Dental Materials Journal 22(4): 425-435, 2003 Original paper

Color Change in Acrylic Denture Base Resin Reinforced with Wire and Cloth

Takahito KANIE, Hiroyuki ARIKAWA, Koichi FUJII and Seiji BAN Department of Biomaterials Science, Field of Oral and Maxillofacial Rehabilitation, Course for Advanced Therapeutics, Kagoshima University Graduate School of Medical and Dental Sciences, 8-35-1 Sakuragaoka, Kagoshima 890-8544,Japan

Received June 9, 2003/AcceptedOctober 3, 2003

In this study, the L*a*b* color system as a color system and light transmittance of the denture base resin reinforced with wire mesh and glass cloth were measured, and the color difference

(ĢE* ab) was calculated using L*, a* and b* values which were measured both on a white cali- bration plate and on a null background. The thicknesses of test specimens, which were rein- forced with wire mesh and glass cloth 0.5 and 1.0mm below the surface, were 3 and 5mm.

L*, a* and b* values of wire mesh reinforcing specimens decreased in comparison with the non-reinforcing specimens (p<0.05). L* values of glass cloth-reinforcing specimens increased compared with the non-reinforcing specimens (p<0.05). The glass cloth is an effective reinforc- ing material and an aesthetically important property of denture base resin, since wire mesh makes the resin appear darker with the background condition greatly altering the color, while glass cloth makes the resin lighter.

Key words: Denture base resin, Reinforcement, Color change

INTRODUCTION

Various methods are used to improve the strength and impact resistance of denture base resin, and these usually use a reinforcing metal rod or mesh. Recent stud- ies1-6) have used carbon, polyethylene, aramid, and glass to strengthen denture base resins. If such materials can be used to reinforce denture base resins for use in po- sitions such as the palate and alveolar ridge, cast dentures may replace denture base resin. This would also increase the use of denture base resins in patients with metal allergies. Transparent polyethylene and glass cloth are the more promising of these materials aesthetically. However, polyethylene does not adhere to denture base resin and the surface treatment of polyethylene to improve adhesion is complicated7,8). The glass cloth used in these materials not only adheres to the resin when silanized, but is also easily cut with scissors and readily shaped. As a result, research9-11) has ex- amined the use of composites reinforced with glass for clasps and orthodontic wires to improve aesthetics and overcome metal allergies. Even though denture base resins and glass cloths or glass fibers are aesthetically pleasing materials, most re- search has examined the strength of the composites and few studies have examined the change in color of reinforced denture base resin. Therefore, we examined the ef- 426 COLOR CHANGE IN REINFORCED RESIN

fect of wire mesh and glass cloth reinforcement on the aesthetics of denture base res- ins.

MATERIALS AND METHOD

Preparation of test specimens

The wire mesh and glass cloth used in this study are shown in Fig. 1. Stainless steel wire mesh (M) (Sankin NETS, Dentsply Sankin, Tokyo, Japan) was used as a rein-

forcing metal . The glass cloth (G) (YEA2306, , Mie Fabric, Tsu, Japan) was cleaned in boiling water for 1 hr, dried in air, and silanized in 2vol% ƒÁ-MPTS

(Shinetsu Chemicals, Tokyo, Japan) in ethanol solution for 10min. After drying, it was heated at 115•Ž for 10min. Then, single or double sheets of the silanized glass cloth were sandwiched between two fluorocarbon resin boards and sealed with alumi-

num foil (S=single, D=double). Next, 1wt% BPO (Nacalai tesque, Kyoto, Japan) was added to the liquid denture base resin monomer (Pour Resin, Shofu Inc., Kyoto,

Japan), which was injected into the space and allowed to polymerize into a thin plate

with an average thickness of 0.31mm for S and 0.56mm for D by heating it at 50

℃ for 30min, as described previously12).

The wire mesh and thin plate containing the glass cloth were put in a pit on a

fluorocarbon resin plate and a mixture of denture base resin consisting of one of three colors (2R, U3 and 3) in a powder/liquid ratio of 9g/5ml was poured into the

mold. This was heated at 50•Ž for 30min. After polymerization, the composite blocks were left out in the air for at least 24hr. The blocks were polished to 3 and

5mm thicknesses (T3 and T5) and finished using wet and dry polishing paper (#1200). The wire mesh or glass cloth was 0.5 or 1.0mm below the surface of the

Fig. 1 Wire mesh and glass cloth used in this investigation (bar: 10mm).

