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Premolar extraction influence on third molar angulation

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Premolar extraction influence on third molar angulation

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Content PREMOLAR EXTRACTION INFLUENCE ON THIRD MOLAR ANGULATION

by

Cabot Denny

A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(CRANIOFACIAL BIOLOGY)

MAY 2014

CABOT DENNY

2

Table of Contents
Abstract 3
List of Tables and Figures 4
Chapter 1. Extraction vs. Nonextraction Treatment
A. History 5
B. Arch Length Considerations 6
C. Nonextraction Treatment 7
D. First Premolar Extraction Treatment 8
E. Second Premolar Extraction Treatment 10
F. Extraction Considerations 13
Chapter 2: Third Molar Impactions
A. General Information 18
B. Previous Studies 21
C. Recent Studies 32
D. Morbidity of Third Molar Extraction 34

Chapter 3: Our Current Study
A. General Overview 40
B. Treatment Mechanics 41

Chapter 4: Hypotheses 43
Chapter 5: Methods 44
Chapter 6: Results 54
Chapter 7: Discussion 56
Chapter 8: Conclusions 59
Bibliography 63

3

Abstract. Premolar Extraction Influence on Third Molar Angulation
Objective: Previous studies evaluating premolar extraction influence on
third molar angulation have combined first and second premolar extractions into
a common extraction group. By combining the extraction groups, individual
influences of each premolar have been negated. Separating the 2 extraction
groups allows the influence of each premolar to be individually evaluated.
Methods: In this retrospective study, 90 patients were divided into 3 groups of 30
patients each. Groups were determined by extraction pattern (nonextraction, first
premolar extraction, second premolar extraction). The long axes of the second
and third molars were traced on initial and final panoramic radiographs. The
angulation change between the molars was determined between initial and final
angulations. Results: There was not a significant difference in uprighting
between the 3 treatment groups. Conclusion: Although the rationale that
extraction of premolars could create a greater amount of retromolar space for
third molars to upright and erupt into occlusion, our findings did not support this
rationale.

4

Tables and Figures
Table 1. Extracting First vs. Second Premolars 14
Figure 1. Buccal view of TSALD 15
Figure 2. Occlusal view of TSALD 16

Figure 3. Anchorage Considerations 17
Figure 4. Sample Cephalogram 37
Figure 5. Sample Panogram 38
Figure 6. Examples of Third Molar Eruption on Panograms 39

Figure 7. Method of Calculating Angulation Change 46

Figure 8. Nonextraction Treatment with Favorable Angulation Change 48

Figure 9. Nonextraction Treatment with Unfavorable Angulation Change 49

Figure 10. Extraction of First Premolars with Favorable Angulation Change 50

Figure 11. Extraction of First Premolars with Unfavorable Angulation Change 51

Figure 12. Extraction of Second Premolars with Favorable Angulation Change 52

Figure 13. Extraction of Second Premolars with Unfavorable Angulation Change 53

Figure 14. Comparing Changes in Third Molar Eruption 55
Table 2. Demographics of Sample Groups 60

Table 3. Average, Max, and Min Angulation Change of Lower Third Molar 61

5

Chapter 1: Extraction vs. Nonextraction Treatment
1A. History
The debate over extraction vs. nonextraction treatment dates back to the
origins of orthodontics as a specialty. Edward Angle was one of the earliest
proponents for nonextraction treatment. He believed that “the best balance, the
best harmony, the best proportions of the mouth in its relation to the other
features require that there shall be a full complement of teeth.” (Angle 1907)
Calvin Case argued the opposing view and defended the practice of extracting
teeth for orthodontic treatment. While Case agreed with Angle that all cases
could theoretically be treated nonextraction, he disagreed that all these cases
would remain stable. (Pollock 1964) Angle's conviction ultimately prevailed and
his influence would shape the specialty for years to come. The extraction
argument continued for years, with other early practitioners like Martin Dewey
and Matthew Cryer joining the debate. In 1941, a practitioner named Charles
Tweed published a paper supporting orthodontic treatment with the extraction of
4 premolars. Tweed attended Angle's orthodontic school and worked closely with
him for the last 2 years of Angle's life. After six and a half years using Angle's
nonextraction philosophy, Tweed became dissatisfied with his case results due to
their protrusive facial profiles and dental instability. (Tweed 1944) Tweed
6

examined his treated cases, and recognized that those with pleasing facial profiles
also had lower incisors that were upright over basal bone. He selected some of his
completed cases that developed protrusive profiles and retreated each with
premolar extractions. He concluded that carefully planned extractions allowed
him to improve facial appearance and dental stability. He presented these
retreated cases at an annual orthodontic meeting, and faced considerable
scrutiny. The controversy continued throughout Tweed's life, and the debate
between extraction and nonextraction treatment continues today.

1B. Arch Length Considerations
The debate over whether it is beneficial to extract teeth initiates from
patients lacking space to align teeth within bone. Successful orthodontic
treatment will introduce several factors that will affect the location of the
dentition within the bone.
Perhaps the most notable problem for an orthodontic practitioner to
resolve is the Tooth Size Arch Length Discrepancy (TSALD). TSALD occurs when
the sum of mesial-distal lengths of the individual crowns is greater than the linear
bone available. These teeth are initially able to exist within bone because they
are misaligned, often overlapping each other. The crown lengths are
7

discontinuous, therefore taking up less space. These crowns will be aligned during
orthodontic treatment, making them continuous, and will require additional
space.
Another obstacle that requires space management is the resolution of the
curve of spee. One of the first steps in orthodontic treatment is to level the
occlusal plane. This step is necessary because orthodontic treatment proceeds by
moving the teeth in relation to one another along this level occlusal plane. Similar
to the affects described in TSALD, as the teeth are leveled along a flat plane, they
will require additional space.
Molar correction using inter-arch elastics is another factor that influences
the amount of room available for the dentition. Class II dentitions will require the
lower dentition to move anteriorly to correct into a class I relationship. As the
lower dentition moves forward, it approaches the limits of the alveolar bone, and
decreases the space available for alignment. The forward movement of class II
dentitions necessitates an increase in space for the lower incisors.

