THE MANDIBULAR THIRD MOLAR, A Method of Predicting its Eruption

Dr Oscar J. Quirós, Associate Professor at the University of Central Venezuela, Faculty of Dentistry.
Dr. Auristhela Palma, Orthodontist

Address of the authors: Apartado postal 47665-1041-A Caracas Venezuela

Email: quiroso@camelot.rect.ucv.ve

ojquirosa@etheron.net

Abstract. One of the problems in Orthodontics and Oral Surgery is determining when to extract third molars or when it is advisable to let them complete their eruption. This study attempts to establish a prediction method using panoramic radiographs to help make that decision.

Key words. Third molar, molar extraction, molar eruption prediction method, unerupted molars.

INTRODUCTION.

It has been a matter of concern to the dental profession to predict when the mandibular third molar will erupt into proper occlusion.^{1,2,3,4,5,6,7,8,9} The factors that must be taken into account are: a) patient uncertainty about having the procedure and; b) the tales and myths people believe about third molar eruption.

Between 9% and 20% of humans do not develop mandibular third molars as an inherited feature. This lack of inheritance is more frequent in men than women.

The mandibular third molar exhibits the highest rate of impaction. The rates, as reported by author:^{13, 14, 15, 16.}

• Hellman 9.5 %
• Bjork 25 %
• Ricketts 50 %
• Richardson 35 %

The etiology of mandibular third molar impaction has many sources which depend upon:

• Multifactorial elements
• Genetic factors
• Lack of space
• Retarded growth process
• Growth direction
• Eruption direction
• Influence of the external oblique line and buccinator

In an attempt to predict the probability of third molar eruption, many studies have been done; most of them using dissected skulls or lateral cephalic radiographs. The lateral cephalic radiograph is an excellent aid in performing cephalometric diagnoses. Because of image superimposition, it is not very precise when trying to measure the third molar's position and its possibility of eruption. After having conducted a careful study and bibliographic review, it was decided to make the prediction by using panoramic radiographs. This allowed one to visualize both sides of the dental arch and measure the structures with the least amount of superimposition.

REVIEW OF THE LITERATURE.

Many authors talk about the eruption of the mandibular third molar, Hellman (1938), Bjork (1956) and Broadbent (1943) which are quoted by Olmos and Beltran¹¹ assert, "the impaction of the mandibular third molars is associated with an insufficient growth of the mandible.

Henry and Morant, who are quoted by Kaplan, (1975)¹², propose that the impaction of the third molar could be predicted by establishing an index of molar space, which could be measured by the mesio-distal width of the third molar and the percentage of space between the anterior edge of the ramus and the second molar.

Bjork (1956)¹³ measured, by using cephalometric radiographs, the distance which separated the anterior edge of the ramus and the distal surface of the second molar and suggested that the probability of impaction decreases as the distance increases.

More Possibility	Less possibility

Rickets (1972)¹⁴ supports Bjork's observation. He evaluated approximately 100 skulls and felt that a successful eruption could be directly correlated with the portion of the third molar that goes beyond the anterior edge of the ramus,. If half of the third molar lies behind the ramus, the possibility of eruption is 50%.

Turley (1974) quoted by Schulhof (1976)¹⁵ evaluated several methods of measuring the available space. He concluded that the most useful was the distance from "Xi", (center of the ramus) to the distal face of the second molar. The average distances proposed by Turley were 21 mm. for the impacted molars, 25 mm, for the erupted molars but out of position and 30 mm. for molars in occlusion. He thinks that the prediction could be performed from the age 8 or 9 years of age with 90% precision. According to Turley in 1974, this was corroborated by Rickets, 1976.

According to Turley in 1971, corroborated by Rickets 1976

Rickets (1976)¹⁶ measured the distance from "Xi" to the distal surface that corresponds to the second molar on the occlusal plane, seen in a lateral cephalometric radiograph. We infer, taking this information into account, that a distance of 30 mm is enough for the eruption of the third molar to occur, and a distance of 20 mm, or less, is not enough. This reveals the occurrence of an error of 2.8 mm. He thinks that the predictions could be made from the age of 8 or 9 years old.

