Adhesive bonding of denture base resins to plastic denture teeth Shiro Suzuki, Mitsuo Sakoh, and Akihiko Shiba Department of Prosthodontics, Faculty of Dentistry, Showa University, 2-1-1, Kitasenzoku, Ota-ku, Tokyo 145, Japan The adhesive bonding of denture teeth to denture base resins in dentures with conventional acrylic teeth and crosslinked plastic teeth was investigated. The dentures with highly crosslinked plastic teeth such as SR-Orthosit, Crystal ND, and MitelOM showed poor bonding at the tooth/ base resin interface using the conventional bonding method. Elimination of the alginate mold lining material in the conven-
tional bonding method effectively improved bonding at the toothlbase resin interface. The application of 4-META adhesive bonding agents to the denture teeth improved the interface bonding of highly crosslinked plastic teeth and the denture base. Dentures with Orthosit and Mitel showed differrnces in bonding ability when two different adhesives were used.
INTRODUCTION
Denture teeth are of obvious importance not only for restoring oral function but also for maintaining appropriate jaw relationships. The two most important factors required of denture teeth are abrasion resistance and bonding between the teeth and denture base material. Porcelain has been a popular substitute for natural teeth, but suffers from lack of bonding to the denture base resin.’ In contrast, conventional acrylic denture teeth prepared from linear polymethylmethacrylate (PMMA) are not suitable as a crown substitute material because of low abrasion resistance.’ Highly crosslinked acrylic denture teeth with good abrasion resistance have been developed and are now used ~linically.~,~ There are two types of crosslinked plastic denture teeth. One is produced by copolymerization of methylmethacrylate (MMA) with a small amount of dimethacrylate (presumably triethylene glycol dimethacrylate (TEGDMA)) in the presence of finely powdered PMMA.5The other is a highly crosslinked acrylic polymer with added filler of colloidal silica instead of PMMA powders. Such crosslinked resins include Orthosit (Ivoclar Co., Liechtenstein).6 Crosslinked networks have been successfully used to improve the mechanical and physical properties of conventional acrylic resins, such as compressive strength, hardness, and abrasion resistance. However, highly crosslinked polymers tend to present new clinical problems when they are used as denture teeth. One of the problems is dislodgement of denture teeth Journal of Biomedical Materials Research, Vol. 24, 1091-1103 (1990) 0 1990 John Wiley & Sons, Inc. CCC 0021-9304/90/081091-13$04.00
SUZUKI, SAKOH, AND SHIBA
1092
from the denture base which may occur due to poor adhesive bonding to the denture base resin. As has been previously demonstrated with porcelain denture teeth, a lack of bonding to the denture base resin represents a sigruficant disadvantage. The purpose of this study was to evaluate and compare the bonding behavior of commercially available highly crosslinked plastic teeth, conventional acrylic denture teeth, and porcelain denture teeth to denture base resins. We also evaluated a new bonding method involving the application of adhesive agents to the highly crosslinked plastic denture teeth in order to improve the bonding between the denture teeth and the denture base. MATERIALS AND METHODS
The trade names, abbreviations, types of plastic, and manufacturers of denture teeth used in this study are shown in Table I. A heat polymerizing acrylic denture base resin (Acron: GC Corp., Japan) was used to prepare the specimens. The base resin selected was colorless and translucent to facilitate dye penetration measurements. A solution of alginate (Acrosep: GC Dental Indust. Corp., Japan) was used as a mold lining material. Two adhesives which contain 4-methacryloxyethyl trimellitate anhydride (4-META)7were used. The compositions of these adhesives are shown in Table II. Both adhesives were prepared by combining components 1 and 2.
Preparation of the specimens The first and second premolar and the first molar were selected from both upper and lower teeth. Denture specimens for upper and lower teeth were made independently. The basal portion of the lower teeth were ground out prior to placement in models. In these teeth, the distance between the central fossa and the bottom is 3 mm. The upper teeth were not ground. The specimens (toothhesin base denture units) were made by three different methods, as follows: In the conventional method, ground or nonground denture teeth are set up on the stone models with paraffin wax. After dewaxing, an alginate mold lining was applied to the stone model prior to packing the base resin dough. Polymerization of the base resin was then completed by heating the mold in water at 60°C for 1 h and 100°C for 30 min. After the specimens were completed, they were polished using a lathe with wet pumice. In addition, two alternate fabrication methods were used. In the first method, the application of the alginate mold lining on the stone model after dewaxing was eliminated. The second method involved the use of adhesive agents. Equal amounts of adhesive components 1 and 2 were well mixed and applied to the cervical and basal portions of the teeth prior to packing the base resin. The alginate mold lining was also used for those specimens.
~~
Livdent FE3 20 Porcelain 100 Wearless Acrylic Teeth Livdent FB 20 Plastic 100 Trubite Bioform IF" 20" Crystal ND SR-Orthosit-PE Mitel-OM"
Denture Teeth, Tradename Poly methyl methacrylate (PMMA) Partially crosslinked Acrylic" with PMMA powders Partially crosslinked Acrylicbwith PMMA powders Highly crosslinked Acrylic' with composite filler' Highly crosslinked Acrylic' with composite fillef Highly crosslinked Acrylicdwith composite filler'
-
Type of Plastic
GC GC GC Dentsply Major Ivoclar Sun-Medical
Manufacturer
"Copolymer of MMA and TEGDMA. bUnknown. 'Poly-dimethacryloxyethyl(trimethy1 hexamethylene diurethane): (PUDMA). dCopolymer of UDMA and 2,2-bis(4-methacryloxypolyethoxyphenyl)prtpane. 'Poly-UDMA with colloidal silica. 'Poly-trimethylol propane trimethacrylate with colloidal silica. *Custom made. Note. GC: G-C Dental Industrial Corp., Tokyo, Japan; Dentsply: Dentsply International INC., Pennsylvania, U.S.A.; Major: Major Dental Industry S.p.A., Torino, Italy; Ivoclar: Ivoclar AG, Schaan, Liechtenstein; Sun-Medical: Sun-Medical Corp., Kyoto, Japan.
