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VOJNOSANITETSKI PREGLED Strana 1
Correspondence to: Ksenija Zelić, Clinic of Restorative Dentistry and Endodontics, Rankeova 4, 11000 Belgrade, Serbia.
Phone: +381 63 238 414. E-mail: ksenijazelic@gmail.com
ORIGINAL ARTICLE
UDC: 616.314.2-053-036.22
DOI:
Size of the lower third molar space in relation to age in Serbian
population
Zavisnost veličine donjeg retromolarnog prostora od uzrasta u srpskoj
populaciji
Ksenija Zelić*, Nenad Nedeljković
†
*Clinic of Restorative Dentistry and Endodontics, School of Dentistry, University of
Belgrade, Belgrade, Serbia;
†
Clinic of Orthodontics, School of Dentistry, University of
Belgrade, Belgrade, Serbia
Abstract
Background/Aim. It is considered that the shortage of
space is the major cause of the third molar impaction.
The aim of this study was to establish the frequency of
insufficient lower third molar eruption space in Serbian
population, to question the differences in this frequency
in the subjects of different age, to determine the influence
of retromolar space size on third molar eruption, and to
investigate a possible correlation between the size of go-
nial angle and the space/third molar width ratio. Meth-
ods. Digital orthopantomograms were taken from 93 pa-
tients divided into two groups: early adult (16–18 years of
age) and adult (18–26) patients. Retromolar space, mesio-
distal third molar crown width, gonial angle and eruption
levels were measured. Results. The space/third molar
width in early adult subjects was smaller (p < 0.0001) and
insufficient space was significantly more frequent (p =
0.0003) than in adult patients. Considerably more third
molars erupted in case of enough space in both age
groups (p < 0.0001). There was no difference between the
means of gonial angle size in relations to the available
space. Conclusions. The retromolar space/third molar
width ratio is more favorable in adult subjects. Gonial an-
gle is not in correlation with the retromolar space/third
molar width ratio.
Key words:
molar, third; tooth eruption; tooth impaction;
adolescent; adult; serbia.
Apstrakt
Uvod/Cilj. Smatra se da je nedostatak prostora glavni uzrok
ukleštenja trećeg kutnjaka. Cilj ove studije bio je da se ustanovi
učestalost nedovoljnog prostora za nicanje umnjaka u srpskoj
populaciji, da se ispitaju razlike u ovoj učestalosti kod mlađih
odraslih i odraslih ispitanika, da se odredi uticaj veličine retro-
molarnog prostora na nicanje umnjaka, kao i da se ispita pove-
zanost između veličine ugla mandibule i odnosa između veliči-
ne retromolarnog prostora i meziodontalne širine umnjaka.
Metode. U istraživanje su bila uključena 93 ispitanika podelje-
na u dve starosne kategorije: mlađi odrasli (16–18 godina) i od-
rasli (18–26 godina) ispitanici. Kod svakog pacijenta na digital-
nom ortopantomogramu mereni su: retromolarni prostor, me-
ziodistalna širina umnjaka, nivo izniklosti umnjaka i ugao
mandibule. Rezultati. Odnos između veličine retromolarnog
prostora i meziodistalne širine umnjaka bio je statistički zna-
čajno manji (p < 0.0001) kod mlađih ispitanika. Takođe, nedo-
statak prostora sretao se znač
ajno češće u istoj starosnoj kate-
goriji (p = 0.0003). Prilikom poređenja nivoa izniklosti u obe
starosne kategorije nađena je visoka statistička značajnost (p <
0.0001) u korist grupe sa dovoljnim prostorom za nicanje um-
njaka. Zaključak. Značajno više umnjaka ima mesta za pra-
vilno smeštanje u zubni niz nakon 18 godina života što navodi
na zaključak da rast retromolarnog prostora nije završen u 16.
godini. Ugao mandibule nije u korelaciji sa odnosom retro-
molarnog prostora i meziodistalnog promera umnjaka.
Ključne reči:
umnjak; zub, nicanje; zub, impakcija; adolescencija;
odrasle osobe; srbija.
Introduction
Surgical extraction of impacted third molar is among
the most frequently performed oral–surgical procedures
1
. It
was reported that the lower third molar is the second most
commonly impacted tooth in the human jaw
2–4
. Insufficient
jaw development will primarily affect the eruption space of
wisdom teeth, as they are the last ones to erupt into the oral
cavity. In addition to inappropriate inclination of the lower
third molar, the lack of space is considerate as main cause of
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its impaction
5
. Because of this, consideration of these teeth
is a part of overall dental examination and treatment plan.
