LITMIR.BIZ

Fig. 2.9 Zygomatic plate. (a) An onplant plate. (b) The plate is held in place by three screws into the inferior surface of the zygomatic arch. (c) The extension portion is drawn through the edges of the fully replaced and resutured surgical flap.

      Zygomatic plates also have excellent application in the vertical plane, as in the treatment of open bite cases that are due to or in association with forward tongue posture and abnormal swallowing behaviour. In these patients there is an increase in the height of the lower third of the face and the maxillary posterior teeth are over‐erupted, with elongated alveolar processes. To close the anterior open bite by extrusion of the incisor teeth is both counterproductive in terms of the face height and highly unstable. On the other hand, it is easy to apply vertically intrusive force to the posterior teeth and achieve a significant reduction in the lower facial height. It should be remembered that 1 mm of molar intrusion is reflected as 3 mm of anterior open bite closure, simply because the incisors are that much further away from the temporomandibular joint centre of the mandibular rotation. The plate should be supported by placing a transpalatal bar between the molars, to prevent buccal ‘rolling’ of these teeth that would occur by applying the intrusive force from the zygomatic onplant plate to an unsupported single molar. The intrusively directed force is then distributed to the bracketed posterior teeth through the agency of an archwire that must reach all the way to the last erupted molar on each side. Intrusion of posterior teeth, rather than extrusion of anterior teeth, may produce more stable results in what seems to offer a greater chance for correction of the abnormal tongue and swallowing anomalies. It should be understood that the prognosis is still in doubt but, if considerable clinical experience (in the declared absence of solid evidence) is anything to go by, probably improved.

      In the immediate context of the treatment of impacted teeth, however, this procedure offers advantages for problems that are difficult to overcome by other means, and nowhere is its efficacy easier to demonstrate than in relation to the resolution of an impacted second mandibular molar. If an attempt is made to elevate an impacted mandibular molar, particular one with a distinct mesial inclination beneath the distal bulbosity of the first molar, a great deal of anchorage potential may be expended in its alignment. To use the remaining teeth as the base from which the extrusive force is to be applied will rapidly cause marked intrusion of these teeth and a strong cant in the occlusal plane. In time, this may then secondarily cause an asymmetrical deterioration in the maxillary occlusal plane.

      Had the opposing teeth been used as the anchorage base, rapid extrusion of these teeth and a cant in the upper occlusal plane would be generated. Furthermore, the maxillary second molar may already have over‐erupted a priori, in the absence of the unerupted mandibular second molar, and may even be impinging on the soft tissue overlying this impacted tooth. Clearly, therefore, intra‐arch or inter‐arch tooth‐borne mechanics are completely inappropriate unless backed up by some form of skeletal anchorage.

      The over‐eruption of an unopposed maxillary second molar is usually recognized by its vertically prominent mesial marginal ridge in relation to the occlusal plane, as represented by the distal marginal ridge of the first molar. This needs to be corrected concurrently with orthodontic eruption of an impacted mandibular molar, to bring it into occlusion at the level of the occlusal plane. To this end, an elastic chain may be drawn across the occlusal surface of the over‐erupted maxillary molar from a palatal screw TAD to the intra‐oral extremity of the zygomatic plate implant.

      At the same time, a vertical intermaxillary elastic may be placed by the patient from the same intra‐oral extremity of the zygomatic plate, to an attachment on the impacted tooth in the mandible. In this manner, it is easy to balance the degree of extrusion of the one with the degree of intrusion of the other, in relation to the occlusal plane. For these movements to be completed, no brackets or other appliances need to be placed on any of the other teeth. Furthermore, no retainer appliances need to be placed at the end of this phase of the treatment.

It will be appreciated that the zygomatic plate is an important device that may be used as a support to move a tooth mesially or distally and to extrude or to intrude a tooth. It may also be employed as an anchor base to safeguard against movement of a block of teeth and to move a large number of teeth at one and the same time. It may be used for intermaxillary up‐and‐down stability in the correction of vertical discrepancies and, antero‐posteriorly, for class II or class III correction.

       Ankylotic, infra‐occluded, implanted or otherwise non‐movable teeth as bone anchors

      There are several situations where an erupted or partially erupted deciduous or permanent tooth cannot be moved. Ankylosis is usually the term given to these teeth, although infra‐occlusion of deciduous or permanent teeth may not necessarily be due to ankylosis. Successfully replanted teeth that have been avulsed as the result of trauma are usually ankylotic. These teeth may often be included in an orthodontic appliance to act as bone anchors, in much the same way as described above regarding the zygomatic plates and screw TADs. Accordingly, they may be used as the base against which orthodontic forces may be applied to other teeth.

Magnets

      One possible source for the application of suitable forces is the rare earth magnet. These magnets were developed more than 60 years ago, and it has more recently become possible to reduce them in size with the introduction of lanthanide alloys, so that now they may be exploited in the present context. The professional literature has presented successful clinical results of the

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