Properties of Acrylic Resins.

• They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
• Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
• Mixing. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed
• Poor distortion resistance at higher temperatures, therefore dentures should not be cleaned in hot water
• Good resistance to color change
• Absorbs water and must be kept hydrated  (stored in water when not in mouth) to prevent dehydration cycling and changes in dimensions
• Not resistant to strong oxidizing agents
• Low strength; however, flexible, with good fatigue resistance
• Poor scratch resistance; clean tissue-bearing surfaces of denture with soft brush and do not use abrasive cleaners

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
 

Dental Implants

Applications/Use
 
Single-tooth implants
Abutments for bridges (freestanding, attached to natural teeth)
Abutments for over dentures

Terms

Subperiosteal- below the periosteum -but above the bone (second most frequently used types)
Intramucosal-within the mucosa
Endosseous into the bone  (80%of all current types)
Endodontics-through the root canal space and into the periapical bone
Transosteal-through the bone
Bone substitutes -replace. Long bone

Classification by geometric form

Blades
Root forms
Screws
Cylinders
Staples
Circumferential
Others

Classification by materials type

Metallic-titanium, stainless steel, and .chromium cobalt
Polymeric-PMMA
Ceramic hydroxyapatite, carbon, and sapphire

Classification by attachment design

Bioactive surface retention by osseointegration
Nonative porous surfaces for micromechanical retention by osseointegration
Nonactive, nonporous surface for ankylosis. By osseointegration 
Gross mechanical retention designs (e.g.. threads, screws, channels, or transverse holes)
Fibrointegration by formation of fibrous tissue capsule
Combinations of the above

Components

a. Root (for. osseointegration)
b. Neck (for epithelial attachment and percutancaus sealing)
c. Intramobile elements (for shock absorption)
d. Prosthesis (for dental form and function)

Manipulation

a. Selection-based on remaining bone architecture and dimensions
b. Sterilization-radiofrequency glow discharge leaves biomaterial surface uncontaminated and sterile; autoclaving or chemical sterilization is contraindicated for some designs

Properties

1. Physical-should have low thermal and electrical conductivity

2. Chemical

a. Should be resistant to electrochemical corrosion
b. Do not expose surfaces to acids (e.g.. APF fluorides).
c. Keep in mind the effects of adjunctive therapies (e.g., Peridex)

3. Mechanical
a. Should be abrasion resistant and have a high modulus
b. Do not abrade during scaling operations (e.g.with metal scalers or air-power abrasion systems like  Prophy iet)

4. Biologic-depend on osseointegration and epithelial attachment

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

CASTING
Melting & Casting Technique Melting & Casting requires Heat source to melt the alloy Casting force, to drive the alloy into the mould

Casting Torch Selection Two type of torch tips: Multi-orifice Single-orifice Multi-orifice tip is widely used for metal ceramic alloys. Main advantage is distribution of heat over wide area for uniform heating of the alloy. Single-orifice tip concentrate more heat in one area.Three fuel sources are used for Casting Torch; Acetylene ,Natural Gas ,Propane

CASTING CRUCIBLES
Four types are available ;
1) Clay .
2) Carbon .
3) Quartz .
4) Zirconia –Alumina .

Casting Machines

It is a device which uses heat source to melt the alloy casting force .

Heat sources can be :
1) Reducing flame of a torch .( conventional alloys & metal ceramic alloys )

2) Electricity .(Base metal alloys )

Advantages of electric heating :
-heating is evenly controlled .
-minimal undesirable changes in the alloy composition .
- Appropriate for large labs .

Disadvantage :
Expensive .
Casting machines use :
1) Air pressure .
2) Centrifugal force .
3) Evacuation technique .

Alloys can be melted by :
1) Alloy is melted in a separate crucible by a torch flame & is cast into the mold by centrifugal force .(centrifugal C M )
2) Alloy is melted by resistance heating or by induction furnace & then cast centrifugally by motor or spring action (springwound CM electrical resistance )
3) Alloy is melted by induction heating cast into mold centrifugally by motor or spring action .(Induction CM )
4) Alloy is vacum melted by an argon atmosphere

Torch melting / Centrifugal casting machine
Electrical resistance /Heated casting machine
Melting of the alloy should be done in a graphite or ceramic crucible .

Advantage :
-Oxidation of metal ceramic restorations on
overheating is prevented .
-Help in solidification from tip of the casting to the button surface .

Induction casting machine
Commonly used for melting base metal alloys.

Advantage :
- Highly efficient .
- Compact machine withlow power consumption
-No pre heating needed ,
- safe & reliable.

Direct current arc melting machine

A direct current arc is produced between two electrodes :
The alloy & the water cooled tungsten electrode .Temp used is 4000 degrees .

Disadvanage :
High risk of overheating the alloy .
Vacuum or pressure assisted casting machine
Molten alloy is drawn into the evacuated mold by gravity or vacuum & subjected to aditional pressure
For Titanium & its alloys vacuum heated argon pressure casting machines are used .

Accelerated casting method

This method reduces the time of both bench set of the investment & burnout .
Uses phosphate bonded investments which uses 15 mnts for bench set & 15mnts for burnout by placing in a pre – heated furnace to 815 degrees .

Effect of burnout on gypsum bonded investments
Rate of heating has influence on smoothness & on overall dimensions of the investment
Rapid heating causes cracking & flaking which can cause fins or spines .
Avoid heating gypsum bonded investment above 700 degrees .Complete the wax elimination below that temp .

Effect of burnout on phosphate bonded investments
Usual burnout temp is 750 -1030 degrees.
Although they are strong they are brittle too .
Since the entire process takes a long time two stage burnout & plastic ring can be used .

Composition of Acrylic Resins.

·        Powder. The powder is composed of a polymethyl methacrylate (PMMA), peroxide initiator, and pigments

·        Liquid. The liquid is a monomethyl methacrylate (MMA), hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)