DML3: Glass ionomer cements Flashcards
What is the composition of GICs
- Ion leachable ionomer glass
2. SiO2-Al2O3-CaF2/SrF2 basic components
Why is the ratio between SiO2 and Al2O3 important
Because it governs reactivity and how fast the cement will form
What components are added to GICs for radiopacity
Sr2+
Ba2+
La3+
What components are added to GICs to boost fluoride release
Na3AlF6, AlF3, NaF
What components in GICs provide phosphate and why are they needed
P2O5, AlPO4 ; governs reactivity
How is the ion leachable ionomer glass prepared for GICs
- Melted at high temperature (1150-1450)
- Rapidly cooled
- Ground into powder
- Acid washed to control reactivity
Why is ion leachable ionomer glass acid washed
Because it controls the reactivity making it easier to mix the cement at the chair side
What is the liquid composition of GIC
- Polyacrylic acid = PAA, 50% aqueous solution
- or a copolymer of acrylic and itaconic acid
- or other polycarboxylic acids e.g. polymaleic acid - Tartaric acid, 10% aqueous solution
How is the liquid composition of GIC treated
Both acids are freeze dried and mixed with the ionomer glass powered, this means that the liquid composition is the water added
How do alkenoic acids modify the strength of GICs
These are aliphatic carboxylic acids - the strength of the cement can be increased by having more than 2 COOH groups
This is because there will be increased cross-linking which enabled higher strengths of the final set
What are the stages involved in the GIC setting reaction
Sets by an acid-base chemical reaction
- Dissolution stage
- Gelation and hardening stage
- Final maturation
What happens in the dissolution stage of GIC setting reaction
Protons dissociate in the presence of water
H+ ions from the poly acid attack the glass to liberate Ca2+ and Al3+ (as well as Na+ and F- and released silicon forms Si(OH)4)
These start forming complexes with tartaric acid and fluoride which are stable up to a set pH
What is the action of tartaric acid in GIC during the setting reaction
Tartaric acid complexes with the liberated ions from the ionised glass and this holds the cement forming ions to increase the working time until the acid is partially neutralised. The ions will then be released from the tartaric acid to five a sharp set
Why does tartaric acid bind cations rather than poly acrylic acid
Because it is stronger
What happens in the gelation and hardening stage of GIC setting reaction
Polymer chain entanglement and cross-linking occurs by Ca2+ initial set and Al3+ final set which will cross link the COOH groups in the polymer chains
This forms hydrated Ca- and Al- polyacrylates
What is the difference in properties produced by Ca2+ and Al3+
Ca2+ = mobile ion, weak bonding, water soluble polyacrylate
Al3+ = less mobile, strong bond, insoluble polyacrylate
What is gelation
It is the formation of cross links via ionic bonds to give a gel like consistency
- Ca2+ gives divalent bonds
- Al3+ gives trivalent bonds causing the cement to harden
What happens in the final maturation stage of the GIC setting reaction
The ratio of bound to unbound water increases which increases strength
Why must the GIC be protected immediately after placement
Because it can take unto 24hours for the final maturation stage where the water is bound to increase strength of the cement
What does early water exposure to GIC result in
Dissolution of reactive components leading to poor properties
What does dehydration of GICs result in
Leads to loss of water which is critical for the continuation of the setting reaction leading to poor properties
Why does the tooth surface have to be conditioned before placement of GIC and how is this done
It removes the smear layer to make the surface chemically active and ready to receive the GIC, this is done by application of PAA solution (10-30%)
How does the GIC adhere and chemically bond to the tooth (3)
Once the smear layer has been removed by PAA, there is chelation of Ca2+ from HAP with COO- from the PAA within the GIC forming a strong ionic bond
There is hydrogen bonding with the amino groups
There is ion exchange between the tooth structure and the cement to form an ion rich interfacial layer
Describe the patters of fluoride release from GICs
- initially high level release from exposed glass particles
- long term lower level release from deeper areas (diffusion controlled)
- uptake of fluoride by enamel and dentine leads to HAF production which inhibits demineralisation
The main source of F- is the unreacted glass particles within GIC
