Bond Strength of SixSoft Denture Liners

Processed Against Polymerizedand Unpolymerized

Poly(methyl Methacryiate)

Fumiaki Ksivano, ODS, PhD'

Tbe University of Tokushima School of Dentistry

Tokushitna, lapan

Erfirjrrf ft. Oootz, CDT**

Andrew Koran til, DDS, MS"'Robert G. Craig, phD*"*The University of Michigan School of DentistryAnn Arbor, Michigan

The bond strength of six commercial soft denfure liners was evaluated by atwo-phase tensile test. The soft denture liners investigated were VinaSoff,Prolastic, Flexor, rMolloplasf-B, Novus, and SuperSoft. The samples werefabricated by processing them (V against polymerized polyimethyimethacryiate), and (2) against unpolymerized polyimethyl methacryiate). Thesoft denfure liners were processed according to the manufacturers'recommendations. The samples were tested using an Inslron Universai TestingMachine. The mode of faiiure, adhesive or cohesive, was also recorded. Thebond strengfh when processed against unpolymerized polyimethylmethacryiatel ranged from 0.48 to 2.60 MPa, and when processed againstpoiymerized polyimefhyi methacryiate) the bond strength ranged from 0.94 to2.56 MPa. A two-way analysis of variance (P= .05) revealed a significantincrease in bond sfrength when the liners were processed against polymerizedpoly(methyl methacryiatel, except for Novus, which had no change, andVinaSoft, which decreased. The Tukey interval between maferials was .22 andbetween methods of polymerization was .08. Four of fhe six linersinvestigated denionstrated increased bond strength when processed againstpolymerized poly(methyl methacryiate). It was concluded that bonding can beinfluenced by fhe processing mefhod. Intj ProsthodonI 1997:10:178-182.

Soft denfure liners are used to assist in producingan even disfribufion of a functional load on the

denture-bearing mucosa, and to improve fhe refen-tion of denfures by engaging undercuts.^ Methacry-lafes, copolymers, silicones, and a polyphosphazinehave been used as long-term soft denture liners.^

able*Assistant Professor, Department of RtProsthodontics.

"Formerly Research Associate. Department of Biologic andMaterisis Science.

'"Professor, Department of Prosthodontics.""Professor Emeritus. Department of Biologic and Materials

Science.

Reprint requests: Dr Fumiaki Kawano, Department ofRemovabie Prosthodontics, The University of TokushimaSchool of Dentistry. 3-KurBmoto-c.ho. Tokushima. 770 lapan.

Presented at the Iniernaiional Association for Denial ResearchCenerai Session, Glasgow. Scotland.

Problems associafed with long-term use of softdenture liners include the loss of softness, staining,absorpfion of odors, poor bond strength, coloniza-fion by Candida sibicans, porosity, and poor tearstrength.^ One of the more serious problems is fail-ure of the bond between the soft lining materialand tbe denture base.̂ The bond failure also cre-afes a potenfial surface for bacterial growth,plaque, and calculus formafion. Tberefore, fre-quent clinical evaluafion and periodic replacemenfof a soft denture liner is generally required.

Several fests have been used fo assess the bondstrengfh of soff denture liners.'""" Craig and Gib-bons'" esfimafed fbe bond strengfh of fen soft dentureliners using a peeling tesf. They concluded fhat anadhesion value of 10 pounds per inch was satisfac-tory for the clinical use of soft denture liner."'^ Batesanci Smith'' assessed the bond sfrength of 12 soft lin-ing maferials using a tensile test. They conciuded

