NEET MDS Lessons
Dental Materials
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
1. Selection-based on strength for models, casts, or dies
2. Mixing
(1)Proportion the water and powder
(2) Sift powder into water in rubber mixing bowl
(3) Use stiff blade spatula to mix mass on side of bowl
(4) Complete mixing in 60 seconds
3. Placement
(1) Use vibration to remove air bubbles acquired through mixing
(2) Use vibration during placement to help mixture wet and flow into the impression
Physical reaction-cooling causes reversible hardening
Chemical reaction-irreversible reaction during setting
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
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
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 .
Dental Implants
Applications/Use
Single-tooth implants
Abutments for bridges (freestanding, attached to natural teeth)
Abutments for over dentures
Terms
Subperiosteal- below the periosteum -but above the bone (second most frequently used types)
Intramucosal-within the mucosa
Endosseous into the bone (80%of all current types)
Endodontics-through the root canal space and into the periapical bone
Transosteal-through the bone
Bone substitutes -replace. Long bone
Classification by geometric form
Blades
Root forms
Screws
Cylinders
Staples
Circumferential
Others
Classification by materials type
Metallic-titanium, stainless steel, and .chromium cobalt
Polymeric-PMMA
Ceramic hydroxyapatite, carbon, and sapphire
Classification by attachment design
Bioactive surface retention by osseointegration
Nonative porous surfaces for micromechanical retention by osseointegration
Nonactive, nonporous surface for ankylosis. By osseointegration
Gross mechanical retention designs (e.g.. threads, screws, channels, or transverse holes)
Fibrointegration by formation of fibrous tissue capsule
Combinations of the above
Components
a. Root (for. osseointegration)
b. Neck (for epithelial attachment and percutancaus sealing)
c. Intramobile elements (for shock absorption)
d. Prosthesis (for dental form and function)
Manipulation
a. Selection-based on remaining bone architecture and dimensions
b. Sterilization-radiofrequency glow discharge leaves biomaterial surface uncontaminated and sterile; autoclaving or chemical sterilization is contraindicated for some designs
Properties
1. Physical-should have low thermal and electrical conductivity
2. Chemical
a. Should be resistant to electrochemical corrosion
b. Do not expose surfaces to acids (e.g.. APF fluorides).
c. Keep in mind the effects of adjunctive therapies (e.g., Peridex)
3. Mechanical
a. Should be abrasion resistant and have a high modulus
b. Do not abrade during scaling operations (e.g.with metal scalers or air-power abrasion systems like Prophy iet)
4. Biologic-depend on osseointegration and epithelial attachment