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Etiology of an infraoccluded permanent molar

Published: April 2018

Bulletin #76 April 2018

Etiology of an infraoccluded permanent molar

An isolated infraoccluded first permanent molar is an enigma. Its etiology is most often inconclusive and the treatment that may be offered is therefore generally empirical and it usually fails. The occurrence of the anomaly is puzzling and, frequently, hardly noticed in the rapidly changing scheme of the dentition in the young child, particularly in its early stages. For the most part, we are unable to offer a remedy with any confidence that it will succeed to resolve the problem, because it is a condition about which we know so very little.

That being the case, let’s begin by discussing what we do know about the eruption of the first permanent molar and see if we can disperse some of the fog that surrounds this odd anomaly.

Teeth develop along a horizontal line on both sides of each of the 2 jaws, which comes to serve as a virtual border between the basal or skeletal bone and the alveolar or tooth-bearing bone. Basal bone is formed independent of teeth, as we see in complete anodontia patients with Ectodermal dysplasia. The characteristic and most important attribute of all normal teeth is that they possess an innate eruptive potential that brings them into the oral cavity and into occlusal contact with an opposing tooth. As they do so, there is an increase in the vertical height of the mandible due to the new alveolar bone that is generated with the teeth move occlusally, which accounts for much of the growth of the lower face in childhood and adolescence.

Simultaneously, but more slowly, there is an apically-directed root development whose long axis is in a fairly straight line, continuous with the orientation of the crown. Yet the teeth move away from the basal bone and from the anatomical limits and barriers of the skeletal units. This is due to a continuous increase in the height of the supporting alveolar bone, despite the fact that the roots of the teeth become 2-3 times longer than crown height. It should be clearly understood that, when root growth occurs in a molar in the mandible, its root apices do not grow downwards. This is because overall eruption and alveolar bone generation carry the tooth upwards to a greater extent than does the downward increase in root length.

What are these limits and barriers? These include the lower border of the mandible, the inferior alveolar canal (nerve and vascular bundles), the ascending ramus, the floors of the maxillary sinus and of the nose, the periosteum of the palate and alveolar ridges and the maxillary tuberosity. When a physical body stands in the way of the advancing tooth, impeding eruption, the root will continue to grow, but its direction may become deflected when the uncalcified root apex hits the obstacle. Initially, it will grow in the same oriented path as before, but the net effect will be directed into basal bone. Thus, for the same mandibular molar, the root will grow down towards the lower border of the mandible and the inferior alveolar canal. Once they meet these anatomic barriers, continuing root development will turn one way or another, to develop a hooked apex.

There are many and varied conditions that create a barrier to the developing root of a tooth. These may be divided into 3 categories. The first relates to physical anomalies that are not the result of morbidity, such as supernumerary teeth, odontomes, infraoccluded deciduous teeth, dental space loss and crowding, an abnormal deviation of the root of an adjacent tooth and a short apical base. These are physical space-occupying obstacles, standing in the eruption path of the crown or the developmental path of the root of the affected tooth.

The second category refers to anomalies that are due to the results of trauma to a tooth or teeth and includes various patterns of premature root apex closure, arrested root development with open apices and classic root dilaceration. The third category comprises teeth developing within dentigerous cysts and invasive cervical root resorption.

An infraoccluded mandibular first permanent molar

76_1a

Fig. 1a. Panoramic view of the 11 year old child with the infraoccluded left mandibular first molar (arrow). Compare its root length with that of the contralateral first molar and the relative distance from each to the compact bone at the inferior border of the mandible.

This month’s bulletin is a case presentation of an 11 year old female patient with an infraoccluded left mandibular first permanent molar who was referred to me by another orthodontist (Fig. 1a). For the purposes of this narrative, we shall refer to an infraoccluded tooth as one which may have erupted fully or only partially and then became arrested, while adjacent teeth continue to grow. The condition generally affects deciduous teeth, when it is generally accepted that the cause of the altered occlusal level is ankylosis of the affected tooth. However, permanent teeth may also be affected, as seen in this case.76_1b

Fig. 1b. An enlarged view of the immediate area of the panoramic film. Note the hooked root apices and their proximity to the inferior border of the mandible.

The referring orthodontist pointed out that the mesial root apex was hooked distally, the distal root hooked mesially and both were very close to the lower border of the mandible (Fig. 1b). She also indicated that she was not prepared to undertake a luxating procedure, which she was sure was necessary, because she “…. did not have the stomach for it” having never performed one previously.

