NEET MDS Lessons
Dental Materials
INVESTING
Mixing investment with distilled water is done according to the manufacturers ratio in a clean dry bowl without entrapment of the air into the mix.
Mixing methods:
a. Hand mixing and the use of the vibrator to remove air bubbles.
b. Vacuum mixing- This is the better method because it removes air bubbles as well as gases that are produced and thus produces a smoother mix.
Methods of investing:
a. Hand investing
b. Vacuum investing
Hand investing:
First the mixed investment is applied on all the surfaces of the pattern with a soft brush. Blow off any excess investment gently, thus leaving a thin film of investment over the pattern, then apply again.
Then the coated pattern can be invested by two methods;
1. Placing the pattern in the ring first and then filling the ring full with investment.
2. Filling the ring with the investment first and then force the pattern through into it.
Vacuum investing :
Vacuum investing unit: This consists of the chamber of small cubic capacity from which air can be evacuated quickly and in which casting ring can be placed.
Evacuation of air can be done by electrically or water driven vacuum pump.
Procedure:
The ring filled with investment is placed in the vacuum chamber. Air entry tube is closed. Then the vacuum is applied. The investment will rise with froth vigorously for about 10-15 sec and then settles back. This indicates that air has been extracted from the ring. The pressure is now restored to atmospheric by opening the air entry tap gradually at first and then more rapidly as the investment settles back around the pattern. Then the ring is removed from the chamber and the investment is allowed to set. Modern investing unit does both mixing and investing under vacuum and is considered better than hand mixing and pouring.
Then there are two alternatives to be followed depending upon what type of expansion is to be achieved in order to compensate for metal shrinkage. They are:
1. If hygroscopic expansion of the investment is to be achieved then immediately immerse the filled ring in water at the temperature of 37C.
Or “under controlled water adding technique”. A soft flexible rubber ring is used instead of usual lined metal ring. Pattern is invested as usual. Then specified amount of water is added on top of the investment in the rubber ring and the investment is allowed to set at room temperature. In this way only enough water is added to the investment to provide the desired expansion.
2. If thermal expansion of the investment is to be achieved, then investment is allowed to set by placing the ring on the bench for 1 hour or as recommended by the manufacturer.
RINGLESS INVESTMENT TECHNIQUE
Used for phosphate bonded investments .
This method uses paper or plastic casting ring .
It is designed to allow urestricted expansion .
Useful for high melting alloys .
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
Mouth Protectors
Use - to protect against effects of blows to chin, top of the head, the face, or grinding of the teeth
Types
o Stock protectors-least desirable because of poor fit
o Mouth-formed protectors-improved fit compared with stock type
o Custom-made protectors-preferred because of durability. low speech impairment, and comfort
I. Components
a. Stock protectors-thermoplastic copolymer of PYA-PE (polyvinyl acetate-polyethylene copolymer)
b. Mouth-formed protectors-thermoplastic copolymer
c. Custom-made protectors- thermoplastic copolymer, rubber. or polyurethane
2. Reaction-physical reaction of hardening during cooling
3. Fabrication
Alginate impression made of maxillary arch. High-strength stone cast poured immediately. Thermoplastic material is heated in hot water and vacuum-molded to cast .
Mouth protector trimmed to within ½ inch of labial fold, clearance provided at the buccal and labial frena, and edges smoothed by flaming. Gagging, taste, irritation. and impairment of speech are minimized with properly fabricated appliances
4. Instructions for use
a. Rinse before and after use with cold water
b. Clean protector occasionally with soap and cool water
c. Store the protector in a rigid container
d. Protect from heat and pressure during storage
e. Evaluate protector routinely for evidence of deterioration
Properties
1. Physical-thermal insulators
2. Chemical-absorbs after during use
3. Mechanical-tensile strength, modulus, and hardness decrease after water absorption, but elongation, tear strength, and resilience increase
4. Biologic-nontoxic as long as no bacterial, fungal, or viral growth occurs on surfaces between uses
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 .
Applications
a. Dentulous impressions for casts for prosthodontics
b. Dentulous impressions for pedodontic appliances
c. Dentulous impressions for study models for orthodontics
d. Edentulous impressions for casts for denture construction
Principles of cutting, polishing, and surface cleaning
- Surface mechanics for materials
Cutting-requires highest possible hardness materials to produce cutting
Finishing-requires highest possible hardness materials to produce finishing, except at margins of restorations where tooth structure may be inadvertently affected
Polishing- requires materials with Mohs ./ hardness that is 1 to 2 units above that of substrate
Debriding-requires materials with Mohs hardness that is less than or equal to that of substrate to prevent scratching
- Factors affecting cutting, polishing. and surface cleaning
- Applied pressure
- Particle size of abrasive
- Hardness of abrasive
- Hardness of substrate
- Precautions
- During cutting heat will build up and change the mechanical behavior of the substrate from brittle to ductile and encourage smearing
- Instruments may transfer debris onto the cut surface from their own surfaces during cutting, polishing, or cleaning operations (this is important for cleaning implant surfaces)