Orthodontic Anchorage: Concept and Complexities
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Clinical Orthodontic anchorage: concept and complexities Limiting unwanted tooth movement, while producing desired positioning of other teeth, is a very important part of orthodontics. In this article, Benjamin Lewis outlines the concept of orthodontic anchorage, explains why it is important, and how to manipulate anchorage techniques to produce the best possible result for your patients. he concept of anchorage, in orthodontic terms, is complex. It relates to techniques that can be used Tby the orthodontist to limit unwanted tooth movement. This article will describe what anchorage is and why an understanding of it is important in orthodontic practice, as well as demonstrating some of the many methods that can be used to reinforce and manipulate anchorage to achieve the best orthodontic result. What is anchorage and why is it important? iStockphoto.com Orthodontic anchorage is a complex Newton’s Law states that for every action there is an equal and opposite reaction. concept that revolves around the The concept of anchorage is built around this. theoretical principles by which orthodontic techniques may be employed attached to the appliance; sometimes resistance of the periodontal ligament, to limit or even prevent unwanted tooth these reactionary tooth movements but the reactionary force which was movement. are not wanted by the orthodontist. produced was distributed over sufficient Newton’s Law states that: Anchorage management is the method teeth so that their periodontal ligaments ‘For every action (in this case, a by which the orthodontist attempts were not pushed over their thresholds, desired tooth movement) there is an to control these undesired tooth then this would result in the movement equal and opposite reaction’. movements. of just the tooth that was intended to be The reactionary force that is created moved (Proffit, 2000). Unfortunately, will move the other teeth that are Achieving tooth movement forces can not currently be placed with To be able to achieve tooth movement, sufficient precision for this to occur. a force needs to be applied to the This is further complicated by the fact Benjamin RK Lewis is a senior tooth that is sufficient to overcome the that the force required to overcome the specialist registrar in orthodontics at St Lukes Hospital, Bradford and resistance of the periodontal ligament periodontal ligament varies between Leeds Dental Institute, Leeds. (see the first article in the series for teeth depending on: more detail; Lewis and Jedynakiewicz, n The type of tooth—incisor, premolar Email: [email protected] 2007). Theoretically, if a force could be or molar. The larger the root surface placed on a tooth that just overcame the area of the tooth, the greater the 320 Dental Nursing June 2008 Vol 4 No 6 Clinical Figure 3. Shows the occlusion initially and following orthodontic treatment. Figure 1. Elastic powerchain over the This involved the extraction of maxillary first premolars to relieve the severe maxillary incisors to close the anterior crowding and to align the canines into a Class I position. The extraction midline diastema. of mandibular second premolars allowed not only the relief to the anterior crowding but also enabled the mesial movement of the molars to correct the molar relationship to Class I. Reciprocal anchorage The amount of space generated by Reciprocal anchorage occurs when a the extraction of a tooth is often not force is placed on two teeth or two exactly the same as the amount of space blocks of teeth that have roughly the required. A premolar is approximately same root surface area, providing 7 mm wide, therefore the extraction similar periodontal ligament thresholds. of a premolar from both sides of a Figure 2. Upper removable appliance to Treatment is planned so that the applied dental arch provides 14 mm space. This expand the maxillary arch. force creates equal tooth movements may be too much, exactly the right force required to move it in both blocks of teeth. This is amount, or insufficient space. Each of n Periodontal condition—a decreased demonstrated in Figures 1 and 2. Figure these outcomes has an impact on the level of periodontal attachment 1 shows a median diastema being closed orthodontist’s anchorage management results in a decreased periodontal with elastic power chain, with each of of the case. If there is still insufficient threshold to tooth movement the central incisors moving the same space, further space has to be created, n Type of orthodontic force that is distance towards the midline. Figure 2 either by additional extractions, moving placed on the tooth—tipping teeth shows an upper removable appliance the buccal teeth distally, expansion of the requires lower forces than those (URA) with a midpalatal expansion arch or interproximal enamel reduction. required to move teeth bodily (see screw. This has been designed to expand If the space