Dental Materials Flashcards
Types + function of removable ortho appliances
Passive: retention post-Tx; maintain position
Active: small movements (tipping) via active components (springs, wires, screws)
Functions of base plate of ROA
Incorporate all components together as functional unit
Anchor appliance in place
Provide support for wires
Distribute forces over larger area
3 stages of making ROA
- Impression
- Casting
- Manufacture
3 DMs used in ROA base plate construction
- Alginate or elastomer (expensive)
- Gypsum
- Acrylic
Discuss orthodontic stone
Gypsum product Weaker cf dental stone - facilitates model trimming Inc. working T Whiteners added: improve model appearance
Discuss final step of ROA base plate construction
After alginate impression cast
Bent wires stuck to model using sticky wax
Acrylic resin added incrementally to desired thickness (1.5-2mm)
Discuss composition of acrylics used for base plate construction
HC/RT Powder - PMMA beads - BP: initiator - colour pigments Liquid - MMA - EDGMA: cross-linker (HC) - hydroquinone: inhibitor - DMPT: activator (RT)
LC
- UDMA
- filler: quartz, silica
- pigments
- camphorquinone
- DMPT or DHPT
Advantages + disadvantages of HC/RT acrylics for ROA base plate
Adv - HC cf RT — stronger — less porous — greater abrasion resistance — less residual monomer (0.5%) - RT cf HC — cheaper — less technician T
Disadv - HC — cost — technician T - RT — H2O uptake — residual monomer
Disadvantages of ROA
Pt can leave out for long T Affect speech Technician’s input req. Inefficient for multiple tooth movements L appliances difficult to tolerate
Discuss fixed ortho appliances and their use
Arch wires attached to teeth by brackets, bands + ligatures
- brackets + bands attached by cement
Fixed: pt can’t remove
Capable of precise + multiple tooth movements
Use
- correct moderate skeletal discrepancies
- in/extrusion
- red. overbite
- space closure
Discuss orthodontic elastics
‘Rubber bands’ hooked onto brackets
- apply force to teeth + jaws
Changed serval times/day by pt
- force of rubber dec. over T
Discuss types, function and uses of orthodontic ligatures
Types
- single: variety of colours, uniform size, lose strength over T
- connected: close space b/w teeth or move specific group(s) of teeth @ same T
Function
- secure arch wire into slot on bracket
- stretched around bracket wings -> wire forced into slot
Use: direct teeth in particular direction
What materials can ligatures be?
Elastic or metal
Types of elastic ligatures
Latex
Synthetic
Discuss latex elastic ligatures
Poly cis-isoprene (natural rubber)
Disadv - O2 (air) attach to C = brittle, lose elasticity — antioxidants incorporated to prevent - oils + sunlight harms - allergy
Discuss synthetic elastic ligatures
Polyurethane rubber
Adv - heat + cold resistance - withstand pressures + stresses - abrasion resistance Disadv - distort permanently w/ T - lose elasticity - chemically degraded by H2O (long term)
What are wrought alloys?
Cast alloys that have been produced by mechanical processes vs being melted and poured
Result of work hardening alloys
Inc.: yield stress, hardness
Dec.: ductility
Req. of ortho wires
Low modulus = high flexibility
High proportional limit = high resilience
High yield stress = high range (distance wire will bend elastically before permanent deformation)
High strength = high formability (amount of permanent deformation before #)
High springback: ability undergo deflection w/o permanent deformation
Low friction
Easy to join
Good corrosion resistance
Biocompatible
Discuss 3 crystalline structure types of carbon steel alloys
Austenite: high temp., face centred cubic
Ferrite (soft): low temp., body centred cubic
Cementite (v hard): Fe3C intermetallic compound
What is the eutectoid point of carbon-steel alloys?
