Failure of eruption and the hooked root apex - cause or effect?

Published: January 2016

Bulletin #51 January 2016 DOI: 10.13140/RG.2.1.4694.2804

Failure of eruption and the hooked root apex – cause or effect?

When you see a good number of patients with impacted teeth, you will experience occasions when the tooth does not respond to extrusive orthodontic forces. So, you pull a little harder, to no avail. You then pull a lot harder, approaching orthopedic force levels. It still does not move and, on the basis of this circumstantial evidence, you feel justified in concluding that the tooth is ankylosed. You refer the patient back to the oral and maxillofacial surgeon for a luxating procedure, by which you mean to break the ankylotic connection that “obviously” exists between an area of the root of the tooth and the alveolar bone of the socket.

This unsubstantiated spot diagnosis of ankylosis may be correct or not. If the tooth is ankylosed and if the surgeon successfully mobilizes it and if you then apply an extrusive force and if you renew this force every week or so to prevent force decay and re-ankylosis, then there is reason to believe that the tooth may move.

On the other hand, in the surgeon’s enthusiasm to materially assist you in erupting the tooth and expecting to have to apply considerable force on the tooth, the OMFS may sometimes be surprised to find the tooth to be excessively mobile. Not expecting this to be the case, he almost extracts it. He sends the patient back to you with a polite note to the effect that you should stop wasting his time………. but not before he casts a surreptitious glance at his chairside assistant implying the conviction that you, the orthodontist, are manifestly incompetent.

At this juncture, you look again at the radiograph of the tooth and notice, perhaps for the first time, that the apical portion of the root of the tooth is configured in the form of a hook. You then rationalize that this abnormally shaped root must be the reason for the failure of the tooth to respond to the orthodontic extrusive forces!

The etiology of hooked root apices

Hertwig’s root sheath is responsible for the development of the roots of a tooth and it starts as a proliferation of epithelial cells located at the cervical loop of the enamel organ. It initiates the formation of dentine in the root of a tooth by causing differentiation of odontoblasts from the dental papilla. For a single-rooted tooth, the direction of root growth is normally in line with the long axis of crown of the tooth, which generally means that it is vertically oriented and continues on this vertical path, strongly linked to the eruptive progress of the tooth. For a multi-rooted tooth, the roots tend to diverge to a limited degree, but otherwise remain on a continuous, vertically-oriented, path. During normal dental development, the unerupted tooth migrates in an occlusal direction as its root grows, concurrently generating alveolar bone. Although the root elongates and because of this vertical alveolar bone growth, the developing apex of a mandibular tooth becomes more and more distant from the lower border of the mandible, while a parallel vertical change occurs in the maxilla. In effect, the tooth becomes propelled occlusally due to these two factors. This is the prime factor in vertical growth of the lower third of the face.

For the root end to become hooked, the dentinogenesis of the root must be induced to alter its directional expression from vertical to include a horizontal component. If we assume that root development is unstoppable for as long as it is uncompleted and unapexified, then any impediment that overpowers the innate eruptive potential will cause the newly developing apical portion of the root to proceed towards the “apical base”, since the impediment prevents eruptive progress. Thus, instead of distancing the tooth from the “apical base”, a still active dental papilla at the apex of the tooth may eventually reach the compact bone of the lower border of the mandible or the periosteum/epithelium that lines the maxillary sinus or the floor of the nose. Because it can go no further, any additional growth potential will be realized in a diverted direction and a hooked apex will likely result.


Fig. 1a. A panoramic view of the dentition of a 7 year old male with unerupted teeth distal to the right mandibular deciduous first molar #84. The unerupted first permanent molar #46 is impacted deep in the corpus of the mandible, with its late developing and wide open root ends in the compact bone of the lower border. The deciduous second molar #85 is similarly impacted and the permanent second premolar is absent. Elsewhere in the mouth, the erupted and unerupted teeth are as would be expected at this age. Unless tooth #46 is brought into the occlusion, the likelihood of root dilacerations is high.


