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Bond strength of permanent soft denture liners bonded to the denture base
B o n d strength of p e r m a n e n t soft d e n t u r e liners b o n d e d to the d e n t u r e base Thomas
J. Emmer,
Jayalakshmi
Jr, DMD, a Thomas
Vaidynathan,
J. Emmer,
Sr, DDS, b
P h D , c a n d T r i t a l a K. V a i d y n a t h a n ,
PhD d
University of Medicine and Dentistry of New Jersey, New Jersey Dental School, Newark, N. J. T h e p u r p o s e o f t h i s s t u d y w a s to c h a r a c t e r i z e d e n t u r e a n d soft l i n e r a d h e s i o n a n d to d e t e r m i n e t h e a d h e s i v e a n d / o r c o h e s i v e s t r e n g t h o f d i f f e r e n t soft t i s s u e l i n e r s b o n d e d to t h e d e n t u r e b a s e b y u s e o f a n e w t e c h n i q u e . T w o g r o u p s of five p e r m a n e n t soft l i n e r s (dry or e x p o s e d to w a t e r for 6 m o n t h s ) w e r e t e s t e d b y u s e o f a t e n s i l e m o d e to c h a r a c t e r i z e t h e f a i l u r e c h a r a c t e r i s t i c s of soft l i n e r s b o n d e d to d e n t u r e b a s e resin. T h e m e t h o d d i f f e r e d f r o m p r e v i o u s t e s t m e t h o d s b e c a u s e o f t h e s p e c i m e n ' s a b i l i t y to align a x i a l l y d u r i n g t h e test. T h e r e s u l t s i n d i c a t e d s i g n i f i c a n t d i f f e r e n c e s in t h e b o n d i n g of l i n e r s to t h e d e n t u r e base, a n d l i g h t - c u r e s y s t e m s e x h i b i t e d t h e g r e a t e s t a m o u n t of s t r e s s n e e d e d for failure. L o w b o n d s t r e n g t h w a s o b s e r v e d w h e n t h e a d h e s i o n w a s p o o r or w h e n t h e c o h e s i v e s t r e n g t h o f t h e soft l i n e r w a s l o w a n d l e a d to p u r e a d h e s i v e or c o h e s i v e failure. W h e n b o t h a d h e s i v e a n d c o h e s i v e b o n d s w e r e strong, f a i l u r e o c c u r r e d at h i g h stresses. C o m b i n a t i o n s of a d h e s i v e a n d c o h e s i v e f a i l u r e s ( m i x e d m o d e ) w e r e also o b s e r v e d i n i n t e r m e d i a t e cases. (J PROSTHET DENT 1995;74:595-601.)
P e r m a n e n t soft d e n t u r e liners have been a Valuable asset for dentists and, because of their viscoelastic properties, they act as shock absorbers and reduce and d i s t r i b u t e the stresses on the d e n t u r e - b e a r i n g tissues. 1-2 Their use for p a t i e n t comfort a n d the t r e a t m e n t of the atrophic ridge, bone undercuts, bruxism, xerostomia, and d e n t u r e s opposing n a t u r a l teeth h a s been known to be clinically beneficial. 3 Although these a t t r i b u t e s are positive, t h e r e are also d i s a d v a n t a g e s to the use of p e r m a n e n t soft liners. One of the major drawbacks of the p e r m a n e n t soft liners is the lack of a durable bond to denture. 4-9 Debonding of soft liners from the d e n t u r e is a common clinical occurrence. Debonding results in localized unhygienic conditions at the debonded regions and often causes functional failure of the prosthesis.I~ Although there are published reports on the bond s t r e n g t h of soft liners bonded to d e n t u r e base resin, different methods such as peel 9, 11 or tensile 12tests have been used to m e a s u r e the bond strength. Although the previously used tests have provided valuable information, there are limitations to some of these methods. In particular, direct gripping of the specimen in the tensile testing machine m a y complicate or compromise the
aResearch Associate, Department of Prosthodontics and Biomaterials. 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 JOURNALOF PROSTHETIC DENTISTRY.
