An Introduction to Orthodontics, 2nd Edition

15. Anchorage, tooth movement, and retention (B. Doubleday)

 

15.1. WHAT IS ANCHORAGE AND WHY IS IT IMPORTANT?

Anchorage has been defined as the source of resistance to the forces generated in reaction to the active components of an appliance. Anchorage is required to prevent unwanted tooth movements.

Anchorage is a difficult concept to grasp, but it may be helpful to consider it initially as the balance between the applied force and the available space. Whenever tooth movement is attempted there will be an equal and opposite reaction to the force(s) applied by the active components (Newton's third law of motion). This reaction force is spread over the teeth which are contacted by the appliance. For example, if both upper canines are being retracted with an upper fixed appliance, which has attachments on all the erupted teeth, an equal and opposite force to that being generated by the active canine retraction will also be acting on the remaining upper arch teeth which comprise the anchorage or resistance to that movement (Fig. 15.1). The amount of forward movement of the anchor teeth will depend upon their root surface area and the force applied (see Section 15.4). However, anchorage is not merely an anteroposterior phenomenon — unwanted tooth movements can also occur in the vertical and transverse dimensions.

Fig. 15.1. Diagram showing the effect upon the anchor teeth of retracting upper canines with a fixed appliance.

The importance of anchorage is perhaps most keenly appreciated when it has been neglected. Anchorage loss may jeopardize a successful result because inappropriate movement of the anchor teeth results in insufficient space remaining to achieve the intended tooth movements. In some cases anchorage loss can result in a worsening of the occlusion, for example, during the canine retraction phase of appliance treatment for a Class II malocclusion, forward movement of the anchor teeth can result in an increase in overjet. However, in some situations loss of anchorage can be used to advantage, for example, in a class Class III malcclusion an increase in overjet can be advantageous. Therefore anchorage requirements need to be assessed at the time of treatment planning.

15.2. THE HISTOLOGICAL BASIS OF TOOTH MOVEMENT

When a point force is applied to the crown of a tooth, it will tilt around an axis approximately at the junction of the apical one-third and the coronal two-thirds of the root (however, this is variable depending upon the size of the force and local anatomy). As a result the force is concentrated at the coronal one-third of the socket wall in the direction of the force and at the root apex in the opposite direction, as shown in Fig. 15.2.

Fig. 15.2. Diagram showing the effect of a tipping force applied to the crown of a tooth (P = pressure T = tension).

When an optimal force is applied, cell proliferation occurs within the periodontal ligament in areas of compression and osteoclasts (bone-resorbing cells) migrate in from the surrounding blood vessels. Direct resorption of the bone of the socket wall adjacent to the areas of pressure takes place within a few days. On the tension side the periodontal fibres are stretched, and proliferation of fibroblasts and osteoblasts (bone-forming cells) is followed by an increase in the length of the periodontal fibres which are subsequently remodelled. Osteoid is deposited on the bony socket wall on the tension side and is then calcified to form woven bone, which in turn is remodelled into mature bone. Thus the tooth moves through the alveolar bone under the influence of an applied force (see Table 15.1).

As these changes are mediated by cells derived from the blood supply, the latter is an important prerequisite for tooth movement to occur. Therefore a force which exceeds capillary pressure and reduces blood flow will not produce optimal movement.

If an excessive force is applied continuously, direct resorption of bone does not take place because compression of the blood vessels within the periodontal ligament results in a sterile necrosis (known as hyalinization because of its homogeneous, glass-like microscopic appearance) and initially a cessation of movement. After a delay of two to three weeks, indirect resorption takes place outwards from the marrow spaces of the adjacent alveolar bone (Fig. 15.3) and then the tooth moves. This is known as undermining resorption (see Table 15.1).