If we base our diagnosis on the panoramic radiograph alone, it would seem that there are several treatment options that are employed in general and may be pertinent in this case. These may be broadly listed as follows:-

1. Orthodontic extrusive mechanics to a maxillary orthodontic appliance

2. Orthodontic extrusive mechanics to a maxillary temporary anchorage device

3. Surgical luxation and options #1 or #2 above

4. Extraction of the tooth

5. Occlusal composite build-ups

6. Coronectomy and auto-resorption of the roots of the affected tooth1 – with later implant-borne prosthodontics.

The referring orthodontist had considered only #3 of my 6 treatment options. However, even on this flimsy radiographic evidence, there are 2 other and less damaging modalities that are worth trying first. Nevertheless, a more certain diagnosis was needed before treatment decisions were made.

Clinical and planar radiographic examination

On examination, the patient was in the late mixed dentition stage, with several indications of asymmetry. It is not the intention here to present the entire case, nor to relate to the other factors that will need to be addressed in the child’s overall orthodontic treatment planning. For the present discussion, we shall ignore these and concentrate on determining the etiology of the infra-occlusion, principally with the help of the initial radiographs and subsequent CBCT imaging.

The panoramic view shows the first permanent molar at a lower level than either of its immediate adjacent neighbors and the over-erupted antagonist is in occlusal contact. The first question that needs to be asked is whether this is sufficient information for us to label the tooth infraoccluded.2

The first permanent molar normally erupts distal to the deciduous second molar, although it may become impacted beneath the distal curvature of the deciduous tooth. Furthermore, the molar erupts about 6 years before the second permanent molar, which is a long period of time during which there is no distal limitation preventing its occlusal migration. The second permanent molar was tipped mesially and its occlusal table had become higher than that of the affected molar because of the latter’s infraocclusion. The distal tipping of the second deciduous molar was secondary to the eruption path and progress of the second premolar. It is also secondary to the influence of the transseptal periodontal fibers that link the tooth to the permanent molar, which is seen typically in most infraocclusion cases.3-5 Over-eruption of the maxillary molar is also secondary to the infraocclusion.

The contralateral right mandibular first permanent molar has markedly longer and straighter roots than the affected molar and yet its apices are further away from the lower border of the mandible. The roots of the affected molar have hooked apices, influenced by their proximity to the lower border.

Therapeutic Diagnosis

Despite the above observations and tentative diagnosis, a bonded multibracketed orthodontic appliance was placed to increase the space between the second deciduous molar and the second permanent molar, followed by the application of extrusive mechanics to the infraoccluded molar. This was done to eliminate doubt and the possibility of misdiagnosis. As expected, the tooth did not respond, confirming the clinical and radiologic evidence of infraocclusion. It was at this point that it was considered important to perform a CBCT examination, in order to seek other possible causes of infraocclusion and pathologic entities that could not be seen on the panoramic view.

CBCT imaging

It is surely axiomatic that a thorough 3-dimensional radiographic examination will be infinitely more informative than a 2-dimensional one. Yet, a reading of many of the recently published studies purports to show that CBCT offers little advantage over planar films. I have learned through the correspondence I have had related to this website and through reading these articles that, by and large, orthodontists treating cases with impacted teeth may perform their own scans or request that scans be performed elsewhere. However, a high percentage of these practitioners do not reap the full benefit of the instrument because they do not demand sufficient interpretive reconstructions that this modality offers.

It is not unusual to meet orthodontists who proudly advertise their new, in-office, CBCT machine, but do not know how to use it to best advantage. There are those who manage with just the raw data DICOMs, the primary reconstruction, which offers hundreds of parallel axial (horizontal) salami-like cuts of the jaw concerned. There are others who demand to see secondary reconstructions in the form of cross-sectional (radial vertical) cuts around the arch with or without a 3D screen shot or two.

Few of us add longitudinal cuts and other less commonly oriented cuts, not to mention customized cuts, that are performed to enhance the information provided for a specific tooth.

On the personal level (and I am sure that I am not alone), I have difficulty keeping up with all that’s new in Orthodontics and, while I might pride myself with a modicum of a little more know-how in one specific area of the field than I have in other areas, I am not sufficiently computer-savvy to be able to acquire the wealth of information that CBCT offers me, without considerable help. So, I stick to what I (arguably) know and I use the expert services of a true CBCT computer genius to give me information that I am unable to prepare for myself. He provides me with the maximum that is possible to attain from a CT scan. I do not have a CBCT machine, so I use the services of one of several radiographic institutes that have sprung up in our locality over the past few years. I have them do the scan and then e-mail the DICOM’s direct to me. I e-mail the DICOM’s on to my expert technician, who is located in the northern-most town in Israel, close to the Lebanese border. Nahariya is a 3-4 hour drive from me in Jerusalem but, a few days later, I receive my beautifully prepared work-up from him, via the internet.

