Lecture 5 Glass Ionomer Cements Flashcards
1
Q
What are Glass Ionomer cements?
A
- Developed 30 years ago by Alan Wilson and John McClean as extension of silicate based cements.
- Also known as glass polyalkenoate
- Acid-base reaction concept: Acid + Base -> Salt + water
- Water soluble polymer system poly-(acrylic acid)
- Base is composed of fluoro-alumino-silicate (FAS).
2
Q
Describe Glass composition
A
- Fluoro-alumino-silicate (FAS) based glasses
- Glass composition melted then quenched into water to retain glass and to prevent crystallisation also to assist with crushing.
- Typical composition: silicon dioxide 30%, aluminium oxide 20%, calcium fluoride 33%, sodium fluoride 4%, aluminium fluoride 3%, aluminium phosphate 10% (by weight%)
- Some companies replace calcium fluoride with strontium fluoride (same ionic radius) that is with strontium fluoride. More radio-opaque.
Strontium fluoride stops growth of bacteria.
3
Q
Describe the polymer component
A
- Water soluble polymer originally poly-(acrylic acid)
- Currently copolymer of acrylic acid with itaconic or maleic acid
- Referred to as poly-alkenoic acid. These are long chain polymers of a few thousand units. If too long then too viscous.
- All contain carboxyl group - COOH
- Inclusion of tartaric acid delays the setting reaction before the viscosity of the mix rises.
- Allows for faster setting once the viscosity increases.
- the tartrate ions sequester the first metal released from the glass and prevent cross-linking of the poly-acid chains.
4
Q
Describe setting reaction of GIC
A
- Two main metal ions are released from glass (powder) by acids (in the mixing liquid): calcium and aluminium.
- Calcium initially has faster setting -> no hydrolysis of covalent bonds.
- Aluminium much slower -> hydrolysis of covalent bonds.
5
Q
Describe acid-base reaction
A
- Poly-(acrylic acid) is the acid component that ionises into hydrogen ions and polyalkenoate anions.
Glass + H+ => calcium ion + SiO4(4-)
- Formation of the silica gel and the calcium ion are chelated by the polyacid chains.
- The set cement consists of an agglomeration of unreacted powder surrounded by a silica gel in an amorphous matrix of hydrated calcium and aluminium polysalts.
- The FAS glass is the source of the base. The outer layer of the glass particles are decomposed releasing calcium ions, aluminium ions and fluoride ions etc.
- Residual silica gel layer on glass particles.
6
Q
Describe setting reactions
A
- Release of anions from the glass
- Initially release of calcium ions. This is followed by aluminium and fluoride ions.
- Release of ions determines the reaction kinetics with the polyalkenoic anions.
- Rate of the reaction also determined by the size of glass particles. Smaller particles are more reactive and can promote a faster set.
7
Q
Describe bonding to enamel and dentine
A
- Two chemical reactions possible with enamel and mineral component of dentine.
- Hydrogen bonding and ionic bonding. The latter via calcium ions in the apatite.
- This suggests that hard etching with phosphoric acid is not required. Simply conditions the surface with polyalkenoic acid to achieve bonding prior to placing GIC mass.
- Some suggestions that bonding with collagen is possible but this has not been proven yet.
8
Q
Describe GIC’s water and shrinkage
A
- Water plays a critical role with acid-base systems as it is the medium for ion transport.
- Volumetric shrinkage for conventional GIC’s is small provided they are kept moist. If the humidity drops below 80%, if they lose water and crazing of the material occurs. Prevent with sealing varnish.
- Too much water initially leaches out the ions (aluminium and calcium) which cross-link the poly-alkenoic chains thus strength reduces.
- The setting cement should be protected from water (saliva).
- The set cement should be protected from loss of water (isolated outside rubber dam for too long, mouth breathers).
- Generally GICs are not advised to cement ceramic crowns since slight absorption of water may create high stress.
9
Q
Describe GIC materials
A
- Wide range of material system are available for GIC’s
- Luting cement
- Fissure sealant
- Occlusal surface in primary dentition
- Cermet-GIC’s, which contain silver or amalgam (5 to 10 um diameter) particles.
10
Q
Describe Resin-modified GIC
A
- Strength of conventional GIC somewhat low, hence the wish to include resin to increase strength and also to command set with lamp.
- Typical resins include hydroxyethyl methacrylate (HEMA) or Bis-GMA together with a photo-initiator such as camphorquinone.
- Alternative is polyalkenoic acid with pendant methacrylate group.
11
Q
Compare the mechanical properties of GIC’s with composite resins
A
Compressive strength (MPa):
- GIC 200-220
- RMGIC 250
- Composite 250-350
Tensile strength (MPa):
- GIC 10-20
- RMGIC 30-40
- Composite 50-60
Tooth bonded conventional GIC’s often fail in a cohesive manner.
- Conventional GIC’s are cariostatic because of fluoride ions released but not anti-bacterial.
- GIC’s have lower hardness and higher wear rate than filled composites. Thermal expansion of unconventional GIC’s lower (11x10^-6 /C), than composites (20 to 40 x10^-6/C). RMGIC is same as the composite.
12
Q
Will GIC stabilise a carious lesion of a primary tooth? That is, will bacteria grow near the restoration?
A
Bacteria do not grow in high fluoride ion areas.
13
Q
GIC provide an excellent prevention of secondary caries
A
True
14
Q
Describe Cements
A
Intermediate restorative materials are agents for pulp protection.
Materials placed in dentine in close proximity to the pulp:
- Permanent placement, not a temporary filling material.
- Cavity varnishes, liners and bases (thicker)
15
Q
Define working time vs setting time
A
- Working time: time from the start of mixing until the time at which the viscosity of the mix is low enough to flow under pressure.
- Setting time: time period during which the matrix formation has reached a point at which external physical forces will not cause permanent dimensional changes.
