Prosthodontics

Clinical application of a soft denture liner: A case reportRussell T. Williamson*

Palienls with thin, nonresilieni mucosa, poor ridge morphology, chronic xerostomia, andacquired or congenital defecis are candidates for soft denliire liners. Soft denture linersmay be placed during fabrication of the neiv denture or during relining of the existingdentures. Soft mandibular denture liners are ideal for senior patients with resorbedmandibular ridges who need replacement ofexi.'iling dentures ro correct excessively closedvertical dimension ofocchision and for patients who have maxillaiy complete dentures andmandibuiar natural teeth, severe loss of maxillary ridge, and clenching habits. Softdenture liners should be used only when needed because of their short service life. However, forthose patients who cannot tolerate hard denture bases, soft liners are an appropriatealternative treatment- (Quintessence im 1995;26:4I3-4¡S.)

Introduction

Patients with severe alveolar ridge résorption, senileatrophy of the mucosa, a history of alveolar surgery,deep anatomic undercuts, bony protuberances,xerostomia, cotnplete dentures opposing natural den-tition, and acquired or congenital defects may requiresoft-lined denture bases. These soft liners distributestress more evenly than do hard denture bases on thesupporting structures under the denture base.

Kawano et al' found that when soft denture linersare not used, the area of greatest stress concentration isthe mylohyoid ridge because the mandibular denturerotates in that direction. The denture studied wassupported on the lingual slope of the residual ridge.When soft denture liners are added to the denture base,the stress distribution in the supporting ridge differsaccording to the design of the soft denture liner.However, the displacement of the denture increaseswhen soft denture liners are used. The direction andamount of movement are related to the design of the

* Assiscanl Professor, Department of Health Practice, University ofKentucky, Coltege of Dentistry. Chandler Medical Center. Lexington,Kentucky.

Reprint requesti: Dr R. T, Williamson, Assislant Professor, Depaiimenlof Heallh Practice, Room D-64K, University of Kentucky, College ofDentistry, Chandier Medical Center, Lexington. Kenlucky 40536-0084.

liner. The four designs Kawano et al ' used were (¡) nosoft denture liner, (2) a soft denture liner covering thesoft mucosa and extending to the periphery of thedenture, (3) a soft denture liner covering the attachedmucosa but not extending to the periphery of thedenture. (4) a soft denture liner between the denturebase and the artificial teeth. The third design results inthe most uniform stress distribution on the softtissue.

During fabrication of the new prosthesis with a softdenture liner, a spacer is needed during the trialpacking of the flasks to create space for the soft liner.Materials that may be used as the spacer includesilicone putty, baseplate wax, spacer paper, tinfoil, or athermoplastic vacuum-formed sheet. Huband" des-cribed a technique of fabricating a spacer from visiblelight-curing resin. The advantages of a light-curingmaterial are that the thickness of the material can bemeasured easily with a periodontal probe prior to lightcuring, the imprint of a denture tooth shows on thelight-curing material during flask closure if there isinadequate space, and the cured spacer can be relievedwith a bur where the denture base is too thin duringtrial packing.

Kutay^ reported use of a silicone rubber spacer todetermine the optimal thickness of both the soft linerand denture base. A minimutn of 2 mm of liner and3 mm of acrylic base is recommended by the author.

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Prosthodontics

An acrylic resin baseplate is adapted over a two-thickness wax spacer. The baseplate has four fingerwidths of acryhc resin extending to the top of the landarea supporting the acrylic resin base after the waxspacer is removed. The correct-thickness acrylic resinbase is then seated over silicone putty, adapted to themaster cast, and trimmed. Conventional waxup andtry-in are then completed on the trial base with thesilicone spacer.

