From Volume 8, Issue 2, April 2015 | Pages 42-54
The success of orthodontic treatment relies upon careful treatment planning, for both desired and unwanted tooth movement. The theory behind anchorage reinforcement will be considered and the current means of anchorage support and creation will be described. Methods by which the orthodontist can reduce anchorage demand and measure anchorage loss will also be discussed.
In order for teeth to undergo planned orthodontic movement it is necessary to apply an appropriate force to the tooth or teeth and for this force to be resisted, either entirely or partially. An understanding of this process is an essential part of orthodontic diagnosis and treatment planning.
Before exploring anchorage in orthodontics it is worth considering the basic principle as to why resistance is required for force application to be effective in achieving the desired tooth movement. Following Newton's third law, it is well known that for every action there is an equal and opposite reaction. This means that forces act not only on the teeth to be moved, but also elsewhere to the same extent, often reciprocally, upon other teeth which are referred to as the anchor units. It is when these reciprocal forces are not managed adequately that unwanted tooth movements may occur, leading to a poor occlusal finish or insufficient orthodontic correction of the initial problem, such as increased overjet.
Anchorage is primarily considered in the antero-posterior plane but also needs to be planned in the vertical and transverse spatial planes.
In order to understand anchorage in orthodontics it is perhaps worth describing current theories of what happens during orthodontic tooth movement.
When an intermittent force is applied to a tooth, such as during normal masticatory function, the periodontal ligament (PDL) is deformed slightly for a short time and the surrounding bone bends in response. If pressure is maintained, the PDL loses its capacity to spring back, as the tissue fluids are squeezed out and bony remodelling occurs adjacent to the periodontal ligament. On the pressure side of the ligament, in the direction of movement, resorption occurs through the action of osteoclasts, whilst on the tension side bone deposition occurs through osteoblastic action. The heavier the force, the greater the amount of compression of the blood vessels within the PDL. Since capillary blood pressure has been measured at 15–20mmHg per 1cm2, traditional theory suggested that the optimal force to move teeth would be light enough to compress but not fully occlude the capillaries.1 The osteoclasts would then be recruited from circulating monocytes within the PDL blood vessels and begin resorbing the alveolar bone that is in direct contact with the PDL. This resorption, known as frontal resorption, would occur progressively and the tooth would gradually move.
Using the same theory, if the applied force exceeds capillary blood pressure, then the blood vessels are occluded and the PDL becomes avascular or necrotic, seen microscopically as a glassy or hyalinized appearance. This is known as undermining resorption and results in a delay in tooth movement whilst the osteoclasts begin resorbing the underside of the bone immediately adjacent to the necrotic area of the PDL. The delay is due to awaiting progenitor osteoclasts since the blood supply to the area has been compromised. Therefore the magnitude of force applied would seem to determine the type of resorption that takes place.2 This is probably somewhat simplistic as it is likely that the application of force to a tooth will always create at least some small areas of avascularity within the PDL.
Testing this theory definitively in a clinical setting is difficult owing to the inherent ability of the PDL to dissipate an applied force to other parts of the tooth, and also because the point of application of a force varies during the process of tipping and uprighting of the tooth. Indeed, a small scale study testing the effect of a four-fold increase in the force applied (200cN) to premolar teeth in adolescents,3 found that teeth moved 50% more in 7 weeks compared with those receiving a low (50cN) force. This is counter-intuitive in light of the above theory, suggesting that the rate of tooth movement may be related to other factors rather than just the applied force.
Higher forces, which exceed capillary blood pressure, are not only associated with the possibility of delayed tooth movement, but have also been associated with an increased risk of root resorption.4 Although the study described above showed no difference in the amount of root resorption of the premolar teeth with higher forces,3 it could be assumed that there was a greater risk of promoting movement of the anchor teeth with higher forces. The optimum force to move a tooth is therefore one that maximizes desired tooth movement, whilst minimizing damage and anchorage loss. The PDL response to the force is dependent on two main factors, namely the type of tooth movement required and the root surface area of the teeth.