In the general area of impacted teeth, most orthodontists use CBCT for the sole purpose of accurately locating the tooth in 3 planes of space and in relation to the adjacent teeth. They also are likely to look for root resorption of adjacent teeth. However, because of the absence of sufficiently varied orientations of the cuts through the teeth and jaws, few use these images specifically to seek pathologic processes, such as medium-sized follicle enlargements, pre-eruption intracoronal resorption and invasive cervical root resorption. This is an important indication for prescribing the use of CBCT, because there is always a reason for the non-eruption and, if present, it will almost certainly show up on careful preparation, interpretation and examination of the CBCT.

From the single CBCT scan, a computer-savvy person should be able to produce axial cuts (parallel to the occlusal plane, cross-sectional cuts (radial cuts around the dental arch and in the vertical plane), coronal cuts (parallel vertical cuts from anterior to posterior), longitudinal cuts (antero-posterior cuts from buccal to lingual) of the alveolar ridges. Additionally, with the appropriate software, it should be possible to produce 3D movie clips, in both solid  and transparent mode, and an MPR simultaneous video of slices presented together on the screen, in the 3 planes of space.

Even though the radiation dosage for a CBCT has been reduced considerably over the past few years in comparison to planar radiography, it is still important to keep exposure to a minimum and to perform CT imaging only when essential for full diagnosis. However, it is probably a greater ethical crime of omission to have performed the imaging and then not have it properly interpreted it for the enormous amount of 3D information that it can yield.

Results of CBCT imaging

76_Fig._2

Fig. 2. A cross-sectional cut across the distal root of the tooth showing the inferior alveolar canal bundle in red (arrow) passing through the mesio-distal horizontal channel created by the encircling root of the tooth.

76_Fig_3a_c

Fig. 3a. An axial cut across the affected tooth, immediately apical to the furca of the root

Fig. 3b. A parallel axial cut across the mesio-distal channel in the root (arrow). The roots have separated into 4 separate branches, two on either side of the channel.

Fig. 3c. A further parallel cut across the apical area, to show the re-united roots of the tooth.

The patient under discussion was subjected to a lateral cephalogram, a panoramic film two bite wings x-rays and a periapical view of the maxillary and mandibular incisor areas by the pediatric dentist and the first orthodontist, prior to her referral. The area covered by the CBCT scan was limited to the left side of the jaw, from distal of the canine to the third molar. The full and comprehensive range of imaging was presented, including axial, cross-sectional and longitudinal cuts, a 3D movie clips in both solid and transparent modes and in an MPR presentation.

76_Fig._4

Fig. 4. A further axial cut at the furca region showing invasive cervical root resorption on the inner surface of the mesial root.

These reconstructions were examined closely and only the critical frames that were directly pertinent to the diagnosis are presented here (Figs. 2-6).

The work-up created 46 cross-sectional cuts through the mesio-angular infraoccluded tooth, at 0.5mm intervals. A representative cut through the distal root is seen in fig. 2. It clearly shows the inferior alveolar canal totally enclosed by the encircled roots of the tooth, to form a longitudinal intra-radicular (durch und durch) channel! Similar numbers of cuts were prepared relating to the other planes of space of the affected tooth.



76_Fig_5a_c

Fig. 5a-c. Three juxtaposed cuts created by “slicing” mesio-distally in the vertical plane from buccal to lingual across the ridge. The roots in 5a are imaged on the buccal side of the ridge and the roots in 5c are imaged on the lingual side, to give the false impression of completeness. However, 5b is cut along the mesio-distal channel area, with root above and below. Each of these cuts shows the invasive cervical root resorption lesion on the inner surface of the mesial root, close to the furca.

As may be seen, the 3D videos below add a great deal to the visualization of the teeth in the 3 planes of space, their relationships with each other and to the structures in their immediate vicinity, particularly the inferior alveolar bundle, as may be seen in this case. Please copy the URL on to your browser.

Video #1 
https://vimeo.com/262270257 

Video #2
https://vimeo.com/262270800

Video #3
https://vimeo.com/262270917

Discussion

There are three etiologic factors at play in the present scenario, none of which were recognized from the initial planar radiographs. The most obvious is the intimate, all-round association between the roots of the teeth and the inferior alveolar bundle.

