Composite Bonding to Enamel and Dentine Flashcards
Why are adhesives used in dentistry
- Tooth tissue is hydrophilic
- Most restorative materials are hydrophobic
- So there is no chemical bonding between the two
What are the benefits of using adhesives
- Better aesthetics
- Conservation of tooth tissue
- Reinforcement of weak tooth structure
- Reduced marginal leakage
- Reduced potential for pulp sensitivity
What are the ideal features of an adhesive
- Provide high bond strength to enamel and dentin
- Immediate and durable bond
- Prevent ingress of bacteria
- Safe to use
- Simple to use
What is considered one of the best techniques for adhesives that there currently is
The acid-etch technique for bonding resin to enamel
What are some of the problems with bonding restoratives to enamel
- Composites have no intrinsic adhesive qualities to tooth tissues
- Composite resins are non-polar (hydrophobic)
- Enamel is hydrophilic
- Surface tension of enamel
How is the surface energy of enamel increased
The pellicle layer must be removed and the enamel surface has to be etched
Why do we want to increase enamel surface energy
So we can have better wetting and more SA between the composite and enamel
How is the pellicle removed and enamel etched
- Pellicle layer and surface of the enamel structure are modified with the acid-etch technique using 30-50% phosphoric acid
What type of adhesive bonding does the acid-etch technique allow
Micromechanical
What effects does the acid-etch technique
- Increased surface roughness of enamel hence increased bonding area
- Surface energy is raised
- Improved Wettability for resin to run into the enamel rods
- Opens inner prism area for interlocking tag formation
How does the acid-etch technique
- 30-50% phosphoric acid is applied to the enamel surface
- Acid-base reaction is initiated, where the acid dissolves the enamel surface hydroxyapatite and this results in loss of enamel prism structure
- When enamel has frosty appearance the etching is adequate
AY BAWS CAN I HABE DE NOTE PLZ
Over etching can breakdown enamel prisms for tag formation
What do you do after the acid etch technique when applying resins
- Apply resin of low viscosity like unfilled BisGMA before placement of composite
- Unfilled resin must be applied to dry/well etched enamel surface
- Unfilled resin flows into etched enamel prisms and forms resin tags that penetrate to a depth of 30um.
AY BAWS CAN I HABE DE NOTE PLZ
Water - highly polar and resin is non polar so it will not adapt to the wet enamel surface
What can cause the adhesive bond to fail
- Poor clinical technique
- Contamination of enamel surface before placement
What procedures can be done on Acid-Etched enamel
- Fissure sealing
- Direct composite restorations
- Ceramic veneers
- Resin bonded bridges
- Orthodontic brackets
- Periodontal splinting
Why does a restorative resin have to be adhesively bonded to dentin
To retain the restoration in the cavity and to eliminate marginal/internal gaps in order to prevent bacterial microleakage
How does the bonding agent affect the dentin
Provide strength
Seal the dentinal tubules to prevent dentin hypersensitivity
Describe the structure of dentin
- Heterogenous structure, 70% inorganic hydroxyapatite, 20% organic material
- Hydrophilic
- Dentinal tubules communicate with the pulp and are permeable to drugs, chemicals that can damage the pulp
What are the some of the difficulties involved in bonding to dentin
- When the dentin is cut, fluid is pumped through the dentin tubules giving a wet surface
- Inorganic hydroxyapatite crystals are broken up, the collagen is stretched, torn and smeared over the cut surface
“Cut” dentin is covered by a smear layer, what is the smear layer composed of
- A layer of denatured collagen and debris covering surface that is weakly bound to the dentin
- It is contaminated with bacteria and cutting debris
Why is cut dentin hard to bond to
Has a wet surface and the smear layer acts as a contaminant
What must we be careful of when we try to remove the smear layer
The dentin tubules are open and exposed to bacteria and irritants so the adhesive resin must act as an effective seal that will seal the tubules
What are the 3 essential components of using a dentin bonding agent and what order should they be used in
- Conditioner - acid modifies/clears smear layer
- Coupling (bonding) agent/primer - adhesive
- Sealer - bond/adhesive that seals the dentin tubules
How is the dentin conditioner used and what does it do
- Acid solution - stronger acid = more effect
- Removes/modifies the smear layer via acid/base reaction with hydroxyapatite that dissolves
- Rinsed with water
- Demineralised dentin surface left
What does the dentin bonding agent/primer do
Acts as an adhesive that bonds hydrophilic dentin to hydrophobic resin
What is the dentin bonding agent/primer made up of
Bi-functional molecule e.g. hydroxyethyl methacrylate (HEMA), which contains a hydrophobic and hydrophilic end that is dissolved in a solvent
What is the general basic structure of dentin bonding agents
Methacrylate molecule - R - X
What does the methacrylate group in dentin bonding agents/primers bond to
The sealer/unfilled resin
What does the R group in dentin bonding agents/primers allow
This is a spacer molecule that provides flexibility to the coupling agent
What does the X group in dentin bonding agents/primers allow
Polar (hydrophilic end)
Why is the polar end of the dentin bonding agent/primer very important
The polar end interacts with polar groups in the dentinal tubules e.g. hydroxyl groups on apatite and amino groups on collagen
Why is the bonding agent/primer dissolved in a solvent
Needs to penetrate and saturate dentin tubules to a reasonable depth so that is seeks out and displaces water
What solvents are dentin bonding agents/primers dissolved in
Ethanol/Acetone (water chasers)
What is the role of HEMA in the bonding agent/primer
It penetrates the demineralised dentin and bonds to the collagen via hydroxyl group
How do you apply the bonding agent/primer
Important to apply enough so might need a few coats.
Is applied to dentin surface and dried
How is the dentin sealer cured
Light
Give examples of dentin sealer
Unfilled BisGMA or UDMA
Recent sealers = Bis-GMA and HEMA mixture to improve adaptation of sealer to dentin surface
What is the hybrid zone
This is the interpretation layer of dentin and resin
What is the type of bonding between resins and dentin
Micromechanical/entanglement to inorganic salts (Ca2+)
Bonds to collagen via hydroxyl and amino groups
What part of the Dentin primer (coupling agent) makes bonds to what
Methacrylate group of primer chemically bonds to methacrylate group of bond (sealer)
What part of the Bond (sealer) makes bonds to what
Methacrylate group of bond chemically bonds to the methacrylate group on filled composite materials
What are some of the biocompatibility disadvantages of dentin bonding agents and composite restorations
- Excessive desiccation of dentin = Post operative pulpal sensitivity and a poor bond
- Pulpal inflammation if bond failure occurs, leading to bacterial leakage
- Avoid regular direct skin contact with primer/adhesive liquids - delayed allergic reaction of contact dermatitis
Name some causes for the breakdown of an adhesive bond
- Polymerisation shrinkage
- Differential thermal expansion and contraction
- Internal stresses from occlusal loading
- Chemical attack (hydrolysis)
What can a breakdown in the adhesive bond result in
- Introduction of bacteria and debris into the cavity margins
- Unsightly marginal staining
- Pulpal sensitivity
- Restoration may be lost
How are etch and rinse systems used
- 3 (acid etch, primer, bond) applied separately
- Acid applied separately followed by primer and adhesive in one bottle
How are self etch systems used
- Acid + primer in one bottle and adhesive in a separate bottle
- Acid, primer and adhesive in one system (2 components, 4 steps)
