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Setting and stress relaxation behavior of resilient denture liners
Setting and stress relaxation behavior of resilient denture liners Hiroshi Murata, DDS, PhD,a Rosalina C. Haberham, DDS,b Taizo Hamada, DDS, PhD,c and Norihiro Taguchi, DDSb Hiroshima University, School of Dentistry, Hiroshima, Japan Statement of problem. Resilient denture liners are widely used for the patients who are not comfortable with correctly made conventional hard-based dentures because of thin and relatively nonresilient mucosa or severe alveolar resorption. There are several materials used for denture liners and the efficacy in their use is influenced by their viscoelastic properties. Purpose. This study evaluated the setting behavior and viscoelastic properties of various types of resilient denture liners and the changes in viscoelasticity with the passage of time. Material and methods. Four types of resilient denture liners were used. Setting behavior of 5 autopolymerizing materials was evaluated with an oscillating rheometer. Stress relaxation tests were conducted to evaluate the viscoelastic properties of 9 materials and changes that occurred over time by means of Maxwell model analogies. Results. Significant differences were found in the setting behavior of the autopolymerizing materials. The acrylic resin and fluoroethylene materials demonstrated viscoelastic properties and the silicone and polyolephin materials were found to be elastic. The acrylic resin materials exhibited the greatest changes in viscoelastic properties over time when compared with silicone, polyolephin, and fluoroethylene materials. Conclusions. The results suggest that it is important to select denture liner materials according to clinical situations because of the wide ranges of setting behavior, viscoelastic properties, and durability over time. (J Prosthet Dent 1998;80:714-22.)
CLINICAL IMPLICATIONS Resilient denture liners are used for edentulous patients to reduce functional and nonfunctional forces and relieve discomfort. Acrylic resin and fluoroethylene materials, which demonstrated viscoelastic behavior, revealed higher levels of cushioning or absorption of functional and nonfunctional forces than silicone and polyolephin materials, which behaved elastically. However, the viscoelastic properties of the silicone and polyolephin materials remained more stable over time. Therefore resilient denture liners should be chosen according to the clinical situations present.
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esilient denture liners are used for patients who cannot tolerate a conventional hard denture base.1-5 Such patients have an irregular and extensively low mandibular alveolar bone, and a thin and relatively nonresilient mucosal tissue. When functional and nonfunctional forces are applied through a hard base, the supporting tissue can be damaged, which results in chronic soreness, abused tissues, and bone loss. From a theoretical standpoint, resilient denture liners should distribute and absorb the functional and nonfunctional forces by means of a cushioning effect.6,7
Research supported in part by a Grant-in-Aid (No. 07771849, 08045065, 08771788) for scientific research from the Ministry of Education, Science and Culture, Japan. aInstructor, Department of Prosthetic Dentistry. bGraduate Student, Department of Prosthetic Dentistry. cProfessor and Chair, Department of Prosthetic Dentistry. 714 THE JOURNAL OF PROSTHETIC DENTISTRY
The efficacy in the clinical use of resilient denture liners is considered to be influenced by their viscoelastic properties, which characterize the ability of the material to achieve the cushioning effect. Several materials are available for use as resilient denture liners, for example, silicone,4 acrylic resin,8 and fluoroethylene type.9 A wide range of viscoelastic properties is found among the commercial resilient denture liners because of the different types of materials. On the other hand, the setting behavior of autopolymerizing resilient denture liners determines working time, manipulation, and adaptation between the supporting mucosa and the fitting surface of denture. Viscoelastic behavior of resilient denture liners has been measured in a variety of ways that include the creep test,10 the dynamic test,10,11 and the Shore-A hardness test.12 McCabe et al.13 devised a simple test based on the measurement of penetration depth to VOLUME 80 NUMBER 6
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Fig. 1. Schematic representation of stress relaxation curve recording characteristic of resilient denture liners.
Fig. 2. Three-element model in which 1 Maxwell element and 1 spring are connected in parallel.
Table I. Resilient denture liners tested Code
SO SR CSS KD MOLL SI TSR MB MOLT
Material
Soften Soft reverse COE super-soft Kurepeet dough Mollosil Simpa Tokuyama soft relining Molloplast-B Molteno
Type
Acrylic—autopolymerizing Acrylic—autopolymerizing Acrylic—laboratory-processed Fluoroethylene—laboratory-processed Silicone—autopolymerizing Silicone—autopolymerizing Silicone—autopolymerizing Silicone—laboratory-processed Polyolephin—laboratory-processed
characterize compliance and viscoelasticity. They reported that the relationship between the actual elastic recovery and apparent elastic behavior was determined by a penetration ratio, and reported this method as being appropriate for the purpose of international standardization of long-term soft lining materials. In this study, the stress relaxation test, a measure of decreasing stress at constant strain, was used to obtain data through application of the Maxwell model analogy. For autopolymerizing denture liners, the setting behavior was also measured with the oscillating rheometer. The purpose of this study was to evaluate the setting and stress relaxation behavior of various types of resilient denture liners, and to determine the influence of immersion in water on the viscoelasticity.
MATERIAL AND METHODS Table I lists manufacturer information for the 9 resilient denture liners evaluated. Two autopolymerizing acrylic resin materials, 1 laboratory-processed acrylic resin material, 1 processed fluoroethylene material, 3 autopolymerizing silicone, 1 processed silicone, and 1 polyolephin material were studied. These materiDECEMBER 1998
Manufacturer
Kamemizu Chem. Ind. Co., Ltd., Osaka, Japan Nissin Dental Products Inc., Kyoto, Japan Coe Laboratories Inc., Chicago, Ill. Kurecha Co., Tokyo, Japan Detax GmbH & Co., Ettlingen, Germany Kettenbach, Dental Eschenburg, Germany Tokuyama Corp., Tokyo, Japan Detax Karl Huber GmbH & Co., Karlsruhe, Germany Molten, Co., Hiroshima, Japan
als were prepared according to the manufacturers’ instructions. The method used for measuring setting time of autopolymerizing materials has been previously reported.14 The apparatus for measuring was the oscillating rheometer (Seiki Co., Tokyo, Japan). Setting time was defined as the time required for a 75% reduction in the width of the rheometer trace.14 Five tests were carried out for each material at 37°C. Materials were kept at 22° ± 1°C before testing. The measuring equipment for the stress relaxation test used in this investigation was also described previously.15 Five specimens of each material were made into disks 2 mm thick and 18 mm in diameter. The specimens were stored in distilled water at 37°C except during the measuring period. A series of stress relaxation tests were conducted at 37°C for 24 hours, 60 days, and 120 days after specimen preparation. On administration of a 20% strain, changes in load were recorded over a period of 5 minutes. Stress relaxation curves of the resilient denture liners (Fig. 1) were evaluated by the analogies of a 3-element model in which 1 Maxwell element and 1 spring are connected in parallel (Fig. 2).15 The equilibrium mod715
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Fig. 4. Setting times of 5 autopolymerizing resilient denture liners at 37°C with standard deviation bars. Connecting bars indicate no significant difference (P>.05).
