15.1 Introduction
Orthodontic anchorage is defined as the resistance to unwanted tooth movement.
Orthodontic tooth movement is achieved through the forces gener- ated by an orthodontic appliance. However, the desired force which is generated has an equal and opposite reactionary force, as described by Newton’s third law, which will be spread over the other teeth that are contacted by the appliance and which can result in unwanted tooth movement. The aim of controlling orthodontic anchorage is to resist these reactionary forces to minimize the unwanted tooth movement and maximize the desired tooth movements.
Orthodontic anchorage is defined as the resistance to unwanted tooth movement.
Orthodontic tooth movement is achieved through the forces gener- ated by an orthodontic appliance. However, the desired force which is generated has an equal and opposite reactionary force, as described by Newton’s third law, which will be spread over the other teeth that are contacted by the appliance and which can result in unwanted tooth movement. The aim of controlling orthodontic anchorage is to resist these reactionary forces to minimize the unwanted tooth movement and maximize the desired tooth movements.
An analogy which may help to simplify this difficult concept is that of ice skating. Imagine that you are standing on the ice and push against the barrier of the rink. You will move backwards and the barrier will remain stationary. The barrier, in this instance, is analogous to absolute anchorage (see Section 15.3), for example, an ankylosed tooth or osse- ointegrated implant.
If two skaters push against each other equally, and they themselves are of similar size, both will move backwards by an equal amount. This is an equal and opposite reaction. If one skater is larger than the other, the smaller skater will be pushed away, whereas the larger skater will only move slightly, or not at all. However, if the larger skater now pushes against two smaller skaters then the larger skater will move more. This can be considered similar to pitting one larger tooth against a smaller tooth, or against two smaller teeth. The more teeth you try to move, the more likely your anchorage unit will move as well.
A clinical scenario is the retraction of upper canines using a fixed appli- ance, with all the available teeth involved in the appliance. An equal and opposite force to that being generated by retracting canines will also be acting on the remaining upper arch teeth to move them anteriorly, which can compromise the anchorage unit causing unwanted forward tooth movement of the rest of the dentition (Fig. 15.1).
15.2 Assessing anchorage requirements
Anchorage requirements should be considered in all three dimensions: anteroposteriorly, vertically, and transversely. Planning anchorage is a fundamental part of treatment planning and needs to be undertaken as part of managing the space requirements of orthodontic treatment. When considering anchorage management, it is important to assess the following factors.
15.2.1 Space requirements
The amount of crowding or spacing should be assessed as part of treat- ment planning. This can be done using either a visual assessment or more formally using a structured space analysis (see Chapter 7, Section 7.7). The results of this assessment will guide the clinician when decid- ing the most appropriate method of space creation, the potential treat- ment mechanics, and also the need for any anchorage reinforcement. For example, the extraction of maxillary first premolar teeth will result in 7 mm of space being generated on each side which can be used for tooth alignment or retraction of the upper labial segment. This may be too much space, the exact amount required, or too little. If it results in too much space, then the clinician needs to plan the controlled loss of anchorage to allow the posterior teeth to move mesially while the ante- rior teeth are being retracted into the optimum position. If the space created by the extractions is just sufficient, then the clinician needs to maintain all the space for tooth alignment or retraction of the incisors, ensuring good anchorage control of the posterior teeth. If the extrac- tion space is insufficient, then additional space needs to be generated with further extractions, expansion, interproximal enamel reduction, or distalization of posterior teeth.
15.2.2 The type of tooth movement to be achieved
There are six different types of tooth movement: tipping, bodily move- ment, rotation, torqueing, intrusion, and extrusion. Tipping occurs when the crown of a tooth moves in one direction and the root apex moves, by a lesser amount, in the opposite direction rotating around the tooth’s centre of resistance (Fig. 15.2). Bodily movement occurs when both the crown and the root move in the same direction equally (Fig. 15.3). In practice, this is often achieved by a series of small tipping and uprighting movements due to the ‘play’ between the archwire and the bracket slot (see Chapter 18). Rotational movement occurs when a force is applied mesially or distally to the labial aspect of a tooth.
Teeth have a centre of resistance around which movement occurs. If force is applied directly through the centre of resistance then the tooth will move bodily, however, because the centre of resistance lies within the root this cannot occur. A simple force applied to the crown of a tooth with a round wire will cause the tooth to tip (see Chapter 18, Fig. 18.1); however, with a fixed appliance in place, the built-in programme of the bracket interacting with a rectangular orthodontic wire causes the force to act as a ‘couple’, which can be used to change the inclination of teeth or to produce bodily tooth movement.
Bodily movement requires more force to be placed onto a tooth than tipping movements and is therefore more anchorage demanding on the remaining teeth.
15.2.3 The number of teeth to be moved
As the number of teeth to be moved increases, so does the anchor- age demand. If the anchorage requirements of treatment are high, then consideration should be given to moving a single tooth in stages or only a few teeth at a time to conserve anchorage. However, although the theoretical principles are sound and this is a commonly applied method of preventing unwanted tooth movement, there is some debate within the literature about the level of anchorage conservation that this tech- nique actually achieves by the end of treatment when all the active tooth movements have been completed.
15.2.4 The distance of the movement required
The greater the distance the teeth are to be moved, the greater the strain on the anchorage, and the greater the risk of unwanted tooth movement.
15.2.5 Aims of treatment
The aims of treatment can fundamentally alter the complexity of treat- ment and also the prognosis for success. The aims of treatment need to be specific and achievable, with the treatment mechanics that will be required to achieve the aims being acceptable to the individual patient. The fewer teeth that need to be moved to achieve the aims of treatment then the less the anchorage demand; however, if treat- ment is complex and multiple teeth are to be moved, there will be a greater anchorage demand and often a greater demand on the level of cooperation from the patient. In cases with a Class II molar relation- ship on initial presentation, the anchorage needs will be far greater if a Class I molar (and canine) relationship is to be achieved rather than a Class II molar (and Class I canine) relationship at the end of treatment (Fig. 15.4). The need to achieve a Class I canine relationship is usually very important for treatment success, therefore it is critical that anchor- age planning should focus not only on the intended molar movements but also on the required movements of the canines to achieve this goal (Fig. 15.5).
15.2.6 Root surface area of the teeth to be moved
The size of the root surface area of the tooth or teeth to be moved influences the anchorage requirements—the larger the root surface area, the greater the demand (Fig. 15.6). Teeth are maintained in a position of balance between the opposing forces of the lips and tongue, with the periodontal ligament, and the surrounding bone, resisting any tooth movement; therefore, for tooth movement to occur, a threshold of force must be achieved. This applies equally to the teeth in the anchorage unit, and if the threshold is exceeded, unwanted movement of the anchor teeth will occur and anchor- age will be lost. As such, the forces applied to achieve the desired tooth movements must be carefully selected to try and ensure they remain below the threshold of the teeth acting as the anchorage unit. Increasing the number of teeth acting in an anchorage unit (e.g. including the second molars in a fixed appliance) is one method of increasing this threshold.
