Dilemmas in treatment planning an infraoccluded mandibular second molar

Published: October 2016

Bulletin #59 October 2016

Dilemmas in treatment planning an infraoccluded mandibular second molar

Written in cooperation with Prof. Refael Zeltser, former Chair, Department of Oral and Maxillofacial Surgery, Hebrew University-Hadassah School of Dental Medicine, Jerusalem, Israel

I have reported on the phenomenon of infraocclusion or impaction of permanent molar teeth in earlier bulletins 1-6. These are relatively rare occurrences and, as such, it is unlikely to be possible to assemble a large enough sample of them to investigate both etiology and response to treatment and produce the evidence-based conclusions needed to advise the hapless orthodontist who has been called upon to treat the problem. Accordingly, operative decisions in regard to planning treatment can only be based on the evidence that is available from individual case reports or, with luck, a short series of them. One’s own past experience with successful and unsuccessful attempts in dealing with this problem may often provide clues as to what may be the best approach. However, the ability of the practitioner to achieve success in a specific case, within the context of the orthodontic modality of treatment, must depend on which of the several known causes for the infraocclusion is responsible. Definitively diagnosing the cause is often impossible, decisions need to be made empirically and the prognosis of the treatment will be correspondingly uncertain.

Of late, several of these cases have been referred to me from different sources and this has brought me to look long and hard at the problem, with the aim of approaching it in a logical, systematic and organized manner, while recognizing the existence of potential pitfalls that are not found in relation to other impacted teeth.

In this month’s bulletin, I present the case of a 15 year old male patient who was referred to me for advice regarding treatment of the teeth in the left mandibular molar region. At the clinical examination, the patient exhibited a well-proportioned face with a minor degree of proclination of the maxillary incisors and a large median diastema. From almost every other viewpoint, the general alignment was good and the occlusion class 1 – essentially a simple, non-extraction, alignment case. However, the patient was referred to me for a solution to the presence of the unerupted mandibular left second molar (#37), discovered in a routine radiographic examination. His medical history was non-contributory and, at the differential diagnosis stage specifically Primary Failure of Eruption (PFE) and Gorlin’s syndrome were negated.


Fig. 1. Panoramic radiograph in March 2016Oct2016_Fig._2_labelled_section_of_pan

Fig. 2 The left side molar region of the same panoramic film with teeth labeled according to the FDI World Dental Federation notation

From the panoramic radiograph (Figs. 1 & 2), the abnormal features of the case in this area of the mouth were as follows:-


1. The crown of the deeply impacted second molar (#37) was encompassed by an enlarged dental follicle/small dentigerous cyst and its roots were located close to the inferior dental canal. The fully closed apex of its curved mesial root raised the suspicion that it was hugging the inferior alveolar nerve.

2. The tips of the cusps of the partially erupted and mesially tipped lower third molar (#38) were barely visible and partially enveloped by the soft tissue covering the distal end of the ridge and the mesial aspect of the ascending ramus of the mandible. It had drifted mesially to reduce the mesiodistal space for the second molar (#37) by about 50% at the occlusal level and it was in occlusal contact with the mildly over-erupted maxillary second molar (#27).

3. In the opposing maxillary area, distal to the second molar, the unerupted third molar (#28) was surrounded by a relatively large eruptive cyst.

4. The eruptive cyst was located entirely in soft tissue with no hard tissue cover, due to extensive bony resorption around the third molar crown, which had proceeded to incorporate the distal and palatal roots of the second maxillary molar (#27).

Primary clinical pathologic entities

It is most important to distinguish between the primary clinical pathologic entities and secondary phenomena that are their direct or indirect consequences. In the present case, there were two apparently unrelated primary anomalies, namely the infraocclusion/impaction of the mandibular second molar (#37) and the eruption cyst of the maxillary third molar (#28). As local pathological entities, each of these needed to be addressed.

Eruption cyst of the maxillary third molar(#28):

Treatment of this cyst is purely surgical and may be achieved by simply exposing the crown of the tooth through removal of the overlying soft tissue. The tooth will likely then continue on its eruption path but, in this specific case, is unlikely to find its place at the end of the dental arch and will need to be extracted. The second treatment option is to extract the third molar (#28) immediately, in the hope that bone will reconstitute around the second molar roots during the healing phase. Alternatively, the over-erupted second molar (#27) may be extracted concurrently with defusing the eruption cyst and the third molar (#28) may then be expected to erupt in a more mesial direction and take up the place of the extracted tooth. Thus, any one of these 3 treatment options will provide a simple and effective answer to the immediate problem and to the retromolar crowding.

