Impact resistance of highly crosslinked plastic denture teeth Shiro Suzuki,, Mitsuo Sakoh, and Akihiko Shiba Department of Prosthdontics, Faculty of Dentistry, Showa University, 2-7-1, Kitasenzoku, Ota-ku, Tokyo 145, Japan The impact resistance of highly crosslinked plastic denture teeth materials such as SR-Orthosit (R), Crystal-ND (R), and Mitel-OM (R) was investigated and compared to that of conventional denture teeth materials. Dentures with upper second molars were placed on a metal model. The thicknesses of the denture teeth were 1.4, 1.0, and 0.4 mm. An impact force was repeatedly applied to the central fossae of the denture teeth with an impact test machine. Impact resistance was defined
as the number of impacts sustained by the tooth material prior to fracture. The results showed that the impact resistance of highly crosslinked plastic teeth decreased with decreasing thickness, but was comparable to that of conventional plastic teeth when the teeth were 1.4 mm thick and supported by denture base resin. It is hypothesized that the layer of PMMA denture base resin beneath the denture teeth functions as a shock absorber during impact.
IN TRODUCTION
Good mechanical and physical properties are required of materials for denture teeth. Three kinds of materials-porcelain, resin, and metal- are used as denture teeth. Porcelain teeth have good abrasion resistance and hardness, but they are not ideal because the material is brittle and has no ability to bond to the denture base.' Metal teeth have better mechanical properties' but are not aesthetically acceptable, and their fabrication technique is time-consuming. Acrylic denture teeth, which are widely used, are advantageous compared to porcelain teeth in terms of fracture toughness and bonding to denture base materials. Conventional acrylic teeth made with PMMA, however, are not suitable for denture teeth because PMMA has poor abrasion resistance. Many efforts have been made to improve the polymers used as tooth crown substitutes? The creation of crosslinked networks of polymer chains and the addition of microparticle inorganic fillers could improve the mechanical and physical properties of acrylic denture teeth.4r5 Currently, highly crosslinked plastic denture teeth are commercially available; improved abrasion resistance has been proven in some st~dies."~ These new materials, how*To whom correspondence should be addressed.
Journal of Biomedical Materials Research, Vol. 24, 1661-1671 (1990) 0 1990 John Wiley & Sons, Inc. CCC 0021-9304/90/121661-11$04.00
SUZUKI, SAKOH, AND SHIBA
1662
ever, still suffer from problems such as poor bonding to the denture base resin.*,9Another problem is fracture during mastication which has occasionally been reported. These teeth had been highly ground and placed on a metal framework during denture fabrication. In the present study, the authors evaluate the impact resistance of highly crosslinked plastic teeth as compared to conventional acrylic and porcelain denture teeth. MATERIALS A N D METHODS
Materials
The denture teeth used in this study (their trade name, abbreviation, manufacturer, and mold number) are shown in Table I. Wearless is a conventional acrylic material, Livdent is a conventional crosslinked acrylic material and Crystal, Orthosit and Mitel are highly crosslinked acrylic materials. In the custom-made denture teeth, Mitel was duplicated from Orthosit by the technique of Suzuki et al."' A heat polymerizing type of acrylic denture base resin (Acron: GC Corp., Japan) was used to complete the specimens. A nickel-chromium model which has the shape of a metal base on an edentulous alveolar ridge was used as a master model (see Fig. 1). Preparation of specimens
Partial dentures were made using the upper second molars of denture teeth. Specimens were prepared on stone models which were duplicated from a master model using a silicone rubber impression material (Toshicon: Toshiba Silicon Corp., Japan). Denture teeth were ground with carborundum points to thicknesses of 1.4,1.0, and 0.4 mm at the basal area. For some specimens, the denture teeth were set up directly on the stone models, without the use of a denture base resin beneath the teeth. For the specimens which did possess the denture base resin beneath the denture teeth, sheet waxes 0.55 mm thick were put on the stone model as spacers, and the dentures were completed with translucent denture base resin by the usual method. The polymerization was carried out by heating in water at 60°C for 1 h and 100°C for 30 min. A schematic picture indicating the thicknesses of both the denture teeth and base in the specimens is shown in Figure 2. Impact testing
The impact test was made by means of a repeating impact machine (Rika Ind. Corp., Japan)." Specimens were placed on the master model and the central fossae of the denture teeth were struck with a steel rod (2 mm tip diame-
Livdent FB 20 Porcelain 100 Wearless Acrylic Teeth Livdent FB 20 Plastic 100 Crystal ND SR-Orthosit-PE Mitel-OM”
Porcelain Wearless Livdent Crystal Orthosit Mite1 PMMA (Poly methyl methacrylate) Crosslinked plastic without filler Crosslinked plastic with composite filler Crosslinked plastic with composite filler Crosslinked plastic with composite filler
-
Type of Plastic
GC GC GC Major Ivoclar Sun-Medical
Manufacturer
29M
-
77N N2
28Ml30
28Ml30
Mold Number
‘Custom-made GC: G-C Dental Industrial Corp., Tokyo, Japan; Major: Major Dental Industry S.p.A., Torino, Italy; Ivoclar: Ivoclar AG, Schaan, Liechtenstein; Sun-Medical: Sun-Medical Corp., Kyoto, Japan.
