BOND STRENGTH OF SOFT DENTURE LINERS FOLLOWING IMMERSION OF DENTURE CLEANSER

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1 BOND STRENGTH OF SOFT DENTURE LINERS FOLLOWING IMMERSION OF DENTURE CLEANSER A. Meşe Dicle University, Dental Faculty, Department of Prosthodontics, Diyarbakır, Turkey ABSTRACT This study investigated the effect of a denture cleanser on tensile bond strength of acrylic and silicone-based soft denture liners. The denture liners investigated were an acrylicbased, heat-cured (Vertex soft), an acrylic-based, auto-cured (Coe soft), a silicon-based, heat-cured () and a silicon-based, auto-cured (Mollosil plus). Soft liner specimens were mm in size and processed for testing according to manufacturers instructions between 2 polymethyl methacrylate (PMMA) blocks. Specimens were stored for 1 day, 1 week and 1, 3 and 6 months in water and Polident at 37 C. Tensile bond strength was measured using a universal testing machine (Testometric Micro 500) at a crosshead speed of 20 mm/min (n=10 specimens per experimental group). ANOVA and Tukey HSD methods were used to analyze the data (α=.05). The results indicated that the tensile bond strength of all lining materials decreased with storage duration (both water and Polident), and no difference between treatments. Introduction Denture soft lining materials are applied to the intaglio surface of dentures to achieve a more equal force distribution, to reduce localized pressures and to improve denture retention by engaging undercuts (1, 2). The most favourable properties of soft liners are resiliency, which is desired over a long period, and good adhesion to the denture base (3). Soft denture liners have been used in dentistry for more than a century; the earliest soft liners were made from natural rubber. One of the first synthetic resins used as a soft liner, a plasticized polyvinyl resin, was developed in 1945 (4). Silicon-based materials were introduced in 1958 (4). Contemporary soft lining materials can be divided into 2 groups: acrylic-and siliconbased. Both groups are available in auto-or heat-cured forms (1, 4). Auto-cured soft lining materials allow the dentist to reline a removable denture directly in the mouth. This method is faster than heat- cured (laboratory-processed) system and the patient is not without the prosthesis during the time required for the laboratory procedures (5). However, it is diffucult to produce liner materials o the optimum thickness with the auto-cured system (6). Acrylic-based soft lining materials generally consist of powder and liquid components. The composition of the powders and liquids is no well documented, but they are generally thought to be methacrylate polymers and copolymers, along with a liquid containing methacrylate monomer and plasticizers (ethyl alcohol and/or phthalate). These materials undergo 2 processes when immersed in water: the leaching of plasticizers and other soluble materials into the water and the absorption of water by the polymer. As a result, the physical and mechanical properties of the materials change with time in the patient s mouth (1, 7). There are several problems associated with the use of soft denture liners including, bond failure between the liner and denture base, loss of softness, colonization Biotechnol. & Biotechnol. Eq. 20/2006/3 184

2 by Candida albicans, porosity and poor tear strength (2). One of the most serious problems with these materials is bond failure between the soft denture liner and the denture base (8). Bond failure creates a potential surface for bacterial growth, and plaque, and calculus formation (2). Any favourable properties of a denture liner are useless in the absence of good bond to the denture base materials. A variety of parameters affect the bond between the resilient lining materials and the denture base, including water absorption, surface primer use, and denture base composition (9). Several tests have been used to assess the bond strength of soft denture liners (10-13). Tensile test differs from the forces that soft denture lining materials are subjected to clinically. Clinically the stress exerted on the interface of two materials is more closely related to shear and tear; however, this invitro tensile test was effective in evaluating bond strength and in ranking the materials (14-17). The increased porosity of resilient lining materials clinical use can lead to plaque accumulation and Candida albicans colonization (18-20). The two methods to control plaque on the intaglio surface of resilient materials to prevent denture stomatitis include mechanical plaque control (21, 22), reported to be performed by a high percentage of patients (23), chemical plaque control (18, 21, 22), and adequate denturewearing habits (24). Brushing is not advisable because it can damage the resilient lining. A chemical soaking tecnique is primarily the method of choice for geriatric patients and for those with poor motor capacity (25). The solution used for denture cleaning can be divided according to their chemical composistion: alkaline peroxide, alkaline hypochlorites, acids, disinfectants, and enzymes (26, 27). The enzymatic solutions, containing protease and mutanase, are able to break down the salivary glyproteins and bacterial polysaccharides of plaque (21) In addition some have reported that they are more effective in preventing microbial invasion and plaque formation (25, 28). However, denture cleaners can cause significant deterioration because they can also cause loss of soluble components and plasticizers, or absorption of water or saliva by the resilient lining materials (29, 32). This process can influence the properties of these materials. Thus the selection of denture cleanser should be considered to avoid or minimize changes in the properties of resilient materials (29, 33). The objective of this study was to evaluate changes in tensile bond strength of four resilient liners bonded to acrylic resin when immersed in a denture cleanser. Materials and Methods Four commercial soft denture lining materials, consisting of two main groups according to chemical composition (acrylic or silicone-based) were chosen for investigation. These materials were chosen because they appear to be among the more successful anda re in common clinical use. The resilient liner materials, denture base resin and denture cleanser used in this study are listed in Table 1. Ten specimens with a cross-sectional area of mm were prepared for each group using a heat-polymerized polymethyl methacrylate (PMMA) denture base material. Two PMMA plates were made by investing brass dies with a 3-mm-thick spacer in a denture flask. All the dies and spacers were calibrated to the same dimensions to standardize the shape of the denture base blocks and the thickness of the soft denture liners. The dies and spacer were invested in hard, but flexible silicone rubber (Lastic Xtra, Kettenbach, D-35713, Eschenburg, Germany) to facilitate removal of the processed specimens from the flask. Specimens were made by processing the soft denture liners against polymerized PMMA blocks. For denture base polymer, the pow- 185 Biotechnol. & Biotechnol. Eq. 20/2006/3

