RESEARCH AND EDUCATION

Presented atImplantologyaResearch AsbProfessor, DcResearch AsdProfessor, DeResearch As

THE JOURNA

Effect of sealer coating and storage methods on the surfaceroughness of soft liners

Ilknur Usta Kutlu, DDS,a Nuran Dinckal Yaniko�glu, DDS, PhD,b Esra Kul, DDS,c Zeynep Yesïl Duymus, DDS, PhD,d

and Nurdan Polat Sa�gsöz, DDSe

ABSTRACTStatement of problem. A soft lining is applied under a removable prosthesis for various reasons.The porosity of the lining material may increase colonization by microorganisms and cause tissueinflammation.

Purpose. The purpose of this in vitro study was to evaluate the effect of sealer coating on thesurface roughness of soft lining materials under 4 different conditions.

Material and methods. A total of 125 specimens were prepared. One high-temperature silicone-based soft lining material and 2 room-temperature-polymerized soft lining materials (1 silicone-based and 1 methacrylate-based) were used. Twenty-five specimens of each room-temperaturesoft lining material were coated with 2 layers of surface sealer. Additionally, 5 specimens of eachmaterial were stored in either distilled water, Coca-Cola, denture cleanser, saliva, or air. Thesurface roughness was measured at baseline and after 1, 7, 14, and 28 days. Surface roughnessvalues were analyzed with repeated measures analysis of variance, and the Bonferroni multiplecomparison test was performed using time-dependent groups and storage methods.

Results. In the time-dependent groups, methacrylate-based sealer-coated soft liners exhibited asignificant increase in roughness (1.74-2.09 mm, P<.001), and silicone-based sealer-coated softliners exhibited a decrease in roughness, but it was not significant (2.16-2.02 mm, P>.05).Therefore, the sealer coating was not effective in reducing surface roughness. Among the time-dependent storage methods, the denture cleanser exhibited an almost significant increase inroughness (1.83-1.99 mm, P=.054). Coca-Cola and artificial saliva did not show a significantdifference (P>.05). However, a significant decrease in roughness was found with distilled water(P=.02) and air (P<.001).

Conclusions. Statistically significant differences in surface roughness were found among thedifferent types of soft liners. The sealer coating had no significant effect, and denture cleanserslightly increased the surface roughness. Contrary to expectations, the roughness did not increasein all groups over time. (J Prosthet Dent 2016;115:371-376)

The use of soft liners for reliningremovable dentures is usuallyadvantageous1 for patients withan atrophic ridge, bony un-dercuts, bruxism, denturesopposing natural teeth, thin andnonresilient mucosa,2 xero-stomia, sensitivity in the mentalforamen region, congenital oracquired defects, or implantplacement surgery.3 Liningmaterials allow the mucosa toheal by distributing functionalloads across the denture-bearing tissues1,4,5 and arepreferred by patients to con-ventional hard denture bases.2,3

Although denture trauma canbe reduced by soft liners, theporosity of the lining materialmay increase colonization bymicroorganisms and cause tis-sue inflammation.6,7

Lining materials can fail forvarious reasons, including lossof smoothness, water sorption,

discoloration, bacterial growth, or loss of adhesion be-tween the liner and the denture base resin.8,9 Colonization

the 38th Annual Conference of the European Prosthodontic Association aAssociation, _Istanbul, Turkey, September 2014.sistant, Department of Prosthodontics, Faculty of Dentistry, Atatürk Univeepartment of Prosthodontics, Faculty of Dentistry, Atatürk University, Erzusistant, Department of Prosthodontics, Faculty of Dentistry, Atatürk Univeepartment of Prosthodontics, Faculty of Dentistry, Recep Tayyip Erdo�gansistant, Department of Prosthodontics, Faculty of Dentistry, Atatürk Unive

L OF PROSTHETIC DENTISTRY

by Candida albicans is one of the most important issueswith soft lining materials.6,10,11 Microbes adhere more

nd 21st Scientific Congress of the Turkish Prosthodondic and

rsity, Erzurum, Turkey.rum, Turkey.rsity, Erzurum, Turkey.University, Rize, Turkey.rsity, Erzurum, Turkey.

