NEWSLETTERS

Banana maxillary third molars

Published: February 2013

Bulletin #19 February 2013

Banana maxillary third molars

It is well known that when teeth develop anomalous crown forms there is a predilection for certain specific teeth. Thus, we often find small or peg-shaped lateral incisors and we find small second premolars. These features are genetic and their prevalence is thus often seen bilaterally, associated with late development of that tooth and with late dental development in general, linked to other hereditary traits such as spaced dentitions, missing teeth and seen also in other family members.

From dental time immemorial, it has been universally recognized that the most frequently affected teeth are the third molars in both jaws. These come in various shapes and sizes and, for the most part, there is no particularly indicative pattern, recurring form or typical location despite the fact that so many are impacted. But, it would be difficult to clinically justify the need to replace an extracted and formerly impacted third molar with an implant, when the rest of the dentition is intact. The existence, abnormality or absence of third molars in general is not considered to be of major importance.

…….. nevertheless there is an exception, namely the Banana Maxillary Third Molar. In common with many other third molars, this tooth is usually unerupted and is an incidental finding in a routine orthodontic examination, in which the orthodontist has the opportunity to review a panoramic radiograph.

……… and why is this specific impacted maxillary third molar significant in the context of this website? Because the banana maxillary third molar causes the non-eruption/impaction of the maxillary second molar, in a good proportion of the cases and it is for this reason that this tooth is the subject of this month’s bulletin.

Figure 3-D PowerPoint_1

Fig. 1. 3-D screen shots of the this case, taken from the cone beam CT of the patient show the extreme crowding that is present at the distal end of the maxillary alveolus. In order to accommodate the unerupted molar teeth, the roots of the second molars are severely displaced mesially and buccally, nestling in the distal concavity at the distobuccal aspect of the first molars. The third molars are developing disto-lingually and inferiorly, apparently confining the second molar with their mesially bulging morphology to prevent any vertical movement.

coronal_cuts

Fig. 2. The vertical (transaxial) “cut” across the alveolus of the left first molar show the location of the 3-rooted second molar on its buccal side. The”cut” that traverses the palatally displaced third molar shows its bulbous mesial cusp and the depression in the CEJ area and how these may trap the more superiorly placed second molar from erupting vertically downward towards its place in the arch. Figs. 1, 2 are of the same patient by courtesy of Dr. Yael Jacoby.

The fact that the maxillary third molar develops at the distal end of the maxilla, in the tuberosity region, means that it grows in a very confined space, bounded by the second molar mesially, the pterygoid area distally, the maxillary sinus superiorly and the narrow alveolar process bucco-lingually. It seems likely that the convolutions of third molar roots are the consequence of their growth in this cramped environment. Perhaps, too, this may be a contributing factor in the anomalous morphology of the crown of the tooth.

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Fig. 3. The dental age of this patient is close to 14 years, given that the root apices of the maxillary canines and maxillary and mandibular second molars are almost fully closed. This labels the canines as exhibiting delayed eruption1, 2. The unerupted maxillary second molars are impeded high in the tuberosity area by the presence of the third molars, which are in the very early stages of root development. By kind permission of Dr. Jeremy Edel.

In the panoramic film, the tooth is frequently seen superimposed on the unerupted second molar and it also appears to be much larger than its mesial neighbor. A “two-dimensional-thinking” orthodontist may be excused for being fooled into diagnosing this as a dens in dente or some form of odontoma, leading him/her to schedule extraction of both teeth. The “three-dimensional-thinking” orthodontist will quickly realize that the enlargement of the third molar is due to the technical geometric properties of panoramic radiography. It will be remembered that all teeth that are lingually displaced from the focal trough have an increased distance from the film and will therefore project an enlarged image on the film.

Typically, the crown of the banana third molar is bucco-lingually narrowed and its occlusal pattern may often be missing one or more of the molar cusps or lacking their usual form. The tooth calcifies later than its more normally-shaped counterparts and one may see a late teenager with only the very earliest beginnings of root formation. The cemento-enamel junction of the tooth and the origin of its root development seem to be located more distally than normal, in relation to the crown of the tooth and this molds a very exaggerated and bulbous mesial contour to the tooth. In turn, it creates a deep hollow or fossa in the mesial surface in the area of the cement-enamel junction (Fig. 2). For reasons unknown, the cramped circumstances of the retromolar area are associated with a displacement of the third molar tooth germ more inferiorly and lingually to the second molar. The peculiar anatomy of its mesial side entraps the second molar against the distal aspect of the first molar. In this way, both second and third molars will usually remain in their unerupted state.

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Fig. 4. In this 18 year old male patient, the banana third molars have erupted with a small space separating them from the first molars. On the right side, the peculiar anatomy of the bulging mesial aspect of the crown of the tooth and the more distally placed site of origin of the early developing root are well illustrated. The follicle surrounding the second molar of the right side has become enlarged and now may be termed a dentigerous cyst. Both second molars are unusually high in the tuberosity area. Extraction of the third molars will almost certainly resolve the cyst and lead to spontaneous eruption of the impacted teeth.

Just occasionally, the third molar will by-pass the unerupted second molar and erupt with a small space between it and the distal surface of the erupted first molar.

If, as we assert here, the reason for the impaction is the presence, form and location of the third molar, then it is logical to suppose that extraction of this anomalous tooth should bring about rapid eruption of the second molar. However, when referring the patient to the oral and maxillofacial surgeon, it is strongly advised to point out to him/her the exact location of the tooth, since it is very easy to err and the wrong tooth may be extracted.

Fig. 5a-d_1

Fig. 5a. The banana third molar is only present on the left side and is clearly intimately associated with impaction of the second molar. Fig. 5b, c. Occlusal and buccal views of the extracted tooth show its irregular form and the very early stage of root development.      Fig. 5d. The panoramic view of the dentition at the completion of orthodontic treatment shows the fully and spontaneously erupted second molar. Eruption had occurred within 4 months of the extraction. This case was first published in Orthodontic Treatment of Impacted Teeth, 3rd edition by Adrian Becker and published by Wiley-Blackwell, Oxford, UK in 2012.

For the most part, the second molar erupts well and relatively rapidly, particularly in the younger patient (Fig. 5). Alternatively, it may happen that, while the second molar may improve its position, it may not have the eruptive power to break through the healed and thickened mucosal tissue covering the ridge, which may need to be excised. Follow-up of the case is advised. By and large, active orthodontic extrusion mechanics is not required with these teeth, although they may erupt into a buccal scissors bite relation with the mandibular second molars.

 References

1.  Becker A: Orthodontic Treatment of Impacted Teeth by Adrian Becker, 3rd ed. 2012, Wiley-Blackwell, Oxford, U.K. Chapter 1.

2.  Demerjian A, Goldstein H, Tanner JM. A new system of dental age assessment. Human Biology 1973; 45: 211–227.