As already mentioned, the distribution of the force throughout the PDL will depend on the type of tooth movement. During simple tipping of a single-rooted tooth, there will be two areas of compression of the PDL (at the alveolar crest and at the root apex) as the tooth is rotated about its centre of resistance. As the force is concentrated at these two points, forces must be kept low at around 35cN.
Bodily movement of the same tooth would need uniform loading of the PDL following the application of two forces to the crown of the tooth, thus doubling the force required for simple tipping. Rotational and extrusive movements will almost always be accompanied by some tipping and so, although theoretically higher forces would be possible, maintaining similar forces to those required for tipping would be appropriate. Intrusion creates a highly concentrated area of force and so the lightest possible force of 10cN must be applied if intrusion is to be successful. For a single-rooted tooth, the optimal forces for different types of tooth movements are shown in Table 1.5
| Movement | Forces (c/N) |
|---|---|
| Extrusion | 35 |
| Tipping | 35 |
| Rotation | 35 |
| Intrusion | 10 |
| Bodily movement | 70 |
When a force is applied to a tooth, there is a threshold level that must be reached in order to overcome the intrinsic resistance of the periodontal ligament6 and before tooth movement will occur.7 As the applied force increases above this threshold value, the amount of tooth movement increases linearly until a plateau is reached, beyond which increasing the force no longer increases the rate of tooth movement (Figure 1).8
These optimum levels of force described earlier are considered to be proportional to the root surface area of the tooth9 being moved and Figure 2 illustrates the average root surface areas in mm of the permanent maxillary and mandibular teeth.10 The optimum force for the movement of a block of teeth will similarly be proportional to the sum of the root surface areas of this block.
Although the forces being applied to the teeth being actively moved should be optimal, in the case of the tooth or teeth not being moved, namely the anchor teeth, the force should be suboptimal or even below the threshold level for tooth movement.
To illustrate this point, Figure 3 compares the effect of an optimal force (70cN [70g]) and a high force (280cN [280g]) for bodily retraction of a maxillary canine against the resistance provided by the second premolar and first molar teeth. The average estimated root surface area of a canine tooth is 273mm2.10 This compares to an average estimated combined root surface area for the anchor teeth on the same side, namely the first molar and second premolar, of 653mm2 and equates to a ratio of root surface areas for the canine and the two anchor teeth of 1:2.39. If 70cN (70g) of force is applied to the single-rooted canine, then the equal and opposite force, over the larger root surface area of the two anchor teeth, will be 17.5cN (17.5g) per root (assuming three roots on the molar and one on the second premolar). This will result in optimal bodily movement of the canine, with only a small amount of mesial bodily movement of the anchor teeth, ie anchorage gain.
If the force applied to the canine was increased to 280cN (280g), then the reciprocal force on the anchor teeth would be 70cN (70g) per root, which would be optimal for promoting movement of the anchor teeth. At the same time, the 280cN (280g) force will lead to hyalinization within the canine PDL, resulting in undermining resorption and slower movement of the canine. The result of this will be that the anchor teeth move mesially more than the canine retracts, ie anchorage loss.
This example refers to the level of root surface encased within bone in a healthy periodontium. Any bone loss will alter the response of the PDL and reduce the threshold required to move the tooth. The location of the bone loss will also alter the tooth's natural ability to resist the force in this area, eg furcation involvement on a molar tooth, and this must be taken into account during treatment planning.11
As part of the diagnosis and treatment planning process, one of the skills required of an orthodontist is the assessment of the anchorage requirements for each individual case. The explanation of anchorage management so far has been simplistic in order to help describe the principles, but other factors will also have important effects.
