Changes in lower third molar position in the young adult Margaret Richardson, MDentSc, D. Orth. Belfast, Northern Ire~and Changes in position of unerupted lower third molars between 18 and 21 years were examined in a group of 17 men and 24 women with intact lower arches. Mesiodistal third molar angulation, molar space, molar space condition, and buccolingual third molar angulation and the changes in these dimensions were measured on 60 ~ cephalograms taken at 18 and 21 years. At 18 years molar space was inadequate by an average of 5.0 mm. Average changes in dimensions were statistically nonsignificant except for an increase in molar space of 0.7 mm. Changes in third molar position ranged from 39 ~ to - 4 6 ~ in the mesiodistal and from 24 ~ to - 2 4 ~ in the buccolingual dimension. Only 10 third molars did not change their mesiodistal angulation. Four third molars erupted fully during the observation period. (AMJ ORTHOD DENTOFAC ORTHOP 1992;102:320-7.)
Lower third molar eruption is an unpredictable event. Hellman' quoted a mean age for eruption as 20.5 years. Haralabakis 2 found a higher mean of 24 years. In African races they may erupt as early as 13 years, and by the age o f 20 years, the majority have already erupted2 '4 Fanning 5 found median ages of 19.8 years in men and 20.4 years in women. Earlier eruption is promoted by extraction o f teeth in the corresponding buccal segment s particularly a molar. 6 The criteria for eruption in some of these studies 35 was the emergence of any part of the crown through the gingival tissues. In the case o f the third molar, this will give misleading results since many o f them do not continue to erupt but remain impacted in a partially erupted position. A mean eruption age with such a wide range is of little clinical significance in an individual subject where the more important issue is whether the third molar is likely to erupt or whether it will remain impacted. The incidence of lower third molar impaction varies considerably in different parts of the world ranging from 9.5% to 39%, ''2'79 and in orthodontic patients up to 50%. ~~ This may be due to genuine population differences, as well as to differences in the definition o f impaction and age at diagnosis. Shiller t~ examined 223 young naval recruits between the ages o f 18 and 21 years in whom 296 lower third molars were diagnosed as mesioangularly impacted. O f these 39% attained a more upright position in a l - y e a r observation period, whereas 12% increased their mesial angulation. Von Wowern and Nielsen'-" and Sewerin and von 811131134
320
W o w e m " also found changes in third molar position in a group of 34 dental students between the ages o f 20 and 24 years. The purpose of the present investigation was to examine and quantify positional changes in lower third molars in a young adult sample.
MATERIAL AND METHOD One hundred and forty 18-year-old students entering dental school were examined. Only those with intact lower arches, 34 men and 32 women, were recorded at the first examination and 3 years later at the beginning of the final year of dental studies. Records included plaster models and cephalometric radiographs taken in the 90 ~ left lateral and 60 ~ left and right lateral positions." Twenty lower third molars (11 in men, 9 in women) were congenitally absent. At age 18 years, 35 lower third molars (26 in men, 9 in women) were erupted. The remaining 77 lower third molars were uncrupted at 18 years. Fourteen men and 22 women had both lower third molars unerupted, and in three men and two women lower third molars were unerupted unilaterally at age 18 years. In the present investigation this unerupted lower third molar group was analyzed with 60 ~ right and left lateral cephalometric radiographs. Follow-up radiographs were not available for I i third molars. At age 18 years, 16 (21%) of the unerupted third molars were diagnosed as vertical, 55 (71%) as mesioangular, and 6 (8%) as horizontal. Seventeen third molars in nine subjects (three men and five women bilaterally and one woman unilaterally) were extracted during the observation period after recurrent episodes of pericoronitis. The stage of development of third molars at age 18 years was classified according to an adaptation of the system devised by Gleiser and HunP 5 for first molars. Stage I. Crypt formation with no calcification.
Volume102
Changes in lower third molar position in young aduhs 321
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Fig. 1. Measurement of mesiodistal lower third molar angulation and molar space and changes in these dimensions between 18 and 21 years. Solidlines, 18 years. Broken fines, 21 years.
Stage Stage Stage Stage Stage Stage
2. 3. 4. 5. 6. 7.
Calcification of cusps. Calcification of half crown. Calcification of full crown. Root formation commenced. Root formation complete. Apices closed.
Numbers 35 m
Males
~
Females
30 25 20
MEASUREMENTS
Mesiodistal angtdation of the lower third molar was measured as the angle formed between the occlusal surface of the third molar and a maxillary horizontal (ANS-PNS) (Fig. 1). Molar space was measured along the maxillary horizontal as the distance between projections of the distal contact point of the lower first molar and the junction of the anterior border of the ramus with the body of the mandible (Fig. I). The size of the second and third molars was measured directly on the 60 ~ cephalograms at their maximum mesiodistal widths. Molar space condition was calculated as the difference between molar space and the sum of second and third molar widths. Bttccolhzgttal attgulation of lower third molars was measured as the linear distance between buccal and lingual cusps on the radiographs. Where these cusps were superimposed, the tooth was upright in the buccolingual direction. The buccolingual dimension of a lower third molar crown is about 8 to 9 mm, so that a difference of 1 mm between buccal and lingual cusps represents approximately I0 ~ to 12" of angulation. The buccal cusps are usually uppermost indicating lingual inclination.