371

Table 1.Materials used

Brand Name Type Manufacturer Batch No.

Ufi Gel P Autopolymerized, siliconebased, definitive

Voco GmbH 1234159-60

GC tissueconditioner

Autopolymerized,methacrylate,interim

GC Corp 1209111

Molloplast B Heat polymerized, siliconebased, definitive

Detax Dental 160108

Ufi Gel Glaze Two components Voco GmbH Base: 1226511Catalyst: 1226512

GC coatingagent

One component GC Corp 1209102

Corega Alkaline peroxides Stafford Miller 5T13093

Clinical ImplicationsDenture cleansers increased the roughness of softliners, and the application of sealer coating did notseem to reduce the roughness.

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easily to soft lining materials than to the acrylic resindenture base because of the surface texture and physicaland chemical affinity.12 Denture stomatitis is causedmainly by microbes and C albicans, which has frequentlybeen isolated from the fitting surface of dentures.13,14

Denture plaque control is important to prevent coloniza-tion and infection by these organisms.13

Dentures should be cleaned with immersion cleansersand brushing.2,15 Chemical soaking is the first choice forelderly patients with poor motor capacity.16 The use ofdenture cleansers is efficient in removing stains and de-posits. Even though denture cleansers should prevent Calbicans colonization and provide denture plaque con-trol,2,17 in daily use, denture plaque bases and soft den-ture lining materials can be adversely affected by thesesolutions.18-21 Denture cleansers can also cause the lossof soluble components and plasticizers or the absorptionof water or saliva by the resilient lining materials.16

Factors that affect the adherence of microorganisms tothe substrate surface include material type, surfaceroughness, and diet ingredients.13 Fungus adheres moreoften to rough surfaces than to smooth surfaces.12,22,23

Three to four days after soft liners are applied, surfaceroughness may increase.24 The rough surface of liningmaterials can easily be colonized by C albicans and otheroral fungi,4 which causes deterioration of soft liners andirritates denture-bearing areas. Surface roughness alsoaffects the viscoelastic properties and the dimensionalstability of soft lining materials.24

Sealers protect soft liners against water sorption anddamage by chemical agents by covering surface defectsand reducing porosity and fissures.25 They also increasethe durability, resilience, and longevity of tissue condi-tioners.3,26 In addition, the smooth surface obtained witha sealer prevent plaque adhesion and bacterial growth.13

However, they seem to be affected by time.27 The use ofdenture cleansers may cause the properties of sealed softliners to deteriorate, but cleansers should still be used toreduce denture plaque formation.24

The hypotheses of this study were that denturecleansers and carbonated drinks such as Coca-Colawould affect the surface roughness of the soft linersand that the surface roughness of the liners would bereduced by the sealer coating. In addition, althoughsurface roughness values increase over time,methacrylate-based liners would increase more thansilicone-based liners.

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MATERIAL AND METHODS

Table 1 lists the 3 soft lining materials, 1 denture cleanser,and 2 sealers tested. One hundred and twenty-fivespecimens were prepared. Fifty specimens were madeof an methacrylate-based room-temperature-polymer-ized soft lining material (GC Tissue Conditioner; GCCorp), 50 specimens were made of a silicone-basedroom-temperature-polymerized soft lining material (UfiGel P; Voco GmbH), and 25 specimens were made of asilicone-based high-temperature-polymerized soft liningmaterial (Molloplast B; Detax Dental).

Each room-temperature soft liner was processedfollowing the manufacturer’s instructions and pouredinto a stainless steel mold to create 16-mm-diameter and2-mm-thick disks (Fig. 1). A glass slab (mean roughnessvalue: 0.009 mm) was placed on top of the mold, anotherwas placed underneath the mold, and both were kept inplace for 90 minutes under a 20 N vertical force. Afterpolymerization, the surfaces of 25 of the room-temperature-polymerized soft lining material specimenswere coated with 2 layers of sealer according to themanufacturer’s directions and allowed to dry without theuse of an air spray.

