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Dynamic Viscoelasticity of Soft Liners and Masticatory Function

¹ Department of Prosthetic Dentistry, Hiroshima University Faculty of Dentistry, 1-2-3 Kasumi, Minami-ku, Hiroshima,734-8553, Japan; and
² Dental Materials Science Unit, The Dental School, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne, NE2 4BW, United Kingdom;

Correspondence: *corresponding author, hmurata{at}hiroshima-u.ac.jp

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Soft denture liners are used for edentulous patients to cushion functional forces. We hypothesized that the application of soft liners having viscoelastic properties would lead to the most marked improvement in masticatory function. The shear storage modulus (G'), shear loss modulus (G''), and loss tangent (tan ) were determined for 6 materials by means of a dynamic viscoelastometer. Masticatory function of ten subjects was evaluated by measurements of maximum bite forces and chewing times and frequencies for 2 food samples, and by the use of visual analogue scales. The acrylic materials exhibited viscoelastic behavior, while the silicones exhibited elastic behavior. The improvement in masticatory function compared with hard resin was found to be in the order: acrylic permanent materials > silicone > acrylic temporary materials. The results suggest that the use of materials with higher tan and G' provides the most optimum masticatory function for patients requiring the provision of soft liners on their dentures.

Key Words: soft denture liners • dynamic viscoelastic properties • masticatory function • visual analogue scales

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Some edentulous patients cannot tolerate a conventional hard denture base due to the presence of a thin and relatively non-resilient mucosa or due to severe alveolar resorption. in these situations, permanent soft liners are sometimes applied to the fitting surface of the denture after the abused tissues are first conditioned over a period of a few days by means of temporary soft liners (Wright, 1984). The influence of the thickness of temporary soft lining on masticatory function—including the maximum bite force, chewing time, and chewing frequency of test food samples—has been studied previously (Glantzet al., 1988). Despite the fact that there are several types of permanent soft lining materials (Mccabe, 1976), which exhibit a wide range of viscoelasticity, it is surprising that little information is available on the effect of their viscoelasticity on masticatory function.

The viscoelastic properties of soft liners have been evaluated by creep tests and stress relaxation methods (Murata et al., 1996, 1998). These static measurements have predicted changes of material behavior over time but have not been able to demonstrate the behavior of materials under high-frequency loading. In practice, materials are exposed both to rapidly applied forces caused by mastication and to long-term, low-level forces caused by functional pressure or changes in the oral mucosa during resting. For a clinically meaningful test to be developed, there must be an evaluation of rheological parameters by the rigorous use of both cyclic and rapid applications of stress and stresses applied under quasi-static conditions. One instrument which meets this requirement is a dynamic viscoelastometer based on the principle of non-resonance forced vibration. The temperature-dependence of the deformation properties of soft liners has been determined at constant frequency by dynamic mechanical thermal analysis (Waters et al., 1996). Although this study has provided useful information on the viscoelastic properties of soft liners, we must determine the frequency-dependent properties of these materials to gain an understanding of the relationship between properties and function.

The purpose of this investigation was to evaluate the dynamic viscoelastic properties of various types of soft denture liners over a wide range of frequencies and, second, to study the effect of dynamic viscoelasticity on masticatory function. It was hypothesized that the improvement in masticatory function would be greater in dentures lined with the soft liners which have viscoelastic properties than in those lined with the soft liners which have elastic properties.

	MATERIALS AND METHODS

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Dynamic Mechanical Tests
Two acrylic temporary soft liners [Hydro-Cast (Kay-See Dental Mfg. Co., Kansas City, MO, USA), Visco-Gel (DeTrey Division, Dentsply Ltd., Weybridge, Surrey, England)], two silicone permanent soft liners [Molloplast-B (Detax Karl Huber GmbH & Co., Karlsruhe, Germany), Tokuyama Soft Relining (Tokuyama Corp., Tokyo, Japan)], and two acrylic permanent soft liners [COE Super-Soft (GC America Inc., Chicago, IL, USA), Soft Reverse (Nissin Dental Products Inc., Kyoto, Japan)] were selected for laboratory evaluation.

Dynamic viscoelastic properties of the test materials were determined by means of an automatic dynamic viscoelastometer (Rheovibron DDV-25FP, Orientec Corp., Tokyo, Japan). This device is based on the principle of non-resonance forced vibration and measures the response of a material to a sinusoidal or other periodic stress. We prepared 5 pairs of specimens of each material (2 mm thickness, 30 mm long, and 20 mm wide) by packing dough into dental stone moulds that had been prepared by investing metal blanks in a flask. Proportioning, mixing, and curing procedures were as recommended by the manufacturers of each product. After setting, specimens were removed from the moulds.