Fig. 15.3. Diagram showing the effect of applying an excessive force: (a) areas of hyalinization (1); (b) undermining resorption (2) and direct resorption in areas where force is less (3).

image(a)

image(b)

Table 15.1 Cellular reactions to the application of an orthodontic force

Optimal force
Pressure areas

1. Cellular proliferation within a few days

2. Osteoclasts migrate into the PDL from blood vessels

3. Resorption of bone and remodelling of PDL fibres

Tension areas

1. Stretching of PDL fibres

2. Cellular proliferation of fibroblasts and osteoblasts

3. Increase in length of PDL fibres

4. Deposition of osteoid

5. Remodelling and reattachment of PDL fibres, and calcification of osteoid into mature bone

Excessive force
Pressure areas

1. Capillary blood vessels are crushed resulting in death of cells in PDL (hyalinization)

2. In areas adjacent to the hyalinized sections of PDL cellular proliferation occurs

3. Resorption occurs deep to hyalinized area from cancellous bone outwards toward lamina dura of PDL (undermining resorption)

4. Tooth movement occurs

Tension areas
As for optimal force

PDL, periodontal ligament.

The optimum force for tooth movement is around 20–25 g/cm2 of root surface area. The size of the force applied to an individual tooth will depend upon its root surface area and the type of tooth movement planned. In bodily tooth movement the applied force is spread over the whole of the root surface in the direction of translation (Fig. 15.4). Thus a larger force is required to achieve the threshold for movement. In contrast, intrusion requires light forces as the applied stress is concentrated at the apex of the tooth and the application of an excessive force runs the risk of occluding the blood supply to the pulpal tissues. Average forces for the common tooth movements are given in Table 15.2, but it should be remembered that the optimal force for a given tooth will depend upon its root surface area.

Fig. 15.4. Diagram showing distribution of the applied force with bodily movement (P = pressure T = tension).

Table 15.2 A guide to force levels for tooth movement

Tipping movements

30–60 g

Bodily movements

100–150 g

Rotational movements

50–75 g

Extrusion

50–75 g

Intrusion

15–25 g

The use of excessive force in orthodontic tooth movement is not advocated for a number of reasons, including the following:

·     delay in tooth movement;

·     the dispersal of an excessive force over the anchor teeth is more likely to reach the threshold for their movement, resulting in an increased risk of anchorage loss;

·     a greater force leads to increased discomfort of the tooth being moved;

·     increased tooth mobility (due to the removal of a greater amount of supporting bone);

·     a greater risk of root resorption.

The success of orthodontic tooth movement also depends upon the duration of the applied force. It has been shown that the chemical mediators of tooth movement appear in the bloodstream within a few hours of a continuous force being applied and that clinical tooth movement will occur with a force duration of as little as 6 hours per day. However, for optimal tooth movement application of a continuous force, for 24 hours per day is preferable (but see Table 15.3for the reasons for more rapid tooth movement in children). Irregularities in the bony socket wall mean that, even though overall an optimal force is applied, excessive forces can develop in small areas. To allow these areas to repair and to limit root resorption, reactivation of the force exerted by an appliance should be undertaken at intervals more than 3 weeks apart.

This discussion has outlined the response of cancellous bone to an orthodontic force. The greater density and reduced vascularity of cortical bone means that, if a force is applied which results in the tooth root contacting bone, resorption of the root rather than bone may result. In addition, although some remodelling of the alveolar process occurs during tooth movement, this is not limitless and it is quite possible to move a tooth root through the labial or palatal cortical plate. This may result in dehiscence of the root, with severe gingival recession and possibly loss of pulp vitality as well as root resorption.

By necessity this section has been a summary of the complex biochemical changes which occur as a result of pressure or tension applied to a tooth and its supporting structures. This interesting area is currently the subject of much research, and the reader is referred to the section on further reading.

Table 15.3 Reasons for more rapid tooth movement in children

· Physiological tooth movement is greatest when the teeth are erupting

· The periodontal ligament is more cellular, and therefore there are more cells available for resorption and remodelling

· The alveolar bone has a greater proportion of osteoblasts

· The cellular response in reaction to an applied force is quicker

· The width of the periodontal ligament is increased in newly erupted teeth, and so a greater force can be applied before constriction of the blood vessels occurs

· Growth can be utilized

15.3. TYPES OF ANCHORAGE

15.3.1. Intra-oral anchorage

Intra-oral anchorage has classically been subdivided as follows:

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

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

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

·     Reciprocal anchorage: two groups of teeth are pitted against each other, resulting in equal reciprocal movement of both. This concept is utilized in appliances to expand the upper arch. Activation of the expansion appliance results in a force acting equally but oppositely on the posterior teeth of both upper quadrants (Fig. 15.5).

