14.3 Tissue conditioners

Tissue conditioners are soft denture liners which may be applied to the fitting surface of a denture (see Fig. 14.1). They are used to provide a temporary cushion which prevents masticatory loads from being transferred to the underlying hard and soft tissues. These materials should undergo a degree of plastic flow for 24–36 hours after mixing to allow for soft tissue changes once ‘trauma’ has been removed and to capture the shape of the supporting tissues in function as opposed to a static or unloaded relationship.

Tissue conditioners have several applications. For example, when the soft tissues have become traumatized due to wearing an ill-fitting denture the dentist would like the tissue to recover before recording impressions for new dentures. Ideally, the patient would refrain from wearing his denture for a period, but this is not popular. In these circumstances, a layer of a cushioning tissue conditioner on the fitting surface of the denture will enable the soft tissue to recover without depriving the patient of their dignity.

Tissue conditioners are often applied to the dentures of patients who have undergone surgery. This reduces pain and helps prevent traumatization of the wound. They are also useful when a tooth or teeth are being added to a denture as an immediate procedure (very shortly after the extraction). The dental technician modifies the cast of the patient’s mouth by removing the teeth that will be extracted and by estimating the amount of change in soft tissue contour. If this estimate is wrong there can be a large gap between the denture base and the socket. A tissue conditioner

can be used to fill this gap to assist with stabilization of the prosthesis at the time of insertion of the immediate denture.

Another application of tissue conditioners is as functional impression materials. A layer of tissue conditioner in the fitting surface of the denture enables a functional impression to be obtained over a period of a few days.

Requirements: Tissue conditioners should remain soft during use in order to maintain an adequate cushioning effect on the underlying soft tissues. The material should be resilient in order that masticatory loads are absorbed without causing permanent deformation of the lining. Paradoxically, when the materials are being used to obtain a functional impression a degree of permanent deformation under load is required. This enables the impression of the soft tissues to be altered during normal function.

Composition: The materials are normally supplied as powder and liquid components (see Fig. 14.2) which are mixed together. Table 14.2 gives the composition of a typical product. The relative amounts of solvent and plasticizer as well as the type of plasticizer used vary significantly from one

Table 14.2 Composition of a tissue conditioner.

product to another. These variations control the softness and elasticity of the set material. Commercial products contain from 7.5% to 40% alcohol in the liquid component, whilst the plasticizer is normally a phthalate or benzoate ester. The powder may be pigmented to give a pink coloured lining similar to that of a pink denture base. It is more common, however, for the powder to be unpigmented, giving a white lining which is easily distinguished from the pink denture base.

It is important to note that the liquid component contains no monomer and the powder no initiator. When the powder and liquid are mixed together, a purely physical process occurs. The solvent dissolves the smaller polymer beads and the larger beads become swollen with solvent which acts as a carrier for the plasticizer. The final ‘set’ material is gel-like, with swollen, platicized spheres being cemented together with a matrix which is a saturated solution of polymer in a solvent/plasticizer mixture. The ‘softness’ of the set material is a function of the use of a higher methacrylate, ethyl methacrylate, coupled with considerable quantities of plasticizer and solvent.

Manipulation: Tissue conditioners are used in chairside techniques in which the freshly mixed material is applied to the fitting surface of the denture. The denture is then seated in the patient’s mouth, whilst the conditioner is still in a fluid state, in order to obtain an impression of the soft tissues. This stage of the procedure is important, since the aim is to form a cushion of reasonable thickness so that it will be effective, but not to increase the ‘height’ of the denture unduly compared with the unlined denture. On completion of setting ideally the tissue conditioner should form a regular layer over the whole of the fitting surface of the denture. It is normal practice to inspect the denture and the patient’s soft tissues after 2–3 days to ascertain whether the tissue conditioning has been successful or, alternatively, whether an adequate functional impression has been obtained.

Properties: Tissue conditioners are initially very soft and viscoelastic. When loaded slowly they undergo permanent deformation even under moderate to low loads. A cylindrical sample of a tissue conditioner can be observed to slump under its own weight if allowed to stand unsupported on a bench top. The lack of a well defined elastic behaviour makes the measurement of rigidity difficult. Under rapid loading a modulus of elasticity value of 0.05 MPa has been estimated. This compares with a value of 2000 MPa for a typical hard acrylic denture base material. The property of compliance is often used to define the deforma- tion-load characteristics of these materials. When this term is used the difference between elastic and viscoelastic strains is often ignored so its scientific value is questionable, although it can give some ‘feel’ for the perceived softness of a material.