The complex dynamic shear modulus (G*), shear storage modulus (G'), shear loss modulus (G''), and loss tangent (tan ) were determined at 37°C on pairs of two-hour-old specimens with the use of a shearing jig. Testing was performed over a frequency range of 0.01 to 100 Hz at a strain of 0.7%.

G*, G', G'', and tan are defined as follows (Ferry, 1980):

(1)

(2)

(3)

(4)

where i = -1.

G' represents the elastic component of material behavior, whereas G'' represents the viscous component of material behavior.

A periodic experiment at frequency is qualitatively equivalent to a transient experiment at time t = 1/. Frequency values of 0.01 Hz and especially 1 Hz are considered to be important for assessment of the clinical significance of the results of dynamic mechanical tests, because these values simulate behavior at long times (i.e., at rest) and at the masticatory rhythm, respectively.

Comparisons of the rheological data were made by one-way Analysis of Variance (ANOVA) combined with a Tukey multiple-range test.

Functional Tests
Ten complete-denture wearers (mean age, 76.0 yrs; range, 72 to 84) were selected as test subjects. The dentures had fulfilled all the selection criteria with respect to good fit, stability, extension of the denture bases, and satisfactory occlusion. However, the subjects all exhibited mandibular alveolar bone loss and complained of masticatory pain. They were not satisfied with conventional hard-based mandibular dentures, even after repeated adjustment by specialists. Consequently, the specialists judged it appropriate to apply a soft denture liner to the subjects' dentures. The protocol for this experiment was approved by the appropriate committee at Hiroshima University, and ethical approval was obtained. Informed consent was obtained from each patient before commencement of the study.

Visco-Gel (temporary soft liner), Tokuyama Soft Relining (silicone permanent soft liner), and Soft Reverse (acrylic permanent soft liner) were selected from among the three types of materials which had been characterized by laboratory testing and were applied, in turn, to the mandibular dentures at a thickness of 2 mm. Hard resin-based dentures were also tested.

Masticatory function in subjects with the denture liners was evaluated by means of measurements of maximum bite force, chewing time, and chewing frequency for test food samples (Glantz et al., 1988) after the subjects had worn the dentures 1 wk after receiving the soft lining.

Maximum bite forces were recorded by means of an occlusal analysis system (Dental Prescale System, Fuji Photo Film Co., Tokyo, Japan) (Hidaka et al., 1999). This system uses 98-µm-thick pressure-sensitive sheets and an analyzing computer, linked to a color scanner. The value of the occlusal force and the area over which this is applied are determined from the color change which occurs in the pressure-sensitive sheet. It has previously been determined that there is a highly significant relationship between the applied load and the readout load of the Dental Prescale System (Hidaka et al., 1999). The subjects were asked to bite as hard as possible for 2 sec. This was repeated 5 times with a 60-second relaxation period between bites.

The chewing times and the number of chewing strokes before swallowing (Glantz et al., 1988) for pickled radish, representing a hard food, and ham, a soft food having dimensions 15 x 15 x 15 mm, were recorded. All the tests were repeated 5 times.

Visual analogue scales (VAS) (Seymour et al., 1985) were used for the patients' subjective assessments of satisfaction with their lined dentures, i.e., comfort and pain-free function. Subjects were asked the following question: "Did you manage your daily meals satisfactorily with your present denture during the past week?" They were given a sheet of paper with a horizontal 100-mm line, marked at the left with "unsatisfactory" (= 0) and on the right with "satisfactory" (= 100). As a guarantee that the subjects understood the question, the left side of this line was also labeled "I cannot chew even soft foods and I feel a marked degree of inconvenience" and the right side "I can chew all the foods that I try and I do not feel any inconvenience". Divisions or numbers, which can interfere with the distribution of the results, were omitted. The subjects were asked to mark the line at a point corresponding to their judgment of satisfaction with the lined dentures.

Differences among these values were tested by non-parametric Friedman two-way ANOVA test with the use of SPSS software (SPSS Inc., Chicago, IL, USA).

	RESULTS

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Dynamic Mechanical Tests
The dynamic viscoelastic behavior of the acrylic soft liners showed sensitivity to changes in frequency (Fig. 1). The acrylic materials exhibited higher values of G' and G'' at higher frequencies. The dependence upon frequency was particularly noticeable for the acrylic permanent materials (COE Super-Soft and Soft Reverse). Tan of the acrylic permanent materials increased as the frequency increased from 0.01 to 10 Hz, then decreased again at higher frequencies. Conversely, tan of the acrylic temporary materials (Hydro-Cast and Visco-Gel) decreased as the frequency increased from 0.01 to 10 Hz, then increased again at higher frequencies. The viscoelastic behavior of the silicone permanent soft liners (Molloplast-B and Tokuyama Soft Relining) was not markedly frequency-dependent (Fig. 1).