Fig. 1b. A new panoramic view of the same patient taken post-operatively at age 9.7 years shows 2 eyelets bonded to the permanent molar, the placement of a zygomatic plate as the anchor base for the placement of extrusive elastics and the elimination of the deciduous second deciduous molar.


Fig. 1c. Seen at age 15.8 years and several years post-treatment, the completed root length of the affected right molar is similar to that of its antimere, due to late catch-up growth, following distancing of the developing apices from the lower border of the mandible. N.B. the first premolar was deliberately moved distally to transpose the intended implant site away from the mental foramen and to increase alveolar bone there, following the extraction of the deciduous second molar and to provide a more favorable future implant site in its former place.

Other factors causing root end alteration

In earlier bulletins on this website, we have discussed unerupted teeth that are to be found deep within basal bone, whose roots have shown diminished and convoluted development due to the cramped anatomic environment in which they are confined. We have also shown anecdotally how, with successful eruptive treatment, there is a “bounce-back” effect in which the uncompleted portion of those with open apices will show rapid compensatory renewed growth (Fig. 1)1, 2


There is no histologic difference between the lining of a normal dental follicle, an eruption cyst and a dentigerous cyst. The only way in which these are distinguished one from another is by their size, as seen on a plane film radiograph. Normal eruption of the tooth into the oral cavity occurs when the lining of a normal dental follicle fuses with the overlying epithelium of the oral mucosa/gingiva, creating an opening into the mouth. An eruption cyst is merely an enlarged follicle, which is no longer covered by bone and can be diagnosed by its fluctuant nature and slightly blue color. For the most part, the eruption cyst spontaneously ruptures to permit the tooth to erupt normally, although it occasionally needs to be incised to achieve this end.

A dentigerous cyst is usually enclosed in bone. As the hydrostatic pressure within the cyst lining increases, adjacent bone is resorbed to form a cystic cavity, while bony apposition on the outer surface leads to a hard swelling which, in the maxilla, may be seen on the patient’s face.3, 4


Fig. 2a. The first permanent molar is impacted deep into basal bone due to the dentigerous cyst encompassing its crown .Its roots are hooked because of their continued development in close proximity to the lower border of the mandible.

Fig. 2b. When the deciduous second molar was shed, the cyst spontaneously drained and the area healed. Despite the hooked root ends, the tooth spontaneously surged upwards until it became lodged between the two adjacent teeth. Courtesy of Dr. Imanuel Gillis.

The intra-cystic pressure displaces the affected tooth in an apical direction, which may therefore lead to an alteration in contour of the uncompleted root apex (Fig. 2a). Adjacent unerupted teeth may also be displaced and end up in the wall of the cyst, often with abnormal root configurations.

In each of these cases, following the resolution of the cyst, the teeth will improve their positions as the formerly cystic area is replaced by new bone (Fig. 2b) and the teeth will often erupt autonomously. Those that do not erupt may then be treated by applying simple orthodontic traction, following exposure and attachment placement. 3, 4

When the intra-cystic pressure builds up, it overcomes eruptive movement, while the root of the tooth continues to grow. Once again, therefore, when the apex of the tooth reaches the compact bone of the lower border of the mandible or the periosteum/epithelium lining the maxillary sinus or the floor of the nose, it will realize any further growth potential in a diverted growth direction and a hooked apex may result. Furthermore, when the cyst is de-fused, the bony fill-in that occurs behind the cyst lining carries the displaced tooth along with it. In time, it may erupt normally without further treatment and quite regardless of a hooked root apex or other form of developmental dilaceration that may have altered the anatomy of its formed root.


Fig. 3a. A permanent second molar is deeply impacted in the mandible, associated with a cystic lesion superior to it. Its root apices are hooked and located in the compact bone of the lower border.

Fig. 3b. At surgery, the cyst was removed and an eyelet attachment bonded to its partially exposed crown. 


Fig. 3c. Extrusive mechanics were applied, with extra-oral anchorage support.

