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

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 

Zinc Phoshate Cement

Uses. Zinc phosphate cement is used both as an intermediate base and as a cementing medium. 

(1) Intermediate base. A thick mix  is used under permanent metallic restoration. This layer of cement protects the pulp from sudden temperature changes that may be transmitted by the metallic restoration. 

(2) Cementing medium. Zinc phosphate cement is used to permanently cement crowns, inlays, and fixed partial dentures upon the remaining tooth structure. A creamy mix of cement is used to seat the restoration or appliance completely into place. The cementing medium does not cement two objects together. Instead, the cement holds the objects together by mechanical interlocking, filling the space between the irregularities of the tooth preparation and the cemented restoration

c. Chemical Composition. 

(1) Powder. primary ingredients - zinc oxide and magnesium oxide. 
(2) Liquid. Phosphoric acid and water in the ratio of two parts acid to one part water. The solution may also contain aluminum phosphate and zinc phosphate Liquids exposed in open bottles will absorb moisture from the air in high humidity. The liquids will lose moisture if humidity is low. Water gain hastens setting; water loss lengthens setting time.
 
PROPERTIES OF ZINC PHOSPHATE CEMENT

a. Advantages. Some advantages of zinc phosphate cement as a cementing medium are:

o    Inconspicuous appearance. 
o    Speed and ease of usage. 
o    Sufficient flow to form a thin layer for the cementing of closely adapted crowns, fixed partial dentures, and inlays. 
o    Low thermal conductivity beneath a metallic restoration.

b. Disadvantages. Some disadvantages of zinc phosphate cement as a cementing medium are:

o    Low crushing strength that varies between 12,000 and 19,000 psi. 
o    Slight solubility in mouth fluids. 
o    Opaque material not suitable for visible surfaces. 

c. Strength. The ratio of powder to liquid increases the strength of phosphate cements to a certain point. For this reason, the dental specialist must use as thick a mix as practical for the work being performed. 

SETTING REACTIONS OF ZINC PHOSPHATE CEMENT 

a. Chemical Reaction. The chemical reaction that takes place between the powder and liquid of setting phosphate cement produces heat. The amount of heat produced depends upon the rate of reaction, the size of the mix, and the amount of heat extracted by the mixing slab. 

b. Powder to Liquid Ratio. The less powder used in ratio to the liquid, the longer the cement will take to harden. Good technique minimizes the rise in temperature and acidity of the setting cement that can injure the pulp. Generally, for increased strength, decreased shrinkage, and resistance to solubility, it is advisable to blend as much powder as possible to reach the desired consistencies. 

c. Setting Time. The setting time of zinc phosphate cement is normally between 5 and 9 minutes. 
 Lower the temperature of the glass mixing slab to between 65° and 75° F (18° to 24° C), if the glass mixing slab is not already cooled below the temperature at which moisture will condense on it. → Blend the powder slowly. →  Mix the powder over a large area of the cool slab. →  Use a longer mixing time, within optimum limits. 
 
Precautions. The following precautions should be observed. 

o    Prevent loss or gain of moisture in liquid cement by keeping bottles tightly stoppered. 
o    Dispense drops only when ready to mix. 
o    Use a cool, dry glass slab (65° to 75° F). 
o    Use the same brand of powder and liquid. 
o    Add increments of powder slowly. 
o    Use the maximum amount of powder to obtain the desired consistency. 

(To incorporate the most powder, the material should be mixed with a moderate circular motion over a large area of the slab, turning the spatula often.) 

Wax elimination (burnout):

Wax elimination or burnout consists of heating the investment in a thermostatically controlled furnace until all traces of the wax are vaporized in order to obtain an empty mold ready to receive the molten alloy during procedure.

• The ring is placed in the furnace with the sprue hole facing down to allow for the escape of the molten wax out freely by the effect of gravity .
• The temperature reached by the investment determines thethermal expansion. The burnout temperature is slowly increased in order to eliminate the wax and water without cracking the investment.
•For gypsum bonded investment, the mold is heated to650 -6870 c )to cast precious and semiprecious
precious alloys.
• Whereas for phosphate-bonded investment, the mold is heated up to 8340 c to cast nonprecious alloys at high fusing temperature.
The ring should be maintained long enough at the maximum temperature (“heat soak”) to minimize a sudden drop in temperature upon removal from the oven. Such a drop could result in an incomplete casting because of excessively rapid solidification of thealloy as it enters the mold.
• When transferring the casting ring to casting, a quick visual check of the sprue in shaded light is helpful to see whether it is properly heated. It should be a cherry-red color .

WAX BURNOUT AND HEATING THE RING

After the investment has set hard, the crucible former and the metal sprue former is removed carefully, and any loose particles at the opening of the sprue hole are removed with small brush.
The purpose of the wax burnout is to make room for the liquid metal. The ring is placed in the oven at 250C with the sprue end down, thus allowing the melted wax to flow, out for 30min or even up to 60min may be a good procedure to ensure complete elimination of the wax and the carbon.

Heating the ring: The object is to create a mold of such dimension, condition and temperature so that it is best suited to receive the metal.

Hygroscopic Low-Heat Technique. 

After the wax elimination the temperature of the same furnace can be set to a higher temperature for heating or else, the ring can be transferred to another furnace, which has already set to the higher temperature. In any case accurate temperature control is essential and therefore these furnaces have pyrometer and thermocouple arrangement. The ring is placed in the furnace with the sprue hole down and heated to 500C and kept at this temperature for 1 hour. In this low heat technique the thermal expansion obtained is less but together with the previously obtained hygroscopic expansion the total expansion amounts to 2.2 percent, which is slightly higher than what is required for gold alloys.

So this technique obtains its compensation expansion from three sources:
(1)   The 37º C water bath expands the wax pattern
(2)   The warm water entering the investment mold from the top adds some hygroscopic expansion
(3)   The thermal expansion at 500' C provides the needed thermal expansion.

High-Heat Thermal Expansion Technique. 

After the wax elimination, the ring should be placed in the furnace which is at room temperature and then the temperature is gradually raised, until it comes to 700C in 1 hour. Then the ring is heat soaked at this temperature for ½ hour. This slow rise in temperature is necessary to prevent 
This approach depends almost entirely on high-heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern.  Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern, and the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.

Physical reaction-cooling causes reversible hardening

Chemical reaction-irreversible reaction during setting

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.