15.2.7 Growth rotation and skeletal pattern
An increased rate of tooth movement has been associated with patients who have an increased vertical dimension or backward growth rota- tion. It has been suggested that space closure or anchorage loss may occur more rapidly in these high-angled cases and so the requirement for anchorage reinforcement needs to be considered in these individu- als. Conversely, in a patient with reduced vertical dimensions or a for- ward growth rotation, space closure or anchorage loss may be slower; therefore, the planned extraction pattern or the treatment mechanics may need to be altered to accommodate this. One hypothesis which has been proposed for this observation is the relative strength of the facial muscles, with individuals with reduced vertical dimensions having a stronger musculature.
15.2.8 Occlusal interdigitation and occlusal interferences
Occlusal interdigitation or occlusal interferences can prevent or slow down tooth movement. This can lead to an increased anchorage demand which may prevent the desired tooth movement and increase the likelihood of unwanted tooth movement. This phenomenon can be utilized to an orthodontist’s advantage, as it has been suggested that if teeth in the anchorage unit have a good interdigitation with the opposing dentition, this may increase their anchorage value by reducing mesial movement of the anchorage unit.
15.2.9 Bone quality
Maxillary bone is less dense than mandibular bone so the threshold for tooth movement in the maxillary teeth is lower than that in the mandib- ular teeth. Teeth move more readily through cancellous bone than cor- tical bone and the anchorage value of teeth can be increased by moving their roots closer to the cortical plate; however, this can put them at increased risk of root resorption, and as such, should be done with care. Certain groups of patients, such as individuals with hypodontia or previ- ous experience of periodontal disease, can have reduced quantity and/ or quality of alveolar bone. This will affect the anchorage balance when undertaking orthodontic treatment and must also be taken into consid- eration when planning the retention phase of treatment.
15.3 Classification of anchorage
• Simple anchorage: one tooth pitted against another.
• Reciprocal anchorage: two teeth or groups of teeth of equal size or equivalent anchorage value are pitted against each other, resulting in movement of both units by a similar amount. For example, a quad- helix used to expand the maxillary arch (Fig. 15.7) or applying power chain on an upper fixed appliance to two central incisors to close a median diastema (Fig. 15.8).
• Anchorage reinforcement: a variety of methods can be utilized:
- Intramaxillary compound anchorage: multiple teeth are used in an anchorage unit within the same arch, for example, using a molar and a second premolar as an anchorage unit for the retrac- tion of a canine.
- Intermaxillary compound anchorage: multiple teeth in oppos- ing arches, for example, the use of intermaxillary elastics (Fig. 15.9).
• Differential tooth movements: tipping movements are less anchorage demanding than bodily movement. This concept can be utilized when planning tooth movements to increase the relative anchorage value of specific teeth.
• Use of intra-oral adjuncts: this incorporates non-tooth structures such as the palatal vault when using a Nance button in conjunction with a transpalatal arch (Fig. 15.10), or bone when using temporary anchorage devices (TADs).
• Use of extra-oral adjuncts: this incorporates devices utilizing extra-oral structures to place a force upon the teeth. For example, headgear systems.
• Stationary/absolute anchorage: this can only be achieved when using an osseointegrated implant or ankylosed tooth as an anchorage unit.
15.4 Intra-oral anchorage
Anchorage reinforcement can be achieved by utilizing the teeth, soft tissues, and skeletal structures intra-orally.
15.4.1 Increasing the number of teeth in the anchorage unit
Incorporating as many teeth as possible into an anchorage unit will aid in reducing anchorage loss. If the anchorage demand is high, then con- sideration should be given to moving one tooth at a time to prevent strain of the anchorage unit (conserving anchorage). This will also allow better monitoring of any anchorage loss, allowing additional anchorage re-enforcing mechanics to be utilized if required.
15.4.2 Differential extraction pattern
The extraction pattern planned for the patient can influence the anchor- age balance of a case. Extracting teeth closer to the area of crowding reduces the amount of tooth movement required and thus the risk of anchorage loss. Also, by careful selection of the appropriate teeth to be extracted, the anchorage balance can be changed in such a way that it is possible to make some teeth more resistant to unwanted movement (Fig. 15.6).
A differential extraction pattern, between arches, can aid in anchor- age management. One such example is the treatment of Class II division 1 cases where the upper first premolars are extracted to aid reduction of an overjet and correction of the canine relationship to Class I. This becomes possible as the anchorage unit is greater posteriorly in the maxilla with the upper first molars and second premolars incorporated within the appliance. In the lower arch, the extraction of the lower sec- ond premolars will not only reduce the retraction of the lower labial segment but also work favourably for molar correction with the lower molars more likely to move mesially. These desired tooth movements can be enhanced with inter-arch (Class II) elastic use. This extraction pattern can be reversed in the treatment of Class III cases with the extraction of lower first premolars and upper second premolars. This differential extraction pattern will aid the camouflage of the reverse overjet with retroclination of the lower labial segment.
15.4.3 Care with initial intra-arch orthodontic mechanics
A number of factors may increase the demands on the anchorage units, such as the engagement of severely displaced teeth in the early stages of alignment. Anchorage loss will also occur if there is friction in the system as a greater force needs to be exerted to overcome friction and achieve the planned tooth movement. As a result of this higher applied force, the reactionary force is greater, which may result in unwanted tooth movement of the anchorage unit.
15.4.4 Bodily movement of teeth
Bodily movement requires more force than tipping movements and is therefore more anchorage demanding. Use of large, rectangular stain- less steel archwires will ensure bodily movement occurs rather than tipping, as the archwire will fill more of the bracket slot (see Chapter 18, Fig. 18.4). Differential tooth movements can be undertaken in an effort to limit unwanted tooth movements. This involves restricting the anchorage teeth to bodily movement while allowing the teeth you do wish to move to tip into place. As tipping movements occur more read- ily, differential tooth movement should occur. This is a concept which is fundamental to certain appliance systems such as Begg and Tip-Edge (see Chapter 18, Section 18.6.2).
15.4.5 Transpalatal and lingual arches
Both a transpalatal arch (usually connecting the upper first molars) and a lower lingual arch (usually connecting the lower first molars) can be used to reinforce anchorage by linking contralateral molar teeth (Figs 15.11 and 15.12). The teeth are joined with an arch bar, usually 1 mm diameter stainless steel, which connects across the vault of the palate or around the lingual aspect of the lower arch. This linking of teeth helps to prevent or reduce unwanted mesial molar movement. By fixing the intermolar width, which aids transverse anchorage, any mesial movement of the molars means that the buccal roots are more likely to come into contact with the cortical plate of the bone which, as mentioned previously, resists subsequent mesial tooth movement, but can also increase the risk of root resorption. Anchorage can poten- tially be further increased by adding an acrylic button, or Nance button, which lightly contacts the anterior palatal mucosa (Fig. 15.10). Caution should be taken with all these appliances, because if anchorage is lost and the molars move mesially then the lingual arch can cause procli- nation of the lower labial segment, or the U-loop of the transpalatal arch or acrylic of the Nance button may become engaged in the palatal mucosa. It is with this in mind that the palatal loop of the transpalatal arch would normally face distally.
The amount of anchorage reinforcement these devices provide has been questioned in the literature and it is advisable for the clinician to carefully monitor for any anchorage loss during treatment.