In the lower jaw, the retromolar space borders are well
defined – the distal surface of the second molar crown and
the anterior border of the mandibular ramus. The mesiodistal
crown width of the third molar should be smaller than this
space if its eruption is to be expected. Ganss et al.
6
claimed
that in this case, almost 70% of wisdom teeth would erupt.
However, this space is insufficient in a significant number of
individuals.
It was considered that the growth of lower retromolar
space should not be expected after the age of 16
5, 7
. On the
other hand, Chen et al.
8
reported that there is a significant
expansion of this space between the age of 16 and 18. This
issue is clinically significant, since possibility to predict im-
paction of lower third molar in an early stage would favor
the decision to remove it easily before the roots are fully
formed. However, if such prediction is based on a wrong as-
sumption that retromolar space will not enlarge in the future,
some of those surgical procedures would not be justified.
Several researchers also investigated the correlation
between the size of gonial angle and the retromolar space
width, as both variables are dependent on mandibular growth
9–11
. As the results are conflicting
9, 11, 12
, it is interesting to
evaluate if size of the gonial angle might be used as a pre-
dictor of the lower third molar impaction.
It can be assumed that facial growth, jaw size and tooth
size differ among races and populations. Since there have
been very few research articles on this issue based on Ser-
bian population
13
, it might be interesting to compare some of
those variables in our material with results from studies re-
ported for other populations.
Therefore, the aims of this study was to establish the
frequency of insufficient space for lower third molar eruption
in Serbian population, to determine the influence of this fact
on third molar eruption, to investigate whether there are dif-
ferences in this variable between different age groups and to
analyze the relationship between the retromolar space and
the gonial angle size.
Methods
A total of 93 subjects (41 males and 52 females) be-
tween 16 and 26 years and with no history of previous ortho-
dontic treatment were included in this study. Exclusion crite-
ria were previous extraction or hypodontia of any tooth and
some particular angulations of the lower third molar (bucco-
oral position and distal angulations for more than 10 de-
grees). The study took place at The Clinic of Orthodontics,
School of Dentistry, University of Belgrade. The participants
were divided into two age groups: the early adult group –
subjects from 16 to 18 years of age and the adult group –
subjects from 18 to 26 years.
The total sample consisted of 164 lower third molars,
85 on the left and 79 on the right side. The early adult group
included 62 third molars (23 from males and 39 from fe-
males), and the adult group included 102 third molars (45
from males and 57 from females).
Digital orthopantomograms (Planmeca, Promax; per-
formed at 66–70 kV; 11–14 mA; 6.2 s exposure time; pulse
x-ray) were taken and, on acetate paper attached to radio-
graphs, the following planes, lines, and angles were drawn
(Figure 1): occlusal plane (OP) – line connecting midpoint of
the vertical overlap of the central incisors and the most distal
contact point of upper and lower teeth; mesiodistal crown
width of the lower third molar (MW) – measured as the
greatest diameter of the crown; tangent line (TL) – drawn
through the most distal points on the crown and root of the
second molar; retromolar space (RS) – measured as a length
of the line drawn along the occlusal plane from the point it
bisects TL to the point it bisects the anterior border of the
ramus; space/third molar width ratio – calculated by dividing
RS with MW; gonial angle formed between the tangent line
to the posterior border of the mandibular ramus and the tan-
gent line to the lower border of the mandibular corpus; erup-
tion level – measured according to the classification of Pell
and Gregory
14
: A level – the occlusal surface of the third
molar is leveled or nearly leveled as the occlusal surface of
the second molar, B level – the occlusal surface of the third
molar is between the occlusal surface of the second molar
and its cervical line, C level – the occlusal surface of the
third molar is below the cervical line of the second molar.
Fig. 1 – Linear and angular measurements on
orthopantomogram
RS – retromolar space; MW – mesiodistal width of the third molar; OP –
occulasal plane; TL – tangent line
After calculating space/third molar width ratio, both age
groups were divided into two subgroups the ES subgroup
with enough space for third molar eruption (space/width ratio
≥ 1), and the NS subgroup with no enough space for third
molar eruption (space/width ratio < 1).
