Denture base materials Flashcards
What are the ideal properties of a denture base material (or any dental material)
Biocompatible Aesthetic Hygienic (Resistant to bacterial contamination) Dimensionally stable High Strength, stiffness, hardness and toughness High thermal conductivity Low density Cost Ease of: Processing Repair/adjust Reproduction of surface detail Radiopacity
What is the most common denture base material?
polymethyl methacrylate
What molecule starts the process of polymerisation?
benzoyl peroxide (initiator) - This molecule readily splits into two, each fragment having one unpaired electron. These fragments attack the C=C bond starting the polymerisation
What does the cross linking agent do?
This forms addition bonds between polymer chains resulting in improved mechanical and physical properties.
-Diethylene glycol dimethacrylate
What processing problems can occur?
Porosity
Polymerisation shrinkage
Processing strains
How does porosity in the acrylic occur?
Volatisation of monomer during curing if the temperature is raised too quickly.
This is because the reaction is exothermic and the monomer has a relatively low boiling point.
How would you solve porosity?
Use a slow heating cycle when curing and keep under pressure.
What can polymerisation shrinkage do to the denture?
Can result in the denture not contacting the hard palate or porosity
- instability/poor fit
How would you solve polymerisation shrinkage?
- Use polymer beads to reduce the necessary polymerisation to a minimum
- Keep denture under pressure and use a slow cool when curing
- Incorporate a post dam on to the denture to compensate
Why may processing strains arise?
These may arise due to dissimilar materials being used (e.g.ceramic teeth or CoCr components)
Also if the polymer is cooled too quickly
What alternative processing routes exist?
‘Injection molding’ of acrylic dough
- Injection molding of acrylic above Tg (glass transition temperature)
- Milling from block
What happens in true injection moulding?
Heat material to above to above Tg
Squeeze into mould using high pressure
- can use PMMA, polyamide, PEEK etc
What is the composition of PMMA powder?
Polymethyl methacrylate granules Initiator Pigments, Dyes & Opacifiers Plasticisers Synthetic fibres (nylon)
What is the composition of PMMA liquid?
Methyl methacrylate monomer
Inhibitor (hydroquinone)
Cross linking agent
What variants of PMMA exist?
Cold cure - auto cure - mainly used for repairs or attaching teeth to a CoCr RPD
- high impact Contains copolymer of butadiene and styrene . Results in a dispersion of rubber inclusions
- colours
- soft-liners
What are the negatives of cold cure?
Has a lower molecular weight more porosity
Therefore is less dense, softer, weaker and is prone to discolouration
Why may adverse reactions to denture materials occur?
Traces of monomer remain in denture base after processing - diffuse out and cause problems - delayed hypersensitivity
- irritant contact dermatitis
Why may bacteria grow on the fitting surfaces?
cant polish surface of fitting surface - lose retention - can encourage bacterial growth on rough surfaces
What are the indications for flexible material?
repeated denture fractures large undercuts occur presence of large tori makes use of rigid matrerials challenging patients cant open wide enough single maxillary denture
What are the advantages of flexible materials?
-
Why might you reline a denture?
carried out when denture becomes ill fitting due to bone resorption using PMMA
- can also use cold cure resin if this is a temporary measure or procedure carried out on an RPD
How would you reline a denture?
- remove undercuts and periphery take impression send lab, model is cast vertical dimension recorded impression material replaced with wax usual processing to replace wax with acrylic
What are tissue conditioners?
Temporary linings that allow traumatised tissue to recover before carrying out definitive treatments