Fig. 3. Rheometer traces of 5 autopolymerizing resilient denture liners at 37°C.
ulus to be Ee, then the relaxation modulus Er(t) for the 3-element model is defined as: Er(t) = σ(t)/γ0 =
σ1(0) exp(-t/τ1)+ σe γ0
= E1 (0) exp(-t/τ1) + Ee where total stress is σ(t), strain is γ0, stress on Maxwell element and degenerated element only with the spring are σ1 and σe, elastic modulus E1, coefficient of viscosity η1, and relaxation time τ1. The instantaneous moduli E0(=Er(0)) and η1, respectively, were represented by: E0 = E1+Ee=Er(0) η1 = E1τ1 The rate of stress relaxation was defined as: rate of stress relaxation = (E0-Ee)/E0 To make comparisons among the materials, the instantaneous modulus E0, elastic modulus E1, coefficient of viscosity η1, relaxation time τ1 of the Maxwell element, equilibrium modulus Ee, and rate of stress relaxation were obtained.
Statistical analyses Comparison of the setting times of the materials was subjected to a 1-way analysis of variance (ANOVA) and 716
the mean values were compared with Tukey’s method at a 5% level of significance. Two-way ANOVAs were performed to determine whether statistically significant differences were present between materials and times for E0, E1, η1, τ1, Ee, and the rate of stress relaxation. The differences of E0 and the rate of stress relaxation among the materials, which were important factors in clinical assessment, were tested 24 hours after specimen preparation with the Tukey’s method at a 5% level of significance.
RESULTS Setting times Rheometer traces and setting times of 5 autopolymerizing resilient denture liners at 37°C are depicted in Figures 3 and 4, respectively. No significant difference was found between the setting times of 2 (1 silicone and 1 acrylic resin material) of the 5 materials, which were under 5 minutes. These 2 materials had significantly shorter setting times (P<.05) than the other 3 materials (1 acrylic resin and 2 silicone materials). A significant difference existed among the setting times of these 3 materials (P<.05). The longest mean setting time (29.9 minutes) was recorded for 1 silicone.
Stress relaxation behavior ANOVA results indicate significant differences between materials for E0, E1, η1, τ1, Ee, and the rate of stress relaxation (Tables II through VII). These results also show significant effects of time of storage for E0, E1, η1, τ1, and Ee. Significant effects of time were not found in the rate of stress relaxation of the materials. Significant interaction between material and time shows that the viscoelastic properties of some materials were affected more by time of storage. Figures 5 and 6 illustrate E0 and the rate of stress relaxation of the 9 resilient denture liners 24 hours VOLUME 80 NUMBER 6
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Table II. Two-way ANOVA for the instantaneous modulus E0 of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
3.07591 2.96376 1.12145 2.76250 2.76250 3.35216 2.05405 3.55756
× × × × × × × ×
1015 1015 1014 1014 1014 1015 1014 1015
DF
10 8 2 16 16 26 108 134
Mean square
3.075909 3.704705 5.607246 1.726563 1.726563 1.289292 1.901894 2.654898
× × × × × × × ×
1014 1014 1013 1013 1013 1014 1012 1013
F
Significance of F
161.729 194.790 29.482 9.078 9.078 67.790
0.000 0.000 0.000 0.000 0.000 0.000
Table III. Two-way ANOVA for the elastic modulus E1 of the Maxwell element of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
3.13855 3.04092 9.76362 2.62745 2.62745 3.40130 1.79575 3.58087
× × × × × × × ×
1015 1015 1013 1014 1014 1015 1014 1015
DF
10 8 2 16 16 26 108 134
Mean square
3.138551 3.801144 4.881810 1.642154 1.642154 1.308191 1.662729 2.672292
× × × × × × × ×
1014 1014 1013 1013 1013 1014 1012 1013
F
Significance of F
188.759 228.609 29.360 9.876 9.876 78.677
0.000 0.000 0.000 0.000 0.000 0.000
Table IV. Two-way ANOVA for the coefficient of viscosity η1 of the Maxwell element of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
8.80009 8.60921 1.90885 7.41141 7.41141 9.54124 7.13429 1.02547
× × × × × × × ×
1016 1016 1015 1015 1015 1016 1015 1017
DF
10 8 2 16 16 26 108 134
after specimen preparation. Figures 7 through 11 and Table VIII show the variation of E0, E1, η1, Ee, the rate of stress relaxation, and τ1 with time of storage for the materials, respectively. There were marked differences in E0, E1, η1, τ1, Ee, and the rate of stress relaxation among the materials. The processed acrylic resin material had the highest E0, E1, η1, modulus, and a rate of stress relaxation 24 hours after specimen preparation. The lowest E0 and Ee values were recorded for 1 autopolymerizing silicone and the lowest E1, η1, τ1, and rate of stress relaxation values were recorded for the processed silicone. The polyolephin material had the highest Ee. No significant differences were found among the E0 of acrylic resin and polyolephin materials. These materials had significantly higher E0 (P<.05) than the other 5 materials (silicone and fluoroethylene materials). No significant differences were found among these silicone and fluoroethylene materials. DECEMBER 1998
Mean square
8.800094 1.076151 9.544244 4.632134 4.632134 3.669706 6.605823 7.652735
× × × × × × × ×
1015 1016 1014 1014 1014 1015 1013 1014
F
Significance of F
133.217 162.909 14.448 7.012 7.012 55.553
0.000 0.000 0.000 0.000 0.000 0.000
That is, there were 2 statistically significant groupings in E0 24 hours after specimen preparation. The acrylic resin and fluoroethylene materials recorded significantly higher rates of stress relaxation than the silicone and polyolephin materials. The E0, E1, and η1 rates of the acrylic resin materials increased most significantly with storage time. Changes in these values for the silicone and polyolephin materials were small and fluoroethylene exhibited intermediate change with time. All materials exhibited almost no change of the rate of stress relaxation. The changes of Ee for all materials were also small. Figure 12 illustrates the variation of the relaxation modulus Er(t) with time for the 9 resilient denture liners 24 hours after specimen preparation. The stress relaxation curves show the transition region and rubbery region of the cross-linked amorphous polymer. Acrylic resin and fluoroethylene materials produced the 717
