NEWSLETTERS

The zygomatic plate: a useful adjunct for the treatment of impacted molars

Published: October 2011

Bulletin #4 - October 2011

The zygomatic plate: a useful adjunct for the treatment of impacted molars

There are several kinds of temporary anchorage device (TAD) available today and they all have their advantages and their disadvantages. It is not the intention here to discuss these in general, but to present just one of them, namely the zygomatic plate. Suggestions will be made as to how it may be placed and what advantages we can expect from its use, specifically with impacted teeth in mind.

Temporary single screw implants are limited in their placement locations to areas where there is attached gingiva, places where there is adequate bone thickness and quality and only within the confines of the alveolar ridge, in the interdental areas1. They must not be inserted through the zone of loose unattached oral mucosa since, in that location, they will inevitably cause ulceration and severe discomfort, not to mention inflammatory soft tissue overgrowth and loss of access. The device may sometimes be inserted into the mesial aspect of the ascending ramus, although there is often an undetermined thickness of soft tissue to penetrate before the screw achieves an adequate “bite” into the bone to provide stability. In the interdental areas, there is always the danger of damage to the roots which may or may not be of any clinical consequence, but certainly will be difficult to explain away to the patient. Perhaps their biggest advantage is that placement is very easy and is most often performed by the orthodontist.

Palatal implants are generally located into the anterior midline portion of the palate and are intended to be osseointegrated. The are generally very successful, but for them to be used as anchors for the eruption of impacted teeth, they generally require an elaborate and custom-designed palatal frame, soldered to 2 or 4 bands on the molar and premolar teeth.

Fig._1

Fig. 1. An onplant plate suitable for placement in the zygomatic ridge.

By contrast, zygomatic plates (fig. 1) may be used as the direct source of anchorage and intermaxillary elastic attachment2. They are surgically demanding for their satisfactory placement and are generally inserted by an oral and maxillofacial surgeon or a periodontist. Nothwithstanding this disadvantage, the zygomatic plate has several inherent advantages over screw TAD’s. In the first place, since it is not placed on the alveolar ridge, it can be used as the base from which to apply traction to move teeth in the mesio-distal plane over long distances and without impediment and without the need to alter their positions in the light of progress. Thus, if placed bilaterally in a young adult patient, they may be employed in place of the much-hated, user-contentious and under-worn extra-oral headgear for rapid horizontal distal movement of all the posterior teeth, en bloc, in the treatment of a class 2 case.

They 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 behavior. 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 counter-productive 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 3mm of anterior open bite closure, simply because the incisors are that much further away from the TMJ center of the mandibular rotation. This may be achieved by placing a transpalatal bar between the molars, to prevent buccal “rolling” of these teeth and applying the intrusive force from the zygomatic onplant plate. The force is then distributed to the bracketed posterior teeth through the agency of the archwire. 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 absence of solid evidence) is anything to go by, probably improved. 

In the immediate context of the treatment of impacted teeth, however, it 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 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 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 asymmetric deterioration in the maxillary occlusal plane.

If we use the opposing teeth as the anchorage base, it will generate a rapid extrusion of these teeth and a cant in the upper occlusal plane. Furthermore, the maxillary second molar may already have over-erupted a priori, in the absence of an erupted mandibular second molar, and may even be impinging on the soft tissue overlying the impacted tooth. Clearly, therefore, intra-arch or inter-arch tooth-borne mechanics are completely inappropriate unless backed up by some form of skeletal anchorage.

Surgery*

Fig._2_composite

Fig. 2a. A 15 year old male, with infraoccluded mandibular first molars whose prognosis was considered hopeless. A zygomatic plate has been adapted to correspond to the shape of the exposed bony area and is held in with 2 screws. The vertical portion of the plate protrudes through the second horizontal incision and its end is bent up to form the traction hook. (Courtesy of Dr. Nardy Caspy-Casap)

Fig. 2b. A clinical view of the left side, following alignment and uprighting of the mandibular teeth, with traction applied from the zygomatic plate to the infraoccluded first molar, following its surgical luxation.

Fig. 2c. The initial panoramic view

Fig. 2d. Follow-up panoramic view of the same patient several months later.

The inferior aspect of the zygomatic process of the maxilla (the key ridge) is palpable and easily accessible in the height of the buccal sulcus and is covered by a thin and mobile layer of oral mucosa. Under local anesthesia, a horizontal incision is made which will reveal the curved contour of its inferior surface. A posterior-to-anterior electrocut is preferable, since it limits the amount of bleeding and thus makes for better visibility. One arm of an L-shaped onplant bracket should be adapted in shape to lie snugly over this contour, while the second arm protrudes vertically downwards into the oral cavity, parallel with the orientation of the long axis of the molar teeth (fig. 2).

A second incision is then made parallel to the first, but this time within or close to the band of attached gingiva covering the buccal side of the molar teeth. The underlying connective tissue is then blunt-dissected away from the periosteum, to interconnect the two incisions. The vertical arm of the onplant is passed through this narrow slit under the attached gingival while its upper part is secured with 2 or, preferably, 3 screws into the zygomatic process (fig. 2). The first incision is now completely closed with sutures, leaving the vertical arm visible emanating from the attached gingiva covering the molar teeth. The second incision is then sutured to close the attached gingiva around the vertical arm, with the intention of avoiding ulceration of the tissue during normal function. A useful traction hook for vertical elastics may then be made either by bending up the end of the vertical arm or by opening up its terminal loop.