Anchorage is most often considered in the antero-posterior plane and requirements are usually classified as low, medium or high. For example, in deciding whether or not to extract teeth to correct a malocclusion, the amount of space required to correct the malocclusion informs this judgement of the anchorage requirements. Clinical experience suggests that, if <30% of the space created by the extraction is required to complete the treatment, it may be considered a low anchorage case, if 30–60% it is medium, and if >60% it would be considered to be a high anchorage case. There are, however, a number of factors that should also be considered when making this judgement, in particular the type of tooth movement required and therefore the angulations and inclinations of both the teeth to be moved and any anchor teeth.
As previously described, tipping teeth around the centroid of the root requires less force (35–60cN, depending on the number of roots) and is achieved much more readily than either bodily movement (70–120cN), or root movement centred on the crown (50–100cN), ie torquing movements. Many orthodontic cases require retraction of canines following the removal of first premolar units. Favourable mesially angulated canines can be tipped easily into the desired position with little strain on anchorage. This is because the estimated 45cN of force required to tip the canine can be dissipated across the roots of the anchor teeth, principally the second premolar and first molar, and will be well below the optimal level for their movement. In the case of a distally angulated canine, the force required to move the root distally will be much greater, around 50–100cN. This in turn will lead to a greater reciprocal force on to the anchor teeth and this is also likely to be for an extended period of time. The resultant reciprocal force may more closely approach the optimal level for movement of the anchor teeth, particularly if the anchor teeth are themselves able to tip mesially. This model can also be applied in the case of proclined incisors, which require simple tipping to reduce an overjet, versus retroclined incisors that require palatal root movement.
Anchorage is not exclusively about preserving space. Good anchorage management will aim to leave the patient with no spaces to close at the end of treatment. This may entail actively encouraging anchor teeth to move as part of the planned mechanics, often called planned anchorage loss.
The most common teeth to be chosen for extraction are the premolar teeth owing to their location close to the site of anterior crowding. First and second premolars, although similar in mesio-distal width, will yield different amounts of space for orthodontic treatment. This is related to the different contributions they will make to anchorage creation.
When the first premolar is extracted, the posterior anchorage block will include the second premolar and molar so that the canine can be retracted directly into the extraction space. If the second premolar is extracted, then an extra unit is added to the anterior teeth block, increasing its anchorage value and, furthermore, the first molar can more easily move mesially into the extraction space. Therefore more space is created to align the labial segment from the extraction of a first premolar compared to a second premolar and more anchorage is ‘created’ for correction of the malocclusion.
If only small amounts of space are required for correction of a malocclusion and the anchorage need is considered to be low, interproximal reduction may be considered for space creation rather than extraction of teeth. Approximately 0.25mm of enamel can be removed from each tooth at each contact point and up to 6.4mm of space can be gained across the molar and premolar contact points, depending on the shape and size of the teeth.12
Sometimes other factors, such as caries and abnormal tooth position, will dictate which teeth need to be extracted as part of the orthodontic treatment. This can transform an averagely demanding case into one that has high and complex anchorage demands, depending on the resultant anchorage balance.
Anchorage can be divided into two principal types, extra-oral and intra-oral and within the latter it can be classified as being within one arch or intra-maxillary, or across two arches, inter-maxillary. The source of any tissue-borne anchorage can be used in the description, namely teeth, soft tissues or bone. We will consider each in turn along with any sub-classification.
Intra-maxillary anchorage from the teeth within the same arch can be sub–classified according to the treatment mechanics used:
So far we have described tooth-borne intra-maxillary anchorage. It should be remembered that the same classification of simple, compound, reciprocal and stationary could also be applied to inter-maxillary anchorage, where the teeth in one arch provide anchorage for those in the opposing arch. This can be as obvious as good interdigitation resisting mesial movement of the opposing buccal segment teeth. More commonly specific treatment mechanics are employed to this effect.
In order to work effectively, elastics are usually required to be worn 24 hours a day. Patients must be relied upon to replace their elastics every day and also additionally when they break. Not all patients comply with this13 and fixed variants have been developed to eliminate the requirement for patient co-operation. They are divided into two broad subtypes, springs and pistons.
Soft tissue can be utilized directly as a source of anchorage whilst bone can be utilized either directly or indirectly.
