REV.CHIM.(Bucharest) 69No. 2 2018 http:www.revistadechimie.ro 463Color Changes and Stainability of Complete Dentures Manufactured [614786]

REV.CHIM.(Bucharest) ♦69♦No. 2 ♦2018 http://www.revistadechimie.ro 463Color Changes and Stainability of Complete Dentures Manufactured
Using PMMA-TiO2 Nanocomposite and 3D Printing Technology – one
Year Evaluation
CORINA MARILENA CRISTACHE1, LUMINITA OANCEA2*, ANDREEA CRISTIANA DIDILESCU2, MIHAI BURLIBASA1,
EUGENIA EFTIMIE TOTU3*
1University of Medicine and Pharmacy Carol Davila, Faculty of Midwifery and Medical Assisting (FMAM), Department of Dental
Techniques, 8 Eroilor Sanitari Blvd., 050474, Bucharest, Romania
2 University of Medicine and Pharmacy Carol Davila, Faculty of Dental Medicine, 37 Dionisie Lupu Str., 020021, Bucharest,
Romania
3 University Politehnica of Bucharest, Faculty of Applied Chemistry and Material Science,1-5 Polizu Str, 11061 Bucharest, Romania
Color stability through wear and time, one of the most important esthetic factor, influencing patient’s
acceptance of an acrylic removable prosthesis, was investigated in vivo in a limited number of studies and
solely on prefabricated denture teeth.The aims of the present paper were to assess the color changes
occurred with the commercially available PMMA for 3D printing technique, improved by doping with titania
nanoparticles, and to evaluate the color stability and stainability of the PMMA with 0.4%TiO2 denture teeth,
on denture wearers, over a period of one year. Ten patients rehabilitated with removable complete dentures
or implant retained overdentures manufactured using a new nanopolymer, PMMA doped with 0.4% by
weight of TiO2 as denture base and artificial teeth material and Digital Light Projection (3D printing)
technology, were evaluated in the present study. The color of each denture was assessed with a
spectrophotometer, before and after use, in CIE L*a*b* system. The before measurement was made on the
positive control – PMMA with nanoTiO2 (PC), while the after was measured the color of the tooth after one
year complete denture use. Negative control – PMMA without nanoTiO2 (NC) was also compared to PC and
the following parameters were registered: V alue (L), Chroma (C), Hue (H), redness/greenness (a) and
yellowness/blueness (b). A color difference ( ∆E) between 1.63 and 5.24 was measured for all patients,
while for the NC ∆E was triple (17.65). The highest ∆E value for all denture wearers with TiO2 nanofiler
inclusions was below the maximum acceptability threshold, in accordance to the patient’s subjective
evaluation, who were unable to identify the color change.
Keywords: nanocomposite PMMA – TiO2, complete denture, color change, spectrophotometry,
acrylic bondings
Color, one of the most important esthetic parameter in
restorative dentistry influences patient’s acceptance of
treatment. The restorative materials should reproduce, asaccurate as possible, the shades of natural teeth and
possess color stability in the oral cavity, over a long period
of time [1].
Polymer poly(methyl methacrylate) (PMMA), the most
popular material for partial and complete denture
manufacturing in daily practice, has several drawbacksmainly due to a great color instability caused by both
intrinsic and extrinsic factors [2]. The intrinsic factors are
mostly related to the physical and chemical changes inthe matrix of the material itself [3,4], the type of
polymerization procedures as well as the contained
inorganic materials, such as amorphous silica and glassfiller [1] and are due to the oxidation of unreacted pendant
methacrylate or chemical bonding breakdown. The
extrinsic factors are related to a process of absorption andadsorption of liquids dependent upon oral environmental
conditions. The absorption is a slowly process over a period
of time and is due primarily to the polar properties of theresin molecules, leading to a staining of the material.
Changes in color can be measured using clinical
methods but instruments such as spectrophotometers,colorimeters and spectroradiometers, can help
overcoming the shortcomings of visual method by bringing
accuracy and facilitating color matching, communicationand reproduction. Instrumental devices reduces errors and
* email: lumioancea@yahoo.com; eugenia_totu@yahoo.com All authors have equally contributed to the manuscrip t and they
should be reg arded as main authorsare more accurate when compared to measurement by
eye, being a valuable tool in shade verification (quality
control) [5].