Fig. 3d. The final film shows the tooth in place.

The presence of other forms of cyst that may exist in the jaws will impede the eruption of teeth, if they interfere with the eruption path of the tooth concerned. In these cases, the tooth will become displaced in the opposite direction, as its roots develop and the apices may become hooked in proximity with the lower border of the mandible (Fig. 3)


Infraoccluded deciduous second molars in either jaw are often over-retained and occasionally associated with severely dis-located second premolars. For the most part, there is no noticeable variation in the root development of the premolars. It is the exception where one may see altered root architecture, yet they almost always erupt without orthodontic traction being applied, provided that space is present, their orientation is favorable and the over-retained infraoccluded tooth removed. Biomechanical assistance is not often necessary.

Invasive cervical root resorption (ICRR)

zap.4a zap.4b

Fig. 4a.A young male with infraoccluded maxillary left first permanent molar

Fig. 4b. Serial cross-sectional CBCT slices show the area of root with invasive “cervical” root resorption which was initiated in the furca area (arrow).Courtesy of Dr. Gabriel Gal.


Fig. 4c. An axial slice from the CBCT showing the ICRR lesion

Fig. 4d. A radiograph of the extracted tooth showing the “woolly” consistency of the resorbed root area.

The case load in my own private orthodontic practice comprises a very high proportion of cases with impacted teeth and it all started when I decided that the buck had to stop somewhere! Judging by the clinical and radiographic records of failing treatment of impacted teeth that are sent to me for advice, through the Clinical Consultation page on this website and generally over the years by e-mail, there is a high frequency of this symptomless but progressively destructive condition among failed impaction cases. I have pointed out elsewhere that ICRR is largely unrecognized by orthodontists and frequently goes undiagnosed, although pediatric dentists, endodontists and dental traumatologists are very much familiar with it (Fig. 4). There can be no question that an advanced ICRR lesion will prevent a tooth from realizing its eruptive potential. It will also resist the application of extrusive traction and, as such, it should not be surprising to find that the affected tooth may develop a hooked root apex.5-9

Pre-eruptive intracoronal resorption (PEIR)

PEIR is a pathologic process which is in many ways similar to ICRR. It is a resorptive process which occurs within the follicle of an unerupted tooth and attacks the dentine of the crown of the tooth through a defect in its enamel layer.8, 10 The favorite locations for this to occur, therefore, are in the depths of occlusal and buccal fissures, incisal edges, cusp tips, buccal pits and dens-in-dente. These lesions are completely asymptomatic and are often mistaken for caries. Pediatric dentists will often examine a 7-8 year old child who has recently erupted a maxillary lateral incisor and diagnosed the existence of a dens-in-dente in its cingulum area. Simple cavity preparation may easily lead to pulp exposure, because of the soft mush within a PEIR lesion, which is commonly found in this location and which had “only yesterday” erupted into the mouth. zap.5a

Fig. 5a. Panoramic film of a male 18 year old patient with erupted third molars The maxillary right and mandibular left canines are both impacted with completed root apices. The mandibular canine has pre-eruptive intracoronary resorption. The apex of the tooth is located close to the lower border of the mandible and is hooked. Courtesy of Dr. Adam Renert.


Fig. 5b. In the cross-sectional view, the PEIR is clearly seen to infiltrate much of the coronal dentine, with the incisal tip port of entry.

Fig. 5c. a longitudinal cut of the same tooth shows its hooked root end and the adcanced resorption process within the crown.

In general, teeth affected by PEIR will erupt normally and the progress of the lesion will then be arrested, because the lesion will have been isolated from its nutritive life-line, which is the intra-follicular fluid. However, there is some anecdotal evidence that an advanced PEIR lesion may interfere with the eruptive process and, therefore, cause the last part of the root end to apexify in an abnormal manner (Fig. 5).