Anchorage can be gained from the soft tissues and one example of where this is the case is the lip bumper, which can be used to resist mesial movement of the lower molars (Figure 10). Using a lip bumper will also remove the lower lip's influence on the lower incisors, which in turn may result in their proclination as a result of tongue pressure.19
The simplest indirect method is to use the vault of the palate to supplement anchorage and to resist mesial movement of the buccal teeth. This effect can be harnessed through removable appliances or fixed transpalatal arches with an acrylic ‘Nance’ button resting against the palatal mucosa.20 The depth and inclination of the palate will affect the anchorage that can be gained, with a deep vertical anterior palate providing the maximum anchorage (Figure 11a, b).
The horizontal part of the palate is also excellent at providing vertical anchorage when using an upper removable appliance to extrude an unerupted ectopic maxillary tooth (Figure 12).
Direct bony anchorage can be gained from a series of devices collectively known as temporary anchorage devices (TADs), although they are not always temporary. These include:
TADs can be used either as an anchor against which the force to move a tooth or teeth is itself directly applied, or alternatively an anchor tooth or teeth can be secured to the TAD. The force of the wanted tooth movement is, in this case, applied to the anchor tooth (or teeth) itself supported by the TAD.
Endosseous dental implants are widely used in restorative dentistry for the replacement of missing teeth and as abutments for bridgework. They are typically constructed from titanium and, once placed, osseointegrate with the patient's alveolar or jaw bone. Placement is a complicated surgical procedure requiring preparation of the implant site and a period of healing of 4–6 months before the implant can be used as an anchor for tooth movements.21 Specific orthodontic abutments have been produced but often a temporary crown with an appropriate attachment is used, later to be replaced by a definitive restoration after completion of the orthodontic treatment. Using endosseous implants in this way requires that the final restorative position for the implant is known and can be accessed, as these are not temporary and will form part of the patient's overall orthodontic/restorative treatment plan.
Specific orthodontic endosseous dental implants have been developed. They are usually 3–4 mm in diameter and 6 mm in length and are not placed in the dental arch, but in the midline of the hard palate. After a period of 13 weeks for osseointegration, the implant is uncovered and the healing abutment is replaced with a treatment abutment. Usually a transpalatal arch is constructed to connect the implant abutment and the buccal segment teeth; often the first molars. The anterior dentition is then retracted against the anchored buccal segment teeth (Figure 13a). Alternatively, the transpalatal arch can connect the canines to the implant abutment and the buccal segment teeth can be moved anteriorly against them (Figure 13b). As these implants osseointegrate, removal can be difficult. A trephine is usually needed to remove the implant in a second surgical procedure, possibly necessitating a general anaesthetic.
Onplants are hydroxyapatite-coated titanium discs, 8–10mm in diameter, placed sub-periosteally on the surface of the palatal bone, once again in the midline22 (Figure 14). The onplant osseointegrates with the bone surface and, after a healing period of 10–12 weeks, it is uncovered and an appropriate abutment is screwed into the central implant thread and impressions are taken for a transpalatal arch. A transpalatal arch is then used in a similar fashion to the conventional midpalatal implant. When the onplant is no longer required, it can easily be removed via soft tissue exposure and direct force application with a chisel, usually under local anaesthesia. Onplants offer an advantage over the conventional mid-palatal implant in that they are easier and less invasive to place and remove and are also cheaper. However, they have not been fully adopted into common orthodontic use due to higher reported failure rates than with other TADs23 and are largely historical now.