1C. Nonextraction Treatment
Nonextraction treatment requires the clinician to preserve a complete
dentition. Conventionally, this treatment modality is possible for cases with
8

spacing, non-crowded cases, and cases with minor TSALD. In cases with minor
TSALD, it is often necessary to decrease the total amount of crown length. There
are various ways to decrease the amount of crown length. One way is to use
Interproximal Reduction (IPR). (Lusterman 1954) Using this technique, the mesial
and distal sides of the anterior teeth are reduced, thus decreasing the total
amount of tooth length. Similarly, reduction in size can be performed on the
posterior teeth in a process called Air-Rotor Stripping (ARS). (Sheridan 1985) IPR
and ARS are limited by the amount of enamel present. Excessive removal causes
misshaped crowns and increased patient sensitivity. While IPR and PNS are both
effective, their limitations restrict their use to only resolving minor TSALD.

1D. First Premolar Extraction Treatment
While nonextraction treatment is possible for cases with minor TSALD,
cases with a moderate to large amount of TSALD require additional methods to
create space for the dentition. Baumrind identified TSALD as "the most important
factor necessitating the decision to extract premolars." (Baumrind et al. 1996)
Several other authors have recognized lower TSALD as one of the most important
factors that influences treatment planning. (Paquette et al. 1992,
Luppanapornlarp et al. 1993, Beattie et al. 1994) The teeth most commonly
9

extracted for orthodontic treatment are the first premolars. First premolars have
both optimal size for extraction space, and arch location so that the clinician can
utilize the maximum amount of space. Since extraction rates vary among
clinicians due to individual preferences, no specific extraction rate can be taken to
represent the entire population. An evaluation of the extraction rate at University
of North Carolina clinic revealed that the rate increased dramatically during the
1960’s and has been declining until present day. (Proffit et al. 2012)
The average mesiodistal length of a first premolar is 7.2 mm. By extracting
2 first premolars the dental arch is benefitted with 14.4 mm of space to align the
teeth. (Black 1902) This space closes through the anterior teeth moving distally,
and the posterior teeth moving mesially. (Williams et al. 1976) Although the
anterior and posterior segments are moving reciprocally, several factors cause the
anterior teeth to close a larger proportion of the extraction space. Large multi-
rooted posterior teeth resist movement more than smaller, single-rooted anterior
teeth. (Jepsen 1963)

The posterior teeth tend to move bodily with less movement,
while anterior teeth have greater movement through a combination of bodily and
tipping actions. (Williams et al. 1976) Also, with the extraction space located
closer to the anterior teeth, it allows a greater amount of incisor retraction.
(Steyn et al. 1997) In general, these factors contribute to the anterior teeth
10

closing 2/3 of the space, and the posterior teeth closing 1/3 of the space.
(Creekmore 1997) The resultant space is used for leveling, aligning, and correcting
the incline of the lower anterior teeth. (Bishara et al. 1995) Due to unequal
closing movements between the anterior and posterior segments, first premolar
extractions are indicated in treatment situations that require a greater amount of
space.

Considerations effecting the decision to extract:
 Evaluation of profile
 Amount of TSALD
 Location of TSALD
 Evaluation of Curve of Spee
 Incisor angulation
 Second molar angulation

Indications for first premolar extractions:
 Protrusive lower facial profiles
 TSALD is moderate to severe
 TSALD is located in the anterior dental arch
 Excessive lower incisor angulations

1E. Second Premolar Extraction Treatment
Facial aesthetics represents an important factor that influences orthodontic
treatment. (Talass et al. 1987, Boley et al. 1998, Ismail et al. 2002, Kim et al. 2003,
11

Stephens et al. 2005, Germec et al. 2008, Konstantonis 2012) Extraction
treatment affects on facial profiles have influenced some orthodontists to extract
second premolars rather than first premolars. These clinicians believe first
premolar extractions provide more space than necessary to align the dentition.
This results in over retraction of the lower anterior teeth to close the space.
(Dewel 1955) The location of these teeth affect the soft tissue and can produce a
flat or retrusive lower facial profile. Clinicians rationalize that extracting second
premolars, when less space is needed, preserves the lower facial profile. De
Castro supports this reasoning in his paper when he recommends to preserve the
facial profile by extracting second premolars in average extraction cases. (De
Castro 1974)
Second premolars tend to be similar in size as first premolars, providing a
similar amount of extraction space. The anchorage requirements for space
closure differ from first premolar extractions due to the location of the extraction
space and number of teeth in the posterior anchorage segment. Second premolar
extractions produce a more distally located extraction space, and do not have a
premolar in the posterior segment. This protocol results in the molars moving
forward a greater distance to close the space than first premolar extractions.
(Brandt et al. 1975) This results in the extraction space closing equally from the
12

anterior and posterior segments. On average, the anterior segment moves distally
7.5mm and the posterior segment moves mesially 7.5mm. (De Castro 1974)

Only
enough space to align and retract the anterior teeth is created, therefore
preventing over-retraction of the anterior teeth. Authors have differing opinions
on when second premolars should be extracted. Schoppe advises second
premolars extractions in cases with TSALD of 7.5 mm or less. (Schoppe 1964) De
Castro advocated extraction of second premolars in cases with TSALD of 5 mm or
less. (De Castro 1974) Regardless of the author, the protocols aim to resolve
moderate crowding while not changing the facial profile. There are certain
treatment situations where it would be preferable to have second premolar
extractions.

