Bonding of tooth coloured restoratives to tooth structure Flashcards
What are the different types of retention?
- Mechanical (micro mechanical and undercut)
- Chemical
When is macromechanical retention used?
Amalgam
When is micromechanical retention used?
Composite resin and compomer
When is chemical retention used?
Glass ionomer cement
What property do resins need to have to be able to form resin tags?
Good surface wetting
What may pre etch prophylaxis pastes contain?
- pumice
- oil containing
- glycerine containing
- fluoride pastes
n.b. used to be thought should just use pumice if etching, the others leave a layer on top that the etch cannot penetrate (fluoroapitite doesnt etch as well) = old wives tail and recent research has shown that it doesnt matter if the prophylactic paste contains the others = achieve equally good bond
What are the most commonly used etchants used today?
- 36% ortho-phosphoric acid :)
- 10% ortho-phosphoric acid
- 10% maleic acid
- 2.5% nitric acid (n.b. v. strong acid with high pKa = more aggressive even with lower %)
Which is better… etchant liquid or etchant gel?
Doesnt really matter -> Gel = easier to control (viscocity) and can see where it is (colour) & penetrates as far into fissure system as liquid
Which etch patterns can be achieved?
- Intra-prismatic (core)
- Inter-prismatic (peripheral)
- Mixed appearance
Which etch pattern is the best?
It doesnt really matter, both are equally effective in their retention
- its also possible that individual etching different teeth can have peripheral or core loss -> even on the same tooth!
Which etch pattern is this?
Interprismatic
Which etch pattern is this?
Intraprismatic
How far does resin need to infiltrate?
A-prismatic enamel (surface enamel) = 10 micrometers removed
Prismatic (further down) = further 40 micrometers removed from core or periphery
= actually over etch (becuase bonding resin only penetrates 25 micrometers)
What does etching of enamel and application of an unfilled resin achieve?
Reduces microleakage (eliminates marginal staining and reduces secondary caries)
Name two etching technique varibales:
- Enamel etching time
- Washing time
How long is etching time?
Gradually been reduced without adversely affecting bond strength = as low as 15 seconds
How long should we wash the etch off for?
At least as long as the etching time (if havent removed the calcium ions etc. then it will reprecipitate - smooth surface again!)
If an etched surface us contaminated by saliva what needs to happen?
Re-etch for 5 seconds (removes the glyco-proteins that will precipitate onto the surface)
How can we isolate the cavity?
Cotton wool rolls and saliva ejector
Rubber dam
What is the effect on bond strength following enamel being etched with rubber dam?
Superior bonding
(Blocks 100% humidity of patients breath instead only being exposed to air cons 40-45% humidity)
The hydrophobic adhesive resin should be….?
- Thin layer (allowed to penetrate the pores for 20s and then air dried)
- should be cured before composite application
What is the strength of enamel bonding?
30 MPa
What reduces the bond efficacy of enamel bonding?
- Clinical technique (lack of bevel)
- Contamination after etching
- Micro-cracks developing from cavity prep
- Unsupported or fractured enamel margins
What is 1 MPa?
For every mm squared you could hang 1kg attached to the resin before it breaks off
Why is bonding to dentine not as reliable as bonding to enamel?
Difference in composition and structure
What is the composition of enamel and dentine?
Denitne contains X12 more water and >2 X the amount of organic material
How do enamel and dentine differ in structure?
- Enamel = homogenous
- Dentine = heterogeneous (different amounts of hydroxyapatite and collagen on peritubular and intertubular dentine)
What does the smear layer consist of?
Smeared hydroxyapatite crystals in a matrix of partially degrade collagen (due to heat from bur), it may also contain bacteria
How thick is the smear layer?
Varies in thickness depending on the burs used to prepare the dentine and the use of water spray:
- water spray + tungsten carbide burs = thinner smear 7 micrometers - relatively homogenous
- Diamond burs = thick loose smear 15 micrometers = less well bound down to underlying tooth structure
How many layers does the smear layer have?
2 =
- Thick outer layer lying on the peri- and inter-tubular dentine (looser)
- thin inner layer = plugs in dentinal tubules
What does etching do?
- Removes the smear layer, opens tubules and exposes collagen in the peri-tubular and inter-tubular dentine
What are the potential adverse effects of removing the smear layer?
- tissue fluid outflow
- bacterial invasion
- dentine sensitivity
- adverse pulpal response
In which two ways can dentine bonding systems be classified?
- chronological = depending on the date of development (doesn’t really tell us anything)
- bonding mechanisms = more acceptable as many materials consist of combinations of systems (rather academic)
What are the 4 ideal properties of adhesive resin?
- non-toxic to the pulp
- hydrophilic (intermix with dentinal fluid)
- good wetting if surfaces (flows across into tubules)
- long term stability
What is the strength of the dentine bond?
20-25 MPa
Why does dentine bond strength vary in studies?
- Roughness of the cut dentine surface (mechanical structure)
- Site from which dentine selected (deep section has more tubules per mm = make it fit their situation
- source of the dentine e.g. bovine or human (different apatite structure)
What is formulae for a dentine bonding resin?
MRX
What does the M in MRX (dentine bonding resin) stand for?
Methacrylate = bonds with overlying composite
What does the R in MRX (dentine bonding resin) stand for?
= organic linking molecule
Some of the modern ones = a radical = acts like a spring so the restoration can give slightly without rupturing the bond
What does the X in MRX (dentine bonding resin) stand for?