Point on phase diagram where 3 solid phases exist
- 0.77% C @ 710C
Effect of inc./dec. C content >/<0.77%
> 0.77%: hyper-eutectoid steels
- hard
- cutting instruments: burs
<0.77%: hypo-eutectoid steels
- soft
- non-cutting: forceps
Effect of cooling rate when cooling from eutectoid point
Pearlite: slow cool
- laminar mix ferrite + cementite
Martensite: fast cool
- distorted body-centred cubic
- v hard + brittle
Composition of stainless steel
Fe
Cr >11%
Ni
C <0.8%
Function of Cr and Ni in SS
Cr: form passive chromic oxide layer; prevent corrosion
Ni: improve strength, corrosion resistance
- dec. Tc; austenite structure remains @ RT on quenching
Effect of annealing SS
Inc.: strength (formability)
Dec.: yield stress (range)
Properties of SS
High proportional limit (resilience)
High yield stress (range)
High modulus (low flexibility)
Lowest frictional resistance
Good springback
Adequate ductility
#: already work hardened thus # if re-bent
Can be welded/soldered; weld decay may occur
Poor biocompatibility: Ni allergy, Cr release
Discuss weld decay of SS
Type of intergranular corrosion
Heated 400-900C; Cr carbides form -> corrosion resistance lost
+ Ti/Nb form stabilised SS; retain corrosion resistance
Effect of overheating SS
Lose fibrous structure = lose springiness
Ortho uses of SS
Removable: bows, springs, Adam’s clasps
Fixed: arch wires (most common)
Composition and function of metals of ortho Co-Cr alloys
Co (40%): hardness, strength Cr (20%): corrosion resistance (passive oxide layer) Ni (15%): inc. ductility, dec. hardness Fe (16%) Mo (7%): solution hardening Other: Mn, C, Be
Classification of Co-Cr ortho alloys
By degree of cold working
Blue: softest, highest formability
Yellow: relatively ductile, more resilient, dec. formability cf blue
Green: resilient, harder cf yellow
Red: hardest, most resilient
Properties of Co-Cr ortho alloys
Excellent corrosion resistance High modulus (low flexibility) High ductility (formability) Adequate springback Prone to work hardening Precipitation hardened post-bending Difficult to solder Expensive: heat Tx
Uses of Co-Cr alloys in ortho
Southend clasps
Arch wires
Composition of beta-Ti alloys
Ti 80%
Mo 11.5%
Zr 6%
Sn 4.5%
Properties of beta-Ti alloys
Low modulus (high flexibility); cf SS, Co-Cr Lower biomechanical forces Improved springback; inc. working range Can be welded - overheating = brittleness Good biocompatibility Good corrosion resistance; TiO2 layer F- exposure degrades
Uses of beta-Ti alloys
Arch wires; intermediate + finishing
Composition of Ni-Ti alloys for ortho
Ni 55%
Ti 45%
Co ~1.6%: improve properties
Discuss the 2 crystalline structures of Ni-Ti alloys
Martensite
- low temp., monoclinic
- low modulus, high strain
- high flexibility + formability
Austenite
- high temp., body-centred cubic
- high modulus, low strain
- low flexibility + formability
What is transition temp. range? Discuss TTR for Ni-Ti alloys
Temp range over which crystalline structure of material changes from one type to another
For Ni-Ti altered by adding metals (C, Cu)
TTR >RT: M-NiTi stable @ RT
TTR
Special properties of NiTi alloys
Shape memory: return to original shape when plastically deformed
- M-NiTi + TTR~mouth temp
Superelasticity
- A-NiTi -> M-NiTi when loaded below yield stress
- low force req. to inc. strain
- revert to original shape when load removed
- up to 10% strain recoverable
Properties NiTi
Low modulus (high flexibility) Excellent springback; higher cf SS Cannot solder/weld Poor biocompatibility: Ni High friction Good corrosion resistance: passive TiO2
Uses of NiTi alloys in ortho
Arch wires
- req. large tooth movements using low force over long T
Composition and properties of wrought Au alloys for ortho
Composition
- Au 60%
- Ag 15%
- Cu 15%
- Pt/Pd 10%: inc. MPt + recrystallisation temp
Properties
- inert
- expensive
- low modulus (high flexibility) cf SS
- low strength + springiness
Effect of wire shape + size of stiffness
Shape
- round: low modulus; initial + intermediate stages
- square: higher modulus; final stages
- rectangle: highest modulus; final stages
Size
- larger size (diameter), higher modulus
What are soldering and brazing?
Soldering: joining of metals by fusion of metal filler (solder) @ <450C
Brazing: temp. >450C
How does soldering work?