Infraocclusion/impaction of the mandibular second molar (#37):

Why this tooth was infraoccluded is a mystery, since there are few signs or clues in the radiographs. It is extremely unlikely that the third molar (#38) was the primary cause of this problem, because it develops inferiorly to the second molar (#37) and it can only rise above the second molar secondarily, if the latter is held down by some other factor. These other factors include ankylosis, encirclement of the inferior alveolar canal by its own developing roots, invasive “cervical” root resorption7-10 or increased intra-cystic pressure from the dentigerous cyst surrounding its crown11 and, possibly, other unknown factors.

Ankylosis is the orthodontist’s favorite empirical assumption assigned to any tooth that has failed to respond to his/her attempts to resolve the impaction. It has been used as a convenient cover-up for failure due to more banal causes, such as traction in the wrong direction8……. and it is very difficult to prove the practitioner wrong! Nevertheless, ankylosis does occur from time-to-time and is rarely diagnosed on planar radiographs, although well-prepared CBCT “slices” in several directions must surely improve diagnostic precision. Breaking the ankylotic adhesion by surgical luxation of the tooth, using extraction forceps will often help, provided it is immediately followed by heavy and continuously-maintained or renewed extrusive traction.

This remedy is obviously entirely inappropriate when there is encirclement of the inferior alveolar (dental) nerve bundle. This is almost always a function of a root or roots that develop after the growing tooth is held down by an obstruction. It is best exemplified by mandibular third molars which are prevented from erupting due to physical limitations, such as their angulation or their location in the vertical ramus, distal to the second molar (#37). The roots then develop too deeply in the basal bone of the jaw and push downward towards the nerve bundle and the compact bone of the lower border, rather than propelling the crown of the tooth occlusally. It will almost always occur when an impacted tooth with an incomplete root is developing in close proximity to a limiting anatomical structure, such as the inferior dental canal, the cortical plate of the mandible, the floor of the nose (for impacted incisors) and the floor of the maxillary sinus (for impacted molars).12

Invasive cervical root resorption (ICRR), as its name implies, usually attacks the area of root closest to the cemento-enamel junction of any tooth. However, in an earlier bulletin on this website, I reported its presence in the furca area of the roots of impacted permanent molars and quite distant from the cervical region3. This resorptive lesion is a potent factor preventing tooth eruption and is a pathologic anomaly which, to this day, appears to be entirely unknown to orthodontists9. Careful examination of cross-sectional, axial and longitudinal “slices” from a CBCT will almost always reveal the lesion, provided the orthodontist knows what to look for. If the lesion is small and accessible, it may be exposed and treated. However, for the most part, the occurrence of ICRR will lead to the demise of the tooth because it is difficult to diagnose in its early stages and rarely surgically accessible.

When a dental follicle changes its character and becomes cystic, it enlarges due to hydrostatic pressure that builds up within it, causing it to expand further. The increase in intra-cystic pressure acts in direct opposition to the force of eruption and may even cause an impacted tooth to back up on its original eruption path. Surgically opening the cyst to the exterior will drain it, reduce the pressure and permit the tooth to realize its eruptive potential.

Secondary phenomena and treatment considerations

As the result of the presence of these two primary clinical entities, adjacent and opposing teeth have been affected and present markedly complicating and limiting factors in the manner in which acceptable treatment may be performed.

Both in the maxilla and the mandible, there is room for only two of the three molars and the difficulty lies in the choice of tooth to be sacrificed in relation to the potential benefit received and damage sustained.

In the maxilla,

1. extracting the third molar (#28) eliminates the cyst and permits bone to regenerate around the roots of the second molar (#27), although the extent of the repair and the chances of a return to normal are difficult to assess. At the same time, however, it leaves the second molar (#27) in its mildly over-erupted state, thereby limiting the further eruption of the mandibular third molar (#38), with which it shares an occlusal contact relationship.