Fig. 15.5. An expansion appliance, showing the use of reciprocal anchorage.

In practice, it may be more helpful to consider intra-oral anchorage in terms of whether it is derived from teeth in the same arch, i.e. intramaxillary anchorage, or whether it is gained from the opposing arch, i.e. intermaxillary anchorage (see Section 15.6).

15.3.2. Extra-oral anchorage

Extra-oral anchorage is achieved by the patient wearing headgear which applies a distal force upon the teeth. Essentially the patient's head is used for anchorage (see Section 15.7).

15.4. FACTORS AFFECTING ANCHORAGE

15.4.1. Type of tooth movement planned

A tipping force results in a concentration of the applied force at the apex and crestal bone margins of a tooth (see Fig. 15.2). In contrast, during bodily movement the force is spread over the root surface in the direction of movement (see Fig. 15.4), and so a greater force is required to achieve tooth movement and consequently a greater strain is placed on anchorage. However, this can be used to advantage as it is possible to increase the value of anchorage teeth by trying to ensure that they can only move bodily.

15.4.2. Root surface area of the teeth used for anchorage

Increasing the root surface area of the anchorage unit means that the reaction to an active orthodontic force is dissipated over a larger area. For this reason molar teeth are preferable to single-rooted teeth. Increasing the number of anchor teeth (e.g. by including second molars when bonding fixed appliances) also increases the root surface area resisting anchorage loss, but by the same token, movement of molar teeth places a greater strain on anchorage.

15.4.3. Skeletal pattern

It has been noted that, in patients with increased vertical skeletal dimensions and a backward pattern of growth rotation, mesial tooth movement and anchorage loss seem to occur more readily than in patients with reduced vertical skeletal proportions and a forward pattern of growth rotation (see Chapter 4, Figs 4.15 and 4.16). One possible explanation for this is the relative ‘strength’ of the facial musculature of the two facial types.

15.4.4. Occlusal interlock

A good buccal occlusion may act to resist tooth movement. This may or may not be an advantage, depending upon whether the tooth or teeth to be moved actively or the anchor teeth are affected.

15.4.5. Tendency for tooth movement in the arch

Anchorage loss is more rapid in the maxillary arch as upper teeth have a greater tendency for mesial drift.

15.5. ASSESSING ANCHORAGE REQUIREMENTS

When planning treatment, the type of tooth movement required (for example tipping or bodily movement) and the demands that this will place upon anchorage should be considered, together with the anticipated final position of both the molars and incisors. As a result of this process the particular malocclusion under consideration will fall into one of the following categories.

1. Excess space will remain following treatment. In this situation either the treatment plan should be re-examined or measures taken to try and ‘burn up’ anchorage.

2. The anchorage available should suffice. However, it is prudent to monitor anchorage throughout treatment.

3. No loss of anchorage can be tolerated. Therefore measures to reinforce anchorage should be instituted from the beginning of treatment.

4. Insufficient anchorage is available even with reinforcement during treatment. In this situation it is necessary to return to the aims of the treatment and to determine if these need to be modified. If not, additional extractions and/or extra-oral traction will be indicated.

15.6. REINFORCING ANCHORAGE

15.6.1. Intra-oral reinforcement of anchorage

Anchorage can be preserved intra-orally during treatment in the following ways.

Increasing the number of teeth in the anchor unit

This means including more teeth in the appliance to try to resist the unwanted effects of active tooth movement. For example, when fixed appliances are used, banding the second molars helps to increase anchorage.

Making movement of the anchor teeth more difficult

With fixed appliances it is possible to ensure that the anchor teeth can only move bodily. As bodily movement requires greater forces, the resistance of the anchorage unit is increased.