Table 1 The preparation conditions for test specimens

Code in parentheses KANIE et al. 427

Fig. 2 Some photographs of the control and reinforced specimens (T5).

test specimens (P1 and P2) (Table 1)12), respectively.

Color measurement

A colorimeter (CR-200, Minolta, Osaka, Japan) was used to measure the color changes with the L*a*b* color system (CIE). The L*, a* and b* values of the test specimens (40•~40mm) (Fig. 2) were measured on a white calibration plate and on a null background. L* indicates the lightness index number, and a* and b* show redness and yellowness in the positive direction, respectively. The color difference

(ΔE*ab) was calculated using the following formula.

ΔE*ab=[(ΔL*)2+(Δa*)2+(Δb*)2]1/2

As controls, test specimens containing no reinforcement were made using the same method (3 and 5mm thicknesses).

Light transmittance measurement

After measuring color change in specimens of the three colors (2R, U3 and 3), the specimens were cut into 15•~15mm squares, and the light transmittance was meas- ured with a compact multi-functional spectral light transmittance meter (TM-1,

TOPCON, Tokyo, Japan). The meter calculated the average light transmittance (%) at wavelengths of 380 to 700nm13). In the calculations, the G-S-P1 and -P2 data were pooled, as were the G-D-P1 and -P2 data. 428 COLORCHANGE IN REINFORCEDRESIN

Statistical analysis Six measurements of three test specimens were made as a group. A two-way analy- sis of variance (ANOVA) using specimen thickness (3 and 5mm) and type of rein- forcement (M-S, G-S and G-D) as the independent variables and L*, a* and b* as the dependent variables was conducted. In addition, a two-way ANOVA was conducted with color code (2R, U3 and 3) and type of reinforcement (M-S, G-S and G-D) as the independent variables and light transmittance as the dependent variable. Tukey's test was used to test for significance.

RESULTS The L*, a* and b* values in P1 and P2 on the white calibration plate and the null background are shown in Table 2 a-d. In P1 and P2, there was an interaction be- tween specimen thickness and type of reinforcement by the two-way ANOVA at the 5% level (p<0.05). For the L*, a* and b* values in P1 and P2 on the white calibration plate (Table 2-a and -b), the values of the T3-control increased more than those of the T5-control for all three colors. The L*, a* and b* values of M-S in P1 and P2 decreased com- pared with the controls for all three colors tested, and the decrease was greatest for a* and b*. The L* values of G-S and G-D in P1 and P2 slightly increased compared with the controls for all three colors, except for the values of T5-G-S-P2 for color

Table 2-a L*, a* and b* values of P1 measured on the white calibration plate

SD in parentheses Mean values designated with the same superscript letter are not significantly different (p>0.05) KANIE et al. 429

Table 2-b L*, a* and b* values of P2 measured on the white calibration plate

SD in parentheses Mean values designated with the same superscript letter are not significantly different (p>0.05)

Table 2-c L*, a* and b* values of P1 measured on the null background

SD in parentheses Mean values designated with the same superscript letter are not significantly different (p>0.05) 430 COLOR CHANGE IN REINFORCED RESIN

Table 2-d L*, a* and b* values of P2 measured on the null background

SD in parentheses Mean values designated with the same superscript letter are not significantly different (p>0.05)

codes 2R and T3-G-S-P2 for color codes 3 (51.6 and 52.3 in Table 2-b). The increase

was especially pronounced for the specimen with two glass cloth sheets (G-D). The

decrease in b* of G-S and G-D in P1 and P2 was smaller than of M-S (Table 2-a

and -b).

The L*, a* and b* values of the control on the null background (Table 2-c and -

d) were lower than the control values on the white calibration plate for all three col-

ors. The L*, a* and b* values of M-S for color code 3 in P1 and P2 were larger

than the values of M-S for the other color codes. The decrease in the L*, a* and

b* values of M-S in P1 and P2 on the null background was smaller than on the white

calibration plate. The L* values of G-S and G-D in P1 and P2 increased as compared

with the control, and the L* values of M-S in P1 and P2 decreased as compared with

the control for all color codes.