Indications for second premolar extractions:
 Acceptable to retrusive lower facial profiles
 TSALD is minor to moderate
 TSALD is located in the posterior dental arch
 Normal and retrusive lower incisor angulations
 TSALD without protrusive profile
 Open bite cases

13

1F. Extraction Considerations
In addition to different space considerations between the premolars, there
are also different clinical deliberations for the extraction of each premolar. It is
clinically accepted that the distal surface of the second premolar has a better
contact with the first molar, and that the mesial surface of the first premolar has
a better contact with the canine. Removing either of these premolars forces the
clinician to decide which contacts will be compromised. Some clinicians dislike the
unusual shape of the lower first premolar and prefer to extract these teeth. These
compromised contacts and morphological differences contribute to the clinician’s
decision on which tooth to extract.
Although general guidelines exist for when to extract first vs. second
premolars, the final decision rests with the practitioner. The decision whether to
extract first or second premolars can be summarized by the following table.

14

Table 1. Extracting First vs. Second Premolars
1st Premolar Extraction 2nd Premolar Extraction
Profile Protrusive Normal
Amount of TSALD Moderate to Severe Mild to Moderate
Location of TSALD Anterior Posterior
Incisor Angulation Excessive Normal
Space for 2nd Molar
Eruption
Adequate Limited

15

Figure 1. Buccal view of TSALD

Minor TSALD

Moderate TSALD

Severe TSALD

16

Figure 2. Occlusal view of TSALD

Mild TSALD

Moderate TSALD

Severe TSALD

17

Figure 3. Anchorage Considerations

Second premolar extractions have a more distal extraction space and a premolar
in the anterior segment.

First premolar extractions have a more anterior extraction space and a premolar
in the posterior segment.

18

Chapter 2. Third Molar Impactions
2A. General Information
Third molars represent a significant concern for the orthodontic profession.
Many children undergo orthodontic treatment while the third molar develops and
begins to erupt. Patients and their parents are often concerned about the status
of these teeth and whether or not these teeth will need to be extracted in the
future. While impaction is not the only reason for their removal, it is responsible
for a yearly average of 10 million third molar extractions in the United States
alone. (Amer. Journal Public Health 2007) Other reasons for extraction include:
prophylaxis, pericoronitis, orthodontics, caries, pulpitis, cysts, tumors, root
resorption, and other factors.

(Lysell et al. 1988)
Third molars are the last teeth to erupt in the adult dentition, often
erupting after jaw growth has been completed. (Rantanen 1967) Some authors
have found race to be an influencing factor in eruption times. Studies have shown
mandibular third molars to erupt as early as 14 years old in Nigerians, (Odusanya
et al. 1991) while other studies have shown them to erupt as late as 26 years old
in Europeans. (Kruger et al. 2001) Race has also been shown to influence
impaction rates, in general, with tooth impactions occurring more commonly in
whites than African Americans. (Brown et al. 1982) Studies have shown gender to
19

be an influencing factor in eruption times as well, with mandibular third molar
eruption 3 to 6 months earlier in males than females. (Hattab 1999) Gender
influences on impaction rate have shown to be controversial. Studies on
impactions, in general, have shown that females have higher rates of impactions
than males. (Sanhu et al. 1982, Narda et al. 1959) Several other authors have
shown similar findings specific to mandibular third molar impactions, with
females having higher rates than males. (Kruger et al. 2001, Hattab et al. 1999,
Yuasa et al. 2004, Hellman 1961) Hellman attributed this to differing gender
growth patterns relative to third molar eruption. On average, females have
stopped growing before the onset of third molar eruption, while males continue
to grow during this time. (Taranger et al. 1980, Engstrom et al. 1983) This
contributes to facial measurements to be significantly smaller in women than in
men during the eruption of third molars. (Forsberg et al. 1976) A Saudi Arabian
study on third molars in general found no gender predilection for impaction.
(Haidar et al. 1986) While this study was not specific to mandibular third molars,
it does bring the previous reasoning into doubt.

Engstrom et al found that third molar crown formation was completed by
14 years of age and root development by the age of 18. (Engstrom et al. 1983)

Third molars generally erupt during the latter stages of root development
20

between 17 to 21 years of age. (Ash 1993, Proffit et al. 1993) These 2 factors
influence the ideal timing for their removal. This occurs when the roots are
between one-third and two-thirds formed, usually during the late teenage years,
between ages 17 and 20 years old. (Hupp et al. 2014)

Alling’s study on impacted teeth showed that 72.7% of the world
population has at least one impacted tooth. (Alling et al. 1993) In a 1989 Swedish
study by Lysell and Rohlin on Swedish Dental students, they found that 33% had
at least 1 third molar impaction. They also found that the frequency of third molar
impaction was higher in the lower jaw than the upper jaw. (Schersten et al. 1989)
Although it is generally accepted that the third molar is the most frequently
impacted tooth in the dental arch, (Hupp et al. 2014, Dachi et al. 1961, Grover et
al. 1985) the reported rate at which they become impacted is varied. Various
researchers have reported different rates of impaction. A few of the rates
reported by authors:
 Hellman 9.5 % (Hellman 1938)
 Bjork 25 % (Bjork et al. 1956)
 Ricketts 50 % (Ricketts 1972)
 Richardson 35 % (Richardson 1977)

Ninety five percent of all young adults have at least 1 third molar and
seventy five percent have all 4 third molars. (Hugoson et al. 1988) Given the high
21

proportion of the population with third molars, and the high rate of impaction, it
shows why some have attributed mandibular third molar impaction as one of the
major problems facing the dental profession. (Silling 1973) This widespread
problem has motivated researchers to further investigate these types of
impactions.