Richardson (1974)¹⁷, reported that the small values that could be appreciated, "in the initial mesial inclination angles" helps the eruption process. He thinks that most of the impacted third molars have been straightening out to some extent and that the degrees of their angles have been augmented.

Olive and Basford (1981)¹⁸. In a lateral cephalometric radiograph, the oclusal plane and two perpendicular tangents to the distal face of the second molar and the external edge of the ramus are traced and the mesio-distal width of the third molar is measured. If this is the same or lower than the available space, the eruption possibilities are good, when it is not, impaction is likely.

Good possibility, if by dividing the value of AB/CD the answer is equal or greater than 1. Less possibility if by dividing the value of AB/CD the answer is less than 1.

Limitations:

All the methods previously described are based on measurements made on dissected skulls or on lateral cephalometric radiographs, which can be more difficult to read. Also, the operator may get confused by the superimposition of images.

PREDICTING ERUPTION.

This study proposes a new method that is based on the results of previous studies by applying them to the panoramic radiograph. To establish an adequate sample to form the prediction, panoramic and lateral cephalometric radiographs of 300 patients were measured.

MATERIALS AND METHODS

To carry out this study 300 patients of both sexes were evaluated. 157 males and 143 females in the age range of 12 to 30 years were selected. All of them had third molars present, either erupted or unerupted and complete dentitions. The use of children less than 12 years old was ruled out since at these early ages it is difficult to measure by using the forming tooth bud, especially in lateral cephalometric radiographs with the superimposition of the images. All the patients belonging to the sample were judged to be a skeletal Class I. They were measured using both panoramic and lateral cephalic radiographs.

Measured on the cephalometric radiographs were:

• The mesio-distal width of the third molar
• The distance from the ramus to the distal surface of the second molar
• The distance from "Xi", or the geographic center of the ramus, to the distal surface of the second molar
• The inclination of the third molar to its apical base

Measured on the panoramic radiographs were:

• The mesio-distal width of the third molar
• The distance from the ramus to the distal surface of second molar
• The distance from "Xi", to the distal surface of the second molar
• The third molar inclination to the apical base

RESULTS.

After having measured the radiographs, the outcomes were analyzed and compared.

• Measurements were performed separately by two different operators and then, the outcomes were compared. In those cases in which there were discrepancies over a given measurement, the measurement was repeated.
• The measurements were written down on a form designed for that purpose. (see attached information).
• It was possible to confirm in 23 patients the predictions both before and after the third molars completed radicular formation.
• Four of the 23 patients were under orthodontic treatment leading to a variation in the final position in the measured position of the second molars. 
• All the radiographs were taken using a Panex panoramic machine from J.J. Morita.

The average third molar width, measured in the on the cephalomatric radiographs, was 12.9 mm, while in the panoramic radiographs it was 15.7 on the right side and 15.8 on the left side. The distance to the anterior edge of the ramus to the distal surface of the second molar in the cephalometric films ranged from 0 to 12 mm., while in the panoramic radiographs, the range was from 2 to 17 mm on the right side and 1 to 14 mm on the left side.

The distance from the center of the mandibular ramus, (Xi) to the distal surface of the second molar ranged from 13 mm to 30 mm in the cephalometric films and in the panoramic firms from 15 to 37 mm on the right side and 18 to 39 mm on the left side.

• Those molars whose angles to their apical base was less than 35 degrees could not erupt properly.

• Among the 23 patients that were followed until the formation of their third molar roots, only 8 were able to obtain complete eruption with good position in the arch, 5 partially erupted, and the rest of them were impacted. The impacted cases were subject to surgery.