Dootz et al'' compared the physical properties of 11soft denture-lining materials after accelerated aging.They reported that three distinctly different types ofsoft liners are used. The largest group of materialstested was plasticizer polymers or copolymers: twomaterials were silicones, and one was a polyphos-phazene fluoroelastomer. They concluded that nodenture liner proved superior to all others and thataccelerated aging dramatically affects the physicalproperties of many of the elastomers.•*

The changing of the physical and mechanicalproperties also affects the color stability of the softliners. Shotwell et aP reported the effect of acceleratedaging of five different soft liners. The critical remarksof color difference range from "slight to very much."The polyphosphazene fluoroelastomer. for example,was rated to exhibit "veiy much" of a color change.Granata and Staffanou* evaluated a new denture hathsolution and its effects on 10 different liners treatedwith oxidizing solutions and inoculated with Candidaalbicans. The authors concluded that the new denturebath material (Oral Safe. Great Lakes Orthodontics),which controls Candida albican.i and bacteria, seemsto preserve the integrity of soft liners.^

As the physicai and chemical properties of the linerchange with age. the adhesive strength is also affected.Sinobad et al' tested bond strengths of three soft linersafter submersion in 31°C water for 7 and 90 days.After saturation in water, acrylic resins exhibit anincrease in tear resistance, and silicone materialsdeteriorate. The results indicated that denture softliners have variable water sorption rates.

Polyzois** reported the shear adhesive strength ofthree soft liners. His findings showed that all of thelining materials tested were acceptable for clinical use,but that a 4-month water storage reduced their bondstrength to visible light-curing resin. The polyphos-phazine fluoroelastomer lining material showed thehighest values for bonding strengths and had mainlycohesive failures.

Clinical useñilness and long-term success are thebest measures of soft liner materials. Ryan ' reported

on 25 years of clinical application of a heat-curingsilicone rubber soft lining material (Molloplast-B,Regneri). The author summarized that heat-curingsilicone soft liner is an important adjunct to removableprosthodontic treatment modalities. The dentist mustfollow good prosthodontic principles, provide closetechnical supervision, and promote good oral hygienein the patient. Silicone soft liner material will promotethe patient's satisfaction with the prosthesis and willprovide serviceability over the normal lifetime of theprosthesis.

Case report

A woman in her mid-60s, who was in good generalhealth and had no contraindications to conventionaldental therapy, requested a new set of dentures. Herchief complaints were: "The lower dentures are looseand I must use a liner in the center. My jaws come tooclose together, it is hard to chew my food, and my chinis too close to my nose." The patient's appearancerevealed classic signs of closed vertical dimension ofocclusion. The chin appeared close to the nose, thecommissures of the lips were turned down, angularcheilitis was apparent, and the lips had no fullness. Themuscles of the face appeared flabby instead of firmand full (Fig 1).

The patient presented with 30-year-old porcelain-tooth maxillary and mandibular complete denturesthat were poorly fitting. The dentures were originallyfabricated so that the mandibular incisors occludedwith the lingual aspect of the maxillary incisors, but,over the years, the résorption of the alveolar ridges hadclosed the vertical dimension of occlusion and themandibular occlusion had moved into an Angle ClassIII relationship in which the mandibular incisors werepositioned labial to the maxillary incisors. The mandi-ble was not in centric relation during maximalintercuspation (Fig 2).

The patient was using a self-applied soft liner in theareas of the resorbed ridge (Fig 3). The soft linersolved the knife-edged ridge soreness by more evenlydistributing the occlusal load on the ridge. However, asthe mandibular alveolar bone continued to resorb, theexposed mental nerve became more and more painfulto increased denture base loading. This was confirmedclinically when finger pressure was placed on the sharpridge and exposed nerve and a painful response waselicited. The same pressure in unaffected areas did notproduce a painful response. Compounding the pro-blem was the thinning of the mucosa (as observed insenile atrophie mucosa).

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Fig 1 The palient's appearance reveals classic signs ofclosed vertical dimension ol occlusion.The chin appearscicse to the nose, the commissures of the lips are turneddown, angular ctieilitis is apparent, and the lips have nofullness.

