1. General principles and indications for the use of removable mechanic appliances.

Although widely utilized in the past as the sole appliance to treat malocclusion, with the increasing availability and acceptance of fixed appliances, the limitation of removable appliances have become more apparent.

Removable appliances work by applying of a single force to the crown of a tooth. A removable appliance induces tipping of a tooth around a point known as the "centroid", "centre of resistance", which locates between 30 and 40% from the root apex of the hole length of a tooth. Tipping can be brought about in the mesio-distal or bucco-lingual direction. The key point to be considered is the position of the root apex before the beginning of treatment. Teeth that have their apexec at the correct position are ideally suited for treatment with removable appliances. In such cases the irregularity is usually due to the crowns being tipped from their correct position and, therefore, responds well to tipping tooth movements enabling the use of removable appliances (RA). RA are generally considered ineffective in bringing about bodily translation, derotation and uprighting of teeth due to the single point of contact of these appliances. Intrusion and extrusion are often induced, using forces of eruption and occlusion.

As a result the role of the RA is changing, it is becoming more widely used to transmit forces to blocks of teeth and as an adjunct to fixed appliance treatment. RA provide a useful means of applying extra-oral traction to segments of teeth, or an en­tire arch, to help achieve intrusive and/or distal movement. RA are also employed for arch expansion, which is another example of their usefulness in moving blocks of teeth. RA are particularly helpful where a flat anterior bite-plane or buccal capping is required to influence development to the buccal segment teeth and/or to free the oc­clusion with the lower arch.

RA are used passively as space maintainers following permanent tooth extrac­tions and also retaining appliances following fixed appliance treatment.

2. Advantages and disadvantages of removable appliances. The advantages and disadvantages of RA are summarized in Table 1.

Table 1

Advantages and disadvantages of removable appliances.
Advantages	Disadvantages
• Can be removed for tooth brushing	• Appliance can be left out
• Palatal coverage increases anchorage	• Only tilting moving possible
• Easy to adjust	• Good technician required
• Can be used for overbite reduction	• Affects speech
in a growing child avoiding a	• Inter-maxillary traction not practicable
lower appliance • Acrylic can be thickened to form	• Lower removable appliances are difficult to tolerate
flat anterior bite-plane or buccal	• Inefficient for multiple individual
capping	tooth movement
• Useful as passive retainer or space
maintainer
• Can be used to transmit forces to
blocks of teeth

3. Removable appliance design.

A design of a removable appliance suggests an active component, retention, anchorage and a baseplate.

Active component is force sources required for desired movement. They are springs, bows, screws and elastics.

Fixation is required to retain RA in the mouth. A number fixation mechanisms are used. They are Adam's cribs, C-clasps, Southend clasps, fitted labial bows, reverse U-loop labial bows, Hawley bows.

Anchorage resists force generated by active component.

Baseplate holds all the elements together. Baseplate can not only hold other elements together, but also be active itself. Heat-cure acrylic is more robust than self-cure.

• Anterior bite-plane. Increasing the thickness of acrylic behind the upper inci­sors forms a bite-plane onto which the lower incisors occlude. A bite-plane is prescribed when the overbite needs to be reduced by eruption of the lower buc­cal segment teeth or reduce possible occlusion interferences to allow tooth movement. Anterior bite-planes are usually flat.

• Buccal capping frees the occlusion on the tooth being moved and allows fur­ther relative eruption of the incisors.

4. Active component and tooth movement.

The components used to apply force to the teeth are:

• Springs are usually made from 0.5, 0.6 or 0.7 mm-diameter stainless steel wire

• Bows are usually made from 0.5 or 0.7 mm-diameter stainless steel wire

• Screws opening 0.2 mm per ¹/₄ turn

• Elastics

Springs

Typical springs used on RA include:

• Z- spring - typically made from 0.5 mm stainless-steel wire on a single tooth and 0.6 mm on two adjacent teeth;

• T-spring - typically made from 0.5 mm stainless-steel wire;

• palatal canine retraction spring - made from 0.5 mm stainless-steel wire;

• finger spring - made from 0.5 mm stainless-steel wire;

• buccal canine retraction spring - made either from supported 0.5 or 0.7-mm un­supported stainless-steel wire;

• Coffin's spring - made from 1.2 mm stainless-steel wire. It is used to bring about slow dento-alveolar arch expansion where the upper arch is constricted or there is a unilateral crossbite. The Coffin's spring consists of a U- or omega-shaped wire placed in the mid-palatal region with the retentive arms incorpo­rated into baseplate.

