Role of third molars in orthodontics /certified fixed orthodontic courses by Indian dental academy

ROLE OF THIRD MOLARS
IN ORTHODONTICS
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com


DEVELOPMENT AND ERUPTION
• There is a great variation in the timing of development
calcification and eruption of third molars
• Development may begin as early as 5 years or as late as
16 years; with the peak formation period at 8 or 9 yrs.
• Calcification can start at age 7 years and as later as age
16 years.
• Enamel formation is normally complete between 12 and
18 yrs and root formation is normally completed
between 18 and 25 yrs.

• In 1992 fanning reported average ages of eruption of
19.8 yrs for females and 20.4 yrs for males.
• Lower 3rd molars normally have their occlusal surface
tilted slightly forwards and lingual during early
clarification. As the mandible increases in length with
bone resorbtion at the inner angulation between the
body and the ascending ramus of the mandible the third
molars become more upright.

• In contrast, upper 3rd molar erupt down word back
ward and often outwards. There is therefore a possibility
of cross bite but tongue pressure on lower crowns and
buccinator pressure on upper crowns will often correct
this.
• If there is lack of space, then normal eruptive paths
cannot be followed, and cross bite can results.

Predication of crown size by factor and multiple
regression analysis (AJO 1996: 109: 79-85)
• Abe et al showed that it is possible to predict the size of
unerupted third molars, based on measurement of erupted
teeth. They found an accuracy of approximately 0.5 mm in
the maxillary arch and 0.4 mm in the mandibular arch. The
accuracy

was highest

when

the maxillary

dentition

measurements were based on the sie of the lateral incisor,
the second bicuspid, and the second molar. In the
mandibular arch the accuracy was highest when the
prediction was based on the size of the central incisor, the
first bicuspid, and the second molar.

BRACKET SPECIFICATIONS
Second molar tubes can used for erupted
third molars, with 0° tip, - 14° torque,
and 10° rotation in the upper and 0° tip
and -10° torque in the lower.
3rd molar seldom required to move
unless high percentage of adult treatment
or 2nd molar extractions.


Eruptive pathways of lower 3rd molars (AJO
1992: 102, 322-327
• Richardson investigated the development of lower 3rd molars
between ages 10 and 15 years.
• In a group of 46 children of average age 10 yrs, she found that
the angle of the occlusal surface of the lower 3rd molars to the
mandibular plane was 41° she found this decreased by 11° by
age 15. But 10% of her sample increased their angulation,
worsening their eruptive position.


Full eruption of 3rd molars
• Successful eruption of the lower third molars occurs by
the tooth continuing to decrease it angle to the
mandibular plane and moving occlusally into sufficient
space.


Type of impaction
• Impaction of 3rd molar occur due to lack of space or to
unfavourable changes in angulation or a combination of
these two problems.
Type A
The tooth can follow the pattern
of an ideally developing third
molar, by decreasing its angle to
the mandibular plane and
becoming more upright, but the
uprighting may not be enough
to allow fully eruption.

Type B
The angular developmental
position relative to the mandibular
plane may remain unchanged.
Type C
The tooth can increase its
angulation to the mandibular
plane, and become more mesially
inclined. There is at present no
reliable way of predicting which
teeth will follow this unfavourable
pattern, which sometimes occurs
unilaterally and leads to horizontal
impaction. However, if the mesial
root develops ahead of the distal
root this may be a favourable
indication.

Type D
The tooth can be seen to make
favourable changes in angulation,
but fail to erupt owing to lack of
space. These are so-called vertical
impactions.

Type E.
The tooth can continue to change
its angulation beyond the ideal
occlusal position, and show
distoangular impaction.


Changes in lower third molar position in the young
adult. (AJO 1992; 102: 320-7).
• Richardson investigated the later development and
mesio-angular impaction of lower third molars.
• For this study she defined an impacted lower third
molar as one which was prevented from eruption
because of its mesio-angular relationship with the
second molar.
• From her study stated 46% became more upright, but
failed to erupt, 13% showed the same angulation, and
41% became more mesially inclined, the change varying

Changes at 18 year and later
In a 1992 investigation, reported on a group of 41 dental
students with intact (no missing teeth) lower arches. At age
18 years only 31 per cent of lower third molars present had
erupted. Between the ages of 18 and 21 the unerupted third
molars were observed to show changes in position ranging
from +39° to -46° in the mesio-distal dimension and from
+24° to -24° in the bucco-lingual dimension. Only 20 per
cent did not change their mesio-distal angulation. He noted
that, in spite of these positional changes, because of lack of
space, only four teeth successfully erupted in the observation
period.