Intermaxillary anchorage

The anchorage available in one arch can be reinforced if the patient wears elastic traction to the opposing arch. For example, in a Class II malocclusion elastics from the upper canine region backwards to the lower first molars on both sides assist overjet reduction. This direction of elastic pull is described as Class II inter-maxillary traction (Fig. 15.6). Class III traction is shown in Fig. 15.7.

Fig. 15.6. Class II intermaxillary traction.

Fig. 15.7. Class III intermaxillary traction.

Elastic intermaxillary traction is difficult with removable appliances and is almost exclusively employed in fixed appliance treatments. Intra-oral elastics (see Chapter 17, Fig. 17.20) are available in a wide variety of sizes and weights.

However, intermaxillary traction is not without its disadvantages. Class II or Class III traction can lead to extrusion of the molar teeth, which has the effect of increasing the lower face height and reducing overbite. In patients with increased vertical proportions this will be counterproductive. Class II traction encourages forward movement of the lower molars, which may be advantageous if there is excess lower extraction space to close. However, the use of this type of traction where no lower arch space exists will have the effect of proclining the lower labial segment.

Intermaxillary traction can also be achieved with functional appliances (see Chapter 18).

Palatal and lingual arches

An arch which connects contralateral molars either across the vault of the palate or around the lingual aspect of the lower arch will help to prevent movement of the molars and thus reinforce anchorage. The arches are usually attached to bands cemented to the molar teeth (Figs 15.8 and 15.9).

Fig. 15.8. Palatal arch.

Fig. 15.9. Lingual arch.

Choice of appliance

Upper removable appliances actually afford more anchorage than fixed appliances because of their palatal coverage.

Implants

Implants act as a fixed structure and are useful for providing anchorage in patients with hypodontia or marked tooth loss.

15.6.2. Extra-oral reinforcement of anchorage

Extra-oral reinforcement of anchorage is discussed in Section 15.7.

15.7. EXTRA-ORAL ANCHORAGE AND TRACTION

15.7.1. General principles

In practice, the distinction between extra-oral anchorage (EOA) and extra-oral traction (EOT) is a matter of degree (Table 15.4), although confusingly the terms are often used interchangeably. Extra-oral anchorage is a method of increasing anchorage and therefore is designed to prevent forward movement of the anchor teeth. Extra-oral traction is a method of achieving tooth movement, most commonly in a distal direction. It is also sometimes used to try to move the maxilla distally and/or vertically, although in reality the net result is rather a restraint of maxillary growth. In order to achieve true (orthopaedic) maxillary movement, prolonged wear with forces in excess of 500 g over the years of active growth is required, followed by prolonged retention to reduce any rebound growth. Perhaps not surprisingly, most patients are unable to sustain this level of cooperation.

Table 15.4 Extra-oral traction and anchorage

 

EOA

EOT

Purpose

Reinforcement of anchorage

Tooth movement

Force

200–250 g

400–500 g

Wear required

10–12 hours

14–16+ hours

In addition to magnitude and duration, the direction of the headgear force also needs to be considered, although this is of more consequence with extra-oral traction. A direction of force below the level of the occlusal plane (cervical-pull headgear) will tend to extrude the upper molar teeth and thus cause an increase in the vertical dimension of the lower face. While this may be an advantage in a patient with a reduced lower facial height, it is contraindicated in a patient with increased vertical proportions. In the latter case, a direction of pull above the occlusal plane (high-pull headgear) is usually preferable, as this will have the effect of intruding the upper buccal segment teeth and will also tend to restrain vertical maxillary development.

To achieve distal movement of the upper first permanent molars, a force directed slightly above the occlusal plane, through the centre of resistance of those teeth, is desirable. It is important to monitor the direction in which the teeth are being translated. For example, if it can be seen that the crowns of the teeth are being tilted distally, the direction of pull needs to be raised to counteract this.

The centre of resistance of the maxilla is estimated to lie at a point approximately above and between the premolar roots. If restraint of maxillary growth is to be attempted, the direction of headgear pull should be adjusted so that the force passes through this area.

Intrusion of the upper incisors can be attempted by applying headgear to the upper labial section of the archwire during fixed appliance treatments, but to avoid root resorption a force of less than 200 g is advisable.

