NEET MDS Lessons
Dental Materials
CAD/CAM Restorations
Applications-inlays, onlays, veneers, crowns, bridges, implants, and implant prostheses
Stages of fabrication
CSD-computerized surface digitization
CAD-computer-aided (assisted) design
CAM-computer-aided (assisted) machining
CAE-computer-aided esthetics (currently theoretic)
CAF-computer-aided finishing or polishing (which are currently theoretic steps)
Classification
Chairside or in-office systems
(1) Cerec (Siemens system)-inlays, onlays, veneers
(2) Sopha (Duret system)-inlays, onlays (and Crowns)
Laboratory systems
(1) DentiCAD (Rekow system)-inlay, onlays, veneers, crowns
(2) Cicero (Elephant system)-porcelain fused-to-metal crowns
Materials
a. Feldspathic oorcelains (Vita)
b. Machinable ceramics (Dicor MGC)
c. Metal alloys limited use)
Cementing
- Etching enamel and/or dentin for micromechanical retention
- Bonding agent for retention to etched surface
- Composite as a luting cement for reacting chemically with bonding agent and with silanated surface of restoration
- Silane for bonding to etched ceramic (or metal) restorations and to provide chemical reaction
- Hydrofluoric acid etching to create spaces for micromechanical retention on surface or restoration
Properties
1. Physical properties
a. Thermal expansion coefficient well matched to tooth structure
b. Good resistance to plaque adsorption or retention
2. Chemical properties-not resistant to acids and should be protected from APF
3. Mechanical properties
a. Excellent wear resistance (but may abrade opponent teeth)
b. Some wear of luting cements but self-limiting
c. Excellent toothbrush abrasion
4. Biologic properties-excellent properties
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
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
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
Finishing and Polishing
Remove oxygen-inhibited layer .Use stones or carbide burs for gross reduction.Use highly fluted carbide burs or special diamonds for fine reduction.Use aluminum oxide strips or disks for finishing. Use fine aluminum oxide finishing pastes. Microfills develop smoothest finish because of small size of filler particles
Properties of Amalgam.
The most important physical properties of amalgam are
- Coefficient of thermal expansion = 25-1 >ppm/ C (thus amalgams allow percolation during temperature changes)
- Thermal conductivity-high (therefore, amalgams need insulating liner or base in deep restorations)
- Flow and creep. Flow and creep are characteristics that deal with an amalgam undergoing deformation when stressed. The lower the creep value of an amalgam, the better the marginal integrity of the restoration. Alloys with high copper content usually have lower creep values than the conventional silver-tin alloys.
Dimensional change. An amalgam can expand or contract depending upon its usage. Dimensional change can be minimized by proper usage of alloy and mercury. Dimensional change on setting, less than ± 20 (excessive expansion can produce post operative pain)
- Compression strength. Sufficient strength to resist fracture is an important requirement for any restorative material. At a 50 percent mercury content, the compression strength is approximately 52,000 psi. In comparison, the compressive strength of dentin and enamel is 30,000 psi and 100,000 psi, respectively. The strength of an amalgam is determined primarily by the composition of the alloy, the amount of residual mercury remaining after condensation, and the degree of porosity in the amalgam restoration.
- Electrochemical corrosion produces penetrating corrosion of low-copper amalgams but only produces superficial corrosion of high copper amalgams, so they last longer
- Because of low tensile strength, enamel support is needed at margins
- Spherical high-copper alloys develop high tensile strength faster and can be polished sooner
- Excessive creep is associated with silver mercury phase of low-copper amalgams and contributes to early marginal fracture
- Marginal fracture correlated with creep and electrochemical corrosion in low-copper amalgams
- Bulk fracture (isthmus fracture) occurs across thinnest portions of amalgam restorations because of high stresses during traumatic occlusion and/or the accumulated effects of fatigue
- Dental amalgam is very resistant to abrasion
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