Acrylic Appliances

Use - space maintenance  or tooth movement for orthodontics and pediatric dentistry

1. Components

a. Powder-PMMA powder. peroxide initiator, and pigments

b. Liquid-MMA monomer, hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)

2. 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

Denture Liners

Use - patients with soft tissue irritation

Types

Long-term liners (soft liners)-used over a period of months for patients with severe undercuts or continually sore residual ridges

Short-term liners (tissue conditioners)-used to facilitate tissue healing over several days

Structure

Soft liners-plasticized acrylic copolymers or silicone rubber

Tissue conditioners-PEMA plasticized with ethanol and aromatic esters

Properties

Liners flow under low pressure, allowing adaptation to soft tissues, but are elastic during chewing forces. 
Low initial hardness, but liner becomes  harder as plasticizers are leached out during intraoral use 
Some silicone rubber liners support growth of yeasts
 

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. 
 

Components 

a. Fillers added to most to control shrinkage
b. Matrix

COMPOSITE RESINS

Types

• Amount of filler-25% to 65% volume, 45% to 85% weight
• Filler particle size (diameter in microns)
  • Macrofill 10 to 100 µm (traditional composites)
  • Midi fill- 1 to 10 µm(small particle composites)
  • Minifill— 0.l to 1 µm
  • Microfill-: 0.01 to  0.1 µm (fine particle composites)
  • Hybrid--blend (usually or  microfill and midifill or minifill and microfill)
• Polymerization method
  • Auto-cured (self-cured)
  • Visible light cured
  • Dual cured
  • Staged cure
• Matrix chemistry
  • BIS-GMA type
  • Urethane dimethacrylate (UDM or UDMA) type
  • TEGDMA-diluent monomer to reduce  viscosity

Casting of glass or ceramic

A castable ceramic is prepared in a similar manner as metal cast preparation .
Glass is heated to 1360 degrees & then cast.
Phosphate bonded investments are used for this purpose .