Porcelain Wearless Livdent IPN Crystal Orthosit Mite1
Abbreviation
TABLE I List of Denture Teeth
b
m
#
W
Component 1 Component 2
Component 2
Component 1
MMA/90,4-METNlO MMAl95
MMAl60, DEGDMN25, 4-METAl15 MMAl30
Silane coupling agenff5
EthylalcohoV80
PMMAI4
Additive (wt%)
-
-
N,N-Dimethyl p-toluidine Sodium p-toluenesulfinate
Benzoyl peroxide
Catalyst
Note. MMA: Methylmethacrylate; DEGDMA: Diethylenglycol dimethacrylate; 4-META: 4-methacryloxyethyl trimellitate anhydride; Silane coupling agent: y-methacryloxypropyl trimethoxysilane.
Adhesive B
Adhesive A
Monomer (wt%)
TABLE I1 Composition of Adhesive Agents
v)
$ 8>
z .
0
z
c
,;r
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC
1095
The measurement of dye penetration To simulate the thermal changes encountered in the oral environment, thermal cycling was performed using a thermal cycling machine (Rika Industrial Corp., Japan). The specimens were alternately soaked in water at 4°C for 1min and 60°C for 1min; this process was carried out 60 times. After thermal cycling, the specimens were immersed in a 0.2% aqueous solution of basic fuchsin dye for 36 h to evaluate the degree of toothbase resin bonding quantitatively. The depths of dye penetration were measured at 10 points: 3 buccal, 3 lingual, and 4 proximal surfaces, as shown in Figure 1. The maximum dye penetration through the base resin at each point was measured to the nearest 0.5 mm. The average depth of dye penetration for each location was then calculated.
Measurement of Knoop hardness The Knoop microhardness test was used to determine the surface hardness of specimens. A diamond indentor was pressed into the specimens under a load of 25 g for 30 s. The areas of indentation were then measured using a ruler under a microscope. Knoop hardness number (KHN) was calculated as the load divided by the area of indentation. Specimens were prepared from sectioned denture teeth in order to evaluate the hardness of the inner structure. Hardness was measured at three points on the specimens such as the occlusal, middle and basal areas. The measurements were made at five points for each area. RESULTS
The percentages of toothhesin base denture units showing dye penetration to various levels at the tooth-resin base denture interface are shown in
h Figure 1. Measurement points of dye penetration depth.
SUZUKI, SAKOH, AND SHIBA
1096
Porcelain
\ \ e a rlcss
li.9
Li vdcnt
___--
---
i_T/--IG*O
C Is 1~a I
Orthosil
Mild
Figure 2. Knoop hardness of sectioned teeth
Table 111. Dentures with conventional acrylic teeth (Wearless) showed good bonding, as evidenced by the small average depths of dye penetration. However, the other denture teeth showed poorer bonding. A t-test was done to evaluate the difference between Wearless and other plastic denture teeth. All the crosslinked denture teeth, except nonground Livdent, showed a significant difference in dye penetration and, therefore, degree of bonding as compared to Wearless at the level of p < 0.01. Dentures with conventional crosslinked teeth such as Livdent and 11" showed better bonding compared to the highly crosslinked group. Dentures with highly crosslinked teeth such as Crystal, Orthosit, and Mitel exhibited poor bonding as shown by large average depths of dye penetration. Prior grinding of the teeth had little effect on bonding as measured by dye penetration except with Livdent and Orthosit; grinding increased the depth of dye penetration for these two materials. The Knoop hardness at each point on the specimens was shown in Figure 2. The conventional acrylic and crosslinked teeth (Wearless, Livdent, and IF") showed less hardness to compare with highly crosslinked teeth (Orthosit, Crystal, and Mitel). Furthermore, the results of the dye penetration analysis showed that bonding between the teeth and the denture base decreased as the Knoop hardness of the teeth increased (see Fig. 2).
1097
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC TABLE 111 Distribution of Depth of Dye Penetration at the Tooth-Resin Base Denture Interface for Various Tooth Materials Penetration (mm) Specimen Wearless NG" G" Livdent NG" G" IPN NG" G" Crystal N G ~ G"
Orthosit NG" G" Mitel
NGb G"
Porcelain NG" G"
0
0.5
1.0
1.5
2.0
2.5
96+ 96 90 62 76 78 20 12 40 24 33 48
4 4 2 30 8 16 15 20 20 38 27
0 0 0 0
0 0 0
0
0 8 26 6 4 10 6
20 8 4 5
26
0 0 8 0 8 6 17 14 26 24 23 20
0
0 8 0 0 25 8 0 6 2 0
0 0
0 0
0 0
0 0
0 0
100 100
8
0 0 0 15
0
AverageDepth * S.E.
* 0.01 * 0.01
0.02 0.02 0.09 f 0.35 2 0.24
0.03 0.09' 0.06' 0.14 0.01' 1.29 k 0.13* 1.23 t 0.10* 0.61 0.08* 0.72 f 0.09* 0.67 2 0.08* 0.42 t 0.06*
* *
*
2.50 2.50
NG = nonground, G = ground, S.E. = standard error. an = 50, bn = 40, 'Expressed as percentage of total number of samples tested. *Significantlydifferent from results for Wearless at p < 0.01 level using t-test.