2B. Previous Studies
Bjork was one of the early researchers to investigate third molar
impactions. In 1956, Bjork conducted a study to determine the factors in
mandibular growth that affected third molar eruption space. (Bjork et al 1956) He
used lateral cephalometric radiographs to measure the distance from the distal
surface of the second molar to the anterior edge of the ramus, along a line
parallel to the occlusal plane of the lower arch. Bjork identified a decreased
amount of eruption space distal to the second molar in 90% of the impaction
cases. (Bjork et al. 1956) He identified 3 developmental factors that affected the
third molar space: (1) amount of increase in the length of the mandible (2)
direction of condyle growth, and (3) direction of tooth eruption. Mandibular
length increases through growth at the posterior ramus and resorption at the
anterior ramus. (Silling 1973) A decreased amount of either of these processes
22

would produce a diminished eruption space for the mandibular third molars. This
reasoning agreed with Broadbent, who also believed that the lower third molar
becomes impacted when there is insufficient growth of the mandible. (Enlow
1968) Changes in condyle shape during development exhibited considerable
influence on the third molar space. Eruption space was benefitted by horizontal
growth of the condyle, while the space was diminished by vertical growth. A
distally erupting dentition, most notably the second molar, will encroach on
eruption space and lead to an increased chance for third molar impaction. Bjork's
studies helped to identify developmental factors that can influence third molar
impactions.
Ricketts was another researcher that investigated the third molar dilemma.
He believed that over 50% of his orthodontic patients had a third molar
impaction. (Ricketts 1972) He also noticed a greater amount of third molar
eruption in extraction cases. He evaluated the anterior-posterior positioning of
developing third molars relative to the external ridge of the ramus. He found that
third molars with a greater portion of the crown mesial to the ridge had a greater
chance of eruption, while those with a greater portion distal, a poorer prognosis
for eruption.
23

Enlow's studies on mandibular growth showed that the remodeling that
occurred at the mandibular ramal surface made it unreliable as a reference.
(Enlow 1968) Rickett's subsequent studies overcame this surface variation
through development of a stable reference point. X
i
point was the result of his
studies. X
i
point's location at the center of the ramus body reduced variation from
surface remodeling. Several authors conducted similar studies to Bjork's, where
they measured the space available between a reference point in the ramus and
the second molar, and attempted to determine the probability of eruption.
(Schulhof 1976, Ricketts 1979)

Rickett's study measured eruption space as the
distance from Xi point to the distal of the lower second molar along the occlusal
plane. Using lateral cephalograms on children at 8-9 years of age and using
growth prediction curves, he predicted the amount of eruption space available.
He calculated that third molars would erupt if this predictive distance was greater
than 30 mm, but impaction was likely if this distance was less than 20 mm.
(Ricketts et al. 1976)
While Ricketts believed the probability of impaction could be diagnosed
with 90% certainty, (Ricketts et al 1976) Olive and Basforth evaluated the
reproducibility of Ricketts study and found that the predictability was unreliable.
(Olive et al. 1981) They determined that prediction of impaction or eruption
24

based on the distance between X
i
point and the lower second molar were
undependable.
As Olive and Basforth's study had shown, it is difficult to predict future
impactions of the third molar. Adding to this difficulty is the fact that the typical
orthodontic patient completes treatment before third molar eruption ceases.
Addressing a desire to solve this problem and better predict third molar
impactions, Forsberg conducted a study on space availability on lateral
cephalograms. He was unsuccessful in his attempts. Contributing to his failure
was the fact that even when the space was judged to be inadequate, mandibular
third molars still erupted half the time. (Forsberg 1988, Forsberg et al. 1989)
Richardson also studied the predictability of third molar impactions. She
attempted to identify measurements on lateral cephalograms at 10-11 years old
that would indicate third molar impactions later in life. Subjects were followed
until their third molars had erupted, or were diagnosed as impacted. Subjects
were then separated into erupted and impacted groups and comparisons were
made. Similar to Bjork's earlier studies on mandibular growth, she found that
impactions were associated with reduced mandibular growth. While she also
found that the developmental angulation of the third molar to the mandibular
plane was higher in the impacted group, she determined that accurate
25

predictions for third molar impaction were not possible at age 10-11 years.

She
doubted that any accurate impaction predictions could be made, even with
accurate estimates of growth and eruption space, due to the variability of the
third molar eruption path. (Richardson 1977)

Some researchers note also address third molar crown angulation and path
of eruption as factors for impaction. Maxillary third molars are distally angulated,
while mandibular third molars are mesially angulated at the time of calcification
and root development. (Sicher 1965, Richardson 1992) Accordingly, maxillary
third molars generally have a distal path of eruption, while mandibular thirds
have a mesially inclined path of eruption. Previous research on mandibular
impactions showed that mesioangular impactions were the most common,
comprising 43-46%. (Peterson 1992, Kan et al. 2002) These impactions had
mesially converging angulations to the second molars. Distoangular impactions,
with distally angled long axes, comprised 6%. 3% of all cases had impacted teeth
perpendicular to the second molar, known as horizontal impactions. (Kan et al.
2002, Hupp et al. 2014)
While third molar impactions are caused by many factors, researchers
agree that one of the most influential factors is the amount of eruption space.
Since third molars are the last teeth to erupt, the eruption space available is
26

dependent on the previously erupted dentition. If the dentition limits the amount
of eruption space, impactions are likely. (Bjork 1963, Bishara et al. 1983)
Following the third molars, the most common impactions are the maxillary
canines and the mandibular premolars. (Hupp et al. 2014) These teeth are the last
teeth to erupt anterior to the first molars in their respected arches. Similar to
third molars, the previously erupted dentition defines the amount of eruption
space available, and results in impaction of these teeth if the space is inadequate.
Also contributing to space availability is the size of the erupting third molar
crown. The average length of the mandibular third molar crown in 10.5 mm. (Ash
1993) Larger third molar crowns require a greater amount of space for eruption,
thus also contributing to spatial considerations. Richardson found this to be true
in her studies, with impacted third molars larger than those that erupted.
(Richardson 1977)
Studies on skull materials have also indicated that third molar impaction is
caused by inadequate spacing. Studies have shown that third molar impactions
were relatively infrequent in primitive populations.(Murphy 1964, Alling et al.
1993) Begg believed that this was due to attrition of primitive man.(Begg 1954)
He believed that attrition in the anterior teeth created space for the dentition to
move forward, and this movement created space for the erupting thirds. As man
27

