COMPOSITE RESINS

Types

• Amount of filler-25% to 65% volume, 45% to 85% weight
• Filler particle size (diameter in microns)
  • Macrofill 10 to 100 µm (traditional composites)
  • Midi fill- 1 to 10 µm(small particle composites)
  • Minifill— 0.l to 1 µm
  • Microfill-: 0.01 to  0.1 µm (fine particle composites)
  • Hybrid--blend (usually or  microfill and midifill or minifill and microfill)
• Polymerization method
  • Auto-cured (self-cured)
  • Visible light cured
  • Dual cured
  • Staged cure
• Matrix chemistry
  • BIS-GMA type
  • Urethane dimethacrylate (UDM or UDMA) type
  • TEGDMA-diluent monomer to reduce  viscosity

Applications/Use

• Load -bearing restorations for posterior  teeth  (class I, II)
• Pinned restorations
• Buildups or cores for cast restorations
• Retrograde canal filling material

 (1) Alloy. An alloy is a solid mixture of two or more metals. It is possible to produce a material in which the desirable properties of each constituent are retained or even enhanced, while the less desirable properties are reduced or eliminated.

(2) Amalgam. When one of the metals in an alloy mixture is mercury, an amalgam is formed. A dental amalgam is a combination of mercury with a specially prepared silver alloy, which is used as a restorative material.

(3) Mercury. Mercury is a silver-white, poisonous, metallic element that is liquid at room temperature

Denture Liners

Use - patients with soft tissue irritation

Types

Long-term liners (soft liners)-used over a period of months for patients with severe undercuts or continually sore residual ridges

Short-term liners (tissue conditioners)-used to facilitate tissue healing over several days

Structure

Soft liners-plasticized acrylic copolymers or silicone rubber

Tissue conditioners-PEMA plasticized with ethanol and aromatic esters

Properties

Liners flow under low pressure, allowing adaptation to soft tissues, but are elastic during chewing forces. 
Low initial hardness, but liner becomes  harder as plasticizers are leached out during intraoral use 
Some silicone rubber liners support growth of yeasts
 

Denture Teeth

Use-complete or partial dentures

Type

a. Porcelain teeth
b. Acrylic resin teeth
c. Abrasion-resistant teeth (microfilled composite)

Structure and properties

1. Porcelain teeth (high-fusing porcelain)
Only bonded into denture base mechanically. Harder than natural teeth or other restorations and abrades those surfaces. Good aesthetics.Used when patients have good ridge support and sufficient room between the arches

2. Acrylic resin teeth (PMMA  [polymethyl methacrylate])

Bonded pseudochemically into the denture base. Soft and easily worn by abrasive foods . Good initial aesthetics
Used with patients with poor ridges and in cases where they oppose natural teeth

3. Abrasion-resistant teeth (microfilled resins)
Bonded pseudochemically into the denture base.Better abrasion resistance then  acrylic resin teeth
 

Mercury hygiene

• Do not contact mercury with skin
• Clean up spills to minimize mercury vaporization
• Store mercury or precapsulated products in tight containers
• Only triturate amalgam components-in tightly- sealed capsules
• Use amalgam with covers
• Store spent amalgam under water or fixer in a tightly sealed jar
• Use high vacuum suction during amalgam alloy placement, setting, or removal when mercury may be vaporized
• Polishing amalgams generally causes localized melting of silver-mercury phase with release of mercury vapor, so water cooling and evacuation must be used

CASTING DEFECTS

Classification :

1) Distortion.
2) Surface roughness .
3) Porosity .
4)Incomplete casting .
5) Oxidation .
6) Sulfur contamination .

Distortion
It is usually due to the distortion of wax pattern.

To avoid this :
Manipulation of the wax at its softening temp
Invest the pattern at the earliest .
If storage is necessary store it in a refrigerator .
Surface roughness

May be due to :
Air bubbles on the wax pattern .
Cracks due to rapid heating of the investment .
High W/P ratio .
Prolonged heating of the mold cavity .
Overheating of the gold alloy .
Too high or too low casting pressure .
Composition of the investment .
Foreign body inclusion.

POROSITY
May be internal or external .
External porosity causes discolouration .
Internal porosity weakens the restoration .

Classification of porosity .
I .Those caused by solidification shrinkage :
a) Localised shrinkage porosity .
b) Suck back porosity .
c) Microporosity .

They are usually irregular in shape .

II ) Those caused by gas :

a) Pin hole porosity .
b) Gas inclusions .
c) Subsurface porosity .

Usually they are spherical in shape .

III ) Those caused by air trapped in the mold :

Back pressure porosity .

Localised shrinkage porosity

Large irregular voids found near sprue casting junction.
Occurs when cooling sequence is incorrect .
If the sprue solidifies before the rest of the casting , no more molten metal is supplied from the sprue which can cause voids or pits (shrink pot porosity )

This can be avoided by -
- using asprue of correct thickness .
- Attach the sprue to the thickest portion of the pattern .
-Flaring of the sprue at the point of atttachment .
-Placing a reservoir close to the pattern .

Suck back porosity

It is an external void seen in the inside of a crown opposite the sprue .
Hot spot is created which freezes last .
It is avoided by :
Reducing the temp difference between the mold & molten alloy .

Microporosity :

Fine irregular voids within the casting .
Occurs when casting freezes rapidly .
Also when mold or casting temp is too low .

Pin hole porosity :
Upon solidification the dissolved gases are expelled from the metal causing tiny voids .
Pt & Pd absorb Hydrogen .
Cu & Ag absorb oxygen .

Gas inclusion porosities

Larger than pin hole porosities .
May be due to dissolved gases or due to gases Carried in or trapped by molten metal .
Apoorly adjusted blow torech can also occlude gases .

Back pressure porosity

This is caused by inadequate venting of the mold .The sprue pattern length should be adjusted so that there is not more than ¼” thickness of the investmentbetween the bottom of the casting .
This can be prevented by :
- using adequate casting force .
-use investment of adequate porosity .
-place the pattern not more than 6-8 mm away from tne end of the casting .
Casting with gas blow holes
This is due to any wax residue in the mold .
To eliminate this the burnout should be done with the sprue hol facing downwards for the wax pattern to run down.

Incomplete casting

This is due to :
- insufficient alloy .
-Alloy not able to enter thin parts of the mold .
-When the mold is not heated to the casting temp .
-Premature solidification of the alloy .
-sprues blocked with foreign bodies .
-Back pressure of gases .
-low casting pressure .
-Alloy not sufficiently molten .

Too bright & shiny casting with short & rounded margins :
occurs when wax is eliminated completely ,it combines with oxygen or air to form carbon monoxide .

Small casting :

occurs when proper expansion is not obtained & due to the shrinkage of the impression .

Contamination of the casting
1) Due to overheating there is oxidation of metal .
2) Use of oxidising zone of the flame .
3) Failure to use a flux .
4) Due to formation sulfur compounds .

Black casting

It is due to :
1) Overheating of the investment .
2) Incomplete elimination of the wax .