The heat-polymerized soft lining specimens wereprepared with 2-mm-thick and 16-mm-diameter spacersin a denture flask. The spacers were made of hard butflexible silicone rubber (Zetaflow; Zhermack SpA) with thestainless steel mold and glass slabs. After the spacers wereremoved from the denture flask, the specimens wereprocessed according to the manufacturer’s directions.

Specimens of each material were immersed indistilled water, denture cleanser solution (Corega, 8hours/day), Coca-Cola (1 hour/day), or artificial saliva(consisting of 0.220 g/L calcium chloride, 1.07 g/L sodiumphosphate, 1.68 g/L sodium bicarbonate, and 2 g/L so-dium azide [0.2% NaN3]) at 37�C16, or were stored in air.Artificial saliva and distilled water were changed daily.Fresh solutions of denture cleanser were prepared ac-cording to the manufacturer’s recommendations, andfresh Coca-Cola was used for every application.

Usta Kutlu et al

Figure 1. Preparation of soft liner disks. Figure 2. Boxing with wax.

Figure 3. Dental stone specimens.

Table 2. ANOVA table

SourceSum ofSquares df

MeanSquare F P

Group 43.138 4 10.784 101.370 <.001

Storage 3.048 4 0.762 7.162 <.001

Group×storage 3.565 16 0.223 2.094 .014

Time, sphericityassumed

5.013 4 1.253 9.456 <.001

Time×group, sphericityassumed

19.235 16 1.202 9.070 <.001

Time×storage, sphericityassumed

5.322 16 0.333 2.510 .001

Time×group×storage,sphericity assumed

11.742 64 0.183 1.384 .034

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The surface roughness of each material wasmeasured after 0 (time [T]0), 1 (T1), 7 (T7), 14 (T14), and28 (T28) days of immersion. Each tissue conditionerspecimen was boxed using wax (Fig. 2), and then mixeddental stone (Moldano; Heraeus Kulzer GmbH) waspoured over the surface of the tissue conditioner28

under gentle vibration and removed 60 minutes aftermixing for indirect measurement (Fig. 3). Dental stonespecimens were stored for 1 day at 23 ±2�C and 70%humidity before measurement.

The surface roughness (Ra) was measured with aprofilometer (Surtronic 25; Taylor Hobson Ltd) calibratedat a cut-off length of 0.8 mm, 2.4 mm percussion ofmeasure, and drive speed of 0.5 mm/s. The mean Ravalues of each group specimen was obtained from 3measurements made by 1 observer (I.U.K.).1 Repeatedmeasures ANOVA was performed to analyze the surfaceroughness of each time-dependent group and condition.Statistically significant subgroups were compared withthe Bonferroni multiple comparison test (a=.05).

RESULTS

Statistically significant differences (P<.001) were foundamong the groups at the baseline (at day zero [T0]). Ac-cording to the repeated measure, there were significant

Usta Kutlu et al

differences in surface roughness among different times,groups, conditions, and interactions (Table 2).

In a comparison between mean values from T0 withthose from T28 for all groups, the Ufis (coated Ufi gel P)and Ufi (uncoated Ufi gel P) groups were not statisticallysignificantly different (P>.05), and no increase was notedin the roughness of either group (Table 3). Statisticallysignificant differences in roughness were found betweenthe Gcs group (coated GC tissue conditioner) and the Gcgroup (uncoated GC tissue conditioner) (P=.02): theroughness of the Gcs group increased over time morethan that of the Gc group. Thus, the sealer was noteffective in reducing the surface roughness of either ofthe soft liners. Although the surface roughness decreasedfor the silicone-based soft liners (Ufi, Ufis, Mol) in theperiod of T28, the decrease in the Ufis group was notstatistically significant (Ufis: P >.05, T0=2.16 mm, T28=2.02mm; Ufi: P=.003, T0=1.58 mm, T28=1.31 mm). The heat-polymerized silicone-based Mol (Molloplast) group hadthe largest decrease (P<.001, T0=2.18 mm, T28=1.45 mm)(Table 3). The roughness of the methacrylate-based softliners increased more than that of the silicone-basedliners (P=.003). This increase was observed at T14 forboth the Gc group (T0=1.58 mm, T14=1.79 mm) and theGcs group (T0=1.74 mm, T14=2.27 mm) (Fig. 4).