15.4.6 Intermaxillary anchorage
Anchorage from one arch can be used to reinforce anchorage in the other. In addition to occlusal interlocking mentioned earlier, the two main adjuncts which can be utilized to achieve this is with the use of intermaxillary elastic traction (see Chapter 18, Section 18.3.4) or Class II correctors, which are metal pistons connected between the arches. Intra-oral elastics are available in a variety of sizes and strengths. Class II elastics will be run from an anterior attachment in the upper arch to a posterior attachment in the lower arch. Class III elastics are the reverse, located anteriorly in the lower to posteriorly in the upper (Figs 15.13 and 15.14).
As with any force applied there can be unwanted effects, and inter- maxillary traction is not without this problem. Class II or Class III elastics can lead to extrusion of the molars, which will in turn reduce the over- bite and can also cause an increase in the face height, this might be beneficial in a few patients but for the majority these movements would be unwanted. The other significant consequence is that in an unspaced lower arch, the use of Class II elastics and Class II correctors will result in proclination of the lower labial segment.
15.4.7 Removable and functional appliances
Removable appliances can be used alone or in conjunction with a fixed appliance to reinforce anchorage (Fig. 15.15). By virtue of their palatal coverage, they have an increased anchorage value to resist unwanted tooth movement. Other design features which reinforce anchorage include the following:
• Anteroposteriorly: by colleting around the posterior teeth with acrylic, inclined bite-blocks, or the use of incisor capping.
• Transversely: the pitting of one side of the arch against the other can reinforce transverse anchorage, typically seen where an expan- sion screw is used for increasing the palatal transverse dimension (Fig. 15.16), usually resulting in reciprocal anchorage.
• Vertically: by either reducing the vertical dimensions during treat- ment of a high-angle patient by intruding the posterior teeth, often aided with high-pull headgear (see Chapter 12, Section 12.3.1); or increasing the vertical dimension by allowing differential tooth erup- tion with the use of an anterior bite-plane.
All of these three-dimensional features can be incorporated into functional appliances, which additionally can be used to gain anchor- age in the anteroposterior direction to aid in the treatment of a Class II malocclusion, often reducing the complexity during the fixed appliance phase of treatment.
15.4.8 Temporary anchorage devices
The use of temporary anchorage devices (TADs), also known as ortho- dontic bone anchorage devices, is becoming increasingly common in contemporary orthodontics. They first became popular in the 1990s and were developed from the dental implants used in restorative dentistry and maxillofacial bone plates. There are three distinct types (Box 15.1):
• Osseointegrated implants
These were modified from dental implants, making them shorter, with a wider diameter than those used in restorative dentistry. Osseointegrated implants can be used to provide maximum anchor- age and are useful if large or difficult tooth movements are required. This type of implant, placed in the mid-palatal region and attached to a palatal arch (Fig. 15.17), was compared with conventional headgear in a randomized controlled trial. The authors concluded that mid-palatal implants are an acceptable technique for reinforcement of anchorage.
They have three principal disadvantages:
(1) They need to be left for 3 months after placement to allow osseointegration.
(2) Due to their size, they are restricted to being used in edentulous areas.
(3) Since the implant osseointegrates, it requires a complex surgical procedure together with bone removal at the completion of treatment and some patients may find this unacceptable.
• Miniplate systems
These are based on maxillofacial bone plates, with a transmucosal portion projecting into the mouth to allow connection to the fixed appliance. They can provide reliable anchorage, but require a surgi- cal procedure to both place and remove them (Fig. 15.18).
• Mini-screws
These were developed from the screws of maxillofacial plating systems, but are smaller in size (typically 6–12 mm in length and 1.2–2 mm in diameter), hence the term mini-screw. No osseointegra- tion is required (or desired) which means that they do not provide absolute anchorage as there is the possibility that they can move during treatment. Their head and neck configuration has been adapted to facilitate placement of auxiliaries to the fixed appliance (Fig. 15.19).
Due to their ease of use—for both the patient and the orthodontist— the mini-screw approach is now the most popular. There is a range of different systems available with various claimed strengths and weak- nesses. Their designs and the associated treatment mechanics are under constant review. They are usually placed under local anaes- thesia and can often be removed at the end of treatment without requiring any anaesthetic.
Bone anchorage devices have the ability to provide anchorage in three dimensions: anteroposteriorly, vertically, and transversely. They can provide anchorage either directly or indirectly:
• Direct anchorage is achieved when forces are applied directly to the TAD (Fig. 15.20).
• Indirect anchorage is achieved when the TAD is linked to the anchor- age teeth, and then the orthodontic forces are applied to this anchor- age unit (Fig. 15.21).
The benefit of using TADs for indirect anchorage is that conven- tional treatment mechanics can be utilized, however, the disadvan- tage is that if the TAD moves or becomes loose between visits, then some anchorage loss could have occurred before it is noticed and rectified.
The use of TADs is allowing different approaches to anchorage, as well as possibly altering the scope of what was previously thought possible with fixed appliances. However, as with all these adjuncts, there are risks associated with their use which the patients must be made aware of. These include the risk of damage to adjacent struc- tures such as blood vessels, nerves, and tooth roots; the risk of fracture; the risk of gingival inflammation; and the risk of them becoming loose with subsequent anchorage loss or need for replacement/use of other means of anchorage reinforcement.
15.5 Extra-oral anchorage
15.5.1 General principles
Headgear can be used for:
• extra-oral anchorage
• extra-oral traction.
Extra-oral anchorage holds the posterior teeth in position, preventing unwanted mesial movement of the anchorage unit.
Extra-oral traction applies a force to the posterior teeth to achieve tooth movement, usually in a distal direction. Traction can also be used to attempt to achieve an orthopaedic effect by using conventional headgear to restrict the growth of the maxilla in a forwards and down- wards direction, or using reverse headgear (see Section 15.5.4) to pro- tract the maxilla. In simple terms, traction requires greater forces, for longer periods of time, than that required to maintain the position of the posterior teeth.
There are three directions of pull that can be achieved with head- gear and these should be considered at the time of treatment planning (Box 15.2): High- or occipital-pull headgear which helps to control the vertical as well as anteroposterior anchorage and is typically used in cases with increased vertical proportions (Fig. 15.22).
• Straight- or combination-pull headgear which controls the anter- oposterior and is typically used in cases with average vertical propor- tions (Fig. 15.23).
• Low- or cervical-pull headgear which aids in the control of anter- oposterior anchorage but is also used to increase the vertical dimen- sion by having an extrusive effect on the molars in cases of reduced vertical proportions (Fig. 15.24). Care should be taken with low-pull headgear in a Class II skeletal pattern as extrusion of the maxillary molars may lead to a clockwise rotation of the mandible making the class II skeletal pattern worse as the mandible rotates downwards and backwards.
The amount of force applied to the headgear is controlled by adjust- ing the attachment straps and should be carefully monitored at each visit. For orthodontic change, it is normal to apply 250–350 g to achieve anchorage reinforcement, increasing to 400–450 g to obtain extra-oral traction. For orthopaedic change, often termed maxillary restraint, the forces are increased to 500 g.