Miniplates are either I-, T-, L- or Y-shaped bone plates that are fixed to the bone with fixation screws.24 They are constructed from titanium and broadly divided into three elements, namely the body, the connecting arm and the head (Figure 15). They are placed as part of a minor surgical procedure, usually under local anaesthesia. A flap is raised and the body of the plate is screwed to the bone using fixation screws. The screws are usually sited apical to the roots of the teeth and the flap is then repositioned. The connecting arm is passed through the attached gingivae to leave the head of the miniplate exposed near to the dental arch. Like other implants, the miniplate head can be used to support direct force application to a tooth or teeth, or it can be used to support the anchor teeth against unwanted movement. The position of the head can be optimized to enable force vectors to be applied for good tooth movement. The miniplate is easily removed with a small surgical procedure at the end of treatment under local anaesthesia. The placement of the fixation screws away from the roots of the teeth confers several advantages (Figure 16) including elimination of the risk of root damage during placement and removing any potential interference with planned tooth movement. Miniplates are versatile since they can be placed in many sites across both jaws to provide direct or indirect anchorage where needed. The sub-apical bone is of a better quality for implant placement than inter-radicular bone and the success rates for miniplates has been reported to be as high as 91.4–100%.25
The temporary anchorage devices discussed so far all require surgical procedures for placement and removal so are often placed by surgeons rather than orthodontists.
Mini-screw implants are much more straightforward to place and can be placed using only topical anaesthesia or a small amount of local anaesthesia. They are subsequently removed, often with no anaesthesia at all. Mini-screw implants are made from either titanium alloy or stainless steel, and they are surgical grade screws that have modified heads to allow attachment of orthodontic auxiliaries. The screw has three components - the intraosseous threaded screw, the smooth transmucosal collar and the head (Figure 17). The intraosseous screw component varies between 1–2mm in diameter and 6–11mm in length.26 Many different companies supply mini-screws and there are many variations in design of the elements that make up the mini-screw. Some screws require a pilot hole to be drilled prior to placement, whilst others are self-drilling, requiring no pilot hole. The latter simplifies the process of insertion and these mini-screws have been shown to give better primary stability.26
In addition to their ease of placement and removal, mini-screws have several advantages over the other available TADs, namely they are more economical, they can be placed in a variety of intra-oral sites and they can be loaded immediately after placement. They do, however, have some disadvantages. Firstly, mini-screw fracture has been reported and as a result it is recommended that screws with a diameter of greater than 1.3mm be used to minimize this risk.27 Secondly, although mini-screws can be placed into the intra-radicular bone, which helps to provide anchorage with a vector as close to the plane of the teeth as possible, it is clearly not possible to move teeth through the site of the screw. Thirdly, mini-screws are often placed near to the roots of the teeth and care must be taken not to place the screw into the periodontal ligament or into the roots themselves.
A recent Cochrane review by Jambi and colleagues28 concluded that surgical anchorage is more effective than conventional anchorage, with mini-screw implants appearing particularly promising. However, a recent multicentre randomized clinical trial,29 comparing TADs, headgear and Nance button palatal arches, found no difference between the three techniques for anchorage support, although better quality orthodontic results were achieved when using TADs.
A naturally occurring bony anchorage device is an ankylosed tooth. This arises when replacement resorption causes the tooth to become fused directly to the bone, often as a sequela of trauma to the tooth.30 Teeth that have become ankylosed are not amenable to orthodontic alignment but can be used for absolute anchorage, against which the rest of the dentition may be moved. The ankylosed tooth may be retained following orthodontic treatment if in a favourable position, or surgically removed once it has served its purpose for anchorage.
So far we have discussed the principles of anchorage management and some of the intra-oral devices that can be used to preserve anchorage utilizing the teeth, soft tissues and bone. There are, however, other means by which the orthodontist can help to manage the anchorage.
By using ‘push-pull’ mechanics it is sometimes possible to move two teeth in opposite directions to create space between them, whilst simultaneously helping to preserve anchorage. One example would be where space is to be created for a palatally excluded upper lateral incisor and the central incisor and canine are in close approximation. By simultaneously using push coil between the central incisor and canine and retracting the canine against the buccal segment teeth, the upper centreline can be corrected and the canine retracted to a Class I relationship, whilst reducing the strain on the buccal segment anchor block on both sides of the arch (Figures 18 and 19).
