Bonding Agents

Applications-composites, resin-modified gIass ionomers, ceramic bonded to enamel restorations, veneers, orthodontic brackets, and desensitizing dentin by covering exposed tubules (Maryland bridges, composite and ceramic repair systems, amalgams and amalgam repair, and pinned amalgams)

Definitions;-

Smear layer - Layer of compacted debris on enamel and/or dentin from the cavity preparation process  that is weakly held to the surface (6 to 7 MPa) , and that limits bonding agent strength if not removed

Etching (or, conditioning)- smear layer removal and production of microspaces for micromechanical bonding by dissolving –minor amounts of surface hydroxyapatite crystals

Priming..- micromechanical (and chemical) bonding to the microspaces created by conditioning step.

Conditioning/priming agent-agent that accomplishes both actions

Bonding- formation of resin layer that connect  the primed surface to the overlying restoration (e.g., composite) .. –

Enamel bonding System-for bonding to enamel (although dentin bonding may be a Second step)

Dentin bonding system  for bonding  to dentin (although  enamel bonding  may have been a first step)

•        First-generation dentin bonding system for bonding to smear layer

•        New-generation dentin bonding system- for removing smear layer and etching intertubular dentin to allow  primer and/or bonding agent to diffuse into spaces between collagen and form hybrid zone

Enamel and dentin bonding system-for bonding to enamel and dentin surfaces with the same procedures

Amalgam bonding  system for bonding to enamel, dentin, and amalgam, dentin and amalgam during an amalgam placement procedure or for amalgam repair

Universal bonding system-for bonding to enamel, dentin, amalgam, porcelain , or any other substrate intraorally that may be necessary for a restorative procedure  using the  same set of procedures and materials

Types

Enamel bonding systems

Dentin bonding systems

Amalgam bonding systems

Universal bonding systems

Structure

o        Components of bonding systems

o        Conditioning agent-mineral or organic acid

Enamel only   37% phosphoric acid

Dentin only or enamel and .dentin---37% phosphoric acid, citric acid, maleic acid, or nitric acid

o        Priming agent

Hydrophobic-solvent-soluble, light cured monomer system

Hydrophilic-water-soluble, light-cured monomer system

Bonding agent

BIS-GMA-type monomer system

UDMA-type monomer system

Reaction

Bonding occurs primarily by intimate micromechanical retention with the relief created by the conditioning step

Chemical bonding is possible but is not recognized as contributing significantly to the overall bond strength

Manipulation-follow manufacturer's directions

Properties

Physical-thermal expansion and contraction may create fatigue stresses that debond the interface and permit micro leakage

Chemical-water absorption into the bonding agent may chemically alter the bonding

Mechanical-mechanical stresses may produce fatigue that debonds the interface and permits microleakage

Enamel bonding-adhesion occurs by macrotags (between enamel prisms) and microtags (into enamel prisms) to produce micromechanical retention

Dentin bonding-adhesion occurs by penetration of smear layer and formation of microtags into intertubular dentin to produce a hybrid zone (interpenetration zone or diffusion zone) that microscopically intertwines collagen bundles and bonding agent polymer

Biologic

Conditioning agents may be locally irritating if they come into contact with soft tissue

Priming agents (uncured), particularly those based on HEMA, may be skin sensitizers after several contacts with dental personnel

Protect skin on hands and face from inadvertent contact with unset materials and/ or their vapors

HEMA and other priming monomers may penetrate through rubber gloves in relatively short times (60 to 90 seconds)

COMPOSITE RESINS

Applications / Use

• Anterior restorations for aesthetics (class III, IV, V, cervical erosion abrasion lesions)
• Low-stress posterior restorations (small class I, II)
• Veneers
• Cores for cast restorations
• Cements for porcelain restorations
• Cements for acid-etched Maryland bridges
• Repair systems for composites or porcelains

Polymerization--reaction of small molecules (monomers) into very large molecules (polymers)

Cross-linking-tying together of polymer molecules by chemical reaction between the molecules to produce a continuous three-dimensional network