The duration of force also varies according to the purpose. Extra-oral anchorage requires a minimum of 10 hours per day, which is usually best achieved at bedtime. Extra-oral traction requires an increased duration and a minimum of 12 hours of wear per day is required though most operators ask for 14 hours. For orthopaedic changes, the duration of wear requested is also normally 14 hours, but the length of treat- ment with the headgear appliance is longer when compared to extra- oral traction (which may be achieved, in a cooperative patient, within 6–9 months).
The effect of a force depends on where it is applied in relation to the centre of resistance. If the force passes directly through the centre of resistance then pure translation will occur. If not, then tipping will arise. The centre of resistance for forces applied to the upper first molars is felt to be at the trifurcation of their roots. The centre of resistance for the maxilla as a whole is further forward, and is suggested to be between the premolar roots. Adjustment of the direction of force applied by the headgear relative to the centre of resistance of the first molar will influence how the molar will move.
In theory, intrusion of the upper incisors can be attempted by apply- ing headgear to the anterior labial segment of a working archwire, though the forces used must be light to avoid possible root resorption of the upper incisors. Due to concerns over safety, this technique has now been largely abandoned.
Headgear can be added to a removable appliance or a functional appliance, such as a twin-block appliance, usually with the aim of achieving orthopaedic changes. In these cases, the forces are kept above the occlusal plane, not only to control the vertical dimension, but also to prevent dislodging the appliance.
15.5.2 Components of headgear
The components of headgear have been modified dramatically in the last 20 years to improve safety (see Section 15.5.3). Where head- gear is to be used in conjunction with a fixed appliance, patients first need to be fitted with upper molar orthodontic bands with buc- cal headgear tubes. Where headgear is being used with a removable or functional appliance then headgear tubes need to be added to either the molar clasps or incorporated into the acrylic (see Chapter 17, Fig. 17.7).
Face-bow: the face-bow slots into the headgear tubes. The original face-bow was called a Kloehn bow, however, due to safety concerns about the unprotected intra-oral ends, its use is now largely historic. Currently the face-bow of choice is a Nitom™ locking face-bow pro- duced with a specialized safety catch (Fig. 15.25). A less complex safety version is the Hamill face-bow with a reverse loop attach- ment (Fig. 15.26); however, although the Hamill type can reduce the degree of soft tissue injury, there is a much higher risk of acciden- tal disengagement from the headgear tube when compared to the Nitom™ face-bow.
• Headcap or neck strap: a headcap or neck strap can be used inde- pendently or together to achieve the direction of pull required.
• High/occipital pull: use of the headcap only (Fig. 15.22). This pro- duces intrusive forces and either leads to intrusion of the molars or prevents them from extruding during treatment. This approach is useful where no increase in the vertical proportions can be toler- ated during treatment.
• Combination pull: uses the headcap and neck strap together (Fig. 15.23). In this situation, it is anticipated that the movements of the molars will be more translational with no intrusive or extru- sive effects. Greater forces are applied with the headcap (250– 300 g) than the neck strap (100–150 g) as teeth are more easily extruded with orthodontic treatment.
• Low/cervical pull: use of neck strap alone (Fig. 15.24). The force produced will be downwards and backwards with an extrusive element. Useful in patients with deep bites and low mandibu- lar plane angles as this direction will tend to extrude the molars resulting in an increase in the lower face height and reduction of the overbite.
• Spring mechanism or strap: this element connects the face-bow to the headcap or neck strap. Adjustment to this allows for an increase in the magnitude of the force applied. Elastics are rarely used due to their tendency to break.
15.5.3 Headgear safety
Injuries associated with headgear have been reported in the past. Most notably these include serious ocular injuries, reportedly result- ing in blindness, which was as a result of the ends of the face-bow coming out of the mouth and causing direct trauma to the eyes. This can occur if the face-bow is pulled out of the mouth and recoils back into the face, but has also been reported after spontaneous disen- gagement at night.
It is therefore essential that safety features are incorporated when fitting headgear to prevent injuries. The British Orthodontic Society recommends that at least two safety features are incorporated into the headgear. These can take several forms including a snap-away, safety release spring strap mechanism, rigid neck strap, locking face-bow, or safety face-bow (Figs 15.27 and 15.28).
The simplest safety element is the Masel strap (Fig. 15.28), which resists dislodgement of the face-bow. However, evidence suggests that this is not a foolproof method as head posture will affect the fit of the strap and still allow detachment of the face-bow (Fig. 15.29).
Success with extra-oral anchorage is dependent on good patient coop- eration. Even with the use of charts for the patients to complete, a number of studies have indicated that patients will often not wear the headgear for the prescribed duration. This has led to a reduction of the use of headgear within clinical practice especially following the introduction of TADs which have been found to have a good level of acceptance by patients and clini- cians, and also reduce the cooperation demands placed upon a patient.
15.5.4 Reverse headgear
A face-mask or reverse/protraction headgear has two uses.
• Tooth movement: moving the posterior maxillary teeth mesially, thereby closing up excess space typically found in hypodontia cases, however this use has diminished due to the increasing use of TADs.
• Skeletal changes: advancement of the maxilla can be achieved in patients, where a face-mask is fitted and worn a minimum of 14 hours per day (Fig. 15.30). This approach is discussed further in Chapter 11.
15.6 Monitoring anchorage during treatment
15.6.1 Single-arch treatments
If an appliance is used in one arch only (e.g. an upper removable appli- ance to retract a buccally placed canine, or a quadhelix to expand the upper arch prior to comprehensive appliance treatment) then the lower arch, which is not actively being altered orthodontically, can be used as a reference guide to monitor anchorage during treatment. In these cases, a record at each visit of the molar and canine relationships bilaterally should be made, together with overjet, overbite, or any centreline changes. In this way, any adverse tooth movements can be identified and dealt with.
15.6.2 Definitive orthodontic treatment with upper and lower fixed appliances
In this situation, tooth movements are occurring in both arches simul- taneously making assessment of any anchorage loss more complex.
Careful recording of molar and canine relationships, together with overjet, overbite, centrelines, and residual spacing, are essential for these cases. Once alignment has been achieved, and prior to com- mencing space closure, anchorage requirements should be reassessed with regard to the original aims of treatment. In many cases anchor- age can be reinforced using either Class II or Class III elastic traction between the arches. Failure to assess the case correctly at this stage may lead to a compromised result. Some clinicians advocate taking a lateral cephalogram at this stage to enable comparison with the pre- treatment cephalogram; however, this is certainly not required for all patients and, in view of the radiation dose, should be prescribed with care and only undertaken if the outcome is likely to alter the planned treatment mechanics.
15.7 Common problems with anchorage
Anchorage problems arise when the following occurs:
• Failure to correctly plan anchorage requirements at the start of treatment.
• Poor patient compliance, repeated breakages, or failure to wear intermaxillary elastics as instructed will have deleterious effects on the treatment. Lack of headgear wear will necessitate finding alter- native methods of anchorage reinforcement, perhaps with the use of TADs or resorting to extractions in a previously non-extraction case. There is no harm in being overcautious with anchorage requirements at the start of treatment. A transpalatal arch can be easily removed when the canines are Class I, headgear wear can be reduced. However, to tell a patient that headgear is needed some 6 months into treatment or extractions are now required, when not previously discussed, makes a compromise more likely.
