NEET MDS Lessons
Dental Materials
METALLURGICAL TERMS
a. Cold Working. This is the process of changing the shape of a metal by rolling, pounding, bending, or twisting at normal room temperature.
b. Strain Hardening. This occurs when a metal becomes stiffer and harder because of continued or repeated application of a load or force. At this point, no further slippage of the atoms of the metal can occur without fracture.
c. Heat Softening Treatment (Annealing). This treatment is necessary in order to continue manipulating a metal after strain hardening to prevent it from fracturing. The process of annealing consists of heating the metal to the proper temperature (as indicated by the manufacturer's instructions) and cooling it rapidly by immersing in cold water. Annealing relieves stresses and strains caused by cold working and restores slipped atoms within the metal to their regular arrangement.
d. Heat Hardening Treatment (Tempering). This treatment is necessary to restore to metals properties that are decreased by annealing and cold working. Metals to be heat hardened should first be heat softened (annealed) so that all strain hardening is relieved and the hardening process can be properly controlled. Heat hardening is accomplished in dental gold alloy by heating to 840o Fahrenheit, allowing it to cool slowly over a 15-minute period to 480o Fahrenheit, and then immersing it in water.
Stages of manipulation
Definitions of intervals
- Mixing interval-length of time of the mixing stage.
- Working interval-length of time of the working stage
- Setting interval-length of time of the setting stage
Definitions of times
- Mixing time-the elapsed time from the onset to the completion of mixing
- Working time-the elapsed time from the onset of mixing until the onset of the initial setting time
- Initial setting time-time at which sufficient reaction has occurred to cause the materials to be resistant to further manipulation
- Final setting time-time at which the material practically is set as defined by its resistance to indentation
[All water-based materials lose their gloss at the time of setting]
Cement Bases
Applications
• Thermal insulation below a restoration
• Mechanical protection where there is inadequate dentin to support amalgam condensation pressures
Types
• Zinc phosphate cement bases
• Polycarboxylate cement bases
• Glass ionomer cement bases (self-curing and light-curing)
Components
o Reactive powder (chemically basic)
o Reactive liquid (chemically acidic)
Reaction
o Acid-base reaction that forms salts or cross linked matrix
o Reaction may be exothermic
Manipulation-consistency for basing includes more powders, which improves all of the cement properties
Properties
Physical-excellent thermal and electrical insulation
Chemical-much more resistant to dissolution than cement liners
Polycarboxylate and glass ionomer cements are mechanically and chemically adhesive to tooth structure
Solubility of all cement bases is lower than cement liners if they are mixed at higher powder- to-liquid ratios
Mechanical- much higher compressive strengths (12,000 to 30,000 psi)
Light-cured hybrid glass ionomer cements are the strongest
Zinc oxide-eugenol cements are the weakest
Biologic (see section on luting cements for details)
Zinc oxide-eugenol cements are obtundent to the pulp
Polycarboxylate and glass ionomer cements are kind to the pulp
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
Chromium Alloys for Partial Dentures
Applications - Casting partial denture metal frameworks
Classification
a. Cobalt-chromium
b. Nickel-chromium
c. Cobalt-chromium-nickel
Composition
a. Chromium-produces a passivating oxide film for corrosion resistance
b. Cobalt-increase~ the rigidity of the alloy
c. Nickel-increases the ductility of the alloy
d. Other elements-increase strength and castability
Manipulation
a. Requires higher temperature investment materials
b. More difficult to cast because less dense than gold alloys usually requires special casting equipment
c. Much more difficult to finish and polish because of higher strength and hardness
Properties
a. Physical-less dense_than gold alloys
b. Chemical-passivating corrosion behavior
c. Mechanical-stronger. stiffer. and harder than gold alloys
d. Biologic
-Nickel may cause sensitivity in some individuals (I % of men and 11 % of women)
-Beryllium in some alloys forms oxide that is toxic to lab technicians
DISTORTION OF THE PATTERN
Distortion is dependant on temperature & time interval before investing .
To avoid any distortion ,
Invest the pattern as soon as possible .
Proper handling of the pattern .
PREREQUISITES
Wax pattern should be evaluated for smoothness , finish & contour .
Pattern is inspected under magnification & residual flash is removed .
ACRYLIC RESINS
Use. Acrylic (unfilled) resins are used as temporary crown material. Temporary crowns are placed to protect the crown preparation and provide patient comfort during the time the permanent crown is being constructed
WETTABILITY
To minimise the irregularities on the investment & the casting a wetting agent can be used .
FUNCTIONS OF A WETTING AGENT
1 . Reduce contact angle between liquid & wax surface .
2 .Remove any oily film left on wax pattern .