Abstract
Objectives
Soft denture lining materials show varied and changeable stress relaxation property and softness under cyclic loading conditions. The purpose of this study was to determine the changes in the stress relaxation property and the softness of commercial soft lining materials after cyclic loading.
Methods
One plasticized acrylic-based material (Coe Soft: COS) and two silicone-based materials (Mucosoft: MUS; Sofreliner S: SFL) were investigated. For each material, 10 cylinder-shaped specimens were subjected to 29 days of cyclic loading, 1800 times per day; other 10 unloaded specimens were used as control groups. Stress relaxation ratio and softness were measured after 1, 4, 8, 15, 22 and 29 days of cyclic loading or storage.
Results
During 29 day period, the stress relaxation ratios of the cyclic loaded and unloaded COS, MUS and SFL specimens were 28.5–31.6% and 30.9–32.6%, 6.4–15.1% and 1.8–15.4%, 14.0–38.2% and 19.8–40.6%, respectively. The softness values of the cyclic loaded and unloaded COS, MUS and SFL specimens were 42.6–60.6% and 56.4–61.8%, 47.6–52.5% and 46.9–51.5%, 56.6–58.4% and 58.0–65.1%, respectively. Based on repeated-measures, 3-way ANOVA, the stress relaxation ratio was influenced by the loaded or stored period and the type of material ( p < 0.05), but not by the application of cyclic loading; the softness value was influenced by the loaded or stored period and the application of cyclic loading ( p < 0.05), but not by the type of material.
Significance
Soft lining materials showed the loaded or stored period and the material type dependent stress relaxation properties regardless of the application of cyclic loading. Softness value was influenced by the loaded or stored period and the application of cyclic loading.
1
Introduction
Soft denture lining materials have been used for denture wearers who cannot tolerate conventional hard acrylic dentures because of thin and relatively non-resilient mucosa or severe alveolar bone resorption . It has been reported that the application of soft lining materials on the mandibular dentures improved masticatory function and created greater maximum biting force without causing an adverse effect on the muscular activity . These results confirmed the clinical usefulness of soft lining materials. It is generally regarded that the efficacy of soft lining materials is determined by their viscoelastic property and durability; therefore, these materials should have proper cushioning effect to distribute and absorb functional stress and should remain stable over time .
Several soft lining materials have been used, such as silicone, plasticized acrylic and fluoroethylene . Among them, silicone- and acrylic-based materials have been more widely used. When using an acrylic-based material, mixing of powder and liquid resulted in the formation of a gel, which showed softness and viscoelastic behavior appropriate for clinical application. However, during functioning in the oral cavity, first ethanol and then plasticizer was lost, resulting in the deterioration of the material in addition to the volumetric change, hardening and color change .
It was suggested that the recovery from deformation of soft lining materials should be slow and not exhibit instantaneous elastic recovery . If the elastic recovery of a material is too fast, the potential for stress relaxation would be low; as a result, pressure on the mucosa would increase. In previous studies, flexibility, compliance and viscoelastic property of soft lining materials were characterized based on static loading protocols such as the puncture strength test, creep test and rheological method . In the creep test, the speed of loading ranged from 0.5 to 5.0 mm/s . It was reported that these static measurements could predict the material behavior changes over time and it was also argued that a slower frequency model might better reflect oral function than a higher frequency one .
In a clinical situation, pressure from the denture base during mastication, as well as thermal aging, supposedly accelerated the degradation of soft lining materials depend on their material type , which could affect their behavior significantly. Therefore, to obtain clinically relevant results, the stress relaxation behavior of these materials should be evaluated after cyclic loading applications that can simulate the mastication. A cycling loading frequency of 2 Hz and 1800 cycles of chewing per day were estimated for complete denture wearers . Occlusal forces of complete denture wearers and average people were reported by several researchers .
Determination of the changes in the stress relaxation and the softness of soft denture lining materials over time under simulated clinical cyclic loading conditions would provide information, which would be useful not only to select a specific material appropriate for each clinical condition but also to decide the regular interval at which these materials should be changed. To examine the stress relaxation property and softness of these materials under a simulated clinical condition, the stress relaxation ratio and the softness value were defined in this study. The purposes of this study were to measure the changes in the stress relaxation property and the softness value of three commercial soft denture lining materials over time, and to determine the influence of cyclic loading on the stress relaxation property and the softness value. Cyclic loading was performed to simulate the masticatory loading cycles in the oral cavity. The null hypothesis assumed in this study was that there would be no differences in the stress relaxation and the softness value of soft denture lining materials by the type of material, application of cyclic loading and the loaded or stored period.
2
Material and methods
2.1
Preparation of specimens
One plasticized acrylic-based soft denture lining material (Coe Soft: COS) and two silicone-based materials (Mucosoft: MUS and Sofreliner S: SFL) were investigated ( Table 1 ).
| Code | Brand name | Classification/packaging | Polymerization process | Lot no. | Manufacturer |
|---|---|---|---|---|---|
| COS | Coe Soft | Plasticized acrylic/powder and liquid | Room temperature | 0710301 | GC Lab Tech. Alsip, Illinois, USA |
| MUS | Mucosoft | Silicone-type/two pastes | Room temperature | MU-05241 | Parkell BioMaterials, Farmingdale, New York, USA |
| SFL | Sofreliner S | Silicone-type/two pastes | Room temperature | Lot 207 | Tokuyama, Tokyo, Japan |
All the materials were polymerized following the manufacturers’ instructions. COS was mixed at a powder/liquid ratio of 5 g/4 ml. MCS and SFL were automatically mixed with a dispensing gun (Dispenser II, Tokuyama, Tokyo, Japan). A fresh mixture of each material was packed into a stainless steel mold (3 mm in height and 10 mm in diameter), and the two open sides of the mold were pressed tightly against glass plates for 15 min. Petroleum jelly was applied as a separating medium between the glass plates and the material. The mold-specimen complex was then stored in 37 °C distilled water for 24 h before specimen removal. Twenty specimens were prepared for each material; 10 were subjected to cyclic loading, and the other unloaded 10 were used as the control group, which were stored in 37 °C distilled water.