View larger version (22K): [in this window] [in a new window]   Figure 1. Dependence of G' (A), G'' (B), and tan (C) on frequency for 6 soft denture liners. Markers indicate the 6 materials: Hydro-Cast (), Visco-Gel (O), Molloplast-B (), Tokuyama Soft Relining (), COE Super-Soft (), and Soft Reverse ().

View larger version (25K): [in this window] [in a new window]   Figure 2. Mean values of G' (A), G'' (B), and tan (C) of 6 soft denture liners at 1 Hz. Error bars are standard deviations (n = 5). Connecting bars indicate no significant difference (p > 0.05) by ANOVA and Tukey tests.

View larger version (24K): [in this window] [in a new window]   Figure 3. Box-Whisker plots of maximum bite forces (A), chewing times (B), and chewing frequencies (C) for the chewing of two test food samples ( , a hard food; , a soft food), and VAS values (D) for ten subjects with lined complete mandibular dentures. N = 10. Friedman test indicated significant differences among the denture liners for maximum bite forces (2 = 23.52, df = 3, p < 0.0001), chewing times for a hard food (2 = 14.25, df = 3, p < 0.005) and a soft food (2 = 7.98, df = 3, p < 0.05, p < 0.05), chewing frequencies for a hard food (2 = 14.88, df = 3, p < 0.005), and VAS values (2 = 23.88, df = 3, p < 0.0001). No significant differences were found among the materials for chewing frequencies for a soft food (2 = 2.43, df = 3, p = 0.49). o = outliers, + = extremes.

The mean rank in terms of chewing times and frequencies for the 4 types of base (1 quickest, 4 slowest) for a hard food was: acrylic permanent soft liner (1.60; 1.70) < silicone permanent soft liner (2.15; 1.90) < temporary soft liner (2.55; 2.70) < hard resin (3.70; 3.70). The ranking of chewing times of a soft food was: acrylic permanent soft liner (1.95) < temporary soft liner (2.15) < silicone permanent soft liner (2.45) < hard resin (3.45).

The VAS values for the patients' assessment of satisfaction with the soft-lined dentures were significantly (p < 0.0001) higher than those with the hard-based dentures. All subjects were more satisfied with the soft-lined dentures. The mean rank (4 most satisfied, 1 least satisfied) in terms of VAS values was: acrylic permanent soft liner (3.50) > temporary soft liner (3.30) > silicone permanent soft liner (2.20) > hard resin (1.00).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
There are many kinds of soft denture liners with various applications. Soft liners fall into two main categories: (1) temporary soft liners (tissue conditioners) and (2) permanent soft liners. Furthermore, permanent soft liners may be divided into four groups according to their composition, as follows: (i) heat-cured silicone, (ii) cold-cured silicone, (iii) heat-cured acrylic resin, and (iv) cold-cured acrylic resin. The acrylic temporary materials contain no initiator and no monomer, and are comprised of non-cross-linked amorphous polymers (McCabe, 1976; Jones et al., 1988). Both the silicone and acrylic permanent materials are comprised of cross-linked amorphous polymers (McCabe, 1976, 1998). The 6 soft liners used in this study were chosen as being representative of these groups.

The acrylic soft liners had higher tan than the silicone soft liners. That is, the acrylic materials demonstrated viscoelastic properties, and the silicones were found to be elastic, with tan approaching zero and almost independent of frequency. However, the acrylic permanent materials differed from the temporary materials through the nature of the dependence of tan on frequency. Large differences in dynamic viscoelasticity among the materials are most likely due to differences in composition and structure. In high-damping materials and especially at the damping peak, much of the energy to deform a material is dissipated directly into heat (Nielsen and Landel, 1994). Therefore, tan is considered to reflect the cushioning effect required in the clinical situation. This parameter is also a sensitive indicator of cross-linking (Murayama and Bell, 1970). The acrylic temporary materials are formed by polymer chain entanglements of non-cross-linked polymers. These entanglements act as temporary and relatively weak cross-links. At temperatures well above the glass transition temperature, damping decreases with an increasing degree of cross-linking. It is difficult for the degree of cross-linking to be directly compared in different kinds of soft liners. However, the ability of the acrylic materials, having higher values of tan , to absorb energy and relieve stress will be greater than that of the silicones with lower tan values.