Despite of a lower accuracy, the most commonly used
shade-matching method is the visual method. Vitapan
Classical (Vita Zahnfabrik, Bad Sackingen, Germany), withdistinctive shades empiric-based organized, is probably the
most popular shade guide in dental practice. The same
manufacturer has also an evidence-based shade guide,Vitapan 3D-Master, described according to the Munsell
color space in terms of Hue, V alue, and Chroma [6].
The perception of tooth color is a complex phenomenon
and can be influenced by a number of factors, including
the type of incident light, the reflection and absorption of
light by the tooth, the adaptation state of the observer andthe context in which the tooth is viewed, leading to
inconsistencies and bias [7].
Spectrophotometers, measuring the amount of light
energy reflected from an object at 1-25 nm intervals along
the visible spectrum, are amongst the most accurate,
useful and flexible instruments for color matching indentistry [8].
A spectrophotometer contains a source of optical
radiation, means of dispersing light, an optical system formeasuring, a detector and means of converting light
obtained to a signal that can be analyzed and transformed
into a form equivalent to a dental shade guide.

http://www.revistadechimie.ro REV.CHIM.(Bucharest) ♦69♦No. 2 ♦2018 464Paul and co-workers [9], found that spectrophotometers
offered a 33% increase in accuracy and a more objective
match in 93.3% of cases when compared with observationsby the human eye or conventional techniques.
Spectrophotometers precisely measures color from
reflectance or transmittance data [5] and have beenextensively used in dental research and applications [10],
including the quantification of color change caused by
processing dental materials [1,11] descriptions of coverageerror of dental shade guides [12], color accuracy and
precision [13], color perceptibility and acceptability [14]
and translucency parameter [15].
To objectively calculate color changes, the CIE-L*a*b*
system, established by the Commission Internationale de
l’Eclairage (CIE), can be use [16]. The CIE L*a*b* systemspecifies color perceptions in terms of a 3-dimensional
space by comparing the color of the tooth surface with the
color of the corresponding control groups throughwavelength vs reflection. The
L axis represents luminosity
and extends from 0 (black) to 100 (perfect white). The a
coordinate represents the amount of red (positive values)
and green (negative values), while the b coordinate
represents the amount of yellow (positive values) and blue
(negative values) [17]. The coordinates a and b coexist in
the same plane of this 3-dimensional space.
To improve the characteristics of the acrylic composite
for complete denture manufacturing, functionalized TiO2nanoparticles have been added, demonstrating better
mechanical, antibacterial characteristics and also high
physicochemical stability [18–20]. In order to increase theaccuracy of the final restoration and also to reduce the
unreacted methacrylate, 3D printing technology with an
improved nanocomposite was used lately in completedenture manufacturing [21,22].
The gloss and color changes after adding 0.4% by weight
TiO
2 nanoparticles and the stainability of the newly obtained
nanocomposite have not been yet investigated.
Therefore, the aims of the present study were:-to assess the color changes occurred after adding 0.4%
TiO
2 nanoparticles in the commercially available PMMA
for stereolithographic technique;
-to evaluate the color stability and stainability of the
PMMA with 0.4%TiO2 denture teeth on denture wearers
over a period of one year.
Experimental part
Ten patients out of the thirty-five enrolled in the cohort
study registered with ClinicalTrials.gov Identifier:
NCT02911038 and approved by the Romanian Research
Bioethical Committee (# 98/2016) have been selected fordenture color stability evaluation.
All the patients were rehabilitated with removable
complete dentures or implant retained overdenturesmanufactured using 3D printing technology. An improved
nano-polymer, PMMA doped with 0.4% by weight of TiO
2was used as denture base and artificial teeth material and
each complete denture/overdenture was fabricated by
Digital Light Projection using the EnvisonTEC Perfactory
3D printer (Gladbeck, Germany) [18]. Dentures wereindividually designed in 3Shape software (Copenhagen,
Denmark) and processed by applying specific
manufacturing stages detailed described elsewhere [18].The materials used for preparing the nanocomposite have
been: poly (methyl methacrylate) polymer mixture used
for 3D printing procedure (eDent 100, Envision TEC GmbH,Germany) which contains: poly (methyl methacrylate),
methyl methacrylate, benzoyl peroxide, silicium and
aluminium oxides; TiO
2 (anatase, Aldrich, Germany) whichwas functionalized according to a previous presented
procedure [19]. The surface morphology of the
nanocomposites obtained have been investigated withscanning electron microscopy (SEM) technique, using an
Oxford Instruments equipment.
