Module 10: Bonding to Enamel + Dentine

Historical Background

In 1949: development of first dental adhesive for dentine: contained glycerol phosphororic acid dimethacrylate

  • con: faced interfacial stresses bc of high polymerization, shrinkage stress and high thermal expansion, partly due to unfilled methacrylate resin

In the 1950s: Michael Bonnacore discovered that phosphoric acid was able to etch enamel

  • modern is less concentrated

  • leaves a durable micro=mechanical retentive interface with resin base, cements and restorative materials

Late 1970s: Fukayama pioneered total etch system

  • etch both enamel and dentin

  • concern: etching dentin can lead to sensitivity

  • however, the seal of the restoration is more important than sensitivity

1980s: Nakabayashi

  • described resin infiltration of dentin collagen to form a hybrid layer

  • with etching, we removed a lot of the inorganic components (calcium + phosphate) leaving collagen exposed

  • he described the penetratuon of the resin monomers with this area as a “hybrid layer”

Over the next 20 years:

  • dentin bonding agents and systems progressed

  • 3-step, 2-step, 1-step systems

Mid 90s: Canker

  • Introduced technique of wet dentin bonding

  • emphasis on ensuring that the area where dentin etching is not desiccated

  • keep moist in order for prevent collapse of dentin collagen fibrils and maintain space for penetration + entanglement of resin monomer

Last 20 years:

  • self-etching and self-adhesive systems

Adhesion:

  • mechanism of adhesion

    • fundamentally an exchange of hydroxy-apatite with synthetic resins at tooth surface

    • remove hydroxyapatite crystals to create micro pores

    • allowing subsequent polymerization

  • Adhesive bonds are dependent on

    • surface energy, contact angle, wettability

      • adhesive needs to spread over the surace of the prepared tooth surface

      • need to displace any air pockets and penetrate by capillary attraction into microscopic and sub microscopic irregularities

      • wettability of any liquid on a solid is defined by the contact angle that forms between the liquid and solid

      • no wetting = contact angle 180 degrees

      • absolute wetting = 0 degrees

    • interpenetration

      • resin tags form and lock to interpenetrate hard tissue

    • micromechanical interlocking

      • penetration of enamel surface in the irregularities created by etching

    • chemical bonding

      • achieved when acidic monomers with phosphate or carboxyl group potentially forms chemical bond with calcium in the tooth surface

Acid Etching

  • Enamel etching

  • removes smear layer 10 microns of enamel surface to expose enamel rods

  • allows resin monomers to readily wet the surface = better infiltration of mnomers into micropores which polymerize to form resin tags 6 microns in length + 10-20 microns in length

  • Fukayaama introduced total etch

  • Nakabayashi demonstrated the ability of hydrophilic resin to infiltrate the layer of demineralized collagen fibres + form hybrid layer

  • Should see a frosty white surface after drying etched surface

Etched Surface

  • honeycomb surface with high surface enrgy increase the potential for wettability with resin

  • dam cuff should be inverted to prevent contamination

  • If you have used Vaseline on patients' lips or topical anaesthetic like Xylocaine ointment that has glycerin, which can also reduce your bone strength on your etched enamel.

  • If there’s any contamination: re-etch for 10 sec and rinse

  • This step is also useful to make sure you’ve removed all previous restoration resin bc you won’t see white frosting

  • bond to enamel with resin composite is reliable bc it exceeds 37 megapascals (of pressure)

  • dentin is different because of tubules

  • outer dentin has 20 000 tubules within a square millimetre of dentin

    • diameter of tubule lumen = 1 micron

  • deeper dentin has 45 000 tubules/mm

    • diameter increases

  • Hydrodynamic theory: heat pressure from a burr will get movement of the dentinal fluid back into the pulpal area

  • Drying the cavity, eating sweet things, breathing in quickly, outward movement of the fluid within tubules occurs

  • fluid is derived from odontoblasts which sit in the pulpal chamber + have odonto odontoblastic processes which extend into tubule

Longitudinal section of dentin tubules

  • longitudinal section of dentin tubules

  • above is the adhesive layer

    • distinguished from resin composite restorative material on top, which has granular components which are the fillers in the resin composite

    • adhesive or bonding agent has visibly less fillers in order to be of lower viscosity to enable entanglement and impregnation of the collagen fibrils after etching

  • lower half extensions of polymerized resin tags extending into tubules + middle of image is the hybrid layer

Bovine Dentin

  • left is covered by a smear layer

  • right has smear layer removed

  • smear layer consists of organic and inorganic debris such as bacteria, hydroxyapatite crystald or collagen bundles

  • generated by mechanical cutting + shattering on the surface of dentin from the use of hand instruments

  • thickness of a smear layer has been reported to be as thin as 0.5 to 10 microns

  • smear layer is loosely attached to dentin surface

  • longitudinal section of dentin that’s been etched w/ smear layer

  • etch has removed hydoxyapatite from the dentin, peritubular dentin + walls of the dentinal tubule

