CAD/CAM Restorations

Applications-inlays, onlays, veneers, crowns, bridges, implants, and implant prostheses    

Stages of fabrication
 
CSD-computerized surface digitization
CAD-computer-aided (assisted) design
CAM-computer-aided (assisted) machining
CAE-computer-aided esthetics (currently theoretic)
CAF-computer-aided finishing or polishing (which are currently theoretic steps)

Classification

Chairside or in-office systems

(1) Cerec (Siemens system)-inlays, onlays, veneers
(2) Sopha (Duret system)-inlays, onlays  (and Crowns)

Laboratory systems

(1) DentiCAD (Rekow system)-inlay, onlays, veneers, crowns
(2) Cicero (Elephant system)-porcelain fused-to-metal crowns

 
Materials

a. Feldspathic oorcelains (Vita)
b. Machinable ceramics (Dicor MGC)
c. Metal alloys limited use)

Cementing

- Etching enamel and/or dentin for micromechanical retention
- Bonding agent for retention to etched surface
- Composite as a luting cement for reacting chemically with bonding agent and with silanated surface of restoration
- Silane for bonding to etched ceramic (or metal) restorations and to provide chemical reaction
- Hydrofluoric acid etching to create spaces for micromechanical retention on surface or restoration

Properties

1. Physical properties

a. Thermal expansion coefficient well matched to tooth structure
b. Good resistance to plaque adsorption or retention

2. Chemical properties-not resistant to acids and should be protected from APF

3. Mechanical properties

a. Excellent wear resistance (but may abrade opponent teeth)
b. Some wear of luting cements but self-limiting
c. Excellent toothbrush abrasion

4. Biologic properties-excellent properties
 

Components 

a. Fillers added to most to control shrinkage
b. Matrix

Investment Materials

Investment is mold-making material

Applications

a. Mold-making materials for casting alloys
b. Mold-making materials for denture production

Classification

a. Gypsum-bonded investments (based on gypsum products for matrix)

b. Phosphate-bonded investments

c. Silicate-bonded investments

Components

a. Liquid-water or other reactant starts formation of matrix binder by reacting with reactant powder
b. Powder-reactant powder, filler, or modifiers

Manipulation

a. P/L mixed and placed in container around wax pattern
b. After setting, the investment is heated to eliminate the wax pattern in preparation for casting
 

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
 

Waxes

Many different waxes are used in dentistry. The composition, form, and color of each wax are designed to facilitate its use and to produce the best possible results.

Applications

o    Making impressions
o    Registering of tooth or soft tissue positions
o    Creating restorative patterns for lab fabrication
o    Aiding in laboratory procedures

Classification

a. Pattern waxes-inlay, casting, and baseplate waxes
b. Impression waxes-corrective and biteplate waxes
c. Processing waxes-boxing, utility, and sticky waxes

Types

1) Inlay wax-used to create a pattern for inlay, onlay or crown for subsequent investing and casting in a metal alloy.
2) Casting wax-used to create a pattern for metallic framework for a removable partial denture
3) Baseplate wax-used to establish the vertical dimension. plane of occlusion. and  initial arch form of a complete denture
4) Corrective impression wax-used to form a registry pattern of soft tissues on an impression
5) Bite registration wax-used to form a registry pattern for the occlusion of opposing models or casts
6) Boxing wax-used to form a box around an impression before pouring a  model or cast
7) Utility wax -soft pliable adhesive wax for modifying appliances, such as alginate impression trays
8) Sticky  wax-sticky when melted and used to temporarily adhere pieces of metal or resin in laboratory procedures

Components

a. Base waxes-hydrocarbon (paraffin) ester waxes    
b. Modifier waxes-carnauba, ceresin, bees wax, rosin, gum dammar, or microcrystalline waxes
c. Additives-colorants

Reaction-waxes are thermoplastic

Properties

Physical

a. High coefficients of thermal expansion and contraction
b. Insulators and so, cool unevenly; should be waxed in increments to allow heat dissipation

Chemical

a. Degrade prematurely if overheated
b. Designed to degrade into CO2and H2Oduring burnout

Mechanical-stiffness, hardness, and strength depend on modifier waxes used
 

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 .