Positional changes in mesio-angular impacted 3rd molars
during a year (JADA 1989 (99) 460-4).
• Shiller

stated

that

positional

changes

in

the

mesioangular inclination of third molars continues
well into mid-20s, and he noted that after second
molar extractions the third molars did not initially
erupt into optimal positions. However, considerable
improvement in angulation, space closure, and
occlusion commonly occurred after eruption.

Mechanism for lower third molar eruption and
impaction
Favourable change in
angulation, to a more
upright position, seemed
to occur in teeth where
the mesial crown surface
and the mesial root
developed ahead of the
distal crown surface and
root.

•Unfavourable mesial tipping, leading to horizontal
impaction, seemed to occur when the distal root
became the same length, and then longer than the
mesial root. The distal root on such teeth was seen to
appear to have a mesial curvature.


Factors influencing the space available for lower 3rd
molars (Angle Orthod. 1987; 57; 155-61).
1. Growth:
Bjork et al measured the distance from the anterior
border of the ramus to the second molar; and
concluded that the bigger the space, the better
chance of eruption. But after 16 years of age there
was negligible growth of the refromolar area.

2. Bone resorbtion:
• Increased space was obtained from both the mesial
movement of the dentition and bone remodelling along
the anterior border of the ramus. On average 2mm of
posterior space was created by bone remodelling
(range 0-6mm).
• The largest increase in 3rd molar space resulted from a
large amount of overall mandibular growth and
forward eruption of the mandibular dentition.

3. Space released by attrition:
• In primitive dentitions, where considerable attrition
takes place, the 3rd molars erupt to take up the space
released.
• Begg felt that lack of this attraction, due to highly
refined diets, was a major cause of 3rd molar
impaction.
• In high caries situations mesial and distal lesions
could possibly also increase space.

4. Space can be increased by 2nd molar extraction
5. Reduction of Space:
• Silling showed that orthodontic therapy in non extraction
cases can increase the probability of 3rd molar impaction by
holding back or distally tipping lower 1st and 2nd molars.
• Distal movement of upper 1st molars with head gear can
reduce the space for upper 3rd molars.
• Magness and Graber have suggested the extraction of upper
second molar in some cases to assist 1st molar positioning and
increase space for upper 3rd molars.
6. Eruption into space released by bicuspid extraction:
• Bicuspid or other extractions increase the chance of 3red
molar eruption.


The effect of bicuspid extractions on 3rd molar eruption:
• Bjork et al observed that extraction of a tooth in a quadrant
considerably lowers the prevalence of 3rd molar impaction.
• Fanning found that 75% of 3rd molar erupted in a growth with
bicuspid extractions but only 57% erupted in a non-extraction
group.
• Faubion compared 40 treated orthodontic patients, 20 of
whom were non extraction and 20 who had four 1st bicuspid
extraction. He found four times as many erupted extraction
group. He concluded that removal of 1st bicuspid for patients
with arch length discrepancy helps to provide space for the
eruption of mandibular 3rd molars.

• Kaplan concluded, where bicuspids have been extracted as part
of orthodontic therapy, there is an increased probability of 3rd
molar eruption.
• Richardson stated that, extraction of a bicuspid has the effect of
reducing the frequency of 3rd molar impaction, and the
extraction of a 2nd molar virtually eliminates it. And also stated
that 2nd bicuspid extraction may be more favourable than 1st
bicuspid extraction, in releasing space for lower 3rd molar
eruption.
• In a class III case, with class III elastics, the lower labial
segment would be retracted into the space released by bicuspid
extractions. However, in a class II uncrowded case, with class II
elastics, an optimal release of space for lower 3rd molar could be
predicted.

In a case with mild crowding,
bicuspid extraction could be expected
to icnreased space for 3rd molars.
However in a bimaxillary protrusion
case, the 7mm of space gained on each
side would be used for retraction of
anterior

teeth

and

minimal

improvement could be predicted for 3rd
molars.

Accelerated eruption of third molars after extractions:
• Fannng, Richardson have reported accelerated eruption of
3rd molar following extraction of teeth further forward in the
arch; with Richardson noting that molar extractions
produced the greatest acceleration.
• Haavikko et al., felt that bicuspid extractions merely
accelerate, but do not promote, eruption of third molars.
• Huggins, in a review of six cases, believed that extraction
stimulated 3rd molars to early eruption and noted that some
erupted as much as seven years earlier than average.
• Accelerated eruption can also occur in association with
missing teeth in the anterior region.