The materials do not remain permanently soft since the alcohol and plasticizer are leached rapidly into saliva. The time taken for the materials to become so hard that they no longer give adequate cushioning varies from a few days to a week or two, depending upon the product used. Those materials which are softer initially, harden more rapidly and vice versa. For adequate conditioning, with a very soft material, the conditioner should be replaced with fresh material every 2–3 days until the tissues have recovered.

The materials are able to perform the functions of both a tissue conditioner and functional impression material due to their viscoelastic properties. The apparent paradox of requiring an elastic material for one purpose and a plastic one for the other is overcome in this way. The viscoelastic properties of the materials may be described by the Maxwell and Voigt models in series as discussed on p. 16 and illustrated in Fig. 2.14. The elastic nature of the products is extremely timedependent. Under the influence of dynamic forces which are applied for a second or less during mastication, the materials are essentially elastic and provide a cushioning effect. Each application of force does, however, cause a small permanent deformation which helps to record the functional impression. Under the influence of smaller, resting loads, further permanent deformation occurs.

One of the most important properties of these materials is that they are non-irritant due to the absence of acrylic monomers from the liquid component.

14.4 Temporary soft lining materials

These materials are very similar to the tissue conditioners. They are supplied as a powder and liquid, the composition of which is equivalent to that given in Table 14.2. The materials are not as soft as the tissue conditioners immediately after setting but they retain their softness for longer, taking up to a month or two to harden. Like the tissue conditioners, they are viscoelastic in nature and give a cushioning effect under dynamic conditions of loading.

The method of manipulation of these products is similar to that discussed for tissue conditioners, but because they take longer to harden they do not require replacing as frequently.

Care should be exercised when selecting a denture cleanser to use with a denture carrying a temporary soft lining or tissue conditioner. The oxygenating-type cleansers, in particular, cause surface degradation and pitting of the materials.

Temporary soft liners are often used in place of tissue conditioners in cases where it is not practicable to replace the conditioner every 2–3 days. In addition, they may be used as a means of temporarily improving the fit of an ill-fitting denture until such a time as a new denture can be constructed. Another use of the products is as a diagnostic aid to ascertain whether the patient would benefit from a permanent soft lining.

Both tissue conditioners and temporary soft lining materials will go hard. When this occurs the surface is both rough and irregular, increasing the risk of trauma. In addition these materials can be relatively easily colonized by Candida in this hardened state, increasing the risk of a denture-induced stomatitis. It is possible to soak such dentures overnight in dilute sodium hypochlorite to help to mitigate infection risk.

14.5 Permanent soft lining materials

Permanent soft lining materials are most commonly used for patients who cannot tolerate a hard base (see Figs. 14.3 and 14.4). This problem generally arises if the patient has an irregular mandibular alveolar ridge covered by a thin and relatively non-resilient mucosa. Not surprisingly it may be very painful when a masticatory load is applied through a hard base on to this type of supporting tissue. In such cases, a soft lining on the denture will help to relieve the pain and increase patient acceptance of the denture.

Fig. 14.3 Acrylic type permanent denture soft liner. This illustration shows an acrylic type denture soft lining material used for applying a permanent soft lining to the fitting surface of an acrylic denture. It consists of a powder and a liquid which are mixed and applied to the fitting surface of the denture. The two other items of equipment shown are used for proportioning the powder and liquid. The powder consists of a polymethacrylate or a higher (e.g. ethyl, butyl) methacrylate polymer whilst the liquid consists of a mixture of a polymerizable acrylic monomer and a plasticizer which is used to lower the glass transition temperature of the resulting polymeric material.

Fig. 14.4 A silicone denture soft lining material. The type of silicone used here is an addition curing product. It is provided in the form of a cartridge containing two pastes which are mixed when the pastes are extruded through the nozzle. The other items shown are those which are required to achieve bonding of the silicone to the acrylic denture base, for trimming the soft lining material and for coating the soft lining material after setting.

