Bond Strength of Permanent Soft Denture Liners Bonded to The

Bond strength of permanent soft denture liners bonded to the denture base

Thomas J . Emmer , J r , DMD, a Thomas J . Emmer , Sr, DDS, b

Jaya lakshmi Va idynathan , PhD, c and Tr i ta la K. Va idynathan , PhD d

University of Medicine and Dentistry of New Jersey, New Jersey Dental School, Newark, N. J.

The purpose of this study was to character ize denture and soft l iner adhes ion and to determine the adhes ive and/or cohesive strength of different soft t issue l iners bonded to the denture base by use of a new technique. Two groups of f ive permanent soft l iners (dry or exposed to water for 6 months) were tested by use of a tensi le mode to character ize the fai lure character ist ics of soft l iners bonded to denture base resin. The method dif fered from previous test methods because of the specimen's abi l i ty to al ign axia l ly dur ing the test. The results ind icated s igni f icant di f ferences in the bond ing of l iners to the denture base, and l ight-cure systems exhib i ted the greatest amount of stress needed for fai lure. Low bond strength was observed when the adhes ion was poor or when the cohes ive strength of the soft l iner was low and lead to pure adhes ive or cohesive fai lure. When both adhes ive and cohesive bonds were strong, fai lure occurred at h igh stresses. Combinat ions of adhes ive and cohesive fa i lures (mixed mode) were also observed in intermediate cases. (J PROSTHET DENT 1995;74:595-601.)

Permanent soft denture liners have been a Valu- able asset for dentists and, because of their viscoelastic properties, they act as shock absorbers and reduce and distr ibute the stresses on the denture-bearing tissues. 1-2 Their use for patient comfort and the treatment of the atrophic ridge, bone undercuts, bruxism, xerostomia, and dentures opposing natural teeth has been known to be clinically beneficial. 3 Although these attr ibutes are posi- tive, there are also disadvantages to the use of permanent soft liners. One of the major drawbacks of the permanent soft l iners is the lack of a durable bond to denture. 4-9 De- bonding of soft l iners from the denture is a common clinical occurrence. Debonding results in localized unhygienic conditions at the debonded regions and often causes func- tional failure of the prosthesis.I~ Although there are pub- lished reports on the bond strength of soft l iners bonded to denture base resin, different methods such as peel 9, 11 or tensile 12 tests have been used to measure the bond strength. Although the previously used tests have provided valuable information, there are l imitations to some of these methods. In particular, direct gripping of the specimen in the tensile testing machine may complicate or compromise the

aResearch Associate, Department of Prosthodontics and Bioma- terials.

bAssociate Clinical Professor of Prosthodontics and Biomaterials. CAssociate Professor of Prosthodontics and Biomaterials. dprofessor of Prosthodontics and Biomaterials. Copyright 9 1995 by The Editorial Council of THE JOURNAL OF

PROSTHETIC DENTISTRY.

0022-3913/95/$5.00 + 0. 16/1/68284

specimen al ignment 1~ and also damage the sample integ- r ity at the gripped regions. There is therefore a need to develop a tensile test method that permits axial self-alignment of the specimen.

This study was designed (1) to characterize the debonding characteristics of soft denture liners bonded to denture resin mater ia l with the following specific objectives, (2) to develop a tensile method to characterize the failure modes and strengths of soft liners bonded to denture base material, and (3) to use this method to evaluate the bonding and/or the cohesive strength of selected permanent soft reline materials bonded to a denture base material.

MATERIAL AND METHODS

The reline materials included selected materials from light- and heat-polymerized systems currently available. There are significant differences in the chemical makeup of different materials (Table I). Whereas Triad (Dentsply/ York Div., York, Pa.) and Astron (Astron Dental, Wheeling, Ill.) reline materials use light polymerized resins based on urethane dimethacrylate and Bis-GMA dimethacrylate monomers, Molloplast-B reline material (Buffalo Dental Mfg. Co., Syosset, N. Y.) is based on silicone. Other systems such as PermaSoft (Nue Dent, Cambridge, Mass.) and Su- per Soft (Coe Laboratories, Chicago, Ill..) are plasticized polymethyl methacrylate (PMMA) that is mixed with polyethyl methacrylate (PEMA). The denture base material used was Lucitone 199 (Dentsply/York Div.), a heat- processed PMMA based system.