Change in mesiodistal atzgulation of lower third molars was measured as the difference in this angle when a tracing of the first film was superimposed on
15 10 5 0
3
4
5
8
Stage of development Fig. 2. Distribution of stage of lower third molar development at 18 years.
the second registering on internal mandibular structures (inner outline o f mandibular symphysis and inferior dental canal, Fig. 1). Change hz molar space was measured along the maxillary horizontal when a tracing of the first film was superimposed on the second film, as before (Fig. 1). Change in buccolingual angulation was calculated as the difference in the distance between buccal and lingual cusps on the first and second films. It was not possible to measure the size and position of two third molars on the 18-year-old films because of poor definition. All measurements were made twice by one observer. If the difference in replicate measurements exceeded 0.5 mm or 0.5 ~ a third reading was made, and the aberrant one discarded. Adjustments for radiographic enlargement were
322
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Am. J. Orthod. Dentofac. Orthop. October 1992
Table I. Means standard deviations and sex differences for all the variables All subjects
Mesiodistal angle 8 (degrees) Molar space (mm) Size 7 + 8 (mm) Molar space condition (mm) Buccolingual angle 8 (mm)
Left Right Left Right Left Right Left Right Left Right
Men
n
Mean
SD
N
Mean
40 35 40 35 40 35 40 35 40 35
41.93 43.69 18.73 18.86 23.60 23.20 -4.86 -4.39 1.46 1.36
18.23 21.80 2.23 2.11 1.43 1.30 2.26 2.46 0.98 0.95
17 13 17 13 17 13 17 13 17 13
42.03 41.12 19.68 19.73 24.26 23.69 -4.59 - 3.96 1.41 1.62
[
SD 24.02 28.46 1.89 2.13 1.40 1.07 2.38 2.64 0.97 1.04
Table II. Means standard deviations and sex differences for the changes in mesiodistal angulation of third molars, molar space, and buccolingual angulation of third molars
Change in mesiodistal angle 8 (degrees) Change in molar space (mm) Change in buccolingual angle 8 (ram)
Left Right Left Right Left Right
All subjects
lffet,
NI 'eo,,ISO
NI 'eo,,ISO
27 22 35 31 27 22
11 8 14 11 11 8
3.13 5. I 1 0.66*** 0.73*** 0.22 0.20
15.88 13.87 0.76 0.95 0.86 0.50
3.86 5.06 0.54* 0.50* 0.36 0.13
21.99 10.10 0.75 0.67 0.74 0.58
*Denotes significance P < 0.05. **Denotes significance P < 0.01. ***Denotes significance P < 0.001.
considered unnecessary since measurements were used only for comparative purposes. Means and standard deviations were calculated for each measurement and change in measurement. Sex differences were tested with independent sample t tests. The changes in mesiodistal third molar angulation, molar space, and buccolingual third molar angulation were tested for significance with one sample Student's t tests. RESULTS
The stage of third molar development at age 18 years is shown in Fig. 2. The majority were in stage 6 with root formation complete, but with apices still open. Eleven, all in men, were at stage 7 with apices closed. Ten, all in women, were at stage 5 with root formation commenced. Three in women and 1 in a man, were at stage 4 with crown formation complete and 1, in a woman was at stage 3 with only half the crown formed. At age 18 years there was a wide variation in mesiodistal angulation of the lower third molar to the max-
illary plane ranging from 9 ~ to 90 ~ averaging 41.9 ~ on the left and 43.7 ~ on the right, with no significant sex difference (Table I). Molar space at age 18 years averaged 18.7 mm on the left and 18.9 mm on the right. There was slightly more space in men than in women, but the difference was not significant (Table I). The average combined size of lower second and third molars was 23.6 mm on the left and 23.2 m m on the right. They were slightly larger in men than in women, but the difference was not significant (Table I). Molar space condition at age 18 years averaged - 4 . 9 m m on the left and - 4 . 4 mm on the right indicating shortage of space for third molars. Women had slightly more molar crowding than men, but the difference was not significant (Table I). The buccolingual angulation of the lower third molars at age 18 years was, on average, 1.5 mm on the left and 1.4 mm on the right. There was no significant sex difference (Table I). The average change in mesiodistal angulation of the
Changes in lower third molar position in young adults 323
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Number 4
Numbers 10
Women
Difference [
Malea
~
Females
men-wome/z
N
Mean
SD
mean
23 22 23 22 23 22 23 22 23 22
41.85 45.21 18.02 18.34 23. I i 22.93 - 5.09 - 4.64 1.50 1.21
13.03 17.33 2.23 1.97 i .26 1.37 2.20 2.37 1.00 0.88
0.18 - 4.09 i .66 1.39 I. 15 0.76 - 0.50 - 0.68 - 0.09 0.41
50/40 40/30 30120 20110 1 0 / 0
0
Negative
0/10 10/20 20/30 30140
Positive
'(Degrees)
Fig. 3. Distribution of changes in mesiodistal third molar angulation between 18 and 21 years.