The percentage of tooth-resin base denture units showing dye penetration when the alginate mold lining between the stone mold and the denture was not used are shown in Table IV. Under these preparative conditions, dentures with highly crosslinked plastic teeth such as Crystal, Orthosit, and Mitel exhibited better bonding (except nonground Orthosit) compared to the conventional method as evidenced by small average depths of dye penetration. Specifically, the decrease in average depth of dye penetration for nonground specimens was 78%for Crystal and 16% for Mitel; for ground specimens, the improvement was 85% for Crystal, 75% for Orthosit, and 57% for Mitel. The percentage of toothhesin base denture units showing dye penetration affected by adhesive agents at the tooth-resin base denture interface are shown in Table V. In general, dentures made using adhesive agents exhibited better bonding compared to both those made by the conventional method and the series in which the alginate mold lining material was eliminated, as evidenced by smaller average depths of dye penetration. In the case of Crystal, bonding was better in the absence of the alginate mold lining than in the presence of adhesives; but both cases were better than the conventional method. Dentures made with Orthosit and Mitel showed better results with adhesive B than with adhesive A as evidenced by smaller average depths of dye penetration.
62' 76 54 80 12 80
Crystal NG 14 6 14 6 20 6
1.0
28(14) 10 22(10) 8
30 46
0.5 36 2 16(6) 4 O(2) 0
1.o
NG = nonground, G = ground, S.E. = standard error. 'Expressed as percentage of total number of samples tested.
G NG G
32+ 52 50(70) 86 76(88) 92
Crystal NG G Orthosit NG
Mitel
0
~
0 0 0 0 2 2
1.5
2 2 8 2 0 2
2.0 0 0 12 0 0 0
2.5
2 0 6(4) 0 2(0) 0
1.5
0 0 O(2) 0 0 0
2.0
Penetration (mm)
0 0 O(2) 0 0 0
2.5
TABLE V Effect of Adhesive A and Adhesive B (in brackets) on Dye Penetration Depth
Specimen
~~~~
10
64
22 16 12 12
0.5
NG = nonground, G = ground, S.E. = standard error. 'Expressed as percentage of total number of samples tested.
G Orthosit NG G Mitel NG G
0
Specimen
Penetration (mm)
TABLE IV Effect of Elimination of Alginate Mold Lining on Dye Penetration Depth
E0 *
89
v,
3
9
.z
*
0.54 0.06 0.25 t 0.03 0.39 2 0.06 (0.28 ir 0.07) 0.09 -C 0.03 0.14 i 0.03 (0.07 ? 0.02) 0.04 0.01
.?i Average Depth (mean f S.E.)
*
0.29 5 0.06 0.18 f 0.05 0.66 ? 0.12 0.16 0.05 0.56 ? 0.04 0.18 f 0.05
Average Depth (mean ? S.E.)
0 \o 00
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC
1099
DISCUSSION
There are two ways to improve the mechanical properties of conventional acrylic resins used in denture teeth. One is to use a crosslinking agent such as Triethylene glycol dimethacrylate, and the other is to include inorganic fillers such as hydrophobic colloidal silica." Highly crosslinked polymers have been in~estigatedll-'~ for use in dental prostheses. The addition of crosslinking agents to MMA has been reported to improve the mechanical properties. On the other hand, the addition of crosslinking agents to MMA did not improve the abrasion resistance of PMMA.l5,I6In an attempt to reduce water uptake in crown and bridge resins, hydrophobic monomers have been investigated. A composite material which is fabricated with dimethacrylate and composite filler is commercially available.6The composite filler is superior to the powdered glass filler which is used in conventional composite filling materials in terms of bonding between the filler and the matrix resin. A very smooth surface is obtained from a polished hard resin with composite filler because of uniformity of the material. Therefore, these types of composite materials are currently being used in denture teeth. Increasing the degree of crosslinking increases the hardness and abrasion resistance of the polymer, On the other hand, the size of the polymer chain networks in highly crosslinked polymers becomes too small for interpenetration of MMA monomer from the denture base into the matrix. This results in poor bonding between the highly crosslinked plastic denture teeth and the denture base resin. Successful crosslinked plastic denture teeth, therefore, must include linear PMMA in order to increase the probability of polymer chain entanglements and thus improve the tooth/denture base interface adhesion. Mich16 reported that Orthosit has PMMA at the basal portion of the tooth in order to facilitate bonding to the denture base resin. From the results of our hardness tests, it is evident that some manufacturers have provided a layer of PMMA at the basaI portion, but this layer is sometimes ground out during clinical manipulation. On this premise, we predicted that substantial grinding at the basal level of the denture tooth would decrease bonding to the denture base resin, but the effect of grinding was not obvious in our experimental results. We suspect that one cause of this apparent contradiction is the increase in the effective surface area for bonding by the roughened surface created by grinding. The different thicknesses of PMMA layers provided by each manufacturer could also be a factor. A sectioned specimen of Orthosit (non-ground) after thermal cycling is shown in Figure 3. Thermal cycling tests were performed in order to evaluate the effect of the expansion and contraction on the denture teeth and base materials. A space could be visualized between the cervical area of the denture tooth and the base resin with higher magnification (see Fig. 3b), but it gradually decreased and eventually disappeared in the direction of the basal area where PMMA is present. At this point, both the material of the tooth and the base material are adherent (Fig. 3c).
1100
SUZUKI, SAKOH, AND SHIBA
(c)
Figure 3. A sectioned specimen with Orthosit (nonground) after thermal cycling: (a) low magnification, (b) a space at the toothhase resin interface (cervical area of tooth), (c) a fusion at the toothhase resin interface (basal area of tooth).
