Biomechanics and Use of Loops in Orthodontics
overview Introduction Advantages of loops Loop principle
Types of loops Force system of loops Clinical application of loops Conclusion
Introduction Concept of loops given by Dr.Ray Day Robinson In1915 in
International journal of Orthodontia Dr.P.R.Begg extensively used vertical loop for rotation
control; space opening & closing Dr.R.H.W Strang introduced Vertical loop Stoner (1962) introduced a horizontal loop
Advantages of loops Increases resiliency of wire Reduces force Increase In range by adding wire In between
interbracket span
Closing loop mechanics Absence of friction between the bracket and the wire Force levels are easier to evaluate clinically Moment/force ratio of the cuspid and the posterior
segment is predictable and controllable during retraction
Loop principles
1)
Function better when activation „closes them‟ instead of „opening them‟
2)
Loops function better when their form is perpendicular to the movement they must perform
3)
The more wire a loop has, less force it will exert
Loop principles
No loop produce continuous force Stiffness of loop may be reduced by helix incorporation or
by reducing the cross-sectional dimension of the wire Elastic range of the loop can be increased by activating in
the loop In the same direction it is fabricated
TYPES OF LOOPS 1.Simple loop loops without helix 2.Springs with helix
Based on shape 1.Vertical loops- movement In horizontal direction 2.Horizontal –vertical axis movements 3.Combination -Vertical loop+ Horizontal loop –deflected In all
three planes Open or closed types
Vertical loop Legs of the loop are directed vertically Activated In any direction
perpendicular to these component Can be single or double Contoured In open or closed loops
Activation is done by compressing legs
CLINICAL USES Space closure -Single closed vertical loop Opening space -Single Open vertical loop Derotation of tooth- Single Open vertical loop
-Double vertical loop
Move labially or lingually displaced tooth into line- Double vertical loop Move teeth bodily In mesial or distal direction -Double vertical loop
MODIFICATION OF VERTICAL LOOPS Twin helical loop
Omega loop
BULL Loop
Ω
Horizontal loop
Legs of the loop are directed horizontally
Used for occluso-gingival tooth movements Leveling the plane of occlusion is readily accomplished
by horizontal loops
Uses
1.Intrusion of ant. teeth that are in supraversion and for extrusion of post. teeth that are in infraversion.(bite opening) 2.Class II div 2 malocclussion Horizontal helical loops place added intrusive and tipping forces on central incisors
3.) Extrude maxillary canines delayed in
eruption 4. Elevation or depression of individual
teeth-Double horizontal loop 5. Axial paralleling of teeth
6. Correction of rotation with a light continuous force
Combined horizontal & vertical loops
Box loop
Two loops with horizontal
segments of wire at the gingival level functioning as the working bases. It is capable of moving teeth
in any direction
Width is equal to the width of single teeth This lies in the same plane as that of the arch wire In occlusogingival plane-deflection is related to the amount
of the wire In the horizontal plane& bending at the corner of the loops Deflection in the labiolingual plane is related to the total
amount of wire contained in the box loop
When employed as root tipping device Rectangular wire should be used to avoid
buccolingual displacement of the apex & to develop rigidity In anchor area The anterior leg of the box should be shorter than
the posterior leg when uprightening the teeth
The crown of the tooth to be moved should be tied
directly to the tooth behind it in order to prevent its tipping in an opposite direction to the root .
Bent in Stop loop
Molar stop- mesial to it Maintain arch length Increase In arch length by slight opening loop
Transverse loop In case of helical loops, when the plane of
the helix is perpendicular to the arch wire, it is referred as transverse loop.
Used -correct axial inclinations or
mediolateral displacements so as to reduce crossbite.
Characteristics of the force system with loop design Force & magnitude Low load deflection rate Proper moment to force ratio Force constancy w.r.t deflection
Burstone CJ & Koening (1976) Described three characteristics feature of retraction
spring 1.Moment-to force ratio 2. Force to deflection rate 3. Maximum strength (Fmax) that the spring is able to
release without permanent deformation.