0022-3913/95/$5.00 + 0. 16/1/68284
DECEMBER 1995
specimen a l i g n m e n t 1~ and also d a m a g e t h e sample integrity at the gripped regions. There is therefore a need to develop a tensile test method t h a t permits axial self-alignm e n t of the specimen. This s t u d y was designed (1) to characterize the debonding characteristics of soft d e n t u r e liners bonded to denture resin m a t e r i a l with the following specific objectives, (2) to develop a tensile method to characterize the failure modes and s t r e n g t h s of soft liners bonded to denture base material, and (3) to use this method to evaluate the bonding and/or the cohesive s t r e n g t h of selected p e r m a n e n t soft reline m a t e r i a l s bonded to a d e n t u r e base material. MATERIAL
AND METHODS
The reline m a t e r i a l s included selected m a t e r i a l s from light- and heat-polymerized systems currently available. There are significant differences in the chemical m a k e u p of different m a t e r i a l s (Table I). Whereas Triad (Dentsply/ York Div., York, Pa.) and Astron (Astron Dental, Wheeling, Ill.) reline m a t e r i a l s use light polymerized resins based on u r e t h a n e dimethacrylate and Bis-GMA dimethacrylate monomers, Molloplast-B reline m a t e r i a l (Buffalo Dental Mfg. Co., Syosset, N. Y.) is based on silicone. Other systems such as PermaSoft (Nue Dent, Cambridge, Mass.) and Super Soft (Coe Laboratories, Chicago, Ill..) are plasticized polymethyl m e t h a c r y l a t e (PMMA) t h a t is mixed with polyethyl m e t h a c r y l a t e (PEMA). The d e n t u r e base material used was Lucitone 199 (Dentsply/York Div.), a heatprocessed PMMA based system. Lucitone 199 denture m a t e r i a l blocks (100 • 80 • 10 mm) were flasked and processed for 6 hours a t 164 ~ F and I hour at 212 ~ F. The blocks were cut into 10 • 10 • 5 m m
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T H E J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
E M M E R E T AL
T a b l e I. D e n t u r e reline m a t e r i a l s Triad
Polymerization mode Material chemical composition
Bonding agent chemical composition How supplied Manufacturers recommended surface preparation
Astron
Molloplast B
PermaSoft
Super Soft
Light
Light
Heat
Heat
Heat
Urethane polyether dimethacrylate
Composite ? (Information not made available)
Silicone
Light cured methylmethacrylate Premixed paste Apply Triad bonding agent
Self
Saline
Plasticized polymethyl methacrylate/ polyethyl methacrylate Self
Plasticized polymethyl methacrylate/ Polyethyl methacrylate Self
Powder liquid Apply "wet" mix of freshly mixed powder liquid
Premixed paste Apply bonding agent
Powder liquid Rinse denture surface with monomer
Powder liquid Rinse denture surface with monomer
T a b l e II. Duncan multiple range tests of subsets Sample group failure strength (MPa)
Stored 24 hours (72 ~ F), dry Stored 6 months (72 ~ F), water
Triad
Super Soft
Astron
Molloplast B
PermaSoft
7.43 12.4
2.94 7.09
2.60 7.80
1.21 2.69
1.50 1.83
squares w i t h a saw, a n d a w a t e r coolant was used. The squares were a t t a c h e d to screws by use of autopolymerizing acrylic resin a r o u n d the screw head. The opposite end t h a t the screw was a t t a c h e d to was roughened with a crosscut carbide b u r (H 72E, Brasseler, Savannah, Ga.) and r a n d o m l y assigned to different groups. For processing the light-polymerized materials, the individual squares were w r a p p e d with clear M y l a r film (Du Pont Co., Wilmington, Del.) and t h e surface of t h e squares was p r e p a r e d according to the m a n u f a c t u r e r ' s recommendations (Table I). The M y l a r m a t e r i a l t h a t was selected h a d a high light t r a n s m i s s i o n in the wavelength necessary for polymerization. Soft liner m a t e r i a l s were introduced to form a 5 m m thick layer between the two squares, and placed in a T r i a d curing unit. The specimen was polymerized for 10 minutes, inverted, and then polymerized for an additional 10 minutes. The M y l a r w r a p was t h e n removed. For processing the heat-polymerized materials, the squares were invested in type III laboratory stone (SnapStone, WhipMix Corp., Louisville, Ky.) The free end of the screw was p a r t i a l l y inserted into a prefabricated plastic jig to ensure t h e i r a l i g n m e n t (Fig. 1). The opposing flask was p r e p a r e d in the s a m e m a n n e r with an identical jig. The height of the Lucitone 199 squares was adjusted by a n u t a t t a c h e d to t h e screw to ensure uniform s a m p l e height.