Perhaps the most dramatic example of this phenomenon is the dilacerate central incisor, which has been described in several earlier bulletins on this website.11-13 Initiated by very early trauma, we have seen how its root becomes tightly curved and its crown tip ends up located adjacent to the anterior nasal spine. We have also described how it can be brought into alignment with simple orthodontic mechanics.2, 3Although the final form of the root is the result of continued but deformed development, its PDL mediated attachment to the surrounding bone is normal, which is why dilacerate incisors teeth will respond to orthodontic forces as with any other tooth.


Ankylosis of an impacted tooth is often very casually determined, with a minimum of clinical information only. It is determined on the empirical basis that the tooth has not responded to extrusive orthodontic forces. In other words, it is an entirely outcome-oriented, fallback, designation. Because, without extracting the affected tooth, it cannot easily be confirmed or denied scientifically, the term has degenerated into a very convenient moniker to explain away orthodontic failure. Ankylosis is only one of many reasons why an impacted tooth does not respond to extrusive traction. We have reported on a study of a large sample of failed cases of impacted teeth and have found that invasive cervical root resorption (ICRR), pre-eruptive intracoronal resorption (PEIR), inappropriate direction of traction and poorly designed auxiliaries can all lead to failure to resolve the impaction.6, 7

So, there are two questions that need to be asked.

1. Is the presence of a hooked root associated with ankylosis? Certainly it could be, because ankylosis also prevents the eruption of the tooth and the root continues to grow in the reverse direction, as outlined above.

2. Is the presence of the hooked root end, per se, the impediment to eruption, as many authors and clinicians argue?

The upshot of the answers to these questions has produced an alternative approach to the treatment of cases in which there is a hooked root end, with or without ankylosis.

The method has been termed “apicotomy” and it involves the surgical removal of the hooked apex using a hammer and chisel.14, 15 The rationale is in the commonly held empirical belief that the hooked root end is indeed the reason the tooth has become impacted and, at the same time, it is claimed that the impact caused by the hammer and chisel may free the tooth from its ankylotic connection and thus enable subsequent traction to erupt the tooth. Dr. Puricelli, the originator of the method, claims that the technique “…. has been successfully used during the past twenty years, for conservative intervention in cases of impacted upper canines with dilaceration or apical root-ankylosis” and “….. the technique aims at freeing the tooth from its dilacerated or ankylosed portion inducing, thus, its traction and eruption”. To my knowledge, there are no English language published studies that provide evidence for the efficacy of apicotomy in this context.

Figures 1 & 3 in this bulletin are copyright and appear in Becker A. Orthodontic Treatment of Impacted Teeth. 3rd edition. Oxford: Wiley-Blackwell Publishers, 2012.


1. Becker A, Shochat S. Submergence of a deciduous tooth, its ramifications on the dentition and treatment of the resulting malocclusion. American Journal of Orthodontics 1982; 81:240-244.

2. Bulletin #42 on this website – March 2015 Root development in impacted teeth

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

4. Bulletin #32 on this website - April 2014 Rescuing teeth impacted in dentigerous cysts.

5. 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.

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

7. Chaushu S, Abramovitch Y, Becker A. Failure in the orthodontic treatment of impacted canines. Journal of the Israel Dental Association 2013,30:45-52.

8. 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.

9. Bulletin #20 on this website March 2013 Invasive cervical root resorption

10. Bulletin #24 on this website July 2013 Pre-eruptive intracoronal resorption

11. Bulletin #10 on this website - April 2012 The “Classic” dilacerate maxillary central incisor.

12. Bulletin #11 on this website May 2012 Treatment of the “Classic” dilacerate maxillary central incisor.

13. Becker A: Early treatment for impacted maxillary incisors. American Journal of Orthodontics and Dentofacial Orthopedics. 2002;121:586-7

14. Puricelli E . Apicotomy: a root apical fracture for surgical treatment of impacted upper canines Head Face Med. 2007; 3: 33.

15. Araujo EA, Araujo CV, Tanaka OM. Apicotomy: Surgical management of maxillary dilacerated or ankylosed canines. American Journal of Orthodontics & Dentofacial Orthopedics, 2013;144:909-915