Denture soft lining materials: clinical indications Peter John Mack, MDS, FDS, RCS, DRD, RCS*

Key words: Clinical performance, dentures, soft linings.

Abstract The success or failure of a soft lining in a denture is dependent not only upon the physical properties of the material employed, but also on an under- standing of the intended function of this group of products, and the physiological and biological properties of the resilient oral tissues upon which the denture rests.

This paper identifies the clinical factors that must be reviewed in order to assess the need for and likely success of a denture soft lining material.

(Received for publication December 1987. Accepted April 1988.)

Introduction Masticatory loads in man are ideally transmitted to and resisted by sound teeth firmly attached via the periodontal ligament to the bone of the jaws. For those who by age, accident or neglect are reduced to an edentulous state, the masticatory loadings transferred by a completely artificial dentition are of necessity borne by the oral mucoperiosteum. The penalty for such a change in tissue function is not only a reduction in chewing efficiency, but also an increase in susceptibility for pathological changes to develop in these tissues.

In efforts to secure some measure of redistribu- tion and reduction of locally damaging loadings by dental prostheses to the underlying oral tissues, increasing use has been made in recent years of both tissue-conditioning and soft lining materials. Soft lining materials may be defined as soft polymers which may be applied to the fitting surface of a

*Senior Lecturer in Restorative Dentistry, The University of Western Australia.

denture for the purpose of reducing and more evenly distributing occlusal loadings on the under- lying mucosal tissues. In one form or another soft lining materials have been in use for many years.

Although at present there exists no internation- ally accepted standard by which this group of materials may be chosen, compared or evaluated, the clinical indications and contra- indications for their use are well established. This paper outlines the clinical considerations that might be reviewed by the clinician who considers a soft liner the treat- ment of choice for his or her prosthetic patient. A detailed appraisal of the materials that have been and are at present available for this purpose is the subject of a second paper.

Indications and applications of soft lining materials 1. Ageing and pathological changes

One major application of soft lining materials is to reduce problems arising from the effects of age changes in the denture bearing tissues of the mouth. The provision of a soft and resilient lining beneath a rigid denture base can improve both masticatory efficiency and oral comfort for patients presenting with a reduced thickness or lessened resilience of the oral mucoperiosteum. A physiological reduction in the thickness of the mucoperiosteal tissues appears in most people to be consequent upon age alone,'-5 but in some it would seem that an addi- tional factor is a constant heavy occlusal Others consider that a reduction in mucosal compli- ance appears to be related to a combination of factors which include local age changes plus more general modifications to systemic hormonal level^.^-^

2. Local relief of pressure The healed mucosa overlying the sockets of

extracted natural teeth may be compromised for many years by rough contours in the underlying

454 Australian Dental Journal 1989;34(5):454-8.

bone. Radiographs of edentulous alveolar ridges commonly show marked bony projections and spicules beneath an apparently smooth mucosal covering. This unevenness of contour is usually (but not invariably) a consequence of irregular bony resorption following traumatic or ill-planned extrac- tion of the natural teeth.

Although surgical techniques for the reduction of such problems are readily available and may even be advised in the more severe cases, a more conser- vative approach is often preferred by both operator and patient. This alternative commonly involves the use of a soft lining material beneath the overlying rigid denture ba~e.'O-'~

At various times soft liners have also been employed for the relief of obtrusive genial tubercle^,^^ mandibular tori,".'6 and sharp mylo- hyoid ridges.I7 Several authors have proposed that soft lining materials could be used to gain relief from direct local pressures resultant from a hard median palatal raphe,'o.12.18 or exposed mental foraminae.6.16

3. Occlusal impact reduction Soft lining materials may be used for the reduc-

tion of occlusal loads impacting upon the mucoperiosteal tissues. In this role the soft lining material is most commonly placed on the fitting surface of a prosthesis directly against the mucosal tissues.'8-21

An alternative technique is to use the soft lining material in a 'sandwich' construction. This system purports to mimic the resiliency of the periodontal membrane whilst maintaining the low frictional qualities of a conventional highly polished acrylic denture b a ~ e . ~ ' . ' ~

4. Undercut engagement The contours of the soft mucosal and hard bony

tissues of the mouth are a product of both natural forces and human intervention. They are not always ideally suited to bear the direct load of a prosthesis. Anatomical areas undercut to the general line of withdrawal of a rigid prosthesis may be utilized in many instances by a judicious choice of the line and rotation of denture insertion.

