Friction mechanics /certified fixed orthodontic courses by Indian dental academy

FRICTION MECHANICS IN ORTHODONTICS

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

Introduction
• Space closure is an important step in
mechanotherapy, solely dictated by clinician trt.
objective, irrespective of method employed
• Space closure should be individually tailored
based on the diagnosis & trt. plan
• Selection of any method should be based on
desired tooth movement

RETRACTION

FRICTIONLESS

TIP AND UPRIGHT

STAGE 1

SLIDING

CANINE

FRICTIONLESS
SIMULTANEOUS
INTRUSION
AND RETRACTION

STAGE 2

ANTERIORS

SLIDING
FRICTIONLESS


SLIDING

INTRODUCTION
Tooth movement occurs by various
mechanisms
EXTRENSIC

Force is basic component
INTRINSIC

Extrinsic - elastics, E-chain, NiTi springs e.t.c

Intrinsic - Loops bend in arch wire

Goals for any space closure method
– Differential space closure capability

– Axial inclination control
– Control of rotation & arch width

– Optimum biological response
– Minimum patient cooperation

– Operator convenience

Single cuspid retrn. Vs En-masse retrn.
Two schools of thoughts
• Separate canine & incisors retraction – less
detrimental to anchorage (enhance anchorage by
adding teeth to pos. segment but anchorage is taxed
twice)
May be true in some methods of s.c , not
necessarily true in all

• En- masse retraction adequately designed
appliances, based on desired biomechanics
significantly ↓ trt. Time

•

Method of anchorage is based on type of tooth
movement on pos. & ant. seg. & does not entirely
depend on no. of teeth (translation of post. seg. Vs
controlled tipping of ant. seg.)

• Differential tooth movement is accomplished by
unequal moments on ant. & pos. seg.
• Separate canine retraction- moderate to severe

ant. crowding, after achieving incisor alignment,
en-masse closure completes the space closure

• Extn. of PMs is commonly believed to be
necessary for proper management of some
malocclusions. 6-7 mm space gained in each
quadrant can be used for
– Relief of crowding
– Retraction of incisors
– Mesial movement of molars

• Determinants of space closure
– Many details of diag. & trt. objectives determine
tooth movement req. during space closure

Determinants of space closure
• Amount of crowding
• Anchorage
• Axial inclination of canine & incisors
• Midline discrepancy & L/R symmetry
• Vertical dimensions

• Amount of crowding :
– in case of severe crowding maintenance of
anchorage is necessary while creating space for
incisor aling.

• Anchorage:
– Anchorage classification & concept of differential
anchorage is imp.
– Using the same mechanics for diff. anchorage need
limits the results
– Reinforcement methods can be used in critical
anchorage sit.
– Using a force system determined appliance design
can improve chances of success.

ANCHORAGE CLASSIFICATION
According to Ravindra Nanda

GROUP A
GROUP B
GROUP C
BIOMECHANICS IN CLINICAL
ORTHODONTICS -RAVINDRA NANDA

GROUP A ANCHORAGE

Maximum (A, Critical) anchorage situation
Critical maintenance of pos. teeth position
75% or more space req. for ant. retraction

GROUP B ANCHORAGE

Moderate (B) anchorage situation
Relatively symm. space closure(50:50 or 60:40)

GROUP C ANCHORAGE

Minimum (C, Noncritical) anchorage situation
75% or more space closure- by mesial
movement of pos. teeth

• Axial inclination of canines & incisors

• Midline discrepancy & L/R symmetry
– Mid line discrepancies with or without an
asymmetric L/R occ. Relationship- corrected as early
as possible
– Asymm. Forces on L/R could result – unilateral
vertical force, skewing of dental arch or asymm.
Anchor loss.

• Vertical dimensions
– Undesired vertical force ass. with class II elastics may
result in ↑ LFH, ↑ interlabial gap & gummy smile.

• Minor & major cuspid retraction
– Depend upon severity of crowding in ant. Seg.,
anchorage req. & axial inclination of canine

• Minor – refers to uncontrolled tipping of canine when
1-2 mm arch length is req. per side (lace back)

• Major –controlled tipping or translation of canine
when more than 3 mm arch length is req. per side.
if canine inclination is ideal then translation is preferred

• Retraction mechanics divided into
– Sliding (Frictional) mechanics involves either

moving the brackets along the arch wire or sliding
the arch wire through bracket & tube

– Loop (Frictionless) mechanics involves movement
of teeth without the brackets sliding along the
arch wire but with the help of loops