Considering each of the storage methods dependingon the change over time, the difference in the denture

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Table 3.Mean value differences of T0 − T28 and standard deviations forall groups

Difference in T0 − T28 (N=5)

Group Storage Mean ±SD n

Ufis Coca-Cola 0.10 0.653 5

Corega -0.70 0.819 5

Water 0.47 0.489 5

Air 0.69 0.557 5

Saliva 0.11 0.762 5

Ufi Coca-Cola 0.19 0.284 5

Corega 0.17 0.333 5

Water 0.25 0.456 5

Air 0.48 0.087 5

Saliva 0.21 0.325 5

Gcs Coca-Cola -0.85 0.792 5

Corega -0.64 0.412 5

Water -0.64 0.425 5

Air 0.42 0.463 5

Saliva -0.01 0.373 5

Gc Coca-Cola -0.05 0.271 5

Corega -0.22 0.490 5

Water 0.15 0.063 5

Air 0.11 0.206 5

Saliva 0.12 0.631 5

Mol Coca-Cola 0.59 1.248 5

Corega 0.57 1.053 5

Water 1.13 0.392 5

Air 0.70 0.306 5

Saliva 0.67 0.201 5

Total Coca-Cola -0.00 0.834 25

Corega -0.16 0.793 25

Water 0.27 0.687 25

Air 0.48 0.400 25

Saliva 0.22 0.518 25

Gc, uncoated GC tissue conditioner; Gcs, sealer coated GC tissue conditioner; Mol, Molloplast;Ufis, coated Ufi Gel P; Ufi, uncoated Ufi Gel P.

t0 t1 t7 t14 t28

Ufis

Ufi

Gcs

Gc

Mol

2.5

Time-Dependent Ra Values for All Materials

2

1.5

1

0.5

0

2.16 2.29 1.88 2.06 2.02

1.58 1.41 1.27 1.26 1.31

1.74 1.66

1.13

1.91 2.27 2.09

1.58 1.66 1.79 1.55

2.18 1.86 1.77 2.04 1.45

Time

(day)

Me

an

Ra

(µ

m)

Figure 4. Mean roughness values by time for all materials. Ra, surfaceroughness.

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cleanser was marginally significant (P=.054), and thesurface roughness increased over time (1.83 mm to 1.99mm). This increase was observed in all the groups exceptfor the Ufi and Mol groups. In artificial saliva, theroughness decreased from T0 to T28 for all the groups(1.875 mm to 1.654 mm). For Coca-Cola, only themethacrylate-based Gc and Gcs groups increased inroughness from T0 to T28 (Gc: T0=1.55 mm, T28=1.79mm; Gcs: T0=1.68 mm, T28=1.72 mm). However, theartificial saliva and Coca-Cola values were not statisti-cally significant (P>.05). The surface roughness ofspecimens immersed in distilled water or air decreasedsignificantly (distilled water: P=.02; air: P<.001)(Table 3, Fig. 5).

In a comparison of the time periods, the roughnessdid not increase over time. Decreases were found insome of the groups. Generally, there was an increasefrom T7 to T14 (1.70 mm to 1.88 mm) and a decrease fromT14 to T28 (1.88 mm to 1.68 mm) in all groups.