created by the extractions is Table 1). the maxillary arch to correct a unilateral just sufficient, the orthodontist must Anchorage is important in orthodontic buccal crossbite with a displacement maintain the position of the posterior treatment because if it is not assessed (see the previous article in the series teeth to allow all the created space to correctly at the outset of treatment and/ for a full description of the diagnosis be used to align the teeth or correct or not carefully monitored throughout and management of crossbites; Lewis, the incisal relationship. This requires treatment, then the final dental and 2008). The teeth opposite each other absolute anchorage, which is discussed facial result could be compromised. are moved the same distance buccally in more detail below. In cases when because they have similar periodontal the extraction of teeth provides more Types of anchorage thresholds. space than is required, the orthodontist There are four basic types of anchorage must then plan the ‘loss of some of that are used in orthodontics: Planned anchorage loss the anchorage’; this will aim to move n Reciprocal anchorage The extraction of teeth is one of the the posterior teeth forwards at the n Planned anchorage loss options an orthodontist has of creating same time as aligning and retracting n Anchorage reinforcement sufficient space to relieve crowding the anterior teeth (Figure 3). This may n Absolute anchorage. or to create an optimum occlusion. be aided during the planning process Dental Nursing June 2008 Vol 4 No 6 321 Clinical Figure 6. A Class II elastic being used to establish a class I canine and molar relationship towards the end Figure 9. Transpalatal arch with Nance of orthodontic treatment. button. Pitting a single tooth against multiple teeth Adding teeth together to create a block of teeth increases their root surface area Figure 4. The maxillary second and their anchorage value. This block premolar and first molar are secured of teeth can then be used to move a together to be pitted against the canine, single tooth while allowing, potentially, resulting in distal movement of the minimal movement of the block of teeth canine with little mesial movement of Figure 7. Transpalatal arch. In this case (Figure 4). the posterior teeth. it has been modified with a mesially extending arm to allow distal traction to be placed to the maxillary right Utilizing different types canine. of tooth movement Tooth movement requires varying levels of force depending on the types of tooth movement which are planned. As can be seen from Table 1, the tipping of teeth requires less force than that needed to Figure 5. Intra-arch elastics being used achieve bodily movement. This can be to correct a centreline discrepancy. utilized by the orthodontist to control anchorage during an individual’s if teeth are extracted closest to the treatment, by allowing the teeth they teeth that the orthodontist wishes to Figure 8. Lower lingual arch. want to move, to tip, while restricting move the furthest. For example, if the orthodontist wanted the molars to move mesially during treatment, to correct TABLE 1. FORCE LEVELS USED FOR the molar relationship, the extraction of the second premolars instead of the first DIFFERENT TYPES OF TOOTH MOVEMENT premolars would make that planned tooth movement more straightforward. Type of movement Force (grams) Intrusion 15–25 Anchorage reinforcement Anchorage reinforcement is required Tipping 30–60 when a large proportion of the extraction space is needed to achieve the aims of Extrusion 50–75 treatment. This reinforcement, which Rotational 50–75 restricts the unwanted tooth movement, can be created in a number of ways. Bodily 100–150 322 Dental Nursing June 2008 Vol 4 No 6 Clinical Figure 10. Cross-sectional view showing the design of a transpalatal arch with a Nance button which rests against the palatal mucosa overlying the palatal vault. those they do not want to move, to bodily movement. This can be achieved by using different types of orthodontic brackets/attachments and specific bends placed into the archwire. Figure 12. A NITOM safety facebow used with orthodontic headgear. (A) The Interarch elastics band on the first molar has a headgear Interarch elastics (Figure 5) allow a tube attached to it; (B) the intraoral connection between the maxilla and arm of the facebow in inserted into the mandible. This connection can the headgear tube on the first molar; be used to pit some of the maxillary (C) the NITOM nickel titanium locking teeth against some of the mandibular attachment secures behind the first teeth, which can ‘alter the anchorage molar tube, preventing the facebow’s balance’ in a particular arch allowing accidental disengagement. the orthodontist to move the teeth they want to achieve a good interarch occlusal of teeth to increase their anchorage relationship; i.e. allowing the mesial value in addition to the maintaining movement of the mandibular molars at intermolar width.