All orthopantomograms were interpreted by the same
examiner.
The arithmetic mean and standard deviation were cal-
culated for each continuous variable. The frequency and per-
centages were displayed for categorical variables. Compari-
son of the continuous variables between genders and sides
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was made using the Student's t-test and Mann Whitney's test.
Statistical differences between frequencies were tested with
Pearson’s χ
2
test and Fisher’s test. Statistical analyses were
performed in R 2.11 statistical software package (R Founda-
tion, Vienna, Austria).
Results
In the early adult group the majority (more than 80%)
of investigated third molars did not have enough space for
eruption. However, in the adult group, this was the case with
about half of the third molars (Table 1).
Table 1.
Distribution of lower third molars in two age groups in rela-
tion to the available space for eruption
Subgroups of patients
Patients
NS
n (%)
ES
n (%)
p
(χ
2
test)
Male
early adult 18 (78.26) 5 (21.74)
adult 24 (53.33) 21 (46.67)
0.04
Female
early adult 34 (87.18) 5 (12.82)
adult 34 (59.65) 23 (40.35)
0.003
Total
early adult 52 (83.87) 58 (56.86)
adult 10 (16.13) 44 (43.13)
0.0003
Early adult – subjects aged 16 to 18 years; Adult – subjects older than 18
years; ES – third molars with enough space for their eruption [RM (retromo-
lar space)/MD (mesiodistal crown with) ≥ 1]; NS – third molars without eno-
ugh space for its eruption (RM/MD < 1)
These differences proved to be statistically significant,
both in the whole sample and when data on genders were
extrapolated. Comparisons between genders and between the
left and right side showed no significant differences. Com-
paring male and female subjects within the these age groups,
the same results were obtained.
In order to confirm these results, mean values of the
space/third molar width ratio for early adult and adult sub-
jects were calculated and the differences between them were
tested (Table 2). The results showed significantly smaller
space/third molar width ratio in younger patients (p <
0.0001). Comparing the means of this parameter between
males and females, no significant difference was observed.
Table 2.
Age dependence of the space/third molar crown width ratio
in males and females
Space/crown ratio
Patients
(ґ ± SD)
p
(t-test)
Male
early adult 0.62 ± 0.44
adult 1.01 ± 0.43
0.0007
Female
early adult 0.67 ± 0.26
adult 0.84 ± 0.37
0.006
Total
early adult 0.64 ± 0.32
adult 0.92 ± 0.40
< 0.0001
Early adult – subjects aged 16 to 18 years; Adult – subjects older than 18
years; space/crown ratio – RM (retromolar space) divided by MD (mesiodistal
crown width)
In the patients from the early adult group, the highest
number of third molars was in the C-position, according to
the Pell-Gregory classification. This was particularly the case
in the third molars with enough space for their eruption in
the NS subgroup, in contrast to the third molars with enough
space for their eruption in the ES subgroup where more of
the third molars were in the A-position (Table 3). On the
other hand, in the adult group, the highest number of the
third molars was in the A-position, clearly indicating their
eruption over time. Despite this, in the NS subgroup more
than half of the investigated teeth were in the C position
while almost 90% of the third molars reached the occlusal
plane in the ES subgroup. Differences between ES and NS
subgroups were statistically significant in both age groups.
There were no differences between the mean values of
the gonial angle size in relation to the available space (Table
4). The average mandibular angle for the whole group was
124.39 on the left and 123.45 degrees on the right side (p >
0.05). There were no significant differences in mean values of
this angle between genders and between left and right sides.
Table 3.
Third molar eruption level in relation to the available space
in the mandible in two age groups
Level of eruption
(the Pells Gregory classification), n (%)
Patients
ABC
p
(χ
2
test)
Early adult
NS 8 (15.38) 15 (28.85) 29 (55.77)
ES 6 (60.00) 1 (10.00) 3 (30.00)
0.008
Adult
NS 14 (24.14) 10 (17.24) 34 (58.62)
ES 39 (88.64) 1 (2.27) 4 (9.09)
< 0.0001
Total
NS 22 (20.00) 25 (22.72) 63 (57.27)
ES 45 (83.33) 2 (3.70) 7 (12.96)
< 0.0001
Early adult – subjects aged 16 to 18 years; Adult – subjects older than 18 years; ES – third
molars with enough space for its eruption [RM (retromolar space)/MD (mesiodistal crown
with) < 1]; NS – third molars without enough space for its eruption (RM/MD < 1)
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Discussion
The lack of space in human jaws has been a topic of
interest for a long time. The mandibular retromolar space is
one of the most investigated parameters for two reasons: the
lower third molars are the second most frequently impacted
teeth
2–4
and the lack of space is considered to be the major
cause of this
9
. Therefore, the analysis of this space should be
carefully performed, especially in young patients.