Women
N 16 14 21 20 16 14
I
Mean 2.63 5.14 0.74*** 0.85** 0.13 0.25
[
SD
Difference inell-WOmelZ irleoll
10.67 16.00 0.77 1.07 0.94 0.47
1.23 -0.08 - 0.20 -0.35 0.23 - 0.12
third molar during the 3-year observation period was small and nonsignificant, 3.0 ~ on the left and 5.0 ~ on the right, with no significant sex difference (Table II). The distribution of the changes shows that large changes did, in fact, occur ranging from 39.0 ~ to - 4 6 . 0 ~ (Fig. 3). There was a significant average increase in molar space of 0.7 mm on each side. The increase was slightly larger in women than in men, but the difference was not significant (Table II). There was no significant change in buccolingua! angulation of lower third molars during the observation period (Table II). DISCUSSION
The average age for lower third molar crypt formation is quoted as 7 years by Banks 16and Garnet a l . j7 Graveley j8 found the peak formation period at 9 years. After a gestation period of 9 or more years, it is to be expected that at age 18 years the majority of lower third molars would be in the later stages of development, with root formation complete or the apices closed. In
view of the wide variation in timing of third molar genesis, ~9 it is not surprising to find some lower third molars still in earlier stages of development at age 18 years. The variation in mesiodistal angulation of lower third molars at age 18 years ranging from 9 ~ to 99 ~ reflected the diagnosis on radiographs of 21% of the lower third molars in a more or less vertical position, 8% in a horizontal position, and the remaining 71% in varying degrees of mesioangular relationship to the adjacent second molar. In all cases the molar space at age 18 years was inadequate to accommodate second and third molars in good alignment. The discrepancy ranged from 0.5 mm to 11.5 mm with the majority of third molars short of room by between 3 and 6 mm. In general teeth tend to erupt before their root formation is complete. The large number of lower third molars in this study remaining unerupted at age 18 years despite advanced root formation is probably largely because of lack of space and their mesioangular and horizontal positions. There is a strong tendency for lower third molars to become more upright during the course of their development, 2~ and if space is available this should lead to their eruption. It is likely therefore that the lack of space in the molar region may also be, at least partly, responsible for the mesioangular position of many of the third molars. On average the buccolingual third molar angulation at age 18 years was about 1.5 mm indicating a lingual inclination of about 18~ The range was from 0.0 mm to 4.0 mm. Only 20% of the third molars were upright in the buccolingual direction, and 30% had lingual inclinations of more than 2 mm, that is from 20 ~ to 50 ~ The change in mesiodistal lower third molar an-
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Richardson
Am. J. Orthod. Dentofac. Orthop. October 1992
Fig. 4. Lower right third molar.with positive change in mesiodistal angulation of 39~ between 18 (left) and 21 (right) yearschanging its position from mesioangular to distoangular impaction.
Fig. 5. Lower left third molar with negative change in mesiodistal angulation of 46~ between 18 (left) and 21 (right) yearschanging its position from mesioangular to horizontal impaction.
gulation during the 3-year observation period was small on average, but the histograms of their distributiont.(F.ig. 3) show that large changes did, in fact, occur. Only 10 third molars (20%) did not change their angulation. Twenty-five (51%) had a positive change resulting in
an uprighting movement. Fourteen (29%) had a negative change that caused the third molar to become more mesioangularly inclined. Fig. 4 shows a third molar that uprighted by 39 ~ changing the position of the tooth from mesioangular to one of distoangular impaction.
Changes ht lower third molar position in young aduhs
Volume 102 Number 4
325
Fig. 6. Lower left third molar that uprighted buccolingually by approximately 24~ between 18 (left) and 21 (right) years.
Fig. 5 shows a third molar that became more mesially inclined by 46 ~ that resulted in horizontal impaction. The average increase in molar space, although statistically significant, was small, 0.5 mm in men and slightly more in women. The range of increase was from zero to 3.5 ram. In only eight quadrants did the increase exceed 2.0 mm. Since molar crowding at age 18 years was, on average, between 4.0 and 5.0 mm, an increase in space of 0.5 mm would have little effect in providing room for third molar eruption except in those few cases where molar crowding was minimal or the increase in space was greater than average. Between the ages of 13 and 18 years an average increase in molar space of 4.0 mm was found in 51 subjects, z5 On average 2.0 mm of this increase occurred anteriorly and 2.0 mm posteriorly. The anterior increase was due to forward movement of the buccal segment, and the posterior increase was due to resorption of bone in the region of the alveolar shelf at the back of the dental arch. In a previous investigation made on the material used for the present study, it was found that there was, on average, no significant forward movement of buccal teeth, ~ so that the increase in molar space is probably due to resorption of bone at the back of the dental arch except in those few cases where there was some mesial drift. On average the buccolingual third molar angulation did not change significantly during the observation period. Ten third molars became more upright in the buc-
colinguaI direction by 1.0 or 2.0 mm (I0 ~ to 24 ~ approximately). Four third molars tipped lingually by similar amounts. Fig. 6 shows a third molar that uprighted buccolingually by approximately 20 ~ to 24 ~ and Fig. 7 one that increased its lingual inclination by the same amount. Of the original 77 third molars, 17 were extracted during the observation period. It is not known exactly what angular changes were experienced by these, but they changed their position sufficiently to become partially erupted and subject to pericoronitis. Of the third molars that became more upright during the observation period, only three, in two women, erupted to full occlusion. One of these had adequate molar space bilaterally at age 18 years, the other had a 2 mm increase in molar space as a result of the mesial drift of buccal teeth. Another five, in two men and one women, became partially erupted, two of these were subsequently removed because of pericoronitis, and the other three remained partly covered by gum 1 year later. These findings are consistent with the degree of molar crowding at age 18 years and the small increase in molar space. It is apparent that, despite lack of space and advanced root formation, some lower third molars undergo considerable positional changes in both the mesiodistal and buccolingual directions after the age of 18 years. Similar changes in mesiodistal angulation of third molars during the earlier stages of their development
:326
Richardson
Am. J. Orthod. Dentofiw. Orthop. October 1992
Fig. 7. Lower right third molar that became more lingually inclined by approximately 24 ~ between 18 (left) and 21 (right) years.
