What are the ideal properties of denture-base materials?
- Ease of fabrication and repair (readily processed).
- Biocompatible and cleansable
- No taste or odour
- Aesthetic
- Chemically stable in oral fluids, drinks, moisture
- Dimensionally stable
- Mechanically stable – properties suitable
- Thermally stable – properties suitable
- Radiopacity – if fractured fragment is inhaled or swallowed, should be x-ray detectable.
What are the mechanical requirements of denture-base materials?
- High elastic modulus to prevent to prevent functional distortion + use in thin sections
- High proportional limit, to prevent deformation
- High flexural (tensile) strength, to resist intra-oral forces
- High fatigue strength, to withstand multiple loading
- High impact strength to withstand fractur if dropped
- Hard, abrasion-resistant surface, polishable, non-scratch.
What are the thermal property requirements of denture-base materials?
- Softening temperature high > 60 degrees Celsius (hot drinks)
- Thermal diffusivity high, to transfer heat rapidly
- Expansion coefficient, equal to that for artificial teeth.
What is the definition of polymers?
- Polymers are large macromolecules resulting from the polymerisation of small monomers.
- Their significance lies in their capacity to replicate the properties of dental tissues, offering aesthetic and functional benefits in various restorative and preventive procedures.
What are the different polymers used for different dental materials?
o Dentures – acrylic resin
o Dental adhesives and sealants
o Elastomers – impressions
o Resin – composites and cements.
What are monomers?
The fundamental units that undergo polymerisation to form larger, more complex structures, their selection is critical in determining the final properties of the polymer.
What are polymers?
Formed from the linkage of monomers, polymers exhibit diverse properties depending on their molecular weight, structure, and cross-linking densities.
What are cross-linking agents?
Substances that interlink polymer chains, enhancing mechanical strength and durability, crucial in ensuring that stability of dental applications.
What are additives?
Incorporated to modify properties such as viscosity, colour, or antimicrobial characteristics, improving the performance of dental polymers in clinical situations.
What are different monomer and polymer structures are there?
- Monomer – single unit
- Polymer – multiple units.
- Simple chain – linear, soluble in solvents
- Branched chains – soluble in solvents
- Cross-linked chains – insoluble in solvents
- Residual monomer – unreacted particles.
- > ramification > crystallinity and < intermolecular cohesion.
- Homopolymer – repeating unit only:
o Polypropylene (PP)
o Poly (Methyl methacrylate) PMMA.
o Polyethylene (PE)
o Polyethylene terephthalate (PET) - Copolymer – a polymer derived from more than one species of monomer:
o Copolymerisation is key to obtain tailored properties compared to parent homopolymers
What is the history of denture polymers?
- Cellulose nitrate (celluloid):
o Problems of warpage, water sorption, colour stability, use of camphor as a plasticiser which leached out with unpleasant taste.
- Cellulose nitrate (celluloid):
- Phenol-formaldehyde resin (Bakelite):
o Very brittle, difficult to mould.
- Phenol-formaldehyde resin (Bakelite):
- Vulcanite:
o Crosslinked polyisoprene rubber – absorbs saliva, unhygienic due to bacterial growth, opaque and non-aesthetic, dimensionally unstable while processing
- Vulcanite:
- Vinyl resins (PVC, PVA, polystyrene):
o Difficult to process, brittle so tended to fracture.
- Vinyl resins (PVC, PVA, polystyrene):
- Polyamide (Nylon):
o High water absorption which impaired mechanical properties.
- Polyamide (Nylon):
- Polycarbonate reinforces with glass fibres:
o Exhibit good toughness but require relatively high temperature, complex inject moulding for forming
- Polycarbonate reinforces with glass fibres:
What are the most commonly used polymers in dentures?
o Polymethyl methacrylate (PMMA)
o Polycarbonate (PC)
o Polyurethane (PU)
o Polyamide (PA)
o Polyoxymethylene (POM)
o Polyaryletherketone (PAEK).
- The PAEK family includes polyetherketone (PEEK) and polyetherketoneketone (PEKK), both used in prosthodontics.
- PMMA plastic polymerisation uses free radical polymerisation.
What is acrylic resin PMMA?
- Current material used for dentures
- Polymer derived from ethylene that contains a vinyl group in its structure.
- Acrylic acid = CH2CHCOOH
- Methacrylic acid = CH2C(CH3)COOI
- First developed in 1932
- Introduced as a denture base material in 1935
- By 1940 PMMA had taken over from vulcanite as the most popular denture base material.
What are physical properties of dental polymers?
- Mechanical properties: strength, hardness, and elasticity of dental polymers are crucial for their functionality in restorations.
- Thermal properties: temperature fluctuations encountered in the oral environment, ensuring longevity and stability.
- Optical properties: translucency and colour matching are essential for the aesthetics of dental materials
- Water sorption and viscoelasticity: they relate to stability, durability and comfort of these materials in the oral environment.
What are the challenges and limitations of polymers?
