Dental Elastomerics Guide: Properties, Uses and Force Decay

Elastics and
elastomerics

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INDIAN DENTAL ACADEMY
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Introduction
• Polymers “Poly” + “merors”

• n(CH2=CH2) (-CH2-CH2-)n
Classification of polymers
• Homopolymers and copolymers
• Natural and synthetic polymers
– Natural rubber is derived from Latex which is a
polymer of 2-methyl buta-1,3-diene (isoprene)

• Linear, branched chain, cross-linked polymers
• Based on type of reaction– Addition polymers (polyethylene, PVC)
– Condensation polymers (nylon)

• Based on inter-particle force
– Mechanical properties of macromolecules like TS,
toughness, elasticity etc. depend on intermolecular
forces – van der Waal’s forces and hydrogen bonds
•
•
•
•

Elastomers
Fibers (nylon)
Thermoplastics ( plasticizers)
Thermosetting plastics


Elastomers
• These type of polymers are held by weakest
attractive forces
• Amorphous in nature and highly elastic
• These polymeric chains are randomly coiled
with few cross links
• When stresses are applied these randomly
coiled structures straighten out and the
polymer gets stretched. When released the
weak intermolecular forces help in regaining
the lost structure.

• Elastomer is a general term that encompasses
materials that, after substantial deformation,
rapidly return to their original dimensions.
• Natural rubber (Incan and Mayan civilizations)
was the first known elastomer.
– unfavorable temperature behavior and water absorption
properties.

• Charles Goodyear(1839)-vulcanization of natural
rubber
• “Vulcan” Roman God of Fire


Natural Rubber
• Rubber is obtained from latex which is a
suspension of rubber particles which oozes out
of the rubber tree
• Polymer of 2-methyl buta-1,3-diene (isoprene)


•
•
•
•

They are derived from a number of plants
“Hevea Brasiliensis”
Chemical structure is Cis-1,4, polyisoprene
One chain contains of 500 units but this may
vary from plant to plant, region to region and
season to season


• Highly resilient
• Absorb water and swell
• Sensitive to ozonization and free radical
ionization


Synthetic rubber
• Synthesized polymerization of “-dienes” other
than isoprene.
• The polymerization is carried out in the
presence of “Zeigler-Natta ” catalyst


• Synthetic rubber polymers, developed
from petrochemicals in the 1920s, have a
weak molecular attraction consisting of
primary and secondary bonds. At rest, a
random geometric pattern of folded linear
molecular chains exists.
• On extension or distortion, these
molecular chains unfold in an ordered
linear fashion at the expense of the
secondary bonds.


• Cross links of primary bonds are
maintained at a few locations along the
molecular chains. The release of the
extension will allow for return to a passive
configuration provided the distraction of
the chains is not sufficient to cause
rupture of these primary bonds. If the
primary bonds are broken, the elastic limit
has been exceeded and permanent
deformation occurs.


• Synthetic polymers are very sensitive to
the effects of free radical generating
systems
– ozone and ultraviolet light.

• The exposure to free radicals results in a
“decrease in the flexibility and tensile
strength” of the polymer.
• Antioxidants and anti-ozonates are added
to retard these effects and extend their
shelf life.


• Elastomeric chains were introduced to the dental
profession in the 1960’s.
• Unitek  Alastiks (1968)
• They are used to generate light continuous forces
for :
•
•
•
•

canine retraction,
diastema closure,
rotational correction,
arch constriction.

• Advantages:
•
•
•
•

Inexpensive
Relatively hygienic
Easily applied
Require little or no patient cooperation.


• Disadvantages:
– When extended and exposed to oral
environment
• Absorb water and saliva
• Permanently stain
• suffer a breakdown of internal bonds that leads to
permanent deformation.

– They experience a rapid loss of force due to
• Stress relaxation resulting in a gradual loss of
effectiveness. This loss of force makes it difficult
to determine the actual force transmitted to the
dentition.


