DMS Flashcards
What concerns may patients have about safety of amalgam?
Aesthetics
Mercury poisoning
Environmental impact
Discolouration of teeth
Metal Allergy
Concerns in pregnancy
What can we tell patients to reassure them about amalgam safety?
Amalgam is a compound- mercury is more stable
Historic material
Amalgam waste is dealt with in special way to protect environment
Describe the mechanism by which composite resin is bonded to teeth?
Smear layer is removed by acid etch- 35% phosphoric acid
-> opens dentine tubules and decalcifies dentine to expose collagen network
DBA- combined primer and sealer
-> hyrdophillic end bonds to dentine
-> hydrophobic end bonds to resin in composite
What are the ideal properties of a denture base ?
Dimensionally accurate
High softening temperature
Non-toxic or irritant
Low density
Easy to repair
High thermal conductivity
Unaffected by oral fluids
Thermal expansion = tooth
What are the constituents of PMMA?
Powder- Benzoyl peroxide, PMMA, plasticiser, pigments, co-polymers
Liquid- Methacrylate monomer, hydroquinone, co-polymer
What are the causes of the different types of porosity/faults in PMMA dentures and why they occur ?
Gaseous porosity- fast curing (monomer boiling) in bulky parts of acrylic
Contraction porosity
-> Too much monomer
-> insufficient clamp pressure, excess material, dough packing
Granular porosity- too little monomer
Internal stresses- thermal expansion, incorrect cooling/curing cycle
What are the advantages of CoCr as denture base?
Radiopaque
Strong
High YM- rigid
Low density
Can be made thin
High thermal conductivity
What are disadvantages of CoCr as denture base?
Difficult to add to/repair
Expensive
Low ductility
Poor aesthetics- not gum coloured like acrylic
What are the ideal properties of an impression material?
Low viscosity
100% elastic recovery (or low viscoelasticity)
High Wettability
Low polymerisation shrinkage
High tear resistance
Acceptable taste and smell
No dimensional change
Compatible with cast material
What are the non-elastic impression materials?
Compound
ZOE
What are examples of elastic impression materials?
Polyether
Silicones
-> addition- PVS
-> condensation silicone- putty
Polysulphide
What are the hydrocolloid impression materials?
Alginate (irreversible)
Agar (reversible)
What are the constituents of alginate?
Sodium Alginate
Calcium sulphate
Trisodium phospate
Fillers
Flavourings
What is the setting reaction for alginate?
Sodium alginate + Calcium sulphate = Calcium alginate and sodium suphate
What are the advantages of alginate?
Flows- replicates surface detail
Acceptable
-> taste/smell
-> setting time
Non-toxic or irritant
Cheap
Easy to use
What are the disadvantages of alginate?
Lacks perfect elastic recovery
Poor tear strength- not suitable for deep undercuts
Storage issues
-> syneresis and imbibition can affect dimensions and accuracy
What are the uses of alginate?
Impressions for study models
Primary/master impressions for dentures
What are the advantages of elastomeric impression materials over alginate?
Better accuracy/surface detail reproduction
Better tear strength
Easier storage- doesn’t dry out
What are the components of GIC?
Powder:
Silica
Alumina
CaF
NaF
Aluminium fluoride
Aluminium phosphate
Liquid:
Polyacrylic acid
Tartaric acid
Describe the setting reaction of GIC?
Dissolution
-> H ions attacks glass surface releasing Ca, Al, Na and F ions
-> Silica gel forms around glassy matrix
Gelation- initial set occurs
-> Ca ions (bivalent) crosslink with poly-acid by chelation with carboxyl groups
Hardening
-> Trivalent Al ions ensure good cross-linking and increased strength (starts after 30 mins and can continue for up to a week)
What are the uses of GIC?
Temporary
Lining material
Luting cement
Fissure sealant
Restorative material
What are 4 advantageous properties of GIC?
Tooth coloured
Fluoride release
Thermal expansion similar to dentine
No contraction on setting
What are 4 negative properties of GIC?
Poor aesthetics
Susceptible to desiccation
Poor mechanical properties
Brittle
What are the advantages of RMGIC over GIC?
On demand set
Better physical properties
Better aesthetics
Easier handling
Lower solubility
Why is GIC a poor luting cement?
Short working time
No on demand set
What luting cement is used for a metal post core?
GIC
What luting cement is used for porcelain veneer?
Light cure resin and DBA
What luting cement is used for Carbon fibre post?
Dual cure composite with DBA
What are the ideal properties of a luting cement?
Low viscosity
Low film thickness- less than 25 microns
Aesthetic
Radiopaque
Low solubility
Cariostatic- F release
Biocompatible
Why is RMGI not used as a luting cement?
Contains HEMA which absorbs water
-> also cytotoxic to pulp
How do you bond a porcelain veneer?
Etch with HF acid
Use DBA to bond composite luting agent to tooth
Use silane to bond porcelain with luting agent
How do you bond a NP metal restoration?
Sandblast
Use DBA to bond composite luting agent to tooth
Use Metal bonding agent containing 4-meta/MDP
-> acid groups bond with oxides in NP metal
What are the components of Temporary Luting agent?
Accelerator- resin, orthodontic EBA, wax, eugenol
Base- ZnO, Starch, mineral oil
Why can’t zirconia be bonded?
It cannot be etched so chemical bond is not possible
-> retention is micromechanical
What makes Lithium Disilicate crowns strong?