Denture Teeth, Tradename
Abbreviation
TABLE I List of Denture Teeth
H
m
m
w
C
4 z
0
z
5
$ m
w
n
.(
2
8
z
2
2 2 8
8
z
4
1664
SUZUKI, SAKOH, AND SHIBA -11.0-r-
Figure 1. Cross-section of master model for impact test.
ter) under a 5008 load. The frequency of impact was 120 times per minute and the rod drop distance was 5 mm (see Fig. 3). The impact resistance was defined as the number of impacts sustained prior to fracture. Five specimens of each thickness were tested independently for each material. RESULTS
The number of impacts sustained prior to fracture for each of the six materials tested at various tooth and base thicknesses is shown in Table 11. Porcelain teeth showed very poor impact resistance under every test condition, as evidenced by the small mean value of impacts sustained. For all of the plastic materials, the thickest (1.4 mm) tooth specimens with the 0.55 mm underlying denture base resin withstood 30,000 cycles without fracturing. Highly crosslinked materials such as Orthosit, Crystal, and Mite1 exhibited poor impact resistance at a tooth thickness of 0.4 mm without denture base resin beneath the teeth. There was a significant difference in impact resistance between specimens having the same tooth thickness, with and without base resin (t-test, p < 0.05); the specimens with base resin showed better impact resistance than those without base resin.
n t u r e Tooth
nture Base
Figure 2. Thickness of denture tooth and base.
1665
IMPACT RESISTANCE OF HIGHLY CROSSLINKED DENTURES
Figure 3. Specimen d u r i n g impaction.
The variation of impact resistance with tooth thickness for all test specimens is shown graphically in Figure 4. All specimens showed a decrease in impact resistance with decreasing tooth thickness. Specimens of highly crosslinked materials (Orthosit, Crystal, and Mitel) without base resin had consistently lower impact resistance than the specimens with base resin. The conventional crosslinked tooth material (Livdent) had the highest impact resistance of all specimens at thicknesses of 1.4 and 1.0 mm, but no significant TABLE I1 Results of Impact Test and Number of Impactions Prior to Fracture Thickness of Denture Teeth (Bases)" Denture Teeth
1.4(0.55)
l.O(O.55)
0.4(0.55)
1.4(0)
1.0(0)
0.4(0)
Orthosit
30000)
Crystal
30000)
Mitel
30000)
Livdent
30000)
Wearless
30000)
Porcelain
3 (3)
12246 (3691) 13372 (4998) 13183 (3524) 26304 (5233) 13219 (6774) 1 (0)
360 (335) 729 (280) 701 (249) 1020 (259) 1985 (749) -
1070 (600) 2446 (775) 1939 (727) 12092 (1526) 14394 (5602) 3
417 (264) 462 (313) 677 (639) 2877 (1486) 2470 (426) 1 (0)
75 (42) 88 (75) 43 (28) 705 (528) 465 (177) -
"unit: mm ( ): S.D.
(0)
SUZUKI, SAKOH, AND SHIBA
1666 40000
a, 3 ci U
m L.
30000
u LO
L. PI (0
-! a,
U
h
V
20000
+
U
m a E
H
w
0
L. Q a,
5 10000
1.4
0.4
1.0 T o o t h T h i c k n e s s ( mm
)
Figure 4. Variation of impact resistance with thickness of denture teeth.
difference compared to the other denture teeth materials was apparent at a tooth thickness of 0.4 mm. The effect of the presence of base resin beneath the teeth on impact resistance is shown in Figures 5 and 6. At a tooth thickness of 1.0 mm (Fig. 5), the specimens without base resin showed lower impact resistance than the ones with base resin, as evidenced by the smaller mean number of impact cycles sustained prior to fracture. The specimens of highly crosslinked tooth materials with underlying base resin showed an impact resistance 20-30 times greater than specimens of the same materials without base resin. Specimens of the conventional crosslinked plastic Livdent with base resin had approximately twice the impact resistance of the specimens without base resin. At a denture tooth thickness of 0.4 mm, a similar trend was observed: the impact resistance of the specimens with denture base resin beneath the teeth was higher than the impact resistance of the same material without base resin (see Fig. 6). The conventional acrylic material (Wearless) showed an impact resistance approximately 2 times that of the conventional crosslinked material Livdent, 3 times that of Crystal and Mitel, and 5 times that of Orthosit when the base resin was present.
