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Journal of Oral Science, Vol. 43, No. 3, 193-205, 2001

Color stability of different denture teeth materials:

an in vitro study

Lamia Mutlu-Sagesen§, Gulfem Ergun§, Yalgln Ozkan† and Bulent Bek§

§Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey

†Department of Pharmaceutical Technology, Gulhane Military Medical Academy,

Ankara, Turkey

(Received l 3 March and accepted 6 September 2001)

Abstract: The aim of this in vitro study was to

compare the color stability of commercially available

porcelain, reinforced acrylic, and conventional acrylic

denture teeth materials used in removable prostheses.

Two brands of porcelain (Unilux-Enta Lactona-Holland

and Vivoperl-Ivoclar-Liechtenstein), 2 brands of

reinforced acrylic (Optodent-Bayer-Germany and

Ivolek-Ivoclar-Liechtenstein), and 2 brands of

conventional acrylic (Isodent-Guney Dis Deposu-Turkey

and Samed-Turkey), were made, for a total of 6 different

denture teeth groups. Denture teeth were subjected to

3 staining solutions (filtered coffee, tea, and cola) and

distilled water. From each group of denture teeth, 4 sets

of maxillary anterior denture teeth were immersed in

each of the 4 solutions. The color values of denture teeth

were measured colorimetrically with the Gardner XL

20 Tristimulus Colorimeter (Gardner Lab. Inc.,

Bethesda, Maryland, USA). Color changes were

characterized in the CIEL*a*b* color space. Color

change values were determined after 1 day, 1 week, 2

weeks, and 4 weeks. The color difference values were

calculated and then evaluated by two-way ANOVA

statistically. The filtered coffee solution was found to

be more chromogenic than the other 2 staining solutions,

while porcelain denture teeth materials were more

color stable. Assuming the color change of ĢE* < 1.0

as a discernible limit and ĢE* = 3.3 as an acceptable

value, the filtered coffee, tea, and cola had slight staining effects on all 6 groups of denture teeth. (J. Oral Sci. 43, 193-205, 2001)

Key words : color stability; acrylic denture teeth;

porcelain denture teeth.

Introduction

Increased demand for aesthetic dentistry has been coupled with a rapid rate of the development of new

denture materials. In partially or completely edentulous

patients, function, phonetics and aesthetics can be provided with removable dentures. These kind of dentures should

also improve the psychological state of edentulous patients

(1). In removable dentures, aesthetics is best obtained by

denture teeth, either porcelain or acrylic resin. While manufacturing a removable denture, the denture teeth can

be selected according to the demands and the financial state of the patient. Rapid development of the dental material

industry could represent many available denture teeth alternatives to dentists. These alternatives might be

porcelain, reinforced acrylic, conventional acrylic denture teeth or combinations of these 3 main groups (2).

Conventional acrylic teeth have been customarily employed as artificial denture teeth attached to a denture

base, because it is relatively easy to arrange such teeth and adjust their occlusion. Failure or success of any aesthetic

approach depends on the color stability of the material. Color stability, the property of a material to retain its color

over a period of time and in a specified environment, is an important physical property of many materials used in dentistry (3). Conventional acrylic teeth are more

Correspondence to Dr. Lamia Mutlu-Sagesen, 35. sokak 7/13 06500 Bahcelievler, Ankara, Turkey Tel: +90-312-215-08-68 E-mail address: sagesene@isbank.net.tr

This study was presented as an oral presentation at the TDA and FDI Joint Congress, Istanbul-Turkey, 21-26/6/1999.

194

susceptible to abrasion and pigments than porcelain teeth

and tend to become gradually discolored by pigments

contained in drinks with long-term use, causing an aesthetic

failure.

To overcome these aesthetic and abrasive shortcomings

of conventional acrylic denture teeth, reinforced acrylic

denture teeth and porcelain denture teeth have been

developed and are now widely employed clinically. Though

reinforced acrylic denture teeth constructed of hard plastic

are still susceptible to pigments, dentists have observed

the aesthetics of removable partial dentures made with such

acrylic teeth gradually become impaired in many patients.

