Vol 4, No 1 (2016) ISSN 2167-8677 (online) DOI 10.5195/d3000.2016.49

Third Eruption Mechanisms and Patterns Winnie Zhang1

1 University of Pisburgh School of Dental Medicine, Pisburgh PA, USA

Abstract Citaon: Zhang, W. (2016) Third Molar Erupon Mechanisms and Paerns. Denstry 3000. Third molars are highly variable in their presence and form. This report focuses on 1:a001 doi:10.5195/d3000.2016.49 a horizontally impacted third molar and analyzes the potenal eology of this sit- Received: May 21, 2016 uaon. Upon a clinical and radiographic examinaon, it was noted that the paent Accepted: June 13, 2016 Published: September 28, 2016 had four third molars present. The paent’s third molars began erupng around Copyright: ©2016 Zhang, W. This is an open ac- the age of 19. Currently, they are asymptomac with incipient caries on the occlu- cess arcle licensed under a Creave Commons sal surfaces. While three of the third molars erupted in a normal orientaon, one Aribuon Work 4.0 United States License. of the third molars (mandibular le) erupted in an orientaon that would be clas- Email: wiz5@pi.edu sified as horizontal and/or mesio-angular. No treatment has been undertaken at the moment, however surgical extracon can be recommended with the progno- sis being very good. For the clinician that has to treat dental complicaons that arise from abnormal erupon, as seen in numerous genec and acquired disorders, knowledge about the basic molecular mechanisms involved may be- come extremely important.

Introduction tially break through/erupt through impacted lower wisdom teeth can the gum. Partial eruption of third be classified into [2]: Third molars, often re- molars provides an opening for ferred to as wisdom teeth, are the bacteria to enter around the 1. Soft Tissue Impaction. most distal (posterior) teeth of the tooth. This could potentially cause When the height of the tooth’s three molars in each quadrant of an infection, which results in pain, contour is above the level of the human dentition. They generally swelling, jaw stiffness, and other surrounding alveolar bone and the erupt between the ages of 17and complications. Partial eruption superficial portion of the tooth is 23. Ideally, the third molars should also makes the third molar more covered only by soft (though this erupt just like all the other teeth susceptible to caries and perio- can be dense and fibrous) tissue. and be properly aligned. However, dontal disease because they are in Soft tissue impaction is usually the often these third molars are misa- hard to reach areas, thus making it easiest type of impacted tooth to ligned which can on occasion lead hard to brush and floss those are- remove. to crowding of the teeth or dam- as [1]. age to adjacent teeth, and rarely 2. Hard Tissue ('Bony') Impac- to cystic pathology or root resorp- There are multiple ways to tion. This is where the wisdom tion [1]. classify impacted third molars. tooth fails to erupt due to being One way is based on the nature of obstructed by the overlying bone. Third molars can also be the overlying tissues. Based on the This can be sub-divided into (a) impacted which means they are nature of the overlying tissue im- partial and (b) complete bony im- enclosed within the soft tissue paction, pactions. and/or jawbone or they only par-

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Vol 4, No 1 (2016) DOI 10.5195/d3000.2016.49 a. Partial Bony. The superfi- ier to remove as the bone covering 7. Inverse (Figure 7): Orienta- cial portion of the tooth is covered the tooth is thinner whereas the tion of the impacted tooth in only by soft tissue but the height palatally positioned tooth requires completely inverted in reference of the tooth's contour is below the bone removal and hence makes to the occlusaly immediate oppo- level of the surrounding alveolar the extraction difficult [2]. site tooth. bone. Apart from cutting the gin- giva (gum) and possible bone re- 1. Mesio-Angular (Figure 1): Familial Aggregation moval from behind the tooth, the The impacted tooth is tilted to- ward the 2nd molar in a mesial Observations of a patient and fam- tooth's roots may need to be di- ily members: vided. direction. A 23 year-old Asian fe- b. Complete Bony. The tooth male with no significant medical is completely encased in bone so history has received regular that when the gingiva is cut and dental check-ups every six reflected back, the tooth is not months throughout her life and seen. Bone removal (large had orthodontic treatment from amounts) together with root sec- age 14 to16. The patient has tioning will be needed to remove class I occlusion. Upon a clinical the tooth. These are often the and radiographic examination, most difficult teeth to remove. it was discovered that the pa- Another way to classify tient had four third molars pre- impacted third molars is using sent (see Figure 8). The pa- 2. Disto-Angular (Figure 2): Winter’s Classification which is tient’s third molars began erupting The long axis of the 3rd molar is based on the inclination of the im- around the age of 19. Currently, angled distally or posteriorly away pacted third molar to the long axis they are asymptomatic with incip- from the second molar. of the second molar [2]. Each type ient discoloration on the occlusal surfaces. While three of the third of impaction has some definite 3. Horizontal (Figure 3): The molars erupted in a normal orien- path of withdrawal of the teeth. long axis of the third molar is hori- tation, one of the third molars Mesially impacted teeth are (can zontal. be) easier to remove whereas dis- (mandibular left) erupted in an tally impacted teeth are (can be) 4. Vertical (Figure 4): The long orientation that would be classi- the hardest to remove. Bucally axis of the third molar is par- positioned maxillary teeth are eas- allel to the long axis of the second molar.