Even small changes in the diameter or length of wire used in the construction of a spring will have a profound impact upon the force delivered. A commonly made mistake is to activate a spring in 0.7 mm wire as the same distance as one of 0.5 mm wire. In fact, for the same design of buccal spring, the 0.5 mm spring should be acti­vated about 3 mm, whereas the 0.7 mm spring requires only 1 mm activation to de­liver 25 - 50 g of force (light or physiological force). Incorporating a coil into the de­sign of a spring increases the length of wire and, therefore, results in the application of a smaller force for given deflection.

Palatal finger springs are the most commonly used active components for mesial or distal movement along the arch. They are fabricated from 0.5 mm wire and are easy to adjust and activate. If boxed out from title acrylic and made with a guard wire they are more stable in the vertical plane than a buccal spring.

For vestibular movement of incisors, Z- springs (for 1 tooth in 0.5mm, for > 1 tooth in 0.6) are used and T- springs are used for premolars (0.5 mm) and molars (0.6). They are all activated only 1-2 mm by pulling them away at 45° from baseplate in the desired direction of movement.

Bows. These types of components are used for both overjet reduction and for providing anterior fixation. An example of a bow used for overjet reduction is the Robert's retractor made from 0.5 mm stainless-steel wire with supporting buccal sleevings. Anterior fixation can also be provided by Hawley bow (made from 0.7 mm wire).

Screws

Basically there are two types: first, the spring-loaded piston (e.g. Landin) with activation by turning with a special screwdriver in the direction of movement, it is used for proclining one incisor; second, the screw type (e.g. Glenross) which is opened or closed by means of a key, a quarter turn separating the two halves by 0.2^ mm. The screws can be used to move teeth:

1. transversely - for the correction of posterior crossbites;

2. anteroposteriorly - for moving teeth over the bite, especially if more than one in­cisor needs to be moved. Ideal tooth movement is achieved by turning the screw a quarter of a turn every 3-7 days. Rapid maxillary expansion is a variation of this type of expansion and normally relates to fixed appliances. However, it is possible to use a removable appliance to deliver this type of force if the appliance is well constructed and the patient is very cooperative, and in these cases, the midline screw is turned twice a week.

Elastics. Using of elastics to reduce an overjet is popular with patients as they are less visible than a metal spring. To reduce an overjet, the design appliance can be made by con­verting a labial bow, cutting it and bending the ends into hooks adjacent to upper ca­nines. Latex elastics are stretched between hooks and lie over the incisors. For most patients a 3.5 oz. 5/8 inch elastic is required. But elastics have a tendency to slide up proclined incisors and retract the gingivae instead.

5. Fixation (retention). Main types of fixation mechanisms.

A number fixation mechanisms are used to retain RA in the mouth. They include:

• Adams' cribs 0.7 mm-diameter SS

• C -clasps (circumferential) 0.7 mm-diameter SS

• Jackson's clasps 0.7 mm-diameter SS

• Southend clasps 0.7 mm-diameter SS

• Triangular clasps, ball-end clasps, Schwarz clasps

• Fitted labial bows 0.7 mm-diameter SS

• Reverse U-loop labial bow 0.7 mm-diameter SS

• Hawley bow 0.7 mm- or 0.8 mm-diameter SS

Clasps act by engaging certain constricted areas of the teeth that are called undercuts. There are two types of undercuts that are found in natural dentition.

1. Buccal and lingual cervical undercuts

2. Mesial and distal proximal undercuts.

Buccal/Lingual cervical undercuts: The buccal and lingual surfaces of molars have a distinct undercut at the cervical margin. Examples of clasps that engage these buccal and lingual cervical undercuts are the circumferential and Jackson's clasps. These undercuts are available for clasp fabrication only in those teeth that are fully erupted. Mesial and distal proximal undercuts: The molars are widest mesio-distally at the contact point and gradually taper towards the cervical margin. Examples of clasps that engage these undercuts are the Adams' clasp and Crozat clasp.

The Adams' clasp is made of the following parts:

1. two arrowheads

2. bridge

3. two retentive arms.