While a number of authors have reported on the color

changes and staining of acrylic denture teeth materials under

Table 1 Denture teeth used in this study

Table 2 Solutions used in this study

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conditions of accelerated aging and exposure to oral fluids

(1,2,4,5), the literature contains few references to the color

stability of both acrylic and porcelain denture teeth materials

(2).

The purpose of this in vitro study was to analyze the

discoloration of commercially available porcelain,

reinforced and conventional acrylic denture teeth used in

removable dentures when exposed to common dietary

fluids using colorimetric techniques.

Materials and Methods

In this study, 2 brands of porcelain [Unilux (UL) and

Vivoperl (VP)], 2 brands of reinforced acrylic [Optodent

(OD) and Ivolek (IL)] , and 2 brands of acrylic denture teeth

[Isodent (ID) and Ultraplus (UP)] were selected. The trade

names, manufacturers, types, shades, forms, and codes of

the denture teeth used in this study are given in Table 1.

Denture teeth were subjected to 3 staining solutions

[filtered coffee (FC), tea (T), cola (C)], and distilled water

(DW) as detailed in Table 2.

From each brand of denture teeth, 4 sets of maxillary

anterior denture teeth were used in this study. Each denture

tooth was bonded on to the modelling pink wax plate

having dimensions of 15 mm •~ 15 mm from the palatinal

side. From each brand of denture teeth, a set of maxillary

anterior denture teeth was prepared for each of the 4

different solutions. Each specimen was suspended in the

solutions with dental floss, so that it did not contact the

container or other specimens. Before immersing each set

of denture teeth into the solutions, they were stored in

distilled water at 37 •} 1•Ž for 24 hours. Then, a set of

maxillary anterior denture teeth was immersed in each of

4 different solutions at 37•}1•Ž for a month in a dark room.

Color measurements were taken just prior to immersion

(Control) and after 1 day, 1 week, 2 weeks, and 4 weeks.

Solutions were freshened at the beginning of every week.

In addition, to reduce the precipitation of particles within

solutions, solutions were stirred twice a day.

Before each measurement, denture teeth were

ultrasonically cleaned(Ultrasonic Cleaner, HS Health-

Sonics CorPoration) in non-ionic multipurpose ultrasonic

solution(HS Health-Sonics Corporation)for 5 min, and

dried using a tissue paper. This cleaning procedure removed

the stains that were not strongly adhered to the samples'

surfaces. Thus, overall color change as a result of entire

matrix discoloration and stain adsorption on the samples'

surfaces was measured.

These denture teeth were tested by the XL-20 Tristimulus

Colorimeter(Gardner Lab. Inc., Bethesda, Maryland,

USA)immediately after removal from the distilled water.

The testing procedure was done with the evaluation of the

light directed and reflected from the tooth surface with a

constant angle. Using a tungsten-halogen lamp as a light

source, all measurements were done according to the CIE

standards(6), from incisal, middle, and cervical thirds of

these denture teeth. These 3 regions were measured in every

tooth.

Before each measurement session, the colorimeter was

calibrated according to the manufacturer's instructions by

using the supplied white calibration standard.

Values were recorded using the Commision

Internationale d'Eclairage(CIE)L*a*b*color system(6).

The CIEL*a*b*system is an approximately uniform color

space with coordinates for lightness, i.e.,white-black(L*),

redness-greenness(a*), and yellowness-blueness(b*)(Fig.

1).The L*, a*, and b*values of each sample(tooth)before

immersion(Control)and after immersion at each specified

time interval(1D,1W,2W, and 4W)were measured 3 times

from insizal, middle, and cervical thirds of these denture

teeth by the same person(5,7,8). The mean values ofΔL*,

Δa*,andΔb*after 3 measurements were automatically

calculated by the colorimeter and recorded.Color difference

(ΔE*)was calculated from the meanΔL*,Δa*, andΔb*

values for each tooth with the following formula(6),

where ĢL*, Ģa*, and Ģb* are the differences in L*, a*,

Fig. 1 Coordinates of the color space: L* (white-black), a* (red-green), and b* (yellow-blue).

