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BDS4 > Dental Materials > Flashcards

Dental Materials Flashcards

1
Q

Types + function of removable ortho appliances

A

Passive: retention post-Tx; maintain position
Active: small movements (tipping) via active components (springs, wires, screws)

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2
Q

Functions of base plate of ROA

A

Incorporate all components together as functional unit
Anchor appliance in place
Provide support for wires
Distribute forces over larger area

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3
Q

3 stages of making ROA

A
  1. Impression
  2. Casting
  3. Manufacture
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4
Q

3 DMs used in ROA base plate construction

A
  1. Alginate or elastomer (expensive)
  2. Gypsum
  3. Acrylic
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5
Q

Discuss orthodontic stone

A 
Gypsum product
Weaker cf dental stone
- facilitates model trimming
Inc. working T
Whiteners added: improve model appearance
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6
Q

Discuss final step of ROA base plate construction

A

After alginate impression cast
Bent wires stuck to model using sticky wax
Acrylic resin added incrementally to desired thickness (1.5-2mm)

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7
Q

Discuss composition of acrylics used for base plate construction

A 
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
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8
Q

Advantages + disadvantages of HC/RT acrylics for ROA base plate

A 
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
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9
Q

Disadvantages of ROA

A 
Pt can leave out for long T
Affect speech
Technician’s input req.
Inefficient for multiple tooth movements 
L appliances difficult to tolerate
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10
Q

Discuss fixed ortho appliances and their use

A

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
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11
Q

Discuss orthodontic elastics

A

‘Rubber bands’ hooked onto brackets
- apply force to teeth + jaws
Changed serval times/day by pt
- force of rubber dec. over T

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12
Q

Discuss types, function and uses of orthodontic ligatures

A

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

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13
Q

What materials can ligatures be?

A

Elastic or metal

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14
Q

Types of elastic ligatures

A

Latex

Synthetic

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15
Q

Discuss latex elastic ligatures

A

Poly cis-isoprene (natural rubber)

Disadv
- O2 (air) attach to C = brittle, lose elasticity 
— antioxidants incorporated to prevent 
- oils + sunlight harms
- allergy
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16
Q

Discuss synthetic elastic ligatures

A

Polyurethane rubber

Adv
- heat + cold resistance 
- withstand pressures + stresses
- abrasion resistance 
Disadv
- distort permanently w/ T
- lose elasticity 
- chemically degraded by H2O (long term)
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17
Q

What are wrought alloys?

A

Cast alloys that have been produced by mechanical processes vs being melted and poured

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18
Q

Result of work hardening alloys

A

Inc.: yield stress, hardness

Dec.: ductility

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19
Q

Req. of ortho wires

A

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

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20
Q

Discuss 3 crystalline structure types of carbon steel alloys

A

Austenite: high temp., face centred cubic
Ferrite (soft): low temp., body centred cubic
Cementite (v hard): Fe3C intermetallic compound

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21
Q

What is the eutectoid point of carbon-steel alloys?

A

Point on phase diagram where 3 solid phases exist

- 0.77% C @ 710C

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22
Q

Effect of inc./dec. C content >/<0.77%

A

> 0.77%: hyper-eutectoid steels

  • hard
  • cutting instruments: burs

<0.77%: hypo-eutectoid steels

  • soft
  • non-cutting: forceps
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23
Q

Effect of cooling rate when cooling from eutectoid point

A

Pearlite: slow cool
- laminar mix ferrite + cementite

Martensite: fast cool

  • distorted body-centred cubic
  • v hard + brittle
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24
Q

Composition of stainless steel

A

Fe
Cr >11%
Ni
C <0.8%

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25
Q

Function of Cr and Ni in SS

A

Cr: form passive chromic oxide layer; prevent corrosion

Ni: improve strength, corrosion resistance
- dec. Tc; austenite structure remains @ RT on quenching

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26
Q

Effect of annealing SS

A

Inc.: strength (formability)

Dec.: yield stress (range)

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27
Q

Properties of SS

A

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

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28
Q

Discuss weld decay of SS

A

Type of intergranular corrosion
Heated 400-900C; Cr carbides form -> corrosion resistance lost

+ Ti/Nb form stabilised SS; retain corrosion resistance

29
Q

Effect of overheating SS

A

Lose fibrous structure = lose springiness

30
Q

Ortho uses of SS

A

Removable: bows, springs, Adam’s clasps
Fixed: arch wires (most common)