A direction of force above the occlusal plane is also advisable when headgear is employed in conjunction with a removable appliance, to aid retention of the appliance.

15.7.2. Components of headgear

Headgear consists essentially of three parts.

Means of attachment to the teeth

This is achieved by using one of the following:

1. A face-bow (Fig. 15.10) which slots into tubes soldered onto the bridge of a removable appliance crib (see Chapter 16, Fig. 16.17), tubes which form an integral part of a molar band attachment (see Chapter 17, Fig. 17.29), or tubes which are incorporated in the design of a functional appliance.

2. J-hooks (Fig. 15.11) which can be directly attached onto the archwire in a fixed appliance or attached to hooks soldered onto the labial bow of a removable appliance.

Fig. 15.10. A face-bow.

Fig. 15.11. J-hooks.

Strap or headcap

A number of different types are available which are mainly described by the direction of pull that the headgear affords:

·     cervical pull which consists of a neck strap (Fig. 15.12);

·     variable pull which consists of a headcap with a variety of positions for the application of force (Fig. 15.13);

·     high pull which is a headcap fitting over the back of the head (Fig. 15.14).

Fig. 15.12. Cervical-pull headgear with the force produced by an elastic strap. The headgear is attached to a face-bow and the patient is also wearing a rigid safety strap.

Fig. 15.13. Variable-pull headgear with force provided by elastic bands between the headgear and the face-bow. A rigid safety strap is also being used.

Fig. 15.14. High-pull headgear attached to a face-bow.

Elastic component or spring mechanism

This connects the two other elements and controls the magnitude of the force applied. Elastic force is produced either by an elastic strap (see Fig. 15.12) or by different sizes of extra-oral elastic bands (see Fig. 15.13). Spring mechanisms are shown in Figs 15.14 and 15.15.

Fig. 15.15. Safety release headgear with a spring mechanism which breaks apart when excessive force is applied.

15.7.3. Headgear safety

Tragically, several cases have been reported where severe ocular injuries, including blindness, have occurred owing to accidents with headgear. These incidents have mainly occurred with face-bows used in conjunction with some form of elastic force, where the face-bow has been pulled out of the mouth and recoiled back into the face or eyes. Various methods of increasing the safety of headgear have been introduced. One of the simplest designs is the rigid safety strap (Fig. 15.16; see also Figs 15.12 and 15.13) which, if correctly fitted, helps to prevent the face-bow from being dislodged. The spring mechanisms have also gained popularity as a safety release feature can be more easily be built into the headgear; if an excessive force is applied, the components come apart thus preventing recoil of the face-bow (see Figs 15.14 and 15.15)., Face-bows with the ends re-curved to form a guard over the sharp end of the intra-oral bow are available (Fig. 15.17). In addition a face-bow has been developed with a small catch to lock it into the molar tubes (Figs 15.18a and b), these are strongly recommended as they prevent the face-bow being pulled out.

Fig. 15.16. Rigid safety strap.

Fig. 15.17. Safety face-bow.

Care is also required with J-hooks as the hook can be dislodged and cause serious injury. It is preferable to bend the hook round so that it forms a circle and is attached onto a hook soldered to the removable appliance or archwire. A relatively large headcap should be used with small heavy elastics so that the distance that the J-hook can travel is minimized.

It would now be considered neglilent to use headgear without safety features. Patients should be warned of the dangers and instructed that headgear should not be worn during any ‘horseplay’. If the headgear dislodges during the night, patients should be advised to discontinue its use and to return for adjustment by the clinician.

15.7.4. Reverse headgear

This type of headgear is also known as a face-mask and is used to try and move teeth mesially to close up excess spacing or in Class III malocclusions in an attempt to move the maxilla forward (see Chapter 11, Fig. 11.15).

15.8. MONITORING ANCHORAGE DURING TREATMENT

15.8.1. Single-arch treatments

Monitoring anchorage during single-arch fixed or removable treatments is relatively straightforward, as it is possible to use the other arch as a reference. This can be done by recording the overjet and molar positions during treatment, preferably at each visit. The progress of the tooth or teeth being moved can be recorded most easily using dividers which can then be imprinted into the record card.