Two main methods have been used for estimation of the
available retromolar space: measurement of the distance
between the center of the ramus (Xi point) and the distal as-
pect of the lower second molar
15, 16
,
and measurement of the
distance between the anterior edge of the ramus and the dis-
tal surface of the lower second molar
6, 12, 17
. Olive and Bas-
ford
17
reported that the use of the first method could not be
supported.
Many studies have demonstrated that orthopantomogra-
phy can give reliable measurements of the skeletal and dental
structures as can lateral cephalogram
6, 18–21
. The advantage
of the orthopantomogram is evident when measuring right
and left side because there is no superimposition, which is
present at lateral cephalograms. Furthermore, digital tech-
nology gives more clear radiograms and analysis on them is
easier. However, possible distortions and magnifications in
the molar region can lead to unreliable linear measurements
on the orthopantomogram
6, 20, 22
. Therefore, the space/third
molar width ratio was used as a parameter for space analyses
because these irregularities will affect the retromolar space
width as well as the third molar width, but the ratio will re-
main constant. Moreover, Olive and Basford
17
concluded
that the space/width ratio provides reliable assessment of the
available retromolar space for the third molar eruption and
that orthopantomogram gives the best estimation of the re-
quired ratio, while the lateral cephalogram is uncertain. Ler-
heim and Svanses
20
showed that orthopantomogram does
not change the size of the gonial angle and Mattila et al.
21
concluded that it is more obvious choice for determination of
the gonial angles than lateral cephalograms.
It is considered that the shortage of space is the major
cause of the third molar impaction
11
. Kahl et al.
23
found that
the majority (97.40%) of impacted teeth did not have enough
space. After 7 years of observation, Ganss et al.
6
concluded
that, if the space/third molar width ratio is larger than 1, most
of wisdom teeth would ultimately enter the arch (almost
70%). Many authors supported this observation. Bjork et
al.
12
reported that the third molar space was reduced in 90%
of cases of its impaction. Hattab and Alihaija
9
found that the
space/third molar width ratio was significantly larger in the
group of teeth that had erupted than in the impacted group. In
addition, in the impacted group, in approximately 80% of in-
vestigated teeth, this ratio was smaller than 1, whereas in the
erupted group, in 69% it was larger than 1
9
. Olive and Bas-
ford
17
concluded that prognosis for the third molar eruption
is favorable if the ratio is equal or greater than 1, while Uth-
man
10
found even smaller minimum values for successful
eruption (0.88 for males and 0.83 for females).
Our results showed significantly more erupted third
molars in the enough space (ES) subgroups, regardless of
patients age (Table 3). In the early adult group, the difference
reached the significance of p = 0.008 and in the adult group
it was even higher (p < 0.0001). It is interesting that these
differences proved to be statistically significant even in the
early adult group, although it is the period of life in which
third molars just begin to erupt. Altogether these results are
in agreement with previous studies, thus supporting the
opinion that the lack of space can delay or disable the third
molar eruption and enough space, among other factors, fa-
vors its eruption.
One of the aims of this study was to investigate the fre-
quency of insufficient retromolar space in Serbian population
as it is considered the main cause of third molar impaction.
Although, there are differences between early adult and adult
subjects, high prevalence of shortage of retromolar space
was evident (Table 1).
The question we also posed was weather the third molar
space can be measured in the age of 16 without making
wrong assessment about the future outcomes. Ganss et al.
6
reported that the space/width ratio remained almost constant
between 16 and 20 years of age in the impacted group and
increased insignificantly in the erupted group. The investiga-
tion of Bjork
24
showed no increase of posterior dental arch
after the age of 14 for girls, and the age of 16 for boys. Le-
dyard
7
also found no expanding
of this area after the age of
16. Niedzielska et al.
5
confirmed this observation and con-
cluded that eruption or non-eruption can be adequately pre-
dicted in young adults.