• Among the 8 patients whose molars erupted satisfactorily and in good position, two presented light anterior crowding, 3 of them were orthodontically treated and only three were naturally free of crowding.
• Among the 5 subjects whose molars partially erupted or were not aligned, 3 presented light to moderate crowding. One of them was treated orthodontically.
• Among the 10 subjects whose molars didn’t erupt, 7 presented crowding, from light to moderate at the anteriors, and one of them presented with severe crowding
• Molars which demonstrated complete eruption were inside the following parameters as they were seen on the original panoramic radiographs (the distance between the anterior edge of the ramus to distal surface of the second molar ranged between 14 and 17 mm., and the distance from Xi to the distal of the second molar ranged from 35 to 39 mm.
• Molars whose eruption was partial to lightly out of alignment had a distance from the anterior edge of the ramus ranging between 11 and 14 mm., and from 31 to 36 mm from Xi.
• In the 23 patients that were followed until completion of radicular development, those molars that erupted successfully measured an angle greater that 38 degrees to their apical plane (on original x-rays).

CONCLUSIONS.

• Reviewing the outcomes generated by this study and comparing them to those obtained by other methods, it could be said that it is easier to obtain the measurements by using panoramic radiographs, since they have less superimposition of tissues than lateral cephalometric radiographs.

• When measurements are taken from a panoramic radiograph to determine the probability of successful third molar eruption the clinician must remember the following:

• The distance from the distal of the second molar to the edge of the ramus should be greater than 14 mm.
  
• The distance from Xi to distal of the second molar should be greater than or equal to 35 mm

.

• The third molar angle should be greater than 40 degrees, as the angle decreases the chances for impaction increases. The orthodontic treatment could increase the probability of eruption when extraction mechanics are performed. Mesial movement of the first and second molars may serve to increase the distance from the distal of the second molar to the edge of the ramus and Xi point. It is not advisable to attempt prediction in patients which have not reached the age of 13 or 14 years because of the difficulty in precisely measuring the distances and apical base angles. The measurements tend to be too subjective.

References:

1. Banks,H.V. Incidence of third molar development, Angle Orthod. : 223-233 1934.
2. Trepp,W. The effect of wisdom teeth (third molars) on the bottom teeth. 1997. URL: www.orthodontics.org/wisdom.htm
3. Mills.W. wisdom teeth. 1997. URL: www.drmills.com/wisdom.htm
4. Tye,C.Wisdom teeth/surgical removal of teeth. 1997. URL: www.crhistophertye.gte.net/teeth.html
5. Dwoskin,M.L. Impacted wisdom teeth. May 1995. URL: www.drdwoskin.com/~world/mld/wisdom.html
6. Butzin,K. Wisdom teeth. 1996. URL: www.butzin.com/wisdom.htm
7. Connecticut Maxillofacial Surgeos,Ilc.home page.Topic: wisdom teeth. 1996. URL: www.cmsllc.com/topics.html
8. Falenders H. Wisdom teeth 1996. URL: www.jawfixed.com/falwt.htm
9. Cleaton-Jones,P. Do those wisdom teeth really need to come out? June 1997. URL: www.mrc.ac.za/pressrel/press16.htm
10. Bishara,S and Andreasen,G. Third molars a review. Am.J. Orthod. Vol 83 Nº 2, 1983
11. Olmos,V., Beltrán,C. estudio sobre la impactación del tercer molar mandibular.Oris Nº 5 Enero - Marzo 1995.
12. Kaplan,Ross G. Mandibular third molars and postretention crowding. Am.J.Orthod. Oct.1974
13. Björk A., Jensen, E. and Palling, M. Mandibular growth and third molar impaction. Acta. Odont. Scand. Vol.14: 231-272, 1956.
14. Ricketts, R. A principle of arcial growth of the mandible. Angle Orthodont. Vol. 42:368-386. 1972
15. Schulhof, R.J.: Third molars and orthodontic diagnosis. J.Clin.Orthodont. 10:273, 1976.
16. Ricketts, R.M., : Third molar enucleation: Diagnosis and technique, J.California Dent. Assoc. 4: 52-57, 1976
17. Richardson, M. Some aspects of lower third molar eruption. Angle Orthodont. Vol 44: 141-145 1974.
18. Richardson, M. The etiology and prediction of mandibular third molar impaction. Angle Orthodont. Vol. 47 Nº3 165-172, 1977
19. Olive, R., and Basford, K.: Reliability and validity of lower third molar space-assessment thecniques, Am.J. Orthod.79: 45-53, 1981