3 Soft lining materials and denture base material used TABLE 1 Type Lot No Product Maufacturer Convensional (heat-cured) Melliodent Heraeus Kulzer, Berkshire, UK denture base polymer Heat-cured Acrylic-based soft liner Auto-cured Acrylic-based soft liner Heat-cured Silicon-based soft liner Auto-cured Silicon-based soft liner Vertex soft Dentimex Zeist, HOOLOND Coe soft Coe Laboratories, Inc, Chicago, Ill Detax GmbH &Co. KG, Ettlingen, Germany Mollosil plus Detax GmbH &Co. KG, Ettlingen, Germany Denture cleanser Polident Block Drug Co, In., Jersey City, N.J. der-liquid ratio used was 23.4 g:10 ml and it was mixed for 1 min, packed into the mold while the single brass spacer was present, and processed in a water bath at a temperature of 100 C for 30 minutes. After this process, the 2 polymerized PMMA specimens were removed from the flask and trimmed, and the surfaces that were to be bonded were smoothed using 240 grit silicone carbide paper (Waterproof Silicone Carbide Paper; A 166 Grade P400, English Abrasives, London, England). The prepared surfaces were then treated with the silicon-based soft lining materials to be studied. For, its own adhesive Primo was applied and allowed to dry for 60 minutes. For Mollosil plus, alcohol ( Isopropanol 99.7%, Voco, Cuxhaven, Germany) was applied to the surfaces, followed by its adhesive Primer, which was allowed to dry for 1 minute. The brass spacers were then removed from the flask. The PMMA blocks were replaced in the mold and the soft denture liners were packed into the space left by the brass spacers, trial packed, and polymerized. For Vertex soft polymerization, the powder-liquid ratio used was 2:1 and it was mixed for 60 seconds. Then, the flasks were placed under pressure in a standard flask press (No.01001; Teledyne Hanau, Buffalo, NY) for 15 min, and immersed in a water bath for 3 hours at 70 C, followed by 30 min at 100 C. For Coe soft polymerization, the powder-liquid ratio used was 1.5 g:8 ml, and the flasks were placed under pressure in a standard flask press for15 minutes. For polymerization, the flasks were placed under pressure in a standard flask press for 15 minutes, and then immersed in a water bath for 2 hours at 100 C. For Mollosil plus polymerization, the flasks were placed under pressure in a standard flask press for 30 minutes. After polymerization, the specimen was removed from the flask and trimmed with a sharp blade. Each group (n=10) of specimens was stored in water and Polident at 37 C for 1 day, 1 week, and 1, 3 and 6 months. Tensile bond strength was determined using a universal testing machine (Testometric Micro 500, Type U4000; Maywood Instruments. Limited, Basingstoke Hants, England) at crosshead speed of 20 mm/min (Figure) and recorded in megapascals (MPa). The changes in the bond strength of each soft denture liner material determined for the 5 test periods (both in water and Polident) and were evaluated statistically using 3-way ANOVA and the Tukey HSD post-hoc test. All statistical testing was performed at a pre-set alpha of The type of failure was observed using Biotechnol. & Biotechnol. Eq. 20/2006/3 186

Figure. The universal testing machine (Testometric Micro 500, Type U4000, Maywood Instruments, Basingstoke Hants, England). an electronmicroscope (Jeol JSM T400, Tokyo, Japan).