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC
1101
The alginate mold lining material reacts with the calcium in the stone mold material to form a film of insoluble calcium alginate.I7This film works successfully as a barrier to prevent MMA monomer from the denture base from penetrating into the mold. This barrier or separator is applied so as to avoid contacting the tooth surface, but it is usually impossible to completely eliminate such contact. Although the alginate solution should not react with the polymer surface, it may be detrimental to strong bonding between the tooth and the denture base. As a result, there could be fewer opportunities for polymer chain entanglements on the tooth surface. The results from the specimens prepared without the alginate mold lining showed good bonding, but this method is not clinically acceptable because it is quite difficult to remove the plaster from the denture after polymerization. The application of adhesive agents to denture teeth before polymerization is the best method for improving bonding strength over the conventional method. Comparisons of the average depths of dye penetration at the tooth/ denture base interface in each condition are shown in Figure 4. A t-test was performed to compare the conventional method with the other two methods for each material. The specimens with adhesive agents showed significant differences ( p < 0.01) compared to those obtained with conventional methods. This shows that the application of adhesive agents is very effective in improving the bonding between the crosslinked plastic denture teeth and the denture base resin. Nakabayashi et a1.I8 have reported the effectiveness of 4-META for increasing the adhesion to tooth substrates. They reported monomers with both hydrophobic and hydrophilic groups, like 4-META, promoted the infiltration of monomers into hard tissue. In our studies, the results of the effect of adhesive agents containing 4-META showed 4-META monomer could promote the infiltration of MMA monomers into crosslinked polymer surfaces. Two types of adhesives were used: adhesive A was a self-curing type while adhesive B was only able to polymerize during the heating process of the denture base resin because it contained no catalyst. The 4-META monomer was coupled to MMA monomer in both adhesive agents. The results indicated that the efficiency of adhesive B was superior to adhesive A. DEGDMA used as a crosslinking agent in adhesive A may have reduced bonding because the presence of a self-cured, crosslinked layer of adhesion inhibited the diffusion of MMA monomer from the denture base. The bonding between the highly crosslinked plastic teeth and the denture base was not perfect even when adhesive agents were used. Further improvements of adhesive methods and agents are necessary. CONCLUSION
The results of this study indicated that highly crosslinked plastic denture teeth showed poorer bonding to denture base resins as compared to conventional acrylic teeth. As the hardness of the teeth increased, the bonding between the teeth and the denture base decreased. The alginate mold lining
0
Adhesive B
U z
a
0.5
*
Orcho s i c
I
b: ground
ConvencFona! method
0
Crysral
7
Figure 4. Comparison of the average depths of dye penetration at the toothidenture base interface.
a : non ground
Adhesive A
I
*
Hitel
... ..... .. .. ... ... ... ... ... 1:. ... ... .. . .. ... ... .... ... .. ... ... ..... . ..... ..... .... ...... .: ...
...
h ir3 e
Hire1
T
w w
0 N
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC
1103
material disturbed and reduced the bonding interface between the denture teeth and the denture bases when it was applied on the denture teeth during preparation. The application of 4-META adhesive agents to the denture teeth significantly improved the interface bonding between highly crosslinked denture teeth and the denture base.
References 1. R. G. Craig and F. A. Peyton, Restorative Dental Materials, Mosby, St. Louis, 1975. 2. N. Nakabayashi and M. Atsuta, The Crown and Bridge Resins, Ishiyaku, Tokyo, 1979. 3. D. J. Whitman, et al. “In vitro wear rates of three types of commercial denture tooth materials,” J. Prosth. Dent., 57, 243-246 (1987).
4. J. A. von Fraunhofer, R. Razavi, and Z . Khan, ”Wear characteristics of high-strength denture teeth,” J. Prosth. Dent., 59, 173-175 (1988). 5. C. E. Schildknect, Polvmer Processes, Interscience, New York, 1956. 6. R. J. Michl, “Isosit, adnew dental material,” Quintessence lnt., 3, 29-33 11978). 7. M. Takeyama, N. Kashibuchi, N. Nakabayashi, and E. Masuhara, “Studies o n dental self-curing resin( 17)-adhesion of PMMA with bovine enamel or dental alloys,” J. Japan SOC. Dent. Appar. Muter., 19, 179-185 (1978). 8. S. Suzuki, and N. Yasuda, “The new concept of the artificial teeth with super hard resin (Mite1 OM),” Quintessence Dentl. Tech., 9, 1329-1338 (1984). 9. M. Atsuta, N. Nakabayashi, Y. Kikuchi, and Y. Uchiyama, “Evaluation of mechanical retention devices for resin veneer crown,” J. !upan Prosth. SOC.,17, 373-380 (1Y74). 10. K. Nagata, S. Suzuki, N. Nakabayashi, and E. Masuhara, ”Preparation of hard crown and bridge resin without PMMA powder,” J . Japan SOC. Dent. Appar. Muter., 8, 20-49 (1979). 11. M. Atsuta, N. Nakabayashi, and E. Masuhara, “Hard methacrylic polymers. 11,” J. Biorned. Muter. Res., 5, 183-195 (1971). 12. M. Atsuta, N. Nakabayashi, and E. Masuhara, ”Hard methacrylic polymers. 111,” J. Biomed. Mater. Res., 6, 479-487 (1972). 13. N. Yasuda, M. Atsuta, N. Nakabayashi, and E. Masuhara, ”Hard methacrylic polymers. V,” Rep. Inst. Med. Dent. Eng., Tobo Med. Dent. Univ., 6, 70-75 (1972). 14. S. Suzuki, N. Nakabayashi, and E. Masuhara, ”The evaluation of new dental resins prepared with polyfunctional methacrylate monomers,” J. Biomed. Muter. Res., 16, 257-287 (1982). 15. E. Masuhara, K. Kojima, and N. Tarumi, ”Studies of reforming dental acrylic resin XI,” Rep. Res. Inst. Dent. Muter., 9, 3 8 4 4 (1958). 16. E. Masuhara, K. Kojima, and N. Tarumi, “Studies of reforming dental acrylic resin XII,” Rep. Res. Inst. Dent. Muter., 10, 39-45 (1958). 17. E. C. Combe, Notes on Dental Materials, Churchill Livingstone, London and New York, 1975. 18. N. Nakabayashi, K. Kojima, and E. Masuhara, ”The promotion of adhesion by the infiltration of monomers into tooth substrates,” J. Biomed. Mater. Res., 16, 265-273 (1982).