Nevertheless, it was also shown that some significant
changes can happen in the size of retromolar space after the age
of 16
8
. It was reported that total increases from 13 to 18 years
Table 4.
Gonial angle size in relation to the available space in the mandible
in two age groups
Early adult Adult
Mandible side
(ґ ± SD) p* (ґ ± SD) p*
Left
NS 126.4 ± 6.63 125.4 ± 8.02
ES 123.2 ± 13.03
0.16
121.1 ± 7.90
0.06
Right
NS 126.4 ± 6.79 120.8 ± 7.2
ES 127.8 ± 13.66
0.80
122 ± 7.76
0.50
Early adult – subjects aged 16 to 18 years; Adult – subjects older than 18 years; ES – third molars
with enough space for its eruption [(RM (retromolar space)/MD (mesiodistal crown with) < 1];
NS – third molars without enough space for its eruption (RM/MD < 1); *Mann-Whitney test
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of age were 5.12 mm for girls and 5.79 mm for boys. Also, sig-
nificant annual increase for boys between 16 and 17 years of
age (average 1.20 ± 0.02 mm) and for girls between 17 and 18
years of age (1.32 ± 0.04 mm) was found. We found that this
increasing is important and we consider that the retromolar
space size cannot be adequately assumed in the age of 16.
Our results show that in early adult patients lack of
space is significantly more frequent than in adults (p = 0.04
in male and p = 0.009 in female subjects) (Table 1). Moreo-
ver, the means of the space/third molar width ratio were sig-
nificantly larger in older subjects (Table 2). For such strong
statistical significance, we find no other explanation than the
fact that retromolar space grows after the age of 16. This
growth will, during time, lead to an improvement of the
space/third molar width ratio. We tested the differences be-
tween means of space/third molar width ratio and frequen-
cies of insufficient space in younger and older subjects, so it
could be more obvious that decision concerning third molar
removal can be unreliable in early adulthood.
Chen et al.
8
found differences between genders, but
this was not observed in our study. However, we divided
subjects in 16–18 years of age as the early adult and from
18–26 years of age as the adult group and compared differ-
ences between them. Chen et al.
8
analyzed differences be-
tween genders annually and found significant retromolar
growth for girls at the age 17 and for boys at the age 16. This
was not observed in our study as both male and female sub-
jects showed significant growth between the age of 16 and
18 (in our study – early adults).
Average gonial angle in our sample was 123.45 degrees
on the right and 124.39 degrees on the left side, whereas in
Finish population, it was 128.3 degrees
25
. In Jordanian
population, Hattab and Alihaija
9
reported smaller average
gonial angle (120.8 degrees). Richardson
11
and Bojrk et al.
12
had reported that smaller gonial angle was more common
among subjects with impacted third molars. On the other
hand, Hattab and Alihaija
9
concluded that there was no rela-
tionship between the size of the gonial angle and impaction
of the third molars. If the size of the gonial angle is different
in subject with impacted than in those with erupted lower
third molars, than the impaction is caused by insufficient
space as these two parameters depend on mandibular growth.
Therefore, we compared sizes of gonial angle of the NS and
ES subgroup, without concerning the eruption status. Our
findings show that the size of gonial angle cannot be an indi-
cator of future outcomes of the space/third molar width ratio
because there was no relationship between these two pa-
rameters (Table 4).
Conclusion
The retromolar space/third molar width ratio differs
between subjects aging from 16 to 18 years and subjects
older than 18 years. Insufficient space was more frequent in
younger group and the mean value of the space/third molar
width ratio was significantly smaller in the same group.
Therefore, the decision about the removal of the third molar
in young adults should be made with caution.
Gonial angle size was not in correlation with the retro-
molar space/third molar width ratio and the use of this pa-
rameter as a predicting factor for future outcomes of this ra-
tio cannot be recommended.
Acknowledgements
Authors wish to thank Professors Saša Cakić and Obrad
Zelić from the Clinic for Periodontology and Oral Medicine
and Dr. Miroslav Andrić from the Clinic of Oral Surgery,
School of Dentistry, University of Belgrade, Serbia for their
help in performing this study.
Funding
This study was supported by the Ministry of Education
Science and Technological Development of the Republic of
Serbia, Project number 45005.
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Received on May 9, 2011.
Revised on June 14, 2011.
Accepted on June 23, 2011.
