Calcium Hydroxide and Zinc Oxide Based Materials Flashcards
What makes up a calcium hydroxide liner
2 pastes, a base and a catalyst
What does the base of a calcium hydroxide liner consist of and what are the purposes of its constituents
Calcium hydroxide 50%
Zinc oxide (filler) 10%
Zinc Stearate (filler) <1%
N-ethyl toluene sulphonamide (plasticiser) 40%
What does the catalyst of a calcium hydroxide liner consist of and what are the purposes of its constituents
Butylene glycol disalicylate (reactive element) 40% Titanium Dioxide (filler) 13-14% Calcium Sulphate (filler) 30% Calcium Tungstate (filler and radiopaquer) 15%
Describe the setting reaction of a CaOH liner
A chelation reaction between the ZnO and the butylene glycol disalicylate
This results in a cement with an initial pH of around 12
Describe the mode of action of a CaOH liner
Bactericidal to cariogenic bacteria as it is alkaline
The cement causes irritation to the odontoblast layer causing necrosis which results in a layer of tertiary dentine being produced. This eventually forms a calcified bridge walling the base of the cavity off from the pulp - the calcium comes from the pulp not the cement
What are the properties of a CaOH liner
Quick setting time Radiopaque Easy to use Low compressive strength Unstable and soluble - if the cavity leaks then the lining will disappear, it may even disappear as it is in contact with moist dentine
What are the different types of zinc oxide based cements
Zinc Phosphate Zinc Polycarboxylate Zinc Oxide Eugenol (ZOE) Resin Modified ZOE Ethoxybenzoic acid (EBA) ZOE
Describe zinc phosphate cement
Acid base reaction
Powder and liquid
Easy to use
Cheap
What does the powder of zinc phosphate cement consist of and what are the purposes of its constituents
Zinc oxide >90% main reactive ingredient
Magnesium dioxide <10% gives white colour and increases compressive strength
Other oxides (Alumina and Silica) improves physical properties and alters shade of set material
What does the liquid of zinc phosphate cement consist of and what are the purposes of the constituents
Aqueous solution of phosphoric acid 50%
Oxides which buffer the solution:
Aluminium oxide - ensures even consistency of set material
Zinc oxide - slows the reaction giving better working time
Describe the reactions that zinc phosphate cement undergoes
The initial acid base reaction followed by a hydration reaction resulting in the formation of a crystallised phosphate matrix - hopiete
Describe the matrix formed in zinc phosphate cement
The matrix is almost insoluble but it is porous and contains free water from the setting reaction
The cement subsequently matures binding this water leading to a stronger, less porous material
What are the problems with zinc phosphate cement
Low initial pH of approx 2 which can cause pulpal irritation as pH can take 24 hours to return to neutral Exothermic setting reaction Not adhesive to tooth or restoration Not cariostatic Final set takes 24 hours Brittle Opaque
Describe zinc polycarboxylate cement
Similar to zinc phosphate but phosphoric acid is replaced by polyacrylic acid
Bonds to tooth surfaces in a similar way to GICs
Less heat of reaction
pH is low to to begin with but pH returns to neutral more quickly
Cheap
What are the problems with zinc polycarboxylate cement
Difficult to mix Difficult to manipulate Soluble in oral environment at lower pH Opaque Lower modulus and compressive strength than zinc phosphate
When are zinc oxide eugenol (ZOE) cements used
Linings/base in deep cavities under amalgam
Temporary restorations - Resin modified or EBA ZOE
Root canal sealer - slow sealer
PDL dressings - fast settings
Describe the reaction of ZOE cement
Acid base reaction
Acid is Eugenol
Base is ZnO
Chelation reaction to form zinc eugenolate matrix
The matrix bonds the unreacted ZnO particles
Describe the properties of ZOE
Adequate working time Rapid setting time Low thermal conductivity Low strength around 20MPa Radiopaque High solubility
Why does ZOE have a low strength and what does this mean for its use
Weak hydrogen bonds between the eugenolate molecules
Not strong enough to use as a base beneath an amalgam filling
Why does ZOE have a high solubility and what are the pros and cons of this
Eugenol is constantly released
Eugenol is replaced by water which causes disintegration of the material
Eugenol when liberated has an obtundant effect on the pulp and can reduce pain
Why should ZOE not be used under composite resin
The release of Eugenol inhibits the set of resin based filling materials
It softens them and causes discolouration
Why are resins added to ZOE
Increase the compressive strength to >40MPa making it suitable as a cavity lining
Greatly decreases solubility
Which resins are added to ZOE
Polymethylmethacrylate in IRM
Polystyrene in Kalzinol
What does the powder in Ethoxybenzoic acid ZOE consist of
ZnO 65%
Quartz or Alumina 35%
Hydrogenated rosin 6%
What does the liquid in EBA ZOE consist of
Eugenol 37%
Ethoxybenzoic acid 63%
Describe the properties of EBA ZOE
Stronger than ZOE or resin modified ZOE at around 60MPa
Less soluble