from 1 1 to 18 years have been described by Richardson. 24 The majority tended to become more upright, but about 12% of all third molars might be expected to become more mesially inclined during this period. Shiller H reported uprighting movement of third molars in a l-year period in men between the ages of 18 and 21 years. In his study 12% of third molars became more mesially inclined. Von Wowern and Nielsen ~2and Sewerin and von Wowem t3 also reported significant changes in mesiodistal third molar angulation in both a positive and a negative direction between 20 and 24 years. Molar space condition was not considered in these studies. H'~3 Positive and negative changes in the buccolingual direction between 1 1 and 19 years have been reported by Svendsen et al. 23 with posteroanterior radiographs. Shiller t' pointed out that, although many third molars became completely upright radiographically, he could not tell if they erupted clinically. In the Danish study ~2"~325 third molars (19% of the original number) completely erupted to the occlusal level between the ages of 20 to 24 years. In the Belfast sample only three third molars (4%) became fully erupted during the observation period. This group was too small to make any meaningful evaluation of the reasons for successful third molar eruption. The fact that adequate molar space was available or become available during the observation period suggests that favorable molar space condition is an important prerequisite for third molar eruption. The pos-
sibility still exists for more third molars to erupt in coming years. It is, however, unlikely that molar space will increase much after the age of 21 years. In view of the degree of molar crowding at 18 years, it is improbable that very many more third molars will erupt in the future. No satisfactory explanation for the third molar that increases its mesial inclination has yet been found. One theory implicates the angle of contact of the third molar with the second molar producing the effect of an inclined plane. '~ This seems unlikely. Third molars can change their angulation before contact with the second molar is established, and some in apparently similar angular relationships with second molars continue their development in opposite directions (Figs. 4 and 5). This can sometimes happen on contralateral sides of the same person. A theory of differential root growth, the mesial root dominating when the third molar uprights, and distal root dominance causing increased mesial inclination has been advanced. 2~This could not be confirmed by Richardson and co-workers. 29 Studies in the transverse dimension 23'29-3~ suggest that the buccolingual position of a third molar may be important in its developmental pattern. The more buccally placed teeth may be influenced by the dense bone of the internal oblique ridge or the pterygomandibular raphe or by the attachment of the buccinator muscle. 23.-'9 Further research is needed on this aspect of lower third molar development.
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Number 4
Changes in lower third molar position in young adults
CONCLUSIONS 1. In this material 3 1 % o f the third molars present had erupted at age 18 years. 2. B e t w e e n 18 and 21 years m a n y o f the unerupted third molars c h a n g e d their position appreciably despite a d v a n c e d root d e v e l o p m e n t . 3. In o n l y a v e r y f e w cases did the positional changes result in clinical eruption because o f inadequate space. 4. It is unlikely that m a n y m o r e l o w e r third molars will erupt in the future because o f the d e g r e e o f m o l a r c r o w d i n g in this material. I am grateful to Professor Andrew Richardson, Miss Jennifer Maltman, Mrs. Marlene Boe, and Mrs. Sally Meehan for the figures and the tables, and to the students for their cooperation.
REFERENCES 1. Hellman M. Our third molar teeth, their eruption presence and absence. Dent Cosmos 1936;78:750-62. 2. Haralabakis H. Observations on the time of eruption, congenital absence and impaction of the third molars. Trans Eur Orthod Soc 1957:308-9. 3. Chagula WK. The age at eruption of third permanent molars in male East Africans. Am J Phys Anthropol 1960;18:77-82. 4. Odusanya SA, Abayomi IO. Third molar eruption among rural Nigefians. Oral Surg Oral Med Oral Pathol 1991;71:151-4. 5. Fanning EA. Third molar emergence in Bostonians. Am J Phys Anthrop 1962;20:339-45. 6. Richardson ME. Some aspects of lower third molar eruption. Angle Orthod 1974;44:141-5. 7. Bj/Srk A, Jensen E, Palling M. Mandibular growth and third molar impaction. Trans Europ Orthod Soc 1956:164-97. 8. Dachi SF, Howell FV. A survey of 3874 routine full mouth radiographs. II. A study of impacted teeth. Oral Surg Oral Med Oral Pathol 1961;14:!165-9. 9. Aitasalo K, Lehtinen R, Oksala E. An orthopantomographie study of prevalence of impacted teeth. Int J Oral Surg 1972;1:117-20. 10. Ricketts RM. A principle of areial growth of the mandible. Angle Orthod 1972;42:368-86. I 1. Shiller WR. Positional changes in mesio-angular impacted third molars during a year. J Am Dent Assoc 1979;99:460-4. 12. von Wowern N, Nielsen HO. The fate of impacted lower third molars after the age of 20. lnt J Oral Maxillofae Surg 1989;18:277-80. 13. Sewerin I, yon Wowern N. A radiographic four-year follow-up
14.
15.
16. 17. 18. 19. 20.
21. 22. 23.
24. 25. 26. 27. 28.
29.
30.