The two arrowheads engage the mesial and distal proximal undercuts. The arrow­heads are connected to each other by a bridge which is at 45° to the long axis of the tooth. They can be used on partially or fully erupted teeth, on molars, premolars and on incisors. The clasp can be modified in a number of ways (Adams with J hook, with additional arrowhead, with soldered buccal tube - it permits use extra-oral an­chorage using face bow and so on).

6. Anchorage and its types.

Anchorage is defined as the source of resistance to the reaction from the active com­ponents) in an appliance. In practice, it is a balance between the applied force and the available space. E.g., in a case where 3 | 3 are being retracted following extraction of 4 | 4, an equal, but the opposite force will also be acting on 65 | 56. The amount of forward movement of these anchor teeth will depend upon their root surface area and the force used.

Anchorage loss can be minimized by limiting the number of teeth being moved at any one time, applying the correct force for the movement required and increasing the resistance of the anchor teeth (e.g. by permitting only bodily movement). In some situations, movement of the anchor teeth is desirable. However, it is important to as­sess the anchorage requirements of a particular malocclusion before embarking on treatment, and if indicated reinforce from the start.

Types of anchorage.

I. Intra-oral anchorage: intra-maxillary, inter-maxillary.

1. Simple anchorage: active movement of one tooth versus several anchor teeth.

2. Compound anchorage: the teeth of greater resistance to movement are util­ized as anchorage for the translation of the teeth which have less resistance to move­ment.

3. Stationary anchorage: this is a misnomer as it is extremely difficult to pre­vent movement of anchor teeth altogether.

4. Reciprocal anchorage: two groups of the teeth are pitted against each other, resulting in equal reciprocal movement of both. This concept is utilized in ap­pliances to expend the upper arch.

II. Extra-oral anchorage. It is achieved by the patient wearing headgear which applies a distal force upon the teeth. Essentially the patient's head is used for anchor­age.

Factors affecting anchorage.

1. Type of tooth movement planned.

2. Root surface area of the teeth used for anchorage.

3. Skeletal pattern.

4. Occlusal interlock.

5. Tendency for tooth movement in the lower and upper arch.

Reinforcing anchorage

1. Intraoral reinforcement: increasing the number of teeth in the anchor unit; increas­ing mucosal coverage, making movement of the anchor teeth more difficult (bodily movement), using of inter-maxillary anchorage (elastic traction to the opposite arch) and palatal and lingual arches (for fixed appliances), using of implants.

2. Extraoral reinforcement (headgear).

Functional appliances (FA).

1. Mode of action of functional appliances

There are three mechanisms of action of functional appliances, namely:

• skeletal changes;

• dentoalveolar changes;

• soft tissue changes.

FA generate forces which are delivered primarily to the teeth. A posteriorly directed force acts upon the upper arch and anteriorly directed force acts upon the lower arch. The sites where FA induces facial changes are the maxillary complex, the mandible and the glenoid fossae.

The skeletal changes can be considered in two dimensions, namely anteroposterior

and vertical, and are:

1. additional overall growth of the mandible;

2. accelerated growth of the mandible, but not necessarily additional growth;

3. a change in the direction of the growth of the mandible;

4. a change in the position of the mandibular condyle and glenoid fossae;

5. restricted growth of the maxilla.

The dentoalveolar changes include:

1. retraction of the upper incisors (palatal tipping);

2. proclination of the lower incisors;

3. overbite reduction by reducing lower incisor eruption while permitting buc­cal segment tooth eruption;

4. mesial movement of lower buccal segment teeth, occurs because they move mesially as they continue to erupt;

5. distal movement of the upper buccal segment teeth - in combination with mesial movement of the lower buccal segment teeth, the molar relationship can be altered to a Class I relationship;

6. expansion of the upper arch.

Some functional appliances have the long-term effect on the soft tissues. Functional regulator appliances in combination with lip exercises might influence the position and functions of lips and cheeks to move the teeth into correct positions.»

2. Timing of treatment with functional appliances.

Traditionally FA have been fitted prior to the pubertal growth. This will usually be somewhere between 10-12 years of age in girls and 12-14 years of age in boys. Once fitted, the overall treatment time lasts 9-12 months, depending on the size of the initial overjet. The average rate of overjet reduction is approximately 1 mm per month.

Recently the treatment has been started at 8 - 9 years of age, but it is supposed to be a two- stage treatment. It begins with an initial phase of FA therapy, followed by a second fixed-appliance phase.