Requirements: The requirements of a permanent soft lining are more critical than those of the tissue conditioner and temporary lining materials since they are expected to function over a much longer period of time.

The materials used should be permanently soft, ideally for the lifetime of the denture. They should be elastic in order to give a cushioning effect and prevent unacceptable distortions during service. The lining should adhere to the denture base. The materials should be non-toxic, non-irritant and incapable of sustaining the growth of harmful bacteria or fungi.

Available materials: Figure 14.5 indicates the types of materials which are available for use as permanent soft linings. Those products described as cold curing acrylic materials are in fact temporary soft lining materials. These materials harden within a period of a few weeks or at best a few months and cannot therefore be seriously considered as permanent soft linings since they would require regular replacement. One advantage of these materials is that they can be readily applied to an existing denture, by the dentist, in a chairside technique.

The heat curing acrylic materials are processed in the laboratory and are normally applied to a new denture at the time of production. They are supplied as a powder and liquid, the composition of which vary from one product to another (Fig. 14.3). They rely, for softness, on the combined use of a higher methacrylate and a plasticizer. A typical powder consists of beads of polyethylor polybutylmethacrylate along with some peroxide initiator and pigment. The liquid is likely to be a mixture of butylmethacrylate and plasticizer. Powder and liquid are mixed to form a type of

‘dough’ which is heat processed simultaneously with the hard acrylic base.

A similar technique is used when applying a heat curing silicone soft lining. These products are supplied as a single paste which consists of a polydimethylsiloxane polymer with pendant or terminal vinyl groups through which cross-linking takes place. The liquid polymer is formulated into a paste by adding inert fillers such as silica. The paste also contains a free radical initiator such as a peroxide which breaks down on heating to initiate the cross-linking reaction.

The structure of the vinyl terminated liquid silicone polymer can be viewed as being similar to that for the hydroxyl terminated material (Fig. 19.5), except that the terminal hydroxyl groups in Fig. 19.5a are replaced by vinyl groups. These vinyl groups undergo chain extension and crosslinking by the mechanism described in Section 12.2. The chain extension reactions cause an increase in viscosity of the liquid polymer, whilst cross-linking causes the material to develop elasticity.

Two types of cold curing silicone elastomers are used as soft lining materials. They are analogous to the two types of silicone elastomers used as impression materials (Sections 19.3 and 19.4), i.e. condensation curing silicones and addition curing silicones. The condensation curing types are generally supplied as a paste and liquid.

The paste contains a hydroxyl-terminated polydimethylsiloxane liquid polymer (Fig. 19.5a) and inert filler. The liquid contains a mixture of a cross-linking agent, such as tetraethyl silicate (Fig. 19.5b) and a catalyst which is normally an organo-tin compound such as dibutyl tin dilaurate. On mixing the paste and liquid a condensation cross-linking reaction takes place. Alcohol is

produced as a byproduct of this reaction (Fig. 19.5c). Cross-linking causes the paste to be converted to a rubber. The addition curing silicones have only recently become available as denture soft lining materials. They are very similar to the equivalent products used for recording impressions (Section 19.4). They are supplied as two pastes which are proportioned and mixed using a cartridge/gun system (Fig. 14.4). The setting reaction for these products is described in Section 19.4.

Although the cold curing silicones are cured at room temperature, they are generally processed in

Fig. 14.6 Penetration under load for (a) an elastic material and (b) a viscoelastic material. The elastic material responds instantaneously to the application and removal of load. The viscoelastic material behaves in a time-dependent (retarded) manner to the application and removal of load. In the example shown the material eventually achieves complete recovery and therefore this type of viscoelastic behaviour is known as ‘retarded elastic’.

the laboratory. The method normally used is to pour casts into the dentures. The casts with dentures are then mounted on an articulator and the fitting surface of the dentures is relieved to make space for the lining. The paste and liquid are mixed together and the fluid mix applied to the fitting surface of the dentures. The dentures are then repositioned on the casts, the articulator closed into occlusion and the material allowed to set. An alternative method is to use an overcast instead of an articulator.