327
study of asymptomatic mandibular third molars in young adults. Int Dent J 1990;40:24-30. Richardson ME. The early developmental position of the lower third molar relative to certain jaw dimensions. Angle Orthod 1970;40:226-30. Gleiser I, Hunt EE. The permanent mandibular first molar: its calcification, eruption and decay. Am J Phys Anthropol 1955; 13:253-83. Banks ttV. Incidence of third molar development. Angle Orthod 1934;4:223-33. Gain SM, Lewis AB, Bonne B. Third molar formation and its developmental course. Angle Orthod 1962;32:270-9. Gravely JF. A radiographic survey of third molar development. Br D J 1965;I 19:397-401. Richardson ME. Late third molar genesis: its significance in orthodontic treatment. Angle Orthod 1980;50:121-8. Bj6rk A, Skeiller V. Facial development and tooth eruption. An implant study at the age of puberty. AM J ORTItOD1972;62:33983. Richardson ME. Development of the lower third molar from 10 to 15 years. Angle Orthod 1973;43:191-3. Richardson ME. Some aspects of lower third molar eruption. Angle Orthod 1974;44:141-5. Svendsen H, Malmskov O, Bj6rk A. Prediction of lower third molar impaction from the frontal cephalometric projection. Eur J Orthod 1985;7:1-16. Richardson ME. The development of third molar impaction and its prevention, lut J Oral Surg 1981;10:Suppl 1:122-30. Richardson ME. Lower third molar space. Angle Orthod 1987;57:155-61. Richardson ME. Lower arch crowding in the young adult. AM J ORTIIODDENTOFACORTItOP 1992 [In press]. Henry CB, Morant GM. A preliminary study of the mandibular third molar tooth in man. Biometrika t936;28:378-427. Silling G. Development and eruption of the mandibular third molar and its response to orthodontic therapy. Angle Orthod 1973;43:271-8. Richardson ER, Malhotra SK, Semenya K. Longitudinal study of three views of mandibular third molar eruption in males. AM J OR'ntOD 1984;86:119-29. Olive RJ, Basford KE. Transverse dento-skeletal relationship and third molar impaction. Angle Orthod 1981;51:41-7.
Reprhlt requests to: Dr. Margaret Richardson Orthodontic Department School of Dentistry Royal Victoria Hospital Grosvenor Rd. Belfast BTI2 6BA Northern Ireland
Lower third molar eruption is an unpredictable event. Hellman' quoted a mean age for eruption as 20.5 years. Haralabakis 2 found a higher mean of 24 years. In African races they may erupt as early as 13 years, and by the age o f 20 years, the majority have already erupted2 '4 Fanning 5 found median ages of 19.8 years in men and 20.4 years in women. Earlier eruption is promoted by extraction o f teeth in the corresponding buccal segment s particularly a molar. 6 The criteria for eruption in some of these studies 35 was the emergence of any part of the crown through the gingival tissues. In the case o f the third molar, this will give misleading results since many o f them do not continue to erupt but remain impacted in a partially erupted position. A mean eruption age with such a wide range is of little clinical significance in an individual subject where the more important issue is whether the third molar is likely to erupt or whether it will remain impacted. The incidence of lower third molar impaction varies considerably in different parts of the world ranging from 9.5% to 39%, ''2'79 and in orthodontic patients up to 50%. ~~ This may be due to genuine population differences, as well as to differences in the definition o f impaction and age at diagnosis. Shiller t~ examined 223 young naval recruits between the ages o f 18 and 21 years in whom 296 lower third molars were diagnosed as mesioangularly impacted. O f these 39% attained a more upright position in a l - y e a r observation period, whereas 12% increased their mesial angulation. Von Wowern and Nielsen'-" and Sewerin and von 811131134
320
W o w e m " also found changes in third molar position in a group of 34 dental students between the ages o f 20 and 24 years. The purpose of the present investigation was to examine and quantify positional changes in lower third molars in a young adult sample.
MATERIAL AND METHOD One hundred and forty 18-year-old students entering dental school were examined. Only those with intact lower arches, 34 men and 32 women, were recorded at the first examination and 3 years later at the beginning of the final year of dental studies. Records included plaster models and cephalometric radiographs taken in the 90 ~ left lateral and 60 ~ left and right lateral positions." Twenty lower third molars (11 in men, 9 in women) were congenitally absent. At age 18 years, 35 lower third molars (26 in men, 9 in women) were erupted. The remaining 77 lower third molars were uncrupted at 18 years. Fourteen men and 22 women had both lower third molars unerupted, and in three men and two women lower third molars were unerupted unilaterally at age 18 years. In the present investigation this unerupted lower third molar group was analyzed with 60 ~ right and left lateral cephalometric radiographs. Follow-up radiographs were not available for I i third molars. At age 18 years, 16 (21%) of the unerupted third molars were diagnosed as vertical, 55 (71%) as mesioangular, and 6 (8%) as horizontal. Seventeen third molars in nine subjects (three men and five women bilaterally and one woman unilaterally) were extracted during the observation period after recurrent episodes of pericoronitis. The stage of development of third molars at age 18 years was classified according to an adaptation of the system devised by Gleiser and HunP 5 for first molars. Stage I. Crypt formation with no calcification.
Volume102
Changes in lower third molar position in young aduhs 321
Number 4
~s
'lq5
Fig. 1. Measurement of mesiodistal lower third molar angulation and molar space and changes in these dimensions between 18 and 21 years. Solidlines, 18 years. Broken fines, 21 years.
Stage Stage Stage Stage Stage Stage
2. 3. 4. 5. 6. 7.
Calcification of cusps. Calcification of half crown. Calcification of full crown. Root formation commenced. Root formation complete. Apices closed.
Numbers 35 m
Males
~
Females
30 25 20
MEASUREMENTS
Mesiodistal angtdation of the lower third molar was measured as the angle formed between the occlusal surface of the third molar and a maxillary horizontal (ANS-PNS) (Fig. 1). Molar space was measured along the maxillary horizontal as the distance between projections of the distal contact point of the lower first molar and the junction of the anterior border of the ramus with the body of the mandible (Fig. I). The size of the second and third molars was measured directly on the 60 ~ cephalograms at their maximum mesiodistal widths. Molar space condition was calculated as the difference between molar space and the sum of second and third molar widths. Bttccolhzgttal attgulation of lower third molars was measured as the linear distance between buccal and lingual cusps on the radiographs. Where these cusps were superimposed, the tooth was upright in the buccolingual direction. The buccolingual dimension of a lower third molar crown is about 8 to 9 mm, so that a difference of 1 mm between buccal and lingual cusps represents approximately I0 ~ to 12" of angulation. The buccal cusps are usually uppermost indicating lingual inclination.