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Table V. Two-way ANOVA for the relaxation time τ1 of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
DF
Mean square
F
Significance of F
1431.998 1288.831 143.167 327.165 327.165 1759.163 1236.619 2995.782
10 8 2 16 16 26 108 134
143.200 161.104 71.583 20.448 20.448 67.660 11.450 22.357
12.506 14.070 6.252 1.786 1.786 5.909
0.000 0.000 0.003 0.042 0.042 0.000
Table VI. Two-way ANOVA for the equilibrium modulus Ee of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
DF
Mean square
F
Significance of F
2.67019 × 1014 2.66026 × 1014 992812 × 105 6.76521 × 1012 6.76521 × 1012 2.73784 × 1014 1.36971 × 1013 2.87481 × 1014
10 8 2 16 16 26 108 134
2.670191 × 1013 3.325329 × 1013 4.964060 × 1011 4.228256 × 1011 4.228256 × 1011 1.053017 × 1013 1.268252 × 1011 2.145384 ×1012
210.541 262.198 3.914 3.334 3.334 83.029
0.000 0.000 0.023 0.000 0.000 0.000
Table VII. Two-way ANOVA for the rate of stress relaxation of 9 resilient denture liners Source of variation
Sum of squares
DF
Mean squares
F
Significance of F
Main effects Material Time 2-way interactions Material time Explained Residual Total
136026.795 135958.120 68.674 2011.982 2011.982 138038.777 3253.023 141291.800
10 8 2 16 16 26 108 134
13602.679 16994.765 34.337 125.749 125.749 5309.184 30.121 1054.416
451.607 564.224 1.140 4.175 4.175 176.264
0.000 0.000 0.324 0.000 0.000 0.000
greatest stress relaxation; conversely, the degree of stress relaxation of the silicone and polyolephin materials were smaller.
DISCUSSION Some edentulous patients cannot be made totally comfortable with conventional dentures because of alveolar bone loss, thin and relatively nonresilient mucosa, sharp alveolar bone, or severe undercuts. In these situations, one of the most effective treatments is to use resilient denture liners in the dentures. There are 2 types of resilient denture liners. One is for chairside use and the other is processed in the dental laboratory. The autopolymerizing materials can be readily applied to an existing denture at chairside. However, it is difficult to produce the optimum thickness of the liner materials. To be effective, the resilient layer must be of sufficient bulk and a thickness of 1.5 to 2 mm is 718
recommended. A wide range of setting times were found among the autopolymerizing materials studied. Mollosil and Simpa autopolymerizing materials had longer setting times. Therefore it would be better to delay placement of the denture lined with these materials and insert them in the mouth after the flow lessens to produce optimal thickness of the liner. On the other hand, the procedure for laboratory-processed materials is more complicated than that for autopolymerizing materials. However, for processed materials, optimal thickness can be controlled more easily. A previous study16 reported that the stress relaxation curves of tissue conditioners can be analogous to the 4-element model in which 2 Maxwell elements are connected in parallel. The noncross-linked amorphous polymers, like tissue conditioners, do flow and their relaxation moduli approach zero in the long term. The equilibrium modulus in the equation is Ee = 0, and the VOLUME 80 NUMBER 6
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Table VIII. Relaxation times τ1 of 9 resilient denture liners Mean relaxtion time τ1 (sec) ± SD Material
SO SR CSS KD MOLL SI TSR MB MOLT
24h
6.71 5.79 5.93 7.62 15.48 12.01 11.42 2.97 13.52
± ± ± ± ± ± ± ± ±
60 days
0.84 0.63 0.62 3.34 10.39 4.80 2.56 1.83 1.06
5.77 5.75 4.93 6.93 5.56 8.92 8.25 0.96 13.12
± ± ± ± ± ± ± ± ±
0.25 0.55 0.42 0.54 2.24 1.08 8.23 0.50 2.24
120 days
5.48 5.81 5.15 7.95 5.56 6.47 8.72 3.58 15.36
± ± ± ± ± ± ± ± ±
0.27 0.47 0.27 1.52 2.24 2.06 2.11 2.69 1.37
degenerated element with the spring only is to be eliminated. Tissue conditioners should flow for an extended period under continuous weak pressure caused by functional pressure and recovery of deformation of the mucosa beneath the denture. Resilient denture liners used in our study were of the cross-linked amorphous polymers,8,9 which do not flow for long periods, and their relaxation modulus, Er(t), reaches a definite value, which is the equilibrium modulus Ee (Fig. 1). Thus, the degenerated element with the spring only is added to the model, and the analogies of 3-element model in which 1 Maxwell element and 1 spring are connected in parallel were made to evaluate the stress relaxation behavior of resilient denture liners in this study (Fig. 2). The stress relaxation curves of 9 materials could be made analogous to this mechanical model. In the 3element model, an instantaneous force, such as mastication or swallowing, works on 2 springs of the Maxwell element and the degenerated element, namely, the instantaneously modulus E0 (=E1+Ee). After applying the instantaneous force, the force on the Maxwell element relaxes, and the force on the degenerated element is held constant. The resilient denture liners should not flow to prevent permanent deformation, which makes the denture ill-fitting and decreases the occlusal vertical dimension. Furthermore, these materials should have a cushioning effect to distribute the stresses and relieve the pain caused by mastication. The structure of the resilient denture liners and tissue conditioners is reasonable for each clinical purpose. Large differences in viscoelastic properties were found among the resilient denture liners. E0 and the rate of stress relaxation are considered to be important factors in clinical assessment. E0 of the materials were divided into 2 classes on the basis of statistically significantly differences. The order of E0 was Coe SuperSoft, Soft Reverse, Molteno, Soften > Kurepeet Dough, Tokuyama Soft Relining, Molloplast-B, Simpa, and Mollosil. The acrylic resin and polyolephin materials exhibited higher elasticity against the instantaneous DECEMBER 1998
Fig. 5. Mean values of instantaneous modulus E0 for 9 resilient denture liners 24 hours after specimen preparation with standard deviation bars. Connecting bars indicate no significant difference (P>.05).