2.2
Cyclic loading and the determination of stress relaxation ratio and softness
Cyclic loading was performed with a universal testing machine (Instron 4465; Instron, Canton, MA, USA) with a cross-head speed of 1 mm/s until the specimen was compressed to two thirds of its original height. Cyclic loading was performed 1800 times per day for 29 days with a total loading number of 52,200. Specimens were stored in 37 °C distilled water when they were not subjected to cyclic loading.
Stress relaxation property and softness of the specimens were determined after 1, 4, 8, 15, 22 and 29 days of cyclic loading or storage with the universal testing machine (Instron 4465). Compressive force was applied on a mounted specimen at a cross-head speed of 1 mm/s until the load reached 68.6 N; this state was maintained for 12 s to determine the stress relaxation property . The amount of compression at the applied load of 68.6 N was recorded; the force exerted at 1 s after the applied load appearance was also recorded. The stress relaxation ratio at 1 s was calculated as (applied load − load exerted at 1 s after the applied load appearance)/applied load (68.6 N). The softness was calculated as the amount of compression at the load of 68.6 N/the original height of the specimen (3 mm).
2.3
Statistical analysis
Repeated measures, 3-way analysis of variance (ANOVA; SPSS 12.0, SPSS, Chicago, IL, USA) was performed, with the type of material (2 types such as acrylic- and silicone-based) and the application of cyclic loading as independent variables and the duration of cyclic loading or storage as a repeated variable. Scheffe’s multiple comparison test was used for a post hoc analysis at a 95% confidence level.
2
Material and methods
2.1
Preparation of specimens
One plasticized acrylic-based soft denture lining material (Coe Soft: COS) and two silicone-based materials (Mucosoft: MUS and Sofreliner S: SFL) were investigated ( Table 1 ).
| Code | Brand name | Classification/packaging | Polymerization process | Lot no. | Manufacturer |
|---|---|---|---|---|---|
| COS | Coe Soft | Plasticized acrylic/powder and liquid | Room temperature | 0710301 | GC Lab Tech. Alsip, Illinois, USA |
| MUS | Mucosoft | Silicone-type/two pastes | Room temperature | MU-05241 | Parkell BioMaterials, Farmingdale, New York, USA |
| SFL | Sofreliner S | Silicone-type/two pastes | Room temperature | Lot 207 | Tokuyama, Tokyo, Japan |
All the materials were polymerized following the manufacturers’ instructions. COS was mixed at a powder/liquid ratio of 5 g/4 ml. MCS and SFL were automatically mixed with a dispensing gun (Dispenser II, Tokuyama, Tokyo, Japan). A fresh mixture of each material was packed into a stainless steel mold (3 mm in height and 10 mm in diameter), and the two open sides of the mold were pressed tightly against glass plates for 15 min. Petroleum jelly was applied as a separating medium between the glass plates and the material. The mold-specimen complex was then stored in 37 °C distilled water for 24 h before specimen removal. Twenty specimens were prepared for each material; 10 were subjected to cyclic loading, and the other unloaded 10 were used as the control group, which were stored in 37 °C distilled water.
2.2
Cyclic loading and the determination of stress relaxation ratio and softness
Cyclic loading was performed with a universal testing machine (Instron 4465; Instron, Canton, MA, USA) with a cross-head speed of 1 mm/s until the specimen was compressed to two thirds of its original height. Cyclic loading was performed 1800 times per day for 29 days with a total loading number of 52,200. Specimens were stored in 37 °C distilled water when they were not subjected to cyclic loading.
Stress relaxation property and softness of the specimens were determined after 1, 4, 8, 15, 22 and 29 days of cyclic loading or storage with the universal testing machine (Instron 4465). Compressive force was applied on a mounted specimen at a cross-head speed of 1 mm/s until the load reached 68.6 N; this state was maintained for 12 s to determine the stress relaxation property . The amount of compression at the applied load of 68.6 N was recorded; the force exerted at 1 s after the applied load appearance was also recorded. The stress relaxation ratio at 1 s was calculated as (applied load − load exerted at 1 s after the applied load appearance)/applied load (68.6 N). The softness was calculated as the amount of compression at the load of 68.6 N/the original height of the specimen (3 mm).
2.3
Statistical analysis
Repeated measures, 3-way analysis of variance (ANOVA; SPSS 12.0, SPSS, Chicago, IL, USA) was performed, with the type of material (2 types such as acrylic- and silicone-based) and the application of cyclic loading as independent variables and the duration of cyclic loading or storage as a repeated variable. Scheffe’s multiple comparison test was used for a post hoc analysis at a 95% confidence level.
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