Adhesion 2 Flashcards
For effective bonding, the bond should …
- form rapidly
- withstand polymerisation shrinkage forces
- overcome hydrophilic/phobic barrier
- withstand oral environment
Important factors in enamel bonding
- effective etching, rinsing and drying
- avoid contamination
- applying low viscosity resin
Composition of enamel
- densely calcified
- 96% mineral/apatite
- 1% organic
- 3% water
- apatite crystals tightly packed into prisms
Composition of dentine
- permeable tubular structure
- 70% mineral (apatite)
- 20% organic (collagen)
- 10% water
- heterogenous (philic and phobic components)
There are pressure difference between the … and the dentine floor. Why?
- pulp
- fluid pumps through tubules - dry field impossible
Is it safe to acid etch dentine?
- may cause post-op pain
- what if there’s pulp exposure
- need an appropriate acid
Is it possible to etch enamel but not dentine?
- difficult/impossible in practice
- results show it leads to low bond strength, leakage, loss of restoration (class V)
Do you rinse and dry dentine?
- see effect of the dentine water concentration
- excessive drying can lead to irreversible damage to vital pulp
- can contamination be avoided?
Dentine is hydrophilic/phobic
philic
2 reasons for failure of early bonding agents
- no acid used on dentine
- smear layer has weak adhesion to dentine
What happens if no acid is used on dentine?
- debris layer forms on surface
- debris from cavity prep
- contaminated with bacteria
- gelatinous layer called the smear layer
Smear layer has strong/weak adhesion to dentine
Explain
- weak
- no mechanical bonding
- chemical attachment is weak
- easily disturbed by composite shrinkage
3 possibilities for dealing with the smear layer
- bond directly to the smear layer
- remove smear layer
- modify smear layer
Why would you usually choose not to bond to smear layer?
- weak bond
- bad idea
How is removing the smear layer a way to deal with it?
- a total etch approach
- requires a strong acid - often phosphoric acid
- rinse acid after use
- bond to bulk dentine
How is modifying the smear layer a way of dealing with it?
- a self-etch approach
- requires weaker acid than if you were removing the layer
- a range of acids are common, with self-etching primers
- don’t rinse acid after use
- bond to mixture of modified smear layer and bulk dentine
Stages of bonding to dentine
- conditioning/acid etching
- priming
- bonding
Define conditioning/acid etching stage
- treatment to remove, disturb or penetrate the smear layer
Explain priming
- dentine is hydrophilic and the monomers in composite are hydrophobic
- drying dentine can damage it
- treatment needed on conditioned dentine to prepare it for bonding
Bonding resin is similar to what concept?
- enamel acid-etch procedure
Structure of primer
- general structure of M-S-R
- M is the methacrylate group
- S - spacer
- R - reactive group
In primer, what does the methacrylate group do?
- bonds to composite
- can polymerise
In primer, what does the spacer do?
- allows primer to be flexible
- low viscosity to enable flow
In primer, what does the reactivegroup do?
affinity for moist dentine
Which primer is the most common?
- HEMA
- hydroxyethyl methacrylate
Features of HEMA
- amphiphilic (hydrophobic and philic parts)
- no chemical bond to dentine or enamel, mechanical bond only
New advancements on HEMA
- newer monomers such as 10-MDP developed
- chemical bonding possible
- some evidence they have higher bond strength in vitro
Conditioned dentine has either … or …
- demineralised collagen layer on surface
- demineralised collagen layer with modified smear layer on surface
Primer must infiltrate what layer?
Partial infiltration means … and complete infiltration means …
- hybrid
- weak bond
- strong bond
The hybrid is a … structure
What’s involved?
- composite
- collagen, primer, smear layer
In enamel, acid is washed away and enamel is dried. Do we do this with dentine?
- no
- drying of dentine can lead to collagen network collapse
- poor bond formation due to difficulty in primer penetrating
- may be able to rehydrate
- may lead to pulp damage
- led to ‘wet bonding approach’
What is the wet bonding approach?
- must rinse strong acid of dentine
- to prevent over etching but leaves the cavity too wet
- bond strength is low as hydrophobic monomers don’t infiltrate
- need to dry dentine but leads to collagen dentine
Infiltration of demineralised layer depends on …
- ability to displace water with primer
- many primers contain water to help with infiltration
- solvents such as ethanol and acetone claimed to displace water for better adhesion possibly?
Cautions with the wet bonding approach
- products differ wildly
- very technique sensitive
An alternative approach to wet bonding
- self-etching primer
Advantage of self-etching primer
- weaker acids only modify smear layer
- don’t need to be rinsed away
- should remove the ‘too wet/too dry confusion’
Early products of self etching primers
- contained maleic acid diluted in HEMA
- limited shelf life
- poor bond strength durability
Modern products of self etching primers
- range of pHs - strong, intermediary strong, mild, ultra-mild
- lower pH more disruption of smear layer
- lower pH more calcium phosphate remains in hybrid layer, leach out over time (soluble), compromise bond durability?