Fig. 6. Mean values of rate of stress relaxation for 9 resilient denture liners 24 hours after specimen preparation with standard deviation bars. Connecting bars indicate no significant difference (P>.05).
pressure caused, for example, by mastication than silicone and fluoroethylene materials. The higher biting force and the ability to crush food may be obtained when the acrylic resin and polyolephin materials are used because of their higher elasticity. On the other hand, acrylic resin and fluoroethylene materials revealed a greater degree of stress relaxation than the silicone and polyolephin materials. The order of the rate of stress relaxation was Coe Super-Soft, Soft Reverse, Soften, Kurepeet Dough > Mollosil > Molteno, Simpa, Tokuyama Soft Relining > Molloplast-B. The acrylic resin and fluoroethylene materials showed viscoelastic behavior after applying the instantaneous force. However, the silicone and polyolephin materials behave elastically. Therefore acrylic resin and fluoroethylene materials may have the ability to distribute stress or stress relaxation. These materials may show 719
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Fig. 7. Variation of instantaneous modulus E0 with time of storage for 9 resilient denture liners.
Fig. 9. Variation of coefficient of viscosity η1 of Maxwell element with time of storage for 9 resilient denture liners.
Fig. 8. Variation of elastic modulus E1 of Maxwell element with time of storage for 9 resilient denture liners.
permanent deformation. The silicone and polyolephin materials retain their shape despite being subjected to functional and nonfunctional forces. A small amount of deformation allows adaptation to changes in the tissues underlying the dentures.11 Changes over time in viscoelastic properties of resilient denture liners are relevant to the liners continued effectiveness. All materials showed almost no changes in rate of stress relaxation over time. However, a wide range in the rate of changes in E0 over time was found among the materials. The acrylic resin materials showed a greater increase in the elasticity with the passage of time. This was probably due to the leaching out of the low molecular weight plasticizer and absorption of water,17 which resulted in the dimensional change, high level of impurities, and deterioration in the viscoelasticity. The fluoroethylene material showed minimal change in elasticity with time. The great changes in E0 with time were not found in the silicone and polyolephin materials. These materials, especially the processed silicone, exhibited low water absorption and solubility of components,17 and resulted in unchanged viscoelasticity for long periods of clinical use. The ideal resilient denture liners would possess higher elasticity during mastication and then behave viscously to designate the functional and nonfunctional
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Fig. 10. Variation of equilibrium modulus Ee with time of storage for 9 resilient denture liners.
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Fig. 11. Variation of rate of stress relaxation with time of storage for 9 resilient denture liners.
forces and relieve the pain. In addition, their durability in the oral environment is necessary over long periods. If the preceding assumptions are correct, the acrylic resin and fluoroethylene materials, which showed viscoelastic behavior and higher levels of cushioning effect, may best meet the requirements for the resilient denture liners from the point of view of the inherent viscoelastic properties. However, from the standpoint of durability, the silicone and polyolephin materials would be better. As stated previously, it is important to understand setting behavior, viscoelastic properties and changes with the passage of time for each resilient denture liner and to choose the material according to the clinical situations.
CONCLUSIONS Setting behavior and static viscoelastic properties of resilient denture liners were evaluated. The results of this study are summarized as follows. 1. There was a wide range of setting behavior for autopolymerizing resilient denture liners. 2. It was feasible to make the stress relaxation curves of the resilient denture liners analogous to the 3-element model in which 1 Maxwell element and 1 spring were connected in parallel. 3. There were significant differences in the viscoelastic properties among the materials. Acrylic resin and DECEMBER 1998
Fig. 12. Variation of relaxation modulus Er(t) with time of storage for 9 resilient denture liners 24 hours after specimen preparation. 721
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polyolephin materials had significantly higher instantaneous modulus E0 than the silicone and fluoroethylene materials. Acrylic resin and fluoroethylene materials exhibited significantly higher rates of stress relaxation than silicone and polyolephin materials. Acrylic resin and fluoroethylene materials exhibited viscoelastic behavior, and the silicone and polyolephin materials exhibited elastic behavior. 4. Of all materials evaluated, the acrylic resin materials demonstrated the greatest changes in viscoelasticity over time. Silicone and polyolephin materials demonstrated smaller changes with time. Change related to time for the fluoroethylene material appeared to be ranked between those of acrylic resin and silicone or polyolephin materials. REFERENCES 1. Storer R. Resilient denture base materials. Part II. Clinical trial. Br Dent J 1962;113:231-9. 2. Bates JF, Smith DC. Evaluation of indirect liners for dentures: laboratory and clinical tests. J Am Dent Assoc 1965;70:344-53. 3. Woelfel JB, Paffenbarger GC. Evaluation of complete dentures lined with resilient silicone rubber. J Am Dent Assoc 1968;76:582-90. 4. Schmidt WF, Smith DE. A six-year retrospective study of Molloplast-Blined dentures. Part I: patient response. J Prosthet Dent 1983;50:308-13. 5. Wright PS. The success and failure of denture soft-lining materials in clinical use. J Dent 1984;12:319-27. 6. Lammie GA, Storer R. A preliminary report on resilient denture plastics. J Prosthet Dent 1958;8:411-24. 7. Jepson NJ, McCabe JF, Storer R. Evaluation of the viscoelastic properties of denture soft lining materials. J Dent 1993;21:163-70. 8. McCabe JF. Soft lining materials: composition and structure. J Oral Rehabil 1976;3:273-8.
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9. Hayakawa I, Kawae M, Tsuji Y, Masuhara E. Soft denture liner of fluoroethylene copolymer and its clinical evaluation. J Prosthet Dent 1984; 51:310-3. 10. Duran RL, Powers JM, Craig RG. Viscoelastic and dynamic properties of soft liners and tissue conditioners. J Dent Res 1979;58:1801-7. 11. Wagner WC, Kawano F, Dootz ER, Koran A. Dynamic viscoelastic properties of processed soft denture liners: part I—initial properties. J Prosthet Dent 1995;73:471-7. 12. Dootz ER, Koran A, Craig RG. Physical property comparison of 11 soft denture lining materials as a function of accelerated aging. J Prosthet Dent 1993;69:114-9. 13. McCabe JF, Basker RM, Murata H, Wollwage GF. The development of a simple test method to characterise the compliance and viscoelasticity of long-term soft lining materials. Eur J Prosthodont Restorative Dent 1996; 4:77-81. 14. Murata H, Iwanaga H, Shigeto N, Hamada T. Initial flow of tissue conditioners-influence of composition and structure on gelation. J Oral Rehabil 1993;20:177-87. 15. Murata H, Shigeto N, Hamada T. Viscoelastic properties of tissue conditioners-stress relaxation test using Maxwell model analogy. J Oral Rehabil 1990;17:365-75. 16. Murata H, Hamada T, Djulaeha E, Nikawa H. Rheology of tissue conditioners. J Prosthet Dent 1998:79:188-99. 17. Kazanji MNM, Watkinson AC. Soft lining materials: their absorption of, and solubility in, artificial saliva. Br Dent J 1988;165:91-4. Reprint requests to: DR HIROSHI MURATA DEPARTMENT OF PROSTHETIC DENTISTRY HIROSHIMA UNIVERSITY, SCHOOL OF DENTISTRY 1-2-3 KASUMI, MINAMI-KU HIROSHIMA 734-8553 JAPAN Copyright © 1998 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/98/$5.00 + 0. 10/1/93125
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CLINICAL IMPLICATIONS Resilient denture liners are used for edentulous patients to reduce functional and nonfunctional forces and relieve discomfort. Acrylic resin and fluoroethylene materials, which demonstrated viscoelastic behavior, revealed higher levels of cushioning or absorption of functional and nonfunctional forces than silicone and polyolephin materials, which behaved elastically. However, the viscoelastic properties of the silicone and polyolephin materials remained more stable over time. Therefore resilient denture liners should be chosen according to the clinical situations present.
