Cavity Lining Materials Flashcards
5 restorative materials you place in cavities
- Composite resin
- Glass ionomer
- Amalgam,
- Precious metal
- Ceramic
3 main disadvantages of restorative materials
May not make intimate contact with the tooth surface (especially dentine)
- Any gap may allow ingress of fluids and bacteria
Common problem - spaces
Heat released during setting/curing e.g. composite (detrimental to pulp)
Release of chemicals
- These may be pulpal irritants and lead to pain or pulpal damage
how to resolve problems caused by restorative materials which may cause pulpal damage?
A lining material
- Prevents gaps
- Acts as a protective barrier
Less need for them as more composite used rather than amalgam
cavity base
Thick mix placed in bulk base is a thick lining
- Thick layer for insulation
Dentine replacement used to minimise the bulk of the material or block out undercuts
More common in metal restorations (direct or indirect) gold, silver amalgam
cavity lining
Thin coating (<0.5mm) over EXPOSED Dentine - Sealing tubules
Van Noort ‘a dentine sealer able to promote the health of the pulp by adhering to the tooth structure or by an anti-bacterial action’ (glass ionomer does to a certain extent)
3 purposes of liners
Pulpal Protection
Therapeutic.
- To calm down inflammation within the pulp and promote pulpal healing. Prior to or at the time of a permanent restoration being placed. Reduce pain
Palliative.
- To reduce patient symptoms prior to definitive treatment being carried out. Most commonly in patients with reversible pulpitis. (More insulative before placing final restoration)
liners are pulpal protection from:
- Chemical Stimuli from unreacted chemicals in the filling material or the initial pH of the filling
- Thermal Stimuli. eg. exothermic setting reaction of composite or heat conducted through metal fillings
- Bacteria and Endotoxins. Microleakage – the penetration of oral fluids and bacteria and their toxins between the restorative and the cavity walls.
(Bond to surface of tooth – impenetrable so bacteria cannot progress in)
key properties needed of lining materials
- Ease of use
- Thermal properties
- Mechanical Properties
- Radiopaque
- Marginal seal
- Solubility
- Cariostatic
- Biocompatible
- Compatible with restorative materials
why linings need to be easy to use?
- Easy to mix
- Working time should be long to allow easy placement (reduce errors)
- Setting time short (Ideally command set)
ideal thermal conductivity of cavity liners
ideally should be low
- want poor to protect the pulp same
How well heat energy is transferred through a material –
Heat flow through a cylinder of unit cross-sectional area and unit length, with a temperature difference of 1oC between the ends
- Units are W/m-1/oC-1
Denture base should be high - want patient to feel heat of food, want good
Restorative material should be low
Cavity lining as low as possible - want poor to protect the pulp same
desired thermal expansion coefficient of dentine
Ideally a liner should match the thermal coefficient of tooth - desired
- Enamel 8.3ppmoC-1 , Dentine 11.4ppmoC-1
- GIC 11ppmoC-1 , RMGIC 20ppmoC-1
- Composite 25ppmoC-1
- Amalgam 25ppmoC-1
thermal expansion
Change in length per unit length for a temperature rise of 1oC
- Units are ppmoC-1
desired thermal diffusivity of liner
Ideally at least as low as tooth – want as low as possible
- Enamel 0.0042cm2/sec
- Dentine 0.0026cm2/sec
All commercially available liners have similar or lower thermal diffusivity than tooth enamel.
- Amalgam is 1.7cm2/sec (500x more than dentine, if have no lining can make you jump)
- Similar to conductivity
- Measured in cm2/sec
compressive strength of liner
high
- To allow placement of filling without it breaking – higher the better
Dentine is around 275MPa
elastic modulas of liner
similar to dentine
- Around 15GPa
As tooth bends and flexes want lining to be same
- Otherwise stresses build, cracks and failures
should liners be radiopaque?
yes
- Should be easy to see the difference between lining and tooth. This makes it easier to see the if there is any leakage or secondary caries
If radiolucent will look like caries - unable to tell difference (used to be case)
marginal seal of liner
Ideally the lining should form a chemical bond to dentine. This bond should be permanent and impenetrable.
solubility of liner
low
cariostatic of liner
Fluoride releasing and/or Antibacterial
This is important in preventing secondary caries around the restoration
should liners be biocompatible?
- Non-toxic
Not damaging to the pulp
- pH neutral
- No excessive heat during setting
Low thermal conductivity
4 types of liner
- 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. Contain antibiotic/steroid)
2 brands of setting calcium hydroxide
life
dycal
constituents in the base of setting calcium hydroxide
- Calcium hydroxide 50%
- Zinc Oxide (filler) 10%
- Zinc Stearate (filler) <1%
- N-ethyl toluene sulphonamide (plasticiser) 40%
catalyst in setting calcium hydroxide
- Butylene glycol disalicylate (reactive element) 40%
- Titanium Dioxide (filler) 13-14%
- Calcium Sulphate (filler) 30%
- Calcium Tungstate (filler and radiopaquer) 15%
what is the setting reaction in calcium hydroxide liner?
chelation reaction between the ZnO and the butylene glycol disalicylate.
