Intro
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What are the materials used in impressions?
Impression compound Impression paste Hydrocolloids (alginate) Elastomers (polysulphides, polyethers, silicones)
What is stainless steel possibly used for?
Denture base Orthodontic appliances
What are the types of force?
Compressive Tensile Shear
What are the mechanical properties of enamel?
Rigid Brittle Strong Hard
What is a typical biting force?
500 to 700N (50-70kg)
What forces are present during grinding/chewing?
Frictional forces -abrasion
What forces are present when biting?
Compressive
What forces can remove material from enamel?
shear force
What is the cavity design for amalgam?
Flared out towards bottom (undercut) to allow secure position and retention (mechanical)
What is the cavity design for composite resin?
Minimal and retention by adhesive
What do stress and strain mean?
stress = force per unit area strain = deformation of material when under stress (change in length/original length)
What is young’s modulus?
the resistance of a material to elastic deformation under an applied force or stress. It is defined as stress/strain
What are the values of young’s modulus in enamel, dentine and composite?
Enamel: 65 GPa Dentin: 20 GPa Composites: 15 GPa
What does a high YM mean?
high stiffness/rigidity
What do fracture, hardness, abrasion resistance, and fatigue mean?
Fracture -large forces cause destruction of materials substance Hardness - ability of surface to resist indentation Abrasion resistance - ability to withstand surface layers being removed due to grinding (loss of material surface layers, roughened surface) Fatigue - repetitive small stresses causing material fracture
What is creep?
The gradual dimensional change or deformation that occurs in a material under the influence of mechanical stress applied over a long time period.
What is permanent deformation?
Applying stress to a material that is larger than the elastic limit resulting in permanent deformation
What is de bonding?
the separation or detachment of a bonded dental material or appliance from the tooth surface to which it was initially adhered. To remove orthodontic appliances, shear force is applied to separate bracket/bonding material from tooth surface
What is ductility, brittleness and tear strength?
Ductility - The ability of a material to deform plastically (permanently) without fracturing when subjected to a tensile stress. Brittleness - The opposite of ductility. Brittleness is the tendency of a material to fracture or shatter with little preceding plastic deformation. Tear strength - The ability of a material to resist the propagation of a crack or tear after initial fracture has occurred.
How is the oral environment varied?
Saliva Temperature variations Ph variations Oral bacteria
What are the chemical properties?
• setting mechanism (setting to hard state -light cure, heat cure etc) • corrosive potential (degradation in oral environment - release of ions) • biocompatibility (does it release toxic materials) • adhesion • solubility/ degradtion (resistance to dissolution and breakdown, especially for biomaterials exposed to oral fluids.)
What are the physical properties of materials?
Viscosity (resistance of a fluid material to flow when a force is applied) Thermal conductivity (ability of a material to transfer heat through its mass.) Thermal expansion (fraction or percentage increase in dimension that a solid material undergoes when heated) Density (The mass per unit volume of a material)