Lucitone 199 denture mater ia l blocks (100 80 10 mm) were flasked and processed for 6 hours at 164 ~ F and I hour at 212 ~ F. The blocks were cut into 10 10 5 mm

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THE JOURNAL OF PROSTHETIC DENTISTRY EMMER ET AL

Table I. Denture reline materials

Triad Astron Molloplast B PermaSoft Super Soft

Polymerization Light Light Heat Heat Heat mode

Material chemical Urethane Composite ? Silicone Plasticized Plasticized composition polyether (Information not polymethyl polymethyl

dimethacrylate made available) methacrylate/ methacrylate/ polyethyl Polyethyl methacrylate methacrylate

Self Self Bonding agent Light cured chemical methyl- composition methacrylate

How supplied Premixed paste Manufacturers Apply Triad

recommended bonding agent surface preparation

Self Saline

Powder liquid Premixed paste Powder liquid Powder liquid Apply "wet" mix of Apply bonding Rinse denture Rinse denture

freshly mixed agent surface with surface with powder liquid monomer monomer

Table II. Duncan multiple range tests of subsets

Sample group failure strength (MPa)

Triad Super Soft Astron Molloplast B PermaSoft

Stored 24 hours (72 ~ F), dry 7.43 2.94 2.60 1.21 1.50 Stored 6 months (72 ~ F), water 12.4 7.09 7.80 2.69 1.83

squares with a saw, and a water coolant was used. The squares were attached to screws by use of autopolymeriz- ing acrylic resin around the screw head. The opposite end that the screw was attached to was roughened with a crosscut carbide bur (H 72E, Brasseler, Savannah, Ga.) and randomly assigned to different groups.

For processing the light-polymerized materials, the in- dividual squares were wrapped with clear Mylar film (Du Pont Co., Wilmington, Del.) and the surface of the squares was prepared according to the manufacturer's recommendations (Table I). The Mylar material that was selected had a high light transmission in the wavelength necessary for polymerization. Soft l iner materials were introduced to form a 5 mm thick layer between the two squares, and placed in a Triad curing unit. The specimen was polymerized for 10 minutes, inverted, and then polymerized for an addit ional 10 minutes. The Mylar wrap was then removed. For processing the heat-polymerized materials, the squares were invested in type I I I laboratory stone (SnapStone, WhipMix Corp., Louisville, Ky.) The free end of the screw was part ial ly inserted into a prefabricated plastic j ig to ensure their al ignment (Fig. 1). The opposing flask was prepared in the same manner with an identical jig. The height of the Lucitone 199 squares was adjusted by a nut attached to the screw to ensure uniform sample height.

The surfaces of the squares were prepared to a thickness of 5 mm according to the manufacturer's recommendations (Table I) before receiving the l iner materials. The test material was packed between the squares with two trial packings with cellophane as a separator. The samples were deflasked with the walnut shell blaster.

Ten samples of each mater ia l were tested at 72 ~ F within 24 hours of processing. Ten similarly prepared samples of each material were also stored in water at 72 ~ F for 6 months and then tested. The samples were placed in an MTS model 810 (MST System Corp., Minneapolis, Minn.) connected to an X-Y recorder. The samples were pulled apart at a crosshead speed of 1 mm/second. Fig. 2 illus- trates the specimen mounted in the machine and ready for testing. The maximum tensile stress before failure, mode of failure, and the total time elapsed preceding failure were recorded. The term'%ond strength" will not be used to describe the maximum stress before fracture. A more accurate term, "failure strength," is used because the samples did not always separate because of interfacial debonding from the denture base (adhesive failure). Tearing within the soft liner itself (cohesive failure) or a mixed mode of failure that involved both cohesive and adhesive failures were also observed.

Fai lure strength was recorded in megapascals (MPa). The mode of failure was characterized as cohesive, adhe-

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EMMER ET AL THE JOURNAL OF PROSTHETIC DENTISTRY

Fig. 1. Al ignment j ig for Lucitone 199 specimens in processing flask.

sive, or mixed mode, dependent on whether the fracture surface was in the soft l iner only, at the denture base-soft l iner interface only, or in both. For evaluating mixed mode of failure, a 10 x 10 mm grid with a total area matching the substrate was placed on the fracture surface, and the surface (with the grid) was imaged on a monitor of the digitiz- ing system (LA-500, Pias Co. Ltd., Osaka, Japan) by a video camera. The area percent of adhesive failure was computed by counting the number of squares of the grid in the denture base free of the liner (namely in the interfacial area of failure). The area mean percentage determined for each sample group was rounded to an interval scale with 20 intervals of 5% each. This interval method of evaluation was considered an excellent way to characterize the mac- roscopic failure features of the fracture surface. The time to failure was determined by a single operator with a stop- watch to record (1) the time from the start of the test (be- ginning arbitrar i ly at an approximate force of 0.1 N) to the time corresponding to maximum stress and (2) the time elapsed between the maximum stress and complete failure. To standardize the testing conditions for uniformity, the same operator performed all of the tests. The time and stress data were used to plot a qualitative deformation profile of each sample group by l inear interpolation between zero stress (at the start of the test) to maximum stress and between maximum stress to zero stress (corresponding to complete failure). This procedure was rela- tively easy and accurate at the strain rate of 1 mm/second used for the tensile test.