596
The surfaces of the squares were p r e p a r e d to a thickness of 5 m m according to the m a n u f a c t u r e r ' s recommendations (Table I) before receiving the liner materials. The test material was packed between the squares with two trial packings with cellophane as a separator. The samples were deflasked w i t h the w a l n u t shell blaster. Ten samples of each m a t e r i a l were tested at 72 ~ F within 24 hours of processing. Ten similarly p r e p a r e d samples of each m a t e r i a l were also stored in w a t e r at 72 ~ F for 6 months a n d t h e n tested. The samples were placed in an MTS model 810 (MST S y s t e m Corp., Minneapolis, Minn.) connected to a n X-Y recorder. The samples were pulled a p a r t at a crosshead speed of 1 mm/second. Fig. 2 illust r a t e s the specimen m o u n t e d in the machine and r e a d y 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. F a i l u r e s t r e n g t h was recorded in megapascals (MPa). The mode of failure was characterized as cohesive, adhe-
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MTS jaws
Hook
Alignment arch attached to nut Screw Autopolymerized acrylic resin Soft liner sample Lucitone 199 blocks F i g . 1. A l i g n m e n t j i g for Lucitone 199 specimens in processing flask.
sive, or mixed mode, dependent on w h e t h e r the fracture surface was in the soft liner only, a t the denture b a s e - s o f t liner interface only, or in both. F o r e v a l u a t i n g mixed mode of failure, a 10 x 10 m m grid with a total a r e a m a t c h i n g the s u b s t r a t e was placed on the fracture surface, and the surface (with the grid) was imaged on a monitor of the digitizing system (LA-500, Pias Co. Ltd., Osaka, J a p a n ) by a video camera. The a r e a percent of adhesive failure was computed by counting the n u m b e r of squares of the grid in the d e n t u r e base free of the liner (namely in the interfacial a r e a of failure). The a r e a m e a n percentage d e t e r m i n e d for each sample group was rounded to an interval scale with 20 intervals of 5% each. This interval method of evaluation was considered a n excellent w a y to characterize the macroscopic failure features of the fracture surface. The time to failure was d e t e r m i n e d by a single operator with a stopw a t c h to record (1) t h e time from t h e s t a r t of the test (beginning a r b i t r a r i l y at an a p p r o x i m a t e force of 0.1 N) to the time corresponding to m a x i m u m stress and (2) the time elapsed between the m a x i m u m stress and complete failure. To s t a n d a r d i z e the testing conditions for uniformity, the s a m e operator performed all of the tests. The time and stress d a t a were used to plot a qualitative deformation profile of each s a m p l e group by l i n e a r interpolation bet w e e n zero stress (at the s t a r t of the test) to m a x i m u m stress a n d between m a x i m u m stress to zero stress (corresponding to complete failure). This procedure was relatively easy a n d accurate a t the s t r a i n rate of 1 mm/second used for the tensile test. RESULTS The m e a n and s t a n d a r d deviation (SD) values of failure s t r e n g t h of both the dry a n d wet groups of samples are shown in Fig. 3. One-way analysis of variance (ANOVA) revealed significant differences of m e a n s (p < 0.001) be-
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F i g . 2. Overall test a r r a n g e m e n t of specimen mounted in MTS machine a n d r e a d y for testing.
tween different b r a n d s in both the fresh and wet sample groups. Duncan multiple range tests (a 0.05) showed distinct homogenous subsets (Table II). Significant differences in failure modes were observed among the sample groups. The percent of the denture base a r e a t h a t was free of a n y liner was recorded as a n a r e a percent of adhesive failure. The results are illustrated 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 illustrates the deformation profiles obtained by l i n e a r interpolation between s t a r t of t e s t at zero load a n d m a x i m u m stress recorded and also between the m a x i m u m recorded stress and complete failure. Although the loading was performed under stroke control in the actual test, the plot a s s u m e s a linear loading a n d unloading r a t e during the t e s t period. Although this assumption m a y not be accurate to describe the deform a t i o n profile, the method is valid to characterize the ductile/brittle failure behavior of the reline m a t e r i a l systems tested. The total time elapsed before complete failure indicates the extent of plastic deformation before failure under the constant s t r a i n r a t e conditions of the test. Significant differences were observed in the failure behavior. Figs. 3 and 4 p r e s e n t trends resulting from w a t e r exposure relative to fresh samples.
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1 .4
1210-
78
MPa.
86-4-
2:9
20Astron
Triad
9
Molloplast-B
PermaSoft
Super Soft
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.
100 100-8060-
%
50 40
40 2O 20-
~ 0
o-
40
35
0
0
~ Triad
Astron
Molloplast-B
PermaSoft
m
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.