15.8.2. Upper and lower fixed appliance treatments

Where tooth movement is occurring in both arches simultaneously it is a little more difficult to determine where the teeth are spatially compared with their starting position. For example, in a Class II, division 1, malocclusion forward movement of the upper arch may occur owing to loss of anchorage, but if the lower labial incisor teeth have also been inadvertently proclined, due to enthusiastic use of Class II traction for example, loss of anchorage is more difficult to detect as the overjet measurement may be unchanged or even reduced. For this reason a lateral cephalometric radiograph should be taken prior to the placement of appliances, and then progress with tooth movement and growth can be evaluated by repeating the radiograph. If necessary, the treatment mechanics can then be modified. It is also advisable continually to bear in mind the final anticipated tooth positions, for example the desired buccal segment occlusion, and to record progress towards this goal at every visit.

Fig. 15.18. Locking face-bow: (a) open; (b) closed.

image(a)

image(b)

15.9. COMMON PROBLEMS WITH ANCHORAGE

The most common reasons for the occurrence of anchorage problems during treatment are as follows.

·     Failure to appreciate fully the anchorage requirements of a particular malocclusion at the treatment planning stage. If this becomes apparent during treatment, it is probably wise to take up-to-date records and reassess the case. It may be necessary to institute extra-oral anchorage or, if problems are marked, extra-oral traction or even additional extractions. It is advisable to explain carefully to the patient and their parents the reasons for the change of treatment plan.

·     Poor patient compliance. It is important during any orthodontic treatment to monitor carefully patient compliance with the appliance, ideally at every visit. The major problem with removable appliance treatment is to ensure that the patient wears the appliance full-time. If compliance is particularly poor, forward movement of the anchor molars owing to mesial drift can occur, leading to loss of anchorage. With fixed appliances, breakages and failure to wear headgear or elastic traction are the most common problems leading to anchorage loss. Sometimes encouragement and an explanation of the effect of the patient's actions upon the success of treatment may be sufficient. However, for a proportion of patients this does not have the desired effect, which emphasizes the need for careful patient selection. Unfortunately, escalating treatment to overcome anchorage loss is often poorly received by this group of patients, and a compromise result may have to be accepted.

15.10. RETENTION

Relapse has been defined by the British Standards Institute as the return, following correction, of the features of the original malocclusion.

After a course of orthodontic treatment a period of retention is usually necessary. This allows the recently formed osteoid and bone to mature and gives time for reorganization of new periodontal fibres. Retention should not be relegated to being an afterthought at the end of treatment; it should be considered at the planning stage and explained to the patient as an integral part of the overall management package. In addition, it is advisable to identify those cases where the prognosis for a stable result has to be guarded and a decision made whether to embark on treatment at all.

15.10.1. Factors to be considered when planning retention

Soft tissues

Where possible, appliance therapy should aim to place the teeth in a position of soft tissue balance following treatment. Sadly, no amount of retention will stabilize an inherently unstable result.

If the lips are incompetent prior to treatment, the method by which a patient achieves an anterior oral seal should be assessed and the probable effect of treatment upon lip competence determined. For example, in a Class II division 1, malocclusion, the lower lip should ideally rest in front of the retracted upper incisors at the end of treatment. This is more likely to be achieved in the patient shown in Fig. 15.19 than in the patient shown in Fig. 15.20, who has grossly incompetent lips which will not act in front of the incisors even if the overjet is reduced.

Fig. 15.19. Potentially competent lips.

Fig. 15.20. Grossly incompetent lips.

In patients with advanced periodontal disease, normal soft tissue pressures may lead to drifting of the teeth. In these cases, permanent retention is usually required following treatment to prevent relapse (see Chapter 19).

