Adhesion - bonding to tooth surface

how does failure occur? 2

1. generally occur either due to fatigue or breaking

2. more commonly, the leakage of the restoration margin

why do we need adhesion? 3

1. prevents leakage at the margins or at the restoration of the tooth-restoration interface 

2. more conservative tooth preparations 

3. strengthen tooth tissue 

what two materials do we need to bond to?

enamel and dentine

how can we describe the texture of enamel?

• dry and brittle 

• very strong in certain directions

• can be dried out to remove most of the water in the structure at the surface where it has been cut 

• we usually try to increase the surface area - this is achieved by etching the surface of enamel 

we usually try to increase the surface area - this is achieved by

• etching the surface of enamel

• high resolution image of etched enamel prisms

first thing we have to do when we do restorations is…

cut a cavity 

cutting cavities - damage

• smooth burs - smooth margins with tungsten carbide, rougher than diamond - rough diamond bur 

• both give sub-surface cracking 

all burs / handpieces cause cracking

cracks may affect the integrity of the restoration 

cracking of enamel is a problem with…

shrinking composites

cracking of enamel is a problem with shrinking composites 

• most composites shrink on polymerisation - can put a strain on cavity margin 

• this will be increased as the volume of the cavity margin increases - and the number of walls which are opposing each other 

• typical shrinkage 3% by volume 

• can pull enamel apart 

• right image - white line in the enamel is caused by stress from the composite

strains on cavity margins may be increased by…

• this will be increased as the volume of the cavity margin increases - and the number of walls which are opposing each other 

microscopic images of cutting dental enamel 

high torque handpiece running with a tungsten  carbide bur - what is to note?

• high torque handpieces cut at slow speeds and with slow advances (through tissues) - this is a very gentle 

• right shows a steady load and displacement graph - showing steady progression through the tissue - very little change in the load 

air turbine handpieces - what is of significance?

• tungsten carbide bur at high speed with a high feed rate  

• air turbine handpiece - pushing hard - then the handpiece - handpiece starts to stall - bur will then stall, then recover, then stall then recover → this creates a rippling effect

• this causes quite a lot of damage to the substrate

• this is uncomfortable for the patient - and is not an efficient was of cutting the tissue

diamond bur rpm

• 200,000

• size of the damage is relate to the roughness of the diamond used in manufacturing the bur 

• can put a lot of subsurface cracking into the enamel 

looking at the tungsten carbide bur - we can look at the type of failure at the enamel margins 

the cracking of the enamel will preferentially go around….

• preferentially go around the enamel prisms and along the prisms from occlusal to cervical 

• going horizontally is a difficult area of the enamel to break - this causes quite significant chipping 

how can you describe the structure of dentine?

• wet + resilient 

• consists of tubules that are approximately a micron in diameter - surrounded by intratubular dentine - which is a composite of collagen - covered with small crystallites of hydroxyapatite 

• bonding to this tissue has always been a challenge 

range of materials - spectrum of adhesive properties 2 main types 

two main types of materials -  those that require adhesive or those that require self adhesive 

1. composites are resin based - adhesives are always needed to make a bond to adhesive tissue 

2. glass ionomer cement - is not resin based and is self adhesive 

3 other types of materials that are mixtures of the two main types (composites or glass ionomer cement)

1. polyacid-modified composites - contains some resin, but has some glass ionomer material incorporated into them and adhesive is needed 

2. resin-modified glass ionomers - some resin included - primer is needed 

3. calcium tri-silicate cements - have a more therapeutic nature - still open to discusssion

what are the general requirements for adhesion?4

1. good substrate wetting

2. a low ‘contact angle’ - material penetrates into structure 

3. a clean substrate - with high surface energy - attractive for molecules to stick to 

4. contamination must be avoided - especially with blood or saliva 

what are the three main mechanisms of bonding? 3

1. mechanical theories

2. adsorption theories

3. diffusion theories 

mechanical theories 

involve the concept of interlocking of the solidified adhesive with irregularities of the surface of the adherend - composites 

adsorption theories

chemical bonding - eg primary (ionic and covalent) and secondary (hydrogen, dipole interaction, or van der Waals) valence forces.