Ideally these three factors should be able to determine
respectively: 1. control of the center of rotation; 2. maintenance of ideal force levels during orthodontic
tooth movement; 3. use of ideal levels of strength for orthodontic tooth
movement.
1.Moment to force ratio
Most important to determine In manner tooth will move The M/F ratio determines the center of rotation of a tooth or
segment of teeth, thus allowing translation, tipping or root movement When force is applied at a distance from Cres Mf causes
tipping- undesirable
1.Moment to force ratio In order to achieve translation counter moment Mc
should be applied The ratio of counter moment to force applied is
called as M/F ratio (expressed in mm)
M/F ratio Uncontrolled tipping < Controlled tipping
< Translation < root movement M/F ratio remains constant with increased
activation for simple springs but varies with complex springs
Factors affecting Moment to force ratio 1.
Height of the loop
2.
Horizontal loop length
3.
Diameter of loop
4.
Apical length of loop
5.
Helix incorporation
6.
Angulations of loop legs
7.
Placement of loop
Load /deflection Thus it represents load needed to produce unit deflection
Or
Force dissipated by the spring when it deactivates by unit
distance Thus spring with low L/D ratio is preferred 1.Maintain constant force levels during retraction. 2.Less force change from 1mm activation to the next
Relationship of force and deflection Hooks law
Within proportional limit of an object deflection is directly proportional
to load R= F/D (Units=gm/mm) R= load deflection rate or spring gradient or spring constant is constant
which is the quotient of applied force (F) divided by deflection(D)
Maximum elastic force (Fmax) which the spring exerts
Must be higher than
the force applied during
activation. Prevents permanent deformation of the spring
during accidental overloads such as with mastication
or following an aggressive activation
Biomechanics of closing loop springs Burstone and Koenig investigated the basic configuration of a
closing loop spring in 1976 It consists of .016" steel wire bent into a vertical loop of
varying length. The closing loop spring lies halfway between the cuspid and
the second premolar and is then activated with varying strengths
1.Loop design Accommodate a large activation, Deliver relatively low and nearly constant forces (i.e.
Exhibit low load/deflection characteristics), Comfortable to the patient,
Easily fabricated.
A. Height of the vertical loop Increased - greatest effect on reducing F/D ratio and
simultaneously increasing the moment Anatomic constraints such as the depth of the vestibule Overcome this problem is by adding wire horizontally to
increase
Horizontal loop length Moment-to-force ratio was not as greatly effected by horizontal
changes compared to vertical changes
B. Diameter of the vertical loop
Minimal influence on the system. An increase in the diameter increases the M/F ratio and decreases the
F/D ratio C.Incorporating helix Decreases the F/D ratio but it doesn't affect the M/F ratio
2.Increasing the inter-bracket distance
Increase in the M/F ratio (less effect, however,
than increasing the height of the loop) and also
increases the F/D ratio
3.Loop Position
Traditionally,closing loops are
typically placed immediately distal to the lateral incisors or canines. Allows for repeated activation
of the loop as the space closes.
T-loop positioned midway between the first molar and the canine.
The preactivation bends provide equal and opposite moments in this
position. Encourage reciprocal space closure,
OFF CENTERING -V-bend principle
6.Loop Preactivation OR Gable bends Gable bends increase root control and, thus, avoid “dumping” of the teeth as the space closes. Increase the moments delivered to the teeth & augment the moments that occur during
activation of the closing loop (residual moment.) Promote anchorage control
How works?? When Gable bends are placed in the occlusal portion of a vertical
loop configuration, an unintended mesiodistal force is introduced This force will alter the desired mesiodistal force originally intended
because of the cross over of the vertical legs This cross over wire shortens the horizontal wire length between the
brackets Functions as a V-bend in the archwire
Disadvantages The teeth must cycle through controlled tipping to translation
to root movement to achieve net translation . loop's neutral position (zero activation position) becomes ill
defined. making it difficult to achieve proper activations. The resulting ever-changing periodontal stress distributions
may not yield the most rapid, least traumatic method of space closure
Clinical application of loops
Alignment & leveling Space closure Finishing
1.Individual tooth movement REQIURED MOVEMENT
Labial Lingual
Double vertical loop –Open Double vertical loop –Open
Depression Elevation Double Horizontal loop or Box
Rotation Root tipping
Double Horizontal loop or Box Double vertical loop –Open or
Box Box or double horizontal (rectangular wire only)
2.MIDLINE CORRECTION Mesial or distal movement : Double vertical loop
Combination of open and
closed loops
3.BITE OPENING T loop mesial to canines. Arch wire in the anterior section between the two
loops should have a reverse curve to transmit the pressure equally to all four incisor
4.Axial inclination correction
Double vertical loop –Open or Box
Box or double horizontal (rectangular wire
only 5.Second molar alignment
Intrusion of anterior segment Horizontal loop can be achieved when it is used to depress the anterior segment. If a pair of these loops is contoured mesial to the canine, the
reciprocal activity with a long range of action will be very effective. Asymmetric T loop- Hilger
SPACE opening (Opening loops/ Expansion loops)
When compressed between adjacent teeth & seated into
their respective brackets, the stored elastic force developed pushes the teeth apart.