Flux dissolves surface oxide layer + wets surface
Solder metals, displaces flux + wets metal surface
Examples of fluxes + flux used for SS alloys
Commonly borates: potassium tetra borate, borax glass, borax, sodium pyroborate
Potassium fluoride only used in SS fluxes as borax may pit surface
Requirements of hard dental solders
Corrosion resistant
High fusion temp.: 50C < solidus temp of alloy
As strong as alloy
Good flow
Types of solder
Au: used for C+B
- Au: 65-80%
- Ag: 8-15%
- Cu: 6-16%
- Zn: 2-4%
- Tn: 2-3%
- Melt: 745-870C
Ag: ortho
- Ag: 40-70%
- Cu: 15-30%
- Cd: 7-24%
- Zn: 16-20%
- Melt: 622-688C
Discuss cast joining
Combine components of RPD
Casting molten metal into interlocking region b/w invested components
Preferred for base metals
What is welding?
Joining of metals w/o use of solder
- actual melting at joint
Discuss: electric spot, pressure, laser welding
Electric Spot
- metals overlapped + pressed b/w Cu electrode
- current pulsed generates enough heat to cause fusion
- metals w/ high thermal and electric conductivity cannot be welded
Pressure: pressure + heat raise temp > recrystallisation temp
Laser: for Au, SS, Co-Cr
Requirements of ortho brackets
Biocompatible Corrosion resistance Aesthetic Not absorb H2O Not be discoloured Min. friction b/w bracket + wire
Why is minimal friction important in ortho?
Friction b/w bracket + wire impedes transferring of movement forces to teeth
3 types of materials ortho brackets can be
Metal
Ceramic
Polymer
Metals ortho brackets can be made of
Au alloys
SS
cpTi/Ti alloys
Discuss Au alloy ortho brackets
Adv
- aesthetic
- corrosion resistance
Disadv
- poor mechanical properties
- expensive
Discuss SS ortho brackets
18/8 SS (same as wire)
Adv
- easy de/bond
- strong
- corrosion resistance
Disadv
- poor aesthetics
- Ni allergy
Discuss cpTi/Ti alloy ortho brackets
Adv
- biocompatible: no Ni
- aesthetic: matte finish
- strength
- corrosion resistance
- good bond
Disadv
- manufacture: high cast temp
- problems from released ions: V, Al
Discuss ceramic ortho brackets
Alumina (single crystal or polycrystalline), Zirconia
Single crystal better as more grain boundaries in polycrystalline acts as crack propagation sites
Adv
- aesthetics (single crystal best)
- biocompatible
- hard, brittle
Disadv - high friction — red.: metal insert - enamel abrasion - debond: enamel # — improve: use w/ polymer base
Discuss polymer ortho brackets
Polycarbonate, polyurethane
Adv: aesthetics Disadv - short lifetime - # easily - soften - poor colour stability - creep (distort)
Discuss reinforcement of polymer ortho brackets
SS slot insert: min. friction
Ceramic filler
- red. friction
- improve colour stability
- strength
Discuss surface modifications of ortho brackets
Textured base (mesh): aid bonding TiO2 coating: red. bacterial adherence Ion implantation (Ni, C): red. friction
Discuss ortho bands
Usually SS
Use: attachments for wires and power products
Cemented on post. teeth
Withstand chewing forces better cf bonded brackets
Compare use of resin and GI cements for ortho
Resin
- use: brackets
- LC, RT, combination
- acid-etch enamel
- prime contaminated surfaces
- HF etch + silanate ceramic brackets
GI - use: bands — RMGIC can be used for brackets - F- release - chemical bond
Compare methods of debonding ortho brackets
Mechanical
- simplest, cheapest
- req. high force
- risk of enamel #
Electrothermal
- softens resin cement
- quick
- less bracket failure
- potential failure @ bracket/cement interface
- potential pulp damage
Laser
- inc. precision
- high cost
- low force
- red. risk enamel damage
- less heat
Ultrasonic
- inc. T
- less enamel damage
- potential soft tissue damage
Discuss invisible braces
Lingual brace
Coated metal arch wires
- PTFE, epoxy resin
- inc. friction
- lower force
- coating can separate
Fibre reinforced resin wire
- brittle
- excessive deformation = crazing
Clear aligner: Invisalign
- thermoplastic resins: polyurethane