• Sliding mechanics - movement of teeth along
arch wire
– The most significant diff. between standard edge
wise mechanics & pre adjusted appliance is in stage
of space closure.
– In sd. Edgewise, rectangular wire could not
effectively slide through bracket slots due to 1st, 2nd
& 3rd order bends in arch wire
– st. wire appliance allows for level bracket slot lined
up & arch wire can more effectively move through
bracket slots. allows effective sliding of canine along
arch wire

• Advantages
– Minimal wire bending time
– More efficient sliding of arch wire through post.
Bracket slots
– No running out of space for activation
– Patient comfort

– Less time consumption for placement

• Disadvantages
– Confusion regarding ideal force level
– Tendency of overactive elastic & spring force 
initial tipping & inadequate rebound time for
uprighting if forces are activated too frequently
– Generally slower than lop mechanics due to friction


• Role of friction in sliding mechanics
– Friction occurs at bracket wire interface

– Some of applied force is dissipated as friction
– Maximum biological tissue response occur only
when the applied force is of sufficient magnitude to
adequately overcome friction & lie with in optimum
range of forces necessary of tooth movement.
– Friction is the function of relative roughness of 2
surfaces in contact

– Described by coff. of friction (constant) related to

surface characterstic of material
– Coff . Static F- reflect force needed to initate
movement
– Coff. Kinetic F – reflects force neede to perpetuate this
motion

– It takes more force to initiate motion than perpetuate

• Variables affecting frictional resistance during
tooth movement
• Physical
– Arch wire
•
•
•
•

Materials
Cross sectional shape/ size
Surface texture
Stiffness

– Ligation of arch wire to bracket
• Ligature wires
• Elastomerics
• Method of ligation, method of tying, bracket design to
limit the force of www.indiandentalacademy.com
ligation, self ligating brackets

– Bracket
•
•
•
•
•

Material
Manufacturing process: cast or sintered s.s
Slot width & depth
Design of bracket: Single or twin
1st, 2nd & 3rd order bends

– Orthodontic appliance
• Interbracket distance
• Level of bracket slot between adjacent teeth
• Force applied for retraction


• Biological
– Saliva
– Plaque
– Acquired Pellicle
– Corrosion


• Inhibitors to canine sliding retraction
– Inadequate levelling resulting in AW binding
– Damaged or crushed bracket
– Soft tissue buid up at extn. Site
– Cortical plate resistance
– Excessive force causing tipping & binding
– Occlu. Interferance
– Insufficient or inconsistant force.


– In some instances, excessive soft-tissue hyperplasia
occurs at the extraction sites This is
• Unhygienic,
• Can prevent full space closure
• Allow spaces to reopen after treatment.

– Local gingival surgery may be necessary in such
cases.

• Effects of Overly Rapid Space Closure
– can lead to loss of control of torque, rotation, and
tip.

– Loss of torque control 
• in upper incisors being too upright
• space closure with spaces distal to the canines
• unaesthetic appearance.
• lost torque is difficult to regain.
– Rapid mesial movement of the upper molars can
allow the palatal cusps to hang down, resulting in
functional interferences, and rapid movement of the
lower molars causes "rolling in"

• Reduced rotation control - mainly in the teeth
adjacent to extn sites, which tend to roll in if
spaces are closed too rapidly

• Reduced tip control produces unwanted movement of
canines, premolars, and molars, along with a tendency
for lateral open bite.
• In high-angle cases, where lower molars tip most
freely, the elevated distal cusps create the possibility of
a molar fulcrum effect.

• Wire selection
– Req. wire that produce less friction
– Rect.> round
– Larger diameter>smaller
– TMA,NiTi > s.s

– 0.016” s.s lowest friction not ideal wire (not offer
control) in three planes
– 0.016X 0.022ss for 0.018 slot
– 0.017x 0.022 or .019x .025 for 0.022 slot


• Methods of canine retraction in sliding
mechanics
– Elastic modules with ligature
– Elastomeric chains
– Coil springs
– J hook head gear
– Mulligan’s V bend sliding mechanics
– Employing tip edge bracket on canines


• Elastic modules with ligature
– Bennett, McLaughlin,
– An .019"´x.025" arch wire in an .022 "-slot system.
– Hooks of .024 " stainless steel or .028 " brass are
soldered to the U & L arch wires The average
distances between hooks— 38mm in the U & 26mm
in the L
– Additional sizes of 35mm & 41mm (U) and 24mm &
28mm (L)
– Force required for space closure is delivered by
elastic "tiebacks"

Active Lace back
Type 1

Type 2

Type 3

– Elastic module stretched by 2-3mm (to twice its
normal length) delivers 0.5 - 1.5mm of space closure
per month( 100- 150 g force).
– About .5mm of incisor retraction and .5mm of mesial
molar movement.
– The tiebacks are replaced every four to six weeks.