RESULTS

The mean and standard deviation (SD) values of failure strength of both the dry and wet groups of samples are shown in Fig. 3. One-way analysis of variance (ANOVA) revealed significant differences of means (p < 0.001) be-

MTS jaws

Hook

Alignment arch attached to nut

Screw

Autopolymerized acrylic resin

Soft liner sample

Lucitone 199 blocks

Fig. 2. Overall test arrangement of specimen mounted in MTS machine and ready for testing.

tween different brands in both the fresh and wet sample groups. Duncan multiple range tests (a 0.05) showed dis- t inct homogenous subsets (Table II).

Significant differences in failure modes were observed among the sample groups. The percent of the denture base area that was free of any liner was recorded as an area percent of adhesive failure. The results are i l lustrated in Fig. 4. Scanning electron microscopy (SEM) revealed typical microstructures of failure surfaces as presented in Fig. 5 (adhesive failure), Fig. 6 (cohesive failure), and Fig. 7 (mixed mode of failure). Fig. 8 i l lustrates the deformation profiles obtained by l inear interpolation between start of test at zero load and maximum stress recorded and also between the maximum recorded stress and complete failure. Although the loading was performed under stroke control in the actual test, the plot assumes a l inear loading and unloading rate during the test period. Although this assumption may not be accurate to describe the deformation profile, the method is valid to characterize the duc- ti le/brittle failure behavior of the reline material systems tested. The total t ime elapsed before complete failure indi- cates the extent of plastic deformation before failure under the constant strain rate conditions of the test. Significant differences were observed in the failure behavior.

Figs. 3 and 4 present trends result ing from water exposure relative to fresh samples.

DECEMBER 1995 597


MPa.

14-- 1 .4

12-

10-

8-

6- -

4-

2-

0-

Triad

78

Astron

2:9

Molloplast-B PermaSoft Super Soft

9 Dry @ 24 Hours.

Wet @ 6 Months

Fig. 3. Graph of mean and standard deviation (SD) of failure strength (in MPa) of dry and wet sample groups of each soft liner system tested.

100 100--

80-

60- 50

% 40 40 40 - 35

m 2O

20- ~ 0 0 0

o- ~ Triad Astron Molloplast-B PermaSoft Super Soft

9 Dry @ 24 Hours.

m Wet @ 6 Months

Fig. 4. Percent area of adhesive failure determined by fracture surface area of denture base free of liner after completion of test.

D ISCUSSION

Bonding material compatibility with denture base, liner material, or both is an important factor to be considered in studying failure strength. Plasticized PMMA (PermaSoft and Super Soft) and PMMA denture base materials (Luci- tone 199) are similar in chemical structure. Bonding agents are considered unnecessary for these materials. Molloplast-B liner material is a silicone and must be cou- pled with silane so that the liner bonds to the silane, which in turn copolymerizes with the denture base resin. Astron liner material uses a thin liquid-powder mix to prepare the denture base surface, which results in bonding by co-

polymerization in addition to the potential mechanical adhesion because of the roughened surface prepared before placement of the full thickness of the liner material. The Triad system uses its own universal bonding agent (unfilled resin) for copolymerization and mechanical bonding.

The tensile strength, tear resistance, and deformation characteristics of each material must also be considered. Triad and Astron liner materials failed immediately after elastic deformation with little stretching or plastic defoe- mation and recorded the greatest failure strength values. Most of these failures were internal (cohesive), which in-

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Fig. 5. SEM shows microstructure of fracture surface of adhesive failure. Absence of l iner material on fracture surface.

F ig. 6. SEM shows microstructure of typical cohesive failure. Entire fracture surface is covered with liner.

dicated that these materials are brittle, strong, and bonded strongly to the denture base. The adhesive strength was higher than the cohesive strength for this material.

Molloplast-B l iner material stretched over t ime and showed a low failure strength. The time elapsed before failure was high. It also failed internal ly with many small fractures toward the end of the elongation. This mater ia l is ductile and weak, and the bonding at the interface is stronger than the cohesive strength of the liner.