The differences in the L*, a* and b* values of controls between T3 and T5 on the

white calibration plate were statistically significant (p<0.05), while the differences in

the L* values in P1 for color codes 2R and U3 on the null background, and for the

a* and b* values in P1 and P2 for color codes 2R and 3 on the null background were

not significant (p>0.05).

The color differences (ĢE* ab) between the control and reinforced specimens

(M-S, G-S and G-D) are shown in Fig. 3 a-d. The ĢE* ab of M-S exceeded 6, except for T5-M-S-P1, T5-M-S-P2 and T3-M-S-P2 for color code U3 on the null background

(Fig. 3-c and -d). The color difference of T3-G-S-P2 for color code 3 on the white KANIE et al. 431

Fig. 3 The color difference between the control and reinforced specimens. (a; T5 on white calibration plate, b; T3 on white calibration plate, c; T5 on null back- ground, d; T3 on null background)

Table 3 Light transmittance (%) of T3-specimems on a 380nm to 700nm of wavelength

SD in parentheses Mean values designated with the same superscript letter are not significantly defferent (p>0.05)

calibration plate was smallest among the specimens tested (ĢE*ab=0.5 in Fig. 3-b).

The light transmittance of T3 is shown in Table 3. The two-way ANOVA showed an interaction between color code and type of reinforcement (p<0.05). The light transmittance of color code U3 was higher than that of the other color codes

(p<0.05). The light transmittance of the reinforced specimens for color code 2R and 432 COLORCHANGE IN REINFORCEDRESIN

U3 was significantly lower than the control. The light transmittance of M-S for all three colors was lower than that of G-S. For color code 3, there was no significant difference in the light transmittance between the control and G-S or between the con- trol and G-D. Since T5 was so thick, it was impossible to measure T5 with the de- vice used.

DISCUSSION Ichiwata et al.14) examined the effect of thickness on color change in denture base resin using a white calibration plate and concluded that the lightness of the resin de- creased as thickness increased. In our study, the lightness (L* value) of the control decreased as thickness increased in test specimens for all color numbers. However, the lightness of G-S-P1 and G-D-P1 for color code 3 increased slightly with thickness (Table 2-a). Furthermore, the lightness of G-S-P2 and G-D-P2, in which the position of the reinforcement differed, decreased as thickness increased (Table 2-b). The lightness may be affected by the light reflected from the reinforcing material within the resin and from the white calibration plate. Therefore, the factors determining lightness are very complex. The lightness of the control on the null background de- creased as thickness increased for all color numbers (Table 2-c and -d). However, the lightness of color code U3 clearly decreased as thickness increased when wire mesh was used. P2 was lighter when glass cloth was used, while the lightness in P1 did not change. The glass cloth made the P1 lighter on the null background, increas- ingly so with an increasing number of layers of glass cloth. Although glass itself is very transparent, irregular reflection may occur at the interface of cloth made from thin twisted glass threads. Therefore, glass cloth placed near the resin surface has a negative effect. It was clear that the position of the reinforcement greatly influ- enced the color properties. In this study, we assumed that the reinforcement would be used in a dental flange position. A white calibration plate was used as the background to simulate an artificial tooth. The L*, a* and b* values of M-S on the white calibration plate decreased compared with the controls for all color codes (Table 2-a and -b). When wire mesh was used, the color became much darker, aesthetically poorer. Moreover, the difference in the L* values between the control and M-S for color code 3 was smaller than the difference for the other color codes, which shows how easy it is to alter the aesthetics of a highly transparent resin. Nakaura et al.15) stated that when a translucent object such as a composite resin is thin, its color is readily affected more by the background color, while when it is thick it is affected more by the resin color. We also showed that color change in- creased with resin thickness. The background affected the color when the denture base resin was less than 3mm thick. The L*, a* and b* values of the control on the null background were lower than on the white calibration plate, which shows that the resin on a null background is darker, with the resin colors fading in the red and yel- low directions. KANIE et al. 433

If aesthetically pleasing prostheses are the main purpose of denture work, the

apparent differences in dentures are very important. When the wire mesh was lo- cated 1.0mm below the resin surface, it was readily apparent (Fig. 2). In compari-

son, the glass cloth was difficult to see, even when it was 0.5mm from the resin surface. Wire mesh is clearly less aesthetically pleasing than glass cloth. As Satoh

et al.16) showed, in the color hue system the discrimination test is most sensitive for

red. However, it may be considerably difficult to detect apparent color changes in comparisons between the control and G-S-P1 and -P2 for all color numbers (Fig. 2).