196

and b* values before and after immersion at each time

interval.

The mean ĢE* value calculated from the 3 measurements

of each tooth were obtained for each solution and each

group of denture teeth.

Average color changes after 1 day, 1 week, 2 weeks, and

4 weeks were also determined. The mean and the standard

deviation values were calculated for each set of teeth and

each solution.

The color system employed in this analysis was L*, a*,

and b* of CIE1986L*a*b* color difference (ĢE*) between

the value at each observation point and the control value

(2). The value of color difference was multiplied by 0.92

in order to get the value of the National Bureau of Standards

(NBS) (2,9,10). Based on the NBS rating system (2,10), which is the way a color change is evaluated by the human eye (Table 3), color changes of the 6 artificial tooth types in the 4 different solutions were determined.

According to the design of the experiment, the most

appropriate statistical analysis is a repeated-measure analysis of variance (ANOVA), with 2 between-units factors (6 materials and 4 solutions) and 1 within-units

factor (5 measurement times). The null hypothesis was that the main effects of the factors involved (solution, material,

and time) as well as possible interactions between them, were equal to zero. Further analyses were carried out at

particular times and for particular materials and solutions. Two-way ANOVA at each immersion time (1 day, 1 week, 2 weeks, and 4 weeks) was used to test the significance

of the factors involved (denture teeth and solutions). These

analyses were performed with the program SPSS for Windows (7.5.1).

Results Table 4 shows the color conditions of 6 denture tooth

groups observed before immersion.

Table 3 NBS Rating

Table 4 Color of denture teeth before immersion

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Table 5 Color of denture teeth after immersion in filtered coffee

Table 6 Color of denture teeth after immersion in tea

Table 7 Color of denture teeth after immersion in cola

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The coloring conditions of 6 denture tooth groups after

1 day, 1 week, 2 weeks, and 4 weeks immersed in the

filtered coffee, tea, cola, and distilled water are shown in

Tables 5, 6, 7, and 8 and also in Figs. 2, 3, 4, and 5,

respectively. The values of color difference (ĢE*) and

their corresponding NBS values for all 6 groups of denture teeth in 4 solutions are given in Table 9.

The results of the repeated-measure ANOVA indicated that the effects of all 3 factors, as well as all of the possible

interactions between them, were statistically significant (P

Table 8 Color of denture teeth after immersion in distilled water

Fig. 2 Color of denture teeth immersed in filtered coffee.

Fig. 3 Color of denture teeth immersed in tea.

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Fig. 4 Color of denture teeth immersed in cola.

Fig. 5 Color of denture teeth immersed in distilled water.

Table 9 Color differences of denture teeth

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<0.0001).Material-solution interaction was also significant

(P<0.0001).Further analyses were carried out at particular

times and for particular materials and solutions. Two-way

ANOVA revealed that denture teeth material and staining

solutions are factors that significantly(P<0.0001)affect

the staining resistance at each immersion period.The

interaction of these factors was also significant(P<

0.0001)at each time period. Table g shows the meanΔE*

values and standard deviations of denture teeth after the

staining procedure at each immersion period.

At the 1-day immersion period, the two-way ANOVA

indicated that UP material Showed the highest ΔE*value

in the filtered coffee solution, and OD material had the

lowest ΔE* value in the distilled water solution(P<

0.0001).Regarding the staining capacity of the solutions

used, the filtered coffee and the cola solutions seem to

provoke greater color changes respectively in UP and ID

materials compared to the tea or the distilled water solutions

(Fig.6).

At the second immersion period(1 week), UP material-

filtered coffee solution combination had the highest ĢE*

value (1.045) compared to the other combinations (P <

0.0001) (Fig. 7).

At the third immersion period (2 weeks), the UP material

combined with 2 staining solutions (filtered coffee and cola)

exhibited a significant color difference (P < 0.0001). This

difference was more than double that of the analogous

combinations of the UL, VP, and OD materials. In the tea

and the distilled water solutions, almost all of the materials

showed similar behavior (Fig. 8).

