5. Dental anatomy: adhesion Flashcards
- why do we need adhesion?
- what will adhesion prevent?
- what do we need to bond to
1
- to get a good seal
2
- prevents leakage at margin and with restoration
- more conservative tooth preparations
- may strengthen tooth tissue
3
- bonding to enamel + dentine
enamel
- strong
- can be dry and brittle
- can be dried out to remove most of water from surface
- try to increase the SA for bonding, that is achieved by etching surface of enamel
why do we etch enamel?
- try to increase the SA for bonding, that is achieved by etching surface of enamel
- image = etched enamel prisms
CUTTING CAVITIES
does diamond or tungsten carbide bur produce a smoother tooth cut?
- cavity surface will relate to structure of bur
- very rough diamond bur = rougher
- tungsten carbide bur= smoother
- both can cause sub surface cracking and chipping
- enamel prisms seen in image
- cracks between prisms
- cracks are important as they can affect the integrity of the restoration
what happens to composite restorations when you polymerise them
what impact does this have on enamel
- composite restorations shrink when polymerised
- typical shrinkage 3% by volume
- this can put strain on cavity margin, and will be increased as the volume of the cavity increases and also on the n.o walls that are opposing each other
- the shrinkage stress can be sufficient to cause cracking of enamel
dentine properties
- v diff structure + properties to enamel
- wet + resilient
- consists of tubules approx micron in diameter,
surrounded by intertubular dentine = composite of collage closely covered with small crystallites of hydroxyapatite - difficult to bond to dentine
materials available for restorations
Composite + GIC are the main ones
- composites
~ all resin
~ adhesive needed - glass ionomer cement
~ no resin
~ self adhesive
the next two incorporate both of the materials
- polyacid-modified composites
~ some resin
~ adhesive needed - resin-modified glass ionomer
~ some resin
~ primer needed
new material
- calcium tri-silicate cements
general requirements for adhesion
- good at wetting substrate of tooth,
- a low ‘contact angle’ (penetrates into structure)
- a clean substrate, with high surface energy (making it attractive for molecules of adhesive molecule)
- contamination must be avoided; especially with blood or saliva
Main mechanism of bonding
- Mechanical theories
- involve the concept of interlocking of the solidified adhesive with the irregularities of the surface of the adherend
- (Composites) - Adsorption theories:
- chemical bonding, e.g. primary (ionic and covalent) and secondary (hydrogen, dipole interaction, or van der Waals) valence forces
- (GICs) - Diffusion theories
- mobile molecules across the interface.
- (GICs, Calcium Tri-silicate cements)
- hard to bond to dentine
- important when looking at gingival margin of a proximal cavity
- severotin cavity seal first used to bond to enamel and dentine
- since then there have been many developments
ADHESION OF COMPOSITES
what does acid etch do?
Acid etch technique
- removal of smear layer
- leaving high energy surface
Enamel: differential dissolution of prism boundaries after 20s etch with H3PO4 (37% acid)
- leaves behind a porous surface which resin will flow into
- once smear layer is removed, dentine structure is opened up
- dentine tubules are widened
- as the highly mineralised intratubular dentine dissolves immediately
- the intertubular dentine loses the mineral due to acidic dissolution, leaving exposed collagen network
lateral walls of proximal cavities
- Prisms parallel with cavity wall:
Can be pulled apart from each other easily
what does the smear layer consist of?
- debris from cutting at tooth
- blood
- saliva
- cells from tissues
- formed by pressure welding technique
- it seals the surface of the tooth
- is hard to penetrate without some sort of prep of tooth
aim of adhsesive?
- to penetrate into that structure
- need to have a layered restoration to result in a satisfactory bond to tooth
- composite restorations = usu hydrophobic but teeth contain water
classification of dentine bonding agents
- based on how they work
- always 3 components, etching, priming, bonding phases
- may not always be 3 separate stages for these components tho
in type 1
- prime = wet the surface that has been etched
- bond = to seal
in type 2
- prime and bond mixed together usu by volatile solvent EG. alcohol, acetones
- once of key ingredients of dentine bonding agents is that they can penetrate wet tissue
total etch technique
- consist of using phosphoric acid to etch enamel and then etching on dentin
- done for short period of time
- in modern adhesives don’t need this stage as have self etch adhesives
- causes rough and porous surface to form (can be seen on microscopic level
what does etching remove?
- removes minerals around collagen fibrils
- this is then replaced with water
- important to not allow surface of tooth to dry out as water helps allow HEMA to penetrate into tooth
EXTRA
- (HEMA displaces water and infiltrates around the collage fibrils)
- any bond will also mix with the HEMA and develop a strong structure in the adhesive interface
- this is then light cured
- hybrid layer will vary in thickness (0.5 - 8 microns thick)
- it is not the dentine tubules filled with resin that makes the bond, it is the intertubular region the makes a strong bond
what happens if we dry surface of collagen free zone (tooth after being etched)
- collagen will collapse
- if that happens the adhesive will just go over the surface and not penetrate into the tooth to make a good hybrid zone
- maintain the wet surface for a good bond
control of bonding / wet moisture
how is this achieved clinically
(photos from pickards)
- proximal cavity with matrix in place
2.
- enamel is etched
- dentine is etched
3.
- washed away
4.
- surplus water washed away with light blow of syringe or cotton pledget or paper towel
- large amount of adhesive put on tooth
- rubbed into it gently
- evaporate all of the solvent that may remain
- you know when you have correct degree of evaporation when you have no rippling on surface under air stream and not shiny
- light cure adhesive
- then ready to place composite
type 3
- primer which is acidic
- sealing resin over surface
- light cured
