Soft Lining Materials and Tissue Conditioners

Soft Lining Materials

• Soft lining materials are resilient polymers used to replace the fitting surface of hard plastic dentures. This is done to improve patient tolerance and denture retention.

Reasons for Using Soft Liners

• Patients may not tolerate hard fitting surfaces.
• To improve denture retention.
• Patients with irritated denture-bearing mucosa, undercut areas, or defects of the palate.

Tissue Conditioners as Soft Liners

• Tissue conditioners are soft elastomers used to treat irritated mucosa supporting a denture.
• They are used for short-term applications and are typically replaced every 3 days.
• This material has a positive effect on inhibiting oral bacterial growth and promoting the healing of inflamed tissues.

Composition of Tissue Conditioners

• Composed of a powder (polymethyl methacrylate) and a liquid (aromatic ester-ethyl alcohol mixture).
• The material is very soft and only undergoes a physical, not a polymerization, reaction.
• They become stiffer due to the loss of the alcohol component.
• Mixed at the chairside and placed in the denture before being fit in the patient's mouth.

Properties of Effective Tissue Conditioners

• Viscous behavior: Enables adaptation to irritated denture-bearing mucosa over several days.
• Visco-elastic and elastic behavior: Cushions the cyclic forces of mastication and distributes the load across the mucosa.

Types of Soft Liners

• Plasticized acrylic: Primarily ethyl methacrylate with a plasticizer (large phthalate ester). The monomer can be methyl or butyl methacrylate.
• Silicone: Two types:
  • Room temperature vulcanizing (RTV): Good resilience but poor tear strength and no intrinsic adhesion to acrylic denture bases. A drawback is the lack of adhesion between the acrylic denture base and the silicone liner and a tendency to support yeast growth in the mouth.
  • Heat-cured: Achieve a greater degree of cross-linking and offer longer clinical lifetimes.

Desirable Properties of Soft Liners

• High bond strength to the denture base.
• Dimensional stability throughout and after processing.
• Permanent softness or resilience.
• Low water sorption.
• Color stability.
• Ease of processing.
• Biocompatibility.

Acrylic Soft Liners (Disadvantages)

• Poor resilience.
• Loses plasticizer.
• Can buckle in water.

Acrylic Soft Liners (Advantages)

• High peel strength to acrylic denture base.
• High rupture strength.
• Can be polished.
• Reasonable resistance to damage by the denture.

Silicone Soft Liners (RTV) (Disadvantages)

• Low tear strength.
• Low bond strength to dentures.
• Attacked by cleansers.
• Buckle in water.
• Poor abrasion resistance.

Silicone Soft Liners (RTV) (Advantages)

• Support fungal resilience.

Silicone Heat-cured (Disadvantages)

• Low tear strength.
• Poor abrasion resistance.

Silicone Heat-cured (Advantages)

• Resilience.
• Adequate bond strength to acrylic.
• More resistant to aqueous environment and cleansers than RTV.

Tissue Conditioners (Disadvantages)

• Low cohesive strength.
• Affected by cleansers.
• Alcohol can sting inflamed mucosa.

Tissue Conditioners (Advantages)

• Ideal viscoelastic properties.
• Can be applied chairside.
• Dentures fit well.

Denture Teeth Materials

• Acrylic resin teeth: Resemble denture base acrylic resin but with more cross-linking agents resistant to crazing. Stronger than the base resin and have tooth-colored pigments.
• Porcelain teeth: Made in similar shapes, sizes and shades as acrylic teeth; significantly harder and more stain resistant. Should not be used with diabetic patients.

Porcelain Disadvantages

• Excessive wear on opposing teeth.
• Cause trauma and bone loss in supporting and opposing alveolar ridges.

Mechanical Attachment of Porcelain Teeth

• Held in the denture by mechanical undercuts or pins embedded in the back of the denture tooth.

Acrylic vs. Porcelain Teeth (Differences)

• Retention: Acrylic – chemically retained; Porcelain - mechanically retained.
• Density: Acrylic – lower density; Porcelain – higher
• Resiliency: Acrylic - resilient; Porcelain – not resilient
• Noise during eating: Acrylic - quite; Porcelain – noisy
• Wear resistance: Acrylic – low; Porcelain – high
• Color stability: Acrylic – low; Porcelain – high
• Staining: Acrylic - easy to stain; Porcelain - difficult to stain
• Adjustment and grinding: Acrylic – can be adjusted, ground, and polished; Porcelain – difficult to grind and glaze
• Impact strength: Acrylic – high; Porcelain – low
• Coefficient of thermal expansion: Acrylic – similar to denture base; Porcelain – different from denture base
• Strength in thin section: Acrylic – strong; Porcelain - brittle

Maxillofacial Materials: Poly(methyl methacrylate) (PMMA)

• Was commonly used for maxillofacial prostheses.
• Readily available, easy to manipulate, and strong.
• Stable, hygienic, and durable.
• Not as flexible as skin.
• Primarily used in cases with little tissue movement.

Maxillofacial Materials: Latex and Latex Alternatives

• Latex is soft, inexpensive and easy to shape.

• Forms realistic prostheses.

• Weakens and changes color rapidly.

• Not a major maxillofacial prosthetic material anymore.

• Plasticized Polyvinyl Chloride: A rigid plastic made more flexible with a plasticizer.

Maxillofacial Prosthesis Materials: Chlorinated Polyethylene

• Introduced in the 1970s/80s as an alternative to silicone.
• Processing involves high-heat curing pigmented sheets in metal molds.
• Less irritating to mucosa, less toxic than thermosetting silicones, and non-carcinogenic.

Maxillofacial Prosthesis Materials: Polyurethane Polymers

• Most recent addition.
• One component is acrylate. Needs careful handling to prevent operator-related toxic reactions.
• Cured at room temperature but requires accurate temperature control.
• Has lifelike feel and appearance, better color stability than polyvinyl chloride.

### Silicone Rubber, Advantages, Disadvantages, Types

• Silicones are widely used in maxillofacial prosthetics.

• Room temperature vulcanizing (RTV): Fluid, semisolid, gel-like, or putty-like material. Commonly transparent/translucent. Available with different base/catalyst ratios.

• Heat Vulcanized (HTV): Stronger than RTV silicones. Can be made with condensation (tin catalyzed) or addition (platinum catalyzed) types. -Advantages of Silicone based Maxillofacial materials over other materials: Good physical properties, easy to manipulate, chemically inert, low toxicity, high thermal oxidative stability, can have natural appearance, resists organic material absorption.

• Disadvantages of silicone based maxillofacial materials: Susceptible to fungal growth and fraying.