DISCUSSION 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 (Lucitone 199) are similar in chemical structure. Bonding agents are considered unnecessary for these materials. Molloplast-B liner material is a silicone and must be coupled 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-
598
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 defoemation and recorded the greatest failure strength values. Most of these failures were internal (cohesive), which in-
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F i g . 5. SEM shows microstructure of fracture surface of adhesive failure. Absence of liner m a t e r i a l on fracture surface.
dicated t h a t these m a t e r i a l s are brittle, strong, and bonded strongly to the d e n t u r e base. The adhesive strength was higher t h a n the cohesive strength for this material. Molloplast-B liner m a t e r i a l stretched over time a n d showed a low failure strength. The time elapsed before failure was high. It also failed i n t e r n a l l y with m a n y small fractures toward the end of the elongation. This m a t e r i a l is ductile a n d weak, a n d the bonding at the interface is stronger t h a n the cohesive s t r e n g t h of the liner. PermaSoft and Super Soft liner systems began to fail adhesively p r e m a t u r e l y . As a result, the r e m a i n i n g interfacial a r e a decreased and resulted in an increase in the stress of the cross section. Because of the configuration of the liner-denture resin interface to the direction of stress, this stress was now closer to a s h e a r type of stress t h a n tensile (Fig. 9). Subsequent failure resulted from s h e a r stress within the liner. This type of failure left a s h a r p cleft of t h e m a t e r i a l over a large area. This m a t e r i a l is brittle a n d weak, a n d the bond strength to the denture base is close to the cohesive s h e a r s t r e n g t h of the material, causing either adhesive or mixed mode of failure in these systems. The changes in the m a t e r i a l properties after 6 months in w a t e r w a r r a n t discussion. The failure strengths i n v a r i a b l y increased on w a t e r exposure a n d this m a y be an indication t h a t the m a t e r i a l s became more brittle and probably less
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F i g . 6. SEM shows microstructure of typical cohesive failure. E n t i r e fracture surface is covered with liner.
Fig. 7. SEM shows microstructure of mixed mode of failure. A r e a A represents portion of fracture surface free of liner a n d a r e a B shows liner m a t e r i a l r e t a i n e d on surface.
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Triad 7.43 b
a
Super Soft
2.94, 2.60 1.21 1.05 MPav
' Time Sec.
I
-
I
10
~
~
20
I_ 30
I' 40
F i g . 8. Deformation profile of time elapsed before failure. Profile is d r a w n by linear interpolation of stress between s t a r t of test a n d m a x i m u m stress (a) a n d between m a x i m u m stress and complete failure (b). Total time to failure is time from s t a r t of test to complete failure.
There is a need to evaluate other effects such as temperature, s t r a i n rate, a n d liner thickness on the adhesive properties, and these were not included in this study. Nevertheless, the differences in failure s t r e n g t h and modes are valuable in u n d e r s t a n d i n g the adhesion characteristics of the soft liners studied. Moreover, the new methods used in this study to characterize soft l i n e r - d e n t u r e adhesion appears to be a valuable approach for future research. CLINICAL
F i g . 9. Transformation of tensile stress to s h e a r stress t h r o u g h initial adhesive failure caused liner to reorient in stress direction.
viscoelastic. This m a y also account for the n e a r l y complete adhesive debonding of some of the m a t e r i a l s (for example, PermaSoft), because they were able to resist deformation caused by increased brittleness. The effect of w a t e r immersion on the bonding agent m a y also be a factor in the adhesive failure of wet samples.
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SIGNIFICANCE
Clinically, the ability of d e n t u r e reline m a t e r i a l s to resist debonding from the denture a n d also internal fracture u n d e r m a s t i c a t o r y stresses are extremely important. In addition, t h e liner m a t e r i a l m u s t r e m a i n stable in the sali v a r y oral environment. In this study, the adhesive and cohesive s t r e n g t h properties of selected soft liners were d e t e r m i n e d in a tensile test method t h a t ensured axial self-alignment of the specimen d u r i n g the test. The changes in the properties listed caused by w a t e r exposure for 6 months were also determined. Typically, Triad a n d Astron liner m a t e r i a l s showed a brittle type of failure t h a t occurred cohesively within the liner material. Molloplast-B liner m a t e r i a l failed in a ductile manner, but cohesively w i t h i n t h e liner material. I n contrast, Permasoft and Super Soft liner m a t e r i a l s failed either adhesively or in a mixed mode. All of the m a t e r i a l s tended to become more brittle on w a t e r exposure for 6 months. These differences in failure characteristics of different m a t e r i a l s 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 a n d modes of failure of soft liner bonded to denture base. There is a significant difference in the bond strength
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between soft liners as function of brands (material types) and curing modes. The failure is characterized by the interrelationships between the properties, chemical characteristics and/or compatibility of the liner, denture base, and bonding materials. Prolonged exposure to water significantly increased the failure strength, introduced brittle behavior to the liner, and changed the mode of failure more toward adhesive failure. REFERENCES
1. Lytle RB. The management of abused oral tissue in complete denture construction. J PROSTHET DENT 1957;7:27-42. 2. Lytle RB. Complete denture construction based on a study of the deformation of the underlying soft tissue. J PROSTHETDENT 1959;9:539-51. 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.
