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Abstract: If the concept of mesial movement of molars to reduce the “wedge effect” and decrease facial vertical dimension (FVD) is valid, it is important to investigate the effect of first (P1) and second premolar (P2) extraction on FVD. This study compares the mesial movement of molars and changes in FVD between P1 and P2 extraction groups in Class I malocclusion with a hyperdivergent facial type. We compared 27 cases with maxillary and mandibular P1 extractions (group 1) and 27 cases with maxillary and mandibular P2 extractions (group 2). To determine FVD changes due to treatment and to compare differences between two groups, paired t-test and independent t-test were performed, respectively. Group 2 showed more mesial movement of the maxillary and mandibular first molars and less retraction of the maxillary and mandibular central incisors than group 1 (P < .05). Both groups showed increased anterior facial height (P < .05), but there were no statistically significant differences in angular and proportional measurements between pre- and posttreatment. There was no significant difference in the amount of FVD change between groups 1 and 2 except in the maxillomandibular plane angle and SN to palatal plane angle (P < .05). These results suggest that there is no decrease in FVD regardless of the maxillary and mandibular P1 or P2 extraction. Therefore, the hypothesis that P2 extraction in hyperdivergent facial types will result in mesial molar movement and decrease FVD by reducing the wedge effect is invalid.
Accepted: March 2004. Submitted: January 2004
Schudy1–3 described facial types as “hypo- and
hyperdivergence” and recommended a nonextraction approach in the treatment of
hypodivergent facial types and an extraction approach “to close down the bite”
in hyperdivergent types. Sassouni and Nanda4 concurred with this
treatment philosophy. However, there is great controversy concerning the effects
of premolar extractions on facial vertical dimension (FVD). Some authors
speculate that first premolar (P1) extractions cause temporomandibular joint
disorder (TMD) by reducing FVD5–8 and overretracting maxillary
anterior teeth.9 However, many other reports offer data to disprove
this hypothesis.10–15
Because the indications for P1 extraction are usually severe
anterior crowding or lip protrusion,16 most of the extraction space
is used for alleviating crowding and retracting incisors. The remaining space is
closed by reciprocal movement of anterior and posterior teeth. The amount of
mesial molar movement can be very little if anchorage is well
maintained.17,18 It has not been shown that the bite is
closed in P1 extraction treatments by mesial movement of the molars. The changes
in FVD occurring with the extraction of maxillary and mandibular P1 were
reported to be no different than the changes in FVD occurring in nonextraction
cases.17–20
It has been shown that in borderline cases with moderate
crowding, fairly well-aligned incisors, and a relatively acceptable profile,
second premolars (P2) can be extracted.16,21–28 According
to the “wedge effect” concept, it is hypothesized that P2 extraction permits the
molar to move more mesially than P1 extraction, resulting in a greater decrease
of the FVD by reducing the wedge
effect.2,3,5,7,8 To
investigate the concept that the bite and FVD can be closed by mesial movement
of the molars, it is more appropriate to evaluate P2 extraction cases than P1
extraction cases.
To observe changes in FVD, it is appropriate to study
hyperdivergent facial type because it is in this group that excess FVD is of
greatest concern. In addition, according to the wedge effect concept, even small
changes in mesial movement of molars result in a pronounced effect on the
mandibular plane angle and FVD in this group. Class I maloccusions were selected
to study in order to eliminate excessive molar movement during treatment of
Class II and III malocclusions.
The null hypothesis is that there is a significant difference
in changes of FVD between P1 and P2 extraction groups with Class I malocclusion
and a hyperdivergent facial type. Therefore, this study investigates FVD change
by orthodontic treatment with P1 or P2 extraction and compares the effects of P1
and P2 extractions on FVD change.
The sample consisted of a total of 54 Class I hyperdivergent
type malocclusion cases (32° < SN-MP < 45°, 24° < FMA < 35°). The
hyperdivergent type was selected because it is easy to identify vertical
dimensional changes. They did not have severe anteroposterior (0° < ANB <
4°) and vertical discrepancies (0 mm < overbite < 4 mm) or TMD
symptoms.
Group 1 was composed of 27 cases (6 male and 21 female cases,
pretreatment age: 15.6 ± 3.9 years, treatment period: 2.3 ± 0.6 years) with
maxillary and mandibular P1 extractions. Group 2 was composed of 27 cases (6
male and 21 female cases, pretreatment age: 16.2 ± 4.0 years, treatment period:
2.5 ± 0.7 years) with maxillary and mandibular P2 extractions. To compare the
amount of mesial movement of the molar, the cases with moderate to severe
crowding were not selected for group 1. Therefore, the arch length discrepancy
of the maxilla and mandible did not show statistically significant differences
(Table 1 
). The anterior Bolton tooth ratio also did not show statistically
significant differences (Table 1
).
