omparisan of bond strength denture base resin
of six soft denture
Kawano, DDS, PhD,a Edward R. Dootz, CDT,b Koran III, DDS, MS,C and Robert G. Craig, PhDd
The bond strength of six commercial soft denture liners was evaluated using a modified tensile test. The soft denture liners investigated were Prolastic, VinaSoft, Flexor, Molloplast-B, NOVUS, and Super-Soft. The samples were processed according to the manufacturers’ instructions to cured denture base resin (polymethyl methacrylate; PMMA). The soft denture liners were 10 x 10 x 3 mm and were processed between two PMMA blocks. The samples were placed in tension until failure. The mode of failure, cohesive or adhesive, was also recorded. The results of this study sbowed that the bond strength is related to the components of the materials. Prolastic, VinaSoft, and Flexor had the lowest bond strength to cured PMMA and ranged from 9.6 to 11.3 kg/cm2. Super-Soft, Novus, and Molloplast-B demonstrated better bond strengths and ranged from 16.7 to 17.6 kg/cm2. The bond strength of Novus could be improved by using the recommended bonding agent and bonded Novus at 26.1 kg/cm2 had the highest bond strength of all materials tested. (J PRQSTHET DENT 1992;68:368-71.)
oft denture liners are used to assist in providing an even distribution of a functional load on the denture-bearing area, avoiding local stress concentrations and improving the retention of the dentures by engaging undercuts.l Laboratory-processed materials are used as long-term denture liners for management of sore or atrophied mueosa, traumatic ulceration, and for obturators after maxillofacial surgery.2 Soft denture liners have several problems associated with their use such as the loss of softness, colonization by Candida albicans, porosity, poor tear strength, and various degrees of softness.2 One of the more serious problems with soft denture liners is the failure of adhesion between the soft denture liner and the denture base.3 Bond failure also creates a potential surface for bacterial growth, plaque, and calculus formation. Therefore frequent clinical evaluation and periodic replacement of a soft denture liner is required. Several tests have been used to assess the bond strength of soft denture liners.4-11 Craig and Gibbons4 estimated the
Supported by National Institutes of Health, National Institute of Dental Research grant no. DE09296 aVisiting Scholar, Department of Biologic and Materials Science, University of Michigan School of Dentistry; and Assistant Professor, Department of Removable Prosthodontics, The University of Tokushima, School of Dentistry, Tokushima, Japan. bSenior Research Associate, Department of Biologic and Materials Science, University of Michigan School of Dentistry. CProfessor, Department of Prosthodontics, University of Michigan School of Dentistry. dProfessor, Department of Biologic and Materials Science, University of Michigan School of Dentistry.
bond strength of 10 soft denture liners using a peeling test. They concluded that an adhesion value of 10 pounds per inch is satisfactory for clinical use of the soft denture lin ers.4, 5 In 1965, Bates and Smith6 assessed the bond strength of 12 soft denture liners by using a tensile test that was similar to the one used in this study. They concluded that the soft denture liners investigated had satisfactory bond strength and showed that the heat-cured soft denture liners have intimate contact with a diffuse boundary when the materials are cured against an acrylic dough.6 Amin et a1.8 measured the bond resistance of four soft denture liners using tensile, shear, compression, and peeling tests. They concluded that roughening the acrylic base before the application of a lining material has a weakening effect on bonding. The purpose of this study was to assess the bond strength of six commercially available soft denture liners to a polymerized polymethyl methacrylate (PMMA) denture base resin by a two-phase tensile test. The ranking of materials will help clinicians select soft denture liners for their patients and the data will provide a comparative data base when new materials are introduced.
Six well-known soft denture liners were chosen on the basis of different chemical compositions (Table I). The bond strength was determined, in tension, after processing to PMMA (Lucitone 199, Dentsply International, York, Pa.). Five specimens with a cross-sectional area of 10 X 10 mm were prepared for each soft liner tested (Fig. 1). Two PMMA samples were prepared by investing brass dies with
Soft Denture Liner
UNIT = mm Fig.
List of materials
1. Sample configuration.
and manufacturers Manufacturer
890701 031115 -
Young Dental, Earth City, MO. NuDansu Inc., Augusta, Ga. Schutz-Dental GmbH, Rosbach, Germany Coe Laboratories Inc., Chicago, Ill.