Many such undercut areas may be engaged for the retention of the denture by the use of a flexible material either within the denture or as part of its peripheral moulding. Clinicians have observed the advantage of resilient polymeric materials in the provision of complete and partial p r o s t h e ~ e s ~ ~ - ~ ~ and in the construction of obturators, both for maxil- lary clefts and subsequent to the effects of the more radical surgical p r ~ c e d u r e s . ~ ~ - ~ ~

5. Aids to denture retention Most soft lining materials show a high surface

coefficient of friction. This physical characteristic has often been cited as a disadvantage, but in a few instances the increase in retention due to the fric- tion between the prosthesis and the oral tissues can prove a welcome adjuvant to the retention of an otherwise excessively mobile prosthesis. Concern has been expressed lest the increased level of surface friction lead to mucosal trauma, but in clinical prac- tice this does not seem to be a p r ~ b l e m . ~ . " . ~ ~ . ~ ~

6. Additional applications Soft lining materials have been used for a multi-

tude of surgical tasks. These have included facial tissue r e ~ t o r a t i o n , ~ ~ - ~ ' mouthguards (both protec- tive and therapeutic), and soft overlay prostheses for tissues recovering from therapeutic radiation9.12.24.38 or soft tissue s ~ r g e r y . ~ . ~ ~

Disadvantages and contra-indications to soft lining materials 1.

The cost of any prosthesis that incorporates a soft lining material is greater than that of the same denture constructed of a single base material. The fabrication of most commercial varieties of 'perma- nent' soft lining materials often involve complex and time consuming laboratory procedures. Some soft lining materials may be processed at the chairside, but this group tend to be the least stable and show a shorter clinical life than those processed in the laboratory.

All soft lining materials are difficult to adjust or modify. They present problems in polishing, espe- cially of the acrylic detail at the junction between the rigid base and the soft polymer lining, and all varieties are complex to repair should fracture or other damage occur to either the hard base or the soft polymer lining.

Costing and problems in fabrication

2. Fracture of the supporting denture base The provision of a soft lining for a denture

inevitably reduces the bulk of acrylic on which the prosthesis is dependent for its overall rigidity and strength. Increased flexing of soft lining lined dentures has been reported by several w ~ r k e r s . ~ ~ . ~ '

The most characteristic consequence of this reduction in acrylic mass is early fracture of the denture; an event that is well recorded in the liter- a t ~ r e . ~ , ~ . ~ ~ - ~ ~ In any clinical situation in which the inter-alveolar distance is reduced, the thickness of a soft lining will also require to be reduced

Australian Dental Journal 1989;34:5. 455

to prevent the denture breaking during the stress of normal function. Because the thickness of the soft lining beneath the hard acrylic base is reduced in this situation the effective compliance of the soft lining material may well be minimal, but the fric- tional qualities of the lining will remain to enhance the retention of the prosthesis. If the inter-alveolar space is reduced below 5 mm, soft lining materials would not normally be recommended.

3. Stability in function Concern was expressed by some earlier research

workers that the addition of a soft lining to a prosthesis would result in a decrease in denture stability during function. Their reasoning was that a tissue borne prostheses was already resting on a mobile base and that the addition of a soft and resilient lining could only exacerbate the clinical problem. Fortunately this has not proved the case.20.45

Doubts were also raised lest an elastic rebound effect would occur during o c c l ~ s i o n . ~ ~ Again, in practice this has not been observed, possibly due to the high damping component of the underlying mucosa and the viscoelastic nature of most soft lining materials.

Finally the possibility was voiced that the cons- tant movement of a loaded soft lining material at the polymer/tissue interface might irritate the underlying tissue^.'^ It would appear that any move- ment that does occur falls well within the physiological tolerance of the mucoperiosteal tissues beneath the prosthesis.