Fig 2 Over the years, the résorption of the alveolar ridgeshas closed the vertical dimension of occlusion and themandibular occlusion has moved into an Angle Class IIIrelationship in which the mandibular incisors are positionedlabially to the maxillary incisors.

Xerostomia further complicates denture treatment.The lack of saliva creates a situation in whichtissue-base friction impedes the displaced denture basefrom easily reseating. This creates pressure on tender,sharp ridges and exposed nerves during functionalloading of the displaced denture. The absence ofnonnal salivary antibacterial enzymes also increasessusceptibility to oral ulcers. Dry mouth can result fromchronic use of drugs (anticholinergics, antidepres-sants, antihistaminés, benzodiazepines. adrenergics.and diuretics), radiation treatment, or diseases{Sjogren's syndrome, acquired immunodeficiency syn-drome, rheumatoid arthritis, or lupus). ClinicaUy, drylips, buccal mucosa, and lack of saliva when glands arepalpated are useful signs. Referral ofthe patient to aspecialist in oral medicine may be appropriate.

The patient agreed that the treatment of choice was anew conventional denture, fabricated at the correctvertical dimension of occlusion, with a polyphos-phazene fluoroelastomer soft liner placed in themandibular denture. The new denture was fabricatedconventionally with a vacuum-formed mandibularocclusion base technique to the compression flaskingstage (Fig 4), During boil-out, ihe vacuum-formedsheet was saved and used for the polyphosphazenefluoroelastomer soft liner spacer (Fig 5). The spacershould be 1,0 to 1,5 mm at the roll, tapering to 2,5 to3,0 mm over the crest ofthe ridge. There should be atleast 1,0 mm of hard denture base resin for strength.With the spacer in place on the cast, denture resin waspacked in the ñask. Trial packing was accomplished asneeded, all excess resin was removed, and then theflask was closed and pressed with the spacer and the

Fig 3 The patient has been using a sell-applied sofi liner inihe resorbed ridge bearing areas around the periphery oiihe hard denture base.

plastic separator sheet still in place. The flask wasplaced into a 165° F (74° C) water bath (30 to 45minutes), cooled, and opened (Fig 6). A strip ofthepolyphosphazene fluoroelastomer soft liner (3,0 to5.0 mm wide) was cut and placed on the crest oftheridge of the stone cast. With a sheet of polyethylenebetween the material and the denture, the flask wasclosed and placed in the press. Pressure was slowlyapplied (1,000 to 1,500 psi for 1 to 2 minutes); theflask was then opened and excess was trimmed. Theroughened denture surface and liner were painted withthe bonding liquid, the flask was clamped, and thedenture was slowly cured at 165'' F (74° C) for 8hours.

After curing, conventional divesting, finishing, and

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Prosthodontics

Fig 4 Ttie new denture has been fabricated conventionallyby uing a vacuum-lormed mandibular occiusion basetechnique to the compression flasking stage.

Fig 5 During boil oui, the vacuum-formed sfieet is savedand used for the polyphosphazene fiuoroelastomer softliner spacer.

Fig 6 With the spacer in place on the cast, denture resin ispaci<ed in the flask and processed.

Fig 7 The final surface will not exhibit a high shine.

polishing were accomplished. Final finishing andadjusting were done with a sharp carbide bur, and asmooth surface was achieved by polishing with a ragwheel and slurry of pumice. The final surface did nothave a high shine (Fig 7). The soft liner denture wasplaced in ice water to cool the material and provide amore solid and easier-to-adjust surface (Fig 8 ). (This isa helpful technique for chairside adjustments. Thecooling of the material prevents the pulling of thematerial when excessively heated by the adjusting in-strument. )

The final prosthesis was adequately adjusted to thepatient's satisfaction within three appointments andwas considered by the patient to be a complete success.

The occlusion was designed to result in end-to-endanterior occlusion and a balanced articulation at thecorrect vertical dimension of occlusion (Fig 9). Thechin was farther from the nose, the commissures of thelips turned up, the muscles of facial expression hadmore tonicity, and the face was firmer and Riller (Fig10). The patient was made aware of the need forreplacement of the soft liner every 1 to 2 years, as wellas the time and the expense involved, before thetreatment began.