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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.
Reprint requests to: DR. THOMASJ. EMMER 15 EASLEYTERRACE CONVENT STATION, NJ 07960
601
J. Emmer,
Jayalakshmi
Jr, DMD, a Thomas
Vaidynathan,
J. Emmer,
Sr, DDS, b
P h D , c a n d T r i t a l a K. V a i d y n a t h a n ,
PhD d
University of Medicine and Dentistry of New Jersey, New Jersey Dental School, Newark, N. J. T h e p u r p o s e o f t h i s s t u d y w a s to c h a r a c t e r i z e d e n t u r e a n d soft l i n e r a d h e s i o n a n d to d e t e r m i n e t h e a d h e s i v e a n d / o r c o h e s i v e s t r e n g t h o f d i f f e r e n t soft t i s s u e l i n e r s b o n d e d to t h e d e n t u r e b a s e b y u s e o f a n e w t e c h n i q u e . T w o g r o u p s of five p e r m a n e n t soft l i n e r s (dry or e x p o s e d to w a t e r for 6 m o n t h s ) w e r e t e s t e d b y u s e o f a t e n s i l e m o d e to c h a r a c t e r i z e t h e f a i l u r e c h a r a c t e r i s t i c s of soft l i n e r s b o n d e d to d e n t u r e b a s e resin. T h e m e t h o d d i f f e r e d f r o m p r e v i o u s t e s t m e t h o d s b e c a u s e o f t h e s p e c i m e n ' s a b i l i t y to align a x i a l l y d u r i n g t h e test. T h e r e s u l t s i n d i c a t e d s i g n i f i c a n t d i f f e r e n c e s in t h e b o n d i n g of l i n e r s to t h e d e n t u r e base, a n d l i g h t - c u r e s y s t e m s e x h i b i t e d t h e g r e a t e s t a m o u n t of s t r e s s n e e d e d for failure. L o w b o n d s t r e n g t h w a s o b s e r v e d w h e n t h e a d h e s i o n w a s p o o r or w h e n t h e c o h e s i v e s t r e n g t h o f t h e soft l i n e r w a s l o w a n d l e a d to p u r e a d h e s i v e or c o h e s i v e failure. W h e n b o t h a d h e s i v e a n d c o h e s i v e b o n d s w e r e strong, f a i l u r e o c c u r r e d at h i g h stresses. C o m b i n a t i o n s of a d h e s i v e a n d c o h e s i v e f a i l u r e s ( m i x e d m o d e ) w e r e also o b s e r v e d i n i n t e r m e d i a t e cases. (J PROSTHET DENT 1995;74:595-601.)
P e r m a n e n t soft d e n t u r e liners have been a Valuable asset for dentists and, because of their viscoelastic properties, they act as shock absorbers and reduce and d i s t r i b u t e the stresses on the d e n t u r e - b e a r i n g tissues. 1-2 Their use for p a t i e n t comfort a n d the t r e a t m e n t of the atrophic ridge, bone undercuts, bruxism, xerostomia, and d e n t u r e s opposing n a t u r a l teeth h a s been known to be clinically beneficial. 3 Although these a t t r i b u t e s are positive, t h e r e are also d i s a d v a n t a g e s to the use of p e r m a n e n t soft liners. One of the major drawbacks of the p e r m a n e n t soft liners is the lack of a durable bond to denture. 4-9 Debonding of soft liners from the d e n t u r e is a common clinical occurrence. Debonding results in localized unhygienic conditions at the debonded regions and often causes functional failure of the prosthesis.I~ Although there are published reports on the bond s t r e n g t h of soft liners bonded to d e n t u r e base resin, different methods such as peel 9, 11 or tensile 12tests have been used to m e a s u r e the bond strength. Although the previously used tests have provided valuable information, there are limitations to some of these methods. In particular, direct gripping of the specimen in the tensile testing machine m a y complicate or compromise the
aResearch Associate, Department of Prosthodontics and Biomaterials. 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 JOURNALOF PROSTHETIC DENTISTRY.
