Denture Soft Liners

• Types of denture soft liners - there are 2 main types:

  • Tissue conditioners and temporary soft liners - these are useful for a few days up to a few weeks
  • 'Permanent' soft liners - a more long-term solution for use over months up to years
• Tissue conditioners and temporary soft liners are often described as 2 different classes but:

  • Chemically almost identical
  • Differences relate to length of working life
  • Tissue conditioners - useful over days
  • Temporary soft liners - useful over weeks
• Tissue conditioners and temporary soft liners - indications for use:

  • Provide temporary support to aid
  • Traumatised tissues
  • Wound healing
  • Improve fit of ill-fitting denture
  • Allow denture wear while new denture is made
  • Diagnostic aid
  • Can be used to identify why the existing denture does not fit
  • Liner can act as a "functional impression"
• Requirements for use - liner needs to be applied chairside:

  • Should be able to be applied to cover denture surface
  • Initial flow on seating in the mouth
  • Soft when set
  • Elastic for cushioning (high resilience required)
  • Plastic for functional impression
• Products come as a combination of:

  • Liquid
  • Plasticiser - phthalate or benzoate ester
  • Solvent - ethanol
  • Powder
  • A higher methacrylate POLYMER - note not a monomer

  Polymer Glass Transitions (Tg)

  1. Polymethyl methacrylate 105°C
  2. Polyethyl methacrylate 65°C
  3. Polybutyl methacrylate 20°C

  Combination of: plasticiser, solvent and low Tg polymer means that the set material is soft.
• Setting reaction for denture soft liners:

  • Mix powder and liquid
  • Dissolution of powder and absorption of liquid
  • Formation of gel

  Note: there is no chemical reaction. So, no exotherm, no residual monomer.
• Technique for using denture soft liners:

  • Mix powder and liquid
  • Apply to fitting surface of denture
  • Seat in mouth - record 'impression'
  • Allow to set
  • Remove and trim excess
• Properties of the set material for denture soft liners:

  • Initially very soft
  • Transition temperature effect of higher methacrylates
  • Solvent and plasticiser soften further
  • Viscoelastic
  • Materials are never fully elastic
  • Will flow under load (i.e. during use)
  • Means liner can act as a functional impression
  • Liner flows away from points of high load
  • New denture can be designed to compensate for these high load points
• Properties of the set material for denture soft liners:

  • Harden after few days/weeks
  • Solvent & plasticiser leached out
  • Separate from base
  • Need replacement
  • Don't use oxygenating cleansers
  • Can cause "pitting" of the surface
• Tissue conditioners and temporary soft liners - advantages:

  • Chair-side procedure
  • Initially soft (few weeks maximum)
  • Materials that are initially the softest harden fastest
• Tissue conditioners and temporary soft liners - disadvantages:

  • Harden: so they need replacing regularly
  • Become highly distorted due to flow: so denture fit becomes worse
  • Difficult to control thickness: so denture may not be comfortable to wear
  • Difficult to clean: so difficult to maintain oral hygiene
• Indications for permanent soft liners:

  • Pain during mastication while wearing a denture
  • Ill-fitting denture
  • Thin and/or non-resilient oral mucosa
  • Severe alveolar resorption
  • Aid to retention - engage hard tissue undercuts
  • Obturators
  • Used when hard and/or soft tissues have been removed
  • Fill space and aid retention
• Requirements of permanent soft liners:

  • Biocompatible - worn with the denture at all times
  • Permanently soft - needs to retain its softness over long periods of time
  • Adhere to denture base - debonding would require a replacement liner
  • Durable - should last for the lifetime of the denture
  • Easy to clean - pt should be able to maintain oral hygiene
  • Elastic/retarded elasticity - debate between dentists as to which is needed
• Types of permanent soft liners:

  • Acrylic-based
  • Powder/liquid - heat-cure type
  • 2 paste - light-activated type
  • Silicone-based
  • Heat cure
  • Self cure
  • Condensation polymerisation
  • Addition polymerisation
• Heat curing acrylic materials - powder/liquid systems:

  • Powder
  • Methacrylate polymer or copolymer (try to reduce transition temperature)
  • Peroxide initiator
  • Pigment
  • Liquid
  • Monomer (methacrylate)
  • Plasticiser (lowers transition temperature)
  • No alcohol
  • Setting by polymerisation
• Heat curing acrylic materials - technique for use:

  • Mix powder/liquid - dough moulding technique
  • Heat cure

  For existing denture - make space

  For new denture - pack with acrylic dough
• Light-activated polymer:

  • 2 paste systems:
  • Mixed in cartridge
  • Light-activated after moulding on reline jig (articulator)
  • Based on urethane dimethacrylate
  • Same type of resin as composite restorative
  • Contains silicone filler
  • Set material is a crosslinked composite
  • UDMA is stiffer than PMMA
  • Difunctional monomer - leads to crosslinking
  • Addition of filler increases stiffness further
  • No advantage of light curing
  • Relative stiffness led to pt dissatisfaction
• Silicones:

  • Poly(dimethyl siloxane)
  • -Si-O-Si-O-
  • Similar to impression silicones
  • Supplied as pastes - liquid silicone polymer + filler
  • Converted to rubber on setting (cross-linking)
• Silicones - 3 types:

  • Heat curing
  • Single paste
  • Methacrylate terminated silicone
  • Peroxide (activated by heat) - similar to denture bases
  • Procedure
  • Existing denture - fitting surface relieved/treated with fresh dough
  • New denture - pack against dough in flask
  • Cold curing - condensation type
  • Cold curing - addition type
  • 2 pastes
  • Modified poly(dimethyl siloxane)
  • Catalyst (platinum)
  • Cartridge mixed
  • Reaction
  • Cross-linking by addition
  • Applied to existing denture
  • Fitting surface relieved to take liner
  • Cured on reline jig (articulator)
• Bonding to acrylic denture bases:

  • For the acrylics
  • Bond 'like with like'
  • Ok for fresh denture bases
  • For silicones
  • Self cure silicones require adhesive
  • Heat cure silicones bond through copolymerisation
  • For new dentures bond to fresh acrylic through methacrylate groups
  • For existing dentures may need to use 'boxing-in'
• Properties of permanent soft liners:

  • Initial softness
  • Light activated product too hard - no commercial products currently available
  • Other materials soft - harder than temporary materials
  • Softness on ageing
  • Silicones remain soft
  • Some acrylics harden
  • Loss of plasticiser
  • Polymerisable plasticisers developed
  • All affected by cleaners
• Denture liner hygiene:

  • All materials difficult to keep clean and hygienic
  • Oxygenating cleansers to be avoided - cause surface "pitting"
  • Brushing with toothpaste - care required
  • Acrylics suffer abrasion leading to rough surface - lower comfort
  • Silicones can debond from the denture
  • Soaking in dilute hypochlorite may help
  • Care needed with hypochlorite concentration
  • Too strong - degrades lining
  • Too weak - ineffective hygiene
• Elastic or retarded elastic:

  • Silicone liners are elastic when set
  • Elastic recovery is immediate
  • Deformation is not permanent
  • Acrylic liners are viscoelastic when set
  • Viscoelastic recovery occurs over time
  • This is termed "retarded elastic"
  • There is no consensus between dentists
  • Both options are available

  
• The effect of liner addition on denture base properties:

  • When a liner is added to a denture base
  • Some of the denture base must be removed
  • Results in a thinner denture base
  • Denture base strength is proportional to thickness
  • Liners are weaker than denture bases
  • So, adding a permanent soft liner will weaken the denture base
  • Denture fracture is common
  • Reinforcement may be needed
• Permanent soft liners:

  • Advantages
  • Give some relief of pain
  • May allow a patient to tolerate denture wear
  • Disadvantages
  • Not permanent
  • Difficult laboratory procedure
  • Weaken base
  • Suspect bonding
  • Difficult to clean