4 Figure. The universal testing machine (Testometric Micro 500, Type U4000, Maywood Instruments, Basingstoke Hants, England). an electronmicroscope (Jeol JSM T400, Tokyo, Japan). Failures that occured at the soft lining-acrylic resin interface were recorded as adhesive, and those that occured within the soft lining were considered to be cohesive. An adhesive/cohesive failure was recorded when there was no complete separation between materials. Results and Discussion Mean and sandart deviation values of bond strength of soft denture liner materials for 5 time intervals are given Table 2. The bond strength of acrylic-based resilient liners (3.50±0.44) was greater than that of silicone-based liners (1.58 ± 0.19) and that the bond strength of heat-polymerized soft liners (3.50 ± 0.44, 1.58 ± 0.19) was greater than that of auto-polymerized liners (0.45 ± 0.18, 1.20 ± 0.14) within both the siliconeand acrylic-based groups in distile water. The bond strength of acrylic-based resilient liners (3.46±0.42) was greater than that of silicone-based liners (1.53 ± 0.18) and that the bond strength of heat-polymerized soft liners (3.46 ± 0.42, 1.53 ± 0.18) was greater than that of auto-polymerized liners (0.42 ± 0.16, 1.17 ± 0.12) within both the silicone- and acrylic-based groups in Polident. There were statistically significant differences of 3.05, 2.05 (P<.001) between Vertex Soft and Coe-Soft, and 0.37, 0.21 (P<.001) between and Mollosil Plus in both treatments (Table 3, 4). The poorest bond strength was seen at 6 months, and the greatest at 24 hours for all four types of resilient liner material. Statistically significant decrease was observed in values of bond strength for all four types of resilient liner material. It was determined that the bond strength of the resilient liner materials decreased with time, and that the decrease was greater in the acrylic resinbased resilient liner materials compared with the silicone-based resilient liner materials (0.67, 0.45); moreover, the decrease in the auto-polymerized resilient liner materials was greater than in the heat-polymerized ones (1.59, 1.32) (Table 3, 4). Comprasion between treatments showed no significant difference between soft lining materials in all periods of time evaluated. In relation to time, when all soft liners were immersed in water or Polident, the data were significantly different with time (p<0.05). Failure mode was adhesive for Vertex soft and Coe soft, whereas cohesive for. Mollosil plus was seen as a mixed mode of failure. Sufficient bond strength (4.5 kg/cm²) between the soft denture lining and the acrylic resin denture base material is required to avoid interfacial separation at the denture borders (10). The bond strength of soft denture liners to PMMA denture base resin is weak, and when the separation takes place, the localized area may become unhygienic and nonfunctional. Ideally, the 187 Biotechnol. & Biotechnol. Eq. 20/2006/3

5 TABLE 2 Means and standard deviation values of the tensile bond strengths (MPa) of resilient liner materials for 5 time intervals (n=10) Vertex Soft Coe-Soft Mollosil Plus In Water In Polident In Water In Polident In Water In Polident In Water In Polident 1 day 3.50± ± ± ± ± ± ± ± week 3.07± ± ± ± ± ± ± ± month 2.57± ± ± ± ± ± ± ± months 1.91± ± ± ± ± ± ± ± months 1.70± ± ± ± ± ± ± ±0.18 soft denture liners should bond sufficiently well to PMMA denture base resin to avoid failure of the interface during the service life of the prosthesis (2). Although maintenance of appropriate denture hygiene is important, many denture wearers fail to maintain a satisfactory level of hygiene. Therefore a wide range of chemical denture cleansers are available to facilitate denture hygiene (23). These solutions not only control plaque on dentures but may also cause significant deterioration of resilient linners as well (18). In this study, the effect of a single denture cleanser on tensile bond strength of soft liners was evaluated. This study showed that, the bond strength of all soft lining materials decreased with both water and Polident immersion duration for four soft liners. The absorption or loss of soluble components may cause failure in bond strength between the soft liner and acrylic resin of the denture base (29). The present results are in agreement with those of others, who suggested that water storage reduced soft liner bond strength (8, 14, 15). Decrease in the bond strength may result from swelling and stress built up at the bond interface, or of a change in the viscoelastic properties of the liner, rendering the material stiffer and thus beter able to transmit the external loads to the bond site (7). The current study produced results disagree with those of others, who reported that bond strength of soft lining materials increased after water storage (11-13, 33). However, a direct comparison of studies cannot be made because of the different research protocols used. Comprasion between Polident and water immersion showed no change in the tensile bond strength of the 4 soft lining materials for the time period tested. These findings are in agreement with Garcia who reported specimens immersed in Polident compared with water, tensile bond strength was unaffected (33). The bonding strength values of acrylicbased liners decreased more than those of the silicone products during the 6-months immersion test, because of the leaching out of plasticizers. This finding agrees with Jepson, who reported that the water storage reduced acrylic-based liner strength more than that of silicone-based products (16). This result indicated that the bond strength values of resilient denture liners vary according to their chemical compositions (1, 7). According to similar chemical compositions, a chemical bond forms between the acrylic-based liners and PMMA denture base polymer (3). Since silicone liners have little or no chemical bond to PMMA, an adhesive is supplied to help bond the liner to the polymerized denture base. Morever, the bonding strength values of auto-cured products decreased more than those of heat-cured materials over the time of this experiment. This finding is in agreement with Waters, who reported that water storage reduced the bond strength of autocured liners more than the heat-cured mate- Biotechnol. & Biotechnol. Eq. 20/2006/3 188