IMPACT RESISTANCE OF HIGHLY CROSSLINKED DENTURES
1667
4000
3000
2000
1000
o
c
M
L
W
MATERIAL
Figure 5. Effect of base resin on impact resistance for denture tooth thickness of 1.0 mm.
The impact resistance of Livdent is greater than that of Wearless without underlying base resin but less than Wearless when base resin was present. Otherwise, the relative rank order of the impact resistances at thicknesses of both 1.0 and 0.4 mm appears to remain constant, i.e., 0 < C < M < W < L, with or without base resin. Alternatively, it can be reported that the impact resistance of highly crosslinked materials is consistently lower than the conventional crosslinked material and the conventional acrylic material, whether base resin is present or not.
DISCUSSION
Highly crosslinked denture teeth, which have the advantage of better abrasion resistance when compared to conventional acrylic denture teeth, are becoming very useful clinically. A metal denture base is better than an acrylic denture base in terms of greater mechanical strength, better thermal conductivity, no water uptake, etc.” A denture made of these highly crosslinked denture teeth and a metal denture base, combining the best properties of
SUZUKI, SAKOH, A N D SHIBA
1668 400
3 $
w i t t i ~ a s cr e s i n
witlrout
base
resin
300
200
* loo
*
€
n o
c
M
L
W
MATERIAL
Figure 6. Effect of base resin on impact resistance for denture tooth thickness of 0.4 mm.
both, has recently been used clinically. Although crosslinked networks make polymers harder and more resistant to abrasion, it has been suggested that the use of highly crosslinked denture teeth sacrifices some of the advantages which are associated with conventional acrylic teeth. One such problem, poor adhesive bonding to the denture base resin, has been investigated by us and other investigator^.^,^ One explanation for the poor bonding of highly crosslinked teeth to the denture base might be inhibition of the infiltration of MMA monomer into the resin by the crosslinked polymer surface. The impact test results in this study indicate that highly crosslinked polymers lose the flexibility that conventional acrylic resins possess. It is very important to evaluate the impact resistance of these highly crosslinked plastic denture teeth when they are used in combination with a metal denture base. The denture teeth are usually ground in the basal region in order to adjust the shape to the metal base structure during laboratory manipulation. The results of this study imply that reducing the thickness of denture teeth by grinding could significantly weaken the teeth.
IMPACT RESISTANCE OF HIGHLY CROSSLINKED DENTURES
1669
We investigated the effects of changing tooth thickness and the presence of denture base resin beneath the teeth in the impact resistance of highly crosslinked plastic and conventional acrylic teeth. The reduction of tooth thickness effectivelyreduced the impact resistance of denture teeth. Excellent impact resistance was shown in all teeth at a thickness of 1.4 mm with denture base resin beneath them. Furthermore, the data indicated that highly crosslinked teeth have essentially the same impact resistance as conventional acrylic teeth under these conditions. For all specimens, the impact resistance fell as tooth thickness was reduced to 1.0 mm, and was even lower at a thickness of 0.4 mm. Specimens made of the conventional acrylic tooth material Wearless and the conventional crosslinked material Livdent consistently demonstrated higher impact resistance than the highly crosslinked materials Orthosit, Crystal, and Mitel. The impact resistance of Livdent is greater than that of Wearless. Livdent is a blend polymer made with PMMA powder in a crosslinked polydimethacrylate matrix. We have previously proposed that this kind of polymer blend has mechanical and physical properties which are the arithmetic mean of the respective values for each c ~ m p o n e n tIn . ~a~ previous study, the polymer blend was shown to have better abrasion resistance than pure PMMA.I3In the present work, a similar trend was noted for impact resistance. PMMA is relatively impact resistant material, although it has low resistance to abrasion. In a polymer blend such as Livdent, impact stress could be absorbed in the PMMA filler beads. The existence of base resin beneath the denture teeth served to improve the impact resistance of the denture teeth. The denture base resin used in this study was PMMA; it contained no crosslinking agent. The data indicated this PMMA layer between the denture teeth and the metal model base acted as a shock absorber. Impact resistance could be compared as the tooth specimen thickness was 1.0 mm and the total tooth specimen thickness with base resin was 0.95 mm (0.55 + 0.4 mm) (Fig. 7). The type of denture base resin used may also affect the impact resistance of the denture teeth. A new acrylic denture base, which functions as an intermediate layer between a metal denture base and acrylic teeth, has been investigated.I4Good bonding to metal has been reported. The impact resistance of denture teeth could be improved using such a metal-adherent denture base with highly crosslinked plastic teeth. Our results have demonstrated that conventional acrylic teeth have impact resistance which is superior to highly crosslinked plastic teeth, but these materials only partially satisfy the requirements for denture tooth materials because the conventional acrylic materials have poor abrasion resistance. During this investigation other unfavorable characteristics of conventional acrylic materials were observed. During impact testing, the surfaces of teeth made of Wearless became scooped out by the steel rod which caused an insignificant increase in the distance of impaction. In contrast, the indentations on the surface of the highly crosslinked teeth remained small because of better abrasion resistance.