R
esilient denture liners are used for patients who cannot tolerate a conventional hard denture base.1-5 Such patients have an irregular and extensively low mandibular alveolar bone, and a thin and relatively nonresilient mucosal tissue. When functional and nonfunctional forces are applied through a hard base, the supporting tissue can be damaged, which results in chronic soreness, abused tissues, and bone loss. From a theoretical standpoint, resilient denture liners should distribute and absorb the functional and nonfunctional forces by means of a cushioning effect.6,7
Research supported in part by a Grant-in-Aid (No. 07771849, 08045065, 08771788) for scientific research from the Ministry of Education, Science and Culture, Japan. aInstructor, Department of Prosthetic Dentistry. bGraduate Student, Department of Prosthetic Dentistry. cProfessor and Chair, Department of Prosthetic Dentistry. 714 THE JOURNAL OF PROSTHETIC DENTISTRY
The efficacy in the clinical use of resilient denture liners is considered to be influenced by their viscoelastic properties, which characterize the ability of the material to achieve the cushioning effect. Several materials are available for use as resilient denture liners, for example, silicone,4 acrylic resin,8 and fluoroethylene type.9 A wide range of viscoelastic properties is found among the commercial resilient denture liners because of the different types of materials. On the other hand, the setting behavior of autopolymerizing resilient denture liners determines working time, manipulation, and adaptation between the supporting mucosa and the fitting surface of denture. Viscoelastic behavior of resilient denture liners has been measured in a variety of ways that include the creep test,10 the dynamic test,10,11 and the Shore-A hardness test.12 McCabe et al.13 devised a simple test based on the measurement of penetration depth to VOLUME 80 NUMBER 6
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Fig. 1. Schematic representation of stress relaxation curve recording characteristic of resilient denture liners.
Fig. 2. Three-element model in which 1 Maxwell element and 1 spring are connected in parallel.
Table I. Resilient denture liners tested Code
SO SR CSS KD MOLL SI TSR MB MOLT
Material
Soften Soft reverse COE super-soft Kurepeet dough Mollosil Simpa Tokuyama soft relining Molloplast-B Molteno
Type
Acrylic—autopolymerizing Acrylic—autopolymerizing Acrylic—laboratory-processed Fluoroethylene—laboratory-processed Silicone—autopolymerizing Silicone—autopolymerizing Silicone—autopolymerizing Silicone—laboratory-processed Polyolephin—laboratory-processed
characterize compliance and viscoelasticity. They reported that the relationship between the actual elastic recovery and apparent elastic behavior was determined by a penetration ratio, and reported this method as being appropriate for the purpose of international standardization of long-term soft lining materials. In this study, the stress relaxation test, a measure of decreasing stress at constant strain, was used to obtain data through application of the Maxwell model analogy. For autopolymerizing denture liners, the setting behavior was also measured with the oscillating rheometer. The purpose of this study was to evaluate the setting and stress relaxation behavior of various types of resilient denture liners, and to determine the influence of immersion in water on the viscoelasticity.
MATERIAL AND METHODS Table I lists manufacturer information for the 9 resilient denture liners evaluated. Two autopolymerizing acrylic resin materials, 1 laboratory-processed acrylic resin material, 1 processed fluoroethylene material, 3 autopolymerizing silicone, 1 processed silicone, and 1 polyolephin material were studied. These materiDECEMBER 1998
Manufacturer
Kamemizu Chem. Ind. Co., Ltd., Osaka, Japan Nissin Dental Products Inc., Kyoto, Japan Coe Laboratories Inc., Chicago, Ill. Kurecha Co., Tokyo, Japan Detax GmbH & Co., Ettlingen, Germany Kettenbach, Dental Eschenburg, Germany Tokuyama Corp., Tokyo, Japan Detax Karl Huber GmbH & Co., Karlsruhe, Germany Molten, Co., Hiroshima, Japan
als were prepared according to the manufacturers’ instructions. The method used for measuring setting time of autopolymerizing materials has been previously reported.14 The apparatus for measuring was the oscillating rheometer (Seiki Co., Tokyo, Japan). Setting time was defined as the time required for a 75% reduction in the width of the rheometer trace.14 Five tests were carried out for each material at 37°C. Materials were kept at 22° ± 1°C before testing. The measuring equipment for the stress relaxation test used in this investigation was also described previously.15 Five specimens of each material were made into disks 2 mm thick and 18 mm in diameter. The specimens were stored in distilled water at 37°C except during the measuring period. A series of stress relaxation tests were conducted at 37°C for 24 hours, 60 days, and 120 days after specimen preparation. On administration of a 20% strain, changes in load were recorded over a period of 5 minutes. Stress relaxation curves of the resilient denture liners (Fig. 1) were evaluated by the analogies of a 3-element model in which 1 Maxwell element and 1 spring are connected in parallel (Fig. 2).15 The equilibrium mod715
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Fig. 4. Setting times of 5 autopolymerizing resilient denture liners at 37°C with standard deviation bars. Connecting bars indicate no significant difference (P>.05).