6 TABLE 3 Mean difference in the values of the tensile bond strengths of the soft liner material groups in water (Tukey HSD) 6 months 3 months 1 month 1 week 1 day First Group Second Group Mean Difference (First Group Second Group) P-values Coe Soft <.001 Vertex Soft <.001 Mollosil Plus <.001 Coe Soft <.001 Mollosil Plus Mollosil Plus Coe Soft <.001 Coe Soft <.001 Vertex Soft <.001 Mollosil Plus <.001 Molloplast- B Coe Soft <.001 Mollosil Plus Mollosil Plus Coe Soft <.001 Coe Soft <.001 Vertex Soft <.001 Mollosil Plus <.001 Molloplast- B Coe Soft <.001 Mollosil Plus Mollosil Plus Coe Soft <.001 Coe Soft <.001 Vertex Soft Mollosil Plus <.001 Coe Soft Mollosil Plus Mollosil Plus Coe Soft Coe Soft <.001 Vertex Soft Molloplast- B Mollosil Plus <.001 Coe Soft <.001 Mollosil Plus Mollosil Plus Coe Soft rials (17). This study showed that for acrylic-based liners, failures were adhesive. These results can be explained by chemical adhesion. Chemical adhesion may be explained by the similar chemical composition of acrylic resin and soft liners. For silicone-based liners, most failures were cohesive, it can be explained by mechanical bonding. The present work tested all material combinations in a laboratory environment. In-vivo testing should be performed next to establish if trends observed in the present work are applicable to the clinical situation. Conclusions Within the limitations of this in vitro study the enzymatic denture cleanser tested, when compared with water, did not cause significant changes in the tensile bond strength of the four soft denture lining materials for the time period tested. 189 Biotechnol. & Biotechnol. Eq. 20/2006/3

7 TABLE 4 Mean difference in the hardness of the resilient liner material groups in Polident (Tukey HSD) First Group Second Group Mean Difference (First Group Second Group) P-values Coe Soft Vertex Soft Mollosil Plus Coe Soft Mollosil Plus Mollosil Plus Coe Soft Coe Soft Vertex Soft Mollosil Plus Molloplast- B Coe Soft Mollosil Plus Mollosil Plus Coe Soft Coe Soft Vertex Soft Mollosil Plus Coe Soft Mollosil Plus Mollosil Plus Coe Soft Coe Soft Vertex Soft Mollosil Plus Coe Soft Mollosil Plus Mollosil Plus Coe Soft Coe Soft Vertex Soft Mollosil Plus Coe Soft Mollosil Plus Mollosil Plus Coe Soft day 1 week 1 month 3 months 6 months REFERENCES 1. Polyzois G.L., Frangou M.J. (2001) J. Prosthodont., 1, Kawano F., Dootz E.R., Koran A., Graig R.G. ( 1992) J. Prosthet. Dent., 68, Al-Athel M.S., Jagger R.G. (1996) Int. J. Prosthodont., 9, El-Hadary A., Drummond J.L. (2000) J. Prosthet. Dent., 83, Cucci A.M., Vergani C.E., Giampaolo E., Afonso S.F. (1998) J. Prosthet. Dent., 80, Murata H., Haberham R.C., Hamada T., Taguchi N. (1998) J. Prosthet. Dent., 80, Meşe A., Güzel K.G., Uysal E. (2005) Acta Odontologica Scandinavica, 63, Sinobad D., Murphy W.M., Huggett R., Brooks S. (1992) J. Oral Rehabil., 19, Kulak Y., Sertgöz A., Gedik H. (2003) J. Oral Rehabil., 89, p Kutay Ö., Bilgin T., Şakar O., Beyli M. (1994) Eur. J. Prosthodont. Rest. Dent., 2, Craig R.G., Gibbons P. (1961) JADA, 1, Dootz E.R., Koran A., Craig R.G. (1993) J. Prosthet. Dent., 69, Dootz E.R. (1995) J. Prosthet. Dent., 74, Biotechnol. & Biotechnol. Eq. 20/2006/3 190

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Evaluation of Bond Strength of Silicone and Acrylic Resin Based Resilient Denture Liners Over A Period of Storage in Water.

DOI: 0.2276/aimdr.208.4..DE4 Original Article ISSN (O):2395-2822; ISSN (P):2395-284 Evaluation of Bond Strength of Silicone and Acrylic Resin Based Resilient Denture Liners Over A Period of Storage in