THE JOURNAL OF PROSTHETIC DENTISTRY

DISCUSSION

Soft liners increase the adaptation of dentures, reducestress by distributing it over the supporting tissues, andimprove patient comfort. However, the length of timesoft liners are in contact with oral tissues is importantsince the surface roughness causes biofilm formation andbacterial accumulation, resulting in deterioration.27,7

Although soft lining materials have more surface ir-regularities and are rougher than denture resins andtherefore allow more bacterial accumulation,25,20 defini-tive soft liners can be used for several months or years. Inaddition, interim lining materials are inexpensive andeasily available. To decrease the deterioration caused byfood, saliva, denture cleansers, and brushing, a sealercoating can be applied. Silicone lining materials arechemically and physically more stable than acrylic resinlining materials. Methacrylate-based liners require asealer coating because they tend to absorb water,dissolve, and lose their plastizer.27,26 However, inmethacrylate-based lining materials, a sealer coating isnot as effective as in silicone-based liners.25 In the pre-sent study, although a decrease was observed in the Ufisgroup (at T7 and T14), an increase was observed in theGcs group.

Similar to this study, Jin et al18 found that thesurface of silicone-based soft liners immersed for 180days in different denture cleansers was less rough thanthat of acrylic-based soft liners. Likewise, Dayrellet al27 suggested that methacrylate-based soft liners arerougher than silicone-based soft liners because of thechemical structure of methacrylate-based liners, resid-ual monomer content, polymerization method, volatilityof the monomers, air bubbles, and the blending tech-nique used.

Usta Kutlu et al

2.5

Time-Dependent Ra Values for All Storage Methods

2

1.5

1

0.5

0t0 t1 t7 t14 t28

1.78Coca-Cola

Cleanser

Water

Air

Saliva

1.75 1.7 1.91 1.78

1.83 1.77 1.68 2.04 1.99

1.9 1.58

1.69

1.74 1.97 1.63

1.86 1.64 1.7 1.37

1.87 1.58 1.74 1.79 1.65

Time

(day)

Me

an

Ra

(µ

m)

Figure 5. Mean roughness values by time for all storage methods. Ra,surface roughness.

March 2016 375

The roughness of methacrylate-based soft liners wascompared by Hong et al.24 Different surface roughnessvalues were associated with the average particle size ofthe polymer, the molecular weight of the plasticizer, thepowder/liquid ratio of soft liners, and the alcoholcontent.28

While the specimens are being prepared, monomervolatility and removal speed of the liner from the glassslab cause changes in the adhesion force and affectroughness.19 Therefore, the specimens were prepared byone researcher, under the same experimental conditions.

In the present study, the mean surface roughnessvalues ranged from 1.10 mm to 2.52 mm and were similarto those of other studies.11,13,14,17 Although the speci-mens in this study were prepared between 2 glass slabs,the surface roughness values at the baseline weredifferent and were more than 0.2 mm, which is thethreshold value for microorganism retention.2,4,22,27 Atthe baseline, the sealer-coated soft liners had highersurface roughness, which was thought to have beencaused by the thickness of the applicator bristles, which isdifferent for each manufacturer’s kit.

Vural et al12 evaluated the surface roughness ofautopolymerizing and heat-polymerizing soft liningmaterials after specimens were submerged in artificialsaliva and stated that the polymerization method affectedroughness. Similar to the present study, the room-temperature-polymerized soft liner was rougher thanthe heat-polymerized Molloplast B. This is because heat-polymerization increases the cross-linking of residualmonomers, thereby making the liner harder and morewear-resistant.13

The hypothesis that sealer coating would reduceroughness was rejected; this agrees with the studies ofSesma et al23 and Dayrell et al,27 which reported crack

Usta Kutlu et al

formation and detachment of sealer-coated specimensover time. Coca-Cola was one of the storage mediatested. The results were not as expected, and no statis-tical differences were found in any of the storage periods.We also tested artificial saliva, and the results remainedstable because of the content of the artificial saliva. Whendentures with soft liners are not in the mouth, theyshould be stored in water. In older patients with poor oralhygiene, chemical cleansers are recommended for plaquecontrol in soft lining materials because ultrasoniccleansers are not effective and brushing can causedamage.14,15