Fig. 3. Rheometer traces of 5 autopolymerizing resilient denture liners at 37°C.
ulus to be Ee, then the relaxation modulus Er(t) for the 3-element model is defined as: Er(t) = σ(t)/γ0 =
σ1(0) exp(-t/τ1)+ σe γ0
= E1 (0) exp(-t/τ1) + Ee where total stress is σ(t), strain is γ0, stress on Maxwell element and degenerated element only with the spring are σ1 and σe, elastic modulus E1, coefficient of viscosity η1, and relaxation time τ1. The instantaneous moduli E0(=Er(0)) and η1, respectively, were represented by: E0 = E1+Ee=Er(0) η1 = E1τ1 The rate of stress relaxation was defined as: rate of stress relaxation = (E0-Ee)/E0 To make comparisons among the materials, the instantaneous modulus E0, elastic modulus E1, coefficient of viscosity η1, relaxation time τ1 of the Maxwell element, equilibrium modulus Ee, and rate of stress relaxation were obtained.
Statistical analyses Comparison of the setting times of the materials was subjected to a 1-way analysis of variance (ANOVA) and 716
the mean values were compared with Tukey’s method at a 5% level of significance. Two-way ANOVAs were performed to determine whether statistically significant differences were present between materials and times for E0, E1, η1, τ1, Ee, and the rate of stress relaxation. The differences of E0 and the rate of stress relaxation among the materials, which were important factors in clinical assessment, were tested 24 hours after specimen preparation with the Tukey’s method at a 5% level of significance.
RESULTS Setting times Rheometer traces and setting times of 5 autopolymerizing resilient denture liners at 37°C are depicted in Figures 3 and 4, respectively. No significant difference was found between the setting times of 2 (1 silicone and 1 acrylic resin material) of the 5 materials, which were under 5 minutes. These 2 materials had significantly shorter setting times (P<.05) than the other 3 materials (1 acrylic resin and 2 silicone materials). A significant difference existed among the setting times of these 3 materials (P<.05). The longest mean setting time (29.9 minutes) was recorded for 1 silicone.
Stress relaxation behavior ANOVA results indicate significant differences between materials for E0, E1, η1, τ1, Ee, and the rate of stress relaxation (Tables II through VII). These results also show significant effects of time of storage for E0, E1, η1, τ1, and Ee. Significant effects of time were not found in the rate of stress relaxation of the materials. Significant interaction between material and time shows that the viscoelastic properties of some materials were affected more by time of storage. Figures 5 and 6 illustrate E0 and the rate of stress relaxation of the 9 resilient denture liners 24 hours VOLUME 80 NUMBER 6
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Table II. Two-way ANOVA for the instantaneous modulus E0 of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
3.07591 2.96376 1.12145 2.76250 2.76250 3.35216 2.05405 3.55756
× × × × × × × ×
1015 1015 1014 1014 1014 1015 1014 1015
DF
10 8 2 16 16 26 108 134
Mean square
3.075909 3.704705 5.607246 1.726563 1.726563 1.289292 1.901894 2.654898
× × × × × × × ×
1014 1014 1013 1013 1013 1014 1012 1013
F
Significance of F
161.729 194.790 29.482 9.078 9.078 67.790
0.000 0.000 0.000 0.000 0.000 0.000
Table III. Two-way ANOVA for the elastic modulus E1 of the Maxwell element of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
3.13855 3.04092 9.76362 2.62745 2.62745 3.40130 1.79575 3.58087
× × × × × × × ×
1015 1015 1013 1014 1014 1015 1014 1015
DF
10 8 2 16 16 26 108 134
Mean square
3.138551 3.801144 4.881810 1.642154 1.642154 1.308191 1.662729 2.672292
× × × × × × × ×
1014 1014 1013 1013 1013 1014 1012 1013
F
Significance of F
188.759 228.609 29.360 9.876 9.876 78.677
0.000 0.000 0.000 0.000 0.000 0.000
Table IV. Two-way ANOVA for the coefficient of viscosity η1 of the Maxwell element of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
8.80009 8.60921 1.90885 7.41141 7.41141 9.54124 7.13429 1.02547
× × × × × × × ×
1016 1016 1015 1015 1015 1016 1015 1017
DF
10 8 2 16 16 26 108 134
after specimen preparation. Figures 7 through 11 and Table VIII show the variation of E0, E1, η1, Ee, the rate of stress relaxation, and τ1 with time of storage for the materials, respectively. There were marked differences in E0, E1, η1, τ1, Ee, and the rate of stress relaxation among the materials. The processed acrylic resin material had the highest E0, E1, η1, modulus, and a rate of stress relaxation 24 hours after specimen preparation. The lowest E0 and Ee values were recorded for 1 autopolymerizing silicone and the lowest E1, η1, τ1, and rate of stress relaxation values were recorded for the processed silicone. The polyolephin material had the highest Ee. No significant differences were found among the E0 of acrylic resin and polyolephin materials. These materials had significantly higher E0 (P<.05) than the other 5 materials (silicone and fluoroethylene materials). No significant differences were found among these silicone and fluoroethylene materials. DECEMBER 1998
Mean square
8.800094 1.076151 9.544244 4.632134 4.632134 3.669706 6.605823 7.652735
× × × × × × × ×
1015 1016 1014 1014 1014 1015 1013 1014
F
Significance of F
133.217 162.909 14.448 7.012 7.012 55.553
0.000 0.000 0.000 0.000 0.000 0.000
That is, there were 2 statistically significant groupings in E0 24 hours after specimen preparation. The acrylic resin and fluoroethylene materials recorded significantly higher rates of stress relaxation than the silicone and polyolephin materials. The E0, E1, and η1 rates of the acrylic resin materials increased most significantly with storage time. Changes in these values for the silicone and polyolephin materials were small and fluoroethylene exhibited intermediate change with time. All materials exhibited almost no change of the rate of stress relaxation. The changes of Ee for all materials were also small. Figure 12 illustrates the variation of the relaxation modulus Er(t) with time for the 9 resilient denture liners 24 hours after specimen preparation. The stress relaxation curves show the transition region and rubbery region of the cross-linked amorphous polymer. Acrylic resin and fluoroethylene materials produced the 717
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Table V. Two-way ANOVA for the relaxation time τ1 of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
DF
Mean square
F
Significance of F
1431.998 1288.831 143.167 327.165 327.165 1759.163 1236.619 2995.782
10 8 2 16 16 26 108 134
143.200 161.104 71.583 20.448 20.448 67.660 11.450 22.357
12.506 14.070 6.252 1.786 1.786 5.909
0.000 0.000 0.003 0.042 0.042 0.000
Table VI. Two-way ANOVA for the equilibrium modulus Ee of 9 resilient denture liners Source of variation
Main effects Material Time 2-way interactions Material time Explained Residual Total
Sum of squares
DF
Mean square
F
Significance of F
2.67019 × 1014 2.66026 × 1014 992812 × 105 6.76521 × 1012 6.76521 × 1012 2.73784 × 1014 1.36971 × 1013 2.87481 × 1014
10 8 2 16 16 26 108 134
2.670191 × 1013 3.325329 × 1013 4.964060 × 1011 4.228256 × 1011 4.228256 × 1011 1.053017 × 1013 1.268252 × 1011 2.145384 ×1012
210.541 262.198 3.914 3.334 3.334 83.029
0.000 0.000 0.023 0.000 0.000 0.000
Table VII. Two-way ANOVA for the rate of stress relaxation of 9 resilient denture liners Source of variation
Sum of squares
DF
Mean squares
F
Significance of F
Main effects Material Time 2-way interactions Material time Explained Residual Total
136026.795 135958.120 68.674 2011.982 2011.982 138038.777 3253.023 141291.800
10 8 2 16 16 26 108 134
13602.679 16994.765 34.337 125.749 125.749 5309.184 30.121 1054.416
451.607 564.224 1.140 4.175 4.175 176.264
0.000 0.000 0.324 0.000 0.000 0.000
greatest stress relaxation; conversely, the degree of stress relaxation of the silicone and polyolephin materials were smaller.