As the a* values in Table 2 show, the reinforcement caused significant fading in the red direction, and the decrease was larger for M-S. The overall effect, however, was

small, because the a* values of the resin were originally small, and the values were decreased by the reinforcement.

Kijima et al.17) reported that the boundary for readily distinguished color differ- ences is 1.2 to 1.4. The color differences within this range included T5-G-S-P2 for

color code 2R and T3-G-S-P2 for color code 3 on the white calibration plate, and

T5-G-S-P2 for color code 2R and T3-G-S-P2 for color code 3 on the null background.

A color difference of less than 3, which is considered •gmoderately different•h, was ob- served for 13 of the 24 test conditions of G-S. In contrast, a color difference of over

6, which is considered •gvery different•h, was observed in 21 of the 24 test conditions

of M-S and in 11 of 12 test conditions of G-D-P1. This indicates that it was more

difficult to discern color changes caused by the glass cloth than those caused by the

wire mesh. In P2 especially, many test specimens (G-S) could not be distinguished from the control, which is aesthetically advantageous. Moreover, the color difference

of G-D-P2 for color code 3, strengthened by two layers of glass cloth, was smaller than that of G-S-P1 for color code 3 incorporating a single cloth layer near the sur- face (0.5mm).

Since the color tone of denture base resins is subtly affected by thickness, and color sense is easily influenced by the environment18), we also measured the light transmittance of the specimens. The light transmittance was highest for color code

U3 (p<0.05) (Table 3). Though the L* values of the control for color codes 2R and

U3 on the white calibration plate were 54.3 and 54.4, the L* values on the null back- ground were 51.3 and 47.2, and L* values for color code U3 differed markedly. Back-

grounds such as artificial teeth or reinforcing rods readily affect transparent resin. Therefore, the relative decrease (M-S/control) in the L* values of T3-M-S-P1 and -P2 for color code U3 on the white calibration plate was 47.0/54.4=0.86 and 48.3/

54.4=0.89 (Table 2-a and -b), respectively. These values are smaller than those of

T3-M-S-P1 and -P2 on null background (44.4/47.2=0.94 and 46.1/47.2=0.98) (Table 2-c and -d). Clearly, the interception of the reflected light by the wire mesh and back- ground has a greater effect than the position of the wire mesh. However, this ten- dency decreases with greater transparency of the resin. That is, the effect of resin thickness is greatest for color code U3.

Recently, there have been many trials of metal-free dental materials in an effort

to avoid metal allergies19,20). If reinforcements become used routinely to strengthen 434 COLORCHANGE IN REINFORCEDRESIN denture base resins, the demands for denture base resin will increase. That is why we examined the aesthetic properties of denture base resins. As mentioned above, the glass cloth was difficult to see when it was 0.5mm from the resin surface. From our previous report12), woven glass was incorporated outside the area of tension as much as possible to increase the flexural strength and flexural modulus. Though the position of the glass cloth should be decided by an adjustment of aesthet- ics and strength, these two facts show that the use of glass cloth is the excellent technique for denture base resin. We showed that wire mesh made the resin appear darker, and the background condition greatly altered the color, while glass cloth makes the resin lighter. The reinforced denture base resin which has the smallest ap- parent change and the smallest color difference will be produced by lower transpar- ent resins with a single layer of glass cloth.