• Elastomerics used in dentistry are made of
polyurethanes and are formed by a stepreaction (condensation) polymerisation.
• Molecular wt. of 500,000

• {-(NH)-(C=O)-O-}  urethane
linkage
• Manufactured by extending a polyester
polyether glycol or a ‘diol’ with di-isocyanide


• Two main methods of manufacturing
– Injection molding technique
– Die stamping

• Pigmenting? (Tg)
• Tg increase makes the polymer more rigid and
hence increase in the modulus of elasticity
• High tensile strength and modulus of elasticity


General properties of Elastomers
• Elongations of 100% and more can be obtained
on rapid stretching with no major loss of
energy
• Maximum values of Tensile strength and
stiffness are obtained after full stretching
• On removal of tensile load it returns to its
original structure rapidly
• Full recovery takes place as long as the elastic
limit is not reached

Elastomeric ligatures
• Conventional ligatures
• Advantages over steel ligature:
–
–
–
–
–

Ease of application
Patient friendly
Aesthetic appearance
Possible release of flourides
Decreased force delivery (almost equal to the steel
ligatures when stretched around a twin bracket)


• According to a study done by Taloumis et al
measuring force decay it can be assumed that
elastic ligatures may be used during initial
leveling and alignment phase but not for
rotational correction as force decay is rapid
• Huge et al have reported that water acts as a
plasticizer and weakens the intermolecular
forces leading to chemical degradation


• Synergistic effect of loading and water
immersion leads hydrolysis of ester or ether
linkages in polyurethanes
• Hence one cannot expect the maintenance full
engagement of the arch wire within the slot
• This led to the introduction of E-modules with
increased Total Diameter: Internal Diameter
ratio (greater wall thickness)  greater initial
force delivery

• Therefore in cases where full engagement of
slot is critical the clinician should:
– Use steel ligatures
– Reduce the time interval for change of E-modules
– Using Fig of ‘8’ configuration

• Probable causes of change in structural and
mechanical properties of E-ligatures:
– Variation in pH and temperature
– Accumulation of plaque (proteinacious film)
– Calciumphosphate formation and possible calcification

Fluoride releasing Elastomerics
• Elution of fluoride from elastomerics was
studied in a different way compared to those
done for other studies on other materials
• The minimal release of fluoride inside the oral
cavity is not as critical as the potential effect
of this release has on their mechanical
properties


• Storei et al have showed that fluoride
releasing elastomerics were not able to deliver
the required force for three weeks as
conventional types
• Hence caution should be exercised on the
frequency of the patient revisit and the need
for reactivation


Elastomeric Chains
• The wide variation seen between E-chains and
E-module although they are made from the
same raw materials is because:
– Manufacturing techniques
– Additives incorporated
– Morphological variation
• Presence or absence of intermodular link
• Ellipsoidal or circular links


• in vitro studies done to measure the rate of
force decay of E-chains employed
– Dry or wet testing states
• Water
• Simulated saliva
• Fluoride media with varying temperatures

– Steady force application or release to simulate
clinical conditions where tooth movement occurs
– Acidic or neutral pH


• The general consensus showed
– Steep decline in force ~ 40%-50% in 24hrs
– Followed by a steady decline in the next 2-3weeks

• Ash and Nikolai have shown a greater decline
in vivo than in vitro.
• Stevenson and Kusy have employed a MaxwellWeichert model which fits the force
degradation rate for elastomers that
represents the two processes
– Rapid loss of force initially
– Slower rate that follows

`


• It has been postulated that since nearly 50%
of the force is lost very early and then a
steady decline is seen it would be logical to
apply a heavier initial force which would
eventually yield the desired force (3x-4x)
• But this has been deleterious to the
Periodontium as it may lead to early
hyalinization and in effect would result in the
same treatment time if not more.


• The use of elastomerics has significantly
reduced over the years because of the advent
of rare earth metals and super elastic coil
NiTi’s that are capable of providing a more
constant force over an extended period of
time


In vivo aging phenomena
• The effect of oral environment on the
structure due to stress absorption is mainly on
– Macromolecular chain orientation and elongation

• It may emanate on the surface as micro-tears
that propagate from the margin to the centre
– Fracture lines  perpendicular to the margins


• In open chains the residual strain correspond
to the link extension pattern

• But in closed elastomeric modules the strain
developed in the modular rings


• Eliades et al showed that after 24hrs of in
vivo exposure, the surface of the modules
were covered with non-continuous
proteinacious film that was rich in alcohol
groups and minimum Na & K mineralization
• After 3weeks well-mineralized proteinacious
films composed of Ca3(PO4)2 with carbonates
and acid phosphate impurities were seen


• Probably due to the entropically favorable
conformational changes that act as nuclei for
microcrystalline growth (Na, K, Cl)


FORCE DELIVERY AND FORCE
DEGRADATION OF
ELASTOMERIC CHAINS


• One characteristic of elastomeric chains is
the inability to deliver a continuous force
level over an extended period of time.