SUZUKI, SAKOH, AND SHIBA
n
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?,
n
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.... __.* Tooth
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T
a .... .... . .
0
.... .... .... .... .._., .... ... ...
C
.... ... .... ... .... ... .... .... ...
i;i 1%-
M
.... .... .... .... .... .... .... .... ... .... ... .... ...
L
...... .... .... .... .... .... .... .... .... .... .... ... ... .... ... .... ...
w
Figure 7. The comparative impact resistance of 1.0 mm tooth specimen and 0.95 mm tooth-base specimen.
The test unit used in this work is a simulation of a denture using a metal base. The impact force was applied to the central fossae of the denture teeth instead of the cusps. There were two reasons for this choice of impact point: first, impact stresses tend to be concentrated in this area, resulting in thinning of the tooth material in this region; second, sliding of the impact machine rod is avoided, simplifying the results. It must be noted that the denture model used in this study is not completely analogous to the clinical situation because there is also a mucous membrane beneath the denture in vivo, which is able to relieve some of the impact stresses sustained during mastication. Furthermore, there are several types of materials, such as natural teeth, metal, resin, and porcelain which may act against denture teeth in the oral environment. In future investigations, these combinations should be considered in addition to the steelhesin combinations used in this work. CONCLUSION
For all materials at a thickness of 1.4 mm, impact resistance was very high, both with and without base resin. For all materials, the impact resistance of all materials was greater when PMMA denture base resin was present between the teeth and metal base than specimens of the same tooth material without the base resin. The base resin thus acts as a shock absorber during impact. The impact resistance of the highly crosslinked materials was consistently lower than both the conventionally crosslinked plastics and the conventional acrylic material, both with and without base resin.
IMPACT RESISTANCE OF HIGHLY CROSSLINKED DENTURES
1671
References 1. S. Winkler, Essentials of Complete Denture Prostkodontics, Saunders, 2.
3. 4.
5. 6.
7. 8.
9.
10. 11.
12. 13.
14.
Philadelphia, 1979. D. Henderson and V. L. Steffel, McCracken’s Removable Partial Prosthodontic, Mosby, St. Louis, 1981. N. Nakabayashi and M. Atsuta, The Crown and Bridge Resins, Ishiyaku, Tokvo, 1979. R. J.‘Michl, “Isosit, a new dental material,” Quintessence Int., 3, 29-33 (1978). S. Suzuki, K. Nagata, N. Nakabayashi, and M. Masuhara, “Preparation of hard crown and bridge resin without PMMA (III),” J. Jpn. SOC. Dent. Apparatus Muter., 21, 89-94 (1980). D. J. Whitman, J. E. Mckinney, R.W. Hinman, R. A. Hesby, and G. B. Pelleu Jr., ”In vitro wear rates of three types of commercial denture tooth materials,” J Prosth. Dent., 57, 243-246 (1987). J. A. von Fraunhofer, R. Razavi, and Z. Khan, ”Wear characteristics of high-strength denture teeth,” J. Prosth. Dent., 59, 173-175 (1988). C.W. Caswell and B.K. Norling, “Comparative study of the bond strengths of three abrasion-resistant plastic denture teeth bonded to a cross-linked and a grafted, cross-linked denture base material,” 1. Prosth. Dent., 55, 701-708 (1986). S. Suzuki, M. Sakoh, and A. Shiba, ‘Adhesive bonding of denture base resin to plastic denture teeth,” 1. Biomed. Muter. Res., 24, 1091-1103 (1990). S. Suzuki and N. Yasuda, ”The new concept of the artificial teeth with super hard resin (Mite1 OM),” Quintessence Dent. Tech., 9, 1329-1338 (1984). S. Suzuki, “Preparation of hard crown and bridge with new composite fillers,” J. Stom. SOC.Jpnk., 48, 261-276 (1981). J. C. Hickey and G. A. Zarb, Boucher‘s Prosthodontic Treatment for Edentulous Patients, Mosby, St. Louis, 1980. S. Suzuki, N. Nakabayashi, and E. Masuhara, “The evaluation of new dental resins prepared with polyfunctional methacrylate monomers,” 1. Biomed. Muter. Res., 16, 257-287 (1982). N. Yasuda, M. Sasaki, T. Mogi, and M. Ai, “Metal adhesive denture base resin (META-DENT),” Quintessence, 1(2), 21-31 (1982).