After 4 weeks of immersion in staining solutions, the

UP material in the filtered coffee and the ID material in

the cola reached the highest AE* value of this period (-

0.950). The tea solution also had almost the same high

staining effect on all of the materials. Ranging the values

from 0.180 (ID material) to 0.295 (OD material), the

distilled water solution had the lowest AE* values of this

period (Fig. 9).

The cumulative results of this in vitro study are

summarized in Fig. 10. After a period of 4 weeks, the most

Fig. 6 Color differences of denture teeth immersed in solutions

after 1 day.

Fig. 7 Color differences of denture teeth immersed in solutions

after 1 week.

Fig. 8 Color differences of denture teeth immersed in solutions

after 2 weeks.

Fig. 9 Color differences of denture teeth immersed in solutions

after 4 weeks.

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staining solution was filtered coffee while the least staining

one was, as expected, distilled water (P < 0.0001).

Discussion The physical properties of porcelain denture teeth may

be best shown when they are compared with the properties

of plastic denture teeth and, when possible, with those of natural teeth. These properties, however, often are so

radically different that they cannot be measured with the same equipment or compared quantitatively (11).

Methodology problems are inherent to the evaluation of color change in vitro, because it is not easy to reproduce the exact conditions of the oral environment in the

laboratory (12). Dentists often come face to face with a

problem of tooth discoloration and because of this phenomenon, filtered coffee, tea and cola were chosen as test agents for they had shown to have greater staining ability on restorations and natural tooth structure (13-16).

Clinical discoloration of aesthetic materials may be

caused by intrinsic and extrinsic factors (17,18). Intrinsic factors are related to the material's chemical stability and

oxidation of polymer matrices. The intrinsic color can be

altered as a result of accelerated aging conditions, mimicking sunlight and water effects, such as ultraviolet

irradiation, temperature, and humidity changes (18-20). Extrinsic factors producing discoloration include staining

by adsorption or absorption of stains or colorants

(15,17,21,22). Colored solutions, coffee, tea, beverages, chlorhexidine, cola and nicotine, are known to cause

staining of oral tissues and dental restorations. Among these, coffee, tea and cola are known to cause heavy staining of

dental restorations and appliances (1,12-6,21-27). Discoloration from staining solutions is found to be

considerably more intensive in terms of coloring than by ultraviolet light or distilled water alone (20,28).

To detect color differences, "evaluation by observation" and "evaluation by color analysis instruments" methods have been used (4,5,15,16,20,22,29,30). In this study, the

latter method was preferred because it is a sensitive and objective method for investigating color change (21,22).

This method achieves a reproducible means for determining when change in color occurs below visual perception levels (7,31). Investigators (7,32) have evaluated the

performance of devices used for color analysis. These authors concluded that all the instruments were able to

assess color with precision, although the measurements varied somewhat depending on the instrument used and the type of material surface being measured. It was shown

that a photometric tristimulus colorimeter had the best overall performance for determining color even on porcelain

surfaces (32). In dentistry, a discoloration that is more than

perceptible (ĢE* < 1.0) will be refered to as acceptable up

to value ĢE* = 3.3 which is considered to be the upper limit

of acceptability in subjective visual evaluations. Color

changes above this level will be refered to as unacceptable

(22).So, none of these denture teeth tested in this study

have shown an unacceptable color change(1.058≧ΔE*

≧0.112),i.e. greater than 3.3.

It is known that obtaining conditions of the oral

environment in vitro is not easy. To simulate in vivo

conditions in this study,3 different coloring solution

(filtered coffee, tea, cola)and the distilled water were

used at a constant temperature of 37±1℃ (14-17,

23,24,27,33-37).Many investigators who evaluated the

coloration effect of tea, coffee, nicotine, erythrosine and

other drinks(8,12-17,21-25,27,28,33-38)and of time-

lapsed artificial aging methods(17-20,28,31,39)either

on natural teeth or on aesthetic dental materials. However,

few investigators compared the color stability of restorative

dental materials and denture tooth materials under in vivo

conditions in their studies(5,7,40).

In general, the results of this in vitro study are in

agreement with earlier works(16,35,38), but it has to be

taken into consideration that the type and the geometry of

materials, the types of solutions and the systematic

techniques of colorimetric instruments were different.