2. On the other hand, extracting the second molar (#27) will eliminate this occlusal interference, together with the eruption cyst and it will leave the unerupted third molar (#28) free to grow down and migrate mesially into the second molar location. Whether the prognosis of this third molar (#28) with its shorter roots and smaller crown is a match for the mildly over-erupted and periodontally-compromised distal and palatal roots of the second molar (#27), is a moot point.

In the mandible, the choice for extraction devolves on the third or the second molar.

1. Extracting the third molar (#38) creates the space needed to accommodate the second molar (#37) in its ideal occlusal location. This would be the optimal line of treatment if we knew, with a reasonable degree of certainty, that this impacted molar would respond to treatment to raise it to the occlusal level. To set up an adequate anchor base against which to apply extrusive forces to the mandibular impacted tooth, a temporary anchorage device (TAD) should be considered de rigeur. My own personal preference would be a zygomatic onplant13, 14 since the opposing area in the maxilla is largely devoid of bone adequate to support a miniscrew. The chances of failure to raise the tooth are potentially high but cannot be assessed since no definitive determination of the cause of the impaction has been established. Should this treatment fail, then both the second (#37) and third molars (#38) would be lost and the tooth in the maxillary second molar location would be unopposed.

Performing a surgical luxation could break an ankylotic connection, but is strongly contraindicated in this case because of the likelihood of damage to the nerve. The only etiologic determination that would be favorable in the present context is a dentigerous cyst, since this can be eliminated easily and completely and the tooth may then be expected to move. Furthermore, applying orthodontic extrusive traction to the tooth is far and away gentler and less traumatic than surgical luxation. Given this etiology, orthodontic extrusive mechanics brought to bear on the second molar (#37) will erupt the tooth if its roots are free of the nerve bundle. If they are not free, the tooth will not respond. Should the root indeed be hugging the nerve bundle, as appeared possible on the 2-D panoramic view, application of even a light extrusive force will generate a more or less immediate parasthesia of the lower lip. The feeling in the lip will rapidly return if the traction mechanism on the tooth is cut. Not so, if the tooth were to be extracted or even luxated! It is therefore essential when first applying traction in this situation, that the orthodontist should be aware and prepared, while not releasing the patient for an hour or so after force application.

2. Extracting the affected second molar (#37) is the next line of treatment to be considered. The advantages of extracting the impacted tooth are, principally, that the major impediment to normalcy will have been eliminated. Furthermore, the third molar (#38) could be drawn mesially and uprighted, provided the over-erupted maxillary second molar (#27) had been the tooth to be extracted in that jaw. If the maxillary second molar (#27) is still in place, it would also be necessary to intrude it, in order to provide occlusal clearance for the mandibular third molar (#38) to be brought to the occlusal level and for the extraction space to be closed.


Fig. 3. Four of the serial cross-sectional cuts from the CBCT. The red arrow and red dot indicate the technologist’s estimation of the most superior point along the inferior alveolar canal. The yellow arrows point to the center of the true canal.

Diagnostic radiographic imaging of the relative positions of the molar root apices and the nerve canal is therefore of crucial importance in this case and the only way to achieve this effectively is with 3-dimensional cone beam CT (Fig. 3). However, the most important complication and contra-indication for extracting the second molar (#37) here, would be the significant risk of long term damage to the inferior alveolar nerve, due to the proximity of its root ends to the mandibular canal. There is also a second major drawback which is the large quantity of bone that would need to be removed on the buccal side of the tooth, which includes the external oblique ridge of the mandible, in order to gain reasonable access for the extraction (Fig. 4).


Fig. 4. The marked lateral thickening of the compact bone on the outer surface of the mandible is termed the buccal shelf. It is bound by the external oblique line and is indicated in the diagram by the red elipse. The impacted second molar tooth (#37) is located lingual and inferior to the buccal shelf.

3. There is a third line of treatment that might be considered and this would involve the coronectomy15 of the impacted second molar (#37) down to the bifurcation of its roots. The two roots would be separated from one another and left in the bone, to be slowly resorbed by the tissues over a long period of time. This achieves two major advantages, the first of which is that the nerve bundle is not endangered. Furthermore and from the panoramic view, it will be seen that the roots of the third molar (#38) are in a plane above the level of the severed roots of the second molar, which will permit mesial movement and uprighting of the third molar, to establish interproximal contact with the first molar (#36). However, as noted in the previous treatment option, access to the tooth to perform this deep coronectomy involves the loss of much buccal bone.