Received February 28,1989 Accepted June 21, 1990
as the number of impacts sustained by the tooth material prior to fracture. The results showed that the impact resistance of highly crosslinked plastic teeth decreased with decreasing thickness, but was comparable to that of conventional plastic teeth when the teeth were 1.4 mm thick and supported by denture base resin. It is hypothesized that the layer of PMMA denture base resin beneath the denture teeth functions as a shock absorber during impact.
IN TRODUCTION
Good mechanical and physical properties are required of materials for denture teeth. Three kinds of materials-porcelain, resin, and metal- are used as denture teeth. Porcelain teeth have good abrasion resistance and hardness, but they are not ideal because the material is brittle and has no ability to bond to the denture base.' Metal teeth have better mechanical properties' but are not aesthetically acceptable, and their fabrication technique is time-consuming. Acrylic denture teeth, which are widely used, are advantageous compared to porcelain teeth in terms of fracture toughness and bonding to denture base materials. Conventional acrylic teeth made with PMMA, however, are not suitable for denture teeth because PMMA has poor abrasion resistance. Many efforts have been made to improve the polymers used as tooth crown substitutes? The creation of crosslinked networks of polymer chains and the addition of microparticle inorganic fillers could improve the mechanical and physical properties of acrylic denture teeth.4r5 Currently, highly crosslinked plastic denture teeth are commercially available; improved abrasion resistance has been proven in some st~dies."~ These new materials, how*To whom correspondence should be addressed.
Journal of Biomedical Materials Research, Vol. 24, 1661-1671 (1990) 0 1990 John Wiley & Sons, Inc. CCC 0021-9304/90/121661-11$04.00
SUZUKI, SAKOH, AND SHIBA
1662
ever, still suffer from problems such as poor bonding to the denture base resin.*,9Another problem is fracture during mastication which has occasionally been reported. These teeth had been highly ground and placed on a metal framework during denture fabrication. In the present study, the authors evaluate the impact resistance of highly crosslinked plastic teeth as compared to conventional acrylic and porcelain denture teeth. MATERIALS A N D METHODS
Materials
The denture teeth used in this study (their trade name, abbreviation, manufacturer, and mold number) are shown in Table I. Wearless is a conventional acrylic material, Livdent is a conventional crosslinked acrylic material and Crystal, Orthosit and Mitel are highly crosslinked acrylic materials. In the custom-made denture teeth, Mitel was duplicated from Orthosit by the technique of Suzuki et al."' A heat polymerizing type of acrylic denture base resin (Acron: GC Corp., Japan) was used to complete the specimens. A nickel-chromium model which has the shape of a metal base on an edentulous alveolar ridge was used as a master model (see Fig. 1). Preparation of specimens
Partial dentures were made using the upper second molars of denture teeth. Specimens were prepared on stone models which were duplicated from a master model using a silicone rubber impression material (Toshicon: Toshiba Silicon Corp., Japan). Denture teeth were ground with carborundum points to thicknesses of 1.4,1.0, and 0.4 mm at the basal area. For some specimens, the denture teeth were set up directly on the stone models, without the use of a denture base resin beneath the teeth. For the specimens which did possess the denture base resin beneath the denture teeth, sheet waxes 0.55 mm thick were put on the stone model as spacers, and the dentures were completed with translucent denture base resin by the usual method. The polymerization was carried out by heating in water at 60°C for 1 h and 100°C for 30 min. A schematic picture indicating the thicknesses of both the denture teeth and base in the specimens is shown in Figure 2. Impact testing
The impact test was made by means of a repeating impact machine (Rika Ind. Corp., Japan)." Specimens were placed on the master model and the central fossae of the denture teeth were struck with a steel rod (2 mm tip diame-
Livdent FB 20 Porcelain 100 Wearless Acrylic Teeth Livdent FB 20 Plastic 100 Crystal ND SR-Orthosit-PE Mitel-OM”
Porcelain Wearless Livdent Crystal Orthosit Mite1 PMMA (Poly methyl methacrylate) Crosslinked plastic without filler Crosslinked plastic with composite filler Crosslinked plastic with composite filler Crosslinked plastic with composite filler
-
Type of Plastic
GC GC GC Major Ivoclar Sun-Medical
Manufacturer
29M
-
77N N2
28Ml30
28Ml30
Mold Number
‘Custom-made GC: G-C Dental Industrial Corp., Tokyo, Japan; Major: Major Dental Industry S.p.A., Torino, Italy; Ivoclar: Ivoclar AG, Schaan, Liechtenstein; Sun-Medical: Sun-Medical Corp., Kyoto, Japan.