i of Prosthodont I 178 MO, Number 2,1997

Bond StrengJi of Denture Liners ind PM,V1A

Table 1 List of Materials Investigated

iianutaciurer TypeFiexor

Prolastic

SuperSott

Mollopiast-B

NovusVinaSott

Schutz-Den tai,Rosbach. Germany

Young Denial,Earth CHy, Missouri

Cos Laboratories,Chicago, iL

Moilopiast Regneri,Karlsruhe, Germany

Hygienic, Akron, OhioNuDansu,

Augusta, Georgia

Copoiymer

Room temperalure-vuicanizect silicone

Plasticized PMMA

Heat-polymerizedsiiicone

PolyphosphazineVinyi

031010

900601

3130187(L)3260889(P)900745

3U89Ano number

0311t5

that the soft denture liners investigated had clinicallysatisfactory bond strength. They also concluded thatthe heat-polymerized lining materiais had an inti-mate contact with a diffuse boundary when theywere polymerized against the acrylic dough. Aminet aK measured the bonding properties of four softdenture liners using tensile, shear, compression, andpeeling tests. They concluded that roughening theacrylic resin base before the application of a liningmaterial has a weakening effect on bonding. Khan etal^" determined the bond strength of three soft den-ture liners to light-polymerized denture base materi-als by using tensile test. They showed that bondstrengths ranged from 21,4 to 193.5 pounds perinch^ and concluded that the materials tested hadsufficient bond strength to light-polymerized denturebase resin for clinical use. In the authors' previousstudy, the bond strength of seven commercial softdenture liners to polymerized poiy(methylmethacrylate) (PMMA) denture base resin rangedfrom 9.6 to 26.1 kg/cm^.'^ It was concluded that thematerials tested had sufficient bond for clinical use.

The purpose of this study was to estimate thebond strength of six commercially available softdenture liners to both polymerized and unpolymer-ized PMMA denture base resin by a two-phase ten-sile test.

Materials and Methods

Six commercially available soft denture liners werechosen on the basis of different chemical composi-tions (Table 1 ). The bond strength of these soft den-ture liners to PMMA (Lucitone 199, DentspiyInternational, York, PA) was evaluated in tension.

All specimens were prepared by investing brassdies with a 3-mm-thick spacer in a dental flask, asreported in a previous study." Ail dies and spacers

Soft denture liner

zz:

Fig 1 Specimen configuration (mm).

were machined to the same dimensions to stan-dardize the shape of the PMMA sample and thethickness of the soft denture liner. Five specimenswith a cross-section area of 10 x 10 x 3 mm wereprepared for each material tested (Fig 1),

Two different groups of specimens were made. Inthe first group, the soft denture liners were processedagainst polymerized PMMA blocks according to theprocess described in the previous study." That is, thePMMA resin was mixed according to the manufac-turer's instructions, packed into the mold with thebrass spacer present, and processed in a water bathat 165°F for 9 hours to produce the resin blocks.After polymerization, the brass spacer was removedfrom the mold, the PMMA resin blocks weretr immed, and the surface to be bonded wassmoothed using 240-grit silicone carbide paper,cleaned, dried, and treated according to the manu-facturers' instructions. The resin blocks were thenrepositioned in the mold, and the soft denture linerwas packed into the space provided by removing thebrass spacer. The liners were then trial packed and

; 10. Njmlier 2, 1997 179 The International iournal of Proslhudorilici

Bord Strenglli ol Demure Lii

Table 2 Bond Strength and Mode of Failure Between Soft Denture Linerand PMMA

i^ateiials

-lexor^ro la SticSuperSoftMolloplast-B-lOVJS

îonded NovusVin a Soft

Un polymerized PMMA(MPa)

0,48 (0,02)0,84(0,11)1,27(0,10)1.41 (0.11)1.75(0.13)

2.01 (0,12)2.60(0,21)

Mode oílai i u re

AA/CCA/CA

AC

Polymerized PMMA(MPa)

1.11 (0.15)0.94 (0.05)1,64(0 12)1,73(0.14)1,71 (0.39)

2.56(0.18)1.09(0,12)

Mode offaiiure

A -C -CcAAA/C

Mean ot five specimens and standard deviation in parentíiesES. A = adhesive failure, C = coliesive tailure.A/C = adhesive tailure and coiiesive failure. Connecting bars sliow no signiticant difference at a 95% levelct confidence, Tukey inten/ai between mateiiab was 0,22 MPa, and that between methods oí polymeriza-tion was 0.08 MPa.

polymerized. Where indicated bonding agents wereused on the PMMA blocks as supplied from the man-ufacturer, Novus samples were made with and with-out the bonding agent (Novus Bond, Hygienic),

In the second group, the sofi denlure liners werepolymerized against unpolymerized PMMA andthen both materials were polymerized simultane-ously in a water bath according to the manufactur-ers' directions. The PMMA was packed with thebrass spacer and stored at room temperature for 4hours. The brass spacer was then removed and thesurface of the unpolymerized resin blocks weretreated according to the manufacturers' instruc-tions. The PMMA and the soft denture liners werethen polymerized.