4. Prof. Refael Zeltser, the referring oral and maxillofacial surgeon, has suggested one other possible solution, which carries with it the advantage of delaying the molar extraction decision until after clarifying whether the impacted second molar (#37) will respond to extrusive mechanics or not. In order to do this, space would first need to be created between first (#36) and third molar (#38) by distal movement of the latter. It is obvious from the panoramic radiograph and the CBCT that the third molar cannot be moved sufficiently because of its proximity to the junction between the distal end of the horizontal alveolar ridge and the anterior border of the vertical ramus. However, surgical modification of that part of the ramus close to the junction can potentially increase the arch length and permit orthodontic enlargement of the second molar space. Subsequently, exposure and bonding of an attachment to the occlusal surface of the impacted tooth would be performed and traction applied from the same zygomatic plate alluded to in option #1 above. Exposure needs to be limited to the occlusal surface only and does not need to go as deep as the bifurcation, as described in options #2 and #3 above and, thus, the surgical procedure will be much less radical in terms of bone loss. As in treatment option #2, the decision which molar to extract would then be based on whether the impacted molar will respond or not. However, with this option the extraction will only be carried out subsequent to knowing the answer.

5. Is there an option for advising no treatment in the mandible and simply leaving the situation as it is? At present, the mandibular third molar (#38) adequately acts as an antagonist to the upper molar but, by leaving the impacted molar (#37) in its place between first and third molars, enlargement of the cyst at the expense of adjacent bone is likely to be significant in the future. Furthermore, the presence of this space-occupying body preclude the presence of interproximal bone and is likely to result in undermining long term periodontal prognosis of each of the 3 molars involved. Draining the cyst without erupting the tooth is a lesson in futility, since it will likely close up quickly after the pressure has been released and will almost certainly become cystic again within a very short time. The chances of the area becoming infected, with accompanying swelling and pain, will be increased when this inaccessible area is opened to the oral environment. The infection will be acute and require immediate treatment, which generally include prescribing antibiotics and then surgery. The attending surgeon will then need to face the same dilemma regarding choice of extraction, as referred to above.

In summary, there is no foolproof way with which to overcome this unusual clinical situation without incurring sizable risks, regardless of which approach is taken.


1. Impacted maxillary first molars Bulletin #30 - February 2014

2. Non-eruption of a developing mandibular first permanent molar Bulletin #38 – November 2014

3. Infra-occlusion or impaction of the maxillary first permanent molar Bulletin #40 – January 2015

4. Root development in impacted teeth Bulletin #42 – March 2015

5. Will these impacted teeth respond to extrusive mechanics Bulletin #50 - December 2015

6. Failure of eruption and the hooked root apex – cause or effect? Bulletin #51 - January 2016 DOI: 10.13140/RG.2.1.4694.2804

7. Brosjo M, Anderssen K, Berg JO, Lindskog S. An experimental model for cervical resorption in monkeys. Endodontics and Dental Traumatology, 1990;6:118-120.Miller PD. A classification of marginal tissue recession. International Journal of Periodontics and Restorative Dentistry, 1985;5:9-13

8. Becker A, Chaushu G, Chaushu A. An analysis of failure in the treatment of impacted maxillary canines. American Journal of Orthodontics & Dentofacial Orthopedics 2010;137:743-54.

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

10. Becker A, Chaushu S. Impacted teeth and the 6 incarnations of resorption. (Les six formes de résorption associées à l’inclusion dentaire) L’Orthodontie Francaise 2015, 86:277–286.

11. Becker, A. The orthodontic treatment of impacted teeth. 3rd edition, 2012. Oxford: Wiley-Blackwell Publishers. 2012, Chapter 11.

12. Bulletin #28 - December 2013 On the immutably of location of the developing root apex.

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

14. The zygomatic plate: a useful adjunct for the treatment of impacted molars. Bulletin #4 - October 2011

15. Pogrel MA, Lee JS, Muff DF: Coronectomy: a technique to protect the inferior alveolar nerve. J Oral Maxillofac Surg. 2004;62