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
 

POLYCARBOXYLATE CEMENT 

Use:. The primary use of polycarboxylate cement is as a cementing medium of cast alloy and porcelain restorations. In addition, it can be used as a cavity liner, as a base under metallic restorations, or as a temporary restorative material. 

Clinical Uses

Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium. 

c. Chemical Composition. 

(1) Powder:. It generally contains zinc oxide, 1 to 5 percent magnesium oxide, and 10 to 40 percent aluminum oxide or other reinforcing fillers. A small percentage of fluoride may be included. 
(2) Liquid. Polycarboxylate cement liquid is approximately a 40 percent aqueous solution of polyacrylic acid copolymer with other organic acids such as itaconic acid. Due to its high molecular weight, the solution is rather thick (viscous). 

d. Properties. 

The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength. 

e. Setting Reactions: 

The setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced. The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix. 

Precautions. 
The following precautions should be observed. 
o    The interior of restorations and tooth surfaces must be free of saliva. 
o    The mix should be used while it is still glossy, before the onset of cobwebbing. 
o    The powder and liquid should be stored in stoppered containers under cool conditions. Loss of moisture from the liquid will lead to thickening. 
 

Properties of Acrylic Resins.

• They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
• Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
• Mixing. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed
• Poor distortion resistance at higher temperatures, therefore dentures should not be cleaned in hot water
• Good resistance to color change
• Absorbs water and must be kept hydrated  (stored in water when not in mouth) to prevent dehydration cycling and changes in dimensions
• Not resistant to strong oxidizing agents
• Low strength; however, flexible, with good fatigue resistance
• Poor scratch resistance; clean tissue-bearing surfaces of denture with soft brush and do not use abrasive cleaners

Effects of Amalgam.

(1) The combined effects of the properties of its ingredients should provide the most satisfactory restorative material.

(2) Quantity of mercury. Too little mercury in the mix results in a grainy, weak, readily tarnished, and corroded amalgam. Too much mercury will cause excessive expansion and weakened amalgam.

 (3) Composition of the alloy. Composition of the alloy must include a minimum of 65 percent silver, a maximum of 29 percent tin, a maximum of 6 to 13 percent copper, and a maximum of two percent zinc by weight

 (4) Correct proportion important. Before use, the silver alloy is mixed with pure and uncontaminated mercury. There are some alloys that are completely zinc free. They can therefore be used more successfully in a moisture-contaminated environment.

 (5) Properties of the finished product.

Silver imparts strength, durability, and color, gives the alloy desirable setting expansion, decreases flow, and accelerates (decreases) the setting time.

Tin makes the amalgam easier to work, controls excessive setting expansion, and increases both flow and setting time.

Copper increases hardness, contributes to setting expansion, reduces flow, and decreases setting time.

Zinc increases workability, and unites with oxygen and other "impurities" to produce a clean amalgam.

Reaction

PMMA powder makes mixture viscous for manipulation before curing. Chemical accelerators cause decomposition of benzoyl peroxide into free radicals that initiate polymerization of monomer

New PMMA is formed into a matrix that surrounds PMMA powder. Linear shrinkage of 5% to 7% during setting. but dimensions of appliances are not critical

Glass Ionomer Cements

Applications

a. Class V restorations-resin-modified glass ionomers for geriatric dentistry
b. Class II restorations-resin-modified glass ionomers, metal-modified glass ionomers in pediatric dentistry
c. Class III restorations-resin-modified glass ionomers
d. permanent cementing of inlays, crowns, bridges, and/or orthodontic band/brackets. In addition, it can be used as a cavity liner and as a base.

Classification by composition

a. Glass ionomer-limited use
b. Metal-modified glass ionomer-limited use
c. Resin-modified glass ionomer-popular use

Components

a. Powder-aluminosilicate glass
b. Liquid-water solution of copolymers (or acrylic acid with maleic, tartaric, or itaconic acids) and water-soluble monomers (e.g., HEMA)

Reaction (may involve several reactions and stages of setting)

a. Glass ionomer reaction (acid-base reaction of polyacid and ions released from aluminosilicate glass particles)
- Calcium, aluminum, fluoride, and other ions released by outside of powder particle dissolving in acidic liquid
- Calcium ions initially cross-link acid functional copolymer molecules
- Calcium cross-links are replaced in 24 to 48 hours by aluminum ion cross-links, with increased hardening of system
- If there are no other reactants in the cement (e.g., resin modification), then protection from saliva is required during the first 24 hours

b. Polymerization reaction (polymerization of double bonds from water-soluble monomers and/or pendant groups on copolymer to form cross-linked matrix)
- Polymerization reaction can be initiated with chemical (self-curing) or light-curing steps
- Cross-linked polymer matrix ultimately interpenetrates glass ionomer matrix 

Manipulation

a. Mixing-powder and liquid components may be manually mixed or may be precapsulated for mechanical mixing
b. Placement-mixture is normally syringed into place
c. Finishing-can be immediate if system is resin-modified (but otherwise must be delayed 24 to 72 hours until aluminum ion replacement reaction is complete)
d. Sealing-sealer is applied to smoothen the surface (and to protect against moisture affecting the glass ionomer reaction)

Properties

1. Physical

-Good thermal and electrical insulation
-Better radiopacity than most composites
-Linear coefficient of thermal expansion and contraction is closer to tooth structure than for composites (but is less well matched for resin-modified systems)
-Aesthetics of resin-modified systems are competitive with composites

2. Chemical

-Reactive acid side groups of copolymer molecules may produce chemical bonding to tooth structure
-Fluoride ions are released
(1) Rapid release at first due to excess fluoride ions in matrix
(2) Slow release after 7  to 30 days because of slow diffusion of fluoride ions out of aluminosilicate particles

-Solubility resistance of resin-modified systems is close to that of composites

3. Mechanical properties

-Compressive strength of resin-modified systems is much better than that of traditional glass ionomers but not quite as strong as composites
- Glass ionomers are more brittle than composites

4. Biologic properties

- Ingredients are biologically kind to the pulp
- Fluoride ion release discourages secondary canes