Change in mesiodistal atzgulation of lower third molars was measured as the difference in this angle when a tracing of the first film was superimposed on
15 10 5 0
3
4
5
8
Stage of development Fig. 2. Distribution of stage of lower third molar development at 18 years.
the second registering on internal mandibular structures (inner outline o f mandibular symphysis and inferior dental canal, Fig. 1). Change hz molar space was measured along the maxillary horizontal when a tracing of the first film was superimposed on the second film, as before (Fig. 1). Change in buccolingual angulation was calculated as the difference in the distance between buccal and lingual cusps on the first and second films. It was not possible to measure the size and position of two third molars on the 18-year-old films because of poor definition. All measurements were made twice by one observer. If the difference in replicate measurements exceeded 0.5 mm or 0.5 ~ a third reading was made, and the aberrant one discarded. Adjustments for radiographic enlargement were
322
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Am. J. Orthod. Dentofac. Orthop. October 1992
Table I. Means standard deviations and sex differences for all the variables All subjects
Mesiodistal angle 8 (degrees) Molar space (mm) Size 7 + 8 (mm) Molar space condition (mm) Buccolingual angle 8 (mm)
Left Right Left Right Left Right Left Right Left Right
Men
n
Mean
SD
N
Mean
40 35 40 35 40 35 40 35 40 35
41.93 43.69 18.73 18.86 23.60 23.20 -4.86 -4.39 1.46 1.36
18.23 21.80 2.23 2.11 1.43 1.30 2.26 2.46 0.98 0.95
17 13 17 13 17 13 17 13 17 13
42.03 41.12 19.68 19.73 24.26 23.69 -4.59 - 3.96 1.41 1.62
[
SD 24.02 28.46 1.89 2.13 1.40 1.07 2.38 2.64 0.97 1.04
Table II. Means standard deviations and sex differences for the changes in mesiodistal angulation of third molars, molar space, and buccolingual angulation of third molars
Change in mesiodistal angle 8 (degrees) Change in molar space (mm) Change in buccolingual angle 8 (ram)
Left Right Left Right Left Right
All subjects
lffet,
NI 'eo,,ISO
NI 'eo,,ISO
27 22 35 31 27 22
11 8 14 11 11 8
3.13 5. I 1 0.66*** 0.73*** 0.22 0.20
15.88 13.87 0.76 0.95 0.86 0.50
3.86 5.06 0.54* 0.50* 0.36 0.13
21.99 10.10 0.75 0.67 0.74 0.58
*Denotes significance P < 0.05. **Denotes significance P < 0.01. ***Denotes significance P < 0.001.
considered unnecessary since measurements were used only for comparative purposes. Means and standard deviations were calculated for each measurement and change in measurement. Sex differences were tested with independent sample t tests. The changes in mesiodistal third molar angulation, molar space, and buccolingual third molar angulation were tested for significance with one sample Student's t tests. RESULTS
The stage of third molar development at age 18 years is shown in Fig. 2. The majority were in stage 6 with root formation complete, but with apices still open. Eleven, all in men, were at stage 7 with apices closed. Ten, all in women, were at stage 5 with root formation commenced. Three in women and 1 in a man, were at stage 4 with crown formation complete and 1, in a woman was at stage 3 with only half the crown formed. At age 18 years there was a wide variation in mesiodistal angulation of the lower third molar to the max-
illary plane ranging from 9 ~ to 90 ~ averaging 41.9 ~ on the left and 43.7 ~ on the right, with no significant sex difference (Table I). Molar space at age 18 years averaged 18.7 mm on the left and 18.9 mm on the right. There was slightly more space in men than in women, but the difference was not significant (Table I). The average combined size of lower second and third molars was 23.6 mm on the left and 23.2 m m on the right. They were slightly larger in men than in women, but the difference was not significant (Table I). Molar space condition at age 18 years averaged - 4 . 9 m m on the left and - 4 . 4 mm on the right indicating shortage of space for third molars. Women had slightly more molar crowding than men, but the difference was not significant (Table I). The buccolingual angulation of the lower third molars at age 18 years was, on average, 1.5 mm on the left and 1.4 mm on the right. There was no significant sex difference (Table I). The average change in mesiodistal angulation of the
Changes in lower third molar position in young adults 323
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Numbers 10
Women
Difference [
Malea
~
Females
men-wome/z
N
Mean
SD
mean
23 22 23 22 23 22 23 22 23 22
41.85 45.21 18.02 18.34 23. I i 22.93 - 5.09 - 4.64 1.50 1.21
13.03 17.33 2.23 1.97 i .26 1.37 2.20 2.37 1.00 0.88
0.18 - 4.09 i .66 1.39 I. 15 0.76 - 0.50 - 0.68 - 0.09 0.41
50/40 40/30 30120 20110 1 0 / 0
0
Negative
0/10 10/20 20/30 30140
Positive
'(Degrees)
Fig. 3. Distribution of changes in mesiodistal third molar angulation between 18 and 21 years.