DISCUSSION Some edentulous patients cannot be made totally comfortable with conventional dentures because of alveolar bone loss, thin and relatively nonresilient mucosa, sharp alveolar bone, or severe undercuts. In these situations, one of the most effective treatments is to use resilient denture liners in the dentures. There are 2 types of resilient denture liners. One is for chairside use and the other is processed in the dental laboratory. The autopolymerizing materials can be readily applied to an existing denture at chairside. However, it is difficult to produce the optimum thickness of the liner materials. To be effective, the resilient layer must be of sufficient bulk and a thickness of 1.5 to 2 mm is 718
recommended. A wide range of setting times were found among the autopolymerizing materials studied. Mollosil and Simpa autopolymerizing materials had longer setting times. Therefore it would be better to delay placement of the denture lined with these materials and insert them in the mouth after the flow lessens to produce optimal thickness of the liner. On the other hand, the procedure for laboratory-processed materials is more complicated than that for autopolymerizing materials. However, for processed materials, optimal thickness can be controlled more easily. A previous study16 reported that the stress relaxation curves of tissue conditioners can be analogous to the 4-element model in which 2 Maxwell elements are connected in parallel. The noncross-linked amorphous polymers, like tissue conditioners, do flow and their relaxation moduli approach zero in the long term. The equilibrium modulus in the equation is Ee = 0, and the VOLUME 80 NUMBER 6
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Table VIII. Relaxation times τ1 of 9 resilient denture liners Mean relaxtion time τ1 (sec) ± SD Material
SO SR CSS KD MOLL SI TSR MB MOLT
24h
6.71 5.79 5.93 7.62 15.48 12.01 11.42 2.97 13.52
± ± ± ± ± ± ± ± ±
60 days
0.84 0.63 0.62 3.34 10.39 4.80 2.56 1.83 1.06
5.77 5.75 4.93 6.93 5.56 8.92 8.25 0.96 13.12
± ± ± ± ± ± ± ± ±
0.25 0.55 0.42 0.54 2.24 1.08 8.23 0.50 2.24
120 days
5.48 5.81 5.15 7.95 5.56 6.47 8.72 3.58 15.36
± ± ± ± ± ± ± ± ±
0.27 0.47 0.27 1.52 2.24 2.06 2.11 2.69 1.37
degenerated element with the spring only is to be eliminated. Tissue conditioners should flow for an extended period under continuous weak pressure caused by functional pressure and recovery of deformation of the mucosa beneath the denture. Resilient denture liners used in our study were of the cross-linked amorphous polymers,8,9 which do not flow for long periods, and their relaxation modulus, Er(t), reaches a definite value, which is the equilibrium modulus Ee (Fig. 1). Thus, the degenerated element with the spring only is added to the model, and the analogies of 3-element model in which 1 Maxwell element and 1 spring are connected in parallel were made to evaluate the stress relaxation behavior of resilient denture liners in this study (Fig. 2). The stress relaxation curves of 9 materials could be made analogous to this mechanical model. In the 3element model, an instantaneous force, such as mastication or swallowing, works on 2 springs of the Maxwell element and the degenerated element, namely, the instantaneously modulus E0 (=E1+Ee). After applying the instantaneous force, the force on the Maxwell element relaxes, and the force on the degenerated element is held constant. The resilient denture liners should not flow to prevent permanent deformation, which makes the denture ill-fitting and decreases the occlusal vertical dimension. Furthermore, these materials should have a cushioning effect to distribute the stresses and relieve the pain caused by mastication. The structure of the resilient denture liners and tissue conditioners is reasonable for each clinical purpose. Large differences in viscoelastic properties were found among the resilient denture liners. E0 and the rate of stress relaxation are considered to be important factors in clinical assessment. E0 of the materials were divided into 2 classes on the basis of statistically significantly differences. The order of E0 was Coe SuperSoft, Soft Reverse, Molteno, Soften > Kurepeet Dough, Tokuyama Soft Relining, Molloplast-B, Simpa, and Mollosil. The acrylic resin and polyolephin materials exhibited higher elasticity against the instantaneous DECEMBER 1998
Fig. 5. Mean values of instantaneous modulus E0 for 9 resilient denture liners 24 hours after specimen preparation with standard deviation bars. Connecting bars indicate no significant difference (P>.05).
Fig. 6. Mean values of rate of stress relaxation for 9 resilient denture liners 24 hours after specimen preparation with standard deviation bars. Connecting bars indicate no significant difference (P>.05).
pressure caused, for example, by mastication than silicone and fluoroethylene materials. The higher biting force and the ability to crush food may be obtained when the acrylic resin and polyolephin materials are used because of their higher elasticity. On the other hand, acrylic resin and fluoroethylene materials revealed a greater degree of stress relaxation than the silicone and polyolephin materials. The order of the rate of stress relaxation was Coe Super-Soft, Soft Reverse, Soften, Kurepeet Dough > Mollosil > Molteno, Simpa, Tokuyama Soft Relining > Molloplast-B. The acrylic resin and fluoroethylene materials showed viscoelastic behavior after applying the instantaneous force. However, the silicone and polyolephin materials behave elastically. Therefore acrylic resin and fluoroethylene materials may have the ability to distribute stress or stress relaxation. These materials may show 719
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Fig. 7. Variation of instantaneous modulus E0 with time of storage for 9 resilient denture liners.