Facial growth

The probable direction of any future growth and its effect should be estimated and taken into consideration at the time of treatment planning. Class III malocclusions and the extremes of the vertical range, i.e. anterior open bites and deep overbites, are most commonly adversely affected by further growth. In these cases it is wise to overcorrect the incisor relationship and, if possible, to continue retention until the end of the teens when the growth rate slows or to defer treatment until this time. In severe class Class III cases particularly it may be appropriate to wait until the slow rate of adult growth has been reached before deciding whether extraction of teeth and orthodontic ‘camouflage’ or surgical correction would be the best treatment.

Although lower incisor crowding is multifactorial, late facial growth has been implicated. Crowding of the lower labial segment is common, even following orthodontic treatment. One study in the USA found that lower incisor crowding increased in around 66 per cent of a sample of 450 post-orthodontic patients. Given these statistics, it is not surprising that, particularly in the USA, there is a growing trend towards permanent retention of the lower labial segment, for example with a bonded retainer.

Supporting tissues

During active tooth movement the periodontal ligament fibres are placed under tension. If the force is removed, the tension in these fibres could result in the tooth's springing back against the newly formed immature bone which is more readily resorbed, resulting in relapse. The rate of turnover of the different groups of periodontal ligament fibres varies. The fibres which run between the socket wall and the cementum are remodelled with the bony changes; however, the supracrestal fibres take over six 6 months to be reorganized. Therefore some retention is advisable to allow the supporting tissues to adapt. However, de-rotation is particularly prone to relapse. This is related to the slow turnover of the free gingival fibres, which remain under tension for months or even years after rotational movements. One method of overcoming this is pericision or circumferential supracrestal fibrotomy, in which a scapel blade is run around the gingival crevice to cut through the supracrestal fibres. Alternatively, rotated teeth can be overcorrected, but relapse is unpredictable and some teeth remain stubbornly overcorrected. In any case prolonged retention, for example with a bonded retainer, is advisable.

An upper midline diastema present after eruption of the permanent canines also has a strong tendency to reopen after closure. It has been suggested that this is due to discontinuity of the transeptal fibres between the central incisors. If the diastema is associated with radiographic and clinical evidence of insertion of the fraenal fibres through the midline suture to the incisive papilla, a fraenectomy during space closure is advisable. Again, prolonged retention is usually required.

Occlusal factors

Achievement of a satisfactory inter-incisal relationship will aid retention: for example establishment of an adequate overbite is essential to retain an anterior tooth (or teeth) which has been proclined from being in crossbite (Fig. 15.21) and correction of the inter-incisal angle is necessary to prevent relapse of a deep overbite (Chapter 10, Fig. 10.11). In contrast, a poor buccal segment interdigitation, particularly associated with a displacement on closure, can contribute to relapse.

Fig. 15.21. If the anterior teeth are proclined to correct the crossbite, lack of overbite will lead to relapse.

15.10.2. Retention regimes

To minimize relapse it is advisable to commence retention immediately after the end of active appliance therapy. It is difficult to lay down rigid rules for retention, as each case should be assessed individually. Therefore the following regimes are for guidance only.

Tilting movements into the line of the arch

Following a course of removable appliance therapy to align one or two teeth, 3 months full-time wear of the passive appliance will usually suffice. If the teeth have been proclined from a crossbite position and there is sufficient overbite, 3 months of night-only wear is usually adequate.

Bodily movements

After correction of more severe malocclusions with a fixed appliance it is common practice to fit removable Hawley retainers (Fig. 15.22) or vacuum-formed thermo-plastic retainers (Fig. 15.23) for 3 to six 6 months of full-time wear followed by 6 months of night-only wear. Alternatively, many operators fit bonded retainers on the lingual aspect of the lower incisors and canines (Fig. 15.24). These are often left in situ until the fate of the third molars is determined. Bonded retainers lie passively against the teeth and are retained using light-cured acid-etch retained composite. They may be fabricated from flexible multistrand wire and bonded to each tooth or formed from more rigid solid stainless steel wire and bonded at either end to the canine teeth only. Pre-formed bonded retainers with pads for bonding to the palatal surfaces of the upper central incisors can be effective in holding a median diastema closed. Micro-magnets have also been used in this situation.

Fig. 15.22. Upper and lower removable retainers.