15.8 Summary
Orthodontic anchorage planning is fundamentally the balance between maximizing the tooth movements required to achieve the correction of a malocclusion and limiting the undesirable movement of any other teeth. The type of tooth movement undertaken will determine the strain placed upon the anchorage unit. Anchorage can be increased by maximizing the number of teeth (and surface area of their roots) resisting the active tooth movement, either within the same arch (intramaxillary anchorage) or in the opposing arch (intermaxillary anchorage). Extra-oral forces with headgear can also be used, although there is an increasing use of TADs which are reducing the need for patient compliance.
If two skaters push against each other equally, and they themselves are of similar size, both will move backwards by an equal amount. This is an equal and opposite reaction. If one skater is larger than the other, the smaller skater will be pushed away, whereas the larger skater will only move slightly, or not at all. However, if the larger skater now pushes against two smaller skaters then the larger skater will move more. This can be considered similar to pitting one larger tooth against a smaller tooth, or against two smaller teeth. The more teeth you try to move, the more likely your anchorage unit will move as well.
A clinical scenario is the retraction of upper canines using a fixed appli- ance, with all the available teeth involved in the appliance. An equal and opposite force to that being generated by retracting canines will also be acting on the remaining upper arch teeth to move them anteriorly, which can compromise the anchorage unit causing unwanted forward tooth movement of the rest of the dentition (Fig. 15.1).
15.2 Assessing anchorage requirements
Anchorage requirements should be considered in all three dimensions: anteroposteriorly, vertically, and transversely. Planning anchorage is a fundamental part of treatment planning and needs to be undertaken as part of managing the space requirements of orthodontic treatment. When considering anchorage management, it is important to assess the following factors.
15.2.1 Space requirements
The amount of crowding or spacing should be assessed as part of treat- ment planning. This can be done using either a visual assessment or more formally using a structured space analysis (see Chapter 7, Section 7.7). The results of this assessment will guide the clinician when decid- ing the most appropriate method of space creation, the potential treat- ment mechanics, and also the need for any anchorage reinforcement. For example, the extraction of maxillary first premolar teeth will result in 7 mm of space being generated on each side which can be used for tooth alignment or retraction of the upper labial segment. This may be too much space, the exact amount required, or too little. If it results in too much space, then the clinician needs to plan the controlled loss of anchorage to allow the posterior teeth to move mesially while the ante- rior teeth are being retracted into the optimum position. If the space created by the extractions is just sufficient, then the clinician needs to maintain all the space for tooth alignment or retraction of the incisors, ensuring good anchorage control of the posterior teeth. If the extrac- tion space is insufficient, then additional space needs to be generated with further extractions, expansion, interproximal enamel reduction, or distalization of posterior teeth.
15.2.2 The type of tooth movement to be achieved
There are six different types of tooth movement: tipping, bodily move- ment, rotation, torqueing, intrusion, and extrusion. Tipping occurs when the crown of a tooth moves in one direction and the root apex moves, by a lesser amount, in the opposite direction rotating around the tooth’s centre of resistance (Fig. 15.2). Bodily movement occurs when both the crown and the root move in the same direction equally (Fig. 15.3). In practice, this is often achieved by a series of small tipping and uprighting movements due to the ‘play’ between the archwire and the bracket slot (see Chapter 18). Rotational movement occurs when a force is applied mesially or distally to the labial aspect of a tooth.
Teeth have a centre of resistance around which movement occurs. If force is applied directly through the centre of resistance then the tooth will move bodily, however, because the centre of resistance lies within the root this cannot occur. A simple force applied to the crown of a tooth with a round wire will cause the tooth to tip (see Chapter 18, Fig. 18.1); however, with a fixed appliance in place, the built-in programme of the bracket interacting with a rectangular orthodontic wire causes the force to act as a ‘couple’, which can be used to change the inclination of teeth or to produce bodily tooth movement.
Bodily movement requires more force to be placed onto a tooth than tipping movements and is therefore more anchorage demanding on the remaining teeth.
15.2.3 The number of teeth to be moved
As the number of teeth to be moved increases, so does the anchor- age demand. If the anchorage requirements of treatment are high, then consideration should be given to moving a single tooth in stages or only a few teeth at a time to conserve anchorage. However, although the theoretical principles are sound and this is a commonly applied method of preventing unwanted tooth movement, there is some debate within the literature about the level of anchorage conservation that this tech- nique actually achieves by the end of treatment when all the active tooth movements have been completed.
15.2.4 The distance of the movement required
The greater the distance the teeth are to be moved, the greater the strain on the anchorage, and the greater the risk of unwanted tooth movement.
15.2.5 Aims of treatment
The aims of treatment can fundamentally alter the complexity of treat- ment and also the prognosis for success. The aims of treatment need to be specific and achievable, with the treatment mechanics that will be required to achieve the aims being acceptable to the individual patient. The fewer teeth that need to be moved to achieve the aims of treatment then the less the anchorage demand; however, if treat- ment is complex and multiple teeth are to be moved, there will be a greater anchorage demand and often a greater demand on the level of cooperation from the patient. In cases with a Class II molar relation- ship on initial presentation, the anchorage needs will be far greater if a Class I molar (and canine) relationship is to be achieved rather than a Class II molar (and Class I canine) relationship at the end of treatment (Fig. 15.4). The need to achieve a Class I canine relationship is usually very important for treatment success, therefore it is critical that anchor- age planning should focus not only on the intended molar movements but also on the required movements of the canines to achieve this goal (Fig. 15.5).
15.2.6 Root surface area of the teeth to be moved
The size of the root surface area of the tooth or teeth to be moved influences the anchorage requirements—the larger the root surface area, the greater the demand (Fig. 15.6). Teeth are maintained in a position of balance between the opposing forces of the lips and tongue, with the periodontal ligament, and the surrounding bone, resisting any tooth movement; therefore, for tooth movement to occur, a threshold of force must be achieved. This applies equally to the teeth in the anchorage unit, and if the threshold is exceeded, unwanted movement of the anchor teeth will occur and anchor- age will be lost. As such, the forces applied to achieve the desired tooth movements must be carefully selected to try and ensure they remain below the threshold of the teeth acting as the anchorage unit. Increasing the number of teeth acting in an anchorage unit (e.g. including the second molars in a fixed appliance) is one method of increasing this threshold.
15.2.7 Growth rotation and skeletal pattern
An increased rate of tooth movement has been associated with patients who have an increased vertical dimension or backward growth rota- tion. It has been suggested that space closure or anchorage loss may occur more rapidly in these high-angled cases and so the requirement for anchorage reinforcement needs to be considered in these individu- als. Conversely, in a patient with reduced vertical dimensions or a for- ward growth rotation, space closure or anchorage loss may be slower; therefore, the planned extraction pattern or the treatment mechanics may need to be altered to accommodate this. One hypothesis which has been proposed for this observation is the relative strength of the facial muscles, with individuals with reduced vertical dimensions having a stronger musculature.