Discussion

The combination of loss of resiliency of the mucosa,

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Fig 8 The reiined denture is placed in ice water lo cool thematerial and to provide a more solid and easier-to-adjustsurlace

Fig 9 The color change associated wilh aging of the sottiiner is accelerated by the patient's use of bleaching agentsto clean the prosthesis. The correct hard and soft linerdesign, liner thickness, bonding technique, and home carewill prevent the tearing of the material that occured in thisliner.

Fig 10 The occlusion has been designed to result inend-to-end anterior occlusion and a balanced articulationat the correct vertical dimension of occlusion.

Fig 11 The new denture has improved the tacial appear-ance. Tîie chin is farther from the nose, the commissures ofthe lips turn up, the muscles ot tacial expression have moretonicity, and the face is firmer and fuller.

exposed mental nerve, and sharp, knife-edged, irregu-lar resorbed ridge creates a very poor prognosis forpatients who need their dentures remade. Usuallythese patients have very little attached gingiva. The softliner design in which only the attached gingiva iscovered would be less desirable than that in which allthe mueosa is covered, because of the exposed mentalnerve and the prominent mylohyoid ridge.

Soft mandibular denture liners are ideal for patientswith resorbed mandibular ridges who need replace-ment of existing dentures. Soft denture iiners shouldbe used only for those patients in need, because softliners have a short life, and additional expense andtime are required for replacement. However, for those

patients who cannot tolerate hard denture bases, softliners are an appropriate alternative treatment to harddenture bases.

The color change associated with aging of soft linersis accelerated by patients' use of bleaching agents forcleaning the prosthesis (Fig 11). Patients are cau-tioned to use only mild soaps as cleaners. One patientwho had a son liner replaced reported that the lifespanof the polyphosphazene fluoroelastomer soft liner wasincreased by cleaning under cold water only. Thepatienl thought that the material, which has a thermo-plastic quality, was stronger and less affected by thenormal abrasion of the cleaning procedure. Use of thecorrect hard-soft liner design, liner thickness, and

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Prosthodontics

bonding technique, as well as appropriate home care,will prevent tearing of the material (Fig 3). All ofthese probably contributed to the liner failure in thisinstance.

Acknowledgment

The author would like to thank William P. Cassady for tiis assistance infabricating the prosthesis and Dr Ldrry C. Breeding for tiis editorialassistance.

References1. Kawano F, Koran A, Asaoka K, Matsiimoto N. Effect of sofl denture

liner on stress distribution in supporting structures under a denture.Int J Prosthoüont l993;6;43-49.

2. Huband ML. Spacer made from visible light-cured resin for proces-sing denture soft liners. J Prosthet Dent 1992:68:542-544.

3. Kutay O. A silicone rubber spacer used to delermine the oplimumthickness for hard resilient materials in comptete dentures. J ProsthetDent 1993:69:329-332.

4. Dootz ER, Koran A, Craig RG. Physical property comparison of i Identure lining materials as a flinctioti of acceterated aging, J ProsthetDent t993:69:lt4-lt9,

5. Stiotweli JL, Razioog MK, Koran A. Color stability of long-term softdenture liner5 J Prosthet Dem 1993:68=836-838.

6. Cranata JS, Staffanou RS. Evaluation of anew denture bath solution. JPruslbetDent 1991;66:790-791.

7. Sinobad D, Murphy WM, Huggctl G, Brooks S. Botid strength andrupture properties of some sofl tiners, J Oral Retiabil 1992;19:15i-160,

8. Polyzois GL. Adhesion properties of resilient litiing materials bondedto iight-cured denture resins. J Prosthet Dent I992;68:854.

9. Ryan JE. Twenty-five years of clinical application of a heat-curedsiiicunc rubber. J Prosthet Dent 1991:65:658-661, D

418

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