Alkaline peroxides, alkaline hypochlorite, acids, dis-infectants, and enzymes are denture cleansers.15 In thepresent study, Corega-containing alkaline peroxide wasused. Gedik et al13 stated that Corega-containing alkalineperoxide is the second most effective denture cleanserafter NaOCl. However, the distortion caused by denturecleansers of the denture resins and soft liners is asimportant as the efficiency of a denture cleanser. Asobserved by Nikawa et al,21 when denture cleansers areused every day, they can affect the physical properties ofsoft liners.

Harrison et al29 indicated that oxidants inside stronglyalkaline solutions change roughness according to theperoxide content or pH of the cleanser. A neutralperoxide with enzymes and alkaline peroxide cleansersare more successful in avoiding bacterial colonizationaccording to Nikawa et al.30 Murata et al19 indicated thatenzyme-type cleansers may be preferable in terms ofsurface conditions. However, another study showed thatperoxide is not the only factor that causes disruption:enzyme denture cleansers can also cause deterioration.18

In this study, Corega increased the surface roughnessof soft lining materials (1.83-1.99 mm). This increase mayhave occurred because of the loss of alcohol and plasti-cizer and because of water absorption. Thus, because ofthe minimal loss of alcohol and plasticizer, the roughnessvalues did not increase in air.24 Similar to the presentstudy, the study by Kazaki and Watkinson16 found thatthe roughness of soft liners kept in denture cleanser washigher than that of liners kept in water because of theionic concentration of the cleansers. Pisani et al20

attributed increased roughness to cavities and bubblesthat occurred because of the loss of the liner’s solublecomponents. However, similar to this study, the lack ofincreased roughness in water was associated with aminimal loss of soluble components and a lack ofbubbles.

Hong et al24 measured the roughness of soft liners at0, 1, 3, 7, and 14 days and observed increases in allgroups over time except for liners stored in air, andDayrell et al27 observed a decrease in roughness in allgroups at the first month. In the present study, adecrease was observed in some of the groups over time.

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The studies’ results varied because of the different liningmaterials, time periods, and solutions used.

Murata et al28,19 used 3 types of tissue conditionerand 2 types of dental stones (Type III and Type IV) intheir study and found that the less important factoraffecting roughness is the type of dental stone. Thus, aType III dental stone was used in the present study. Nottesting other stones is a limitation of this study. Otherlimitations include the sole use of the profilometer toevaluate the surface roughness of the materials. Using ascanning electron microscope before and after storageunder different conditions might have given more in-formation regarding the nature of the surface change inthe specimens.

Factors that can affect the surface conditions of softliners include masticatory force, thermocycling,19 tissueirregularities, the presence of saliva during polymeriza-tion, and microbial factors.2 These factors could beinvestigated in future studies.

CONCLUSIONS

Based on the findings of this in vitro study, the followingconclusions were made:

1. Methacrylate-based soft liners had rougher sur-faces than silicone-based liners. Denture cleanserincreased the surface roughness, especially withmethacrylate-based liners. Surprisingly, sealercoating had no significant effect on surfaceroughness.

2. All roughness values did not increase over time. Insome groups, the surface roughness decreased. Theincrease was observed especially in the T7-to-T14period, and there was a decrease in the T14-to-T28period in every group.

3. Polymerization type affected surface roughness,with heat-polymerized liners having lower surfaceroughness

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Corresponding author:Dr Nuran Dinckal Yaniko�gluDepartment of Prosthodontics, Faculty of DentistryAtatürk University, ErzurumTURKEYEmail: nyanikoglu@gmail.com

Copyright © 2016 by the Editorial Council for The Journal of Prosthetic Dentistry.

Usta Kutlu et al