0022-3913/95/$5.00 + 0. 16/1/68284
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specimen a l i g n m e n t 1~ and also d a m a g e t h e sample integrity at the gripped regions. There is therefore a need to develop a tensile test method t h a t permits axial self-alignm e n t of the specimen. This s t u d y was designed (1) to characterize the debonding characteristics of soft d e n t u r e liners bonded to denture resin m a t e r i a l with the following specific objectives, (2) to develop a tensile method to characterize the failure modes and s t r e n g t h s of soft liners bonded to denture base material, and (3) to use this method to evaluate the bonding and/or the cohesive s t r e n g t h of selected p e r m a n e n t soft reline m a t e r i a l s bonded to a d e n t u r e base material. MATERIAL
AND METHODS
The reline m a t e r i a l s included selected m a t e r i a l s from light- and heat-polymerized systems currently available. There are significant differences in the chemical m a k e u p of different m a t e r i a l s (Table I). Whereas Triad (Dentsply/ York Div., York, Pa.) and Astron (Astron Dental, Wheeling, Ill.) reline m a t e r i a l s use light polymerized resins based on u r e t h a n e dimethacrylate and Bis-GMA dimethacrylate monomers, Molloplast-B reline m a t e r i a l (Buffalo Dental Mfg. Co., Syosset, N. Y.) is based on silicone. Other systems such as PermaSoft (Nue Dent, Cambridge, Mass.) and Super Soft (Coe Laboratories, Chicago, Ill..) are plasticized polymethyl m e t h a c r y l a t e (PMMA) t h a t is mixed with polyethyl m e t h a c r y l a t e (PEMA). The d e n t u r e base material used was Lucitone 199 (Dentsply/York Div.), a heatprocessed PMMA based system. Lucitone 199 denture m a t e r i a l blocks (100 • 80 • 10 mm) were flasked and processed for 6 hours a t 164 ~ F and I hour at 212 ~ F. The blocks were cut into 10 • 10 • 5 m m
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E M M E R E T AL
T a b l e I. D e n t u r e reline m a t e r i a l s Triad
Polymerization mode Material chemical composition
Bonding agent chemical composition How supplied Manufacturers recommended surface preparation
Astron
Molloplast B
PermaSoft
Super Soft
Light
Light
Heat
Heat
Heat
Urethane polyether dimethacrylate
Composite ? (Information not made available)
Silicone
Light cured methylmethacrylate Premixed paste Apply Triad bonding agent
Self
Saline
Plasticized polymethyl methacrylate/ polyethyl methacrylate Self
Plasticized polymethyl methacrylate/ Polyethyl methacrylate Self
Powder liquid Apply "wet" mix of freshly mixed powder liquid
Premixed paste Apply bonding agent
Powder liquid Rinse denture surface with monomer
Powder liquid Rinse denture surface with monomer
T a b l e II. Duncan multiple range tests of subsets Sample group failure strength (MPa)
Stored 24 hours (72 ~ F), dry Stored 6 months (72 ~ F), water
Triad
Super Soft
Astron
Molloplast B
PermaSoft
7.43 12.4
2.94 7.09
2.60 7.80
1.21 2.69
1.50 1.83
squares w i t h a saw, a n d a w a t e r coolant was used. The squares were a t t a c h e d to screws by use of autopolymerizing acrylic resin a r o u n d the screw head. The opposite end t h a t the screw was a t t a c h e d to was roughened with a crosscut carbide b u r (H 72E, Brasseler, Savannah, Ga.) and r a n d o m l y assigned to different groups. For processing the light-polymerized materials, the individual squares were w r a p p e d with clear M y l a r film (Du Pont Co., Wilmington, Del.) and t h e surface of t h e squares was p r e p a r e d according to the m a n u f a c t u r e r ' s recommendations (Table I). The M y l a r m a t e r i a l t h a t was selected h a d a high light t r a n s m i s s i o n in the wavelength necessary for polymerization. Soft liner m a t e r i a l s were introduced to form a 5 m m thick layer between the two squares, and placed in a T r i a d curing unit. The specimen was polymerized for 10 minutes, inverted, and then polymerized for an additional 10 minutes. The M y l a r w r a p was t h e n removed. For processing the heat-polymerized materials, the squares were invested in type III laboratory stone (SnapStone, WhipMix Corp., Louisville, Ky.) The free end of the screw was p a r t i a l l y inserted into a prefabricated plastic jig to ensure t h e i r a l i g n m e n t (Fig. 1). The opposing flask was p r e p a r e d in the s a m e m a n n e r with an identical jig. The height of the Lucitone 199 squares was adjusted by a n u t a t t a c h e d to t h e screw to ensure uniform s a m p l e height.
