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.
Manipulation
Selection
o Microfilled composites or hybrids for anterior class III, IV, V
o Hybrids or midifills for posterior class I, II, III, V
Conditioning of enamel and / or dentin
Do not apply fluorides before etching.-->Acid-etch --> Rinse for 20 seconds with water --> Air-dry etched area for 20 seconds but do not desiccate or dehydrate --> Apply bonding agent and polymerize
Mixing (if required)--> mix two pastes for 20 to 30 seconds
o Self-cured composite-working time is 60 to 120 seconds after mixing
o Light-cured composite-working time is unlimited (used for most anterior and some posterior composite restorations)
o Dual-cured composite-working time is > 10 minutes
o Two-stage cured composite-working time is >5 minutes
Placement
use plastic instrument or syringe --> Light curing --> Cure incrementally in <2 mm thick layers. Use matrix strip where possible to produce smooth surface and contour composite .Postcure to improve hardness
Tooth Polishing and Cleansing Agents
1. Cleansing-removal of exogenous stains, pellicle, materia alba, and other oral debris without causing undue abrasion to tooth structure
2. Polishing-smoothening surfaces of amalgam, composite, glass ionomers, porcelain, and other restorative materials
Factors influencing cleaning and polishing
- Hardness of abrasive particles versus substrate
- Particle size of abrasive particles
- Pressure applied during procedure
- Temperature of abrasive materials
Structure
Composition
-contain abrasives, such as kaolinite, silicon dioxide, calcined magnesium silicate, diatomaceous silicon dioxide, pumice. Sodium-potassium
-aluminum silicate, or zirconium silicate; some pastes also may contain sodium fluoride or stannous fluoride, but they have never been shown to produce positive effects
Reactions-abrasion for cleansing and polishing
Properties - Mechanical
- Products with pumice and quartz produce more efficient cleansing but also generate greater abrasion of enamel and dentin
-Coarse pumice is the most abrasive
-The abrasion rate of dentin is 5 to 6 times faster than the abrasion rate of enamel, regardless of the product
-Polymeric restorative materials, such as denture bases, denture teeth, composites, PMMA veneers, and composite veneers, can be easily scratched during polishing
-Do not polish cast porcelain restorations (e.g., Dicor) that are externally characterized or the color will be lost
Manipulation
Mixing
o P/L types mixed in bowl (plaster and alginate)
o Thermoplastic materials not mixed (compound and agar-agar)
o Paste/paste types hand mixed on pad (zinc oxide-eugenol, polysulfide rubber, silicone rubber, polyether rubber. and poly-vinylsiloxane)
o Paste/paste mixed through a nozzle on an auto-mixing gun (poly-vinylsiloxane)
Placement
o Mixed material carried in tray to mouth (full arch tray, quadrant tray. or triple tray)
o Materials set in mouth more quickly because of higher temperature
Removal - rapid removal of impression encourages deformation to take place elastically rather than permanently (elastic deformation requires about 20 minutes)
Cleaning and disinfection of impressions
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
Properties of Amalgam.
The most important physical properties of amalgam are
- Coefficient of thermal expansion = 25-1 >ppm/ C (thus amalgams allow percolation during temperature changes)
- Thermal conductivity-high (therefore, amalgams need insulating liner or base in deep restorations)
- Flow and creep. Flow and creep are characteristics that deal with an amalgam undergoing deformation when stressed. The lower the creep value of an amalgam, the better the marginal integrity of the restoration. Alloys with high copper content usually have lower creep values than the conventional silver-tin alloys.
Dimensional change. An amalgam can expand or contract depending upon its usage. Dimensional change can be minimized by proper usage of alloy and mercury. Dimensional change on setting, less than ± 20 (excessive expansion can produce post operative pain)
- Compression strength. Sufficient strength to resist fracture is an important requirement for any restorative material. At a 50 percent mercury content, the compression strength is approximately 52,000 psi. In comparison, the compressive strength of dentin and enamel is 30,000 psi and 100,000 psi, respectively. The strength of an amalgam is determined primarily by the composition of the alloy, the amount of residual mercury remaining after condensation, and the degree of porosity in the amalgam restoration.
- Electrochemical corrosion produces penetrating corrosion of low-copper amalgams but only produces superficial corrosion of high copper amalgams, so they last longer
- Because of low tensile strength, enamel support is needed at margins
- Spherical high-copper alloys develop high tensile strength faster and can be polished sooner
- Excessive creep is associated with silver mercury phase of low-copper amalgams and contributes to early marginal fracture
- Marginal fracture correlated with creep and electrochemical corrosion in low-copper amalgams
- Bulk fracture (isthmus fracture) occurs across thinnest portions of amalgam restorations because of high stresses during traumatic occlusion and/or the accumulated effects of fatigue
- Dental amalgam is very resistant to abrasion
Acrylic Appliances
Use - space maintenance or tooth movement for orthodontics and pediatric dentistry
1. Components
a. Powder-PMMA powder. peroxide initiator, and pigments
b. Liquid-MMA monomer, hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)
2. Reaction
PMMA powder makes mixture viscous for manipulation before curing . Chemical accelerators cause decomposition of benzoyl peroxide into free radicals that initiate polymerization of monomer . New PMMA is formed into a matrix that surrounds PMMA powder. Linear shrinkage of 5% to 7% during setting. but dimensions of appliances are not critical