PFM Alloys

Applications-substructures for porcelain-fused-to-metal crowns and bridges
 
Classification
o    High-gold alloys
o    Palladium-silver alloys
o    Nickel-chromium alloys

Structure

Composition
o    High-gold alloys are 98% gold. platinum. And palladium
o    Palladium-silver alloys are 50% to 60% palladium and 30 to 40% silver
o    Nickel-chromium alloys are 70% to 80% nickel and 15% chromium with other metals

Manipulation
o    Must have melting temperatures above that of porcelains to be bonded to their surface
o    More difficult to cast (see section on chromium alloys)

Properties - Physical

Except for high-gold alloys, others are less dense alloys
Alloys are designed to have low thermal expansion coefficients that must be matched to the overlying porcelain

Chemical-high-gold alloys are immune, but others passivate

Mechanical-high modulus and hardness
 

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

Manipulation

Mixture of powder and liquid is painted onto working cast to create shape for acrylic appliance à  After curing of mixture, the shape and fit are adjusted by grinding with burrs and stones with a slow-speed handpiece .Acrylic dust is irritating to epithelial tissues of nasopharynx and skin and may produce allergic dermatitis or other reactions.  Grinding may heat polymer to temperatures that depolymerize and release monomer vapor. which may be an irritant

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.) 

Composition of Acrylic Resins.

·        Powder. The powder is composed of a polymethyl methacrylate (PMMA), peroxide initiator, and pigments

·        Liquid. The liquid is a monomethyl methacrylate (MMA), hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)

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