NEET MDS Lessons
Dental Materials
COMPOSITE RESINS
Components
- Filler particles-colloidal silica, crystalline silica (quartz), or silicates of various particle sizes (containing Li, AI, Zn, Yr)
- Matrix-BIS-GMA (or UDMA) with lower molecular weight diluents (e.g., TEGDMA) that correct during polymerization
- Coupling agent- silane that chemically bonds the surfaces of the filter particles to the polymer matrix
Waxes
Many different waxes are used in dentistry. The composition, form, and color of each wax are designed to facilitate its use and to produce the best possible results.
Applications
o Making impressions
o Registering of tooth or soft tissue positions
o Creating restorative patterns for lab fabrication
o Aiding in laboratory procedures
Classification
a. Pattern waxes-inlay, casting, and baseplate waxes
b. Impression waxes-corrective and biteplate waxes
c. Processing waxes-boxing, utility, and sticky waxes
Types
1) Inlay wax-used to create a pattern for inlay, onlay or crown for subsequent investing and casting in a metal alloy.
2) Casting wax-used to create a pattern for metallic framework for a removable partial denture
3) Baseplate wax-used to establish the vertical dimension. plane of occlusion. and initial arch form of a complete denture
4) Corrective impression wax-used to form a registry pattern of soft tissues on an impression
5) Bite registration wax-used to form a registry pattern for the occlusion of opposing models or casts
6) Boxing wax-used to form a box around an impression before pouring a model or cast
7) Utility wax -soft pliable adhesive wax for modifying appliances, such as alginate impression trays
8) Sticky wax-sticky when melted and used to temporarily adhere pieces of metal or resin in laboratory procedures
Components
a. Base waxes-hydrocarbon (paraffin) ester waxes
b. Modifier waxes-carnauba, ceresin, bees wax, rosin, gum dammar, or microcrystalline waxes
c. Additives-colorants
Reaction-waxes are thermoplastic
Properties
Physical
a. High coefficients of thermal expansion and contraction
b. Insulators and so, cool unevenly; should be waxed in increments to allow heat dissipation
Chemical
a. Degrade prematurely if overheated
b. Designed to degrade into CO2and H2Oduring burnout
Mechanical-stiffness, hardness, and strength depend on modifier waxes used
Dental Solders
Applications-bridges and orthodontic appliances
Terms
Soldering -joining operation using filler metal that melts below 500° C
Brazing -joining operation using filler metal that melts above 500°C
Welding-melting and alloying of pieces to be joined
Fluxing
-Oxidative cleaning of area to be soldered
- Oxygen scavenging to prevent oxidation of alloy being soldered
16- 650 -- 650 fineness solder to be used with 16-karat alloys; fineness refers to the gold content
Classification
a. Gold solders-bridges
b. Silver solders-gold-substitute bridges and orthodontic alloys
Structure of gold solders
Composition-lower gold content than of alloys being soldered
Manipulation-solder must melt below melting temperature of alloy
Properties
1. Physical-similar to alloys being joined
2. Chemical-more prone to chemical and electrochemical corrosion
3. Mechanical-similar to alloy being joined
4. Biologic-similar to alloys being joined
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
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