The mesio-distal intra-radicular channel is located in the apical third of the roots of the tooth, which indicates that two thirds of the root had developed before this factor could have been operative. Two thirds root development is the age at which the tooth will normally develop, yet this tooth had not exploited the chance to migrate upwards away from the area and remained deep in basal bone. As the result, the roots were growing downward in close proximity to and on either side of the inferior alveolar bundle. The roots then turned inwards, under the bundle, as they grew closer to the lower border.

It is quite clear that the tooth is held down from below by its being traversed by a nerve and vascular bundle, like a bead on a necklace. This will not permit the tooth to erupt naturally. Attempts to draw the tooth biomechanically will fail, regardless of the extrusive force level brought to bear on it. However, within minutes of the application of extrusive forces, the patient is likely to complain of numbness in the lip, due to strangulation of the nerve and vascular bundle. This disturbing phenomenon will disappear if the forces are eliminated in timely fashion. Any procedure aimed at luxating or extracting the tooth will likely sever the inferior alveolar nerve.

76_Fig._6

Fig. 6. A cross-sectional view across the inner part of the mesial root to show the invasive cervical root resorption lesion (arrow) in addition to the mesio-distal channel, which appears on each of the cross-sectional cuts.

The second factor is the invasive cervical root resorption lesion, which has only been reported in this context since CBCT imaging has come to be used in clinical practice, since ICRR is rarely diagnosable from 2-dimensional planar radiography. When it occurs, it will arrest eruption of the affected tooth. 6, 7

In the present case, therefore, the most likely course of events would appear to have been as follows:-

1. The ICRR lesion began at some time before the age of 9 years, since this is the age at which the ½ to 2/3 of the root will have developed. It will have prevented the tooth from migrating occlusally. This is the primary etiologic trigger.

2. Only then will further root growth have proceeded at and below the level of the inferior alveolar nerve and vascular bundle. This is a secondary effect of the tooth’s deep location.

3. The roots then enclosed the bundle from below, under the influence of the inferior border of the mandible. This was the final contributory “nail in the coffin”.

Treatment

Given the assumptions made above, what approach should be made to treatment of this case?

Faced with the anatomic reality of the trapped bundle and the presence of the ICRR lesion, options #1 and #2 are futile. Options #3 and #4 will produce permanent disability to the patient due to severance of the blood vessels and nerve. Option #5 would provide an immediate and simple remedy, but the composite build-ups would lose their occlusal contacts within a few short months, as the adjacent teeth of this 11 year old patient continue their vertical development. At the end of the growth period, the disparity between the height of the natural crown of the tooth and its neighbors would be marked and would create a parallel disparity between their CEJ levels – with accompanying periodontal complications.

Only option #6 stands out as an acceptable manner in which to treat the problem. Coronectomy, with its accompanying auto-resorption of the roots of the affected tooth in this case, would be advantageous. Moreover, the ICRR lesion will materially help in the eventual disappearance of the remnants of the doomed tooth. Appropriate space evaluation, space maintenance and artificial replacement will then need to be considered as part of the overall orthodontic treatment.

References

1. Pogrel MA, Lee JS, Muff DF. Coronectomy: A technique to protect the inferior alveolar nerve. Journal of Oral and Maxillofacial Surgery, 2004, 62:1447-1452

2. Kurol J. Infraocclusion of primary molars: an epidemiologic and familial study. Community Dental and Oral Epidemiology. 1981;9:94-102.

3. Becker, A. and Karnei-R'em R.M.:The effects of infraocclusion: part 1 - tilting of the adjacent teeth and space loss. American Journal of Orthodontics 102: 257-264,1992.

4. Becker, A. and Karnei-R'em R.M.:The effects of infraocclusion: part 2 - the type of movement of the adjacent teeth and their vertical development. American Journal of Orthodontics 102:302-309,1992

5. Becker, A., Karnei-R'em R.M. and Steigman, S.:The effects of infraocclusion: part 3 - dental arch length and the midline. American Journal of Orthodontics 201:427-433, 1992.

6. Becker A, Abramovitz I, Chaushu S. Failure of treatment of impacted canines associated with invasive cervical root resorption. The Angle Orthodontist, 2013;83;870-876.

7. Becker A, Chaushu G, Chaushu S. Analysis of failure in the treatment of impacted maxillary canines. American Journal of Orthodontics and Craniofacial Orthopedics, 210;137:743-754.