It is of interest that in one experimental series47 mandibular dentures were constructed (excepting the teeth) entirely of a resilient material. The denture was thus able to flex during normal func- tion. The majority of patients who were provided with these resilient prostheses accepted them very well, but unfortunately no repeat series has been since undertaken to confirm this result which appar- ently cuts across present prosthodontic opinion.

4. Clinical problems in use Because of their softness, soft lining materials and

in particular the tissue conditioning group, do not react well to physical abuse nor to the more abra- sive varieties of denture c l e a n ~ e r . ~ ~ , ~ ~ - ~ ~ Early and unsightly staining of the soft lining material surface appears to be an unavoidable response to the inability of some materials to accept a high p ~ l i s h . ' ~ . ~ . ~ Other workers have noted an increased tendency for plaque and calculus formation on the surface of soft liners.'." Possibly in efforts to reduce

the effects of staining, surface accretions and colour changes some denture wearers have resorted to harsh chemical cleansers which bleach most soft lining material^.'^.^^.^^.^^

Most soft lining materials take up oral fluids. In the absence of an adequate regimen of denture hygiene these fluids stagnate and can cause the liner to become offensive both to the wearer and to

Fungal colonization of soft lining materials is well d o ~ u m e n t e d , ~ . ~ ~ . ~ ~ - ~ ~ whilst others have noted an increased tendency for plaque and calculus formation on the surface of soft liners.55 In the past anti-fungal agents have been included in some tissue conditioning materials to prevent such c o l o n i ~ a t i o n . ~ . ' ~ . ~ ~ ~ ~ ~

5. Dimensional stability The clinical comfort and longevity of function of

the ideal soft lining material is often assumed to be dependent upon its dimensional stability. Most soft lining materials are inherently dimensionally unstable: water absorption and plasticizer loss lead to a continuing variation and change during intra-

Nevertheless, many soft lining materials which show a small degree of dimensional instability are successful in clinical practice. The success of these prostheses is probably dependent upon compensa- tory plastic flow (creep) occurring within the soft lining material lining the denture. This 'balance of disadvantages' can probably be accepted only when the soft liner is incorporated within a dimension- ally stable and rigid base. Tissue conditioning materials must creep to perform their function and to permit the underlying mucosal tissues to regain their ideal form.

oral wear.5. 10. I I .48.51.56.57

6. Surface integrity The ideal soft lining material would show a

resistance to surface wear at least equal to that of the most commonly employed base material. The acrylic based soft lining materials, and to a slightly lesser extent the silicones, demonstrate a much reduced abrasion resistance when compared with a correctly cured conventional acrylic Surface vegetative eruptions have been reported to form from within some soft lining material^.'^.'^.^' These eruptions appear to be exacerbated by some proprietary denture cleaners, notably the p e r b o r a t e ~ . ~ ~ . ~ ~ . ~ ~ . ~ ~

The surface integrity of many 'permanent' soft lining materials appears to be less stable than might be expected. Microscopic surface fractures, or even macroscopic cracking has been reported by several

456 Australian Dental Journal 1989345

~ o r k e r s . ~ ~ ~ ~ - ~ ~ This has been considered by some to be related to the high surface frictional resistance exhibited by this group of material^.^.^^,^^.^' Conclusion

‘Permanent’ or ‘semi-permanent’ soft denture lining materials of various types are supplied to as many as 5 per cent of all new complete d e n t u r e ~ . * ~ ~ ~ ’ ~ ~ ~ This figure does not include dental prostheses fitted with a temporary tissue condi- tioning or soft lining material. These figures indicate that despite the considerable concern over the physical behaviour of this group of dental materials, their dimensional instability and their poor longevity, they hold an important place in the treatment of the edentulous or semi-edentulous dental patient.

It is hoped that the present resurgence of interest in the soft denture lining materials will lead to the early formulation of a material that is simple to form, stable in function and closely similar in visco- elastic properties to the load-bearing mucoperiosteal tissues of the mouth.

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Address for correspondenceheprints: Division of Restorative Dentistry,

Dental School, The University of Western Australia,

179 Wellington Street, Perth, Western Australia, 6000.

458 Australian Dental Journal 1989;34:5.