5. Buccal / Lingual Obliquity (Figure 5): The tooth can be buccally (tilted towards the cheek) or lin- gually (tilted towards the tongue) impacted.

6. Transverse (Figure 6): fied as horizontal and/or mesio- This type is where the tooth is in angular. effect horizontally impacted but in the cheek or tongue direction.

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Vol 4, No 1 (2016) DOI 10.5195/d3000.2016.49

alveolus. The genes potentially play a role in regulating that are involved in the eruption [3]. This is an example of process of tooth erup- how a gene seemingly unrelated tion are found all over to tooth development can influ- the human genome. ence a very crucial step in denti- tion development. Therefore, it is Mononuclear cells (osteoclast precur- sors) must be recruited into the dental follicle prior to the onset of The patient’s father had eruption. These cells, in turn, fuse four erupted third molars, all with to form osteoclasts that resorb normal eruption orientation. The alveolar bone, forming an eruption patient’s mother has congenitally pathway for the tooth to exit its missing third molars. The patient bony crypt [3]. There are many has a younger sibling who is 19 different genes that are involved and third molars have not erupted in . Some of the but one of them is slightly mesially molecules possibly involved in the possible that a mutation anywhere tipped, and impacted (Figure 9). signaling cascades of eruption along the genome can easily influ- have been proposed in studies ence a tooth to develop and erupt If one considers third mo- from null mice, osteopetrotic ro- in an abnormal pattern such as lar impaction and agenesis as vari- dents, injections of putative erup- horizontal eruption. ation of the same clinical presen- tion molecules, and cultured den- tation, this family would fit an au- might also in- tal follicle cells. In particular, re- tosomal dominant mode of inher- fluence the process of eruption, cruitment of the mononuclear itance, suggesting a major gene the most important physiologic cells to the follicle may require effect. role likely being at the eruptive colony-stimulating factor-one site, in the formation of osteo- (CSF-1) and/or clasts through signaling via the monocyte chemo- RANKL/OPG pathway. If this sig- tactic protein-1 naling pathway were interrupted (MCP-1) [3]. If the by another protein, osteoblasts recruitment of function could potentially be al- mononuclear cells tered and interfered with. This to the follicle were could subsequently affect how misguided and the bone was deposited. If bone was dental follicle of a Mechanisms Controlling Tooth deposited only underneath the third molar were disoriented to Eruption distal portion of the third molar, it begin with, it could potentially re- could potentially cause the tooth sult in the tooth erupting in a hori- The etiology of the mecha- to tip over, giving it the horizontal zontal orientation. nisms behind eruption patterns of orientation it is in now. Evidence third molars is currently not very Paracrine signaling by par- thus far supports a role for an os- well understood. Tooth eruption is athyroid-hormone-related protein teoblast-specific transcription fac- a complex and tightly regulated and interleukin-1α, produced in tor, CBFA1 (RUNX2), in molecular process that involves cells of the the stellate reticulum adjacent to events that regulate tooth erup- tooth organ and the surrounding the follicle, has also been found to tion [3]. In the case presented

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Vol 4, No 1 (2016) DOI 10.5195/d3000.2016.49