• Alternate systems found to be disadv. to this in
following aspects
– Power chain- variable force, difficult to keep clean,
some times falls off

– Elastic bands- Applied by patient, inconsistent results
due to cooperation factor
– Stainless steel coil spring- deliver excessive
force,unhygenic
– Niti coil spring generally achieve faster & more
consistent space closure

• Elastomeric Chains
– Introduce in 1960’s
– Can be used for canine retraction, diastema closure,
rotation corr.

• Adv.
– Inexpensive
– Relatively hygienic
– Easily applied without arch wire removal
– Not depend on pt. cooperation

• Disadv.
– Absorb water & saliva
– Permanent staining after few days in oral cavity
– Stretching - breakdown of internal bonds –permanent
deformation
– Force degradation- variable force levels-↓effectiveness
– Can untie or break if not placed with care

• Tooth movement, pH & temp. change, fluoride rinse,
salivary enzymes & masticatory forces- deformation,
force degradation and relaxation
– When E-chain first applied produces 300- 350 gms of
force but lose 50- 70% of initial force during 1st day
at 3 weeks retain 30-40% of original force
– To overcome the problem of rapid force decay prestretching of E-chain by this ↑in residual force after
3 weeks is only 5%

• Configurations
– Closed loop chain
– Short filament chain
– Long filament chain

• Clinical considerations
– M/F is lowest at initial placement of E-chain distal
crown tipping of canine
– As tooth retracted M/F ↑es due to dissipation of E
force & by binding the arch wire produces moment
results in uprighting of tooth.
– For optimize tooth movement sufficient time should
be allowed for distal root movement
– A common mistake to change elastic too oftenmaintaining high force & M/F which produce tipping
– Hyalinization around canine & direct resorption of
pos.  anchor loss
– E-chain or module should be changed at interval of
4-6 weeks.

• Closed coil springs
– 1931

– Various materials
• Stainless steel
• Co-Cr-NI alloy
• Ni Ti

– Stainless steel coil spring
• Before s.s made avail. In 1930’s – precious metals
• 1854 T.W Evans- retr. Maxillary incisors precious
metal c.c springs

• Apply more predictable level of force than force
elastics
• Easy to apply
• But have high LDR as compare to NiTi, so as space
closes, some force degradation due to lessening
activation

• NiTi close coil spring
– Produce more consistent space closure than elastics
– Indicated if large spaces need to close or infrequent
adjustment opportunities
– Samuels et al (1998)optimum force for space closure
with this spring – 150 gm

• Two sizes avali. – 9 mm & 12 mm
• Springs should not be extending beyond manuf.
Recomm. (22mm for 9 mm spring, 36 mm for 12 mm
springs)
• Deliver constant force till reach the terminal end of
deactivation stage
• Can be easily placed & removed without Aw removal
• Don't reactivation at each appointment
• Pt. cooperation not needed
• Relatively unhygienic as compare to elastic system


• Problems during sliding mechanics with elastics
or coil springs
– Occl. Interference can hinder distalization
– Friction & binding due to improper angulation of
canine bracket to wire
– Cortical plate resistance
– Excessive force
– Rotation of canine (MB) & molar (DB)


• Direct Head gear retraction
– J hook head gear( st. pull or high pull) Four hooked
for both the arches, clipped mesial o canine
– St. pull- swifer canine retraction than high pull, may
cause ant. Extrusion
– High pull more bodily retraction, bite opening, not
efficient for distal movement

• Adv.
– Extremely conservative to anchorage
– can be applied to both arches simult. (Hickham’s)


• Disadv.
– Force application intermittent –slower method
– Pt. cooperation
– Canine tipping & ant. Extrusion in st. pull

• Problems
– Occl. Interference (bite opening, heavy wire in lower
arch, ABP)
– MB rotation of canines (rotation wedge)
– Flaring of canine in buccal cortex (AW cons. Across
canine)
– One canine may retract faster than other
– Trauma to corner www.indiandentalacademy.com
of mouth

• Mulligan’s V bend sliding mechanics
– Principle – apply differential moments to teeth via
bends in continuous AW while force is applied by
aux. like E-chain, coil spring etc.
– 18 – slot – 0.016” ss wire
– 22 – slot - 0.016, 0.018 or 0.020 wire
– Incisors are not engaged
– 45 degree V bend are added to wire and 200 g force
between canine & molar
– V bend diff. moments on canines & molars
– In max. anch. case near molar(2 PM not banded
intially)

Friction mechanics /certified fixed orthodontic courses by Indian dental academy

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