As discussed previously, the rate of tooth movement will be affected by several factors, including the density of the surrounding bone. Since mandibular bone is more dense than the maxillary bone, especially in the molar regions,31
and maxillary teeth have been shown to move significantly more than mandibular teeth in animal models,32 it would be expected that more anchorage is to be gained from the mandibular molars than the maxillary molars. Similarly, cortical bone is more dense than cancellous bone and manoeuvring the roots of anchor teeth into the cortical plate is thought to slow their movement and therefore increase their anchorage value.33 Active management of anchorage demanding tooth movement can be achieved using the following methods.
We have already discussed tipping versus bodily movement in anchorage preservation. By actively tipping teeth in the opposite direction to their preferred movement, it is possible to reinforce the anchorage further, although it is not always as effective and straightforward as it may seem. Second order ‘tip back’ bends can be placed into stainless steel archwires, usually between the second premolar and first permanent molar (Figure 20) with the aim of limiting the molar to bodily movement. However, they may often end up tipping the molar too far distally, which then has to be corrected towards the end of treatment with an increased risk of anchorage loss at this late stage.
‘Toe in’ bends are likewise placed for molars in stainless steel archwires, but are first order bends. They are placed to rotate the molars distally (Figure 21) and thereby counteract the buccal flaring and rotation of the molars that might otherwise occur as a result of the buccally applied forces during retraction, again ideally restricting the teeth to bodily movement and limiting the rotation. This utilizes the higher anchorage value of bodily movement compared with rotational movements.
Transpalatal and lingual arches come in a number of forms and are either commercially available or can be custom-made. They are usually fabricated from 0.9mm diameter hard stainless steel wire soldered to bands on the first molars (Figure 22) and, by maintaining the inter-molar width, they have three theoretical modes of action. Firstly, as the anchor teeth try to move mesially into the narrower anterior part of the arch, the roots of the first molars will end up hitting the buccal cortical plate, increasing the amount of anchorage present. Secondly, any mesiolingual/mesio-palatal rotation of the molars will be prevented and, thirdly, for either molar to tip mesially, the other molar cemented to the palatal arch on the opposite side will have to tip by exactly the same amount, which may not be possible. Although the theory sounds plausible, in practice it has been shown that transpalatal arches have little effect in reducing anchorage loss,34 and may even contribute to it,35 possibly by lulling the orthodontist into a false sense of security. However, as discussed previously, a transpalatal arch may be used in conjunction with other elements, such as a mid-palatal implant, to good effect. They may also be helpful at increasing vertical anchorage, such as required during mechanical eruption of an ectopic canine where they are used to support the extrusive force from a sectional rectangular NiTi wire.36
Similar to the simple palatal arch, lingual arches are not very effective in reducing anchorage loss as an unwanted effect can be proclination of the lower incisors.37 They are, however, effective in maintaining arch length whilst awaiting tooth eruption. Nance, in 1947, described a modified palatal arch where the arch is extended forward to the anterior palate and where an acrylic button rests against the most vertical aspect of the palatal mucosa20 (Figure 23). The intention of this appliance is to gain antero-posterior anchorage from the vertical component of the vault of the palate in a similar manner to a removable appliance. However, Nance buttons can become embedded into the palatal mucosa (Figure 24) with some anchorage loss.38
The bones of the cranium and facial skeleton can also be used to provide anchorage via the use of a headgear or a face mask in conjunction with either fixed or removable appliances. Headgear is connected to the teeth via a facebow and the force is usually primarily directed to distalize the teeth. The vertical component of force can be altered by varying the direction of pull - occipital, cervical or combination pull (Figure 25).
The duration and the magnitude of force required for anchorage is 250–300cN of force per side for 8–10 hours a day.