15.2.8 Occlusal interdigitation and occlusal interferences
Occlusal interdigitation or occlusal interferences can prevent or slow down tooth movement. This can lead to an increased anchorage demand which may prevent the desired tooth movement and increase the likelihood of unwanted tooth movement. This phenomenon can be utilized to an orthodontist’s advantage, as it has been suggested that if teeth in the anchorage unit have a good interdigitation with the opposing dentition, this may increase their anchorage value by reducing mesial movement of the anchorage unit.
15.2.9 Bone quality
Maxillary bone is less dense than mandibular bone so the threshold for tooth movement in the maxillary teeth is lower than that in the mandib- ular teeth. Teeth move more readily through cancellous bone than cor- tical bone and the anchorage value of teeth can be increased by moving their roots closer to the cortical plate; however, this can put them at increased risk of root resorption, and as such, should be done with care. Certain groups of patients, such as individuals with hypodontia or previ- ous experience of periodontal disease, can have reduced quantity and/ or quality of alveolar bone. This will affect the anchorage balance when undertaking orthodontic treatment and must also be taken into consid- eration when planning the retention phase of treatment.
15.3 Classification of anchorage
• Simple anchorage: one tooth pitted against another.
• Reciprocal anchorage: two teeth or groups of teeth of equal size or equivalent anchorage value are pitted against each other, resulting in movement of both units by a similar amount. For example, a quad- helix used to expand the maxillary arch (Fig. 15.7) or applying power chain on an upper fixed appliance to two central incisors to close a median diastema (Fig. 15.8).
• Anchorage reinforcement: a variety of methods can be utilized:
- Intramaxillary compound anchorage: multiple teeth are used in an anchorage unit within the same arch, for example, using a molar and a second premolar as an anchorage unit for the retrac- tion of a canine.
- Intermaxillary compound anchorage: multiple teeth in oppos- ing arches, for example, the use of intermaxillary elastics (Fig. 15.9).
• Differential tooth movements: tipping movements are less anchorage demanding than bodily movement. This concept can be utilized when planning tooth movements to increase the relative anchorage value of specific teeth.
• Use of intra-oral adjuncts: this incorporates non-tooth structures such as the palatal vault when using a Nance button in conjunction with a transpalatal arch (Fig. 15.10), or bone when using temporary anchorage devices (TADs).
• Use of extra-oral adjuncts: this incorporates devices utilizing extra-oral structures to place a force upon the teeth. For example, headgear systems.
• Stationary/absolute anchorage: this can only be achieved when using an osseointegrated implant or ankylosed tooth as an anchorage unit.
15.4 Intra-oral anchorage
Anchorage reinforcement can be achieved by utilizing the teeth, soft tissues, and skeletal structures intra-orally.
15.4.1 Increasing the number of teeth in the anchorage unit
Incorporating as many teeth as possible into an anchorage unit will aid in reducing anchorage loss. If the anchorage demand is high, then con- sideration should be given to moving one tooth at a time to prevent strain of the anchorage unit (conserving anchorage). This will also allow better monitoring of any anchorage loss, allowing additional anchorage re-enforcing mechanics to be utilized if required.
15.4.2 Differential extraction pattern
The extraction pattern planned for the patient can influence the anchor- age balance of a case. Extracting teeth closer to the area of crowding reduces the amount of tooth movement required and thus the risk of anchorage loss. Also, by careful selection of the appropriate teeth to be extracted, the anchorage balance can be changed in such a way that it is possible to make some teeth more resistant to unwanted movement (Fig. 15.6).
A differential extraction pattern, between arches, can aid in anchor- age management. One such example is the treatment of Class II division 1 cases where the upper first premolars are extracted to aid reduction of an overjet and correction of the canine relationship to Class I. This becomes possible as the anchorage unit is greater posteriorly in the maxilla with the upper first molars and second premolars incorporated within the appliance. In the lower arch, the extraction of the lower sec- ond premolars will not only reduce the retraction of the lower labial segment but also work favourably for molar correction with the lower molars more likely to move mesially. These desired tooth movements can be enhanced with inter-arch (Class II) elastic use. This extraction pattern can be reversed in the treatment of Class III cases with the extraction of lower first premolars and upper second premolars. This differential extraction pattern will aid the camouflage of the reverse overjet with retroclination of the lower labial segment.
15.4.3 Care with initial intra-arch orthodontic mechanics
A number of factors may increase the demands on the anchorage units, such as the engagement of severely displaced teeth in the early stages of alignment. Anchorage loss will also occur if there is friction in the system as a greater force needs to be exerted to overcome friction and achieve the planned tooth movement. As a result of this higher applied force, the reactionary force is greater, which may result in unwanted tooth movement of the anchorage unit.
15.4.4 Bodily movement of teeth
Bodily movement requires more force than tipping movements and is therefore more anchorage demanding. Use of large, rectangular stain- less steel archwires will ensure bodily movement occurs rather than tipping, as the archwire will fill more of the bracket slot (see Chapter 18, Fig. 18.4). Differential tooth movements can be undertaken in an effort to limit unwanted tooth movements. This involves restricting the anchorage teeth to bodily movement while allowing the teeth you do wish to move to tip into place. As tipping movements occur more read- ily, differential tooth movement should occur. This is a concept which is fundamental to certain appliance systems such as Begg and Tip-Edge (see Chapter 18, Section 18.6.2).
15.4.5 Transpalatal and lingual arches
Both a transpalatal arch (usually connecting the upper first molars) and a lower lingual arch (usually connecting the lower first molars) can be used to reinforce anchorage by linking contralateral molar teeth (Figs 15.11 and 15.12). The teeth are joined with an arch bar, usually 1 mm diameter stainless steel, which connects across the vault of the palate or around the lingual aspect of the lower arch. This linking of teeth helps to prevent or reduce unwanted mesial molar movement. By fixing the intermolar width, which aids transverse anchorage, any mesial movement of the molars means that the buccal roots are more likely to come into contact with the cortical plate of the bone which, as mentioned previously, resists subsequent mesial tooth movement, but can also increase the risk of root resorption. Anchorage can poten- tially be further increased by adding an acrylic button, or Nance button, which lightly contacts the anterior palatal mucosa (Fig. 15.10). Caution should be taken with all these appliances, because if anchorage is lost and the molars move mesially then the lingual arch can cause procli- nation of the lower labial segment, or the U-loop of the transpalatal arch or acrylic of the Nance button may become engaged in the palatal mucosa. It is with this in mind that the palatal loop of the transpalatal arch would normally face distally.
The amount of anchorage reinforcement these devices provide has been questioned in the literature and it is advisable for the clinician to carefully monitor for any anchorage loss during treatment.
15.4.6 Intermaxillary anchorage
Anchorage from one arch can be used to reinforce anchorage in the other. In addition to occlusal interlocking mentioned earlier, the two main adjuncts which can be utilized to achieve this is with the use of intermaxillary elastic traction (see Chapter 18, Section 18.3.4) or Class II correctors, which are metal pistons connected between the arches. Intra-oral elastics are available in a variety of sizes and strengths. Class II elastics will be run from an anterior attachment in the upper arch to a posterior attachment in the lower arch. Class III elastics are the reverse, located anteriorly in the lower to posteriorly in the upper (Figs 15.13 and 15.14).