has evolved, better diets and oral hygiene have led to a decrease in attrition.
(Lombardi 1982) According to Begg's theory, this lack of anterior space limits the
amount of forward movement by the dentition, and decreases the amount of
eruption space available. (Mehta et al. 1966)
Faubion also studied the spatial effects of a mesially drifting dentition, but
used extractions for space creation rather than attrition. He was able to show that
the prevalence of third molar impaction was reduced, but not eliminated, by
premolar extractions. (Faubian 1968) Richardson led a similar study on the effects
of lower second molar extractions on developing third molars. She evaluated 63
subjects that had second molars removed and found that in all the cases, the third
molars erupted 3-10 years after extractions. She found that ninety-nine percent
of the third molars uprighted mesiodistally, but few as upright as the second
molars they were replacing. (Richardson et al. 1993) Separate studies by
Cavanaugh and Artun supported Richardson's findings. These researchers also
evaluated third molars after second molars were extracted. Each found that third
molar impaction was rarely observed in these cases. (Gooris et al. 1990,
Cavanaugh 1985)

Based on these findings, it could be suggested that given
enough space, third molars will find a way to erupt. Foubian, Richardson, Artun,
28

and Cavanaugh all found independently that extractions provided additional room
for eruption space and decreased the third molar impaction rate.
Artun lead several studies that evaluated the affects that extraction and
nonextraction treatment had on third molars. In one of these studies, he
evaluated 157 patients from a post-retention sample at the University of
Washington. He used lateral cephalograms and compared mesial molar
movement and eruption space in nonextraction and extraction patient samples.
He calculated third molar angulation by measuring the angle between the occlusal
plane and a line bisecting the occlusal surface of the third molar. Linear mesial
molar movement was measured by calculating the distance between the distal
surface of the lower second molar and a point on the ramus. Artun's study found
that premolar extraction therapy reduced the frequency of third molar impaction.
He confirmed that impaction of mandibular third molars occurred twice as often
in nonextraction patients compared to extraction patients. (Kim et al. 2003) He
attributed this to the increased eruption space due to mesial molar movement
during space closure. His study was unique because in addition to initial and final
records, it also included records from 10 years post treatment. Panoramic and
cephalometric radiographs were valuable in determining impactions
29

radiographically rather than clinically. This allowed the clinician to determine
impactions based on apex formation and cessation of eruption.
This study had several limitations. His use of lateral cephalograms
introduced possible inaccuracies through superimpositions. Superimpositions
exist that make it difficult to accurately trace the third molars and the distal
aspect of the second molars. Another study limitation was the use of the occlusal
plane as a reference plane. One of the first steps in orthodontic treatment is to
level the occlusal plane, therefore altering the reference plane between initial and
final treatment. In addition, he combined both first and second premolar
extractions into one group. This negated any additional eruption space gained by
second premolar extraction. Due to the more posterior location of the second
premolars in the dental arch, one could rationalize that second premolar
extractions would offer a better chance for eruption than first premolars. His
study could be improved by using panograms for more clarity, using a more stable
reference plane, and by separating the extraction group into individual first and
second premolar extraction groups.
As discussed in the previous sections on premolar extractions, the first
molar moves forward different amounts depending on which premolar is
extracted. Nonextraction treatment offers minimal space creation, and has little
30

expected molar movement. Premolar extraction treatment offers greater space
creation, with differing molar movement based on extraction location and
anchorage considerations. Cumulatively, these factors lead to greater mesial
molar movement in second premolar extractions. Since the molar is moving
forward a greater distance, it is also increasing the space behind it, and therefore
increases the eruption space available for the third molar. Artun's study evaluates
the angulation affects that are changed because of this distance. The mandibular
retromolar space can increase about 2 mm from age 15 to adulthood, due to
growth alone. (Ledyard 1953) Richardson found that there was an average change
of 11.2 degrees in the mandibular third molar between the ages of 10 and 15
years of age. (Richardson 1970) Based on this information, one could reason that
the best prognosis for third molar eruption is to maximize the amount of MMM
and allow for greater uprighting of the third molar. Nance et al suggested that if
unerupted third molars were more vertical, eruption to the occlusal plane was
more likely than if the third molars were inclined more mesially. (Nance et al.
2006) Extraction of second premolars would cause more mesial molar movement
and could offer a better prognosis for third molar eruption.
Turkoz and Ulusoy conducted a retrospective study on the mandibular third
molars of 44 patients at Gazi University. The study evaluated the linear and
31

angular changes of the third molar in extraction and nonextraction treatment. It
used the lower occlusal plane to measure the amount of space available from the
distal of the lower second molar to the anterior ramus. It also used the long axis
of the second molar to evaluate the angulation of the erupting third molar. They
found that in the nonextraction group, 81.8% of third molars were impacted,
while only 63.6% of third molars were impacted in the extraction group. (Turkozet
al 2013) They also found that retromolar distance increased significantly in the
extraction group with a mean distance of 1.30 ± 1.25mm.
In addition to studies showing a decreased rate of impaction in extraction
cases, (Dierkes 1975) other studies have shown an increased rate of impaction in
nonextraction cases. (Kaplan 1975, Richardson 1975) This has led some to believe
that third molar impactions are significantly influenced by extraction therapy. This
relationship is controversial due to other studies showing only small differences
between extraction and nonextraction treatment. (Haavikko et al. 1978, Graber et
al. 1981) Contributing to the controversy is whether uprighting of the third molar
is benefitted by extraction therapy. Developing third molars continually change
their angular positions and pre-eruptive rotational movements. (Silling 1973,
Richardson 1978, Huggins 1962)

Longitudinal studies have shown that the
mandibular third molar uprights during early adolescence without orthodontic
32

treatment. (Richardson et al. 1984, Richardson 1973) Some researchers have
found that premolar extractions improve the angulation of the developing third
molar, (Al Kuwari et al. 2013, Feng et al. 2013, Saysel et al. 2005) and rationalize
that the increase in space is responsible for the uprighting. Staggers led a similar
study and found that there were no significant differences in the change in third
molar angulation in either group. (Staggers et al. 1992) In addition, Capelli's study
found that it is unclear whether angulation can be used as a predictive factor for
impaction. (Capelli 1991)

