POLYMERS & APPLICATIONS FOR ORTHODONTIC ALIGNERS

Question 1

Orthodontic Archwires: Ideal Properties
(5)

Answer 1

a. High strength
b. Low stiffness
c. High elastic range
d. High formability
e. Weld or solder

Question 1

Tooth movement during wire unloading or recovery
Tooth movement related to elastic strain recovery during wire unloading

Answer 1

Archwire inserted.
Elastic strain in archwire.
Archwire elastic strain
recovery moves teeth to
original shape of archwire.

Question 2

Property & functional relationship - stage of treatment
a. More crowding initial stage:
b. Final stage—fine adjustments:
c. Similar stiffness considerations when using —

Answer 2

low stiffness, higher
resilience

high stiffness

aligners

Question 3

1. Stainless Steel
   a. Advantages
   (1)
   c. Disadvantages
   (3)

Answer 3

1) Lowest cost; Excellent formability

1) High stiffness: high E modulus = 168-180 GPa; Highest force delivery
2) Require frequent adjustments (not low continuous force delivery)
3) Corrosion susceptibility

Question 4

2. Beta Titanium
   a. Advantages
   (1)
   b. Disadvantages
   (2)

Answer 4

1) Intermediate stiffness: E modulus = ~65 GPa;
Intermediate delivery force

1) Expensive
2) Higher bracket friction

Question 5

3. Nickel-Titanium (NiTi)
   a. Ni: 55%; Ti: 45%
   b. Advantages
   (2)
   c. Disadvantages
   (3)

Answer 5

1) Lowest stiffness: E modulus = ~35 GPa; Lowest force delivery
2) Low continuous unloading force at varying deflection, less frequent
adjustments

1) Expensive
2) Second highest bracket friction
3) Lowest in vivo corrosion resistance (Ni release

Question 6

Orthodontic Aligner Therapy Background
1. 1946: Clear orthodontic appliance concept first introduced
(Kesling HD, Am J Orthod Oral Surg, 1946)
2. 1998: Align Technology released Invisalign
3. Appropriate case selection important component for success
4. Incremental movement of teeth using a series of aligners
5. Tooth movement typically no more than — at one time
a. Why —?
6. More complex case, more aligners, more time to completion

Answer 6

0.2mm

Question 7

7. Treatment effectiveness
   a. Aligner therapy and conventional fixed orthodontics are both
   effective overall.
   b. However, conventional fixed (braces/archwires) more effective
   to produce adequate (3) (Ye et al 2019)
8. Similar biomechanical process to wires, aligner is under load when placed onto misaligned teeth
   a. Teeth move during — recovery of the polymer

Answer 7

occlusal contracts, control tooth torque,
and retention.

elastic strain

Question 8

Tooth movement related to elastic strain recovery during wire or aligner
strain recovery

Answer 8

Aligner inserted

Elastic strain in aligner
Series of aligners
* Elastic strain recovery moves teeth
to ~original shape of each aligner
* Incremental movement with each
aligner that is worn ~1 wk

Question 9

Based on ADA survey of general dentists (n=373 responses) :
a. –% of responding GDs provide orthodontic care
1) –% used clear aligner therapy
2) –% used fixed orthodontic appliances
b. –% reported increasing orthodontic care provided over last
5 years

Answer 9

42
96
28
61

Question 10

c. 2 most important factors considered whether to treat or refer:

Answer 10

1) Case complexity (41%)
2) Patient age (21%)

Question 11

Aligner therapy treatment management comparison
between orthodontists and general dentists
Based on survey responses from 374/1000 orthodontists and
229/1000 general dentists:
a. Both groups reported confidence using aligner therapy
b. Orthodontists more likely to use additional techniques
such as elastics, extractions, and combination of fixed
and aligner therapy
c. Ortho more likely to tell patient their malocclusion was
too complex for — therapy

Answer 11

aligner

Question 12

Thermoplastic polymer sheets
1. Polymer component options:
(6)

Answer 12

• Polyurethane (PU)
• Polypropylene (PP)
• Polycarbonate (PC)
• Polyethylene terephthalate (PET)
• Polyethylene terephthalate glycol (PETG)

Question 13

For comparison, monomer/polymer components of restorative
composites:
Bis-GMA, Bis-EMA, TEGDMA, UDMA, HEMA
(2)

Answer 13

a. Methacrylate-based (MA) monomers that form a polymer resin network via
polymerization reaction, not heat-forming
b. Polymer resin network with fillers, i.e. ‘composite’ not a polymer only

