📖 Dental Materials
PHYSICAL PROPERTIES OF MATERIALS
Dental MaterialsPHYSICAL PROPERTIES OF MATERIALS
Definite and precise terms are used to describe the physical properties of dental materials.
a. Hardness. Hardness is the measure of the resistance of a metal to indentation or scratching. It is an indication of the strength and wearability of an alloy or metal.
b. Ductility. Ductility is the measure of the capacity of a metal to be stretched or drawn by a pulling or tensile force without fracturing. This property permits a metal to be drawn into a thin wire.
c. Malleability. Malleability is the measure of the capacity of a metal to be extended in all directions by a compressive force, such as rolling or hammering. This property permits a metal to be shaped into a thin sheet or plate.
d. Flexibility and Elasticity. These terms differ in their technical definition but they are very closely related. Flexibility is the characteristic of a metal, which allows it to deform temporarily. The elasticity of a metal is used when it returns to its original shape when the load or force is removed.
e. Fatigue. Fatigue is the property of a metal to tire and to fracture after repeated stressing at loads below its proportional limit.
f. Structure (Crystalline or Grain Structure). Metals are crystalline and many of their physical properties depend largely upon the size and arrangement of their minute crystals called grains.
(1) Grain size. The size of the grains in a solidified metal depends upon the number of nuclei of crystallization present and the rate of crystal growth. In the practical sense, the faster a molten is cooled to solidification, the greater will be the number of nuclei and the smaller will be the grain size. Generally speaking, small grains arranged in an orderly fashion give the most desirable properties.
(2) Grain shape. The shape of the grains is also formed at the time of crystallization. If the metal is poured or forced into a mold before cooling, the grains will be in a flattened state. Metal formed by this method is known as cast metal. If the metal is shaped by rolling, bending, or twisting, the grains are elongated and the metal becomes a wrought wire.
g. Crushing Strength. Crushing strength is the amount of resistance of a material to fracture under compression.
h. Thermal Conductivity. Thermal conductivity is defined as the ability of a material to transmit heat or cold. A low thermal conductivity is desired in restorative materials used on the tooth whereas a high thermal conductivity is desirable where the material covers soft tissue.
CRUCIBLE FORMER
Dental MaterialsCRUCIBLE FORMER
It serves as a base for the casting ring during investing .Usually convex in shape.
May be metal , plastic or rubber .
Shape depends on casting machine used .
Modern machines use tall crucible to enable the pattern to be positioned near the end of the casting machine .
COMPOSITE RESINS -Bonding Agents
Dental MaterialsBonding Agents
Applications-composites, resin-modified gIass ionomers, ceramic bonded to enamel restorations, veneers, orthodontic brackets, and desensitizing dentin by covering exposed tubules (Maryland bridges, composite and ceramic repair systems, amalgams and amalgam repair, and pinned amalgams)
Definitions;-
Smear layer - Layer of compacted debris on enamel and/or dentin from the cavity preparation process that is weakly held to the surface (6 to 7 MPa) , and that limits bonding agent strength if not removed
Etching (or, conditioning)- smear layer removal and production of microspaces for micromechanical bonding by dissolving –minor amounts of surface hydroxyapatite crystals
Priming..- micromechanical (and chemical) bonding to the microspaces created by conditioning step.
Conditioning/priming agent-agent that accomplishes both actions
Bonding- formation of resin layer that connect the primed surface to the overlying restoration (e.g., composite) .. –
Enamel bonding System-for bonding to enamel (although dentin bonding may be a Second step)
Dentin bonding system for bonding to dentin (although enamel bonding may have been a first step)
• First-generation dentin bonding system for bonding to smear layer
• New-generation dentin bonding system- for removing smear layer and etching intertubular dentin to allow primer and/or bonding agent to diffuse into spaces between collagen and form hybrid zone
Enamel and dentin bonding system-for bonding to enamel and dentin surfaces with the same procedures
Amalgam bonding system for bonding to enamel, dentin, and amalgam, dentin and amalgam during an amalgam placement procedure or for amalgam repair
Universal bonding system-for bonding to enamel, dentin, amalgam, porcelain , or any other substrate intraorally that may be necessary for a restorative procedure using the same set of procedures and materials
Types
Enamel bonding systems
Dentin bonding systems
Amalgam bonding systems
Universal bonding systems
Structure
o Components of bonding systems
o Conditioning agent-mineral or organic acid
Enamel only 37% phosphoric acid
Dentin only or enamel and .dentin---37% phosphoric acid, citric acid, maleic acid, or nitric acid
o Priming agent
Hydrophobic-solvent-soluble, light cured monomer system
Hydrophilic-water-soluble, light-cured monomer system
Bonding agent
BIS-GMA-type monomer system
UDMA-type monomer system
Reaction
Bonding occurs primarily by intimate micromechanical retention with the relief created by the conditioning step
Chemical bonding is possible but is not recognized as contributing significantly to the overall bond strength
Manipulation-follow manufacturer's directions
Properties
Physical-thermal expansion and contraction may create fatigue stresses that debond the interface and permit micro leakage
Chemical-water absorption into the bonding agent may chemically alter the bonding
Mechanical-mechanical stresses may produce fatigue that debonds the interface and permits microleakage
Enamel bonding-adhesion occurs by macrotags (between enamel prisms) and microtags (into enamel prisms) to produce micromechanical retention
Dentin bonding-adhesion occurs by penetration of smear layer and formation of microtags into intertubular dentin to produce a hybrid zone (interpenetration zone or diffusion zone) that microscopically intertwines collagen bundles and bonding agent polymer
Biologic
Conditioning agents may be locally irritating if they come into contact with soft tissue
Priming agents (uncured), particularly those based on HEMA, may be skin sensitizers after several contacts with dental personnel
Protect skin on hands and face from inadvertent contact with unset materials and/ or their vapors
HEMA and other priming monomers may penetrate through rubber gloves in relatively short times (60 to 90 seconds)
COMPOSITE RESINS -Manipulation
Dental MaterialsManipulation
Selection
o Microfilled composites or hybrids for anterior class III, IV, V
o Hybrids or midifills for posterior class I, II, III, V
Conditioning of enamel and / or dentin
Do not apply fluorides before etching.-->Acid-etch --> Rinse for 20 seconds with water --> Air-dry etched area for 20 seconds but do not desiccate or dehydrate --> Apply bonding agent and polymerize
Mixing (if required)--> mix two pastes for 20 to 30 seconds
o Self-cured composite-working time is 60 to 120 seconds after mixing
o Light-cured composite-working time is unlimited (used for most anterior and some posterior composite restorations)
o Dual-cured composite-working time is > 10 minutes
o Two-stage cured composite-working time is >5 minutes
Placement
use plastic instrument or syringe --> Light curing --> Cure incrementally in <2 mm thick layers. Use matrix strip where possible to produce smooth surface and contour composite .Postcure to improve hardness