DML1: Bonding to dentine Flashcards
Why are adhesives used in dentistry
Because the tooth tissue is hydrophilic and restorative materials are hydrophobic; thus adhesives allow chemical bonding to occur which is key for the longevity of the restoration
What are the advantages of using adhesives
- better aesthetics
- tooth tissue conservation
- reinforces weak tooth structure
- reduced marginal leakage
- reduced potential for pulp sensitivity
What are the ideal requirements for adhesives
- Provide high bond strength to enamel/dentine
- Immediate and durable bond formation
- Prevent ingress of bacteria (typically at margins)
- Safe and simple usage
What type of adhesive is used for bonding resin to enamel
Acid-etch technique which is used in the placement of anterior and posterior composites, compomers, resin-bonded bridges, veneers and orthodontic appliances
What issues arise when bonding adhesive to enamel
- surface tension of enamel is lower than adhesive resin
- this means perfect wetting cannot take place
- enamel is covered by a pellicle layer which has an even lower surface energy which will also prevent perfect wetting
How are the issues arising when bonding adhesive resin to enamel resolved
- Raise the surface energy of enamel by removing pellicle layer through etching
- This roughens the surface
How is the pellicle layer removed when the enamel is etched
By the use of 30-50% Phosphoric acid (acid primer) and there is an acid bade reaction where the acid dissolves enamel surface hydroxyapatite giving a frosty appearance
Therefore there is micromechanical retention between the enamel and the resin because there is an increased SA for adhesive influx
What are the main effects of the acid-etch technique
Removal of the pellicle layer and contaminants with acid results in
- increased surface roughness of enamel
- bonding area is increased
- surface energy is raised
- improved wettability for resin to run into enamel rods
- opens inner prism area for interlocking tag formation
- increased area of contact and micromechanical bonding
What is the problem with over etching
It can break down enamel prisms needed for tag formation and this compromises the bonding
How is the resin applied after acid-etching the enamel
- low viscosity resin (unfilled Bis-GMA or dentine adhesive resin) is used prior to composite
- this is applied to the dry etched surface
- it will form resin tags which penetrate 30um deep
- a filled resin is applied on top of this unfilled resin
Why does a restorative resin material have to be adhesively bonded to dentine
To retain the restoration in the cavity and to eliminate marginal/internal gaps in order to prevent bacterial microleakage
Why is a bonding agent used to bind a restorative resin to dentine
Because dentine is even more hydrophilic than enamel and this bonding agent will provide added strength to the restored tooth by sealing the dentinal tubules
- this prevents dentine hypersensitivity
Why is it important that the dentinal tubules are sealed
Because they communicate with the pulp and are permeable to drugs and chemicals which can injure the pulp
What happens to the dentine structure when it is cut
- Fluid is pumped through dentinal tubules giving a wet surface
- Inorganic hydroxyapatite crystals are broken up, the collagen is stretched, torn and smeared over the cut surface forming smear plugs
What is the smear layer which covers cut dentine
A layer of denatured collagen and debris which is weakly bound to the dentine ; it is contaminated with bacteria (carious) and cutting debris
Why is it important that the adhesive resin acts as an effective seal for the dentinal tubules
Because when the smear layer is removed the dentine tubules are open and so exposed to bacteria and irritants - this has to be removed because it is a wet layer which makes bonding of the resin difficult
What are the three components of dentine bonding agents
- Conditioner (modifies/clears smear layer)
- Coupling agent/primer (bonding adhesive)
- Sealer (seals the dentinal tubules)
Order = Etch-prime-bond
What is a dentine conditioner
Component of the dentine bonding agent which is an acid solution and so removes/modifies the smear layer
It has to then be rinsed with water to leave the demineralised dentine surface and exposed tubules
What is the coupling agent/primer and what does it consist of
This acts as an adhesive in the dentine bonding agent and bings the hydrophilic dentine to hydrophobic resin
It consists of
- Bi-functional HEMA molecule that is dissolved in a solvent
- Spacer molecule which provides flexibility to the coupling agent
- Ethanol/acetone solvent (water chasers) this displaces water form tubules so the OH groups can saturate them via a molecular entanglement network
What is a dentine sealer and what does it do
It is the third component of the dentine bonding agent which contains unfilled Bis-GMA/UDMA and adhesion promoters to dentine
- it makes the dentine surface more hydrophobic
- it is light cured
What is the hybrid zone
The interpenetrating layer of dentine and resin
What issues arise with excessive desiccation of dentine
Post operative pulpal sensitivity and a poor bond
- pulpal inflammation will occur if there is bond failure leading to bacterial leakage
- there could be delayed allergic reaction or contact dermatitis with HEMA
What are the causes of breakdown in the adhesive bond
- Polymerisation shrinkage
- Differential thermal expansion and contraction
- Internal stresses from occlusal loading
- Chemical attack (hydrolysis)
What does the breakdown in the adhesive bond result in
- unsightly marginal staining
- pulpal sensitivity
- the restoration may be lost