There are rare reports of injuries as a result of wearing headgear and a number of safety features have been incorporated into the appliance to help prevent this occurring.39 They aim to prevent release of the facebow from the molar tube, limit the elastic recoil if the facebow is inappropriately pulled away from the teeth, or to blunt the ends of the facebow to minimize the risk of penetration injuries. The British Orthodontic Society guidelines recommend at least two separate safety features must be in place whenever a patient is wearing headgear.40
A reverse pull headgear or facemask uses the frontal bone and the mandible for anchorage and can be used to mesialize the upper arch teeth in a Class III malocclusion. In this case, elastics are connected directly between the intra-oral appliance and the cross member of the facemask, without the need for an intervening facebow. The challenge with the use of the facemask is patient compliance with wear.
So far we have discussed methods of anchorage preservation and management. In cases of very high anchorage demand it may be desirable to create additional anchorage and this is usually achieved by distalizing the maxillary buccal segments, in addition to any planned extractions.
The use of headgear for maintaining anchorage has already been discussed but, by increasing the force to 400–500g per side and increasing the hours of wear to a minimum of 12–14 hours per day, distalization of the buccal segments can be achieved. This can be optimized through the additional use of a ‘nudger’ appliance.41Figure 26 shows a simple ‘nudger’ design that could be used in conjunction with molar bands with headgear tubes. The use of the ‘nudger’ means that additional anchorage is incorporated using the vertical component of the palate. However, patient satisfaction with headgear is low and non-compliance can be an issue.42 If the headgear is not worn whilst using a ‘nudger’ appliance, this may result in the overjet increasing as antero-posterior anchorage is lost. This is particularly difficult to detect if a lower fixed appliance has been placed at the same time and some lower incisor proclination has taken place.
Fixed molar correctors are also available to achieve molar distalization and create anchorage. Commonly used molar correctors include the pendulum, Distal jet (Figure 27) and Jones jig appliances. They are similar in principle to the ‘nudger’ appliance, but instead are fixed to the teeth. They are designed to utilize the vault of the palate to act against the active components distalizing the buccal segments. Despite this, anchorage can be lost and can be seen as unwanted proclination of the upper incisors with an increase in the overjet along with mesial tipping of the premolars.43 In a recent Cochrane review it was concluded that greater molar distalization is achieved with intra-oral appliances than with headgear, but that it comes with a greater loss of anterior anchorage.44
Skeletal anchorage, obtained through using TADs, can also be used to distalize teeth and create anchorage. Force can be applied directly from the TAD to distalize teeth, or indirectly from teeth that are anchored to the TAD, against which the buccal teeth are distalized.
The ability to assess anchorage loss is an important part of clinical orthodontics. Regularly assessing the anchorage requirements during treatment is good practice as it allows judicious changes to be made to treatment mechanics, where necessary. Clinically, the simplest assessment of anchorage is to check that planned tooth movements are occurring. Regularly reviewing the remaining space within the arch, checking the molar and canine relationships, position of the centrelines and any overjet correction all contribute to this ongoing assessment.
This clinical impression may also be supplemented with radiographic assessments, most often using two or more lateral cephalometric views taken at different time points during treatment. The changes in tooth position can be assessed against stable reference points, such as those described by Björk.45 Commonly, lower incisor inclination relative to the mandibular plane and the change in molar position are used as indicators of anchorage loss. The disadvantage of using cephalometric radiographs to assess anchorage loss is that features such as the molar relationship can be difficult to judge due to superimpositions in addition to the increased radiation exposure risk to the patient.
Study models, either plaster or 3D, can also be used to monitor anchorage. They are used routinely in the clinic to assess changes in tooth relationships and arch form. It is also possible to measure changes against supposedly stable structures such as the palatal rugae.46 These have not always been found to be completely stable, especially during periods of facial growth.47 In addition, identifying the medial aspect of the most stable third palatal rugae, in itself, can be difficult. As a result, this method of assessing anchorage loss is usually only used for research purposes. With the increasing use and reliability of digitally scanned models48 and further development in scanning technology, this may however change.
It can be seen that anchorage is a complex subject and there will always be challenging cases that test the orthodontist's understanding of anchorage demands and means. However, careful pre-treatment assessment of anchorage requirements with regular in-treatment monitoring and management of anchorage is key to obtaining excellent orthodontic results.