Fig. 9. Variation of coefficient of viscosity η1 of Maxwell element with time of storage for 9 resilient denture liners.
Fig. 8. Variation of elastic modulus E1 of Maxwell element with time of storage for 9 resilient denture liners.
permanent deformation. The silicone and polyolephin materials retain their shape despite being subjected to functional and nonfunctional forces. A small amount of deformation allows adaptation to changes in the tissues underlying the dentures.11 Changes over time in viscoelastic properties of resilient denture liners are relevant to the liners continued effectiveness. All materials showed almost no changes in rate of stress relaxation over time. However, a wide range in the rate of changes in E0 over time was found among the materials. The acrylic resin materials showed a greater increase in the elasticity with the passage of time. This was probably due to the leaching out of the low molecular weight plasticizer and absorption of water,17 which resulted in the dimensional change, high level of impurities, and deterioration in the viscoelasticity. The fluoroethylene material showed minimal change in elasticity with time. The great changes in E0 with time were not found in the silicone and polyolephin materials. These materials, especially the processed silicone, exhibited low water absorption and solubility of components,17 and resulted in unchanged viscoelasticity for long periods of clinical use. The ideal resilient denture liners would possess higher elasticity during mastication and then behave viscously to designate the functional and nonfunctional
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Fig. 10. Variation of equilibrium modulus Ee with time of storage for 9 resilient denture liners.
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Fig. 11. Variation of rate of stress relaxation with time of storage for 9 resilient denture liners.
forces and relieve the pain. In addition, their durability in the oral environment is necessary over long periods. If the preceding assumptions are correct, the acrylic resin and fluoroethylene materials, which showed viscoelastic behavior and higher levels of cushioning effect, may best meet the requirements for the resilient denture liners from the point of view of the inherent viscoelastic properties. However, from the standpoint of durability, the silicone and polyolephin materials would be better. As stated previously, it is important to understand setting behavior, viscoelastic properties and changes with the passage of time for each resilient denture liner and to choose the material according to the clinical situations.
CONCLUSIONS Setting behavior and static viscoelastic properties of resilient denture liners were evaluated. The results of this study are summarized as follows. 1. There was a wide range of setting behavior for autopolymerizing resilient denture liners. 2. It was feasible to make the stress relaxation curves of the resilient denture liners analogous to the 3-element model in which 1 Maxwell element and 1 spring were connected in parallel. 3. There were significant differences in the viscoelastic properties among the materials. Acrylic resin and DECEMBER 1998
Fig. 12. Variation of relaxation modulus Er(t) with time of storage for 9 resilient denture liners 24 hours after specimen preparation. 721
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polyolephin materials had significantly higher instantaneous modulus E0 than the silicone and fluoroethylene materials. Acrylic resin and fluoroethylene materials exhibited significantly higher rates of stress relaxation than silicone and polyolephin materials. Acrylic resin and fluoroethylene materials exhibited viscoelastic behavior, and the silicone and polyolephin materials exhibited elastic behavior. 4. Of all materials evaluated, the acrylic resin materials demonstrated the greatest changes in viscoelasticity over time. Silicone and polyolephin materials demonstrated smaller changes with time. Change related to time for the fluoroethylene material appeared to be ranked between those of acrylic resin and silicone or polyolephin materials. REFERENCES 1. Storer R. Resilient denture base materials. Part II. Clinical trial. Br Dent J 1962;113:231-9. 2. Bates JF, Smith DC. Evaluation of indirect liners for dentures: laboratory and clinical tests. J Am Dent Assoc 1965;70:344-53. 3. Woelfel JB, Paffenbarger GC. Evaluation of complete dentures lined with resilient silicone rubber. J Am Dent Assoc 1968;76:582-90. 4. Schmidt WF, Smith DE. A six-year retrospective study of Molloplast-Blined dentures. Part I: patient response. J Prosthet Dent 1983;50:308-13. 5. Wright PS. The success and failure of denture soft-lining materials in clinical use. J Dent 1984;12:319-27. 6. Lammie GA, Storer R. A preliminary report on resilient denture plastics. J Prosthet Dent 1958;8:411-24. 7. Jepson NJ, McCabe JF, Storer R. Evaluation of the viscoelastic properties of denture soft lining materials. J Dent 1993;21:163-70. 8. McCabe JF. Soft lining materials: composition and structure. J Oral Rehabil 1976;3:273-8.
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9. Hayakawa I, Kawae M, Tsuji Y, Masuhara E. Soft denture liner of fluoroethylene copolymer and its clinical evaluation. J Prosthet Dent 1984; 51:310-3. 10. Duran RL, Powers JM, Craig RG. Viscoelastic and dynamic properties of soft liners and tissue conditioners. J Dent Res 1979;58:1801-7. 11. Wagner WC, Kawano F, Dootz ER, Koran A. Dynamic viscoelastic properties of processed soft denture liners: part I—initial properties. J Prosthet Dent 1995;73:471-7. 12. Dootz ER, Koran A, Craig RG. Physical property comparison of 11 soft denture lining materials as a function of accelerated aging. J Prosthet Dent 1993;69:114-9. 13. McCabe JF, Basker RM, Murata H, Wollwage GF. The development of a simple test method to characterise the compliance and viscoelasticity of long-term soft lining materials. Eur J Prosthodont Restorative Dent 1996; 4:77-81. 14. Murata H, Iwanaga H, Shigeto N, Hamada T. Initial flow of tissue conditioners-influence of composition and structure on gelation. J Oral Rehabil 1993;20:177-87. 15. Murata H, Shigeto N, Hamada T. Viscoelastic properties of tissue conditioners-stress relaxation test using Maxwell model analogy. J Oral Rehabil 1990;17:365-75. 16. Murata H, Hamada T, Djulaeha E, Nikawa H. Rheology of tissue conditioners. J Prosthet Dent 1998:79:188-99. 17. Kazanji MNM, Watkinson AC. Soft lining materials: their absorption of, and solubility in, artificial saliva. Br Dent J 1988;165:91-4. Reprint requests to: DR HIROSHI MURATA DEPARTMENT OF PROSTHETIC DENTISTRY HIROSHIMA UNIVERSITY, SCHOOL OF DENTISTRY 1-2-3 KASUMI, MINAMI-KU HIROSHIMA 734-8553 JAPAN Copyright © 1998 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/98/$5.00 + 0. 10/1/93125
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