Received February 1989 Accepted January 26, 1990
tional bonding method effectively improved bonding at the toothlbase resin interface. The application of 4-META adhesive bonding agents to the denture teeth improved the interface bonding of highly crosslinked plastic teeth and the denture base. Dentures with Orthosit and Mitel showed differrnces in bonding ability when two different adhesives were used.
INTRODUCTION
Denture teeth are of obvious importance not only for restoring oral function but also for maintaining appropriate jaw relationships. The two most important factors required of denture teeth are abrasion resistance and bonding between the teeth and denture base material. Porcelain has been a popular substitute for natural teeth, but suffers from lack of bonding to the denture base resin.’ In contrast, conventional acrylic denture teeth prepared from linear polymethylmethacrylate (PMMA) are not suitable as a crown substitute material because of low abrasion resistance.’ Highly crosslinked acrylic denture teeth with good abrasion resistance have been developed and are now used ~linically.~,~ There are two types of crosslinked plastic denture teeth. One is produced by copolymerization of methylmethacrylate (MMA) with a small amount of dimethacrylate (presumably triethylene glycol dimethacrylate (TEGDMA)) in the presence of finely powdered PMMA.5The other is a highly crosslinked acrylic polymer with added filler of colloidal silica instead of PMMA powders. Such crosslinked resins include Orthosit (Ivoclar Co., Liechtenstein).6 Crosslinked networks have been successfully used to improve the mechanical and physical properties of conventional acrylic resins, such as compressive strength, hardness, and abrasion resistance. However, highly crosslinked polymers tend to present new clinical problems when they are used as denture teeth. One of the problems is dislodgement of denture teeth Journal of Biomedical Materials Research, Vol. 24, 1091-1103 (1990) 0 1990 John Wiley & Sons, Inc. CCC 0021-9304/90/081091-13$04.00
SUZUKI, SAKOH, AND SHIBA
1092
from the denture base which may occur due to poor adhesive bonding to the denture base resin. As has been previously demonstrated with porcelain denture teeth, a lack of bonding to the denture base resin represents a sigruficant disadvantage. The purpose of this study was to evaluate and compare the bonding behavior of commercially available highly crosslinked plastic teeth, conventional acrylic denture teeth, and porcelain denture teeth to denture base resins. We also evaluated a new bonding method involving the application of adhesive agents to the highly crosslinked plastic denture teeth in order to improve the bonding between the denture teeth and the denture base. MATERIALS AND METHODS
The trade names, abbreviations, types of plastic, and manufacturers of denture teeth used in this study are shown in Table I. A heat polymerizing acrylic denture base resin (Acron: GC Corp., Japan) was used to prepare the specimens. The base resin selected was colorless and translucent to facilitate dye penetration measurements. A solution of alginate (Acrosep: GC Dental Indust. Corp., Japan) was used as a mold lining material. Two adhesives which contain 4-methacryloxyethyl trimellitate anhydride (4-META)7were used. The compositions of these adhesives are shown in Table II. Both adhesives were prepared by combining components 1 and 2.
Preparation of the specimens The first and second premolar and the first molar were selected from both upper and lower teeth. Denture specimens for upper and lower teeth were made independently. The basal portion of the lower teeth were ground out prior to placement in models. In these teeth, the distance between the central fossa and the bottom is 3 mm. The upper teeth were not ground. The specimens (toothhesin base denture units) were made by three different methods, as follows: In the conventional method, ground or nonground denture teeth are set up on the stone models with paraffin wax. After dewaxing, an alginate mold lining was applied to the stone model prior to packing the base resin dough. Polymerization of the base resin was then completed by heating the mold in water at 60°C for 1 h and 100°C for 30 min. After the specimens were completed, they were polished using a lathe with wet pumice. In addition, two alternate fabrication methods were used. In the first method, the application of the alginate mold lining on the stone model after dewaxing was eliminated. The second method involved the use of adhesive agents. Equal amounts of adhesive components 1 and 2 were well mixed and applied to the cervical and basal portions of the teeth prior to packing the base resin. The alginate mold lining was also used for those specimens.
~~
Livdent FE3 20 Porcelain 100 Wearless Acrylic Teeth Livdent FB 20 Plastic 100 Trubite Bioform IF" 20" Crystal ND SR-Orthosit-PE Mitel-OM"
Denture Teeth, Tradename Poly methyl methacrylate (PMMA) Partially crosslinked Acrylic" with PMMA powders Partially crosslinked Acrylicbwith PMMA powders Highly crosslinked Acrylic' with composite filler' Highly crosslinked Acrylic' with composite fillef Highly crosslinked Acrylicdwith composite filler'
-
Type of Plastic
GC GC GC Dentsply Major Ivoclar Sun-Medical
Manufacturer
"Copolymer of MMA and TEGDMA. bUnknown. 'Poly-dimethacryloxyethyl(trimethy1 hexamethylene diurethane): (PUDMA). dCopolymer of UDMA and 2,2-bis(4-methacryloxypolyethoxyphenyl)prtpane. 'Poly-UDMA with colloidal silica. 'Poly-trimethylol propane trimethacrylate with colloidal silica. *Custom made. Note. GC: G-C Dental Industrial Corp., Tokyo, Japan; Dentsply: Dentsply International INC., Pennsylvania, U.S.A.; Major: Major Dental Industry S.p.A., Torino, Italy; Ivoclar: Ivoclar AG, Schaan, Liechtenstein; Sun-Medical: Sun-Medical Corp., Kyoto, Japan.