GICs (glass ionomer cements)

diffusion theories 

• mobile molecules across the interface 

• GICs Calcium Tri-silicate cements

historically, dentine has been difficult to bond to

image is showing the gingival margin of a proximal cavity

serviton was the first to bind to both dentine and enamel 

rather than using ‘generations’ we should classify materials based on their composition 

Adhesion of Composites - Acid etch technique - enamel

• removal of smear layer, high energy surface

• Enamel: differential dissolution of prism boundaries after 20s etch with H3PO4

• this will leave behind a surface which is porous - into which a resin will then flow 

• this only works if the etching pattern is end on to the enamel prisms 

the lateral walls of proximal cavities have prisms which are parallel to the cut margin 

these prisms can be pulled apart easily - prisms are not strong in their long axis - can bend easily 

in the acid etch technique - the ‘smear layer’ contains …

• debris from the cutting process - blood, saliva, cells 

• formed from a pressure welding technique - not from heat 

• seals the surface of the tooth - this is difficult to penetrate without any preparation 

Adhesion of Composites - Acid etch technique - dentine

• Acid etch technique: removal of smear layer, high energy surface

• Dentine: dissolution of mineral phase, leaving exposed collagen network

• when we remove the smear layer over dentine - we open up the dentine structure

• the dentine tubules are widened - because the intratubular which is highly mineralised, dissolves immediately

• intertubular dentine loses the mineral - leaving a collagen network    

Resin Bond achieved with micromechanical retention into dentine

composite restoration is usually hydrophobic - layered interface to achieve a satisfactory bond to the tooth

dental bonding agents must therefore be classified based on how they work - rather than when they were developed 

always have etching phase, priming phase and a bonding phase 

classification of older DBA

priming system is to wet the surface which has been etched , bonding is to seal it

type two has a smear phase then a prime and bond mixed together usually with volatile alcohol - eg acetone  

one of the key ingredients of DBAs is that they are able to penetrate into wet tissue 

• the resin infiltration is helped by the presence of HEMA - hydroxyethylmethacrolate to form the hybrid zone between the tooth and the restoration - it is neither tooth nor restoration 

• this provides a strong structure - making a bond between the composite and the tooth tissue - but the bond is mainly achieved not through penetration down the dentinal tubules but the intertubular penetration - and bonding to the dentinal collagen 

what is the oxygen inhibition layer?

• adhesive layer is formed above the layer of collagen impregnated with resin 

• the oxygen inhibition layer - allows the composite to make a strong bond with the resin 

total etch technique

• phosphoric acid on the enamel

• followed by etching of the dentine also with phosphoric acid 

• dentine etching is only done for a very short period of time 

• with modern self-etch adhesives it is not necessary 

with the total etch technique we therefore produce a …

a very rough / porous technique at a microscopic level in both enamel and dentine 

• resin penetrates intimately within the collagen fibrils

• can see the cross banding of the collagen in the TEMUs

schematic of collagen that has been etched 

can see that the dentine has been removed from the collagen fibrils 

this is then replaced with water - its important that the surface of the tooth doesn’t dry out 

dentinal collagen is then replaced with water - its important that the surface of the tooth doesn’t dry out - why is this important (etching)

this allows the HEMA to enter and bind to the collagen 

HEMA displaces the water and infiltrates around the collagen fibrils 

• the bond will also mix with the (yellow) HEMA an forma strong structure at the adhesion interface 

• this is then light cured and then the hybrid layer will vary in thickness

range of thickness in the hybrid layer 

0.5-0.8um thick - remember its the intertubular region, not the dentinal tubules 

what happens if we dry the surface of the collagen?

• collagen will collapse

• no HEMA can penetrate - hybrid zone not achieved

• whereas if you maintain the wet surface, the bond can displace the water - good adhesive interface 

red labelled adhesive - sitting on the top of the tooth 

bond can displace water in wet 

how is control of moisture/ wet bonding achieved clinically?

image shows proximal cavity 

1. enamel is etched 

2. dentine is etched 

3. this is washed away with a good stream of water 

4. surplus water is removed using a cotton pledget or paper towel or air syringe 

5. a large amount of adhesive is placed onto the tooth and rubbed into it gently

6. critical step - evaporate all the solvent that was within the adhesive you just applied - you know you’ve achieved the correct degree of evaporation when there is no longer any rippling on the surface under the air stream 

7. once the surface is shiny but not rippling - light cure the adhesive - it’s ready for the composite to be placed

top left - composite (starry)

MDP aids in chemical bonding of the resin

type3 

consists of a primer (acidic)

and sealing resin over the surface 

class V wear cavity 

• isolated with a rubber dam 

• rubber dam not essential if there is good isolation by other means - if adequate moisture control 

once the primer is applied and the air is thinned - primer layer is much thinner and is penetrating into the dentinal tubules

• after air blowing - the layer of adhesive becomes thicker 

• controlling some of these resins may not be as straightforward as you may assume 

final restoration after 10 years 

• can see some staining around the margins 

• etching effect of the self etch adhesive is quite poor - especially in the enamel bonding 