2C. Recent Studies
In a 2009 study, Jain and Valiathan used panoramic radiographs to evaluate
third molar angulation changes referenced to the palatal plane. The two sample
groups were patients that underwent either first premolar extractions or
nonextraction treatment. Their study used a modified version of the midline
reference plane (MRP) that was developed by Elsey and Rock. (Elsey et al. 2000)
The MRP was determined by bisecting the nasal septum and the anterior nasal
spine. A line was then drawn perpendicular and defined as the horizontal
reference plane (HRP). A line parallel to the third molar occlusal plane was then
traced and initial and final angulations between third molars and HRP were
33

calculated. They found a significant difference in angulation between the
extraction and nonextraction groups. Extraction treatment resulted in a
significant improvement in third molar angulation on both right and left sides.
(Jain et al. 2009)

A study performed by Proffit addressed the need to separate the 2
extraction groups. In this study, 3 treatment sample groups were formed (1st
premolar extractions, 2nd premolar extractions, nonextraction). The long axes of
the second and third molars were then traced on initial and final panoramic
radiographs. Third molar angulations were then calculated using the long axis of
the second molar as a reference plane. This study found that the average
angulation change during treatment was not significantly affected by group, but a
higher proportion of third molars were more vertical by at least 5° in the second
premolar extraction group. (Russell et al. 2013)
An advantage of the Proffit study is that it is one of the few that isolates
second premolars into their own extraction group. Other studies combined first
and second premolar extractions in the same group. Isolating second premolars
allows a greater amount of space to be recorded, and the different affects of this
space to be expressed. The additional space provided by second premolar
extractions could allow additional uprighting of the developing third molar.
34

Limitations of this study were that the reference planes used to evaluate
the angulations were the long axes of the second and third molars. Second molars
are commonly involved in orthodontic treatment. Once a bracket is placed and
engaged with an archwire, the reference plane changes from before and after
treatment. Some practitioners decide against engaging second molars for various
reasons. For some it is because they are already reasonably aligned, others wish
to avoid vertically opening the bite. Even in situations when a bracket is not
placed, second molars will still shift due to the rest of the dentition shifting,
especially during space closure in the extraction groups. Using the long axis of the
developing third molar is limiting due to the changing morphology. The third
molar develops additional root structure, possibly changing the long axis of this
tooth between initial and final radiographs.

2D. Morbidity of Third Molar Extractions
If clinicians were better able to predict third molar eruption, it would
improve the strategy for their treatment. Kaminishi found that between 1997 and
2002, there was an increase in patients over the age of 40 requiring third molar
removal. Out of all age groups requiring third molar extractions, the percentage
35

of over 40 year olds requiring extraction went from 10.5% in 1997 to 17.3% in
2002. (Kaminishi et al. 2006)

The increase in extractions for older patients is alarming due to research
that shows that complications with extractions increase with increasing patient
age. A study on the symptoms of third molars showed that the frequency of the
symptoms increased with age. (Punwutikorn et al. 1999)

Bruce evaluated
operative and postoperative morbidity associated with the removal of impacted
mandibular third molars in patients of various ages. He found a significant
increase in surgical morbidity as patients became older. He found all risks
associated with third molar removal to increase in patients over the age of 35.
(Bruce et al. 1980) Similar studies have shown a generalized increase in risk
correlated with increasing age. (Mercier et al. 1992, Bui et al. 2003, Valmaseda-
Castellon et al. 2001) These studies have collectively shown an increased
extraction risk in older patients.
Studies specific to periodontal defects after third molar extraction have
also shown an increased age related risk. These defects have been shown to occur
twice as often in patients over 26 years old, compared to patients under 25,
following third molar removal. (Kugelberg 1990) Risks for second molars were
also increased in older groups. Comparing groups of patients that were older and
36

younger than 25, periodontal defects were 2-3 times more common after third
molar extraction in the older group, and the persistence of the defects were age
related. (Kugelberg et al. 1991, Kugelberg et al. 1985)

The risk of jaw fracture may
also be age related. A study by Krimmel showed the major risk factor for fracture
to be advanced age and a full dentition, rather than degree of tooth impaction.
(Krimmel et al. 2000) Possible nerve damage is an additional treatment
complexity that needs to be considered. Studies have shown an existence of
persistent nerve involvement after third molar extraction. (Valmaseda-Castellon
et al. 2001, Schultze-Mosgau et al. 1993, Gulicher et al. 2001, Queral-Godoy et al.
2005) Although some of these cases spontaneously recover, some cases need
additional surgeries for repair, and other cases completely fail to recover.

Identifying a future impaction would allow the clinician to take a
prophylactic approach to their removal. A germectomy procedure removes the
developing tooth before the tooth has formed. Studies of germectomy
procedures in younger patients have shown no cases of periodontal defects,
nerve damage, or second molar damage. (Ash et al. 1962, Chiapasco et al. 1995,
Chossegros et al. 2002) From these studies it appears that a germectomy
procedure may be beneficial in lowering the incidence of third molar morbidity.

37

Figure 4. Sample Cephalogram

38

Figure 5. Sample Panogram

39

Figure 6. Examples of Lower Third Molar Eruption on Panograms

Ideal Eruption to Occlusion

Mesial Impactions

Distal Impaction (Lower Left)

40

Chapter 3. Our Current Study
3A.General Overview
The purpose of this study was to reproduce an existing study by Proffit et
al. We address a limitation of Proffit's study by using all digital radiographs from
the same machine. We evaluated the change in mandibular third molar
angulations relative to the adjacent 2nd molars in nonextraction, 1st premolar
extraction, and 2nd premolar extraction treatment.
After having conducted a careful study and bibliographic review, it was
decided to evaluate third molars using panoramic radiographs. This allowed one
to visualize both sides of the dental arch and measure the structures with the
least amount of superimposition.
The majority of extraction studies have tended to focus on the anterior
segment and the resolution of crowding issues. The few that evaluate the
posterior segment each have their limitations. This study evaluates the posterior
segment and examines the effects caused by the mesial molar movement of the
second molars. This study examines the additional third molar retromolar space
that is created by premolar extractions and what effect this additional space has
on the eruption of third molars.