REFERENCES 1) Stafford, G.D., Huggett, R., MacGregor, A.R. and Graham, J.: The use of as a denture-base material, J Dent 14: 18-22, 1986. 2) Fukuda, H. and Miyairi, H.: Effectiveness of reinforcement by carbon aramid fiber-cloth to upper complete denture bases (Part 1) thinning of palatal area and flexural strength, J J Dent Mater 7 (2): 167-175, 1988. (in Japanese) 3) Berrong, J.M., Weed, R.M. and Young, J.M.: Fracture resistance of -reinforced poly (methyl methacrylate) resin: A preliminary study, Int J Prosthodont 3: 391-395, 1990. 4) Ladizesky, N.H., Cheng, Y.Y., Chow, T.W. and Ward, L.M.: Acrylic resin reinforced with chopped high performance polyethylene fiber-properties and denture construction, Dent Mater 9: 128-135, 1993. 5) Vallittu, P.K.: Flexural properties of acrylic resin polymers reinforced with unidirec- tional and woven glass fibers, J Prosthet Dent 81: 318-326, 1999. 6) Kanie, T., Arikawa, H. Fujii, K. and Ban. S.: Impact strength of acrylic denture base resin reinforced with woven , Dent Mater J 22 (1): 30-38, 2003. 7) Ramos, V.J., Runyan, D.A. and Christensen, L.C.: The effect of plasma-treated poly- ethylene fiber on the fracture strength of polymethyl methacrylate, J Prosthet Dent 76: 94- 96, 1996. 8) Samadzadeh, A., Kugel, G., Hurley, E. and Aboushala, A.: Fracture strengths of provi- sional restorations reinforced with plasma-treated woven polyethylene fiber, J Prosthet Dent 78: 447-450, 1997. 9) Jancar, J., Dibenedetto, A.T., Hadziinikolau, Y., Goldberg, A.J. and Dianselmo, A.: Measurement of the elastic modulus of fiber-reinforced composites used as orthodontic wires, J Mater Sci Mater Med 5: 214-218, 1994. 10) Freilich, M.A., Karmaker, A.C., Burstone, C.J. and Goldgerg, A.J.: Development and clinical applications of a light-polymerized fiber-reinforced composite, J Prosthet Dent 80: 311-318, 1998. 11) Kishita, C., Hamano, T., Tsuru, K., Nishi, Y., Fujii, K. and Nagaoka, E.: Apprication of a glass fiber-reinforced composite materials to clasp, QDT 26: 15-23, 2001. 12) Kanie, T., Arikawa, H., Fujii, K. and Ban, S.: Mechanical properties of reinforced den- ture base resin: The effect of position and the number of woven glass fibers, Dent Mater J 21 (3): 261-269, 2002. 13) Arikawa, H., Kanie, T., Fujii, K., Homma, T., Takahashi, H. and Ban, S.: Optical properties of paint-on resins for shade modification of crown and bridge resins -Light transmittance characteristics-, Dent Mater J 22 (3): 272-279, 2003. KANIE et al. 435

14) Ichiwata, T., Ueda, K., Takeda, T., Goto, H., Shirane, M., Nakamura, A., Ibonai, T., Morita, E., Satoh, Y. and Ohki, K.: Research into the colors of denture base resin -In fluenced of difference in shade and thickness-, J Nihon Univ Sch Dent 26 (1): 30-38, 1984. 15) Nakaura, K., Kijima, S., Yamazaki, N., Ikeji, M., Enya, T., Katayama, I. and Motonomi, T.: Color change of visible light-cured composite resin by thickness (Part 1), Jpn J Conserv Dent 30 (4): 1185-1191, 1987. (in Japanese) 16) Satoh, Y., Iwasawa, A., Nagai, E., Ohyama, T., Uchida, K., Okada, N., Toyoma, H., Ohwa, M. and Ohki, K.: A study on the ability of distinguish color, J Jpn Prosthodont Soc 38: 856-861, 1994. (in Japanese) 17) Kijima, S., Mori, S., Ichimura, Y., Ubukawa, N., Nakaura, K., Watanabe, H., Katayama, T. and Katayama, I.: Study of color evaluation on the difference distance - on the case of background color with gingival color of red-, Jpn J Conserv Dent 36 (2): 430-435, 1993. (in Japanese) 18) Oka, T., Kurahi, M., Sugimoto, Y., Oota, Y. and Yamada, S.: A color tone discrimina- tion of resin for denture base by reflection spectrum data, J Gifu Dent Soc 19 (1): 55-63, 1992. (in Japanese) 19) Freilich, M.A., Karmaker, A.C., Burstone, C.J. and Goldberg, A.J.: Development and clinical applications of a light-polymerized fiber-reinforced composite. J Prosthet Dent 80 (3): 311-318, 1998. 20) Nakamura, T., Saito, O., Mizuno, M., Kinuta, S. and Ishigaki, S.: Influence of abutment substrates on the colour of metal-free polymer crowns. J Oral Rehabil 30 (2): 184-188, 2003.