- Acid base reaction between calcium hydroxide and acid catalyst
This results in a cement with an initial pH of around 12.
mode of action of CaOH liner
Bactericidal to cariogenic bacteria
Irritation leads to reparative dentine formation
bactericidal action of CaOH liner
Cariogenic bacterial survive in an acidic environment. The highly alkaline liner kills the bugs. pH12 good as bugs that cause caries like acid environment - die
why tertiary dentine forms due to CaOH?
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 off from the pulp. (The calcium comes from the pulp not the cement).
- Cause a degree of irritation in pulp which initiates tertiary dentine being laid down, increasing the distance between cavity base and pulp
3 advantages of CaOH
- Quick setting time
- Radiopaque
- Easy to use
4 disadvantages of CaOH
- Low compressive strength (can’t put amalgam on top, will crack)
- Unstable and soluble
- If the cavity leaks then the lining will disappear – break restoration
- It may even disappear just because it is in contact with moist dentine
5 types of zinc oxide based cements
- Zinc Phosphate
- Zinc Polycarboxylate
- Zinc Oxide Eugenol (ZOE)
- Resin Modified ZOE
- Ethoxybenzoic acid (EBA) ZOE
clinical properties of zinc phosphate cement
In use for 100+ years, under silver amalgams
Acid base reaction
Powder and liquid
Excellent clinical service
- Over time hardens
- Decent base
- Insulating
Under many restorations may need to remove
Need to understand that extremely strong and hard-wearing material
Easy to use
Cheap
constituents of zinc phosphate cement powder
Zinc Oxide >90%
- Main reactive ingredient)
Magnesium Dioxide <10%
- Gives white colour;
- Increases compressive strength
Other Oxides (Alumina and Silica)
- Improve physical properties,
- Alter shade of set material
constituents of zinc phosphate cement liquid phase
Aqueous solution of phosphoric acid (approx. 50%)
Oxides which buffer the solution
- Aluminium oxide
(Ensures even consistency of set material )
- Zinc Oxide
(Slows the reaction giving better working time)
reaction of zinc phosphate cement
The initial reaction is acid base
ZnO + 2 H3PO4 –> Zn(H2PO4)2 + H2O
This is followed by a hydration reaction resulting in the formation of a crystalised phosphate matrix – makes stronger
ZnO + Zn(H2PO4)2 + 2H2O –> Zn3(H2PO4)2.4H2O
The aluminium oxide prevents crystalisation leading to an amorphous glassy matrix of the acid salt surrounding unreacted ZnO powder. (ZnO powder surrounds zinc phosphate)
matrix is almost insoluble, but it is porous (with time) and contains free water from the setting reaction. But still strong
The cement subsequently matures binding this water leading to a stronger, less porous material
physical properties of zinc phosphate cement
matrix is almost insoluble, but it is porous (with time) and contains free water from the setting reaction. But still strong
The cement subsequently matures binding this water leading to a stronger, less porous material
7 problems with zinc phosphate cement
Low initial pH approx. 2
- Can cause pulpal irritation as pH can take 24hrs to return to neutral
Exothermic setting reaction – gives off heat
Not adhesive to tooth or restoration
- Retention may be slightly micromechanical due to surface irregularities of cavity
- No chemical bond to tooth
Not cariostatic
Final set takes 24hrs (reasonably hard within 5 minutes, could probably pack on top of then)
Brittle
Opaque
good use of zinc phosphate cement in modern clinics
good as temporary dressing which will remove and replace with resin based restorative e.g. inlay
- No need to drill as doesn’t stick to tooth
Glue for sticking on crowns - temporary crown worried about cementation can use this to stick on better, since wont bond to tooth can remove all cement before placing final crown
zinc polycarboxylate cement advantages
material had the advantage of bonding to tooth surfaces in a similar way to glass ionomer cements.
- Main advantage – sticks to dentine, not as well as glass ionomer though
There is less heat of reaction.
The pH is low to begin with but returns to neutral more quickly and longer chain acids do not penetrate dentine as easily.
- Higher pH so less toxic to pulp
- Polyacrylic acid is bigger so less penetrable
Cheap.