genes influencing the final out- somal dominant mode of inher- come of a horizontally oriented itance (Figure 10). third molar, whereas the case presented here exemplifies one In a research study done of the instances a single gene on patterns of third-molar agene- form of inheritance is in play. sis and associated dental anoma- lies, it was found that permanent Hence, an alternative tooth agenesis, microdontia of hypothesis for the etiology of maxillary lateral , and total this case could be that the hori- dental anomalies are more fre- zontal impaction of the third quently associated with agenesis molar is a variation of tooth of all four third molars than with above, one possibility is that since agenesis. The mother of the their presence [4]. Horizontal im- neither of the patient’s parents patient described in this case ex- paction of the third molar could be had any third molar eruption ab- hibits agenesis of the third molars. categorized as a dental anomaly. normalities, the genetic factors The agenesis alleles could have This association could suggest a that caused this horizontal orien- been passed on to the patient, variable phenotypic expression of tation was probably a polygenic which then exhibited a variable the same genes or set of genes for manifestation that resulted from a phenotype of agenesis: horizontal tooth agenesis. unique combination of the par- impaction of the third molar. This ents’ genetic make-up. The pa- hypothesis is further supported by In addition, third molar tient’s mother had congenitally the fact that the patient’s sister agenesis seems to predispose for reduced size of the remaining teeth and the delayed develop- ment of certain teeth and has also been linked to diminished stability of specific molar cuspal patterns [5,6]. A relationship between tooth agenesis and abnormal morphology of remaining teeth has been observed within other types of agenesis. This goes to fur- ther support the fact that these dental anomalies are all interre- lated and thus likely to have a sim- ilar genetic pathway or mode of inheritance. It is very possible that third molar agenesis is a trait that missing third molars, so it is possi- also has a mesially tipped and im- shows varying phenotypes, such ble that some of the genes which pacted third molar, which could as the anomalies listed above and contribute to missing third molars likely be another variable pheno- perhaps horizontal impaction of also play a role in tooth eruption type of the tooth agenesis allele third molars. and influenced the eruption of the passed down from the mother. If horizontal third molars in her off- this were the case, a pedigree Management of the Condition spring. The etiology of third molar drawn of this particular case study The optimal treatment impaction in the population is would strongly suggest an auto- plan for this case would be extrac- most likely complex with multiple

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Discussion and Practical References Implications 1. Prevalence of impacted teeth While the mecha- and associated pathologies--a ra- nisms and etiology of third diographic study of the Hong Kong molar eruption are not well Chinese population. Chu FC, Li TK, understood currently, con- Lui VK, Newsome PR, Chow RL, ducting more research in Cheung LK. Hong Kong Med J. this aspect can yield bene- 2003 Jun;9(3):158-63. fits for both the field of PMID:12777649. dentistry as well as medi- cine as a whole. Skeletal 2. Mandibular third molar impac- biologists view the process tion: review of literature and a of tooth eruption as a valu- proposal of a classification. tion of the horizontal third molar. able model to study bone remod- Third molars often cause crowding Juodzbalys G, Daugela P. J Oral eling, since the emergence of a in the mouth and therefore are Maxillofac Res. 2013 Jul 1;4(2):e1. tooth into the oral cavity involves recommended for extraction. Indi- PMID:24422029. both coupled and uncoupled bone viduals with impaction showed turnover events [3]. For cell and 3. Cellular, molecular, and genetic more moderate to extreme molecular biologists, tooth erup- determinants of tooth eruption. crowding than those with agenesis tion provokes several questions Wise GE, Frazier-Bowers S, D'Sou- [7]. In addition, the horizontal concerning the tightly pro- za RN. Crit Rev Oral Biol Med. eruption of the mandibular left grammed series of signaling inter- 2002;13(4):323-34. third molar has caused the mesial actions between cells of the con- PMID:12191959. marginal ridge of the tooth to nective tissue sac surrounding the come in contact with the distal tooth (dental follicle) and the sur- 4. Patterns of third-molar agenesis cervical portion of the crown on rounding alveolus [3]. and associated dental anomalies the mandibular left second molar. in an orthodontic population. Ce- The orientation of the second and For a clinician that has to likoglu M, Bayram M, Nur M. Am J third left mandibular molars cre- treat dental complications that Orthod Dentofacial Orthop. 2011 ates a food trap that could poten- arise from abnormal tooth erup- Dec;140(6):856-60. tially lead to caries on the distal of tion, as seen in numerous genetic PMID:22133951. the second molar or periodontal and acquired dis- orders, 5. Third molar agenesis and size knowledge about reduction of the remaining teeth. the basic molecu- Garn SM, Lewis AB, Kerewsky RS. lar mechanisms Nature. 1963 Nov 2;200:488-9. involved is ex- PMID:14076752. tremely im- 6. Third molar agenesis and varia- portant. Knowing tion in size of the remaining teeth. the etiology can Garn SM, Lewis AB, Kerewsky RS. not only help the Nature. 1964 Feb 22;201:839. clinician formu- PMID:14161232. inflammation. Therefore, a possi- late a better treatment plan for ble treatment for this case is sur- the patient, but it can help den- 7. Third molar impaction and gical extraction of the third molar. tists better educate the patient in agenesis: Influence on anterior their dental health. crowding. Esan T, Schepartz LA.

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Ann Hum Biol. 2016 Feb 8:1-29. PMID:26856343.

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