A hydrophilic molecule that interacts with the etched dentine surface
How do we bond to the dentine surface?
Mainly by entanglement or micro-mechanical retention -> small gain in bonding to apatite salts in etched dentine -> further small gain in bonding to the organic collagen matrix of etched dentine
What is bonding by entanglement?
Removal of smear and apatite cyrstallites
- > resin flows around triple helix of collagen bundles exposed in the inter-tubular dentine and polymerise
- > bonding resin penetrates dentinal tubules displacing the water
= polymerises in the pores
= Hybrid zone = produces about 80% of bond strength value
How does additional bonding with apatite salts occur?
The materials bond ionically to calcium ions in the enamel, dentine and glass ionomer cements = materials consist mainly of phosphated esters of bis GMA
e.g.
PENTA (in prime and bond, 5 phosphates related to 1 molecule = ionic interaction with tooth)
and
MDP (only just out of liscence)
How does additional covalent bonding with organic matrix occur?
Materials generally contains the following hydrophilic resins:
- HEMA (2 hydroxy ethyl methacrylate)
- PMDM (pro mellitic acid, di-anhydride & methacrylate)
- Aldehyde (risk of contact dermatitis)
What is the total etch technique?
- Etchant gel for 15-30 seconds on enamel and 15 seconds on dentine
- Wash
- Dry
What is critical in etching?
- The tooth is washed for at least as long as it was etched
- The dentine is not dessicated
What are the 3 steps of smear remocing bonding systems?
Primer, etchant and adhesive
What were the first smear removing bonding systems?
3 steps = Etch + Primer + adhesive
- = simultaneous ETCHING of enamel and dentine with phosphoric acid
- = application of hydrophilic PRIMER
- = ADHESIVE application
What was the primer resin dissolved in in the first smear removing bonding systems? Why?
Dissolved acetone or alcohol (butanol) which displaced water from collagen due to volatility (and carry the bonding resin in around it)
What were the second smear removing bonding systems?
2 Steps = Self etching primer + Adhesive
- = simultaneous ETCHING and PRIMING of tooth structure using weaker e.g. maleic or itaconic acids
- = ADHESIVE application
What does etching do?
Smear layer removal and exposure of collagen scaffold in inter-tubular dentine
What does primer do?
Primer resin infiltration of exposed collagen scaffold and dentinal tubules
What does adhesive application do?
Co-polymerises with primer
What is the consequence of the 2 step bonding system on bond strength?
Much weaker = tend to break under stress
Give some examples of self etching primers:
NRC
Clearfil
SE bond
What are the third smear removing bonding systems?
2 steps = Etch + combined primer and adhesive (‘one bottle adhesive’)
- = Simultanous ETCHING of dentine and enamel using phosphoric acid etch
- = Application of a ‘one bottle’ combined PRIMER and ADHESIVE
What is the advantage of the third smear removing bonding systems?
Only one bottle of adhesive resin needs to be applied following the traditional etch
What are the fourth smear removing bonding systems?
1 step = Etch + Primer + Adhesive
- =simultaneous ETCHING, PRIMING and ADHESIVE application to enamel and dentine = smear layer removal, neutralisation of etch and forms a hybrid layer all in one
Restorations placed with which technique have the longest life?
1 step = not stronger but less risk of contamination
Name an example of the 1 step smear removing bonding system we have used:
Xeno
How does the Xeno 1 step smear removing bonding system work?
Xeno liquid B = Pyro-EMA
- splits when water is added to form 2 phosphoric acid groups attached to methacrylate = hydrolysis -> where ever carries down the methacrylate with it into the structure = 30 microms of penetration and depth of resin
- Calcium ions from etched hydroxyapatite neutralises phosphoric acid groups = calcium phosphate
What is the hybrid layer?
Mix of resin with collagen with dentine with tubules
Tomorrows world…
Trying to produce composite resins incorporating resins that will etch and bond to the tooth structure without the need for a separate etching bonding regime (MDP plays a huge part in this)
How do polyalkenoate cements bond?
The do NOT rely on micro-mechanical retention = utilises ion exchange at the enamel or dentine surface to produce a dynamic ionic bond (2 X COO- groups in Glass ionomer bond to Ca 2+ ion)
How is the smear layer removed but the plugs left in the tubule ends?
By application of a 10% polyacrylic acid for 10 seconds = then washed with water for 15 seconds and the area dried with oil free compressed air
What is the benefit of removing the smear layer but leaving plugs in tubule ends?
Limits fluid movement and allows dry surface
How is the polyalkenoate cement bond (GIC) formed?
- free polyakenoic acid penetrates the enamel and dentine surfaces, carboxyl groups displace phosphate groups
- Phosphates take the calcium ions with it to maintain electrical neutrality
- phosphates and calcium ions combine with the cement matrix = rich mineral soup at the interface of tooth and cement = firmly bound to both the tooth and cement (calcium binds to strontium & phosphate to alkenoate = crystaline structure)
How to polyalkenoate cements bond (GIC) with organic component?
Slow ongoing chemical reaction = eventually results in a degree of union with the collagen in the dentine
- hydrogen bonding
- metallic ion bridges between carboxyl groups on polyacid and amino acids of collagen
What can cause failure of the bond (GIC)?
Weakest material = the matrix holding the glass particles together
-> when stressed = cracks cohesively between glass particles and not along the ionic interface forming the bond
What is the typical GIC bond strength?
5 - 9 MPa
= usually test cohesive strength of cement