Denture Teeth, Tradename
Abbreviation
TABLE I List of Denture Teeth
H
m
m
w
C
4 z
0
z
5
$ m
w
n
.(
2
8
z
2
2 2 8
8
z
4
1664
SUZUKI, SAKOH, AND SHIBA -11.0-r-
Figure 1. Cross-section of master model for impact test.
ter) under a 5008 load. The frequency of impact was 120 times per minute and the rod drop distance was 5 mm (see Fig. 3). The impact resistance was defined as the number of impacts sustained prior to fracture. Five specimens of each thickness were tested independently for each material. RESULTS
The number of impacts sustained prior to fracture for each of the six materials tested at various tooth and base thicknesses is shown in Table 11. Porcelain teeth showed very poor impact resistance under every test condition, as evidenced by the small mean value of impacts sustained. For all of the plastic materials, the thickest (1.4 mm) tooth specimens with the 0.55 mm underlying denture base resin withstood 30,000 cycles without fracturing. Highly crosslinked materials such as Orthosit, Crystal, and Mite1 exhibited poor impact resistance at a tooth thickness of 0.4 mm without denture base resin beneath the teeth. There was a significant difference in impact resistance between specimens having the same tooth thickness, with and without base resin (t-test, p < 0.05); the specimens with base resin showed better impact resistance than those without base resin.
n t u r e Tooth
nture Base
Figure 2. Thickness of denture tooth and base.
1665
IMPACT RESISTANCE OF HIGHLY CROSSLINKED DENTURES
Figure 3. Specimen d u r i n g impaction.
The variation of impact resistance with tooth thickness for all test specimens is shown graphically in Figure 4. All specimens showed a decrease in impact resistance with decreasing tooth thickness. Specimens of highly crosslinked materials (Orthosit, Crystal, and Mitel) without base resin had consistently lower impact resistance than the specimens with base resin. The conventional crosslinked tooth material (Livdent) had the highest impact resistance of all specimens at thicknesses of 1.4 and 1.0 mm, but no significant TABLE I1 Results of Impact Test and Number of Impactions Prior to Fracture Thickness of Denture Teeth (Bases)" Denture Teeth
1.4(0.55)
l.O(O.55)
0.4(0.55)
1.4(0)
1.0(0)
0.4(0)
Orthosit
30000)
Crystal
30000)
Mitel
30000)
Livdent
30000)
Wearless
30000)
Porcelain
3 (3)
12246 (3691) 13372 (4998) 13183 (3524) 26304 (5233) 13219 (6774) 1 (0)
360 (335) 729 (280) 701 (249) 1020 (259) 1985 (749) -
1070 (600) 2446 (775) 1939 (727) 12092 (1526) 14394 (5602) 3
417 (264) 462 (313) 677 (639) 2877 (1486) 2470 (426) 1 (0)
75 (42) 88 (75) 43 (28) 705 (528) 465 (177) -
"unit: mm ( ): S.D.
(0)
SUZUKI, SAKOH, AND SHIBA
1666 40000
a, 3 ci U
m L.
30000
u LO
L. PI (0
-! a,
U
h
V
20000
+
U
m a E
H
w
0
L. Q a,
5 10000
1.4
0.4
1.0 T o o t h T h i c k n e s s ( mm
)
Figure 4. Variation of impact resistance with thickness of denture teeth.
difference compared to the other denture teeth materials was apparent at a tooth thickness of 0.4 mm. The effect of the presence of base resin beneath the teeth on impact resistance is shown in Figures 5 and 6. At a tooth thickness of 1.0 mm (Fig. 5), the specimens without base resin showed lower impact resistance than the ones with base resin, as evidenced by the smaller mean number of impact cycles sustained prior to fracture. The specimens of highly crosslinked tooth materials with underlying base resin showed an impact resistance 20-30 times greater than specimens of the same materials without base resin. Specimens of the conventional crosslinked plastic Livdent with base resin had approximately twice the impact resistance of the specimens without base resin. At a denture tooth thickness of 0.4 mm, a similar trend was observed: the impact resistance of the specimens with denture base resin beneath the teeth was higher than the impact resistance of the same material without base resin (see Fig. 6). The conventional acrylic material (Wearless) showed an impact resistance approximately 2 times that of the conventional crosslinked material Livdent, 3 times that of Crystal and Mitel, and 5 times that of Orthosit when the base resin was present.