• Andreasen and Bishara(1970) compared latex
elastics and Unitek C-1 AlastiK modules
(Unitek, Monrovia, Calif.) with respect to
simulated intra-arch space closure and interarch forces.
• They found that, after 24 hours of load,
Alastiks suffered a 74% loss of force delivery
capability, whereas latex elastics only lost
42%.


• Subsequent testing showed that after the
first day, the force degradation declined
in a relatively stable manner. These results
led Andreasen and Bishara to recommend
an initial extension of the chain of four
times the desired force level to
compensate for this inherent force loss.


• Bishara and Andreasen found a 50% force loss
after the first day, with 40% of the original
force remaining after 4 weeks. With simulated
tooth movement of 0.25 mm and 0.5 mm per
week, the amount of original force remaining
after four weeks decreased to 25% and 33%,
respectively.


• Their study also showed that consistent force
was produced from chains manufactured by
stamping process as compared with injection
molded chains.


• In a study by Wong two manufacturer’s chains
distracted to and maintained at 17mm while
stored in water at 37° C were compared.
• Greatest amount of force loss took place in
the first 3 hours and initial force loss of 50%
to 75% occurred in the first 24 hours.
• Considerable variation in the initial force
delivery of chains from different
manufacturers was seen.

• Latex showed greatest amount of strength
• Ormco power chains remained more constant
in strength and resiliency than Unitek’s
Alastik power chains
• Ormco  342gms (12.0 oz.)  171 after 21
days
• Unitek  641gms (22.5 oz.)  171 after 21
days


• Modulus of elasticity
– Latex 22gmsmm
– Ormco 46gmsmm
– Unitek 89gmsmm


• Kovatch et al evaluated initial force values
and force degradation of Unitek AlastiKs
stretched to 30% of their original length
at rates of 0.2”, 2.0”, and 20” min.
• Rapidly extended chains showed greater
initial force levels than those slowly
stretched.
• At 1 week the chains stretched at the slow
rate exhibited less force decay.
Therefore slowly stretching the modules
to position is recommended.

• They also calculated a formula that
predicted the force values of a chain at a
given time because, after the first 5
seconds of force decay, the force decay
rate followed a straight line on a log-log
graph.
• This formula is a parabolic equation of the
form: load = constant x (time)-n where n is a
fixed exponent for a given set of
variables.


• In 1978 Ash and Nikolai compared force
decay of chains extended and stored in
air, water, and in vivo. Chains exposed to
an in vivo environment exhibited more
force decay after 30 minutes than those
kept in air. No difference was noted
between the chains maintained in water
and those in vivo until 1 week.


• After 3 weeks, the chains stored in vivo
had a greater force loss than those stored
in water, but both still a force of 160gm
was maintained. They postulated that the
effects of mastication, oral hygiene,
salivary enzymes, and temperature
variations within the mouth influenced the
degradation rates of in vivo chains.


• De Genova et al(1985) investigated force
degradation of chains from 3 companies that
were maintained at a constant length and
stored in artificial saliva.
– Ormco Power Chain ll
– Rocky Mountain Energy Chain
– TP Elast-O Chain


• In the first study, one set of specimens was
maintained at 37° C and another was thermal
cycled between 15° C and 45° C.
• Results thermal-cycled chains displayed
significantly less force loss after 3 weeks.
• Initially force level of 300 to 400 gm for all
three specimens
• Difference of only 7 – 10gms was seen
between them at the end of the test


• A second study compared force decay rates of
thermal-cycled chains held at a constant
length to those subjected to simulated tooth
movement of 0.25 mm per week. The chains
subjected to tooth movement retained 9% to
13% less force than those held at a constant
length.


• Rock et al tested commercially available
elastomeric chains for initial force extension
characteristics and reported that, regardless
of the number of loops, the force values at
100% extension were constant for each
individual material.
• Hence it is recommended to extend chains to
50% to 75% of their original length to provide
the desired force of approximately 300 gm.