• seal is not as good as it should be 

can be difficult to thin down resin layers

• 1st molar - can see a dark line associated with the white composite filling 

• the dark line may be a residual caries or it may be the resin of the adhesive bond - inadequately air thinned 

type 4 adhesive 

comprises all the stages in one - may be two solutions that need to be mixed - goes on in one go 

self etch adhesives 2

• smear layer - the debris which plugs into the dentinal tubules 

• when we add the self etch adhesive - this fluffs up and becomes solubilised in the acidic primer 

• dentine is modified, smear plug is modified 

• resin penetrates into the smear layer 

• if you apply enough, and rub it rigorously - you can dissolve the smear layer  

• can achieve a reasonable bond to the tooth 

dentine bonding - evaporation of solvent in the bonding and light curing

• light curing effect comes from the blue light

• air inhibited layer and a bonding layer on the surface 

• allows the composite to bind to the air inhibited layer 

after placement of the restoration 

restoration and the adhesive will make a tight junction 

are the dentinal tubules in black or red?

what adhesive was used? serviton cavity seal 

Adhesion of conventional Glass Ionomer Cements

Condition the dentine with Poly Acrylic Acid

(PAA) - active ingredient

• Cleans pellicle from tooth.

• 'Pre-wets' tooth surface

• Removes smeared layer

Dynamic bonding with tooth

• ionic exchange with tooth

GIC Maturation

Setting reaction – acid base reaction of poly-acrylic acid / alumino-silicate glass. round - PAA, jagged - glass, green - enamel/dentine 

Maturation – proceeds rapidly over 24 h. – maximal at 6 months.

Ionic exchange – between glass particles and matrix.

Evidence of dynamic GIC bonding:

bonding is so strong to the enamel its capable of cracking the enamel - this will happen if the GIC is allowed to dry out 

the cement bonds strongly to tooth tissue

many of the failures occur within the GIC - this called a cohesive failure of cement 

Calcium trisilicate cement: ‘Biodentine’

A highly alkaline cement with pulpal therapeutic properties, suitable as a dentine replacement.

Adhesion resin-modified glass-ionomer cements.

• Contain HEMA & GIC Components

• Mechanism of adhesion as for Conventional GICs

Limited mechanical penetration of resin into conditioned, primed, surface

'Absorption layer' next to dentine compensates for shrinkage of resin, by swelling following water up-take.

Benefits of restoration swelling: Resin-Modified Glass Ionomer Cements

compensates for the resin shrinking by the resin itself swelling in water 

this forms a seal at the adhesion interface 

Mapping of HEMA distribution in a resin modified glass ionomer

Absorption layer mapping of resin-modified GIC

nb - their colour stability is less ideal - once swollen they may stain easier 

Polyacid modified composites (compomers)

• Composites with some GIC components

• These can be considered as composites for the purposes of adhesion

Adhesion of Composites - acid etch - enamel

Acid etch technique: removal of smear layer, high energy surface

formation of a honeycomb layer within the enamel which is then infiltrated with the resin 

– Enamel: differential dissolution of prism boundaries after 20s etch with H3PO4

incisal edge being etched - green phosphoric acid 

etchant is then thoroughly washed away by water syringe 

Adhesion of Composites - acid etch - dentine

Acid etch technique: removal of smear layer, high energy surface

Dentine: dissolution of mineral phase, leaving exposed collagen network

excess water is removed - but not over dried - resin infiltration which forms the hybrid zone 

applied generously using a microbrush

adhesive is blown away - primer is evaporated - look for no further rippling 

once there is no further rippling

surface is light cured

to restore the contours we are using a wedge and a clear matrix

dentine shade is being built up at the core of the tooth using a clean instrument 

then, light cured to give the shape of the dentine core 

composite - in the enamel shade is built up in excess 

with the matrix removed we can see a little bit of excess 

excess is trimmed with a water jet and fine diamond burs to recontour the restoration 

better done with the rubber dam removed - gives access to all the tooth tissue 

restoration is polished using 

a silicone rubber disc 

final restoration has good contour

colour match is less good because the natural tooth tissue has dried out from being isolated from the mouth by the dental dam - shade needs to be taken before the dental dam is applied 

‘Post sealing’ may help to reduce leakage

Re-etch margins after polishing, then apply resin helps to seal any minor cracks that come from cavity preparation or any cracks in the margins of the restoration 

etchant is briefly applied then washed away 

resin placed over the surface of the tooth 

contour matches the tooth well - with an irregular surface as teeth are not always dead flat