Cavity linings Flashcards
what are the disadvantages of restorations
- May not make intimate contact with the tooth surface (especially dentine) and any gap may allow ingress of fluids and bacteria
- Heat is released during setting/curing
- Release of chemicals that may be pulpal irritants and lead to pain or pulpal damage
what is the solution to the disadvantages
The solution to these disadvantages is using an intermediate restorative material - a lining material prevents gaps and acts as a protective barrier
what is a cavity base
A cavity base is a thick mix of material that is placed in bulk. A definition of a cavity base is a dentine replacement used to minimize the bulk of the restoration or black out undercuts
what is a cavity liner
A cavity lining is a thin coating (<0.5mm) which is placed over exposed dentine. A definition of a cavity liner is a dentine sealer able to promote the health of the pulp by adhering to the tooth structure or by an anti-bacterial action
what is the purpose of a liner
provide pulpal protection
therapeutic
palliative
what does a liner protect the pulp from
chemical stimuli
thermal stimuli
bacteria and endotoxins
what is the chemical stimuli that can harm the pulp
o Chemical stimuli from unreacted chemicals in the filling material or the initial pH of the filling as some materials have an adverse pH
what is the thermal stimuli that can harm the pulp
it can protect from the heat of some setting reactions which are exothermic e.g composite or heat conducted through metal fillings
what is the bacteria and endotoxins that can harm the pulp
microleakage (the penetration of oral fluids and bacteria and their toxins between the restorative and the cavity walls. It does this by providing an impermeable bond which bacteria cannot pass through
how are cavity liners therapeutic
calm down inflammation within the pulp and promote pulpal healing prior to or at the time of a permanent restoration being placed
how are cavity liners palliative
o Reduce patient symptoms prior to definitive treatment being carried out – it is most commonly used in patients with reversible pulpitis
o Rather than putting them under the filling you put them in on their own
what are the properties of lining materials
- Easy to use
- Thermal properties
- Mechanical properties
- Radiopaque
- Marginal seal
- Solubility
- Cariostatic
- Biocompatible
- Compatible with restorative material
describe ease of use
- It should be easy to mix
- The working time should be long to allow easy placement
- The setting time should be short and ideally command set
what are the thermal properties
thermal conductivity
thermal expansion coefficient
thermal diffusivity
what is thermal conductivity
o The thermal conductivity is how well heat energy is how well heat energy is transferred through a material
o It is the heat flow through a cylinder of unit cross-sectional area and unit length, with a temperature difference of 1 degrees celcius between the ends
o The units are W/m-1/degrees celcius-1
what should thermal conductivity be
o It should be high for a denture base so that patients can tell whether what they are eating is hot or cold
o It should be low for a restorative material
o It should be as low as possible for a cavity lining as the whole point of the cavity lining is to protect the pulp from thermal stimuli, so we want the poorest thermal conductivity
what is thermal expansion coefficient
o Thermal expansion is change in length per unit length for a temperature rise of one degree Celsius
o Units are ppm degrees celcius-1
what should thermal expansion coefficient be
similar to dentine
what are the thermal expansion coefficients
o It is 11 for GIC
o 20 for RMGIC
o 25 for amalgam
what is thermal diffusivity
o Similar to the conductivity
o Measured in cm2/sec
what should thermal diffusivity be
o Ideally at least as low as tooth if not lower
Enamel is 0.0042
Dentine is 0.0026
what is the thermal diffusivity of liners
o all commercially available liners have similar or lower thermal diffusivity than tooth enamel
what are the mechanical properties of liners
• High compressive strength to allow placement of filling without breaking it
o Dentine has a compressive strength of around 275MPa, closer the values are to the tooth the better
• We want a modulus similar to dentine – around 15GPa – so that it moves the same way the tooth does
what are the radiopacity of liners
- Should be easy to see the difference between lining and tooth as this makes it easier to see if there is leakage or secondary caries
- If it is radiolucent then the restoration can look like caries and we cannot see if there is caries or if it is the lining