31
Q

Composition and function of metals of ortho Co-Cr alloys

A 
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
32
Q

Classification of Co-Cr ortho alloys

A

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

33
Q

Properties of Co-Cr ortho alloys

A 
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
34
Q

Uses of Co-Cr alloys in ortho

A

Southend clasps

Arch wires

35
Q

Composition of beta-Ti alloys

A

Ti 80%
Mo 11.5%
Zr 6%
Sn 4.5%

36
Q

Properties of beta-Ti alloys

A 
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
37
Q

Uses of beta-Ti alloys

A

Arch wires; intermediate + finishing

38
Q

Composition of Ni-Ti alloys for ortho

A

Ni 55%
Ti 45%
Co ~1.6%: improve properties

39
Q

Discuss the 2 crystalline structures of Ni-Ti alloys

A

Martensite

  • low temp., monoclinic
  • low modulus, high strain
  • high flexibility + formability

Austenite

  • high temp., body-centred cubic
  • high modulus, low strain
  • low flexibility + formability
40
Q

What is transition temp. range? Discuss TTR for Ni-Ti alloys

A

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

41
Q

Special properties of NiTi alloys

A

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
42
Q

Properties NiTi

A 
Low modulus (high flexibility)
Excellent springback; higher cf SS
Cannot solder/weld
Poor biocompatibility: Ni
High friction
Good corrosion resistance: passive TiO2
43
Q

Uses of NiTi alloys in ortho

A

Arch wires

- req. large tooth movements using low force over long T

44
Q

Composition and properties of wrought Au alloys for ortho

A

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
45
Q

Effect of wire shape + size of stiffness

A

Shape

  • round: low modulus; initial + intermediate stages
  • square: higher modulus; final stages
  • rectangle: highest modulus; final stages

Size
- larger size (diameter), higher modulus

46
Q

What are soldering and brazing?

A

Soldering: joining of metals by fusion of metal filler (solder) @ <450C
Brazing: temp. >450C

47
Q

How does soldering work?

A

Flux dissolves surface oxide layer + wets surface

Solder metals, displaces flux + wets metal surface

48
Q

Examples of fluxes + flux used for SS alloys

A

Commonly borates: potassium tetra borate, borax glass, borax, sodium pyroborate
Potassium fluoride only used in SS fluxes as borax may pit surface

49
Q

Requirements of hard dental solders

A

Corrosion resistant
High fusion temp.: 50C < solidus temp of alloy
As strong as alloy
Good flow

50
Q

Types of solder

A

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
51
Q

Discuss cast joining

A

Combine components of RPD
Casting molten metal into interlocking region b/w invested components
Preferred for base metals

52
Q

What is welding?

A

Joining of metals w/o use of solder

- actual melting at joint

53
Q

Discuss: electric spot, pressure, laser welding

A

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

54
Q

Requirements of ortho brackets

A 
Biocompatible 
Corrosion resistance 
Aesthetic
Not absorb H2O
Not be discoloured
Min. friction b/w bracket + wire
55
Q

Why is minimal friction important in ortho?

A

Friction b/w bracket + wire impedes transferring of movement forces to teeth

56
Q

3 types of materials ortho brackets can be

A

Metal
Ceramic
Polymer

57
Q

Metals ortho brackets can be made of

A

Au alloys
SS
cpTi/Ti alloys

58
Q

Discuss Au alloy ortho brackets

A

Adv

  • aesthetic
  • corrosion resistance

Disadv

  • poor mechanical properties
  • expensive
59
Q

Discuss SS ortho brackets

A

18/8 SS (same as wire)

Adv

  • easy de/bond
  • strong
  • corrosion resistance

Disadv

  • poor aesthetics
  • Ni allergy
60
Q

Discuss cpTi/Ti alloy ortho brackets

A

Adv

  • biocompatible: no Ni
  • aesthetic: matte finish
  • strength
  • corrosion resistance
  • good bond

Disadv

  • manufacture: high cast temp
  • problems from released ions: V, Al
61
Q

Discuss ceramic ortho brackets

A

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
62
Q

Discuss polymer ortho brackets

A

Polycarbonate, polyurethane

Adv: aesthetics
Disadv
- short lifetime
- # easily
- soften
- poor colour stability 
- creep (distort)
63
Q

Discuss reinforcement of polymer ortho brackets

A

SS slot insert: min. friction

Ceramic filler

  • red. friction
  • improve colour stability
  • strength
64
Q

Discuss surface modifications of ortho brackets

A 
Textured base (mesh): aid bonding
TiO2 coating: red. bacterial adherence
Ion implantation (Ni, C): red. friction
65
Q

Discuss ortho bands

A

Usually SS
Use: attachments for wires and power products
Cemented on post. teeth
Withstand chewing forces better cf bonded brackets

66
Q

Compare use of resin and GI cements for ortho

A

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
67
Q

Compare methods of debonding ortho brackets

A

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
68
Q

Discuss invisible braces

A

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

BDS4 (10 decks)

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