In previous in vitro studies, many investigators(12,22,26)

compared the staining effect of coffee and tea, and on the

contrary to our findings, they reported that the tea had a

superior discoloration effect on polymeric dental materials

than the coffee. Um and Ruyter(22)reported that the tea

produced a yellow-brown stain while coffee stain was

yellowish. The discoloration from the tea was probably due

to adsorption of polar colorant from the tea at the surface

of composite resin materials. However, in the present

Fig. 10 Color differences of denture teeth immersed in

solutions after 4 weeks in general.

202

study and other studies (16,21,23,24,37) the coffee was found to be a more chromogenic material than the tea. The

preparation and concentration of the coffee and the tea may change the results. The solutions used in the present study were prepared closer to "real" drinks. The discoloration

from the coffee was due to both surface adsorption and absorption of colorants. The less polar colorants from the coffee had penetrated into the materials, probably because

the colorants were compatible with the polymer matrices of the composite resin materials. The reason color

differences of acrylic denture teeth, either conventional or reinforced, were higher than the porcelain denture teeth

in the filtered coffee is considered to be the result of relatively large water sorption value for the former group.

In in vitro color stability studies, a period of 4 weeks immersion may be considered to be too long. However,

to reach the cumulative staining results of these solutions,

it was decided to continue testing until the end of a period

of 4 weeks (2,18). The color difference values of this in vitro study were

converted into NBS units in order to be able to make a

comparison in the clinics. Based on the NBS rating system

(Table 9), UL, VP, and OD denture teeth showed extremely slight changes and IL, ID, and UP materials which showed

only slight changes in color after 4 weeks in the filtered coffee. Thus, the color change of porcelain (UL and VP) and one of the reinforced acrylic (OD) denture teeth in the

filtered coffee were less than that of the reinforced acrylic denture teeth (IL), and conventional acrylic denture teeth

(ID and UP). Moreover, the UL, VP, OD, and IL denture teeth showed extremely slight changes after 1 day, 1 week, 2 weeks, and slight changes after 4 weeks in the tea solution. The ID denture teeth revealed extremely slight

changes after 1 day and 4 weeks, and slight changes after 1 and 2 weeks, whereas the UP denture teeth exhibited slight

changes after 1 day and 1 week and extremely slight changes after 2 weeks and 4 weeks in the tea solution. Thus,

the color change of the porcelain (UL and VP) and the reinforced acrylic (OD and IL) denture teeth in tea were less than that of the conventional acrylic (ID and UP)

denture teeth. When denture teeth after the filtered coffee immersion were compared with those after the tea

immersion, the filtered coffee had a greater tendency to color acrylic denture teeth than the tea, and the tea had a

greater tendency to color porcelain denture teeth than the filtered coffee.

According to the NBS rating system (Table 9),

conventional acrylic denture tooth (ID and UP) specimens revealed color difference values greater than reinforced

acrylic and porcelain denture tooth specimens in the cola solution. The color changes of ID specimens after 1 day

and 4 weeks and those of UP specimens after 1 week and 2 weeks in the cola were considered as slight changes.

Finally, as expected, all of the denture tooth specimens exhibited extremely slight color changes in the distilled

water. As seen in Fig. 10, for one of the reinforced acrylic

denture teeth (IL material) and one of the conventional

acrylic denture teeth (UP material), the filtered coffee

produced the highest staining effect; while for the other conventional acrylic denture teeth (ID material) it was the cola and for porcelain denture teeth (UL and VP

materials) and the other reinforced acrylic denture teeth

(OD material), it was the tea. For all 6 groups of denture teeth materials the distilled water, as expected, exhibited the least color difference means.

The predisposing factors for staining of aesthetic dental

materials are: contamination of materials, porosity

depending on the technique, insufficient oral hygiene, dietary habits, surface failures, and polishibility of the

materials (1,13,14,20,22,28,41). Comparing the conventional acrylic denture teeth group

with the other 2 groups, color change of the former group was greater than those of the latter in all 3 staining solutions

(filtered coffee, tea, and cola). Rough surfaces mechanically retain surface stains more than smooth surfaces. To achieve less color change, special attention should be paid to

obtain a perfect surface finish. Roughening of the surface caused by wear and chemical degradation can also increase extrinsic staining (17).