• Silling sampled 100 non-extraction orthodontic patients
and found 67 per cent males and 69 per cent females
developed impacted 3rd molars.
• Ricketts claimed that more than 50 per cent of orthodontic
patients required extraction of lower 3rd molars.
• Richardson found 55 per cent of lower 3rd molars were
impacted in the Belfast group of patients, who had had no
lower arch extractions.
• The incidence of impaction of lower 3rd molars is probably
25 per cent or higher in many populations. It is also
probable that between 30 and 70 per cent of so called non
extraction treatments will have impaction of lower third
molars.

Etiology of lower 3rd molar impaction:
 Poor mandibular bone growth
 3rd molar who had lighter in weight and had
smaller cranial dimensions.
 Vertical grower with short mandibular lengths.
 Retarded facial development
 Retarded 3rd molar development
 Late mineralization of the lower 3rd molar
crowns.
 Less space b/w second molar to the anterior
border of the ramus.

Width of the mandible:
(Transverse dento-skeletal relationships and
3rd molar impaction. – Angle orthod 1981;
51: 41-7).
Olive and Basford showed a
relationship b/w lower 3rd
molar impaction and ratio
b/w bilateral molar and
ramus widths. They stated
that low ramus/ molar ratio
leads to impaction.

The development of 3rd molar impaction
and its prevention. AJO 1981; 122-130
Richardson showed that 12% of
all lower 3rd molars became
more mesially angulated and
also concluded that
radiographic measurement at
age 10 or 11 years could not be
used to predict impaction
accurately.

3rd molars and lower incisor crowding
Facial growth
Bjork’s implant studies showed a
distal migration of the lower teeth
near the end of the mandibular
growth spurt. This facial growth
has been suggested as a possible
reason for lower incisor crowding.
Impacted 3rd molars may, in theory,
impede this distal migration, and
contribute to incisor crowding.


Pressure from behind theory
• Vego found more crowding in a group with erupting 3rd
molars than in a group with developmentally absent lower
3rd molar.
• Lindquist and Thilander extracted lower 3rd molars on
one side only in a group, and found less crowding on the
extraction sides than on the non-extraction side.
• Bergstrom and Jensen studied 60 dental students and
concluded that 3rd molars exerted some influence on
the development of lower arch crowding. but not enough
to recommend extraction or enucleation of 3rd molars
except in exceptional circumstances

• Schwarze reported that 56 orthodontically treated patients for
whom lower 3rd molars had been enucleated were more resistant
to late lower incisor crowding than a group with lower 3rd
molars.
• Bergstrom and Jensen examined 30 dental students with
unilateral agenesis of lower 3rd
molars and found more
crowding on the side with the 3rd molar present compared with
the side where it was absent.
• Richardson's Belfast 3rd molar study has produced further
evidence to support the 'pressure from behind' theory. A group
of 51 subjects with intact lower arches and both lower 3rd
molars present were examined at ages 13 and 18 years. Only
16% showed no increase in crowding. The average increase in
crowding was slightly more than 1mm on each side over 5
years. In some quadrants the crowding increased by 4 mm.

•

More recently, Richardson found that later on
(between ages 18 and 21 years) the lower arch is
stable in terms of tooth alignment and mesial
drift, regardless of continuing mandibuiar growth
and 3rd molar status.

• It

should

also

be

noted

that

Richardson's

observation that there is greater lower incisor
stability

after

extraction

of

2nd

molar,

supportive of the pressure from behind theory

is

No pressure from behind theory
•Kaplan studied 75 orthodontically treated patients on average
9.3 years after treatment. The mean age of Kaplan's sample was
26.6 years; it included extraction and non-extraction cases,
which introduced another variable. During the post-retention
period he found that no more lower crowding and rotational
relapse occurred in cases with 3rd molars than in those with
agenesis of 3rd molars.
•Sampson,
Richards,
and
Leighton
studied
54
nonorthodontically treated Caucasians and found no significant
difference in the crowding among groups with erupted, impacted,
or missing 3rd molars. They concluded that it did not seem to be
important whether the mandibular 3rd molar erupted vertically,
remained mesio-angularly impacted, or was absent.


• Southard reported in 1992 on the tightness of contact
points in the mandibuiar arch after unilateral extraction of
one lower 3rd

molar. The presence of a 3rd

molar was not

found to affect contact point tightness.
• This suggests that the pressure from a lower 3rd molar is not
continuous and may occur only when the tooth is trying to
erupt, slopping after the tooth becomes impacted. He
concluded that extraction of 3rd
prevent incisor

molars did not help to

crowding.

• He noted that the general tendency for lower incisors to
crowd with age occurs well into adulthood, with or without
the presence of 3rd

molars, and past the time when

unerupted 3rd molars would exert any influence.