41

3B. Treatment Mechanics
Our study incorporates segmental mechanics for second premolar
extractions. While segmental mechanics does not offer an increase in anchorage
compared to traditional mechanics, it does offer other advantages. Segmental
mechanics allows for immediate space closure in a fresh extraction site. The
clinician can immediately close space, rather than waiting for aligning and
leveling. These steps take additional time and allow bone apposition, which
causes increased resistance during space closure. In addition, aligning and leveling
a crowded anterior dentition results in distalization of these teeth into the
extraction space to relieve crowding. This partially closes the extraction space
before any molar traction is applied to close the space. A decreased extraction
space means that there is less distance for the molar to mesialize. By starting
space closure before leveling and aligning, a greater space exists and possibly
greater mesial molar movement.
Closing space immediately also allows for the maximum amount of time for
eruption space to influence third molar angulation. The earlier this space is
created, the lower the developing third molar tooth bud is located, and the longer
amount of time that this space will influence the eruption of the third molar.
42

Segmental mechanics offers several advantages for the developing third molar
over traditional mechanics.

43

Chapter 4. Hypotheses
There is no significant difference in third molar angulation between
nonextraction, first premolar extraction, and second premolar extraction
treatment.

44

Chapter 5. Materials and Methods
This was a retrospective study that included 3 patient sample groups that
completed orthodontic treatment. Each patient included had complete initial and
final orthodontic records, and 2 lower third molars at the start and end of
treatment. Patients were obtained from a university archive with no restriction on
ethnicity or gender. Patients selected were between the ages of 9 and 16 years
old at the time of initial records. Exclusion criteria included: craniofacial
anomalies, surgical cases, missing teeth, and extraction of third molars prior to
final records. A review of the records began in 2014 and then working backward
until the desired number of subjects (30) were obtained in each category. Sample
groups were nonextraction, first premolar extraction, and second premolar
extraction.
Using Dolphin Imaging software (Dolphin Imaging and Management
Solutions, Chatsworth, Calif), a vertical axis for the second and third molars were
digitized on panoramic radiographs. This was performed on each patient's initial
and final radiograph. Differences between initial and final angulations were then
calculated. An decrease in angulation of 5° or more was considered clinically
favorable.
45

Patients did not differ significantly at the start of treatment. The subjects
were an average of 12.4 years at the start of treatment, and similarly divided by
gender.

46

Figure 7. Method of Calculating Angulation Change

Initial Panogram

Final Panogram

Ɵ°
Final
- Ɵ°
Initial
= ∆Ɵ° Change in Angulation

Positive changes represent a more parallel orientation of the teeth, while
negative changes represent a more perpendicular orientation.
-21°
-12°
-43°
-40
-47°
47

Example Angulation Change Calculation

From Figure 7, the initial and final angulations for the lower left third molar
are -47° and -12°, respectively.

Negative final value indicates a converging path with the 2nd molar

(-12°) - (-47°) = +35°

Positive angulation change indicates improvement and a more
parallel orientation with the second molar

Negative initial value indicates a converging path with the 2nd molar

From Figure 7, the initial and final angulations for the lower right third
molar are -43° and -21°, respectively.

Negative final value indicates a converging path with the 2nd molar

(-21°) - (-43°) = +22°

Positive angulation change indicates improvement and a more
parallel orientation with the 2nd molar

Negative initial value indicates a converging path with the 2nd molar

48

Figure 8. Nonextraction Treatment with Favorable Angulation Change

-50.6° -47°
-19.5°
-26°
49

Figure 9. Nonextraction Treatment with Unfavorable Angulation Change

-18°
-20.5°
-52.4°
-36.4°
50

Figure 10. Extraction of First Premolars with Favorable Angulation Change

-35.2°
-17.6°
-16.1°
-39.1°
51

Figure 11. Extraction of First Premolars with Unfavorable Angulation Change

-62.2° -53.1°
-15.8° -13.5°
52

Figure 12. Extraction of Second Premolars with Favorable Angulation Change

-15.3°
-13.2°
-43.2°
-41.7°
53

Figure 13. Extraction of Second Premolars with Unfavorable Angulation Change

-4.9°
-27.6°
-66.5°
-70.9°
54

Chapter 6. Results
The average change in third molar inclination did not differ significantly
among the three study groups. The average angulation change for the lower right
third molar was -0.9° for nonextraction, -5.9° for first premolar extractions, and
-4.2° for second premolar extractions. The average angulation change for the
lower left third molar was -6.7° for nonextraction, -1.9° for first premolar
extractions, and -4.0° for second premolar extractions. Although not significantly
different, the extraction groups had more instances of third molar uprighting than
the nonextraction group. Out of a total of 60 third molars in each group, positive
uprighting was seen in 23 in the second premolar group, 25 in the first premolar
group, and 22 in the nonextraction group. The first and second premolar
extraction groups also experienced a higher proportion of third molars that
underwent a 5° more vertical change than the nonextraction group. Each
extraction group had 17 third molars that uprighted more than 5°, while there
were only 14 in the nonextraction group. Extraction treatment was not a
statistically significant explanatory factor for a clinically favorable change in
angulation.