The following criteria have been applied for the inclusion
in the present study:
-color A2, according to Vitapan Classical shade guide,
was choose as teeth shade for denture manufacturing;
-patient should be at least rehabilitate with maxillary
denture/overdenture;
-complete maxillary dentures were required to be in
good condition, with intact dental anatomy of the maxillary
central incisors and with no cracks or fractures in the
denture base;
-at least one year of uninterrupted wearing of 3D printed
complete dentures.
All the patients signed a consent form and data were
assessed during the one year follow-up scheduled visit.
A portable spectrophotometer (Easyshade VITA
Zahnfabrik, H. Rauter GmbH, Germany) was used toevaluate the denture teeth color.
Prior to the evaluation, biofilm removal and disinfection
of the dentures was performed applying the followingprotocol:
-immersion for 15 min in a container containing an
effervescent tablet, Corega (Glaxo SmithKline, Romania)dissolved in 250 mL of warm water and rinsing afterward
in running water;
-washing with neutral soap, thoroughly scrubbing with
it by hand friction for 30 s and rinsing in running water [23];
-disinfecting with chlorhexidine 2% (Gluco-Chex,
Cerkamed, Poland) by immersing in the solution for 10min and rinsing in running water.
This procedure is described to be effective only on the
biofilm and do not remove the stains that occurred on the
acrylic teeth [24].
Measurement protocol
Calibration of the measuring device:
To evaluate the repeatability and the reliability of the
measuring instrument, 12 set of measurement of the same
tooth have been performed. After each set of
measurements the device was turn off, restarted andrecalibrated. A good agreement among all color
components
L, a, and b (repeatability >.91 and reliability
>.89) have been obtained, in agreement to other studies[1,25,26].
The Vita Easyshade spectrophotometer has an
embedded fiber optic light and can record the tooth shadeunder any light condition. However, to avoid bias, color was
evaluated extra orally with a black background, standard
ambient lighting and temperature on the middle third ofthe buccal surface of the maxillary right central incisor for
all dentures. All measurements were made by a single
trained operator. Before any measurement, the device wascalibrated on its own white ceramic block, according to
the manufacturer instructions. The single tooth option was
selected from the menu and probe tip was securedperpendicular to tooth surface flushing the whole surface.
After the initiation of the measurement procedure, tip was
kept stable until the long beep and the tooth shade foreach complete denture was recorded with
l, a, b , values.
To evaluate the color stability, e-DENT 100 sample, with
no addition of TiO2 and polymerized with EnvisonTEC
Perfactory 3D printer, using the protocol described by the
manufacturer, was considered as negative control (NC).
The positive control (PC) was a complete maxillary denture

REV.CHIM.(Bucharest) ♦69♦No. 2 ♦2018 http://www.revistadechimie.ro 465obtained from e-DENT 100 with 0.4% nanoTiO2 inclusions.
Both positive and negative control have been processed
12 month ahead of the measuring date and stored underideal conditions (in a dark, dry place, at room temperature).
The positive control denture teeth as well as the negative
control and all the ten assessed dentures were A2,according to the manufacturer described parameters [27].
Differences between colors were calculated in the CIE
L*a*b* system using the color distance between thecoordinates of two stimuli with the following Euclidean
formula [17]:
color match (A2), when compared to the unmodified
PMMA (NC). On the screen of the spectrophotometer (fig.
1b), the difference between the color space positioning of
the PC tooth tristimulus values and the corresponding shadetab, could be seen. The bar graphs show that the tooth is
lighter, slightly higher chromatic, and yellowish than A2.
The overall evaluation was
Fair, meaning that an expert at
shade matching may see a noticeable but acceptable
difference between the restoration and the target shade to
which it has been verified. NC’s overall evaluation wasAdjust, meaning that a noticeable difference could be
observed between the restoration and the target shade to
which it has been verified, an important
mismatch appears
in hue.