how is the marginal seal of liners
- Ideally the lining should form a chemical bond to dentine
* This bond should be permanent and impenetrable
what do we want the solubility of liners to be
• We want it to be low as we do not want it to dissolve away
what do we mean when we want a liner that is cariostatic
• We want it to be fluoride releasing and antibacterial – this is important in preventing secondary caries around the restoration
what do we mean by biocompatibility
• We want it to be non-toxic • Not damaging to the pulp by o Having a neutral pH o No excessive heat during setting • Low thermal conductivity
what are the types of intermediate materials
- Setting calcium hydroxide: liner
- Zinc oxide based cements: base
- Glass ionomer and Resin Modified Glass Ionomer cements: base or liner
- Palliative cements: base (seldom used nowadays, mainly historic)
what is setting calcium hydroxide liner
- Consists of two pastes
* Examples include Life, Dycal
what are the constituents of setting calcium hydroxide liner
base
catalyst
what is in the base of setting calcium hydroxide liner
Calcium hydroxide 50% - active ingredient
Zinc oxide (filler) 10%
Zinc stearate (filler) <1%
N-ethyl toluene sulphonamide (plasticizer)
what is in the catalsyst of setting calcium hydroxide liner
Butylene glycol disalicylate (reactive element) 40%
Titanium dioxide (filler) 13-14%
Calcium sulphate (filer) 30%
Calcium tungstate (filler and radiopaquer) 15%
what is the setting reaction of setting calcium hydroxide liner
• The setting reaction is a chelation reaction between the ZnO and the butylene glycol disalicylate - this results in a cement with an initial pH of around 12
what is the mode of action of CaOH liner
o It is bactericidal to cariogenic bacteria as cariogenic bacteria survive in acidic environment and the highly alkaline liner kills them
o Irritation – the cement causes irritation to the odontoblast layer, necrosis follows which in turn results in a layer of tertiary dentine being produced. This eventually forms a calcified bridge walling the base of the cavity from the pulp (the pulp comes from the pulp not the cement)
what are the good properties of CaOH liner
o Quick setting time
o Radiopaque
o Easy to use
what are the bad properties of CaOh liner
o Low compressive strength
o Unstable and soluble meaning if the cavity leaks then the lining will disappear and may even disappear just because it is in contact with moist dentine
what are the zinc oxide based cements
- Zinc phosphate
- Zinc polycarboxylate
- Zinc oxide eugenol (ZOE)
- Resin modified ZOE
- Ethoxybenzoic acid (EBA) ZOE
what is zinc phosphate cement made up of
powder and liquid
powder is base and liquid is acid
what is the powder made up of in zinc phosphate
o Zinc oxide >90% - the main reactive ingredient
o Magnesium dioxide <10% - it gives the white colour and increases compressive strength
o Other oxides (alumina and silica) – improve physical properties and alter shade of set material
what is the liquid made up of in zinc phosphate
o Aqueous solution of phosphoric acid (approximately 50%)
o Oxides which buffer the solution which are:
Aluminium oxide – ensures even consistency of set material
Zinc oxide – slows the reaction giving better working time
what is the initial reaction in zinc phosphate cement
The initial reaction is acid/base
o ZnO + 2H3PO4 > Zn(H2PO4) + H2O
what is the second reaction in zinc phosphate cement
The following reaction is a hydration reaction resulting in the formation of a crystalized phosphate matrix
o ZnO +Zn(H2PO4)2 + 2H2O Zn3(H2PO4)2.4H2O
what happens after the second reaction in zinc phosphate cement
The aluminum oxide prevents crystallization leading to an amorphous glassy matrix of the acid salt surrounding unreacted ZnO powder. This 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. The final material consists of unreacted powder particles held together by a salt matrix.
what is zinc phosphate cement also used for
It is also used for cementation of inlays, crowns, bridges and orthodontic appliances and occasionally as a temporary restoration
what are problems with zinc phosphate cement
- Has a low initial pH of approximately 2 which can cause pulpal irritation as the pH can take 24 hours to return to neutral
- Exothermic setting reaction
- Not adhesive to tooth or restoration – retention may be slightly micromechanical due to surface irregularities of cavity
- Not cariostatic
- Final set takes 24 hours
- Brittle
- Opaque
what is zinc polycarboxylate cement
They are a similar material to the zinc phosphate cement but the phosphoric acid has been replaced by polyacrylic acid.