To create space for alignment of Single or a no. of teeth
Various expansion loops made of round stainless steel wire & differing only in no. of turns in the helix
½ Turn helix 1 ½ Turn helix 2 ½ Turn helix 1 ½ Turn helix with 2 moment arms
Molar distalisation-K loop Made of .017”×.025” TMA wire
Used for molar distalization along with a
Nance button. Each loop of K should be 8mm long 7 1.5
mm wide. The legs of K should be bent down 20˚ &
inserted into molar tube & premolar bracket.
Space closure
Two methods Friction the teeth slide along the archwire Frictionless incorporated loops which produce forces
and moment to move the teeth in desired position
The forces generated In the loop or the arch wire will be
transmitted to tooth through the bracket attachment as a tipping or bodily force depending upon the contour or mode of activation.
Loop designs for retraction Close vertical loop I-loop T-loop or Segmented "T" loop (Burstone;1976) Bull loop (Henry bull,21x25 ss) Keyhole loop ( R.Roth) orDouble key or DKL arch
Mushroom loop (with or without pre-activation bends)
Loop designs for retraction † PG spring (Poul Gjessing, 16x22 ss,1985) † Opus loop (Reymond Siatkowsky,1997) † Rickets retractor (R.Rickets,1979,.16x.16 blue elgiloy) † Drum spring canine retractor † K-sir arch wire (modification of burston and Nanda
segmented loop)
T loop Phases of tooth movement Tipping
Translation
Root movement
Symmetrically centered spring
M/F ratio of 6/1 to the teeth 2mm of deactivation -- M/F ratio increases 10/1 1-2 more mm of space closure -- M/F increases to
12/1 high (root movement)
Modifications of T- loop
Broussard Loop 5m m
2mm
5mm
Asymmetrical T loop
Opus loop It is capable of delivering a nonvarying target M/F within the
range of 8.0-9.1 mm inherently, without adding gable bends
Delta loop It is half of a box loop, distorted so that
it can fit into a single interproximal space. The range of this loop can be increased
by adding helices at the lower corners.
Keyhole loop
Open "I" loop
Ricketts' maxillary canine retractor
It is a combination of a double closed helix and an extended crossed
'T'. The retractor is fabricated using.016" .016" blue elgiloy wire In critical anchorage cases, 45 gable bends and 30-50 g/mm
activation is recommended . For lower canine retraction, double closed helix is used. This
delivers50g/mm of activation
THE POUL GJESSING CANINE RETRACTION SPRING
0.016 X 0.022 inch stainless steel
active element = double helix loop extending 10 mm
genty rounded = avoids effect of sharp bends on load/deflection
use of the greater amount of wire in the vertical direction = minimizing horizontal wire increases rigidity in the vertical plane
Mechanics associated with loops are used improperly
Loss of anchorage, Excessive verticalization of incisors, Increase of overbite,
Mechanics associated with loops are used improperly
Dental mobility, Root resorption, Increase in treatment time may result, Irreversible damage to the patient
Conclusion It is important to prevent undesired tooth movement
, ensure optimal tooth movement and effective space closure, frictionless mechanics (loops)must be understood and controlled.