The mean values of the parameters measured (V alue,
Chroma, Hue, redness/greenness and yellowness/blueness) and also the overall estimated color according
to Vitapan Classical and Vitapan 3D-Master shade guides,
for all the analyzed samples, are displayed in table 2.
As could be observed in table 2, none of the analyzed
samples had the original color (A2). The grater color
difference appears at the negative control (overall color
Fig. 1. Vita Easyshade spectrophotometer was used to perform
measurements: a – A2 shade tab from the Vitapan Classical shade
guide was verified prior to any measurement. b-Positive control
(PC) was compared to A2 shade using the option Restoration. The
differences of V alue, Chroma and Hue are represented and Fair is
the overall color accuracy. c- The screen shows the positioning of
the tooth’s color measured with respect to the color space
coordinates for the A2 shade. ∆E is the color difference calculated
using CIE L*a*b* system and ∆ELC is calculated excluding H.
d -Color coordinates in CIE L*a*b* system (L, a, b, C, H) for the
measured specimen using the Tooth single option are displayed
on the screen
∆E = [(∆L)2 + (∆a)2 + (∆b)2]1/2 (1)
Sequence of measurements:
-Positive and negative control were compared to A2
predefined color Vitapan Classical and the threshold valueÄE was registered (fig. 1 a, b and c).
-The color of each denture was evaluated before and
after use. The
before measurement was made on the
positive control (PC), while the after ” measurement was
made on the tooth (central incisor) after one year complete
denture use. Negative control (NC) was also compared topositive control (PC). Three consecutive measurements
were performed for each specimen and the following
parameters were registered: V alue (L), Chroma (C), Hue(H), redness/greenness (a) and yellowness/blueness (b)
(fig. 1 d ).
A questionnaire regarding dentures color assessment
was used to subjectively evaluate color change in the
artificial teeth. The subjective data collected were
compared to the objective data obtained from thespectrophotometric analysis.
Results and discussions
The color assessment of the negative control (e-DENT
100 3D printed as indicated by the manufacturer) and the
positive control (e-DENT 100 with nano-TiO
2 unused
denture) evaluated with the option Restoration of the
spectrophotometer are indicated in table 1.
As could be observed in figure 1 b and table 1, the
modified material PMMA – nano TiO2 (PC) had a better
Table 1
COMPARISON OF POSITIVE AND NEGATIVE CONTROL TO SHADE A2
PC=positive control, NC=negative control, ∆L=differences in V alue
between specimen and A2, ÄC=differences in Chroma between
specimen and A2, ∆H=differences in Hue between specimen and
A2, ∆E= color difference between specimen and A2 calculated as
Euclidean distance between the two points (colors) in the three-
dimensional space, ∆ELC= ∆E calculated excluding H.
PC=positive control, NC=negative controlTable 2
MEAN VALUES OF THE MEASURED PARAMETERS

http://www.revistadechimie.ro REV.CHIM.(Bucharest) ♦69♦No. 2 ♦2018 466All performed studies evaluated denture prefabricated
teeth and to our knowledge, this is the first study evaluating
3D printed denture teeth color changes.
However, in all cases evaluated for a one year period,
except for the negative control, without nanofiller
inclusions, the highest ∆E value was 5.24 (table 3),
significant lower than the maximum reported by Barão
and co-workers [1] on dentures manufactured with
commercially available prefabricated teeth.
The important consideration is the range of color
changes and if the eye perceives these changes. In the CIE
L*a*b*color space, this range of color change is shown by∆E, which is the algebraic difference between the two
colors and almost all the studies have used this system
after its introduction [31], leading to measurements in anobjective way.
Several evaluation of visual judgments of dental
restorations reported a 50:50% CIE L*a*b* perceptibilitythreshold to human eye ( ∆E) of 1.0 [32], 1.8 [10] or 2.6
[33], different than corresponding 50:50% acceptability
threshold ( ∆E) of 3.5 [10] or 5.5 [33]. However, the highest
∆E value (5.24) for the specimens with TiO
2 nano-filler
inclusions is below the maximum acceptability threshold,
in accordance to the patient’s subjective evaluation. Thesubjective data obtained from all the participants asked
whether they had noticed any changes in the coloring of
the artificial teeth in their dentures were negative (none ofthe subjects noticed color changes) and, therefore, could
not be correlated to the objective data obtained from the
spectrophotometric analysis.