- no open dentine tubule left on surface
What pH is considered
- strong
- intermediary strong
- mild
- ultra-mild
in self-etching products?
- less than 1
- around 1.5
- around 2
- more than or equal to 2.5
What’s good with leaving no open dentine tubule left on the surface?
- leads to less sensitivity post-op
- limited evidence
Bonding agents have similar monomers to composites. Explain
- methacrylate-based
- bonds to primer and composite
- seals dentine surface
Explain filler level of bonding agents
- traditionally unfilled
- filler particle size to big
- stopped penetration of demineralised dentine
- filled sealers nano-size fillers
- potentially improve compressive strength
- no clinical evidence of advantage
Could the conditioning, priming, bonding be simplified?
- using fewer steps could be more consistent
- seems a logical assumption but not always true
3 dentine bonding adhesive categories
- 3 stage
- 2 stage
- 1 stage
Explain the different categories of dentine bonding adhesive
- 3 stage is conditioning, priming, bonding
- 2 stage is conditioning (priming and bonding), self-etching primer, bonding
- 1 stage is all in one - conditioner, primer, bond
Don’t confuse stages with steps. What’s the difference?
- many steps are needed regardless of how many stages
- mixing of material
- air thinning
- light curing
Explain conditioning (priming and bonding)
- one bottle systems (confusing as actually 2 bottles)
- separate acid step (similar acid to enamel etchants - 30-40% phosphoric acid)
- combined primer and bonding agent (similar monomers to composite with HEMA)
- repeat applications needed (saturate demineralised dentine, optimise hybrid layer)
- reduce steps but not necessarily time
Explain (conditioning and priming) , bonding
- two bottle systems
- etching and priming step combined
- demineralise and infiltrate dentine simultaneously, no rinsing step, modifies smear layer and pH of acid varies between products, typically requires repeated applications
- bonding resin has similar monomers to composites, some resins contain HEMA
Explain (conditioning, priming and bonding)
- single step adhesives
- more than 1 step
- typically 4 steps
- dispense and mix components, apply to enamel and dentine, air dry, light cure
Why is there variable results with (conditioning, priming and bonding)?
- technique sensitivity
- acid strong enough to etch enamel
- air diffuses through thin layers quickly (potential for oxygen inhibition)
How is enamel etched?
- 10-60s
- rinsed
- air dried
- chalky white appearance
How is dentine etched?
- 10-15s
- rinsed
- carefully dried
- not clear appearance difference
Can enamel etching and dentine etching be combined?
- total-etch approach acid is the same as acid-etch approach
- are self-etching primers strong enough?
- evidence suggests you need a full acid-etch procedure of unprepared enamel or bonding will fail
Does long exposure to acid irritate pulp?
- acid penetration typically 4-5 micrometres dentine
- evidence shows normally no pulp irritation
- proper seal of cavity more important - prevents access to dentine tubules
Classifications of bonding agents
- generations (1st or 2nd etc) but clincially unhelpful
- one bottle/two bottles - but number of bottles not the same as stages so no indication of how to use
- total etch/self etching primer - some indication of clinical procedures
The smear layer must be effectively … and expose …
conditioned
demineralised dentine surface
The hybrid layer is … and … establishes the bond
- effective infiltration of demineralised layer with primer/resin
- polymerisation
Current issues in use of enamel and dentine adhesion
- consequences for enamel bonding
- differences in dentine
Consequences of enamel bonding
- simultaneous bonding preferred by dentists (total etch is okay, self-etching primers may not be sufficient)
- wet or dry conditions needed? (enamel needs drying before bonding, dentine damaged by excessive drying)
- enamel condition important factor (is enamel freshly cut?)
Differences in dentine
- sclerotic dentine (highly mineralised, acid well tolerated)
- carious dentine (not highly mineralised, acid less well tolerated)
Post operative problems
- pulp inflammation
- shrinkage of composite
How is pulp inflammation a post-op problem?
- acids no longer seen as a major issue
- bond failure more important
- leads to bacterial access to pulp
Shrinkage of composite depends on…
- filler content
- monomer type
- technique
- cavity size
Shrinkage of adhesives
- bond strength takes 24hrs to develop
- polymerisation of composites - 20s
- marginal failure may occur before bond strength reaches maximum value
Explain how shrinkage effects C-factor
- more bonded surfaces more potential shrinkage effect - adhesive must compensate
- more compensation for shrinkage by adhesive - more cuspal displacement of tooth to compensate
Why is inconsistence of performance a current issue?
- wide range of products (lots of steps and technique sensitvitity)
- bond strength durability (hydrophilic components, may lead to plasticisation of bond, hydrolysis of components, enzymatic degradation of collagen)
- questionable shelf-life (bond strength decrease over time, only in vitro but worrying)