PermaSoft and Super Soft liner systems began to fail adhesively prematurely. As a result, the remaining interfacial area decreased and resulted in an increase in the stress of the cross section. Because of the configuration of the l iner-denture resin interface to the direction of stress, this stress was now closer to a shear type of stress than tensile (Fig. 9). Subsequent failure resulted from shear stress within the liner. This type of failure left a sharp cleft of the mater ia l over a large area. This mater ia l is britt le and weak, and the bond strength to the denture base is close to the cohesive shear strength of the material, caus- ing either adhesive or mixed mode of failure in these systems.

The changes in the material properties after 6 months in water warrant discussion. The failure strengths invariably increased on water exposure and this may be an indication that the materials became more brittle and probably less

Fig. 7. SEM shows microstructure of mixed mode of failure. Area A represents portion of fracture surface free of l iner and area B shows liner material retained on surface.

DECEMBER 1995 599


7.43 Triad

b

a Super Soft 2.94,

2.60 1.21

1.05

MPav ' I - I ~ ~ I_ I' Time Sec. 10 20 30 40

Fig. 8. Deformation profile of time elapsed before failure. Profile is drawn by l inear interpolation of stress between start of test and maximum stress (a) and between maximum stress and complete failure (b). Total time to failure is t ime from start of test to complete failure.

F ig . 9. Transformation of tensile stress to shear stress through initial adhesive failure caused liner to reorient in stress direction.

viscoelastic. This may also account for the nearly complete adhesive debonding of some of the materials (for example, PermaSoft), because they were able to resist deformation caused by increased brittleness. The effect of water im- mersion on the bonding agent may also be a factor in the adhesive failure of wet samples.

There is a need to evaluate other effects such as temper- ature, strain rate, and liner thickness on the adhesive properties, and these were not included in this study. Nev- ertheless, the differences in failure strength and modes are valuable in understanding the adhesion characteristics of the soft l iners studied. Moreover, the new methods used in this study to characterize soft l iner-denture adhesion appears to be a valuable approach for future research.

CL IN ICAL S IGNIF ICANCE

Clinically, the abil ity of denture reline materials to resist debonding from the denture and also internal fracture under masticatory stresses are extremely important. In addition, the liner material must remain stable in the sal- ivary oral environment. In this study, the adhesive and cohesive strength properties of selected soft liners were determined in a tensile test method that ensured axial self-alignment of the specimen during the test. The changes in the properties l isted caused by water exposure for 6 months were also determined.

Typically, Triad and Astron l iner materials showed a britt le type of failure that occurred cohesively within the l iner material. Molloplast-B liner mater ia l failed in a duc- ti le manner, but cohesively within the liner material. In contrast, Permasoft and Super Soft l iner materials failed either adhesively or in a mixed mode. All of the materials tended to become more brittle on water exposure for 6 months. These differences in failure characteristics of different materials should be considered in evaluating their clinical performance.

CONCLUSIONS

The tensile method developed in this study appears to be a valuable procedure to characterize the stress magnitudes and modes of failure of soft l iner bonded to denture base. There is a significant difference in the bond strength

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EMMER ET AL THE JOURNAL OF PROSTHETIC DENTISTRY

between soft liners as function of brands (material types) and curing modes. The failure is characterized by the in- terrelationships between the properties, chemical characteristics and/or compatibility of the liner, denture base, and bonding materials. Prolonged exposure to water sig- nificantly increased the failure strength, introduced brittle behavior to the liner, and changed the mode of failure more toward adhesive failure.

REFERENCES

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3. Boucher CO, Hickey JC, Zarb GA, eds. Prosthodontic treatment for edentulous patients. St Louis, C V Mosby; 1975:37-8.

4. Craig RG, ed. Restorative dental materials. St Louis: CV Mosby, 1989:542-4.

5. Sauve JL. A clinical evaluation of Silastic 390 as lining material for dentures. J PROSTHET DENT 1966:16:650-60.

6. Wright PS. Soft lining materials: their status and prospective. J Dent 1976;4:247-56.

7. Wright PS. The success and failure of denture soft-lining materials in clinical use. J Dent 1981;9:336-46.

8. Bates JF, Smith DC. Evaluation ofindirect resilient liners for dentures: Laboratory and clinical tests. J Am Dent Assoc 1965;70:344-53.

9. Amin WM, Fletcher AM, Ritchie GM. The nature of the interface between polymethyl methacrylate base materials and soft, linings materials. J Dent 1981;9:336-46.

10. Kawano F, Dootz ER, Koran A 3d, Craig RG. Comparison of bond strength of six sol% denture liners to denture base resin. J PROSTHET DENT 1992;68:368-71.

11. Wood WE, Johnson DL, Duncanson MG. Variables affecting silicone polymethyl lnethacrylate interracial bond strength. J Prosthodont 1993;2:13-8.

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.

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