For the NC (PMMA without nano-inclusions), ∆E was
17.65 (table 3), when compared to PC (PMMA with nano-
inclusions) and 5.7 when compared to A2 color (table 1),greater than the human acceptability threshold.
The ability of denture teeth to remain color stable through
wear and time is of critical importance and most of thetime measures the longevity of patient acceptance of a
removable prosthesis.
The etiology of artificial tooth discoloration is
multifactorial. Some of the most important factors that
contribute to intrinsic and extrinsic staining are: denture
wear, lack of patient maintenance, the effect of compositestructure and chemical characteristics, exposure to stains,
and many others [30,34].
Poly (methyl methacrylate) slowly absorb water in time,
which is a property attributable to the polar nature of the
resin molecules. Color changes of materials might be
Table 3
STUDY VARIABLE RELATED TO COLOR CHANGE
NC=negative control, ∆L=differences in V alue between
specimen and PC, ∆C=differences in Chroma between
specimen and PC, ∆H=differences in Hue between specimen
and PC, ∆E= color difference between specimen and PC
calculated with the Euclidean formula (1)
Fig. 2 Differences in V alue ( ∆L), Chroma ( ∆C) and Hue ( ∆H)
between the specimen analyzed, negative control (NC) and
positive control (PC)
Fig 3 Differences in color on a axis (red-green) between the
specimen analyzed, negative control (NC) and positive control (PC)
Fig. 4 Differences in color on b axis (yellow-blue) between the
specimen analyzed, negative control (NC) and positive control
(PC)
turns to B3), mainly due to a lower chromatic and a higher
color tendency to greenish. However, after one year, all of
the samples with nanofillers inclusions registered as maincolor A3.5. The reason of this color mismatch could be
explained by the optical tooth properties, different of those
of acrylic resin. Light scattering and absorption withinenamel and dentin give rise to the intrinsic color of the
tooth, enamel being relatively translucent confers dentin a
major role in the overall color [17,28].
One of the aims of the present study was to evaluate
the color stability and stainability of the PMMA with
0.4%TiO
2 denture teeth on denture wearers over a period
of one year, therefore, each specimen, including negative
control (PMMA without nanofiller) was compared to
positive control (un-wear denture teeth). The studyvariables related to the color change for all enrolled
participants and also for the negative control, compared to
the positive control are listed in table 3 and represented infigure 2-4.
Acrylic resins undergo color changes in the oral cavity.
Alteration in the color of acrylic resin denture teeth weredescribed in several in vitro and in vivo studies [1,16,29].
The highest ∆E values observed in these studies varied
from 1.82 to7.94 for the in vitro experiments [16,29,30]and up to 11.03 over a period between 6 months and 4
years, in clinical environment [1].

REV.CHIM.(Bucharest) ♦69♦No. 2 ♦2018 http://www.revistadechimie.ro 467mediated by an accelerated oxidation or by penetration of
colored solutions.
The sorption of various water soluble metabolites with
ionizable groups onto the polymeric dental device surface
could be a reason of color change.
The poly (methyl methacrylate) used as polymeric
matrix for the dental devices under study present a certain
degree of hydrophilicity due to solutions diffusion through
intermolecular space. It is known that residual monomerpresent in the polymerized compound might increase the
water sorption [35]. However, due to the technology
applied for the denture manufacturing, namely 3D printing,the amount of residual monomer is reduced, therefore its
influence is not significant. On the other hand, the
polymeric web chains are characteristic for syndiotacticPMMA, therefore it is expected that the pendent groups of
PMMA (fig.5) would not allow the molecular chains to get
Fig. 5. Poly (methyl
methacrylate) -pendent
groups
close each other and in consequence would hinder the
adjacent chemical groups.
As consequence, within amorphous PMMA the
polymeric chains almost do not slip between them. Beingrigid, the inner PMMA structure also embraces void areas
where adsorbed solutions/soluble metabolites could be
accommodated. These diffusing metabolites could causethe color change of the PMMA made dental devices. In the
meantime, it is important to underline that through the 3D
printing technology the PMMA crosslinking is almostcomplete and thus, a lower level of fluids sorption would
be expected. In such case, one should consider other
possible reasons for color change, as well.
