Calcium Hydroxide and Zinc Oxide Based Materials

Question 1

What makes up a calcium hydroxide liner

Answer 1

2 pastes, a base and a catalyst

Question 2

What does the base of a calcium hydroxide liner consist of and what are the purposes of its constituents

Answer 2

Calcium hydroxide 50%
Zinc oxide (filler) 10%
Zinc Stearate (filler) <1%
N-ethyl toluene sulphonamide (plasticiser) 40%

Question 3

What does the catalyst of a calcium hydroxide liner consist of and what are the purposes of its constituents

Answer 3

Butylene glycol disalicylate (reactive element) 40%
Titanium Dioxide (filler) 13-14%
Calcium Sulphate (filler) 30%
Calcium Tungstate (filler and radiopaquer) 15%

Question 4

Describe the setting reaction of a CaOH liner

Answer 4

A chelation reaction between the ZnO and the butylene glycol disalicylate
This results in a cement with an initial pH of around 12

Question 5

Describe the mode of action of a CaOH liner

Answer 5

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

Question 6

What are the properties of a CaOH liner

Answer 6

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

Question 7

What are the different types of zinc oxide based cements

Answer 7

Zinc Phosphate
Zinc Polycarboxylate
Zinc Oxide Eugenol (ZOE)
Resin Modified ZOE
Ethoxybenzoic acid (EBA) ZOE

Question 8

Describe zinc phosphate cement

Answer 8

Acid base reaction
Powder and liquid
Easy to use
Cheap

Question 9

What does the powder of zinc phosphate cement consist of and what are the purposes of its constituents

Answer 9

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

Question 10

What does the liquid of zinc phosphate cement consist of and what are the purposes of the constituents

Answer 10

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

Question 11

Describe the reactions that zinc phosphate cement undergoes

Answer 11

The initial acid base reaction followed by a hydration reaction resulting in the formation of a crystallised phosphate matrix - hopiete

Question 12

Describe the matrix formed in zinc phosphate cement

Answer 12

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

Question 13

What are the problems with zinc phosphate cement

Answer 13

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

Question 14

Describe zinc polycarboxylate cement

Answer 14

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

Question 15

What are the problems with zinc polycarboxylate cement

Answer 15

Difficult to mix
Difficult to manipulate
Soluble in oral environment at lower pH
Opaque
Lower modulus and compressive strength than zinc phosphate

Question 16

When are zinc oxide eugenol (ZOE) cements used

Answer 16

Linings/base in deep cavities under amalgam
Temporary restorations - Resin modified or EBA ZOE
Root canal sealer - slow sealer
PDL dressings - fast settings

Question 17

Describe the reaction of ZOE cement

Answer 17

Acid base reaction
Acid is Eugenol
Base is ZnO
Chelation reaction to form zinc eugenolate matrix
The matrix bonds the unreacted ZnO particles

Question 18

Describe the properties of ZOE

Answer 18

Adequate working time
Rapid setting time
Low thermal conductivity
Low strength around 20MPa
Radiopaque
High solubility

Question 19

Why does ZOE have a low strength and what does this mean for its use

Answer 19

Weak hydrogen bonds between the eugenolate molecules

Not strong enough to use as a base beneath an amalgam filling

Question 20

Why does ZOE have a high solubility and what are the pros and cons of this

Answer 20

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

Question 21

Why should ZOE not be used under composite resin

Answer 21

The release of Eugenol inhibits the set of resin based filling materials
It softens them and causes discolouration

Question 22

Why are resins added to ZOE

Answer 22

Increase the compressive strength to >40MPa making it suitable as a cavity lining
Greatly decreases solubility

Question 23

Which resins are added to ZOE

Answer 23

Polymethylmethacrylate in IRM

Polystyrene in Kalzinol

Question 24

What does the powder in Ethoxybenzoic acid ZOE consist of

Answer 24

ZnO 65%
Quartz or Alumina 35%
Hydrogenated rosin 6%

Question 25

What does the liquid in EBA ZOE consist of

Answer 25

Eugenol 37%

Ethoxybenzoic acid 63%

Question 26

Describe the properties of EBA ZOE

Answer 26

Stronger than ZOE or resin modified ZOE at around 60MPa

Less soluble