Techniques for uprighting lower 3rd molars
• Uprighting and detail positioning of 3rd molars is
likely to be needed for some patients if 2nd
molars have been extracted.
• It may also be required for routine orthodontic
patients where 3rd molars have erupted in a less
than ideal position after four bicuspids have
previously been extracted, to avoid leaving the
patient with only 24 teeth.


Shallow mesio-angular impaction:
• A one-stage method is used. A 2nd molar tube can normally be
bonded onto the buccal aspect of a partly erupted lower 3rd
molar, if enough enamel is visible.

It is then possible to

include the tooth in full treatment, if other teeth are already
bonded and bracketed.

If the case is not fully banded, the

lower 2nd or 1st molars alone can be used with a lingual arch
for support.


Decp mesico-angular impactions
• A two-stage method is used. If it is not possible to bond
onto the buccal surface of an impacted lower 3rd molar owing
to the level of the gingival tissue, then a different technique
should be considered. This can be delayed until 18-19 years
of age, to allow lime for the tooth to improve its position
spontaneously as sometimes occurs.
• The 1st stage involves bonding a 2nd molar lube on to the
occlusal surface of the lower 3rd molar. The hook is removed
from the tube, before bonding. Lower 1st or 2nd molars are
banded, with a lingual arch, using 1st molar bands and
brackets. Capping is removed from the molar brackets.


• A small sectional archwire- with a compressed coil
spring, is used to provide a disializing and
uprighting force to the crown of the impacted
molar. After some uprighting using this method, it
is normally possible to bond a tube buccally for the
2nd stage.


The Extraction of 3rd molars
Extraction before treatment
• Henry described enucleation as a simple, rapid technique,
with minimal trauma, removing the lower 3rd molar tooth
germ when it is a mere uncalcified sac between ages 7 and
10 years.
• Schulhof recommended that enucleation should be
considered for any lower 3rd molars which, after careful
diagnosis, had a greater than 50% chance of impaction.
• Liddle" reported early success with enucleation, but noted
one case of interference with the posterior facial nerve. He
mentioned this possibility, and the chance of damage to the
developing lower 2nd molars, as contraindications

Lateral trepanation:
• Henry recommended a deep lateral approach, calling it
lateral trepanation' for tower 3rd molars in an early stage of
partial development. This was in contrast to the
conventional approach downwards through the external
oblique ridge.
• He recommended lateral trepanation as an easier technique,
with less complications and more rapid healing. The
technique of lateral trepanation, with its possible
importance to orthodontics, was also described by Burgess
et al. Nevertheless, it can be a traumatic procedure.
• In 1973, Schwarze reported positively on 56 percent who
had had early removal of 3rd
molars, describing the
technique as 'germectomy'. He compared them with 49
patients who did not have extraction, and concluded that
upper and lower 1st
molars subsequently drifted less
mesially in the extraction group.

Extraction during treatment:
• If orthodontic treatment includes orthognathic
surgery to one or both jaws, and 3rd molars also
require extraction, some surgeons prefer to do all
the necessary surgery, including any 3rd molar
extractions, at the same operation.
• However, other surgeons prefer to remove lower 3rd
molars 6 months before orthognathic surgery, so
that bone healing can occur in the surgical site.

Extraction after orthodontic treatment:
• Extraction of 3rd molars after orthodontic treatment with a
view to prevention of relapse should seldom be necessary. If
their impaction causes problems they can be removed by the
conventional approach.
• Richardson has warned that a final prediction of mesioangular impaction should not be made before age 18 years,
because lower 3rd molars continue to show rotational change
up to and beyond this time.
• The lower 3rd molars are just one of many factors which may
contribute to late lower incisor crowding, and there has been
a tendency to favor extraction soon after placing retainers,
instead of waiting to give them a chance to erupt. This
tendency should Schwarze reported that lower 1 st
drifted less be resisted.

molars

Conclusions:

.

The influence of the 3rd molars on the alignment of the
anterior dentition maybe controversial, but there is no
even the major etiologic factor in the post treatment
changes in incisor alignment. The evidence suggests that
the only relationship between these two phenomena is that
they occur at approximately the same stage of
development. ie., in adolescence and early adulthood. But
this is not a cause and effect relationship.

. The clinician has to have a justifiable reason to recommend
the extraction of any tooth.

.
.

The clinician has to consider the impact of the extraction
decision on any future treatment plan from an
orthodontic, surgical, periodontic, or prosthodontic aspect.
If extraction is indicated, 3rd molars should be removed in
young adulthood rather than at an older age.

Thank
“U”

Role of third molars in orthodontics /certified fixed orthodontic courses by Indian dental academy

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