IMPACT RESISTANCE OF HIGHLY CROSSLINKED DENTURES
1667
4000
3000
2000
1000
o
c
M
L
W
MATERIAL
Figure 5. Effect of base resin on impact resistance for denture tooth thickness of 1.0 mm.
The impact resistance of Livdent is greater than that of Wearless without underlying base resin but less than Wearless when base resin was present. Otherwise, the relative rank order of the impact resistances at thicknesses of both 1.0 and 0.4 mm appears to remain constant, i.e., 0 < C < M < W < L, with or without base resin. Alternatively, it can be reported that the impact resistance of highly crosslinked materials is consistently lower than the conventional crosslinked material and the conventional acrylic material, whether base resin is present or not.
DISCUSSION
Highly crosslinked denture teeth, which have the advantage of better abrasion resistance when compared to conventional acrylic denture teeth, are becoming very useful clinically. A metal denture base is better than an acrylic denture base in terms of greater mechanical strength, better thermal conductivity, no water uptake, etc.” A denture made of these highly crosslinked denture teeth and a metal denture base, combining the best properties of
SUZUKI, SAKOH, A N D SHIBA
1668 400
3 $
w i t t i ~ a s cr e s i n
witlrout
base
resin
300
200
* loo
*
€
n o
c
M
L
W
MATERIAL
Figure 6. Effect of base resin on impact resistance for denture tooth thickness of 0.4 mm.
both, has recently been used clinically. Although crosslinked networks make polymers harder and more resistant to abrasion, it has been suggested that the use of highly crosslinked denture teeth sacrifices some of the advantages which are associated with conventional acrylic teeth. One such problem, poor adhesive bonding to the denture base resin, has been investigated by us and other investigator^.^,^ One explanation for the poor bonding of highly crosslinked teeth to the denture base might be inhibition of the infiltration of MMA monomer into the resin by the crosslinked polymer surface. The impact test results in this study indicate that highly crosslinked polymers lose the flexibility that conventional acrylic resins possess. It is very important to evaluate the impact resistance of these highly crosslinked plastic denture teeth when they are used in combination with a metal denture base. The denture teeth are usually ground in the basal region in order to adjust the shape to the metal base structure during laboratory manipulation. The results of this study imply that reducing the thickness of denture teeth by grinding could significantly weaken the teeth.
IMPACT RESISTANCE OF HIGHLY CROSSLINKED DENTURES
1669
We investigated the effects of changing tooth thickness and the presence of denture base resin beneath the teeth in the impact resistance of highly crosslinked plastic and conventional acrylic teeth. The reduction of tooth thickness effectivelyreduced the impact resistance of denture teeth. Excellent impact resistance was shown in all teeth at a thickness of 1.4 mm with denture base resin beneath them. Furthermore, the data indicated that highly crosslinked teeth have essentially the same impact resistance as conventional acrylic teeth under these conditions. For all specimens, the impact resistance fell as tooth thickness was reduced to 1.0 mm, and was even lower at a thickness of 0.4 mm. Specimens made of the conventional acrylic tooth material Wearless and the conventional crosslinked material Livdent consistently demonstrated higher impact resistance than the highly crosslinked materials Orthosit, Crystal, and Mitel. The impact resistance of Livdent is greater than that of Wearless. Livdent is a blend polymer made with PMMA powder in a crosslinked polydimethacrylate matrix. We have previously proposed that this kind of polymer blend has mechanical and physical properties which are the arithmetic mean of the respective values for each c ~ m p o n e n tIn . ~a~ previous study, the polymer blend was shown to have better abrasion resistance than pure PMMA.I3In the present work, a similar trend was noted for impact resistance. PMMA is relatively impact resistant material, although it has low resistance to abrasion. In a polymer blend such as Livdent, impact stress could be absorbed in the PMMA filler beads. The existence of base resin beneath the denture teeth served to improve the impact resistance of the denture teeth. The denture base resin used in this study was PMMA; it contained no crosslinking agent. The data indicated this PMMA layer between the denture teeth and the metal model base acted as a shock absorber. Impact resistance could be compared as the tooth specimen thickness was 1.0 mm and the total tooth specimen thickness with base resin was 0.95 mm (0.55 + 0.4 mm) (Fig. 7). The type of denture base resin used may also affect the impact resistance of the denture teeth. A new acrylic denture base, which functions as an intermediate layer between a metal denture base and acrylic teeth, has been investigated.I4Good bonding to metal has been reported. The impact resistance of denture teeth could be improved using such a metal-adherent denture base with highly crosslinked plastic teeth. Our results have demonstrated that conventional acrylic teeth have impact resistance which is superior to highly crosslinked plastic teeth, but these materials only partially satisfy the requirements for denture tooth materials because the conventional acrylic materials have poor abrasion resistance. During this investigation other unfavorable characteristics of conventional acrylic materials were observed. During impact testing, the surfaces of teeth made of Wearless became scooped out by the steel rod which caused an insignificant increase in the distance of impaction. In contrast, the indentations on the surface of the highly crosslinked teeth remained small because of better abrasion resistance.
