Cost comparison
polyether > PVS > condensation silicone > poly sulfide > reversible hydrocolloid > irreversible hydrocolloid (alginate)
Dimensional Stability comparison
PVS (1 week) > polyether (few hrs) > poly sulfides (1 hour) > condensation silicone > hydrocolloids
Wettability comparison
hydrocolloids > poly ether > PVS > poly sulfide > Hydrophobic PVS > condensation silicone
Stiffness comparison
Polyether > PVS > condensation silicone > polysulfide > hydocolloid
Tear strength comparison
polysulfide > PVS > poly ether > condensation silicone > hydrocolloid
castability comparison
Hydrocolloids > hydrophilic PVS > poly ether > polysulfide > hydrophobic PVS > condensation silicone
working time comparison
Reversible hydrocolloid > polysulfide > PVS > condensation silicone > polyether = Irreversible hydrocolloid
setting time comparison
Poly sulfide (8-12 min) > condensation silicone (6-8) > PVS (3-7) > rev. hydrocolloid (5) > polyether > Irrversible hydrocolloid (3.5)
Cement Luting Agents
- provide a non-permeable marginal seal
- Do not add to retention of crown, only increase frictional resistance.
- Add cement to both tooth and crown
Types:
- ZnPhosphate
- ZnPolycarboxylate
- GI
- Resin modified GI
- Resin
Define the process in processing alloys:
- Heat treatment
- Quenching
- Annealing
- Tempering
- Fritting
- Degassing
- Pickling
Processing Alloys
Heat treatment = controlled heating and cooling to relieve internal stress to improve physical properties (Ex: Quench, anneal, tempering)
Quenching = rapid cooling post high temperature that is meant to maintain the mechanical properties with the crystalline structure, while softening the alloy for finishing. The casting is also more easily cleaned while maintaining its malleability and ductility. Ex: Type III Gold should be quenched within 30-40 sec of fabrication
Annealing = controlled heat and slow cool of the metal to manipulate, and make the metal tougher and less brittle (increase ductility and strength)
Tempering = hardening something by heat treatment
NOTE: Heat treating is annealing + hardening + tempering
Fritting = manufacturing low/medium fusing porcelains where the porcelain is grounded into fine powder so that it can be added over other metals to make a certain color.
Degassing = heat treatment = necessary for all gold-porcelain systems = heating a porcelain to 980 C to burn off any remaining impurities prior to adding porcelain
Pickling = An acidic solution which removes surface oxides from cast prior to polish
Coefficient of Thermal Expansion
- Tendency of a material to change shape during temperature change (eat or drink hot)
- When the difference in coefficient is high between a tooth and the restorative material, a break in the margins is high. This will risk percolation = cyclic ingress and egress of fluid at the margin, thus increasing recurrent marginal decay.
- Thus, the BEST restorative material will always be AMALGAM or direct GOLD, even on a Class III-DL on a canine
Tooth 11.4 Direct Gold 14.4 Amalgam 22-28 Composite 28-35 Unfilled resin 81-92
Adhesion
van der walls and chemisorption
Smaller the angle, the greater the gettability, the higher the adhesion potential
Toughness
- Difficult to break
- Affected by yield strength, elongation percent and modulus of elasticity
Brittle
amalgam
- opposite of toughness
- fracture near its proportional limit
- High compressive strength but low tensile strength. This is why amalgam must have a butt margin, and no bevel.
Strain
is the actual shape or deformation that accompanies stress
• Elastic = before permanent deformation occurs
• Plastic
Creep
- Strain relaxation
- Deformations over time in response to a constant stress
- Susceptible to creep: weak materials or close to their melting point
- Higher the creep, the greater risk of marginal discrepancy
- Change the Creep in Amalgams:
o High copper (low Mercury) = decrease creep
o Undertitrate AND over-titrate = increase creep
o Delay in time between condensation and titration = increase creep
o Decrease condensation force = increase creep (since it increases final [Hg])
o Note: Marginal leakage decrease over time where corrosion products (tin oxide, tin sulfide) seal gaps
Modulus of elasticity
- measure of the stiffness or rigidity of a material
- Ratio of stress to strain below the elastic limit
- The higher the modulus of elasticity, the more rigid and the less strain exhibited for a given stress
Elastic Limit
Elastic Deformation takes place prior to plastic deformation. The difference between the two is called elastic limit.
- The greatest stress a material can take and still be able to return to its original form.
- If a material (i.e. dental wire) does not return to its original form after bending, its said to pass its elastic limit
yield strength
- Strength measured at the stress at which a small amount of plastic strain occurs
- the stress slightly higher than the proportional limit.
Resilience
- Energy a material can absorb before the onset of deformation.
Ductile
- Ability to easily shape a metal to a form (extensive plastic deformation prior to fracture during tension)
- % elongation = amount an object stretches (strain) = elastic deformation + plastic deformation
- Depends on both plasticity and tensile strength
Malleable
- Ability to compress a metal/hammer it
- Depends on plasticity
Gold vs Silver vs copper vs platinum. Most malleable and most ductile.
Most Ductile: Gold > Silver > Platinum
Most Malleable Gold > Silver > copper
What is the anterior determinant of occlusion?
Overjet and overbite