Porcelain Wearless Livdent IPN Crystal Orthosit Mite1
Abbreviation
TABLE I List of Denture Teeth
b
m
#
W
Component 1 Component 2
Component 2
Component 1
MMA/90,4-METNlO MMAl95
MMAl60, DEGDMN25, 4-METAl15 MMAl30
Silane coupling agenff5
EthylalcohoV80
PMMAI4
Additive (wt%)
-
-
N,N-Dimethyl p-toluidine Sodium p-toluenesulfinate
Benzoyl peroxide
Catalyst
Note. MMA: Methylmethacrylate; DEGDMA: Diethylenglycol dimethacrylate; 4-META: 4-methacryloxyethyl trimellitate anhydride; Silane coupling agent: y-methacryloxypropyl trimethoxysilane.
Adhesive B
Adhesive A
Monomer (wt%)
TABLE I1 Composition of Adhesive Agents
v)
$ 8>
z .
0
z
c
,;r
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC
1095
The measurement of dye penetration To simulate the thermal changes encountered in the oral environment, thermal cycling was performed using a thermal cycling machine (Rika Industrial Corp., Japan). The specimens were alternately soaked in water at 4°C for 1min and 60°C for 1min; this process was carried out 60 times. After thermal cycling, the specimens were immersed in a 0.2% aqueous solution of basic fuchsin dye for 36 h to evaluate the degree of toothbase resin bonding quantitatively. The depths of dye penetration were measured at 10 points: 3 buccal, 3 lingual, and 4 proximal surfaces, as shown in Figure 1. The maximum dye penetration through the base resin at each point was measured to the nearest 0.5 mm. The average depth of dye penetration for each location was then calculated.
Measurement of Knoop hardness The Knoop microhardness test was used to determine the surface hardness of specimens. A diamond indentor was pressed into the specimens under a load of 25 g for 30 s. The areas of indentation were then measured using a ruler under a microscope. Knoop hardness number (KHN) was calculated as the load divided by the area of indentation. Specimens were prepared from sectioned denture teeth in order to evaluate the hardness of the inner structure. Hardness was measured at three points on the specimens such as the occlusal, middle and basal areas. The measurements were made at five points for each area. RESULTS
The percentages of toothhesin base denture units showing dye penetration to various levels at the tooth-resin base denture interface are shown in
h Figure 1. Measurement points of dye penetration depth.
SUZUKI, SAKOH, AND SHIBA
1096
Porcelain
\ \ e a rlcss
li.9
Li vdcnt
___--
---
i_T/--IG*O
C Is 1~a I
Orthosil
Mild
Figure 2. Knoop hardness of sectioned teeth
Table 111. Dentures with conventional acrylic teeth (Wearless) showed good bonding, as evidenced by the small average depths of dye penetration. However, the other denture teeth showed poorer bonding. A t-test was done to evaluate the difference between Wearless and other plastic denture teeth. All the crosslinked denture teeth, except nonground Livdent, showed a significant difference in dye penetration and, therefore, degree of bonding as compared to Wearless at the level of p < 0.01. Dentures with conventional crosslinked teeth such as Livdent and 11" showed better bonding compared to the highly crosslinked group. Dentures with highly crosslinked teeth such as Crystal, Orthosit, and Mitel exhibited poor bonding as shown by large average depths of dye penetration. Prior grinding of the teeth had little effect on bonding as measured by dye penetration except with Livdent and Orthosit; grinding increased the depth of dye penetration for these two materials. The Knoop hardness at each point on the specimens was shown in Figure 2. The conventional acrylic and crosslinked teeth (Wearless, Livdent, and IF") showed less hardness to compare with highly crosslinked teeth (Orthosit, Crystal, and Mitel). Furthermore, the results of the dye penetration analysis showed that bonding between the teeth and the denture base decreased as the Knoop hardness of the teeth increased (see Fig. 2).
1097
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC TABLE 111 Distribution of Depth of Dye Penetration at the Tooth-Resin Base Denture Interface for Various Tooth Materials Penetration (mm) Specimen Wearless NG" G" Livdent NG" G" IPN NG" G" Crystal N G ~ G"
Orthosit NG" G" Mitel
NGb G"
Porcelain NG" G"
0
0.5
1.0
1.5
2.0
2.5
96+ 96 90 62 76 78 20 12 40 24 33 48
4 4 2 30 8 16 15 20 20 38 27
0 0 0 0
0 0 0
0
0 8 26 6 4 10 6
20 8 4 5
26
0 0 8 0 8 6 17 14 26 24 23 20
0
0 8 0 0 25 8 0 6 2 0
0 0
0 0
0 0
0 0
0 0
100 100
8
0 0 0 15
0
AverageDepth * S.E.
* 0.01 * 0.01
0.02 0.02 0.09 f 0.35 2 0.24
0.03 0.09' 0.06' 0.14 0.01' 1.29 k 0.13* 1.23 t 0.10* 0.61 0.08* 0.72 f 0.09* 0.67 2 0.08* 0.42 t 0.06*
* *
*
2.50 2.50
NG = nonground, G = ground, S.E. = standard error. an = 50, bn = 40, 'Expressed as percentage of total number of samples tested. *Significantlydifferent from results for Wearless at p < 0.01 level using t-test.
The percentage of tooth-resin base denture units showing dye penetration when the alginate mold lining between the stone mold and the denture was not used are shown in Table IV. Under these preparative conditions, dentures with highly crosslinked plastic teeth such as Crystal, Orthosit, and Mitel exhibited better bonding (except nonground Orthosit) compared to the conventional method as evidenced by small average depths of dye penetration. Specifically, the decrease in average depth of dye penetration for nonground specimens was 78%for Crystal and 16% for Mitel; for ground specimens, the improvement was 85% for Crystal, 75% for Orthosit, and 57% for Mitel. The percentage of toothhesin base denture units showing dye penetration affected by adhesive agents at the tooth-resin base denture interface are shown in Table V. In general, dentures made using adhesive agents exhibited better bonding compared to both those made by the conventional method and the series in which the alginate mold lining material was eliminated, as evidenced by smaller average depths of dye penetration. In the case of Crystal, bonding was better in the absence of the alginate mold lining than in the presence of adhesives; but both cases were better than the conventional method. Dentures made with Orthosit and Mitel showed better results with adhesive B than with adhesive A as evidenced by smaller average depths of dye penetration.