  • orfices are lateral cannals that communicate the tubules w/ eachother

    • resin tags can form here

  • dentinal tubular fluids

  • concentration of acid in primer is highest at the top and gets lower

  • priming ensures sufficient wetting of exposed collagen fibrils + removal of water from dentinal tubule

  • wetting causes impregnation of the resin in the tubules + removing water helps

  • low molecular weight = popular, easily distributed

  • if collagen fibres collapse, it reduces entanglement

  • squiggly line = collagen surface

  • forms bonds with calcium

  • these bonds form during priming

  • solvents removed by gently blow-drying, too hard desiccates + collapses collagen

  • dentine surface is hydrophilic

  • resin material is hydrophobic

What happens to the hybrid layer over time?

  • Hybrid layer is susceptible to water sorption, hydrolytic degradation and resin leaching

Enzyme Inhibitors:

  • thus longevity of adhesive interface is increased when nonspecific enzyme-inhibiting strategies are used such as:

    • chlorhexidine

    • benzalkonium chloride

  • These inhibitors are used in therapeutic primers, W in the resin bonds

Future Direction:

  • may focus on reducing collagen degradations

  • increasing extent of collagen cross-linking prior to application of adhesive

  • carbodiimide, glutaraldehyde, proanthocyanidin

  • on the left, dentin has smear layer + smear plug

  • self-etch with mild primer (2) + resin impregnated smear

  • stronger intermediary = remove smear + plug

  • with etch and rinse, more of the smear layer has been removed

  • resin tags are more extensive w/ a funnel shape

  • self-etch not as effective bc not as much smear removed and less extensive resin tags

Clinical Performance:

  • three-step etch-rinse adhesives and two-step self-etch adhesives show clinically reliable and preditably good clinical performance

  • two-step etch-and-rinse adhesives were less clinically effective

  • ineffecient clinical performance was found for one-step self-etch adhesives

  • cervical restorations placed with an etch-and-rinse adhesive shower higher retention than an all-in-one adhesive

    • clinical evaluation of different adhesives used in restoration of non-carious cervical lesions: 24 month results

Class Notes:

Etch:

  • rinse etch w same amount of time etch sat

  • dry thoroughly = white frosty appearance

    • if not, there are contaminates or previous resin

  • why is hydrophilic resin important?

    • dentinal fluid from odontoblasts

    • don’t dessicate bc collagen fibres collapse + prevent entanglement

    • want a hybrid layer to form (demineralized collagen + infiltrated resin) - nakabayashi

  • primer has an acid + has an etchant itself

  • maintain the smear layer (primer modifies the smear layer while making sure to expose too much collagen fibres)

  • etch activates enzymes that are imbedded into dentin (can disintegrate collagen)

  • collagen is wet so we need a hydrophilic component in bond, and resin

  • when placing primer, don’t dessicate (gentle airdry)

  • primer on dentin for 20s, modifies the smear layer, gently agitate to remove solvents (or airdry)

  • place bond, blow to thin layer + remove air bubbles

  • light cure, then place resin

  • Self-etch adhesive systems combine etching and priming into one step

  • these principals apply to cementing crowns, bridges, other appliances

Primer:

  • funnel shape of dentinal tubule because the etchant concentration lowers the further down you go

  • orfices in the dentinal tubule allow resin to seep in?

  • entangle + impregnate

  • bc of liquid from dentinal fluid, fluid is similar to interstitial fluid makes it difficult for resin to enter (causes outwards pressure)

  • fluid in dentinal tubule would be reduced with adrenaline from local anaesthetic

  • hybrid layer should be stable

  • discovery of having HEMA as hydrophilic substance in two step etch and rinse

  • hydrophobic part of HEMA binds with the resin (interface)

Solvents:

  • water, ethanol, acetone

  • water ionizes the acidic monomers as well as expand the collagen network

  • conditioning the collagen fibres upright to encourage entanglement

  • ethanol and acetone have water chasing properties to penetrate the tubules

  • REMOVE SOLVENT

  • after primer, use microbrush to agitate to evolve the solvent + use for 20 sec + air dry

  • place bond/adhesive + form interface between collagen fibres + resin composite

  • 3-5% of resin undergoes polymerization shrinkage (leaves a gap between tooth + restorative material)

  • resin may also pool onto hybrid (causes pulling away from dentin)

  • flowable composite tends to pool

  • pulsing cure light allows periods of relaxation and curing

Bond:

  • bond is intermediary layer between tooth + resin composite

  • fill interfibrular space + form resin tags

  • adhesive needs to be both hydrophobic and hydrophilic

    • resin is hydrophobic

    • dentinal tubules are hydrophilic

Longitudinal section of dentin tubules

Initiators:

  • champoroquinone is a photoinitiator

  • self cure systems exist aswl (may not be able to light cure smtms)

    • extremely deep cavity

    • crown

Classifications:

  • image in slides