After polymerization the samples were removedfrom the flask and any flash was trimmed using asharp blade. The samples were then tested in ten-sion using an Instron Universal Testing Machine(Instron, Canton, MA) at a crosshead speed of 2 cmper minute. To ensure that the samples were placedpurely in tension, the longitudinal axis of all speci-mens was made perpendicular using a square rule.During the clamping of the sample in the grips,care was taken to align the longitudinal axis of allspecimens with the direction of grip travel.

The bond strength was calculated as the stress atfailure divided by the cross-sectional area of thesample and expressed in MPa, Means and standarddeviations were determined for all materials underboth groups. The failure site was observed using anoptical microscope (Stereomicroscope SV8, CarlZeiss, Oberkochen, Germany) and recorded aseither cohesive, adhesive, or cohesive/adhesive fail-ure. Five samples were used for each material forboth groups. The bond strengths were compared bya two-way analysis of the variance and Tukey's mul-tiple comparison lest at a 95% level of confidence.'^

Results

Table 2 indicates bond strength values, standarddeviations, and the bond failure mode. The bondstrength of the soft denture liner ranged from 0.48MPa (Flexor) to 2,60 MPa (VinaSoft) when pro-cessed against unpolymerized PMMA, The modeof failure for Flexor, Novus, and bonded Novuswas adhesive, Prolastic and Molioplast-B failedboth adhesively and cohesively. SuperSoft andVinaSoft failed cohesively.

When the soft denture liners were processed onpolymerized PMMA, the bond strength rangedfrom 0,94 MPa (Prolastic) to 2,56 MPa (bondedNovus). Flexor, Novus, and bonded Novus failedadhesively. Cohesive fai lure was found forProlastic, SuperSoff, and Molloplast-B, VinaSoftfailed adhesively and cohesively.

Table 2 shows the results of fhe Tukey interval(P = ,05) for comparison of means among materials.When the soft denture liners were processedagainst unpolymerized PMMA, a significant differ-ence existed between all materials except SuperSoftand Molloplast-B, All materials except NoviJSshowed significant differences when the processingmethods were compared (unpolymerized/polymer-ized), VinaSoft had the highest bond strength whenprocessed against unpolymerized PMMA, BondedNovus had the highest bond strength when proc-essed against polymerized PMMA.

Discussion

Ideally, a soft denture liner should bond suffi-ciently well to the PMMA denture base resin toavoid failure of the interface during service.However, the bond strength between the soft den-ture liner and PMMA denture base resins is often

Tiie International loiirnal of Prosthodontic 180 Volume 10,Niimher 3, 1997

weak, and debonding of lining materials is oftenobserved before the materials have lost elasticity.The soft denture liner then has to be replaced be-cause the area of debonding becomes unhygienicor the material will simply peel off of ihe denture.

The peel test^'^ and tensile test^''-" have beenused to measure the bond strength of soft dentureliner to PMMA. Although tbe peel test closely simu-lates the force applied at the interface between thesoft denture liner and the dcntiire base, direct grip-ping of the soft liner in the peel test may damage thesample integrity at the gripped region, and the forcemay not be applied at the interface between bothmaterials directly because the applied force may de-pend on the properties of the soft denture liner.

The tensile bonding test did not simulate the clin-ical forces that may induce separation between thesoft denture liner and the denture base resin; how-ever, the force was applied to the bonding area ofboth materials. This test was effective in ranking thematerials and in evaluating the failure mode.

The highest bond strength was seen withVinaSoft denture liner when processed against un-polymerized PMMA, This bond strength was about2.4 times larger than that when processed to poly-merized PMMA. Also, the bonding mode shiftedfrom adhesive/cohesive to completely cohesivewhen processed against unpolymerized PMMA,This result indicates that the bond strength be-tween VinaSoft and unpolymerized PMMA isgreater than the tensile strength of the VinaSoft lin-ing material. VinaSoft apparently forms an inter-penetrating polymer network with the PMMA den-ture base resin,'5 which suggests that VinaSoftshould be processed against unpolymerizedPMMA to achieve the greatest bond.