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The surfaces of the squares were p r e p a r e d to a thickness of 5 m m according to the m a n u f a c t u r e r ' s recommendations (Table I) before receiving the liner materials. The test material was packed between the squares with two trial packings with cellophane as a separator. The samples were deflasked w i t h the w a l n u t shell blaster. Ten samples of each m a t e r i a l were tested at 72 ~ F within 24 hours of processing. Ten similarly p r e p a r e d samples of each m a t e r i a l were also stored in w a t e r at 72 ~ F for 6 months a n d t h e n tested. The samples were placed in an MTS model 810 (MST S y s t e m Corp., Minneapolis, Minn.) connected to a n X-Y recorder. The samples were pulled a p a r t at a crosshead speed of 1 mm/second. Fig. 2 illust r a t e s the specimen m o u n t e d in the machine and r e a d y 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. F a i l u r e s t r e n g t h was recorded in megapascals (MPa). The mode of failure was characterized as cohesive, adhe-
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MTS jaws
Hook
Alignment arch attached to nut Screw Autopolymerized acrylic resin Soft liner sample Lucitone 199 blocks F i g . 1. A l i g n m e n t j i g for Lucitone 199 specimens in processing flask.
sive, or mixed mode, dependent on w h e t h e r the fracture surface was in the soft liner only, a t the denture b a s e - s o f t liner interface only, or in both. F o r e v a l u a t i n g mixed mode of failure, a 10 x 10 m m grid with a total a r e a m a t c h i n g the s u b s t r a t e was placed on the fracture surface, and the surface (with the grid) was imaged on a monitor of the digitizing system (LA-500, Pias Co. Ltd., Osaka, J a p a n ) by a video camera. The a r e a percent of adhesive failure was computed by counting the n u m b e r of squares of the grid in the d e n t u r e base free of the liner (namely in the interfacial a r e a of failure). The a r e a m e a n percentage d e t e r m i n e d for each sample group was rounded to an interval scale with 20 intervals of 5% each. This interval method of evaluation was considered a n excellent w a y to characterize the macroscopic failure features of the fracture surface. The time to failure was d e t e r m i n e d by a single operator with a stopw a t c h to record (1) t h e time from t h e s t a r t of the test (beginning a r b i t r a r i l y at an a p p r o x i m a t e force of 0.1 N) to the time corresponding to m a x i m u m stress and (2) the time elapsed between the m a x i m u m stress and complete failure. To s t a n d a r d i z e the testing conditions for uniformity, the s a m e operator performed all of the tests. The time and stress d a t a were used to plot a qualitative deformation profile of each s a m p l e group by l i n e a r interpolation bet w e e n zero stress (at the s t a r t of the test) to m a x i m u m stress a n d between m a x i m u m stress to zero stress (corresponding to complete failure). This procedure was relatively easy a n d accurate a t the s t r a i n rate of 1 mm/second used for the tensile test. RESULTS The m e a n and s t a n d a r d deviation (SD) values of failure s t r e n g t h of both the dry a n d wet groups of samples are shown in Fig. 3. One-way analysis of variance (ANOVA) revealed significant differences of m e a n s (p < 0.001) be-
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F i g . 2. Overall test a r r a n g e m e n t of specimen mounted in MTS machine a n d r e a d y for testing.
tween different b r a n d s in both the fresh and wet sample groups. Duncan multiple range tests (a 0.05) showed distinct homogenous subsets (Table II). Significant differences in failure modes were observed among the sample groups. The percent of the denture base a r e a t h a t was free of a n y liner was recorded as a n a r e a percent of adhesive failure. The results are illustrated 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 illustrates the deformation profiles obtained by l i n e a r interpolation between s t a r t of t e s t at zero load a n d m a x i m u m stress recorded and also between the m a x i m u m recorded stress and complete failure. Although the loading was performed under stroke control in the actual test, the plot a s s u m e s a linear loading a n d unloading r a t e during the t e s t period. Although this assumption m a y not be accurate to describe the deform a t i o n profile, the method is valid to characterize the ductile/brittle failure behavior of the reline m a t e r i a l systems tested. The total time elapsed before complete failure indicates the extent of plastic deformation before failure under the constant s t r a i n r a t e conditions of the test. Significant differences were observed in the failure behavior. Figs. 3 and 4 p r e s e n t trends resulting from w a t e r exposure relative to fresh samples.
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1 .4
1210-
78
MPa.
86-4-
2:9
20Astron
Triad
9
Molloplast-B
PermaSoft
Super Soft
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.
100 100-8060-
%
50 40
40 2O 20-
~ 0
o-
40
35
0
0
~ Triad
Astron
Molloplast-B
PermaSoft
m
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.