3. Classification of functional appliances.

FA can be classified according to:

• their component parts;

• whether they are primarily tooth-borne or tissue-borne;

• the malocclusion in which they are used (Class II or Class III incisor relation­ship);

• the degree of soft tissue stretch they induce when in place. There are two basic types of FA:

1. Myotonic FA.

2. Myodynamic FA.

4. Myotonic functional appliances. Their components and main types.

In myotonic FA the elastic recoil within the stretched soft tissues generates the forces that move the teeth. The main components of FA are cheek shields, lip pads, pilots and tongue cribs. One of the typical myotonic FA is the Frankel appliance or the functional regulator (FR). It has acrylic shields in the buccal sulci and little or no acrylic lingually, but in common with all FA it induces mandibular posturing. The buccal shields are intended to cause expansion of the arches by holding the cheeks away from the teeth and also to enlarge the alveolar process by stretching the perio­steum in the depth of the sulcus, thus causing bone to be laid down on the buccal as­pect.

There are three main variants of the appliance. The FR I is for treatment of Class II division 1 malocclusions and incorporates lip pads labial to the lower incisors to al­low forward development of the mandibular alveolar process. The cheek shields al­low normalizing the width of the upper arch. In case of infantile swallowing it can include a tongue crib. The FR II has in addition a palatal wire to procline the upper incisors and is intended for Class II division 2 malocclusions. The FR III is for the treatment of Class III malocclusions, having lip pads labial to the upper incisors, which together with a palatal arch, procline them, and a lower labial bow retroclining the lower incisors.

5. Myodynamic functional appliances. Their components and main types.

The forces that move the teeth are generated by stimulation of masticatory muscles and use of their strength while wearing FA. The main components of myodynamic FA are inclined planes, occlusal shelves and bite planes. As an example of the myodynamic FA is a twin-block appliance. It is constructed in two parts, as, separate upper and lower appliances. Forward mandibular posturing is achieved by incorporat­ing buccal blocks with interlocking inclined planes, with the lower blocks engaging in front of the upper ones. The appliance is often used with headgear to the upper arch.

Fixed appliances

1. General principles and indications for the use of.

Fixed appliances, also referred to as braces, are combination of bands, brackets and auxiliaries that can move a tooth on six directions: mesially, distally, lingually, facially, apically, and occusially. Fixed appliances are attached to the teeth and, thus, are capable of greater range of tooth movements than is possible with removable appliances. Not only does the attachment on the tooth surface (called a bracket) allow the tooth moved vertically or tilted, but also a force couple can be generated by the interaction between the bracket and archwire running through the bracket. The interplay between the archwire and the bracket slot determines the type and direction of movement achieved.

The edgewise bracket is rectangular in shape and is typically described by the width of bracket slot, usually 0.018 or 0.022 inch. The depth of the slot is commonly between 0.025 and 0.032inch.

A round wire in a rectangular slot of bracket will give a degree of control of mesio-distal tilt, vertical height, and rotational position. The closer the fit of the archwire in the bracket, the greater is the control gained. However, with a round wire only tipping movements in a bucco-lingual direction are possible. When a rectangular wire is used in rectangular slot, a force couple can be generated by the interaction between the walls of the slot and sides of the archwire and bucco-lingual apical movement produced. Thus fixed appliances can be used in conjunction with a rectangular archwires to achieve tooth movement in all three spatial planes. In orthodontics these are described by the types of bend that are required in an archwire to produce each type of movement:

• First-order bends are made in the plane of the archwire to compensate for differing tooth width.

• Second-order bends are made in the vertical plane to achieve correct mesiodistal angulation or tilt of the tooth.

• Third- order bends are applicable to rectangular wires only. They are made by twisting the plane of the wire so that when it is inserted into the rectangular bracket slot the bucco-lingual force is exerted on the tooth apex. This type of movement is also known as torque.

In the original edgewise appliance these bands were placed on the archwire during treatment so that the teeth were moved into their correct position. Modern bracket systems have average values for tip and torque built into the bracket slot itself, and the bracket bases are of different thicknesses to produce an average bucco-lingual crown position (known as in-out). These pre-adjusted systems have the advantage that the amount of wire bending required is reduced. For initial alignment flexible archwire are used , but to minimize unwanted movements, progressively more rigid archwires are necessary.