NEET MDS Lessons
Dental Materials
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.
Root canal sealers
Applications
Cementation of silver cone gutta-percha point
Paste filling material
Types
Zinc oxide-eugenol cement types
Noneugenol cement types
Therapeutic cement types
properties
Physical-radiopacity
Chemical-insolubility
Mechanical-flow; tensile strength
Biologic-inertness
Gingival tissue packs
Application-provide temporary displacement of gingival tissues
Composition-slow setting zinc oxide-eugenol cement mixed with cotton twills for texture and strength
Surgical dressings
1.Application-gingival covering after periodontal surgery
2. Composition-modified zinc oxide-eugenol cement (containing tannic, acid. rosin, and various oils)
Orthodontic cements
Application-cementation of orthodontic bands
Composition-zinc phosphate cement
Manipulation
Zinc phosphate types are routinely mixed with cold or frozen mixing slab to extend the working time
Enamel bonding agent types use acid etching for improved bonding
Band, bracket, or cement removal requires special care
Solution Liners (Varnishes)
Applications
o Enamel and dentin lining for amalgam restorations
o Enamel and dentin lining for cast restorations that are used with non adhesive cements
o Coating over materials that are moisture sensitive during setting
Components of copal resin varnish
o 90% solvent mixture (e.g., chloroform, acetone, and alcohol)
o 10% dissolved copal resin
Reaction
Varnish sets physically by drying → Solvent loss occurs in 5 to 15 seconds (a film forms the same way as drying fingernail polish)
Manipulation
Apply thin coat over dentin. enamel. And margins of the cavity preparation → Dry lightly with air for 5 seconds Apply a second thin coat → Final thickness is 1 to 5 µ.m
Properties
o Physical
Electrically insulating barrier that prevents shocks. Too thin to be thermally insulating. Decreases degree of percolation attributable to thermal expansion
o Chemical
Forms temporary barrier that prevents microleakage into dentinal tubules until secondary dentin formation occurs. Decreases initial tendency for electrochemical corrosion
o Mechanical
Very weak and brittle film that has limited lifetime
Film adheres to smear layer
Physical reaction-cooling causes reversible hardening
Chemical reaction-irreversible reaction during setting
Classification of Dental amalgam
1. By powder particle shape .
- Irregular (comminuted, filing, or lathecut)
- Spherical (spherodized)
- Blends (e.g., irregular-irregular, irregularspherical, or spherical-spherical)
2. By total amount of copper
- Low-copper alloys (e.g., conventional, traditional); <5% copper
- High-copper alloys (e,g. corrosion resistant); 12% to 28% copper
3.By presence of zinc
Examples
- Low-copper, irregular-particle alloy-silver (70%)-tin (26%)-copper (4%)
- High-copper, blended-particles alloy-irregular particles, silver (70%) –tin (26%) -Copper (4%); spherical particles, silver (72%)-copper (28%)
- High-copper, spherical-particles alloy-silver (60%) - tin (27%)-copper (13%)
Manipulation
1. Selection-based on strength for models, casts, or dies
2. Mixing
(1)Proportion the water and powder
(2) Sift powder into water in rubber mixing bowl
(3) Use stiff blade spatula to mix mass on side of bowl
(4) Complete mixing in 60 seconds
3. Placement
(1) Use vibration to remove air bubbles acquired through mixing
(2) Use vibration during placement to help mixture wet and flow into the impression
Mercury hygiene
- Do not contact mercury with skin
- Clean up spills to minimize mercury vaporization
- Store mercury or precapsulated products in tight containers
- Only triturate amalgam components-in tightly- sealed capsules
- Use amalgam with covers
- Store spent amalgam under water or fixer in a tightly sealed jar
- Use high vacuum suction during amalgam alloy placement, setting, or removal when mercury may be vaporized
- Polishing amalgams generally causes localized melting of silver-mercury phase with release of mercury vapor, so water cooling and evacuation must be used