Lost Wax Process

The lost wax casting process is widely used as it offers asymmetrical casting withnvery fine details to be manufactured relatively inexpensively. The process involves producing a metal casting using a refractory mould made from a wax replica pattern.
The steps involved in the process or the lost wax casting are:

1 . Create a wax pattern of the missing tooth / rim
2 . Sprue the wax pattern
3 . Invest the wax pattern
4 . Eliminate the wax pattern by burning it (inside the furnace or in hot water). This will create a mould.
5 . Force molten metal into the mould - casting.
6 . Clean the cast.
7 . Remove sprue from the cast
8 . Finish and polish the casting on the die .

The lost-wax technique is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burned out ("lost") to create a space into which molten metal is placed or cast. The entire lost-wax casting process . 

Wax pattern removal:

Sprue former can be used to remove the pattern. If not the pattern is removed with a sharp probe. Then the sprue former is attached to it. The pattern should be removed directly in line with the principle axis of the tooth or the prepared cavity. Any rotation of the pattern will distort it. Hollow sprue pin is advisable because of its greater retention to the pattern.

Properties

I. Physical

a. Excellent thermal and electrical insulator
b. Very dense
c. Excellent dimensional stability
d. Good reproduction of fine detail of hard and soft tissues

2. Chemical

a. Heating will reverse the reaction (decompose the material into calcium sulfate hemihydrate, the original dry component)
b. Models, casts, and dies should be wet during grinding or cutting operations to prevent heating

3. Mechanical

a. Better powder packing and lower water contents at mixing lead to higher compressive strengths (plaster < stone < diestone)
b. Poor resistance to abrasion

4. Biologic

a. Materials are safe for contact with external - epithelial tissues
b. Masks should be worn during grinding or polishing operations that are likely to produce gypsum dust

Chromium Alloys for Partial Dentures

Applications - Casting partial denture metal frameworks

Classification

a. Cobalt-chromium
b. Nickel-chromium
c. Cobalt-chromium-nickel

Composition

a. Chromium-produces a passivating oxide film for corrosion resistance
b. Cobalt-increase~ the rigidity of the alloy
c. Nickel-increases the ductility of the alloy
d. Other elements-increase strength and castability

Manipulation

a. Requires higher temperature investment materials
b. More difficult to cast because less dense than gold alloys usually requires special casting equipment
c. Much more difficult to finish and polish because of higher strength and hardness

Properties

a. Physical-less dense_than gold alloys
b. Chemical-passivating corrosion behavior
c. Mechanical-stronger. stiffer. and harder than gold alloys
d. Biologic

-Nickel may cause sensitivity in some individuals (I % of men and 11 % of women)
-Beryllium in some alloys forms oxide that  is toxic to lab technicians

Investment Techniques 

Single step investing technique:
The investing procedure is carried out in one step either by brush technique or by vacuum technique.

a). Brush technique:
The accurate water-powder ratio is mixed under vacuum. A brush is then used to paint the wax pattern with mix then the casting ring is applied over the crucible former and the ring is filled under vibration until it is completely filled.

b). vacuum technique:
• The mix in first hand spatulated, and then with the crucible former and pattern is place, then ring is attached to the mixing bowl.
• The vacuum hose is then attached to the assembly. The bowel is inverted and the ring is filled under vacuum and vibration

Two-step investing technique:

The investing procedure is carried out in two steps:

• First, the wax pattern is painted with a thick mix andis left till complete setting, the set investment block(first cost) is immersed in water for about tenminutes . the casting ring is then applied over the crucible former and filled with the properly mixedinvestment (second coat) till the ring is completely filled and the mix is left to set.The two-step investing technique is recommendedwhenever greater amount of expansion is required. Thistechnique also minimizes the distortion of the waxpattern and provides castings with smoother surfaces.

• The investment is allowed to set for the recommendedtime (usually one-hour) then the crucible former is removed. If a metal sprue former is used, it is removedby heating over a flame to loosen it from the wax pattern. Any loose particles of investment should beblown off with compressed air should be placed in a humidor if stored overnight.