Fig. 15.23. Vacuum-formed thermoplastic upper removable retainer.

Fig. 15.24. Bonded retainer.

Opening space for prosthetic replacement of missing teeth

This has been considered separately for the following reasons:

1. If a removable partial denture cum retainer is to be used following active tooth movement, C-clasps or stops should be placed mesial and distal to the pontic tooth to help prevent relapse.

2. It has been shown that if acid-etch retained bridgework is placed immediately after the end of active tooth movement there is a higher rate of bond failure. Therefore it is advisable to retain with a removable retainer for at least 9 months.

De-rotation

Prolonged retention is wise after correction of rotations. This is perhaps most easily accomplished with a bonded lingual/palatal retainer (see Fig. 15.24).

Orthopaedic movement

After growth modification with a functional appliance or with the use of headgear, it is advisable to continue retention with the appliance, at least for nights only, until the growth rate has slowed to the low levels of adulthood, in the late teens.

15.11. SUMMARY

Anchorage is the balance between the tooth movements desired to achieve correction of a malocclusion and the undesirable movement of any other teeth. The strain placed upon anchorage depends upon the type of tooth movement to be carried out and the applied force(s). Anchorage can be increased by maximizing the number of teeth (and root surface area) resisting the active tooth movement, either within the same arch (intramaxillary anchorage) or in the opposing arch (intermaxillary anchorage). Extra-oral forces can also be utilized with headgear. It is important to map out anchorage requirements at the planning stage and monitor throughout treatment.

Retention is usually necesssary to overcome the elastic recoil of the periodontal supporting fibres and to allow remodelling of the alveolar bone. Retention requirements should be planned prior to the start of treatment.

PRINCIPAL SOURCES AND FURTHER READING

Bowden, D. E. J. (1978). Theoretical considerations of headgear therapy: a literature review. British Journal of Orthodontics5, 145–52.

Bowden, D. E. J. (1978). Theoretical considerations of headgear therapy: a literature review. Clinical response and usage. British Journal of Orthodontics5, 173–81.

These two papers provide an authoritative review of the principles of headgear.

Firouz, M., Zernik, J., and Nanda, R. (1992). Dental and orthopedic effects of high-pull headgear in treatment of Class II, division 1, malocclusion. American Journal of Orthodontics and Dentofacial Orthopedics102, 197–205.

Hill, P. A. (1998). Bone remodelling. British Journal of Orthodontics25, 101–7

Kaplan, H. (1988). The logic of modern retention procedures. American Journal of Orthodontics and Dentofacial Orthopedics93, 325–40.

Little, R. M., Reidel, R. A., and Ĺrtun, J. (1988). An evaluation of changes in mandibular anterior alignment from 10 to 20 years postretention. American Journal of Orthodontics and Dentofacial Orthopedics93, 423–8.

Meghji, S. (1992). Bone remodelling. British Dental Journal172, 235–42.

Melrose, C. and Millett, D. T. (1998). Toward a perspective on orthodontic retention? American Journal of Orthodontics and Dentofacial Orthopedics113, 507–14.

Nanda, R. S. and Nanda, S. K. (1992). Considerations of dentofacial growth in long-term retention and stability. Is active retention needed? American Journal of Orthodontics and Dentofacial Orthopedics101, 297–302.

Postlethwaite, K. (1989). The range and effectiveness of safety headgear products. European Journal of Orthodontics11, 228–34.

Proffit, W. R. (2000). Contemporary Orthodontics (3rd edn). Mosby, St Louis, MO.

Samuels, R. H. (1996) A review of orthodontic face-bow injuries and safety equipment. American Journal of Orthodontics and Dentofacial Orthopedics110, 269–72.

Samuels, R. H. (1997) A new locking face-bow Journal of Clinical Orthodontics31, 24–7.

Sandy, J. R. (1992). Tooth eruption and orthodontic movement. British Dental Journal172, 141–9.

An erudite paper detailing the cellular and biochemical theories of the mechanisms involved in tooth movement.

Seel, B. D. S. (1980). Extra-oral hazards of extra-oral traction. British Journal of Orthodontics7, 53–5.



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