• Killiany and Duplessis (1986) studied the force
delivery and force decay characteristics of
the Rocky Mountain “ Energy” chain (RMO,
Denver, Colo.) compared with short loop chain
from American Orthodontics.
• The initial force levels (330 gm) of the new
“Energy” chain at 100% extension were lower
than those of the short loop chain (375 gm).
• After 4 weeks of storage in a simulated oral
environment, the “ Energy” chain retained
66% of its initial force, whereas the short
loop chain possessed only 33% of its original
force.

• Storie and von Fraunhofer investigated the
initial force delivery and force degradation of
a gray chain and a recently marketed fluoridereleasing chain from Ortho Arch.
• Fluoride-releasing chain possessed a higher
initial force level at 100% extension
• Gray chain retained 38% of its initial force


• Fluoride-releasing chain delivered only 14%
after 1 week in 37° C distilled water. After 3
weeks only 6% of the original force level was
observed.
• Evaluation of the flouride release capacity
showed

– Single four-loop piece of chain  3 mg of fluoride
during the 3-week testing period.
– 50% of the total fluoride released(24hrs)
– 90% had been leached out in 1 week of fluid
immersion.

Colour coded chains
• The initial force delivery and effects of fluid
immersion of colored chains were studied
(Baty and von Fraunhofer). They compared
three colors of elastomeric chains with the
standard gray chain from three different
manufacturers, and the data indicated that
the coloring of the chains had little effect on
the initial force delivery levels of the chains.


• Force Degradation in Elastomeric
Chains
Stuart D. Josell, Jeffrey B. Leiss, and E. Dianne
Rekow


• TP Orthodontics closed chain and Rocky
Mountain Orthodontics closed and open chains
maintained the highest percentage of initial
force.
• Dentaurum's closed and open chain decayed to
the lowest percentage of initial force.
• There were significant differences between
closed and open chains in five of the six
companies investigated when comparing 28day mean forces (RMO's closed and open
chains were not different).

PRESTRETCHING EFFECTS


• Attempts to alleviate the large initial force
degradation and improve the constancy of
force delivery have led several investigators
to look at the effects of prestretching the
elastomeric chains before placement.


• Pre-stretching was done to eliminate the
force loss
• Two modes of pre-stretching have been
proposed

– Instantaneous pre-stretching (Sandrik, Chang & Young)
– Extended-time technique of pre-stretching (Brantley
et al)


Extended-time technique of prestretching
Lexan plastic

0.880”~ 22.4mm


• Samples tested were
– Unitek Alastik Chain
– Ormco power chain

• 5 batches with each batch containing ten
samples
• Group A, B, C, D, E
• Group A- control batch
• Groups B & C- 370 distilled water
• Groups E & F- air

Results
• Three week pre-stretching  nearly constant
forces on immediate usage
• Pre-stretching in air not effective
• Unitek vs Ormco
• Force = constant x (time)-n


• Kuster et al (1986) compared the chains of
two companies stored in air and in vivo. Chains
stored in air were extended to 82% and 115%
their original length and, after 4 weeks, had
maintained 70% to 75% of their initial force
level.


• Chains placed in vivo at approximately 100%
extension retained 43% to 52% of their initial
force level after 4 weeks. At 100% extension,
the force levels of the two chains were 315
gm and 279 gm, respectively. These results do
not recommend the extending the chains by
50% to 75% of the original length to provide
an optimal force level.


• Williams and von Fraunhofer  prestretching
effects on force decay at 1 week,
prestretching chains to 100% of their original
length for 10 seconds before loading. Their
results displayed a statistically significant
difference in some prestretched chains
compared with the controls. But this
improvement was only 4% to 6% and clinically
importantance is questinable.


• Prestretching of elastomeric chains has been
suggested as a means of reducing the rapid
force decay rate and providing for a more
constant and consistent force delivery.
• The increased residual force at 3 weeks is
generally about 5%. Therefore, with a 50% to
75% reduction in the initial force, it is
questionable whether this improvement is of
any clinical benefit.


Conclusion
• All chains delivered reduced force over time.
• The shape of the degradation curve was
constant for all types of chains and for chains
from all suppliers.
• The force dropped rapidly for the first 2 to 4
days then remained approximately constant.


• There was a difference between the amount
of
– Initial force delivered
– Percentage of degradation from initial to final
force delivered. Chains delivering the highest initial
forces delivered higher forces after degradation.


Thank you
Leader in continuing dental education


Dental Elastomerics Guide: Properties, Uses and Force Decay

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