Women
N 16 14 21 20 16 14
I
Mean 2.63 5.14 0.74*** 0.85** 0.13 0.25
[
SD
Difference inell-WOmelZ irleoll
10.67 16.00 0.77 1.07 0.94 0.47
1.23 -0.08 - 0.20 -0.35 0.23 - 0.12
third molar during the 3-year observation period was small and nonsignificant, 3.0 ~ on the left and 5.0 ~ on the right, with no significant sex difference (Table II). The distribution of the changes shows that large changes did, in fact, occur ranging from 39.0 ~ to - 4 6 . 0 ~ (Fig. 3). There was a significant average increase in molar space of 0.7 mm on each side. The increase was slightly larger in women than in men, but the difference was not significant (Table II). There was no significant change in buccolingua! angulation of lower third molars during the observation period (Table II). DISCUSSION
The average age for lower third molar crypt formation is quoted as 7 years by Banks 16and Garnet a l . j7 Graveley j8 found the peak formation period at 9 years. After a gestation period of 9 or more years, it is to be expected that at age 18 years the majority of lower third molars would be in the later stages of development, with root formation complete or the apices closed. In
view of the wide variation in timing of third molar genesis, ~9 it is not surprising to find some lower third molars still in earlier stages of development at age 18 years. The variation in mesiodistal angulation of lower third molars at age 18 years ranging from 9 ~ to 99 ~ reflected the diagnosis on radiographs of 21% of the lower third molars in a more or less vertical position, 8% in a horizontal position, and the remaining 71% in varying degrees of mesioangular relationship to the adjacent second molar. In all cases the molar space at age 18 years was inadequate to accommodate second and third molars in good alignment. The discrepancy ranged from 0.5 mm to 11.5 mm with the majority of third molars short of room by between 3 and 6 mm. In general teeth tend to erupt before their root formation is complete. The large number of lower third molars in this study remaining unerupted at age 18 years despite advanced root formation is probably largely because of lack of space and their mesioangular and horizontal positions. There is a strong tendency for lower third molars to become more upright during the course of their development, 2~ and if space is available this should lead to their eruption. It is likely therefore that the lack of space in the molar region may also be, at least partly, responsible for the mesioangular position of many of the third molars. On average the buccolingual third molar angulation at age 18 years was about 1.5 mm indicating a lingual inclination of about 18~ The range was from 0.0 mm to 4.0 mm. Only 20% of the third molars were upright in the buccolingual direction, and 30% had lingual inclinations of more than 2 mm, that is from 20 ~ to 50 ~ The change in mesiodistal lower third molar an-
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Am. J. Orthod. Dentofac. Orthop. October 1992
Fig. 4. Lower right third molar.with positive change in mesiodistal angulation of 39~ between 18 (left) and 21 (right) yearschanging its position from mesioangular to distoangular impaction.
Fig. 5. Lower left third molar with negative change in mesiodistal angulation of 46~ between 18 (left) and 21 (right) yearschanging its position from mesioangular to horizontal impaction.
gulation during the 3-year observation period was small on average, but the histograms of their distributiont.(F.ig. 3) show that large changes did, in fact, occur. Only 10 third molars (20%) did not change their angulation. Twenty-five (51%) had a positive change resulting in
an uprighting movement. Fourteen (29%) had a negative change that caused the third molar to become more mesioangularly inclined. Fig. 4 shows a third molar that uprighted by 39 ~ changing the position of the tooth from mesioangular to one of distoangular impaction.
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Fig. 6. Lower left third molar that uprighted buccolingually by approximately 24~ between 18 (left) and 21 (right) years.
Fig. 5 shows a third molar that became more mesially inclined by 46 ~ that resulted in horizontal impaction. The average increase in molar space, although statistically significant, was small, 0.5 mm in men and slightly more in women. The range of increase was from zero to 3.5 ram. In only eight quadrants did the increase exceed 2.0 mm. Since molar crowding at age 18 years was, on average, between 4.0 and 5.0 mm, an increase in space of 0.5 mm would have little effect in providing room for third molar eruption except in those few cases where molar crowding was minimal or the increase in space was greater than average. Between the ages of 13 and 18 years an average increase in molar space of 4.0 mm was found in 51 subjects, z5 On average 2.0 mm of this increase occurred anteriorly and 2.0 mm posteriorly. The anterior increase was due to forward movement of the buccal segment, and the posterior increase was due to resorption of bone in the region of the alveolar shelf at the back of the dental arch. In a previous investigation made on the material used for the present study, it was found that there was, on average, no significant forward movement of buccal teeth, ~ so that the increase in molar space is probably due to resorption of bone at the back of the dental arch except in those few cases where there was some mesial drift. On average the buccolingual third molar angulation did not change significantly during the observation period. Ten third molars became more upright in the buc-
colinguaI direction by 1.0 or 2.0 mm (I0 ~ to 24 ~ approximately). Four third molars tipped lingually by similar amounts. Fig. 6 shows a third molar that uprighted buccolingually by approximately 20 ~ to 24 ~ and Fig. 7 one that increased its lingual inclination by the same amount. Of the original 77 third molars, 17 were extracted during the observation period. It is not known exactly what angular changes were experienced by these, but they changed their position sufficiently to become partially erupted and subject to pericoronitis. Of the third molars that became more upright during the observation period, only three, in two women, erupted to full occlusion. One of these had adequate molar space bilaterally at age 18 years, the other had a 2 mm increase in molar space as a result of the mesial drift of buccal teeth. Another five, in two men and one women, became partially erupted, two of these were subsequently removed because of pericoronitis, and the other three remained partly covered by gum 1 year later. These findings are consistent with the degree of molar crowding at age 18 years and the small increase in molar space. It is apparent that, despite lack of space and advanced root formation, some lower third molars undergo considerable positional changes in both the mesiodistal and buccolingual directions after the age of 18 years. Similar changes in mesiodistal angulation of third molars during the earlier stages of their development
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Am. J. Orthod. Dentofiw. Orthop. October 1992
Fig. 7. Lower right third molar that became more lingually inclined by approximately 24 ~ between 18 (left) and 21 (right) years.
