dental Materials Flashcards
What are the different uses for gypsum bonded investment, phosphate bonded investment and silica bonded investments?
Gypsum bonded investment:
- cast gold alloy inlays, onlays, crowns and FPD
Phosphate bonded:
- for alloys used to produce copings or frameworks for metal ceramic prostheses
- for some base metal alloys
- can split into carbon containing phosphate bonded investments and non carbon containing phosphate bonded investments. Carbon containing phosphate bonded investments are recommended for gold based metal ceramic alloys and should not be used with palladium, nickel or cobalt based alloys because these alloy systems have the potential to absorb available carbon to form carbides and porosity due to physical carbon inclusions
- phosphate bonded investments appear coarse in texture compared to gypsum bonded investments
Silica bonded investments
- casting of removal partial dentures with base metal alloy (cobalt based, nickel based alloy)
3 common noble metals in dental casting alloys and what does noble mean
Noble metals are defined by their resistance to corrosion even under extreme conditions that occur in the oral cavity.
Common ones in dental casting alloys are gold, palladium and platinum
Common base metals in dentistry and what does the term mean
Is a loose term referring to metals that commonly undergo oxidation and corrosion processes.
Common ones in dent are titanium, nickel, copper, silver, cobalt, zinc
Base metals are required in alloys to ensure strength, flexibility and wear resistance necessary for dental restorations
But in their pure form, base metals have a greater tendency to corrode in oral environment than noble metals and hence pure base metals are almost never used for dental restorations
What are the percentages of metals in high noble, noble, predominantly base and titanium alloys?
High noble: at least 60% noble metals and at least 40% gold
Noble: at least 25% noble metal
Predominantly base: noble metal content <25%
Titanium: at least 85% titatnium
How does the ceramic bond to metal
1) compressive forces (slightly higher coefficient of thermal expansion for metal, metal cools first and contracts slightly more, pulls porcelain in)
2) chemical bonding
- when fired in air, trace elements (base metal) in alloy form oxides
- oxides bond to similar oxides in opaque layer of porcelain
- to achieve a strong bond at the interface, a high noble alloy also includes a small proportion of base metal. The base metal is embedded within the alloy but those of its atoms located at the surface will bind oxygen, thereby forming a small oxide layer at the surface, since the matching ceramics contain significant proportions of SiO2 or other metal oxides, the oxygen atoms from both sources will start combining and secure the ceramic to the alloy surface
3) mechanical bonding
- air abrasion eliminates irregularities and increase SA
4) van der waals forces
- these forces help to decrease the distance between metal and porcelain until it is short enough for chemical bonding
Disadvantages of all ceramic crowns compared to PFMs
1) reduced strength
2) more aggressive tooth prep (for the whole ceramic vs the metal portions of the PFM)
3) greater wear potential (in vitro studies show that ceramic materials have a greater potential to cause wear than metals)
4) increased technique sensitivity eg during adhesive cementation
5) cannot be used as surveyed crown because the absence of metal substructure does not allow for RPD design features like rests to be incorporated
4 classifications of dental ceramics and their features
1) traditional feldspathic (glassy)
- composed primarily of amorphous phase with some leucite crystal embedded
- amorphous phase confers excellent translucency and esthetic properties but the lack of crystalline phase makes these materials weak
- flexural strength only 70-90 MPa so mainly used as veneers over alloys or high strength ceramic substructures that impart strength
2) glass dominated
- contains increased amounts of crystalline phase and the crystals may be leucite or fluoroapatite
- increased crystalline phase gives ceramic higher strength, but still sufficient translucency to serve in esthetic applications
- increased strength allows ceramics to occasionally be used for anterior all ceramic crowns that are not under excessive occlusal forces
- should not be used in posterior regions
3) crystalline dominated
- mostly crystalline phase
- crystalline phase is generally spinel, zirc, alumina or lithium disilicate
- ceramics themselves are almost opaque and cannot be used by themselves as veneers. Instead, they serve as cores for anterior and posterior all ceramic crowns. Onto which veneering porcelain (feldspathic/glass dominated) is added
4) crystalline
- newest and strongest of the ceramics used in dent
- have no glassy phase and hence are opaque
defn of MOE
is the ratio of stress to strain along the elastic region of the stress strain curve
higher MOE means greater stiffness/rigidity for elastic deformation and is preferred for long span FDP to reduce amount of bending deflection under loading
defn of proportional limit
is the value of stress at which permanent deformation occurs
is the elastic limit
