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Dental Materials Flashcards

(68 cards)

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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
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
26
Effect of annealing SS
Inc.: strength (formability) | Dec.: yield stress (range)
27
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
28
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
29
Effect of overheating SS
Lose fibrous structure = lose springiness
30
Ortho uses of SS
Removable: bows, springs, Adam’s clasps Fixed: arch wires (most common)
31
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 ```
32
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
33
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 ```
34
Uses of Co-Cr alloys in ortho
Southend clasps | Arch wires
35
Composition of beta-Ti alloys
Ti 80% Mo 11.5% Zr 6% Sn 4.5%
36
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 ```
37
Uses of beta-Ti alloys
Arch wires; intermediate + finishing
38
Composition of Ni-Ti alloys for ortho
Ni 55% Ti 45% Co ~1.6%: improve properties
39
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
40
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
41
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
42
Properties NiTi
``` Low modulus (high flexibility) Excellent springback; higher cf SS Cannot solder/weld Poor biocompatibility: Ni High friction Good corrosion resistance: passive TiO2 ```
43
Uses of NiTi alloys in ortho
Arch wires | - req. large tooth movements using low force over long T
44
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
45
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
46
What are soldering and brazing?
Soldering: joining of metals by fusion of metal filler (solder) @ <450C Brazing: temp. >450C
47
How does soldering work?
Flux dissolves surface oxide layer + wets surface | Solder metals, displaces flux + wets metal surface
48
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
49
Requirements of hard dental solders
Corrosion resistant High fusion temp.: 50C < solidus temp of alloy As strong as alloy Good flow
50
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
51
Discuss cast joining
Combine components of RPD Casting molten metal into interlocking region b/w invested components Preferred for base metals
52
What is welding?
Joining of metals w/o use of solder | - actual melting at joint
53
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
54
Requirements of ortho brackets
``` Biocompatible Corrosion resistance Aesthetic Not absorb H2O Not be discoloured Min. friction b/w bracket + wire ```
55
Why is minimal friction important in ortho?
Friction b/w bracket + wire impedes transferring of movement forces to teeth
56
3 types of materials ortho brackets can be
Metal Ceramic Polymer
57
Metals ortho brackets can be made of
Au alloys SS cpTi/Ti alloys
58
Discuss Au alloy ortho brackets
Adv - aesthetic - corrosion resistance Disadv - poor mechanical properties - expensive
59
Discuss SS ortho brackets
18/8 SS (same as wire) Adv - easy de/bond - strong - corrosion resistance Disadv - poor aesthetics - Ni allergy
60
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
61
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 ```
62
Discuss polymer ortho brackets
Polycarbonate, polyurethane ``` Adv: aesthetics Disadv - short lifetime - # easily - soften - poor colour stability - creep (distort) ```
63
Discuss reinforcement of polymer ortho brackets
SS slot insert: min. friction Ceramic filler - red. friction - improve colour stability - strength
64
Discuss surface modifications of ortho brackets
``` Textured base (mesh): aid bonding TiO2 coating: red. bacterial adherence Ion implantation (Ni, C): red. friction ```
65
Discuss ortho bands
Usually SS Use: attachments for wires and power products Cemented on post. teeth Withstand chewing forces better cf bonded brackets
66
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 ```
67
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
68
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

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