As with any force applied there can be unwanted effects, and inter- maxillary traction is not without this problem. Class II or Class III elastics can lead to extrusion of the molars, which will in turn reduce the over- bite and can also cause an increase in the face height, this might be beneficial in a few patients but for the majority these movements would be unwanted. The other significant consequence is that in an unspaced lower arch, the use of Class II elastics and Class II correctors will result in proclination of the lower labial segment.
15.4.7 Removable and functional appliances
Removable appliances can be used alone or in conjunction with a fixed appliance to reinforce anchorage (Fig. 15.15). By virtue of their palatal coverage, they have an increased anchorage value to resist unwanted tooth movement. Other design features which reinforce anchorage include the following:
• Anteroposteriorly: by colleting around the posterior teeth with acrylic, inclined bite-blocks, or the use of incisor capping.
• Transversely: the pitting of one side of the arch against the other can reinforce transverse anchorage, typically seen where an expan- sion screw is used for increasing the palatal transverse dimension (Fig. 15.16), usually resulting in reciprocal anchorage.
• Vertically: by either reducing the vertical dimensions during treat- ment of a high-angle patient by intruding the posterior teeth, often aided with high-pull headgear (see Chapter 12, Section 12.3.1); or increasing the vertical dimension by allowing differential tooth erup- tion with the use of an anterior bite-plane.
All of these three-dimensional features can be incorporated into functional appliances, which additionally can be used to gain anchor- age in the anteroposterior direction to aid in the treatment of a Class II malocclusion, often reducing the complexity during the fixed appliance phase of treatment.
15.4.8 Temporary anchorage devices
The use of temporary anchorage devices (TADs), also known as ortho- dontic bone anchorage devices, is becoming increasingly common in contemporary orthodontics. They first became popular in the 1990s and were developed from the dental implants used in restorative dentistry and maxillofacial bone plates. There are three distinct types (Box 15.1):
• Osseointegrated implants
These were modified from dental implants, making them shorter, with a wider diameter than those used in restorative dentistry. Osseointegrated implants can be used to provide maximum anchor- age and are useful if large or difficult tooth movements are required. This type of implant, placed in the mid-palatal region and attached to a palatal arch (Fig. 15.17), was compared with conventional headgear in a randomized controlled trial. The authors concluded that mid-palatal implants are an acceptable technique for reinforcement of anchorage.
They have three principal disadvantages:
(1) They need to be left for 3 months after placement to allow osseointegration.
(2) Due to their size, they are restricted to being used in edentulous areas.
(3) Since the implant osseointegrates, it requires a complex surgical procedure together with bone removal at the completion of treatment and some patients may find this unacceptable.
• Miniplate systems
These are based on maxillofacial bone plates, with a transmucosal portion projecting into the mouth to allow connection to the fixed appliance. They can provide reliable anchorage, but require a surgi- cal procedure to both place and remove them (Fig. 15.18).
• Mini-screws
These were developed from the screws of maxillofacial plating systems, but are smaller in size (typically 6–12 mm in length and 1.2–2 mm in diameter), hence the term mini-screw. No osseointegra- tion is required (or desired) which means that they do not provide absolute anchorage as there is the possibility that they can move during treatment. Their head and neck configuration has been adapted to facilitate placement of auxiliaries to the fixed appliance (Fig. 15.19).
Due to their ease of use—for both the patient and the orthodontist— the mini-screw approach is now the most popular. There is a range of different systems available with various claimed strengths and weak- nesses. Their designs and the associated treatment mechanics are under constant review. They are usually placed under local anaes- thesia and can often be removed at the end of treatment without requiring any anaesthetic.
Bone anchorage devices have the ability to provide anchorage in three dimensions: anteroposteriorly, vertically, and transversely. They can provide anchorage either directly or indirectly:
• Direct anchorage is achieved when forces are applied directly to the TAD (Fig. 15.20).
• Indirect anchorage is achieved when the TAD is linked to the anchor- age teeth, and then the orthodontic forces are applied to this anchor- age unit (Fig. 15.21).
The benefit of using TADs for indirect anchorage is that conven- tional treatment mechanics can be utilized, however, the disadvan- tage is that if the TAD moves or becomes loose between visits, then some anchorage loss could have occurred before it is noticed and rectified.
The use of TADs is allowing different approaches to anchorage, as well as possibly altering the scope of what was previously thought possible with fixed appliances. However, as with all these adjuncts, there are risks associated with their use which the patients must be made aware of. These include the risk of damage to adjacent struc- tures such as blood vessels, nerves, and tooth roots; the risk of fracture; the risk of gingival inflammation; and the risk of them becoming loose with subsequent anchorage loss or need for replacement/use of other means of anchorage reinforcement.
15.5 Extra-oral anchorage
15.5.1 General principles
Headgear can be used for:
• extra-oral anchorage
• extra-oral traction.
Extra-oral anchorage holds the posterior teeth in position, preventing unwanted mesial movement of the anchorage unit.
Extra-oral traction applies a force to the posterior teeth to achieve tooth movement, usually in a distal direction. Traction can also be used to attempt to achieve an orthopaedic effect by using conventional headgear to restrict the growth of the maxilla in a forwards and down- wards direction, or using reverse headgear (see Section 15.5.4) to pro- tract the maxilla. In simple terms, traction requires greater forces, for longer periods of time, than that required to maintain the position of the posterior teeth.
There are three directions of pull that can be achieved with head- gear and these should be considered at the time of treatment planning (Box 15.2): High- or occipital-pull headgear which helps to control the vertical as well as anteroposterior anchorage and is typically used in cases with increased vertical proportions (Fig. 15.22).
• Straight- or combination-pull headgear which controls the anter- oposterior and is typically used in cases with average vertical propor- tions (Fig. 15.23).
• Low- or cervical-pull headgear which aids in the control of anter- oposterior anchorage but is also used to increase the vertical dimen- sion by having an extrusive effect on the molars in cases of reduced vertical proportions (Fig. 15.24). Care should be taken with low-pull headgear in a Class II skeletal pattern as extrusion of the maxillary molars may lead to a clockwise rotation of the mandible making the class II skeletal pattern worse as the mandible rotates downwards and backwards.
The amount of force applied to the headgear is controlled by adjust- ing the attachment straps and should be carefully monitored at each visit. For orthodontic change, it is normal to apply 250–350 g to achieve anchorage reinforcement, increasing to 400–450 g to obtain extra-oral traction. For orthopaedic change, often termed maxillary restraint, the forces are increased to 500 g.
The duration of force also varies according to the purpose. Extra-oral anchorage requires a minimum of 10 hours per day, which is usually best achieved at bedtime. Extra-oral traction requires an increased duration and a minimum of 12 hours of wear per day is required though most operators ask for 14 hours. For orthopaedic changes, the duration of wear requested is also normally 14 hours, but the length of treat- ment with the headgear appliance is longer when compared to extra- oral traction (which may be achieved, in a cooperative patient, within 6–9 months).
The effect of a force depends on where it is applied in relation to the centre of resistance. If the force passes directly through the centre of resistance then pure translation will occur. If not, then tipping will arise. The centre of resistance for forces applied to the upper first molars is felt to be at the trifurcation of their roots. The centre of resistance for the maxilla as a whole is further forward, and is suggested to be between the premolar roots. Adjustment of the direction of force applied by the headgear relative to the centre of resistance of the first molar will influence how the molar will move.