Polyphosphazine fluoroelastomers have recently become available for use as denture soft lining materials. They are supplied in sheet form and are manipulated in a similar manner to the heat cured silicone products. Recommended curing is either at 74ºC for 8 hours or 74ºC for 21/2 hours, followed by 100ºC for 30 minutes. There is very little detailed information available on the composition of these products.

Properties: The ISO Standard for ‘long-term’ resilient lining materials (ISO 10139-2) utilizes a penetration test to evaluate the softness and elastic properties of the materials. A 4 mm thick specimen of material is indented with a 2 mm diameter probe under a load of 100 g for 30 seconds. Depth of penetration is monitored as a function of time to produce a result of the type shown in Fig. 14.6. The depth of penetration after 5 seconds loading is used to characterise softness. Materials are described as stiff, medium or soft depending on the extent of penetration. The elastic or viscoelastic properties of the materials are defined by the time dependence of the penetration. For elastic materials (Fig. 14.6a) penetration is essentially independent of time – it increases immediately on application of load and recovery is instantaneous after removal of the load. The penetration ratio, which is the ratio of the penetration after 30 seconds of loading to that at 5 seconds of loading, is close to one for these materials. For viscoelastic materials (Fig. 14.6b) penetration under load is time dependent and the ratio of the penetration at 30 seconds to that at 5 seconds is greater than unity. The limits set out in the ISO Standard are shown in Table 14.3.

Acrylic products tend to react to surface indentation in a manner which can be described by the curve in Figure 14.6b. This type of viscoelastic behaviour can be described as retarded elastic behaviour as both the deformation and recovery

processes are damped rather than instantaneous. The recovery may be virtually complete but may involve some degree of permanent deformation which would result in a dimensional change of the material. The silicone and polyphosphazine materials behave in a manner depicted in Fig. 14.6a, i.e. they have a high resistance to flow and are more elastic. Dentists cannot agree on which, between elastic or retarded elastic, is the preferred behaviour for a soft material. A perfectly elastic material may offer a better cushioning effect but a material with retarded elastic behaviour may have the potential for better retention. All types of soft lining material are sufficiently soft on insertion to give an adequate cushioning effect. The softest of the four materials initially are the cold curing acrylic materials. These products harden, however, through rapid loss of alcohol and slow leaching of plasticizer. They should not be considered suitable for anything other than short term use. Softness of the other products varies from brand to brand even within specific classes of materials. Some acrylic products are classified as stiff and others as soft, depending on the type and amount of plasticizer used. The heat curing acrylic products, though not as soft as the cold curing products initially, retain their softness for longer. They too eventually become hard due to gradual leaching of plasticizer into the oral fluids. The silicone materials remain permanently soft and the modulus of elasticity value may, in fact, decrease due to water absorption. This may cause problems with some silicones since water absorption may be followed by bacterial or fungal growth in the soft lining. The silicones have good elastic properties and retain their shape after setting despite being subjected to masticatory loading. The acrylic materials, on the other hand, are viscoelastic and

gradually become distorted. There is a tendency for the materials to ‘flow away’ from areas of greatest stress causing the cushioning effect to be lost.

The durability of the bond between the denture base and the soft lining is adequate for the acrylic materials and the heat cured silicone products. In the case of the cold curing silicone material, however, there is a tendency for the lining to peel away from the base. This occurs despite the use of an adhesion primer supplied by the manufacturers. The problem is often reduced by ‘boxing in’ the soft lining. Clinically the techniques used to add a soft lining vary according to the material chosen. The acrylic based materials can either be added to an existing denture, using a technique analogous to that used when relining the denture with hard acrylic, or they can be incorporated into the denture base when making a new prosthesis. The results in terms of both resilience and durability are similar. Conversely, whilst the silicone rubber materials can be processed onto an existing denture, the quality of the attachment between the acrylic and the rubber is much better if it is processed against ‘fresh’ acrylic.

The cushioning effect of a soft lining depends on the thickness of the soft material. The greater the thickness of the soft material the greater the cushioning effect perceived by the patient. There are limits to the total thickness of the combined denture base and soft lining and this means that the use of a soft lining inevitably involves the use of a thinner residual acrylic denture base. Since a thickness of soft lining of 2–3 mm is required for adequate cushioning the residual hard acrylic base may become more flexible and weakened such that the occurrence of fracture is increased compared to dentures constructed solely from hard acrylic material.