RTV silicone Vinyl Copolymer Plasticized PMMA
The Hygienic Corp., Akron, Ohio Molloplast
& Co. KG,
900601 None 031010 60189A(L) 3260889(P) 31489A
Prolastic VinaSoft Flexor Super-Soft
Karlsruhe, Germany RTV,
a 3 mm thick spacer in a denture flask. The dies and spacers were invested in hard but flexible silicone rubber to allow for easy removal of the processed samples from the flask, All the dies and spacers were machined to the same dimensions to standardize the shape of the PMMA samples and the thickness of the soft denture liners. Specimens were made by processing the soft denture liners against polymerized PMMA blocks. The Lucitone 199 denture resin was mixed according to the manufacturers’ instructions, packed into the mold with the brass spacer present, and processed in a water bath at 165’ F for 9 hours. After polymerization, the brass spacer was removed from the mold, the two PMMA resin samples were trimmed, and the surfaces to be bonded were smoothed on 240 grit silicone carbide paper and treated according to the manufacturers’ instructions for each soft denture liner. The PM&IA blocks were placed back into the molds (Fig. 2) and the soft denture liners were then packed into the space made by the brass spacer, trial packed, and cured according to the manufacturers’ instructions. Where indicated, bonding agents were used on the PMMA blocks as
supplied from the manufacturer. Novus samples were made with and without the bonding agent (Novus Bond, The Hygienic Corp., Akron, Ohio). After polymerization the samples were removed from the flask and any flash was trimmed with a sharp blade. The samples were placed under tension, until failure, in an Instron Universal Testing Machine (Instron Corp., Canton, Mass.) using a crosshead speed of 2 cm/min (Fig. 3). The bond strength was calculated as stress at failure divided by the cross-sectional area of the sample. Means and standard deviations were determined for all materials. The type of failure was observed using an optical microscope (Stereomicroscope SV8, Carl Zeiss, Oberkochen, Germany) and was recorded as cohesive or adhesive failure. Five samples were used for each material. The bond strengths were compared using one-way analysis of variance and calculating the Scheffe interval at a 95% confidence 1evel.r’
RESULTS Table rankings
II shows mean values, standard deviations, and of the tensile bond strengths of the soft denture
Fig. 2. Prepared spacer removed.
reset in mold
liners to cured PMMA resin and the type of bond failure. The mean bond strength to PMMA resin ranged from 9.6 to 26.1 kg/ems. The lowest bond strength was seen with Prolastic at 9.6 kg/cm2. The highest bond strength was seen with bonded Novus at 26.1 kg/cm2. The Scheffe interval for comparisons of mean values among materials was 4.7 kg/cm2 at a 95 % confidence level. There were three distinct groups of materials that were statistically different. The first group, Prolastic, VinaSoft, and Flexor, had the lowest bond strengths and ranged from 9.6 to 11.3 kg/cm2. A second group, Super-Soft, Novus, and Molloplast-B demonstrated better bond strength and ranged from 16.7 to 17.6 kg/cm2. The third group, bonded Novus, was statistically different from all the other materials tested and had the highest bond strength at 26.1 kg/cm2. In the group with the lowest bond strength, Prolastic failed cohesively under tensile stress. Adhesive failure was observed for Flexor. VinaSoft failed both adhesively and cohesively. In the second group, Super-Soft and Molloplast-B showed cohesion failure. Both Novus and bonded Novus failed adhesively.
DISCUSSION In service, debonding of the soft denture lining materials is a common occurrence. The bond strength of soft denture liners to PMMA denture base resins is weak, and when the separation takes place the localized area may become unhygienic and nonfunctional. Ideally, the 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. In this study, the bond strength of the soft denture liners to cured PMMA resin was determined by a tensile test. This test differs from the forces that soft denture lining
Fig. 3. Tensile machine.
in the Instron
materials are subjected to clinically. Clinically the stress exerted on the interface of two materials is more closely related to shear and tearlo; however, this invitro tensile test was effective in evaluating bond strength6 and in ranking the materials. The results of this study indicated that the force for failure was 9.6 kg/cm2 or higher for all materials investigated. It has been reported that soft denture lining materials that have a 10 pounds per inch (4.5 kg/cm2) bond strength are acceptable for clinical use.ll Considering this criterion, all materials tested had a satisfactory bond strength to cured PMMA denture base resin. Prolastic, a room temperature vulcanizing (RTV) silicone, had the weakest bond strength to the denture base resin. Since the failure was cohesive, this indicates that the tensile strength of the soft liner material is weaker than the bond strength to the PMMA substrate. Flexor, Novus, and bonded Novus materials failed adhesively, which implies that the tensile strength of the soft liner materials was greater than the bond strength to the PMMA resin. Only VinaSoft failed both adhesively and cohesively. This may indicate that the tensile strength of the soft liner and the bond strength are nearly the same. Bonded Novus, a polyphosphazine fluoroelastomer, showed the highest bond strength (26.1 kg/cm2). This bond strength was weaker than the tensile strength of the soft denture liner, since Novus and bonded Novus failed adhesively. Novus without the bonding agent demonstrated