Although dental porcelain is thought to be a color-stable aesthetic material, in previous studies (31,34) and also in

the present study, surprisingly, porcelain material was found to be stained by the coloring solutions. It is well-known that the integrity of the glaze of a dental porcelain

restoration decreases the roughness of the porcelain surface and thus minimizes staining. However, occlusal wear or

the omission of a proper porcelain adjustment protocol may leave an unglazed porcelain surface that encourages staining. Studies have shown that an unglazed porcelain

surface was affected more than a glazed porcelain surface from staining (41). A nonglazed surface provides for the

seepage of staining materials through the porosities and surface defects of the porcelain. Although the staining medium used (methylene blue) can be considered rather

aggressive in such studies, clinical equivalents could be stains of coffee, tea, or cigarette tar. These substances

could cause possible discoloration of porcelain materials if adjustments are performed without polishing and glazing.

It is therefore suggested that clinical corrections warrant

glazing of the porcelain prior to cementation to decrease the possibility of staining such restorations (41). From this

203

point of view, it can be said that color variation of denture

teeth made of acrylic resin material may be due to the

absorption and adsorption of colorants by the teeth, whereas

color variation of porcelain denture teeth may be due to

only the adsorption of colorants. The variation in

discoloration is directly proportional to ĢE* (28,42).

For all groups of denture teeth used in this study, the

filtered coffee (with a mean AE* value of 0.999) was the

most staining agent, followed by the tea and the cola, and

the distilled water was the least intense staining agent for

all types of denture teeth.

Water bleaching and sunlight, to some extent, have no

effect on porcelain, but repeated cycles of drying and

water immersion may cause whitening and loss of color

both in acrylic and porcelain denture teeth. Continued

exposure to ultraviolet light may also cause a slight

yellowing (11). This phenomenon could explain why the

b* values of porcelain denture teeth were slightly changed

in this present study. Porcelain is resistant to the action of

solvents, with only hydrofluoric acid being known to have

any significant effect on it. The cross-linked plastics are

relatively craze resistant and are immune to reasonable

amounts of ordinary solvents. They may, however, be

softened to some extent by a number of organic solvents.

When plastic is softened by solvents, organic dyes penetrate

the outer layer and cause discoloration (11).

According to the results of this in vitro study, while

selecting a denture teeth material in construction of a

removable prosthesis, from the point of color stability, a

reinforced acrylic or a porcelain denture teeth material may

be recommended.

It can be concluded that this kind of in vitro experiments

should be supported by planned in vivo observations.

Conclusions

Six commercially available denture teeth materials were

evaluated after 1 day, 1 week, 2 weeks, and 4 weeks of

immersion in various staining solutions. Within the

limitations of this in vitro study, these conclusions can be

drawn:

1. Clinical discoloration is affected by many parameters,

including the type of denture tooth, dietary habits, and

oral hygiene of patients. Clinical discoloration in denture

teeth is also because of extrinsic staining.

2. Among the denture teeth materials investigated in this

study, the UP denture teeth material had the highest color

difference values. At the 1 day, 1 week and 4 weeks

measurements in the filtered coffee and at the 2 weeks

measurements in the cola, the UP material revealed the

highest color difference values.

3. Although the most staining solution was the filtered

coffee, the highest color difference mean was obtained

in the cola at the 2 weeks measurement of UP material. However, when the results were evaluated in general,

the filtered coffee was found to be the most staining solution and UP material was found to have the highest discoloration value.

Acknowledgments The authors wish to thank Dr. Arife Dogan (Department

of Prosthodontics of Faculty of Dentistry of Gazi University) for her constructive criticism of this manuscript,

Mr. Mustafa Semiz (MSc) (Department of Statistics of Faculty of Arts and Sciences of Gazi University) for his

statistical evaluations, the Guney Di§ Deposu Company and Mr. Atakan Orhon for kindly supplying materials

used in this study.

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