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.) 

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

Model. Cast. and Die Materials

Applications
- Gold casting, porcelain and porcelain-fused–to metal fabrication procedures
- Orthodontic and pedodontic appliance construction
- Study models for occlusal records

Terms
a. Models-replicas of hard and soft tissues for study of dental symmetry
b. Casts-working replicas of hard and soft tissues for use in the fabrication of appliances or restorations
c. Dies :-  working replicas of one tooth (or a few teeth) used for the fabrication of a restoration
d. Duplicates-second casts prepared from original  casts

Classification by materials

a Models :- (model plaster or orthodontic stone; gypsum product)
b. Stone casts (regular stone; gypsum product)
c. Stone dies (diestone; gypsum product)-may electroplated
d. Epoxy dies (epoxy polymer)-abrasion-resistant dies

Properties-improve with filler content

Physical

Radiopacity depends on ions in silicate glass or the addition of barium sulfate (many systems radiolucent)
Coefficient of thermal expansion is 35 to 45 ppm/C and decreases with increasing filler content
Thermal and electrical insulators

Chemical

Water absorption is 0.5 % to 2.5% and increases with polymer level)
Acidulated topical fluorides (e.g., APF) tend to dissolve glass particles, and thus composites should be protected with petroleum jelly (Vaseline) during those procedures
Color changes occur in resin matrix with time because of oxidation, which produces colored by-products

Mechanical

Compressive strength is 45,000 to 60,000 lb/ in2, which is adequate
Wear resistance-improves with higher filler content, higher percentage of conversion in curing, and use of microfiller, but it is not adequate for some posterior applications
Surfaces rough from wear retain plaque and stain more readily

Biologic

Components may be cytotoxic, but cured composite is biocompatible as restorative filling material

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

Applications/Use

• Load -bearing restorations for posterior  teeth  (class I, II)
• Pinned restorations
• Buildups or cores for cast restorations
• Retrograde canal filling material

 (1) Alloy. An alloy is a solid mixture of two or more metals. It is possible to produce a material in which the desirable properties of each constituent are retained or even enhanced, while the less desirable properties are reduced or eliminated.

(2) Amalgam. When one of the metals in an alloy mixture is mercury, an amalgam is formed. A dental amalgam is a combination of mercury with a specially prepared silver alloy, which is used as a restorative material.

(3) Mercury. Mercury is a silver-white, poisonous, metallic element that is liquid at room temperature