Novus (polyphosphazine fiuoroelastomer), whenprocessed without a bonding agent, forms a satis-factory bond to PMMA (1,71 MPa processed/1,75MPa unprocessed), and its bond strength was notchanged by processing methods. However, when abonding agent (bonded Novus} was used, the bondstrength increased by 50% {2.56 MPa) whenprocessed against polymerized PMMA and by 157«(2,01 MPa) when processed against unpolymerizedPMMA, These results support the use of the bond-ing agent with Novus for maximum bond strength.

Proiastic is an room temperature-vu I can i zed sili-cone, and wben used as a denture lining material itis processed in a water bath. Since silicones haveless bond strength to more substrates, a bondingagent is required. This material had a low bondstrength and failed adhesively/cohesively whenprocessed against unpolymerized PMMA, This in-dicates that the bond strength is almost the same as

Bond Strength of Denture Liners ,ind PMMA

the tensile strength of the material. When Prolasticwas joined to polymerized PMMA it failed cohe-sively,

Molloplast-B (also a silicone) had similar charac-teristics, although Molloplast-6 had higher bondstrengths than Prolastic, The bond strength was1,41 MPa when processed against unpolymerizedPMMA, and failure was adhesive/cohesive. Thebond strength increased by 227o when processedagainst polymerized PMMA, and the failure modewas cohesive.

Flexor (a copolymer) had the lowest bond strength(0,48 MPa) of all tested materials when processedagainst unpolymerized PMMA, However, the bondstrength was increased by 130% when processedagainst polymerized PMMA, Flexor failed adhesivelyregardless of the processing method. SuperSoft(methyl/ethyl methacrylate) has a chemical compo-sition similar to the PMMA denture base resin andwill form a bond without a bonding agent. The bondstrength of SuperSoft (1,27 MPa¡ increased by 29%(1,64 MPal when processed against polymerizedPMMA, Both methods of processing produced a co-hesive mode of failure, which indicated good bond-ing. However, the bonding appeared to improvewhen processed against polymerized PMMA.

These results do not support the results of Aminet a l , ' who indicated that the strongest bondstrength resulted when the liners were processedagainst unpolymerized PMMA, This difference maybe the result of different experimental designs andthe time interval between joining the soft dentureliners to the unpolymerized PMMA,

All liners demonstrated increased bond strengthwhen processed to polymerized PMMA, with theexception of Novus, which had no significant dif-ference, and VinaSoft, which decreased whenprocessed against polymerized PMMA, SuperSofthas the potential of forming an interpenetratingmolecular network across the interface through thetwo chemically similar polymers,'^ However, thebond strength of SuperSoft decreased whenprocessed against unpolymerized PMMA, The re-sults of this study suggested that the interminglingof the monomers (SuperSoft or PMMA) might im-prove the mechanical properties of SuperSoft whenprocessed to unpolymerized PMMA.

The results of this study are heipfui in determiningwhich materials have Ihe best bond strength be-tween polymerized and unpoiymerized PMMA den-ture base resins and wili serve as a benchmark forthe study of new processed soft denture iiners andbonding agents. Water sorption may also affectbond strength and softness, and further investigationis required to determine the infiuence of this factor.

Volume 10, Nurrber 2, 1997 1 8 1 The Intematiunal Joiimal ot Prosthcidontics

Bond Strengtti of Dejiiure Lir

Conclusion

Six soft resin liners were tested for adherence toPMMA denture base resin using a tensile stressmethod. The liners were placed on both polymer-ized and unpolymerized resin samples. Within theparameters of the materials used and the study de-sign, the following conclusions may be made:

1. The bond strength for all soft denture linerstested was from 0,48 to 2,60 MPa when proc-essed against unpolymerized PMMA.

2. VinaSoft and bonded Novus had the highestbond strengths when processed against unpoly-merized PMMA,

3. The range of bond strength for all soft dentureliners was from 0,94 to 2,56 MPa when pro-cessed against polymerized PMMA,

4. Bonded Novus had the highest bond strength ofall materials when processed against polymer-ized PMMA, and VinaSoft had the highest bondstrength when processed against unpolymerizedPMMA,

5. All bond strengths increased when the linerswere processed against polymerized PMMA ex-cept for Novus, which had no significant differ-ence, and VinaSoft, which had a bond strengthdecrease.

Acknowledgments

This study was supported by NIt-t-NIDR Grant no. OEÛ9296,

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