Polymeric Denture Base Materials Flashcards
What is a denture base?
The component of a removable denture which is contact with the denture bearing mucosa and which carries the teeth and replaces lost oral tissues
Name 4 ideal properties of a denture base material
- Biological / Chemical properties
- Physical properties
- Mechanical properties
- Other properties
Describe 3 ideal biological / chemical properties of a denture base material
- Biocompatible (non-toxic, non-irritant)
- Chemically inert (impermeable to oral fluids, tasteless and odourless)
- Chemical bond to prosthetic teeth
Describe 6 ideal physical properties of a denture base material
- Aesthetic
- Glass transition value which is high enough to prevent distortion
- Minimal thermal expansion if alloy
- Dimensionally stable
- Light in weight
- Radio-opaque
Describe 4 mechanical properties of a denture base material
- Good rigidity - High MoE and proportional limit
- High fatigue strength
- High impact strength
- Hard surface and resistant to abrasion
What is the difference between fatigue and impact strength?
High fatigue strength is resistance to fracture on function but high impact strength is resistance to fracture on impact
Name 4 other properties which are ideal for denture base materials
- Easy to process and accurate manipulation
- Relatively inexpensive
- Easy to repair
- Easy to clean
Name 4 common denture base materials in current use
- Polymethylmethacrylate (acrylic resin)
- Cobalt-chrome alloy
- Thermoplastic polyamides (nylon)
- Titanium alloy
Name 4 historically used denture base materials
- Wood, ivory or porcelain
- Celluloid, rubber
- Gold and stainless steel
- Polycarbonate
Describe 4 possible “upgrades” to PMMA as a denture base material
- Flexible
- Injection moulded
- High impact
- Fibre reinforced
Describe PMMA resin as a material
- Glassy, colourless, hard, brittle acrylic
- Developed as a glass reinforcement
- Easy to colour
- Adequate strength
- Low water sorption, low solubility and non-toxic
- Categorized by its method of curing / activation
Describe 3 applications of heat-cured acrylic
- Denture bases
- Gum-work on metallic denture bases
- Artificial teeth
What is the composition of PMMA?
Liquid monomer and preformed polymer powder
What is the monomer found in the liquid which makes up PMMA?
Methylmethacrylate monomer
What is the stabiliser found in the liquid which makes up PMMA?
Hydroquinone
What is the cross linking agent found in the liquid which makes up PMMA?
Ethylene glycol dimethacrylate
What is the initiator found in the powder which makes up PMMA?
Benzoyl peroxide initiator
What is the pigments are found in the powder which makes up PMMA?
Cadmium (red) and Fe salts / titanium dioxide (white)
What is the plasticiser found in the powder which makes up PMMA?
Dibutylphthalate
Name 3 parts of the liquid part of PMMA
- Monomer
- Stabiliser
- Cross-linking agent
Name 5 parts of the powder part of PMMA
- Preformed PMMA beads
- Initiator
- Pigments
- Nylon
- Plasticiser
What is the most common manipulation technique of PMMA?
Dough moulding technique
What occurs if the polymer : monomer ratio is too high in manipulation of PMMA?
Granular acrylic forms which is weak and opaque
What occurs if the polymer : monomer ratio is too low in manipulation of PMMA?
Polymerization shrinkage occurs to a higher extent
Describe 7 steps to dough moulding method
- Mixing
- Mould lining
- Packing
- Curing
- Cooling
- Deflasking
- Finish and polishing
Describe what physical changes occur when mixing polymer and monomer in the manipulation of PMMA
- Wet sand
- Tacky
- Doughy (snappy)
What is the function of mould-lining in dough moulding technique?
Prevents monomer entering plaster and mould prevents water entering the resin
What is the function of packing in the dough moulding method?
Fill to excess to avoid contraction porosity
Name 3 methods of curing when using heat-cured PMMA and the dough moulding method
- Water bath
- Air oven
- Microwave oven
What occurs if PMMA is under cured?
High residual monomer content leading to weakened acrylic
What occurs if a temperature is raised too fast during the curing of PMMA?
Gaseous porosity
What is the function of cooling in the dough moulding technique?
Allow stress relief
Name 6 critiques of injection moulding
- Denser base material - reduced risk of porosity
- Reduced dimensional change
- No risk of increase in OVD
- Finer reproduction of gingival wax work
- No monomer exposure
- Expensive equipment
Describe 6 properties of heat cured PMMA
- Residual monomer of around 0.2-0.5%
- Water content of 2%
- Susceptible to crazing
- Low impact and fatigue strength
- Tg of heat cured PMMA of 120 degrees
- Allergy to PMMA possible
Name 3 reasons for crazing of heat cured PMMA
- Stress on repeating drying and wetting
- Stress concentration around inserts
- Monomer from repairs
Name 5 properties of chemically cured PMMA
- Content same as heat cured but liquid contains chemical activator dimethyl-p-toluidine
- Greater porosity
- Reduced strength
- Residual monomer content of 2%
- Poor colour stability
Name 3 components of chemically cured PMMA powder
- Pre-polymerized PMMA spheres
- Benzoyl peroxide (initiator)
- Pigment
Name 4 contents of chemically cured PMMA liquid
- Methylmethacrylate monomer
- Hydroquinone (inhibitor)
- Ethylene glycol dimethacrylate (cross linking agent)
- Dimethyl-p-toluidine (activator)
Name 5 uses of chemically cured PPMMA
- Denture repairs and additions
- Denture copy technique
- Impression trays
- Relines
- Orthodontic appliances
What are 2 main reasons for modifying PMMA
- Improve low impact strength (brittle with low ductility)
2. Improve fatigue strength (susceptible to crack propagation)
Name 2 high impact acrylics
- Trevalon Hi
2. Acron Hi
Describe how PMMA is modified to improve fatigue and impact strength
- Polymer bead are made by co-polymerizing PMMA and butadiene styrene rubber in an emulsion
- Coat resultant beads in PMMA producing micro-dispersed rubber phase polymers
- Absorb energy and reduce crack propagation
Name 3 types of reinforcing fibres found in modified PMMA
- Carbon fibre inserts
- Glass fibres
- Polyethylene fibres
What is a drawback of PMMA during major trauma?
It is radio-lucent so will not show up on x-rays
Describe 3 ways PMMA may be modified to become radio-opaque
- Metal inserts (poor appearance and weakens base)
- Barium salts (considerable weakens base)
- Bromine-containing additives (expensive and not used commerically)
What is the most commonly used flexible denture base material?
Nylon
Name 2 major issues with flexible dentures
- Design
2. Health
Describe 2 reasons health is compromised in flexible dentures
- Pressure is exerted on soft tissues
2. Coverage of gingival margins