what are the advantages of zinc polycarboxylate cement
- This material had the advantage of bonding to tooth surfaces in a similar way to glass ionomer cement
- There is less heat of reaction
- In pH is low to begin with but returns to neutral more quickly and longer chain acids do not penetrate dentine easily
- They are cheap
what are the disadvantages of zinc polycarboxylate cement
- Difficult to mix unlike zinc phosphate
- Difficult to manipulate
- Soluble in oral environment at lower pH
- Opaque
- Lower modulus and compressive strength than zinc phosphate
what is zinc eugenol materials used in
- Linings/base in deep cavities e.g under amalgam restorations
- Temporary restorations – either resin modified or EBA ZOE
- Root canal sealer – slow setting 24 hours used
- Periodontal dressings – fast setting version, 5 minutes
what is ZOE
It is still an acid base reaction where the base is ZnO and the acid is Eugenol
what happens in setting of ZOE
During setting there is a chelation reaction of zinc oxide with the eugenol to form zinc eugenolate matrix and this matrix bonds the unreacted ZnO particles
what are the advantages of ZOE
- Adequate working time
- Relatively rapid setting time – sets quicker in the mouth due to moisture and heat but can be modified by addition of accelerators
- Low thermal conductivity
- Radiopaque
what are the disadvantages of ZOE
- Low strength of around 20MPa due to weak hydrogen bonds between the eugenolate molecules and so is not strong enough to use as a base beneath an amalgam filling – the packing pressure would damage it
- High solubility as eugenol is constantly released and this is good and bad as eugenol is replaced by water which results in the disintegration of the material BUT eugenol when liberated has an obtundent effect on the pulp and can reduce pain
- The released eugenol inhibits the set of resin based filling materials – it softens them and can cause discoloration
- ZOE materials should not be used under composite resin materials
what are resin modified ZOE
• Resins are added to the powder and the liquid
o Polymethylmethacrylate in IRM
o Polystyrene in kalzinol
what is different about resin modified ZOE
• These do not take part in the reaction but give a stronger backbone to the set material and this increases the compressive strength to >40MPa making it suitable as a cavity lining as well as decreasing the solubility
what is EBA cement
It is another modified ZOE
what does the powder in EBA cement consist of
- ZnO 65%
- Quarts or alumina 35%
- Hydrogenated rosin 6%
what does the liquid in EBA cement consist of
- Eugenol 37%
* Ethoxybenzoic acid (EBA) 63% - reactive
what is the setting reaction in EBA cement
The setting reaction is the same as with ZOE but EBA encourages crystalline structure which imparts greater strength to the set material
what are the properties of EBA cement
The properties is that it is stronger than ZOE or resin modified ZOE with a strength of around 60 MPa and it is less soluble
what are the properties of GIC cement
- Thermal conductivity and diffusivity are lowr than dentine for both GIC and RMGIC
- Thermal expansion is similar to dentine for GIC
- Compressive strength is greater than 170MPa which is higher than any of the ZnO based materials
- Most materials are radiopaque, radiopacity varies between materials
- The marginal seal is better than any of the other materials as there is a chemical bond to enamel and dentine
- They are the only material to predictably seal dentinal tubules – this decreases microleakage and prevents post treatment sensitivity
- Solubility is greater for GIC than RMGIC and is greatest initially
- However GIC materials are less soluble than any of the other liners apart from zinc phosphate cement but RMGIC is less soluble than any other cement
how is GIC cariostatic
- Cariostatic as fluoride release from GIC materials is possible and may be cariostatic
- Benzoyl iodides and benzoyl bromides are released during the polymerization reaction of RMGICs – these are cytotoxic and can be effective against residual cavity bacteria
why should complete cure of RMGIC be ensured
any unreacted HEMA may damage the pulp
what is unique about GIC
- GIC is the only lining able to bond to restorative materials – some conventional GI materials may require to be etched prior to bonding however RMGICs require no surface treatment
- It may be possible to bond amalgam to tooth using RMGIC
