What are the benefits of adding palladium to dental alloys?
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Palladium hardens the alloy, confers tarnish-resistance to Ag-Au alloys, whitens gold, decreases greening of silver and red color of copper, and lowers the coefficient of thermal expansion.
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What are the benefits of adding palladium to dental alloys?
Palladium hardens the alloy, confers tarnish-resistance to Ag-Au alloys, whitens gold, decreases greening of silver and red color of copper, and lowers the coefficient of thermal expansion.
What are some examples of legacy cements?
Legacy cements include zinc polycarboxylate, zinc phosphate, and conventional glass ionomer cement (GIC).
What gold content percentage is found in Type I (soft) gold alloy?
Highest gold content is 88%, lowest is 66%.
What should you do if a veneer comes off and the patient needs to look good for the next day?
Sandblast the area (e.g., with ProphyJet), use a metal primer, apply a layer of resin cement followed by a layer of composite, and then go through the normal workflow when the patient returns.
What are the primary components of Co-Cr alloys used in RDPs?
35-65% cobalt, 28-30% chromium, and 3-6% molybdenum
What is the yield stress range for Type III (hard) gold alloy?
200-340 MPa
What is the minimum thickness required for alloys under ceramics?
0.3 mm
How does the modulus of elasticity of titanium compare to gold and chromium?
The modulus of elasticity of titanium is higher than gold but lower than chromium.
What are the benefits and limitations of Au-Pt-Pd alloys?
Increased hardness but low sag resistance, limited to crowns on 3-unit FPD
What are the benefits of adding molybdenum to dental alloys?
Molybdenum acts as a grain-refiner, increases corrosion resistance and strength, and decreases the thermal expansion coefficient, which is good for compatibility with porcelains.
What element in Au-Cu alloys can cause discolouration?
Copper (Cu)
What are the two main types of bonding mechanisms in metal-ceramic systems?
Mechanical bonding and chemical bonding
What is the purpose of having a higher alkali content in porcelain used in PFMs?
To raise thermal expansion to be compatible with the metal substructure
What is the role of silicon and manganese in dental alloys?
Silicon and manganese improve castability by increasing the fluidity of the molten alloy and act as deoxidizers.
What are contemporary cements used in dental restorations?
Contemporary cements include resin, RMGIC (resin-modified glass ionomer cement), self-adhesive composite resins, and compomers.
What role do indium, iron, and tin play in dental alloys?
They form an oxide layer that acts as an adherend for porcelain bonding.
What is the benefit of using both precious and non-precious metals together in dental alloys?
Better adaptation of the crown.
What is the elongation percentage for Type II (medium) gold alloy?
18%
What are Ni-Cr alloys mainly used for in dentistry?
Ni-Cr alloys are mainly used for small castings such as crowns and fixed partial dentures (FPDs) due to their ease of finishing and polishing compared to Co-Cr.
What are the ideal properties of metal-ceramic alloys?
High rigidity, high sag resistance, and a coefficient of thermal expansion that matches porcelain
What issues can arise from using silver in dental alloys?
Silver can pick up oxygen when molten, leading to pitting and surface roughening, cause green discoloration of porcelain, and lower tarnish resistance.
Why is copper important in Au-Cu alloys?
Copper is important for heat treatment.
For what dental applications is Type IV (extra hard) gold alloy used?
Post and core, crowns, bridges, and partial denture frameworks.
What types of cements can be used for bonding PFM crowns?
Any water-based cements that undergo an acid-base setting reaction, as well as resin cements.
What effect does carbon have in dental alloys?
Carbon forms discontinuous carbides at the grain boundaries, increasing hardness, strength, and yield strength, but decreases ductility.
Why is gold considered an expensive material in dental alloys?
Gold is expensive due to its tarnish and corrosion resistance, ductility, but it has low elastic modulus, low sag resistance, and low strength and hardness.
What is the most common alloy used to cast full crowns?
Au-Cu (Gold-Copper) alloy
What are the properties of cobalt in dental alloys?
Cobalt increases strength, hardness, and modulus of elasticity.
Why are certain base metals good for RPD frameworks?
Because of their excellent biocompatibility, corrosion resistance, and mechanical properties.
What is the role of titanium in Co-Cr-Mo-Ti alloys?
Titanium can be added up to 5% in Co-Cr-Mo-Ti alloys.
What are the phases that commercially pure titanium (CP-Ti) can undergo?
