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
CASTING: casting is the process by which the wax pattern of a restoration is converted to a replicate in a dental alloy. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges and removable partial dentures.
Objectives of casting
1) To heat the alloy as quickly as possible to a completely molten condition.
2) To prevent oxidation by heating the metal with awell adjusted torch .
3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold.
STEPS IN MAKING A CAST RESTORATION
1. TOOTH PREPARATION
2. IMPRESSION
3. DIE PREPARATION
4. WAX PATTERN FABRICATION
5. SPRUING
Acrylic Denture Bases
Use - used to support artificial teeth
Classification
a. PMMA/MMA dough systems
b. PMMA/MMA pour resin systems
1. Components
a. Powder-PMMA polymer, peroxide initiator, and pigments
b. Liquid-MMA monomer, hydroquinone inhibitor, and cross-linking agents
2. Reaction
a. Heat (or chemicals) is used as an accelerator to decompose peroxide into free radicals
b. Free radicals initiate polymerization of MMA into PMMA
c. New PMMA is formed as a matrix around residual PMMA powder particles
d. Linear shrinkage is 5% to 7% of monomer on polymerization
3. Manipulation
a. P/L mixed to form dough or fluid resin to fill mold
b. Mold heated to start and control reaction
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