55

Figure 14. Comparing Changes in Third Molar Eruption

90 80 70 60 50 40 30 20 10 0 10 20 30 40 50 60
Left Nonextraction
Left 2nd Premolar Extraction
Left 1st Premolar Extraction
Right Nonextraction
Right 2nd Premolar Extraction
Right 1st Premolar Extraction
Improved-->
<--Worsened
% of Third Molars
56

Chapter 7. Discussion
While patients often inquire at the start of treatment whether they will
need their third molars extracted, practitioners are often reluctant to evaluate
the status of the third molars until the conclusion of treatment. There is a
considerable benefit to the patient if practitioners were able to predict the fate of
the third molar early in treatment. While practitioners commonly calculate the
amount of arch perimeter necessary to align the dentition, they seldom calculate
the distance necessary to facilitate 3rd molar eruption. Premolars are commonly
extracted to aid in cases involving anterior arch length discrepancy, but the same
cannot be said in aiding third molar eruption. Posterior arch considerations, such
as the third molar impactions, should be considered early on in treatment
planning to avoid unnecessary extractions and surgery.
Data from this study indicated a slight increase in the number of third
molars that uprighted in both extraction groups, however the difference between
the groups was not significant. Out of a total of 60 third molars in each group,
positive uprighting was seen in 23 in the second premolar group, 25 in the first
premolar group, and 22 in the nonextraction group. From Proffit's study, he
considered a decrease of 5° or more as clinically favorable, and an increase of 5°
57

or more as clinically detrimental. Using Proffit's 5° definition of clinically
favorable, a higher proportion of mandibular third molars were more vertical by
at least 5° in the both the first and second premolar extraction groups. Each
extraction group had 17 third molars that uprighted more than 5°, compared to
14 in the nonextraction group. Again, the difference between the groups was not
significant. Similar to Proffit's study, there were large standard deviations for each
group in average angulation change, which represents a wide variability in
outcomes for each group. This study supports the findings originally made by
Richardson and supports her statement that due to the variability of third molar
eruption path, accurate predictions for third molar impaction are not possible.
The data we reported has limitations. Using the long axis of the second
molar as a reference plane introduces possible inaccuracies. The second molar is
commonly moving during treatment, both as a result of the dentition shifting and
due to the second molar itself commonly being involved in treatment. The ideal
reference plane for the third molar would be one that is stationary throughout
treatment. Another limitation involves using the long axis of the developing third
molar. Since the third molar is developing at the time of the initial radiograph, a
short distance defines the long axis. This shorter distance can cause small linear
inaccuracies to be recorded as much larger discrepancies in degree calculations. A
58

longer axis causes measurement errors to have a lesser impact when calculating
degrees of angulation. Root development can also cause inaccuracies, if root
growth deviates from the initial long axis. This causes a change in angulation in
the final radiograph that is falsely recorded as tooth rotation.
A follow up study would use the occlusal surface of the third molar as a
reference plane. Third molar crown formation is completed before root
development and avoids the inaccuracies of root growth during treatment. A
second reference plane would involve a structure that is stable throughout
treatment, so that any change in angulation can be solely attributed to the third
molar.

59

Chapter 8. Conclusions
This study evaluated third molar angulation changes in three different
treatment modalities. Although the rationale that extraction of premolars could
create a greater amount of retromolar space for third molars to upright and erupt
into occlusion, our findings do not support this rationale. While clinicians need to
be able to better predict third molar impactions, clinical judgment should still
determine the decision to extract premolars.

This study was approved by the IRB. ID: UP-13-00551

60

Table 2. Demographics of Sample Groups

Female

Male

Age at start of
Treatment (years)

Length of Tx
(months)

Nonextraction

22

8

12.4 (SD 1.0)

35.6 (SD 11.9)

First
Premolar
Extraction

15

15

12.6 (SD 1.4)

37.4 (SD 10.3)

Second
Premolar
Extraction

18

12

12.2 (SD 1.3)

40.3 (SD 9.9)

61

Table 3. Average, Maximum, and Minimum Angulation Change of Lower Third Molars

Nonextraction

First Premolar
Extraction

Second Premolar
Extraction

Right 3rd Molar
(degrees)

-0.9

-5.9

-4.2

Standard
Deviation

15

16.5

19.2

Minimum

-34.4

-46.4

-61.6

Maximum

33

21.5

35.6

Nonextraction

First Premolar
Extraction

Second Premolar
Extraction

Left 3rd Molar
(degrees)

-6.7

-1.9

-4

Standard
Deviation

11

15.8

16.1

Minimum

-23.1

-39.6

-43.3

Maximum

20.6

29

29.5

62

Table 4. Initial Angulation of Third Molars

Nonextraction

1
st
Premolar Ext

2
nd
Premolar Ext
Lower Left 3rd Molar
Mean, SD (degrees)

-22.9 (14.3)

-26.0 (12.5)

-25.7 (13.6

Lower Right 3rd Molar
Mean, SD (degrees)

-26.5 (14.1)

-26.3 (14.6)

-29.1 (15)

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Abstract (if available)

Abstract Objective: Previous studies evaluating premolar extraction influence on third molar angulation have combined first and second premolar extractions into a common extraction group. By combining the extraction groups, individual influences of each premolar have been negated. Separating the 2 extraction groups allows the influence of each premolar to be individually evaluated. Methods: In this retrospective study, 90 patients were divided into 3 groups of 30 patients each. Groups were determined by extraction pattern (nonextraction, first premolar extraction, second premolar extraction). The long axes of the second and third molars were traced on initial and final panoramic radiographs. The angulation change between the molars was determined between initial and final angulations. Results: There was not a significant difference in uprighting between the 3 treatment groups. Conclusion: Although the rationale that extraction of premolars could create a greater amount of retromolar space for third molars to upright and erupt into occlusion, our findings did not support this rationale.

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Creator Denny, Cabot (author)

Core Title Premolar extraction influence on third molar angulation

School School of Dentistry

Degree Master of Science

Degree Program Craniofacial Biology

Publication Date 04/02/2014

Defense Date 05/07/2014

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag OAI-PMH Harvest,premolar extraction,third molar angulation

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Language English

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Advisor Sameshima, Glenn T. (committee chair)

Creator Email cabotdenny8@gmail.com

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