CP-Ti can undergo alpha, near-alpha, alpha-beta, or beta phases.
What happens if the chromium content exceeds 30% in an alloy?
If the chromium content exceeds 30%, it causes casting difficulties and forms a more brittle sigma phase.
What is the effect of aluminum in Ni-Cr alloys?
Aluminum forms Ni3Al, which increases tensile and yield strength.
What is thought to cause the gradual discolouration of certain base metals in dental applications?
Segregation of aluminum during casting that corrodes during service and the use of alkaline denture cleaners.
What is a potential disadvantage of all-ceramic crowns in patients who brux?
Their high hardness can be problematic as it may wear down the natural tooth in the opposing arch.
What is the role of chromium in dental alloys?
Chromium is essential for corrosion resistance as it forms a passive Cr2O3 layer which is thin, uniform, non-porous, and adherent.
Why is zinc phosphate considered a good choice for cementing crowns?
Zinc phosphate is considered a good choice because it is cheap, easy to use, and suitable for crowns that are already opaque.
Which metals are used at much lower concentrations in dental alloys?
Copper, silver, iron, and tin.
What are the challenges associated with casting titanium for dental applications?
Casting titanium is problematic due to internal porosity within the clasp assembly, which can lead to fractures.
What are the advantages of all-ceramic crowns over metal-ceramic crowns?
All-ceramic crowns have better aesthetic qualities, can be supragingival, withstand corrosion better, have greater biocompatibility, bond to tooth structure for extra strengthening, and provide better thermal and electrical insulation.
What protective layer does titanium form and what is its drawback?
Titanium forms a protective oxide layer (~10 nm) which promotes plaque adherence to frameworks more than other base metal frameworks.
What is the significance of adding different stabilizers to titanium?
Adding different stabilizers allows temperature manipulation to achieve desirable properties in titanium.
What are potential exam questions for DENT4206?
What is the role of the opacifier layer in PFM crowns?
The opacifier layer, often containing ZnO and other oxides, is crucial for bonding PFM crowns.
Why should the coefficient of thermal expansion of the alloy match that of the porcelain?
To ensure that the porcelain is placed under compression if it shrinks slightly less than the metal
How does beryllium affect dental alloys?
Beryllium lowers the fusion temperature of the alloy by 100°C, improves fluidity and castability, refines grain size, increases strength, and promotes the formation of a stable metal oxide for porcelain bonding.
What is the function of iridium in dental alloys?
Iridium acts as a grain refiner, adding strength to the alloy.
Why is nickel less preferred than cobalt in dental alloys?
Nickel provides strength, hardness, modulus of elasticity, and ductility but to a lesser extent than cobalt, and it can cause nickel allergies.
What is a disadvantage of base metals with low yield strength and tensile strength in dental applications?
They have a higher percentage of elongation, making clasps easily adjustable but requiring careful denture design to achieve rigidity.
How is mechanical bonding achieved in metal-ceramic systems?
By ceramic locking on cooling due to the difference in coefficients of thermal expansion
What properties must the oxides have for effective chemical bonding in metal-ceramic systems?
Wettable, soluble in porcelain, and should not react with or discolour the porcelain
What is a notable problem with silver-containing alloys in metal-ceramic systems?
Silver discolours ceramics and has a high coefficient of thermal expansion
How does nitrogen from ambient air affect dental alloys?
Nitrogen from ambient air improves the quality of casting.
How can gold be classified commercially?
By carat (24 parts by weight) and fineness (pieces per thousand, e.g., 999.9).
How can the superficial reaction layer be removed from cast dental metals?
Chemically using hydrofluoric acid (HF) or mechanically using grit-blasting or rotary instruments.
What elements are added to form the metal oxide layer for chemical bonding in metal-ceramic systems?
Fe, Indium, Sn
What are the main types of metal-ceramic alloys?
Noble metals, Au-Pt-Pd, Au-Pd-Ag, Pd-Ag
What are the aesthetic and functional differences between metal collars and ceramic collars in dental restorations?
Metal collars are more conservative but unaesthetic, suitable only posteriorly and subgingivally, while ceramic collars offer a balanced compromise of aesthetics and conservation, ideally equigingival.
What is the most biocompatible metal used in dental alloys?
Titanium is the most biocompatible metal used in dental alloys.
What are the benefits of Pd-Ag alloys?
Cheaper, high elastic modulus, and less saggy