Preferably at the same surgical episode, exposure and attachment bonding of the impacted mandibular molar should be made, with consideration given to the need for the opportune extraction of the adjacent third molar. For the patient, access to a mandibular molar for the placement of an intermaxillary vertical elastic is often difficul. It is recommended to try to place two eyelets (preferably) or buttons on the exposed molar, because of its large surface area and to use a twisted soft steel ligature of 0.012” gauge, in order to be able to form it into a traction hook which will not easily open up under tension. A gold chain is not appropriate, since its collapses down into the exposed area and is difficult to retrieve and to apply elastic traction.

If the patient is relatively young or if the circumstances demand exposure of the impacted molar and extraction of the third molar, attachment bonding and a palatal screw device to treat the case, in addition to placement of the zygomatic device, then there is much to be said for a single general anesthetic/sedation session to compete all these goals in a single surgical session.

Orthodontics

Intermaxillary elastics need to be applied by the patient between the ligature hook on the exposed tooth and the hooked end of the zygomatic plate. It is required on a full time basis, including during meals.

Strictly, raising an impacted molar needs no orthodontic appliances, other than these two points of application of force, unless the adjacent teeth have drifted over it to block its movement. Thus, for those cases where the tooth has a direct and unimpeded route, de-impaction may be sufficient to achieve an adequately functioning, healthy dentition. It may also satisfy the patient’s requirements, since appearance will not have been influenced by the initial condition and neither will it be adversely affected by this limited form of treatment. Thus, the mechanism for de-impaction can be divorced from orthodontic treatment per se, since the two force systems are not interdependent and the two only combined if the patient elects to do this. However, it is more common to see cases in which the adjacent teeth have tipped and closed over the impacted tooth (fig. 2).

Fig._3_composite

Fig. 3a. Panoramic view of a horizontally impacted left second mandibular molar, in a 16 year old male patient. The maxillary second molar is unerupted.

Fig. 3b. Panoramic view taken after alignment and leveling of the teeth in both arches and following surgery (1) to remove third molars, (2) to expose and bond two attachments to the mandibular second molar and (3) to insert a zygomatic plate.

Fig. 3c. Progress film after 1 year of treatment

As a mesially tipped second mandibular molar erupts, its orientation may improve, although an initial strong mesial tilt will almost always requires much distal uprighting (fig. 3). This must be addressed with a more comprehensive approach and may be achieved by placing a molar mini-tube in place of the eyelet/button, in any convenient location on the buccal aspect and, at least initially, at a convenient supragingival angle to produce some uprighting. The mini-tube may then be progressively relocated as the correction occurs.

Fig. 4 composite_1

Fig. 4a. Occlusal view of overerupted second molar, with an elastic chain stretched across its occlusal surface between a palatal screw implant and a zygomatic plate on the buccal side. A button has been bonded on the occlusal surface to prevent slippage of the chain interproximally. 

Fig. 4b. The same case showing the intermaxillary elastic which the patient places between the zygomatic plate and the twisted wire hooks, emanating through the tissues. These wire hooks are ligated to the eyelets bonded to an impacted second molar (not seen).

Reproduced from Becker A, The Orthodontic Treatment of Impacted Teeth, 3rd edition, published by John Wiley-Blackwell, Abingdon, Oxford, 2011 in press.


In the event that the maxillary first or second molar has over-erupted, a useful approach is to place a screw TAD on the palatal side of the tooth concerned and to stretch a chain module between the palatal and the buccal TAD’s, across the occlusal surface of the molar (fig. 4). After 2 or 3 changes of chain module, intrusion will have been achieved. If needed, the chain may be secured in place against its slippage into the mesial or distal interproximal contact areas by bonding a small amount of composite or a button on the occlusal surface (fig. 4).

In young children, we have found that the quality of the bone in the area of the zygomatic arch may be too soft to support the holding screws and these may simply turn freely even in a self-tapped hole, without “biting”, rendering the onplant unserviceable. Otherwise and taking into account the relatively small sample of patients in our care employing this modality, the device is generally reliable and much freer from failure, in our hands, than screw implants.

Some patients find difficulty manipulating intermaxillary elastics in the lower retromolar area. In these cases, it is often useful to apply extrusive force direct from the main arch, by extending it distal to the first molar, to ensnare the twisted wire ligature that is linked to the eyelet on the buried second molar. In order to prevent secondary intrusion of the molar and premolars and the establishment of a cant in the occlusal plane, vertical elastics may be applied from hooks on the first molar and premolar in the mandibular arch to the zygomatic plate. This may be described as indirect supporting anchorage.

Removing the plate at the end of treatment is a simple procedure and involves relatively little discomfort. However, before this is done, a re-evaluation should be made to see if the TAD may be exploited to achieve other goals that remain in the overall orthodontic treatment of the case.

References

1. Ludwig B, Glasl B, Kinzinger GSM, Leitz T, Lisson JA: Anatomical guidelines for miniscrew insertion: vestibular interradicular sites. J.Clin.Orthod. 2011;45:165-173

2. Erverdi N, Usumez S, Solak A, Koldas T. Non-compliance open-bite treatment with zygomatic anchorage. Angle Orthod. 2007;77:986-90.

*Thanks are due to Dr. Nardy Casap-Caspi for his advice on the surgical aspects of zygomatic plate insertion.