DISCUSSION 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 (Lucitone 199) are similar in chemical structure. Bonding agents are considered unnecessary for these materials. Molloplast-B liner material is a silicone and must be coupled 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-
598
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 defoemation and recorded the greatest failure strength values. Most of these failures were internal (cohesive), which in-
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F i g . 5. SEM shows microstructure of fracture surface of adhesive failure. Absence of liner m a t e r i a l on fracture surface.
dicated t h a t these m a t e r i a l s are brittle, strong, and bonded strongly to the d e n t u r e base. The adhesive strength was higher t h a n the cohesive strength for this material. Molloplast-B liner m a t e r i a l stretched over time a n d showed a low failure strength. The time elapsed before failure was high. It also failed i n t e r n a l l y with m a n y small fractures toward the end of the elongation. This m a t e r i a l is ductile a n d weak, a n d the bonding at the interface is stronger t h a n the cohesive s t r e n g t h of the liner. PermaSoft and Super Soft liner systems began to fail adhesively p r e m a t u r e l y . As a result, the r e m a i n i n g interfacial a r e a decreased and resulted in an increase in the stress of the cross section. Because of the configuration of the liner-denture resin interface to the direction of stress, this stress was now closer to a s h e a r type of stress t h a n tensile (Fig. 9). Subsequent failure resulted from s h e a r stress within the liner. This type of failure left a s h a r p cleft of t h e m a t e r i a l over a large area. This m a t e r i a l is brittle a n d weak, a n d the bond strength to the denture base is close to the cohesive s h e a r s t r e n g t h of the material, causing either adhesive or mixed mode of failure in these systems. The changes in the m a t e r i a l properties after 6 months in w a t e r w a r r a n t discussion. The failure strengths i n v a r i a b l y increased on w a t e r exposure a n d this m a y be an indication t h a t the m a t e r i a l s became more brittle and probably less
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F i g . 6. SEM shows microstructure of typical cohesive failure. E n t i r e fracture surface is covered with liner.
Fig. 7. SEM shows microstructure of mixed mode of failure. A r e a A represents portion of fracture surface free of liner a n d a r e a B shows liner m a t e r i a l r e t a i n e d on surface.
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Triad 7.43 b
a
Super Soft
2.94, 2.60 1.21 1.05 MPav
' Time Sec.
I
-
I
10
~
~
20
I_ 30
I' 40
F i g . 8. Deformation profile of time elapsed before failure. Profile is d r a w n by linear interpolation of stress between s t a r t of test a n d m a x i m u m stress (a) a n d between m a x i m u m stress and complete failure (b). Total time to failure is time from s t a r t of test to complete failure.
There is a need to evaluate other effects such as temperature, s t r a i n rate, a n d liner thickness on the adhesive properties, and these were not included in this study. Nevertheless, the differences in failure s t r e n g t h and modes are valuable in u n d e r s t a n d i n g the adhesion characteristics of the soft liners studied. Moreover, the new methods used in this study to characterize soft l i n e r - d e n t u r e adhesion appears to be a valuable approach for future research. CLINICAL
F i g . 9. Transformation of tensile stress to s h e a r stress t h r o u g h initial adhesive failure caused liner to reorient in stress direction.
viscoelastic. This m a y also account for the n e a r l y complete adhesive debonding of some of the m a t e r i a l s (for example, PermaSoft), because they were able to resist deformation caused by increased brittleness. The effect of w a t e r immersion on the bonding agent m a y also be a factor in the adhesive failure of wet samples.
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SIGNIFICANCE
Clinically, the ability of d e n t u r e reline m a t e r i a l s to resist debonding from the denture a n d also internal fracture u n d e r m a s t i c a t o r y stresses are extremely important. In addition, t h e liner m a t e r i a l m u s t r e m a i n stable in the sali v a r y oral environment. In this study, the adhesive and cohesive s t r e n g t h properties of selected soft liners were d e t e r m i n e d in a tensile test method t h a t ensured axial self-alignment of the specimen d u r i n g the test. The changes in the properties listed caused by w a t e r exposure for 6 months were also determined. Typically, Triad a n d Astron liner m a t e r i a l s showed a brittle type of failure t h a t occurred cohesively within the liner material. Molloplast-B liner m a t e r i a l failed in a ductile manner, but cohesively w i t h i n t h e liner material. I n contrast, Permasoft and Super Soft liner m a t e r i a l s failed either adhesively or in a mixed mode. All of the m a t e r i a l s tended to become more brittle on w a t e r exposure for 6 months. These differences in failure characteristics of different m a t e r i a l s 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 a n d modes of failure of soft liner bonded to denture base. There is a significant difference in the bond strength
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between soft liners as function of brands (material types) and curing modes. The failure is characterized by the interrelationships between the properties, chemical characteristics and/or compatibility of the liner, denture base, and bonding materials. Prolonged exposure to water significantly increased the failure strength, introduced brittle behavior to the liner, and changed the mode of failure more toward adhesive failure. REFERENCES
1. Lytle RB. The management of abused oral tissue in complete denture construction. J PROSTHET DENT 1957;7:27-42. 2. Lytle RB. Complete denture construction based on a study of the deformation of the underlying soft tissue. J PROSTHETDENT 1959;9:539-51. 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.
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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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