DML2: Acid-base cements Flashcards
What are the general uses of acid-base dental cements
- Cavity liner/base (used in deep caries coming close to the pulp or placed beneath permanent restorations)
- Permanent or temporary fillings
- Root canal sealer
- Permanent or temporary luting of crowns or bridges
- Attachment of orthodontic appliances
What are the ideal properties of acid-base cements
- non irritant and non toxic
- insoluble in oral fluids
- adhesive to tooth
- adequate mechanical properties to withstand mastication
- thermally, chemically and electrically insulating
- providing therapeutic effect (bacteriostatic and obtundent)
- good aesthetics
- thermal expansion coefficient similar to tooth
- radiopaque
- should be easy to manipulate
Define an acid-base cement
Formed on mixing the powder and liquid through which an acid-base reaction will produce a solid matrix which binds the mass together
What is the consistency of the set cement like
This consists of a cored-structure; the unreacted powder acts as a filler and this increases the strength of the cement
What factors affect the setting of acid-base cements
- Particle size; smaller = larger SA = faster set
- Powder/liquid ratio; more powder = faster set
- Temperature; higher = sets faster; because the reaction is exothermic, as it progresses it becomes faster
- Moisture; accelerates/slows depending on the cement
Classify the acid-base cements by type (6)
- ZInc phosphate cements
- Zinc polycarboxylate cements
- Glass ionomer cements
- Zinc oxide eugenol cements
- Calcium hydroxide cements\
- Ethoxybenzoic acid cements
Classify the acid-base cements by application
Type I
= luting, particle size = 25um
- this is used for veneers, crowns and bridges because it is thinner
Type II
= restorative/lining, particle size = 40um
Type III
= lining or base, particle size = 40um
Classify the acid-base cements by chemical bonding
- Phospate bonded
- zinc phosphate cements - Polycarboxylate bonded
- zinc polycarboxylate cements
- glass ionomer cements - Phenolate boned
- calcium hydroxide cements
- zinc oxide eugenol
- ethoxybenzoic acid based cements
Classify the acid base cements by liquid
Water based
- zinc phosphate
- zinc polycarboxylate
- GIC
Oil based
- calcium hydroxide
- ZOE
- EBA
Classify the acid-base cements by powder
Zinc oxide
- zinc phosphate
- zinc polycarboxylate
- ZOE
- calcium hydroxide
- EBA
Ion leachable glass
- GIC
What is the powder component of zinc phosphate cements
90% ZnO
10% other oxides; MgO, Al2O3, SiO2
Why is zinc oxide used in cements
- Low cost
- Heavy metal so good radiopacity and whiteness
- Low toxicity ad body can tolerate zinc in excess
- Reactivity can be controlled
How can zinc oxide powder be modified to control reactivity in acid base cements
It can be heated meaning that oxygen ions are lost and the structure becomes zinc rich; this slows down the reaction to give a longer mixing time so the desired properties can be achieved
What does MgO in the powder component of zinc phosphate cements provide
Increased strength
Whiteness
What does Al2O3 and SiO2 in the powder component of zinc phosphate cements provide
Mechanical reinforcement
What is the role of SnF2 in cements
Provides short term fluoride release
What is the liquid component of zinc phosphate cements
Phosphoric acid (45-60% aqueous solution) Partially neutralised with Al3+ and Zn2+ ions to slow the setting reaction
What does the addition of water to zinc phosphate cements result in and as a result, what precautions should be taken
Accelerated setting
As the phosphoric acid is hygroscopic and easily absorbs moisture, the bottle should be tightly closed otherwise the concentration will be changed which can affect setting time and end properties
Outline the setting reaction occurring in zinc phosphate cements
- ZnO + 2H3PO4 –> Zn(H2PO4)2 + unbound H2)
- ZnO + Zn(H2PO4) + 2H20 –> Zn(PO4)2.4H2O
Formation of hopetite occurs because there is unreacted ZnO powder
How is zinc phosphate cement mixed
Setting reaction is exothermic so a cool mixing slab is used to incorporate the powder in small incriments (upto 16 parts)
- mixing time = 90-120 seconds
What are the strengths of zinc phosphate cements (7)
- Adequate strength
- Sharp set
- Rapid hardening (strength gained in 3hrs)
- Good retentive properties (by mechanical interlocking)
- Good thermal insulator so could be used as cavity base
- Cheap
- SnF2 provides short term fluoride release
What are the disadvantages of zinc phosphate cements (7)
- Low initial pH causing enamel solubility and pulp irritation
- Slow to neutralise
- Weaker when it contains SnF2
- Brittle (low tensile strength of 5-7MPa)
- High acidic solubility
- Linear set shrinkage
- No chemical adhesion (only mechanical)
What is the primary and secondary use of zinc phosphate cements
- Luting cement for restorations and orthodontic appliances due to adequate strength
- Thermal insulating base under amalgams or as a temporary restorative (not usually chosen due to high acid solubility)
What is the powder composition of zinc polycarboxylate cements and the reasons for their incorporation
- ZnO
- MgO, SnO, Al2O3, SiO2 = mechanical reinforcement
- Bismuth salts = radiopacity and modifies set
- SnF2 or tannin fluoride = fluoride release, improved mixing and strength
