orthodontic appliances

There are as many orthodontic appliances available as there are clinicians that will use

them. Two broad categories of appliances: REMOVABLE and FIXED.

1- ACTION:

- As a general rule, removable appliances tilt (or tip) the teeth, providing mainly a

movement of the crown, possibly followed -(and only to a certain extent)- by the apices.

- Extrusion

- Controlled movement of the roots cannot be easily achieved with removable appliances.

2- TYPES:

Fundamentally, two kinds of removable appliances may be recognized

A. One making use of intrinsic forces, or forces from within the appliance itself,

through wires, screws, or a combination of both (ACTIVE PLATES). The device is rigidly

fixed to a variable number of teeth.

B. The other utilizes extrinsic forces, i.e. muscle forces, causing these forces to exert

pressure on the teeth and alveolar process. In this category, are included: the removable

inclined plane, the vestibular oral screen, simple bite plates, activators and other

functional appliances.

*** A third kind would combine intrinsic and extrinsic force application in one appliance

(A.C.C.O., some functional appliances).

A. Removable appliances with intrinsic forces

1. ACTIVE PLATES

BASIC ELEMENTS: - Baseplate

- Clasps

ACTIVE ELEMENTS: - labial wire

- springs screws

- elastics

--Pressure is usually applied to a tooth using a spring, essentially a cantilever spring.

--There is no attachment on the tooth, and it is impossible to grasp a tooth with a spring.

Therefore, the spring must impinge on the correct point on any tooth that is to be moved.

--There is virtually NO FRICTION in that movement, since both the spring wire and the

tooth surface are hard and polished. This means that at that point of application, the

pressure is perpendicular to the tooth surface.

--The direction in which the tooth is being pushed is, therefore, determined by the point

at which the spring bears and not always by the direction of movement of the free end of

the spring.

--A low-load/deflection rate of the wire increases the flexibility or range of action of the

wire, with the advantage that the dimensions of the spring are not increased. This can be

done, for example, by incorporating a coil at the point of attachment of a cantilever

spring, or the addition of an extra limb, so forming the double cantilever spring.

--Anchorage in active plates is provided by all the teeth that are not being moved, (and

the palatal surface in the maxilla), since the acrylic lies against these surfaces.

2. APPLIANCES WITH SCREWS

The appliance has two parts. A screw is embedded in both parts of the appliance.

When the screw is turned - by means of a key or pin - the two sections of the appliance

move apart. Normally, the screw is given one quarter of a turn at a time, and the rate of

tooth movement is regulated by the frequency with which this adjustment is made (e.g.

Rapid Palatal Expansion v/s Slow Palatal Expansion). Removable appliances can be

used only with a slow rate of expansion (1 x 3 or 4 days); otherwise, they dislodge.

B. Removable appliances using extrinsic forces:

1. LIP BUMPER: In case of gaining space, use lip pressure to exert distal force on

molars, and relieve labial force off incisors.

2. INCLINED PLANE: In case of anterior crossbite, the inclined plane guides maxillary

incisors labially.

3. MAXILLARY BITE PLATE (without active parts): Widely used in conjunction with

fixed appliances and by itself for the control of excessive overbite; the mandibular

incisors contact the palatal acrylic. The posterior teeth are out of occlusion and

tend to erupt.

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APPLIANCES:

A. Advantages

1. NO FRICTION

2. Easy to wear

3. Hygiene

4. Cost (less than fixed)

B. Disadvantages

1. NO precise control of movement: no torque or bodily movement

2. Breakage

3. Relies on a lot of cooperation from patient

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APPENDIX OF DEFINITIONS

TYPES OF TOOTH MOVEMENT

BODILY MOVEMENT: A tooth is moved bodily when the action of the force exerted on it is pure

translation. The force acts through the center of resistance of the tooth. The center of resistance in a single

rooted tooth with a parabolic shape is at approximately 1/3 the distance for the alveolar crest to the apex.

In case of bodily movement (pure translation) the center of rotation is at infinity. Practically, a

force cannot be placed to act through the center of resistance. It can only be exerted at the level of the

crown, through a bracket or a tube. Therefore, when a force is exerted on the crown,

TIPPING of the tooth is expected to occur = no pure translation. There is distal (or mesial) movement, but also rotation (moment) = (F x d). The center of rotation is not at infinity, but approaches toward the apex

(between apex and center of resistance).

PURE ROTATION is obtained when the center of rotation is AT the center of resistance. In this case, we

are dealing with a pure moment or a couple. If a pure moment (couple) is placed on a tooth-anywhere on a

tooth-,a center of rotation occurs at the center of resistance of the tooth. Generally speaking, a torque,

torsion or twisting motion applied to a tooth to maintain or produce a root (or crown) movement, involves a

couple or pure moment + a force to restrain the unwanted movement of the crown (or root). In this

instance, we are moving the center of rotation from the center of resistance toward the incisal surface of the

tooth.

TRANSLATION (center of rotation at infinity) and PURE ROTATION (center of rotation at the

center of resistance) can be considered as the two basic types of tooth displacement. Other centers of

rotation are a combination of pure rotation and pure translation.

ANCHORAGE

Anchorage is the degree of resistance to displacement offered by an anatomical unit when used for the

purpose of effecting tooth movement.

-An active member of an orthodontic appliance is the part of the appliance concerned with tooth movement

(e.g. retracting anterior teeth).

-The reactive member of an appliance = teeth supporting (anchoring) the appliance, that are not supposed

to be displaced (e.g. molars).

-A member of an appliance may function as both active and reactive in case reciprocal anchorage is used

(retract anterior teeth, protract posterior teeth; close an anterior diastema between two central incisors).

Where a single tooth, or a group of teeth, is to be moved in one direction only, care must be taken to ensure

that the reaction does not also produce tooth movement.

Anchorage may be obtained from 3 sources:

1. within the same dental arch in which tooth movements are being carried out. In this case, as

many teeth as possible are incorporated in the anchorage section of the appliance, and as few teeth as

possible are moved at a time.

2. between the dental arches, by intermaxillary traction to the opposing dental arch (class 2, or class

3, elastics).

3. by extraoral means (head gear, chin cap).

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FUNCTIONAL APPLIANCES

Functional appliances use extrinsic (muscle) forces to effect tooth movement. Numerous

appliances have been advocated for the treatment of occlusal problems. Because of the

success of functional appliances mainly in the treatment of distoclusions (Class II), this

lecture will deal with this malocclusion.

A- DEVELOPMENT OF FUNCTIONAL APPLIANCES

B- TYPES OF FUNCTIONAL APPLIANCES

(most commonly used in USA)

1. Activator (original and variations).

A- Principles and Construction

B- Mode of Action, Types

C- Posttreatment Studies

2. Bionator

A- Standard

B- Modified

3. Function regulator or corrector (Frankel).

4. Herbst (fixed).

5- Combined Functional Appliances/Headgear

C- MODE OF ACTION OF FUNCTIONAL APPLIANCES

1. Muscular behavior.

2. Adaptability of condylar cartilage.

3. Clinical studies.

4. Concepts of growth stimulation.

5. Is the response the same with different functional appliances or other orthodontic

appliances (e.g. Headgear)?

D- ROLE OF FUNCTIONAL APPLIANCES IN CONTEMPORARY ORTHODONTICS

Functional appliances as adjunct to orthodontic therapy.