figure 5- E4D CAD/CAM machine and pre-crystallized lithium disilicate glass ceramic.
Figure 6- Isomet 1000 machine used to cut the cylinder into disks with 1.2 mm thickness.
Figure 7- BL1 shade groups.
BL1 shade Group
High Translucency
(HT)
HT ND1 (reference) HT ND4
Low Translucency
(LT)
LT ND1 LT ND4
40
Figure 8- A2 shade groups.
Figure 9- C3 shade groups.
A2 shade Group
High Translucency
(HT)
HT ND1 (reference) HT ND4
Low Translucency
(LT)
LT ND1 LT ND4
C3 shade Group
High Translucency
(HT)
HT ND1 (reference) HT ND4
Low Translucency
(LT)
LT ND1 LT ND4
41
Figure 10- Left: Pre-crystallized A2 LT lithium disilicate glass ceramic specimen. Middle: Programat P700 furnace. Right: Crystalized A2 LT lithium disilicate glass ceramic specimen.
Figure 11- IPS e.max Ceram Glaze Paste.
Figure 12- Digital caliper (Dentaguage 1) confirming the specimen thickness 1.2mm.
Figure 13- ND1 and ND4 shade IPS Natural die material.
42
Figure 14 Left: ND4 background with specimen holder. Right: Specimen, ND1 and specimen holder in the dark box.
Figure 15- Spectrophometer (Crystaleye) with custom-positioning jig and dark box.
Figure 16- Left: ΔE* and Lab* of the target (specimen) and the reference (control group) of group 7 (A2 LT ND1). Right: 2 mm in diameter positioned over the middle region of the specimen.
43
Figure 17- Boxplot illustration of the results of BL1 groups ΔE*.
Figure 18- Boxplot illustration of the results of A2 groups ΔE*.
Figure 19- Boxplot illustration of the results of C3 groups ΔE*.
44
Figure 20- Mann-Whitney U test results for HT and LT of BL1 groups. Dependent variable= ΔE
Figure 21- Mann-Whitney U test results for HT and LT of A2 groups. Dependent variable= ΔE
Figure 22- Mann-Whitney U test results for HT and LT of C3 groups. Dependent variable= ΔE
45
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