satisfactory chemical bonding to PMMA; however, using an adhesive (Novus Bond) increased the bond strength to processed PMMA by 50 % . These results support the use of the bonding agent when using Novus. For silicone-based soft denture liners, an adhesive is supplied to aid in bonding to the denture base resin because silicone denture base liners have little or no chemical adhesion to PMMA resin. Therefore the bond strength of silicone denture base liners depends upon the tensile strength of the materials and the adhesive used. When Prolastic was processed to PMMA, the bond failed cohesively and the bond strength was only 37 % of the bond strength of bonded Novus. One heat-cured silicone, Molloplast-B, showed a higher bond strength than that of the RTV silicone, Prolastic. This value was higher than that observed by Bates et aL6 (13.6 kg/cm2) and lower than that observed by Khan et aLI (23.9 kg/cm2). However, this difference may be the result of different denture base resins used, the shape of the samples, and the crosshead speed of the tensile testing machine. Molloplast-B also requires a bonding agent when joined to cured PMMA. Using the adhesive increased the bond strength to cured PMMA and Molloplast-B failed cohesively, indicating that the tensile strength of the soft liner was weaker than the bond strength to the PMMA. Flexor is a copolymer that has a similar chemical composition to PMMA. However, Flexor showed low bond strength and failed adhesively. Since many soft denture liners contain plasticizers, this might explain in part why the bond strength was low. Other factors such as the degree of cross-linking must also be considered. To achieve adequate softness, the tensile strength of the elastomer may suffer. Super-Soft is a methyl/ethyl methacrylate soft denture liner. The chemical composition is also similar to that of the PMMA denture base resins. Thus bonding agents are not required to achieve a bond with PMMA denture base resin. When Super-Soft was processed to PMMA, the bond strength was reasonable at 16.7 kg/cm2. The cohesive failure observed would indicate a good bond to the denture base resin. Molloplast-B, a silicone with a bond strength of 17.6 kg/cm2, demonstrated similar behavior. These results are helpful in determining which materials have the best bond strength when processed to new denture base resin. However, it must be pointed out that the bond strength will likely be lower when old dentures are relined, since the denture base may be contaminated by microorganisms and other materials absorbed into the denture base from food or cleansing agents. The results of this study will serve as a benchmark for studying new materials and other variables that can affect bond strength. Factors such as processing methods, water sorption, bonding agents, and changes in the bond strength in the harsh oral environment need further investigation to predict which materials will provide the best service clinically.
II. Bond strength and type of failure of soft lining materials to processed PMMA
Prolastic VinaSoft Flexor Super-Soft
9.6 (0.53) 11.0 (1.21) 11.3 (1.52)
16.7 17.4 17.6 26.1
(1.24) (3.95) (1.45) (1.82)
C A/C A C
Mean of five specimens and standard deviations in parentheses. Materials connected with a vertical bar show no significant difference 96 % confidence level. C, Cohesive failure; A, adhesive failure.
CONCLUSIONS 1. The tensile test used in this study was effective in evaluating and ranking the bond strength of six soft denture liners to a cured PMMA denture base resin. 2. Prolastic, VinaSoft, and Flexor had the lowest bond strength to cured PMMA and ranged from 9.6 to 11.3 kg/ cm2. 3. Super-Soft, NOVUS, and Molloplast-B demonstrated better bond strengths and ranged from 16.7 to 17.6 kg/cm2. 4. Bonded Novus at 26.1 kg/cm2 had the highest bond strength of all materials tested. REFERENCES 1.
2. 3. 4. 5. 6. 7.
Heartwell CM, Rahn AO. Syllabus of complete dentures. 4th ed. Philadelphia; Lea & Febiger, Inc, 1986423~56. Craig RG. Restorative dental materials. 7th ed. St Louis. CV Mosby Co, 1986:496-S. Sauve JL. A clinical evaluation of Silastic 390 as a lining material for dentures. J PROSTHET DENT 1966;16:650-60. Craig RG, Gibbons P. Properties of resilient denture liners. J Am Dent Assoc 1961;63:383-90. Eick DJ, Craig RG, Peyton FA. Properties of resilient denture liners in simulated mouth conditions. J PROSTHET DENT 1962;12:1043-52. Bates JF, Smith DC. Evaluation of indirect resilient liners for dentures: Laboratory and clinical tests. J Am Dent Assoc 1965;70:344-53. Matsuo J, Kohmoto K, Nagasawa T, Tsuru H. Study on the bonding strength of silicone lining materials to methyl methacrylate. J Jpn Prosthodont Sot 1976;20:173-8. Amin WM, Fletcher AM, Ritchi GM. The nature of the interface between polymethyl methacrylate denture base materials and soft denture liners. J Dent 1981;9:336-46. Wright PS. Characterization of adhesion of soft denture liners to poly (methyl methacrylate). J Dent Res 1982;61:1002-5. McMordie R, King GE. Evaluation of primers used for bonding silicone to denture base materials. J PROSTHET DENT 198%61:636-g. Khan Z, Martin J, Collard S. Adhesion characteristics of visible lightcured denture base materials bonded to resilient lining materials. J PROSTHET DENT 1989;62:196-200. Guenther WC. Analysis of variance. Englewood Cliffs, N.J.: PrenticHall, 1964:31-63.
DR. ANDREW KORAN DEPARTMENT OF PROSTHODONTICS THE UNIVERSITY OF MICHIGAN SCHOOL OF DENTISTRY ANN ARBOR, MI 48109