62' 76 54 80 12 80
Crystal NG 14 6 14 6 20 6
1.0
28(14) 10 22(10) 8
30 46
0.5 36 2 16(6) 4 O(2) 0
1.o
NG = nonground, G = ground, S.E. = standard error. 'Expressed as percentage of total number of samples tested.
G NG G
32+ 52 50(70) 86 76(88) 92
Crystal NG G Orthosit NG
Mitel
0
~
0 0 0 0 2 2
1.5
2 2 8 2 0 2
2.0 0 0 12 0 0 0
2.5
2 0 6(4) 0 2(0) 0
1.5
0 0 O(2) 0 0 0
2.0
Penetration (mm)
0 0 O(2) 0 0 0
2.5
TABLE V Effect of Adhesive A and Adhesive B (in brackets) on Dye Penetration Depth
Specimen
~~~~
10
64
22 16 12 12
0.5
NG = nonground, G = ground, S.E. = standard error. 'Expressed as percentage of total number of samples tested.
G Orthosit NG G Mitel NG G
0
Specimen
Penetration (mm)
TABLE IV Effect of Elimination of Alginate Mold Lining on Dye Penetration Depth
E0 *
89
v,
3
9
.z
*
0.54 0.06 0.25 t 0.03 0.39 2 0.06 (0.28 ir 0.07) 0.09 -C 0.03 0.14 i 0.03 (0.07 ? 0.02) 0.04 0.01
.?i Average Depth (mean f S.E.)
*
0.29 5 0.06 0.18 f 0.05 0.66 ? 0.12 0.16 0.05 0.56 ? 0.04 0.18 f 0.05
Average Depth (mean ? S.E.)
0 \o 00
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC
1099
DISCUSSION
There are two ways to improve the mechanical properties of conventional acrylic resins used in denture teeth. One is to use a crosslinking agent such as Triethylene glycol dimethacrylate, and the other is to include inorganic fillers such as hydrophobic colloidal silica." Highly crosslinked polymers have been in~estigatedll-'~ for use in dental prostheses. The addition of crosslinking agents to MMA has been reported to improve the mechanical properties. On the other hand, the addition of crosslinking agents to MMA did not improve the abrasion resistance of PMMA.l5,I6In an attempt to reduce water uptake in crown and bridge resins, hydrophobic monomers have been investigated. A composite material which is fabricated with dimethacrylate and composite filler is commercially available.6The composite filler is superior to the powdered glass filler which is used in conventional composite filling materials in terms of bonding between the filler and the matrix resin. A very smooth surface is obtained from a polished hard resin with composite filler because of uniformity of the material. Therefore, these types of composite materials are currently being used in denture teeth. Increasing the degree of crosslinking increases the hardness and abrasion resistance of the polymer, On the other hand, the size of the polymer chain networks in highly crosslinked polymers becomes too small for interpenetration of MMA monomer from the denture base into the matrix. This results in poor bonding between the highly crosslinked plastic denture teeth and the denture base resin. Successful crosslinked plastic denture teeth, therefore, must include linear PMMA in order to increase the probability of polymer chain entanglements and thus improve the tooth/denture base interface adhesion. Mich16 reported that Orthosit has PMMA at the basal portion of the tooth in order to facilitate bonding to the denture base resin. From the results of our hardness tests, it is evident that some manufacturers have provided a layer of PMMA at the basaI portion, but this layer is sometimes ground out during clinical manipulation. On this premise, we predicted that substantial grinding at the basal level of the denture tooth would decrease bonding to the denture base resin, but the effect of grinding was not obvious in our experimental results. We suspect that one cause of this apparent contradiction is the increase in the effective surface area for bonding by the roughened surface created by grinding. The different thicknesses of PMMA layers provided by each manufacturer could also be a factor. A sectioned specimen of Orthosit (non-ground) after thermal cycling is shown in Figure 3. Thermal cycling tests were performed in order to evaluate the effect of the expansion and contraction on the denture teeth and base materials. A space could be visualized between the cervical area of the denture tooth and the base resin with higher magnification (see Fig. 3b), but it gradually decreased and eventually disappeared in the direction of the basal area where PMMA is present. At this point, both the material of the tooth and the base material are adherent (Fig. 3c).
1100
SUZUKI, SAKOH, AND SHIBA
(c)
Figure 3. A sectioned specimen with Orthosit (nonground) after thermal cycling: (a) low magnification, (b) a space at the toothhase resin interface (cervical area of tooth), (c) a fusion at the toothhase resin interface (basal area of tooth).