In theory, intrusion of the upper incisors can be attempted by apply- ing headgear to the anterior labial segment of a working archwire, though the forces used must be light to avoid possible root resorption of the upper incisors. Due to concerns over safety, this technique has now been largely abandoned.
Headgear can be added to a removable appliance or a functional appliance, such as a twin-block appliance, usually with the aim of achieving orthopaedic changes. In these cases, the forces are kept above the occlusal plane, not only to control the vertical dimension, but also to prevent dislodging the appliance.
15.5.2 Components of headgear
The components of headgear have been modified dramatically in the last 20 years to improve safety (see Section 15.5.3). Where head- gear is to be used in conjunction with a fixed appliance, patients first need to be fitted with upper molar orthodontic bands with buc- cal headgear tubes. Where headgear is being used with a removable or functional appliance then headgear tubes need to be added to either the molar clasps or incorporated into the acrylic (see Chapter 17, Fig. 17.7).
Face-bow: the face-bow slots into the headgear tubes. The original face-bow was called a Kloehn bow, however, due to safety concerns about the unprotected intra-oral ends, its use is now largely historic. Currently the face-bow of choice is a Nitom™ locking face-bow pro- duced with a specialized safety catch (Fig. 15.25). A less complex safety version is the Hamill face-bow with a reverse loop attach- ment (Fig. 15.26); however, although the Hamill type can reduce the degree of soft tissue injury, there is a much higher risk of acciden- tal disengagement from the headgear tube when compared to the Nitom™ face-bow.
• Headcap or neck strap: a headcap or neck strap can be used inde- pendently or together to achieve the direction of pull required.
• High/occipital pull: use of the headcap only (Fig. 15.22). This pro- duces intrusive forces and either leads to intrusion of the molars or prevents them from extruding during treatment. This approach is useful where no increase in the vertical proportions can be toler- ated during treatment.
• Combination pull: uses the headcap and neck strap together (Fig. 15.23). In this situation, it is anticipated that the movements of the molars will be more translational with no intrusive or extru- sive effects. Greater forces are applied with the headcap (250– 300 g) than the neck strap (100–150 g) as teeth are more easily extruded with orthodontic treatment.
• Low/cervical pull: use of neck strap alone (Fig. 15.24). The force produced will be downwards and backwards with an extrusive element. Useful in patients with deep bites and low mandibu- lar plane angles as this direction will tend to extrude the molars resulting in an increase in the lower face height and reduction of the overbite.
• Spring mechanism or strap: this element connects the face-bow to the headcap or neck strap. Adjustment to this allows for an increase in the magnitude of the force applied. Elastics are rarely used due to their tendency to break.
15.5.3 Headgear safety
Injuries associated with headgear have been reported in the past. Most notably these include serious ocular injuries, reportedly result- ing in blindness, which was as a result of the ends of the face-bow coming out of the mouth and causing direct trauma to the eyes. This can occur if the face-bow is pulled out of the mouth and recoils back into the face, but has also been reported after spontaneous disen- gagement at night.
It is therefore essential that safety features are incorporated when fitting headgear to prevent injuries. The British Orthodontic Society recommends that at least two safety features are incorporated into the headgear. These can take several forms including a snap-away, safety release spring strap mechanism, rigid neck strap, locking face-bow, or safety face-bow (Figs 15.27 and 15.28).
The simplest safety element is the Masel strap (Fig. 15.28), which resists dislodgement of the face-bow. However, evidence suggests that this is not a foolproof method as head posture will affect the fit of the strap and still allow detachment of the face-bow (Fig. 15.29).
Success with extra-oral anchorage is dependent on good patient coop- eration. Even with the use of charts for the patients to complete, a number of studies have indicated that patients will often not wear the headgear for the prescribed duration. This has led to a reduction of the use of headgear within clinical practice especially following the introduction of TADs which have been found to have a good level of acceptance by patients and clini- cians, and also reduce the cooperation demands placed upon a patient.
15.5.4 Reverse headgear
A face-mask or reverse/protraction headgear has two uses.
• Tooth movement: moving the posterior maxillary teeth mesially, thereby closing up excess space typically found in hypodontia cases, however this use has diminished due to the increasing use of TADs.
• Skeletal changes: advancement of the maxilla can be achieved in patients, where a face-mask is fitted and worn a minimum of 14 hours per day (Fig. 15.30). This approach is discussed further in Chapter 11.
15.6 Monitoring anchorage during treatment
15.6.1 Single-arch treatments
If an appliance is used in one arch only (e.g. an upper removable appli- ance to retract a buccally placed canine, or a quadhelix to expand the upper arch prior to comprehensive appliance treatment) then the lower arch, which is not actively being altered orthodontically, can be used as a reference guide to monitor anchorage during treatment. In these cases, a record at each visit of the molar and canine relationships bilaterally should be made, together with overjet, overbite, or any centreline changes. In this way, any adverse tooth movements can be identified and dealt with.
15.6.2 Definitive orthodontic treatment with upper and lower fixed appliances
In this situation, tooth movements are occurring in both arches simul- taneously making assessment of any anchorage loss more complex.
Careful recording of molar and canine relationships, together with overjet, overbite, centrelines, and residual spacing, are essential for these cases. Once alignment has been achieved, and prior to com- mencing space closure, anchorage requirements should be reassessed with regard to the original aims of treatment. In many cases anchor- age can be reinforced using either Class II or Class III elastic traction between the arches. Failure to assess the case correctly at this stage may lead to a compromised result. Some clinicians advocate taking a lateral cephalogram at this stage to enable comparison with the pre- treatment cephalogram; however, this is certainly not required for all patients and, in view of the radiation dose, should be prescribed with care and only undertaken if the outcome is likely to alter the planned treatment mechanics.
15.7 Common problems with anchorage
Anchorage problems arise when the following occurs:
• Failure to correctly plan anchorage requirements at the start of treatment.
• Poor patient compliance, repeated breakages, or failure to wear intermaxillary elastics as instructed will have deleterious effects on the treatment. Lack of headgear wear will necessitate finding alter- native methods of anchorage reinforcement, perhaps with the use of TADs or resorting to extractions in a previously non-extraction case. There is no harm in being overcautious with anchorage requirements at the start of treatment. A transpalatal arch can be easily removed when the canines are Class I, headgear wear can be reduced. However, to tell a patient that headgear is needed some 6 months into treatment or extractions are now required, when not previously discussed, makes a compromise more likely.
15.8 Summary
Orthodontic anchorage planning is fundamentally the balance between maximizing the tooth movements required to achieve the correction of a malocclusion and limiting the undesirable movement of any other teeth. The type of tooth movement undertaken will determine the strain placed upon the anchorage unit. Anchorage can be increased by maximizing the number of teeth (and surface area of their roots) resisting the active tooth movement, either within the same arch (intramaxillary anchorage) or in the opposing arch (intermaxillary anchorage). Extra-oral forces with headgear can also be used, although there is an increasing use of TADs which are reducing the need for patient compliance.
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