Question 14

nvisalign thermoplastic polymers:
first material
1. Align 1 st material:

2. Modifications:

Answer 14

single layer polyurethane (Exceed-30, EXE30)

multilayer polyurethane/copolyester (SmartTrack, LD30)
a. Increased flexibility (more elastic)
b. More constant forces
c. Based on peer assessment rating (PAR) score:
no difference in clinical outcomes between the 2 materials

Question 15

Besides Polyurethane (PU), other subsequent aligner manufacturers and materials:
a. Polyethylene terephthalate (PET); Polyethylene terephthalate glycol (PETG),
Polycarbonate (PC), Polypropylene (PP)
b. PETG (most common):
c. PETG easier to

Answer 15

Polyester modified with ethylene glycol to link polymer
components (similar to ethylene glycol added to denture base PMMA to increase
polymer stability)

print (if not using thermoplastic process)

Question 16

Orthodontic Aligner Fabrication Techniques
1. Thermoforming thermoplastic
polymer sheets
(2)

Answer 16

a. First used and most common
b. Commercial lab vs in-office

Question 17

3D printing newer approach
a. Options
(2)

Answer 17

1) For now, most promising is photo-polymerization from clear resin
2) Other possibilities: Fused filament fabrication (FFF), selective laser
sintering (SLS), selective laser melting (SLM), stereolithography
apparatus (SLA)

Question 18

Aligner material options: Thermoformed
1. Essix Plus; Essix C+ (SureSmile/Dentsply Sirona): Polypropylene/ethylene copolymer
2. Smart Track (Invisalign/Align Technology): Polyurethane/copolyester multilayer
3. Clarity (3M ESPE): proprietary 5-layer copolymer blend
4. Clear Correct (Straumann): Zendura polyurethane tri-layer
No content information available for the following:
5. GT Flex (GoodFit)
6. Comfort Track (Great Lakes)
7. DX30 (Tru-Tain)
8. Taglus Tuff (Taglus)
9. Erkodur (Erkodent)
10. Biocryl (Sheu Dental)
11. ATMOS (American Ortho)
12. The list goes on……..

Question 19

Tera Harz TC-85DAC photopolymer (Graphy):
(2)

Answer 19

a. Manufacturer does not list content
1) Some speculation is polyurethane
b. Shape-memory polymer
(shape memory mentioned, not clearly explained)
1) Other aligner materials up to now are not shape memory

Question 20

Aligner vs Ortho archwire properties
1. With aligners or orthodontic archwires, tooth movement
occurs during unloading or elastic strain recovery
2. Property relationship: stage of treatment
a. More crowding initial stage:
b. Final stage—fine adjustments:

Answer 20

low stiffness
high stiffness

Question 21

Unlike ortho archwires, aligners significantly affected by stress
relaxation
a. Stress (force) applied by the aligner is reduced when the aligner is subjected
to …
b. Similar issue with orthodontic —
c. Loss of —

New and improved ‘developmental’ Zendura TM aligner, less stress relaxation.
No units of stress or time on the graph? (Zendura TM)

Answer 21

prolonged strain over time
elastics
effectiveness

Question 22

Aligner properties affected by:
1. Material chemical structure:
2. Material thickness:
3. Thermoforming process:
a. More affect high-pressure thermoform than vacuum-formed
4. Storage & loading conditions:
a. Clinical application environment: (2)
5. Recommendation to report properties only after storage in — conditions

Answer 22

a. increase cross-linking, increase ring structures: less flexible (lower elasticity)

thicker is stiffer (higher modulus)

increase stiffness & force

Wet, loaded
1)decrease force and modulus

wet,
loaded

Question 23

Time and aligner removal frequency
(limitation consideration)
1. Each aligner of sequential series is worn for
~
2. Patients must wear aligners at least – hrs/day
(ideally –) to optimize treatment efficiency
3. Modified (higher modulus) material less affected by
aligner removal frequency

Answer 23

1 week
20, 22

Question 24

Other potential limitations of aligners
1. Concerns with — breakdown products into oral
cavity over time (phthalate release), ‘phthalate-free’ polymers
may be preferable
a. Phthalate-free: PU, PC, PP
b. Phthalate-containing:
Polyethylene terephthalate (PET); Polyethylene terephthalate glycol
(PETG)
2. Typically, resins for printing are more — than
thermoforming polymers
a. Post-curing and processing reduces toxicity; toxicity decreases with
time

Answer 24

eluted polymer

cytotoxic