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC
1101
The alginate mold lining material reacts with the calcium in the stone mold material to form a film of insoluble calcium alginate.I7This film works successfully as a barrier to prevent MMA monomer from the denture base from penetrating into the mold. This barrier or separator is applied so as to avoid contacting the tooth surface, but it is usually impossible to completely eliminate such contact. Although the alginate solution should not react with the polymer surface, it may be detrimental to strong bonding between the tooth and the denture base. As a result, there could be fewer opportunities for polymer chain entanglements on the tooth surface. The results from the specimens prepared without the alginate mold lining showed good bonding, but this method is not clinically acceptable because it is quite difficult to remove the plaster from the denture after polymerization. The application of adhesive agents to denture teeth before polymerization is the best method for improving bonding strength over the conventional method. Comparisons of the average depths of dye penetration at the tooth/ denture base interface in each condition are shown in Figure 4. A t-test was performed to compare the conventional method with the other two methods for each material. The specimens with adhesive agents showed significant differences ( p < 0.01) compared to those obtained with conventional methods. This shows that the application of adhesive agents is very effective in improving the bonding between the crosslinked plastic denture teeth and the denture base resin. Nakabayashi et a1.I8 have reported the effectiveness of 4-META for increasing the adhesion to tooth substrates. They reported monomers with both hydrophobic and hydrophilic groups, like 4-META, promoted the infiltration of monomers into hard tissue. In our studies, the results of the effect of adhesive agents containing 4-META showed 4-META monomer could promote the infiltration of MMA monomers into crosslinked polymer surfaces. Two types of adhesives were used: adhesive A was a self-curing type while adhesive B was only able to polymerize during the heating process of the denture base resin because it contained no catalyst. The 4-META monomer was coupled to MMA monomer in both adhesive agents. The results indicated that the efficiency of adhesive B was superior to adhesive A. DEGDMA used as a crosslinking agent in adhesive A may have reduced bonding because the presence of a self-cured, crosslinked layer of adhesion inhibited the diffusion of MMA monomer from the denture base. The bonding between the highly crosslinked plastic teeth and the denture base was not perfect even when adhesive agents were used. Further improvements of adhesive methods and agents are necessary. CONCLUSION
The results of this study indicated that highly crosslinked plastic denture teeth showed poorer bonding to denture base resins as compared to conventional acrylic teeth. As the hardness of the teeth increased, the bonding between the teeth and the denture base decreased. The alginate mold lining
0
Adhesive B
U z
a
0.5
*
Orcho s i c
I
b: ground
ConvencFona! method
0
Crysral
7
Figure 4. Comparison of the average depths of dye penetration at the toothidenture base interface.
a : non ground
Adhesive A
I
*
Hitel
... ..... .. .. ... ... ... ... ... 1:. ... ... .. . .. ... ... .... ... .. ... ... ..... . ..... ..... .... ...... .: ...
...
h ir3 e
Hire1
T
w w
0 N
ADHESIVE BONDING OF DENTURE BASE RESINS TO PLASTIC
1103
material disturbed and reduced the bonding interface between the denture teeth and the denture bases when it was applied on the denture teeth during preparation. The application of 4-META adhesive agents to the denture teeth significantly improved the interface bonding between highly crosslinked denture teeth and the denture base.
References 1. R. G. Craig and F. A. Peyton, Restorative Dental Materials, Mosby, St. Louis, 1975. 2. N. Nakabayashi and M. Atsuta, The Crown and Bridge Resins, Ishiyaku, Tokyo, 1979. 3. D. J. Whitman, et al. “In vitro wear rates of three types of commercial denture tooth materials,” J. Prosth. Dent., 57, 243-246 (1987).
4. J. A. von Fraunhofer, R. Razavi, and Z . Khan, ”Wear characteristics of high-strength denture teeth,” J. Prosth. Dent., 59, 173-175 (1988). 5. C. E. Schildknect, Polvmer Processes, Interscience, New York, 1956. 6. R. J. Michl, “Isosit, adnew dental material,” Quintessence lnt., 3, 29-33 11978). 7. M. Takeyama, N. Kashibuchi, N. Nakabayashi, and E. Masuhara, “Studies o n dental self-curing resin( 17)-adhesion of PMMA with bovine enamel or dental alloys,” J. Japan SOC. Dent. Appar. Muter., 19, 179-185 (1978). 8. S. Suzuki, and N. Yasuda, “The new concept of the artificial teeth with super hard resin (Mite1 OM),” Quintessence Dentl. Tech., 9, 1329-1338 (1984). 9. M. Atsuta, N. Nakabayashi, Y. Kikuchi, and Y. Uchiyama, “Evaluation of mechanical retention devices for resin veneer crown,” J. !upan Prosth. SOC.,17, 373-380 (1Y74). 10. K. Nagata, S. Suzuki, N. Nakabayashi, and E. Masuhara, ”Preparation of hard crown and bridge resin without PMMA powder,” J . Japan SOC. Dent. Appar. Muter., 8, 20-49 (1979). 11. M. Atsuta, N. Nakabayashi, and E. Masuhara, “Hard methacrylic polymers. 11,” J. Biorned. Muter. Res., 5, 183-195 (1971). 12. M. Atsuta, N. Nakabayashi, and E. Masuhara, ”Hard methacrylic polymers. 111,” J. Biomed. Mater. Res., 6, 479-487 (1972). 13. N. Yasuda, M. Atsuta, N. Nakabayashi, and E. Masuhara, ”Hard methacrylic polymers. V,” Rep. Inst. Med. Dent. Eng., Tobo Med. Dent. Univ., 6, 70-75 (1972). 14. S. Suzuki, N. Nakabayashi, and E. Masuhara, ”The evaluation of new dental resins prepared with polyfunctional methacrylate monomers,” J. Biomed. Muter. Res., 16, 257-287 (1982). 15. E. Masuhara, K. Kojima, and N. Tarumi, ”Studies of reforming dental acrylic resin XI,” Rep. Res. Inst. Dent. Muter., 9, 3 8 4 4 (1958). 16. E. Masuhara, K. Kojima, and N. Tarumi, “Studies of reforming dental acrylic resin XII,” Rep. Res. Inst. Dent. Muter., 10, 39-45 (1958). 17. E. C. Combe, Notes on Dental Materials, Churchill Livingstone, London and New York, 1975. 18. N. Nakabayashi, K. Kojima, and E. Masuhara, ”The promotion of adhesion by the infiltration of monomers into tooth substrates,” J. Biomed. Mater. Res., 16, 265-273 (1982).
Received February 1989 Accepted January 26, 1990
