Denture Base Materials

• Denture bases must be biocompatible and easy to manufacture:

  • Biocompatibility
  • Dentures worn for long periods
  • Must maintain health of soft tissues (non-toxic and non-irritant)
  • Ease of manufacture
  • Must be mouldable to individual shape
  • Fitting errors may effect comfort and performance
  • May lead to non-wearing of denture
  • Must be cheap to make; 1000s produced each year
• Ideal mechanical properties of denture bases:

  • Sufficient strength - everyday activities (biting, chewing, talking) shouldn't lead to denture fracturing
  • Sufficient toughness - wearers tend to have reduced dexterity - should be able to survive being dropped
  • Sufficient stiffness - denture shouldn't deform during everday activities
• Ideal mechanical properties of denture bases:

  • Resist permanent deformation (proportional limit) - if there's any deformation it must be elastic - maintain shape & fit to pt anatomy
  • Sufficient hardness - notch sensitive materials are weakened by surface scratches - high hardness allows cleaning with abrasive cleaners
• Physical requirements of denture bases - density:

  • High density materials may be difficult to retain in place for an upper denture
  • Ideally should have low density and high strength
  • Typically strength and density are proportional (higher the density, lower the strength) - needs a compromise
• Physical requirements of denture bases:

  • Heat transfer
  • Maintains health of underlying soft tissue - PMMA is an insulator
  • Thermal diffusivity and conductivity - warn against potential to scald
  • Radio-opacity
  • Detect fragments when swallowed or inhaled
• Physical requirements of denture bases:

  • Accuracy and dimensional stability
  • Good fit when provided and over life-course of denture
  • Should not absorb water
  • Should not be affected by dietary agents and cleaning products
  • Ability to maintain in clean and hygienic state
• Acrylic denture bases are based on the monomer methyl methacrylate (MMA). There are 2 main types of acrylic denture base materials:

  • Chemically the two types are nearly identical
  • Difference is due to method of polymerisation activation
  • 1 = Heat-curing denture acrylic
  • Requires heat to activate polymerisation
  • Common methods: water bath, dry oven - bath most common
  • 2 = Self-curing denture acrylic
  • Doesn't require heat to activate
  • Can be called: cold-curing, auto-polymerising
  • Different types available: dough-moulded, pourable, injection-moulded, light-cured
• Composition of commercial products:

  • Powder
  • Beads (~50μm) of PMMA (polymethylmethacrylate)
  • Peroxide
  • Pigment
  • Liquid
  • Methacrylate monomer
  • Di-methacrylate (cross-linker)
  • Hydroquinone (stabiliser)
  • Amine compound (activator) - self-curing only (only in cold cure)
• Setting reaction - free radical addition polymerisation:

  • Activation
  • Break down of initiator
  • Production of free radicals
  • Requires either heat or an amine
  • Initiation
  • Radical reacts with monomer
  • Propagation
  • Continuing reactions to form long chains
  • Includes cross-linking
  • Termination
  • Reaction ends, residual monomer remains

  
• Acrylic denture manufacture:

  • Made using a dough moulding process
  • The dough may form before polymerisation occurs
  • Heat cure acrylic - polymerisation activated at elevated temperature
  • Cold cure acrylic - polymerisation activated when powder and liquid mix
  • Series of clear stages with distinct physical properties
  • Mix the powder with the liquid to form the dough
  • Stages are: slurry/creamy, sandy, stringy, dough

  
• Acrylic denture manufacture:

  • Made using a dough moulding process
  • Stages are: slurry/creamy, sandy, stringy, dough
  • Slurry/creamy
  • Typical consistency of powder/liquid mix
  • Sandy
  • The monomer starts to soak into the PMMA
  • Some small beads may dissolve
  • Most beads swell
  • Stringy
  • The swollen beads start to join together
  • These are not polymer chains!
  • Dough
  • The mass is now sufficiently cohesive to form a paste
  • Does not stick to the mixing vessel
  • Does not mean polymerisation has occurred

  
• Dough-moulded denture production:

  • Dough is placed into the gypsum mould - contained in a metal flask
  • Mould is sealed and pressurised
  • Force dough into all of mould
  • Allows trial closure
  • Is there enough dough to fill the mould
  • Only possible with heat cure materials; cold cure materials already polymerising
  • Repressurise mould
  • Heat cure: place flasks into oven/bath
  • Cold cure: may use a pressure pot, not always
  • Remove and de-flask after setting time - will require grinding and polishing
• Alternative production methods:

  • Injectable materials
  • Dough injected into the mould under pressure
  • Pourable materials
  • Poured into a mould when sandy then polymerised
  • Special mould made from agar possible
  • Dough forms in the mould
  • Light-cured materials
  • Place dough and mould into special oven
  • Light (and heat) applied once in oven
• Effect of curing process on properties - two main processes used:

  1. Heat activation
  2. Initiator breaks down above 65°C
  3. Typically requires hours to make denture
  4. Chemical activation
  5. Initiator breaks down at room temperature (because chemical activator is added)
  6. Typically requires less than 1 hour

  So why is heat activation used more...
• The heat cure process:

  • Dough formed at room temperature - trial closure possible
  • Flasks placed in oven/bath
  • Peroxide activated at 65-70°C
  • Exothermic reaction - rapid temperature rise
  • Monomer boils at 100.3°C - turns to gas
  • Gaseous porosity - leads to weakness
  • Different heating cycles possible
  • Often use slow rise to 70°C
  • If only 70°C used then high residual monomer found
  • Add a final phase with heating to around 100°C
  • Maximise polymerisation
  • Reduce residual monomer
  • Reduce gaseous porosity
• The cold cure process:

  • Activation occurs on mixing powder and liquid - no trial closure possible
  • Polymerisation occurs while dough forming - shorter working time
  • Exothermic reaction
  • Temperature never reaches 100.3°C
  • No gaseous porosity
  • Residual monomer
  • Related to curing temperature
  • No final high temperature curing phase used

  
• Curing conditions effect degree of polymerisation:

  • Degree of polymerisation = amount of monomer that converts to polymer
  • Heat curing produces a higher degree of polymerisation
  • This happens because:
  • PMMA glass transition temperature (Tg) is around 100°C
  • In chemical activation, temperature is always below 100°C
  • Polymer is always glassy so monomer finds it hard to flow to active chains
  • In heat activation, temperature can be close to or above Tg
  • Monomer finds it easier to flow to active chains
  • Beware Tg is close to the monomer vaporisation temperature
• Residual monomer and molecular weight:

  • High average molecular weight and low residual monomer content - obtained with high temperature cure (around 100°C)
  • Low average molecular weight and high residual monomer content - obtained with low temperature cure
  • Glass transition temperature (Tg) controlled by:
  • Molecular weight and residual monomer
  • Affects dimensional stability - think boil-and-bite gum shields
  • If Tg is close to mouth temperature
  • Denture may be dimensionally unstable
  • Quality of fit may reduce over time
• Polymerisation shrinkage:

  • Methyl methacrylate
  • Low molecular weight monomer
  • Conversion to a polymer - 20% volume shrinkage
  • This would be too much to make dentures
  • Adding powder reduces shrinkage
  • Think dental composites
  • Powder:liquid ratio of 2.5:1 used
  • Leads to 6% shrinkage, still large but manageable
  • Too much powder or poor mixing - granular porosity
  • Shrinkage can still occur
  • Contraction porosity
  • Need to add excess material to mould
  • Trial closure - check sufficient material added - only possible with heat cured materials
• PMMA is in engineering terms:

  • A weak and flexible material
  • Dentures suffer complicated stress in the mouth - flexure
  • Increasing thickness would increase strength
  • Thickness limited by pt requirements
  • It has low toughness
  • Shatters when dropped
  • Notch sensitive
  • Notches where teeth are attached
  • Beware scratches - act as extra notches
• Mechanical properties of acryclic denture bases - PMMA:

  • Higher toughness PMMA materials available - but more expensive
  • Has low fatigue strength
  • Denture bases "flex" during biting and chewing
  • Most deformation occurs in the midline
  • Is a soft material
  • Has low abrasion resistance
  • Scratches easily
• Effect of cleaning on dentures:

  • Regular cleaning vital for denture hygiene
  • Two types of cleaning regime available:
  • Mechanical - brush and paste - use specific products
  • Chemical
  • Soaking
  • Oxygenating tablet
  • Sodium hypochlorite solution
  • Current evidence suggests both types are needed
• Care must be taken when brushing:

  • Scratching of denture base is possible
  • PMMA is a soft material
  • Cleaning pastes are abrasive
  • Normal toothbrushes are abrasive
  • Scratches make surface rough
  • May affect comfort - rough surfaces can be abrasive to soft tissues
  • May provide surfaces for microbial colonisation - may lead to stomatitis
• Care must be taken when soaking:

  • Soaking often requires warm water
  • Difficult to define "warm" - may lead to poor compliance with manufacturer's instructions
  • Poor cleaning can lead to low transition temperature - denture may change dimensions in "warm water"
• Care must be taken when soaking:

  • Change in pigmentation can occur
  • Often termed "denture bleaching"
  • Previously thought to be due to NaOCl cleaners
  • Now known to be due to:
  • Too hot water when soaking
  • Atmospheric solvents
  • Poor curing
  • Lead to refractive index change
  • Results in a white appearance
  • Effect has been replicated in pigment-free dentures
• Other acryclic denture base properties - biocompatibility:

  • Residual monomer - irritant and cytotoxic - well-conducted heat cure procedure should reduce residual monomer
  • Individual allergies - monomer is a sensitiser - take a careful pt history
  • Pigments - some concerns over cadmium use but no longer used
• Other acryclic denture base properties - aesthetics/appearance:

  • Pigments - good colour match with soft tissues
  • Fibres -  give the appearance of blood vessels
  • Staining - dietary factors (turmeric, coffee, etc.)
  • Cleaning - "denture bleaching"
• Dentures need to be visible on diagnostic radiographs:

  • Radio-opacity is atomic number dependent
  • PMMA contains C, O, and H - radiolucent
  • So heavy atoms are required
  • Common agents are bases around Ba or I
  • Adding reduces strength
  • Compromise between radio-opacity and strength
• Artificial teeth:

  • Added to denture bases during production
  • Requirements
  • Mechanical and physical properties
  • Need to be similar to bases
  • Likely to be subjected to greater stress (eg incisal edges)
  • Bonding to denture base
  • Need a strong bond to prevent debonding in service
  • Debonding can be dangerous - swallowing by pt
  • Types:
  • Porcelain - traditional choice
  • Acrylic - more common today
• Porcelain teeth:

  • Made from a ceramic: porcelain
  • Can be mass-produced by standard shapes/sizes
  • Bonding to denture is mechanical - achieved using pins and holes
  • Porcelain is hard, rigid, brittle - abrasion resistant, abrasive, chipping
  • Natural appearance, unnatural sound
  • Difficult adjustment
• Acrylic teeth:

  • Made from the same material as the base
  • Can be produced in standard shapes/sizes
  • Polymerised in metal moulds
  • Polymerised using heat and pressure
  • Bonding is easily achieved (like to like)
  • Properties similar to acrylic bases - eg poor abrasion resistance