The used initial polymeric matrix contains partially
polymerized methyl methacrylate (under 25% conversion)
and poly (methyl methacrylate) and a peroxide (benzoylperoxide, fig. 6) as photoinducer of the complete
polymerization according to the scheme 1 where a general
scheme for PMMA photo-polymerization is introduced.
It is known that peroxides are photoinitiators for different
radicalic reactions in organic compounds or composites.
For instance, the benzoyl peroxide absorbs light of 378 nmwhich causes their excitation and further cleavage into
radicals as it splits at O-O bond generating two radicals.
Thus, it might initiate in the system under discussion, a
free radical polymerization up to a complete reticulated
polymer following the known stages of initiation,
propagation and termination, when the poly(methyl
Fig. 6. Benzoyl peroxide
cleavage under UV radiation
(λ = 378nm)methacrylate) radicals undergo mainly disproportionation,
see equation 2 [36].
Regarding the initiation of the free radical polymerization
mechanism, it is very likely that no matter the initial radicalformation, it is possible to be produced hydroperoxydes
following a reaction with oxygen [37]. Such hydro-
peroxides are photolabile and are decomposing generatingradicals which could subtract hydrogen atoms from poly
(methyl methacrylate) and in this way to lead to
photooxidation of the polymer. It is then possible a chemicalchange of color of the polymeric matrix due to its oxidation.
Much more, if there is any unpolymerized methyl
methacrylate than oxidation at the pendant methacrylategroups level will occur. The consequence would be a
change in color.
The material used for polymeric matrix, e-Dent 100 also
contains SiO
2 filler. Such filler could adsorb liquid
metabolites onto the surface. An important amount of
liquids adsorbed hydrolyzes the silica filler. This might causethe appearance of micro crevices and cracks into the
polymeric matrix (fig. 7.a).
The existence of developed defects as presented could
be the reason for stain penetration and significant color
change of the dental devices studied. Another reason for
color change of the nanocomposite after 3D printingprocedure could be assigned to the possible porosities of
the surface [38,39]. However, as our study proved, the 3D
printed dentures using the PMMA – 0.4%TiO
2 nano-
composite was less affected by color change compared
with the 3D printed dentures manufactured from PMMA.
This improved behavior could be due to a more compact
Scheme 1. Photopolymerization mechanism.
BPO (benzoyl peroxide) photosensitizer;
MMA (methylmethacrylate) monomer;
R = free radical or polymer fragmentFig. 7 SEM images of the 3D printed dental devices using different
polymeric matrix. a. PMMA; b. nanocomposite PMMA- TiO2
(2)

http://www.revistadechimie.ro REV.CHIM.(Bucharest) ♦69♦No. 2 ♦2018 468structure of the printing material when no crevices are
present (fig. 7b).
Conclusions
To our knowlege, this is the first study assesssing the
color changes and stainability of the 3D printed dentureteeth in vivo.
Despite the color instability ( ∆E) between 1.63 and 5.24,
measured in all denture wearers with TiO
2 nanofiler
inclusions, the color change over one year period was
below the maximum acceptability threshold, in
accordance to the patient’s subjective evaluation, who wereunable to identify the color change. However, for the NC,
without nano-titania inclusions, ÄE was triple (17.65),
leading to the conclusion that color stability increases withdoping the polymer for dental usage with 0.4% by weight
TiO
2 nanoparticles.
Acknowledgements: The authors wish to thank Dr. Mihai Andrei for
assisting in data collection. Also, warm thanks are directed towards
Ozlem Tavukcuoglu from Department of Biomedical Engineering,
Istanbul Gelisim University, Ýstanbul, Turkey, and Ismail Agir, Mustafa
Nigde, Rýdvan Yildirim from Department of Bioengineering, Yildiz
Technical University, Istanbul, Turkey for running SEM analysis.This
work was supported by a grant of the Romanian National Authority for
Scientific Research and Innovation, CCDI- UEFISCDI, project number
30/2016, within PNCDI III. In addition, this research was partially
supported by the scýentific and technological research council of
Turkey, TUBITAK, project number 9150165, MANUNET ER A-NET.
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Manuscript received: 23.11.2017