5 disadvantages of 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
4 uses of zinc oxide eugenol (ZOE) cements
Linings/base in deep cavities
- under amalgam restorations
Temporary restorations
- Resin modified or EBA ZOE
Root canal sealer
- Slow setting 24hrs
Periodontal dressings
- Fast setting , 5 minutes
reaction of zinc oxide eugenol
base acid reaction
- base is ZnO
- acid is Eugenol
Base + Acid –> Salt + Water
Chelation reaction of zinc oxide with the eugenol to form zinc eugenolate matrix.
This matrix bonds the unreacted ZnO particles same idea as zinc phosphate
ZOE properties
Adequate working time
Relatively rapid setting time
- Sets quicker in the mouth due to moisture and heat
- Can be modified by addition of accelerators
- Expands on setting
Low thermal conductivity
Low strength around 20MPa
- Weak hydrogen bonds between the eugenolate molecules
- Not strong enough to use as a base beneath an amalgam filling. The packing pressure would damage it. But can be used as a long term filling
Radiopaque
High solubility
- Eugenol is constantly released
- This is good and bad
Eugenol is replaced by water which leads to disintegration of the material (more brittle and break up)
BUT
Eugenol when liberated has an obtundant effect on the pulp and can reduce pain
can zinc oxide eugenol be used as a liner for resin based liners
no
softens them and can cause discoloration.
- Acts as a plasticizer
ZOE materials should NOT be used under composite resin materials – as they will not set
advantage and disadvantage of high solubility of ZOE
Eugenol is constantly released
- This is good and bad
Eugenol is replaced by water which leads to disintegration of the material (more brittle and break up)
BUT
Eugenol when liberated has an obtundant effect on the pulp and can reduce pain
what resins are added to resin modified ZOE
- Polymethylmethacrylate in IRM
- Polystyrene in Kalzinol
added to the powder and liquid
what do resins do in resin modified ZOE
These do not take part in the reaction but give a stronger backbone to the set material (act as a filler)
- increases the compressive strength to >40MPa making it suitable as a cavity lining. – resin chains make whole material stronger
Greatly decreases solubility
Ethoxybenzoic acid) EBA cement
modified ZOE
Same acid base reaction
- Quarts and alumina in matrix – stronger, reinforcing
- EBA longer chain eugenol – stronger, reactive
Powder
- ZnO 65%; Quartz or Alumina 35%; Hydrogenated rosin Around 6%
Liquid
- Eugenol 37%; Ethoxybenzoic acid EBA 63%
properties of Ethoxybenzoic acid) EBA cement
Stronger than ZOE or resin modified ZOE, around 60MPa
- Suitable for amalgam filling, medium-long term temporary dressing
Less soluble
encourages crystalline structure which imparts greater strength to the set material
what are the most widely used lining materials
glass ionomers
glass ionomers can bond to….
can bond to and seal the dentine
It then in turn can bond to the composite resin filling placed over it
10 advantages of glass ionomer linings
cariostatic - releases fluoride
easy to use (clicker system)
command set
Thermal conductivity and diffusivity are lower than dentine for both GIC and RMGIC
Thermal expansion is similar to dentine for GIC, RMGI is less but still better than filling material
Compressive strength is > 170MPa, higher than any of the ZnO based materials
Most materials are radiopaque, the radiopacity varies between materials.
- Good as can tell difference between them and caries
Marginal seal is better than any of the other materials as there is a chemical bond to enamel and dentine.
- Good as prevents secondary preventative barrier from microleakage of pulp
They are the only material to predictably seal dentinal tubules.
- This decreases microleakage and helps prevent 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.
- RMGIC is less soluble than any other cement.
why light cure is good for GIC?
- This gives a long working time and a conveniently short setting time (30 seconds)
- Unlike all previous material which where own rate set, glass ionomer is command set
why complete cure of RMGICs is important?
Benzoyl iodides and benzoyl bromides are released during the polymerisation reaction of RMGICs.
- These are cytotoxic and can be effective against residual cavity bacteria.
complete cure of RMGICs is required as any unreacted HEMA may damage the pulp too
- As cytotoxic to both bacteria and pulp so need fully cured to prevent damage to pulp but provide protection from bacteria
unique advantageous property of GICs
only lining able to bond to restorative materials.
Some conventional glass ionomer materials may require to be etched prior to bonding.
- RMGICs require no surface treatment
linings in cavities in dentine
Consider use of a lining of RMGIC (such as vitrabond) for amalgam and for larger cavities to be filled with composite
what should be used when cavity approached the pulp?
calcium hydroxide
- Either as a direct or indirect pulp cap over the deepest part of the cavity
because:
- Protect from hot and cold e.g. heat of composite setting)
- place as dentine isn’t good for bonding point of view
(Glass ionomer bonds to Ca ions instead of mesh way of composite)
what should calcium hydroxide be covered with?
RMGIC prior to a final restoration being placed.
- to protect it and stop CaOH being dissolved away