- Degradation issues:
o Polymers can undergo degradation when exposed to oral conditions, affecting their performance and lifespan, necessitating ongoing evaluation and improvement - Wear resistance:
o Limited wear resistance of some polymers can lead to premature failure, requiring the development of more durable materials for long-term use. - Colour stability:
o Colour change over time can affect the aesthetic appeal of dental restorations, presenting a challenge in selecting suitable polymer materials for various applications. - All of them are linked to their biocompatibility, as monomers can be released as a result of these challenges.
What is the processes of making the denture base polymerisation?
- Chemically activated (self-curing)
- Thermally activated
- Photoactivated
- Activated by microwave energy
- 3D printed
- Milled
What is an example of chemically cured acrylic resin?
- Methyl methacrylate (MMA)
o Clear colourless liquid of intense odour
o Ester of methacrylic acid: CO-O-CH3
o Stabilised by hydroquinone or hydroquinone (derivatives)
o Polymerizable C=C double bond
o Boiling point – 100 degrees Celsius
o Density – 0.943 g/cm3 at 20 degrees Celsius
o Vapour pressure – 38kPa at 20 degrees Celsius
.
o Molecular weight – 100g/mol
o Odour threshold – 0.2ppm
What is the 5 sequential stage of heat cured resin?
o Activation – generation of free radicals by cleavage of an initiator molecule (by either heat or mixing of reactants) e.g. benzoyl peroxide
o Initiation – free radical plus monomer = activated monomer (attack of the first radicals on the monomer molecules).
o Propagation – (polymer) free radical + monomer = chain (the auto-accelerating growth of each linear polymer chain).
o Termination – (polymer) free radical + free radical … = polymer chain (until there are no more free radicals). The reaction comes to a stop by either various kinds of radical-annihilation or by inhibition of segmental movement.
o Chain transfer – inactive molecules become reactive.
What is the stages of processing?
- Use a wax pattern.
- Need to make sure fill it up in sections, or else teeth will move positions and fall out, ruining the denture.
- Then you open it and remove the wax.
- Dough moulding
- Flasking
- Deflasking after boiling.
- Mixing of powder and liquid (ratio P/L wt%, 1.6-1 vol%) cause monomer diffusion and softening of the surface of the powder producing the following gelling stages:
o Stage 1 – Sandy (initial melting of beads). 5 minutes it is elastic.
o Stage 2 – stringy or sticky (entanglements with swollen beads and thickened interstitial monomer)
o Stage 3 – dough (gelation – useful). This stage determines the working time – 5 minutes.
o Stage 4 – rubbery (monomer penetrates to the core of beads, plasticising them).
What is heat cured acrylic resin?
- When heated above 60 degrees Celsius, molecules of benzoyl peroxide decompose to yield electrically neutral species containing unpaired electros; free radicals which initiate the chain growth
- Heat is termed as the activator and is applied to the materials by immersing a flask in a water bath.
What are the different polymerisation cycles?
- Long cycle:
o Temperature – 74 degrees Celsius for 8 hours, without reaching 100 degrees Celsius. - Short cycle:
o Temperature – 74 degrees Celsius for 2 hours, later rising to 100 degrees Celsius for another 1 hour. - End of cycles:
o The muffle is cooled to room temperature (30 min) and then placed in room temperature water (15 min).
What are some denture failures?
- Denture fracture:
o Repairing dentures costs the NHS about £9 million/year
o 68% of dentures fracture within the first 3 years. - PMMA disadvantages:
o Poor impact strength
o Poor thermal conductor.
What does polymer shrinkage lead to?
- Porosities
- Temperature above the boiling point of the monomer (100.8 degrees Celsius).
- Inadequate mixing
What are some examples of types of porosity in acrylic?
o Gaseous – monomer has boiled inside the mix. Causes fine bubbles, especially in thicker sections
o Granular – insufficient monomer was added to the powder
o Shrinkage porosity – irregular voids on surface and throughout the material. Too low a pressure or insufficient material in the flask.
-
Revision of endodontic principles27
-
Success/failure in endodontic treatment12
-
Restoring endodontically treated teeth26
-
Extra coronal restorations - crowns and laboratory stages17
-
Principles of tooth preparation16
-
Shade selection in dentistry26
-
Impression techniques and soft tissue management27
-
Extra-coronal restorations - treatment planning17
-
Temporary crowns18
-
Elastomeric impression materials37
-
Crown fitting and cementation36
-
Periapical pathology33
-
Medical devices (MDR) regulations23
-
Jaw movements and articulators36
-
The TMJ32
-
An introduction to partial dentures 131
-
Materials for cementation31
-
Partial dentures 2 - clinical and laboratory stages34
-
Partial dentures 3 - partial denture design34
-
Partial dentures 4 - impressions and recording the occlusion35
-
Partial dentures 5 - anterior saddles and aesthetics20
-
Fixed bridges25
-
Partial dentures 6 - managing free-end saddles, try in and fit22
-
Partial dentures 7 - Denture polymers27
-
An introduction to oral surgery30
-
Fascial spaces and the spread of infection30
-
Clinical occlusal investigations12
-
Occlusal factors in restoration failures9
-
Complications of extractions in oral surgery32
-
Abnormal bleeds21
-
Inlays/onlays31
-
Materials for crown and bridgework25
