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 Ω

BULL Loop

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

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

Uses

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

Depression Elevation

Rotation Root tipping

Double vertical loop –Open Double vertical loop –Open

Double Horizontal loop or Box 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

½ Turn helix

1 ½ Turn helix

2 ½ Turn helix

1 ½ Turn helix with 2 moment arms

Various expansion loops made of round stainless steel wire & differing only in no. of turns in the helix

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

Asymmetrical T loop

5mm

5mm 2mm

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.