SUZUKI, SAKOH, AND SHIBA
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Figure 7. The comparative impact resistance of 1.0 mm tooth specimen and 0.95 mm tooth-base specimen.
The test unit used in this work is a simulation of a denture using a metal base. The impact force was applied to the central fossae of the denture teeth instead of the cusps. There were two reasons for this choice of impact point: first, impact stresses tend to be concentrated in this area, resulting in thinning of the tooth material in this region; second, sliding of the impact machine rod is avoided, simplifying the results. It must be noted that the denture model used in this study is not completely analogous to the clinical situation because there is also a mucous membrane beneath the denture in vivo, which is able to relieve some of the impact stresses sustained during mastication. Furthermore, there are several types of materials, such as natural teeth, metal, resin, and porcelain which may act against denture teeth in the oral environment. In future investigations, these combinations should be considered in addition to the steelhesin combinations used in this work. CONCLUSION
For all materials at a thickness of 1.4 mm, impact resistance was very high, both with and without base resin. For all materials, the impact resistance of all materials was greater when PMMA denture base resin was present between the teeth and metal base than specimens of the same tooth material without the base resin. The base resin thus acts as a shock absorber during impact. The impact resistance of the highly crosslinked materials was consistently lower than both the conventionally crosslinked plastics and the conventional acrylic material, both with and without base resin.
IMPACT RESISTANCE OF HIGHLY CROSSLINKED DENTURES
1671
References 1. S. Winkler, Essentials of Complete Denture Prostkodontics, Saunders, 2.
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Philadelphia, 1979. D. Henderson and V. L. Steffel, McCracken’s Removable Partial Prosthodontic, Mosby, St. Louis, 1981. N. Nakabayashi and M. Atsuta, The Crown and Bridge Resins, Ishiyaku, Tokvo, 1979. R. J.‘Michl, “Isosit, a new dental material,” Quintessence Int., 3, 29-33 (1978). S. Suzuki, K. Nagata, N. Nakabayashi, and M. Masuhara, “Preparation of hard crown and bridge resin without PMMA (III),” J. Jpn. SOC. Dent. Apparatus Muter., 21, 89-94 (1980). D. J. Whitman, J. E. Mckinney, R.W. Hinman, R. A. Hesby, and G. B. Pelleu Jr., ”In vitro wear rates of three types of commercial denture tooth materials,” J Prosth. Dent., 57, 243-246 (1987). J. A. von Fraunhofer, R. Razavi, and Z. Khan, ”Wear characteristics of high-strength denture teeth,” J. Prosth. Dent., 59, 173-175 (1988). C.W. Caswell and B.K. Norling, “Comparative study of the bond strengths of three abrasion-resistant plastic denture teeth bonded to a cross-linked and a grafted, cross-linked denture base material,” 1. Prosth. Dent., 55, 701-708 (1986). S. Suzuki, M. Sakoh, and A. Shiba, ‘Adhesive bonding of denture base resin to plastic denture teeth,” 1. Biomed. Muter. Res., 24, 1091-1103 (1990). S. Suzuki and N. Yasuda, ”The new concept of the artificial teeth with super hard resin (Mite1 OM),” Quintessence Dent. Tech., 9, 1329-1338 (1984). S. Suzuki, “Preparation of hard crown and bridge with new composite fillers,” J. Stom. SOC.Jpnk., 48, 261-276 (1981). J. C. Hickey and G. A. Zarb, Boucher‘s Prosthodontic Treatment for Edentulous Patients, Mosby, St. Louis, 1980. S. Suzuki, N. Nakabayashi, and E. Masuhara, “The evaluation of new dental resins prepared with polyfunctional methacrylate monomers,” 1. Biomed. Muter. Res., 16, 257-287 (1982). N. Yasuda, M. Sasaki, T. Mogi, and M. Ai, “Metal adhesive denture base resin (META-DENT),” Quintessence, 1(2), 21-31 (1982).
Received February 28,1989 Accepted June 21, 1990