The use of soft liners produced a dramatic improvement in masticatory function and satisfaction compared with hard-based dentures. Furthermore, the viscoelasticity of the lining material was found to be an important factor in bringing about this improvement. The improvement was greater in dentures lined with the acrylic permanent materials (having high tan and G' at 1 Hz) than in those lined with the silicone permanent materials (having low tan and high G'), which in turn showed improvement greater than that observed with the acrylic temporary materials (having high tan and low G'). That is, the improvement in masticatory function was found to be greater in the soft liners with higher tan and G'. The hypothesis that materials having viscoelastic properties (i.e., high tan ) improve masticatory function when compared with those having elastic properties (i.e., low tan ) can be supported by the results in this study. However, surprisingly, it was also found that G' affects masticatory function. Clinically, higher G' and G'' at 1 Hz would indicate a greater ability of the dentures to crush food instantaneously, combined with a lower potential for permanent deformation during mastication. The damping which results from a higher value of tan is likely to produce a degree of stress relief. Soft liners should ideally exhibit elastic behavior against masticatory forces to transmit the energy required for the comminution of foods. At the same time, they should behave viscously to distribute forces, absorb energy, and prevent transmission of forces to the denture-bearing tissues, thus preventing masticatory pain by means of a cushioning effect. Hence, both G' and tan at 1 Hz should be high if the desired combination of properties is to be achieved. G'' is also a clinically meaningful parameter, because it characterizes the viscous deformations of the material. Soft liners should ideally have a high value of G'' at 1 Hz to prevent permanent dimensional change. On the other hand, permanent materials and temporary materials have different requirements for viscoelasticity at lower frequencies. The permanent materials should not flow, to maintain the dimensional integrity of the lining, while the temporary materials should flow under continuous weak functional pressure to allow mucosal tissues to change shape during tissue healing. Therefore, permanent materials should have low tan at 0.01 Hz, and temporary materials should have high tan . This assumption is consistent with the observation of the dynamic mechanical tests. The rheological properties of each soft liner were found to be reasonably in line with what is required for each clinical application.

Young' modulus (E) of the oral mucosa ranges from approximately 0.66 to 4.36 MPa (Inoue et al., 1985).

For a viscoelastic solid, E is related to the shear modulus (G) as follows:

(5)

where µ is the Poisson' ratio.

In a polymeric system, µ is very close to 0.5 (Ferry, 1980), and Eq. 5 becomes

(6)

According to Eq. 6, E of the temporary materials is calculated (approximately) to be less than 0.28 MPa, while that of both permanent materials is calculated (approximately) to be from 0.81 to 0.92 MPa, which is within the range of E for mucosa. Although both acrylic materials showed viscoelastic behavior, the masticatory function was significantly better with the permanent materials than with the temporary materials. Therefore, it is considered that the elastic moduli of the soft liners should be about equal to that of the oral mucosa, because these materials can be considered to be replacing the missing mucosa.

VAS was used for the patients' subjective assessment of denture comfort and function. All except one subject reported that the acrylic temporary materials with high tan were more comfortable than the silicones with low tan , regardless of the result of masticatory functional tests. Improved subject satisfaction scores were found to be correlated with higher damping, i.e., higher degree of stress relief. The subjects' own evaluations were not exactly consistent with the results of the functional test.

The effectiveness of soft liners in the clinical situation depends both on their viscoelasticity and on durability. In the case of acrylic materials, the low-molecular-weight plasticizer and ethyl alcohol are leached out, and, at the same time, water is absorbed into the polymer (Jones et al., 1988). Silicones exhibit low water absorption and low solubility of components (Kalachandra et al., 1995). Thus, the silicone permanent materials remain more stable over time, while the acrylic permanent materials and especially the temporary materials undergo a more marked loss of cushioning effect over time (Murata et al., 1998, 2000). In terms of durability, the silicones are therefore preferred. However, from the standpoint of rheological properties, the cross-linked acrylic materials, which showed viscoelastic behavior, i.e., better cushioning and almost the same elasticity as the oral mucosa, will best meet the requirements for permanent soft liners. It is suggested that permanent soft liners which have viscoelastic properties and acceptable durability should be developed to combine the advantageous characteristics of both the acrylic-permanent materials and the silicones.

In conclusion, the application of a soft denture liner having a relatively high value of both tan and G' to the mandibular complete denture leads to the most marked improvement in masticatory function.

	ACKNOWLEDGMENTS

 
This research was supported in part by Grants-in-Aid (Nos. 10557184, 11671936) for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

Received for publication July 20, 2000. Revision received November 27, 2001. Accepted for publication November 29, 2001.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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Journal of Dental Research, Vol. 81, No. 2, 123-128 (2002)
DOI: 10.1177/154405910208100208


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