from 1 1 to 18 years have been described by Richardson. 24 The majority tended to become more upright, but about 12% of all third molars might be expected to become more mesially inclined during this period. Shiller H reported uprighting movement of third molars in a l-year period in men between the ages of 18 and 21 years. In his study 12% of third molars became more mesially inclined. Von Wowern and Nielsen ~2and Sewerin and von Wowem t3 also reported significant changes in mesiodistal third molar angulation in both a positive and a negative direction between 20 and 24 years. Molar space condition was not considered in these studies. H'~3 Positive and negative changes in the buccolingual direction between 1 1 and 19 years have been reported by Svendsen et al. 23 with posteroanterior radiographs. Shiller t' pointed out that, although many third molars became completely upright radiographically, he could not tell if they erupted clinically. In the Danish study ~2"~325 third molars (19% of the original number) completely erupted to the occlusal level between the ages of 20 to 24 years. In the Belfast sample only three third molars (4%) became fully erupted during the observation period. This group was too small to make any meaningful evaluation of the reasons for successful third molar eruption. The fact that adequate molar space was available or become available during the observation period suggests that favorable molar space condition is an important prerequisite for third molar eruption. The pos-
sibility still exists for more third molars to erupt in coming years. It is, however, unlikely that molar space will increase much after the age of 21 years. In view of the degree of molar crowding at 18 years, it is improbable that very many more third molars will erupt in the future. No satisfactory explanation for the third molar that increases its mesial inclination has yet been found. One theory implicates the angle of contact of the third molar with the second molar producing the effect of an inclined plane. '~ This seems unlikely. Third molars can change their angulation before contact with the second molar is established, and some in apparently similar angular relationships with second molars continue their development in opposite directions (Figs. 4 and 5). This can sometimes happen on contralateral sides of the same person. A theory of differential root growth, the mesial root dominating when the third molar uprights, and distal root dominance causing increased mesial inclination has been advanced. 2~This could not be confirmed by Richardson and co-workers. 29 Studies in the transverse dimension 23'29-3~ suggest that the buccolingual position of a third molar may be important in its developmental pattern. The more buccally placed teeth may be influenced by the dense bone of the internal oblique ridge or the pterygomandibular raphe or by the attachment of the buccinator muscle. 23.-'9 Further research is needed on this aspect of lower third molar development.
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Changes in lower third molar position in young adults
CONCLUSIONS 1. In this material 3 1 % o f the third molars present had erupted at age 18 years. 2. B e t w e e n 18 and 21 years m a n y o f the unerupted third molars c h a n g e d their position appreciably despite a d v a n c e d root d e v e l o p m e n t . 3. In o n l y a v e r y f e w cases did the positional changes result in clinical eruption because o f inadequate space. 4. It is unlikely that m a n y m o r e l o w e r third molars will erupt in the future because o f the d e g r e e o f m o l a r c r o w d i n g in this material. I am grateful to Professor Andrew Richardson, Miss Jennifer Maltman, Mrs. Marlene Boe, and Mrs. Sally Meehan for the figures and the tables, and to the students for their cooperation.
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study of asymptomatic mandibular third molars in young adults. Int Dent J 1990;40:24-30. Richardson ME. The early developmental position of the lower third molar relative to certain jaw dimensions. Angle Orthod 1970;40:226-30. Gleiser I, Hunt EE. The permanent mandibular first molar: its calcification, eruption and decay. Am J Phys Anthropol 1955; 13:253-83. Banks ttV. Incidence of third molar development. Angle Orthod 1934;4:223-33. Gain SM, Lewis AB, Bonne B. Third molar formation and its developmental course. Angle Orthod 1962;32:270-9. Gravely JF. A radiographic survey of third molar development. Br D J 1965;I 19:397-401. Richardson ME. Late third molar genesis: its significance in orthodontic treatment. Angle Orthod 1980;50:121-8. Bj6rk A, Skeiller V. Facial development and tooth eruption. An implant study at the age of puberty. AM J ORTItOD1972;62:33983. Richardson ME. Development of the lower third molar from 10 to 15 years. Angle Orthod 1973;43:191-3. Richardson ME. Some aspects of lower third molar eruption. Angle Orthod 1974;44:141-5. Svendsen H, Malmskov O, Bj6rk A. Prediction of lower third molar impaction from the frontal cephalometric projection. Eur J Orthod 1985;7:1-16. Richardson ME. The development of third molar impaction and its prevention, lut J Oral Surg 1981;10:Suppl 1:122-30. Richardson ME. Lower third molar space. Angle Orthod 1987;57:155-61. Richardson ME. Lower arch crowding in the young adult. AM J ORTIIODDENTOFACORTItOP 1992 [In press]. Henry CB, Morant GM. A preliminary study of the mandibular third molar tooth in man. Biometrika t936;28:378-427. Silling G. Development and eruption of the mandibular third molar and its response to orthodontic therapy. Angle Orthod 1973;43:271-8. Richardson ER, Malhotra SK, Semenya K. Longitudinal study of three views of mandibular third molar eruption in males. AM J OR'ntOD 1984;86:119-29. Olive RJ, Basford KE. Transverse dento-skeletal relationship and third molar impaction. Angle Orthod 1981;51:41-7.
Reprhlt requests to: Dr. Margaret Richardson Orthodontic Department School of Dentistry Royal Victoria Hospital Grosvenor Rd. Belfast BTI2 6BA Northern Ireland
