NEET MDS Lessons
Dental Materials
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
Components
a. Fillers added to most to control shrinkage
b. Matrix
Mercury bioactivity
- Metallic mercury is the least toxic from and is absorbed primarily through the lungs rather than the GI tract or skin
- Mercury in the body may come from air, water, food. dental (a low amount). Or medical sources
- Half life for mercury elimination from body is 55 days .-
- mercury toxicity is <50 µm / m3 on average per 40-hour work week.
- Mercury hypersensitivity is estimated as less than 1 per 100,000,000 persons
- Indium-containing amalgams can have lower Hg vapor pressures than conventional dental amalgam
Effects of Amalgam.
(1) The combined effects of the properties of its ingredients should provide the most satisfactory restorative material.
(2) Quantity of mercury. Too little mercury in the mix results in a grainy, weak, readily tarnished, and corroded amalgam. Too much mercury will cause excessive expansion and weakened amalgam.
(3) Composition of the alloy. Composition of the alloy must include a minimum of 65 percent silver, a maximum of 29 percent tin, a maximum of 6 to 13 percent copper, and a maximum of two percent zinc by weight
(4) Correct proportion important. Before use, the silver alloy is mixed with pure and uncontaminated mercury. There are some alloys that are completely zinc free. They can therefore be used more successfully in a moisture-contaminated environment.
(5) Properties of the finished product.
Silver imparts strength, durability, and color, gives the alloy desirable setting expansion, decreases flow, and accelerates (decreases) the setting time.
Tin makes the amalgam easier to work, controls excessive setting expansion, and increases both flow and setting time.
Copper increases hardness, contributes to setting expansion, reduces flow, and decreases setting time.
Zinc increases workability, and unites with oxygen and other "impurities" to produce a clean amalgam.
Reaction
a. Calcium sulfate hemihydrate(one-half water) crystals dissolve and react with water
b. Calcium sulfate dihydrate(two waters) form and precipitate new crystals
c. Unreacted (excess) water is left between crystals in solid
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
CLEANING AND PICKLING ALLOYS
The surface oxidation or other contamination of dental alloys is a troublesome occurrence. The oxidation of base metals in most alloys can be kept to a minimum or avoided by using a properly adjusted method of heating the alloy and a suitable amount of flux when melting the alloy . Despite these precautions, as the hot metal enters the mold, certain alloys tend to become contaminated on the surface by combining with the hot mold gases, reacting with investment ingredients, or physically including mold particles in the metal surface. The surface of most cast, soldered, or otherwise heated metal dental appliances is cleaned by warming the structure in suitable solutions, mechanical polishing, or other treatment of the alloy to restore the normal surface condition.
Surface tarnish or oxidation can be removed by the process of pickling. Castings of noble or high-noble metal may be cleaned in this manner by warming them in a 50% sulfuric acid and water solution . . After casting, the alloy (with sprue attached) is placed into the warmed pickling solution for a few seconds. The pickling solution will reduce oxides that have formed during casting. However, pickling will not eliminate a dark color caused by carbon deposition
The effect of the solution can be seen by comparing the submerged surfaces to those that have still not contacted the solution. the ordinary inorganic acid solutions and do not release poisonous gases on boiling (as sulfuric acid does). In either case, the casting to be cleaned is placed in a suitable porcelain beaker with the pickling solution and warmed gently, but short of the boiling point. After a few moments of heating, the alloy surface normally becomes bright as the oxides are reduced. When the heating is completed, the acid may be poured from the beaker into the original storage container and the casting is thoroughly rinsed with water. Periodically, the pickling solution should be replaced with fresh solution to avoid excessive contamination.
Precautions to be taken while pickling
With the diversity of compositions of casting alloys available today, it is prudent to follow the manufacturer's instructions for pickling precisely, as all pickling solutions may not be compatible with all alloys. Furthermore, the practice of dropping a red-hot casting into the pickling solution should beavoided. This practice may alter the phase structure of the alloy or warp thin castings, and splashing acid may be dangerous to the operator. Finally, steel or stainless steel tweezers should not be used to remove castings from the pickling solutions. The pickling solution may dissolve the tweezers and plate the component metals onto the casting. Rubber-coated or Teflon tweezers are recommended for this purpose.