All cases were diagnosed and treated by one operator (Dr J.-T.
Kim) with a 0.022-inch slot preadjusted edgewise appliance (SWA, Ormco Corp.,
West Collins Orange, Calif) and closing loop mechanics. Confounding mechanics
that could influence molar extrusion, such as extraoral anchorage and interarch
elastics, were not used. Any expansion appliance, such as a quad helix or
intraoral anchorage such as a Nance appliance or transpalatal arch, was not used
to avoid significant vertical dimensional change. The pre- and posttreatment
lateral cephalograms were taken on the same radiographic unit (Panex-EC, J.
Morita Corporation, Osaka, Japan) and traced and digitized by another operator
(Dr T.-K. Kim). A Graphtec Digitizer KD4300 (Graphtec Corporation, Yokohama,
Japan) and IBM compatible computer were used for digitization and
measurements.
The amount of incisor and molar movement was determined by
superimposing the maxilla on ANS and the palatal plane19 and the
mandible by the structural method.29 Retraction of maxillary (U1
loss) and mandibular central incisor (L1 loss) and mesial movement of maxillary
(U6 loss) and mandibular first molar (L6 loss) were measured on the
posttreatment occlusal plane from the projection point of the maxillary (U1E)
and mandibular central incisor edge (L1E) and mesial contact point of the
maxillary (UM) and mandibular first molars (LM) from pre- and posttreatment
lateral cephalograms (Figures 1
and 2
). Because the amount of incisor retraction depends on the angulation
of maxillary and mandibular incisors, the angulation of the upper incisor (U1 to
FH plane and U1 to SN plane) and lower incisor (IMPA) at the beginning of the
treatment and angulation changes were measured (Figure 3
). Fourteen angular (defined in Figure 3
) and seven linear and proportional measurements (defined in Figure 4
) were selected to evaluate vertical dimensional changes.
To determine FVD changes with treatment, pre- and
posttreatment parameters from groups 1 and 2 were evaluated by the paired
t-test. To compare differences in FVD change between groups 1 and 2,
independent t-tests were done.
U1-FH and U1-SN did not show statistical differences in the
pre- and posttreatment measurements between groups 1 and 2. However, there were
statistical differences in the IMPA in the pre- (a < .05) and posttreatment
(b < .01) measurements between groups 1 and 2 (Table 2
).
Group 2 showed more mesial movement of the maxillary and
mandibular first molars and less retraction of the upper and lower central
incisors than group 1 (* indicates <.05, Table 3
). Comparison of incisor angulation change according to treatment
between groups 1 and 2 showed no significant differences (Table 3
).
Differences in the pretreatment FVD between groups 1 and 2
were analyzed by the independent t-test. Although SN-MP angle and AB-MP
angle showed differences (* indicates <.05, Table 4
), there were no differences in other pretreatment measurements of FVD
between groups 1 and 2.
Parameters of facial height in group 1 were significantly
increased after treatment (P < .05), but angular and proportional
measurements were not statistically different before and after treatment (Table
5 ). Group 2 showed similar results. Facial height measurements were
significantly increased after treatment (P < .05) (Table 5
). Although the maxillomandibular plane angle (MMA) and lower facial
height ratio (LFHR) were statistically different in group 2 (P < .05),
the amount of increase was too small to have clinical significance (Table 5
). There were no significant differences in other angular and
proportional measurements before and after treatment (Table 5
).
When the amount of change in FVD during treatment between
groups 1 and 2 were compared, there were no significant differences in linear
and proportional measurements. Although SN to palatal plane angle (SN-PP) and
MMA showed significant changes (** indicates <.05, Table 5
), the amount of increase was too small to have clinical significance.
There were no differences in other angular measurements between groups 1 and 2
(Table 5 ).
Previous reports on the relationship between extraction with
orthodontic treatment and FVD have shown that the former does not significantly
change the latter. Staggers17 showed that there was no significant
difference in the vertical dimension changes between P1 extraction and
nonextraction groups, and orthodontic treatment produced increases in the
cephalometric vertical dimensions in both groups. Chua et al20
examined the effect of extraction and nonextraction on lower anterior facial
height (LAFH, ANS-Me) with a standardized score to account for effects due to
growth and concluded that nonextraction treatment was associated with a
significant increase in LAFH, but extraction treatment was not associated with
any significant changes in LAFH. Cusimano et al19 found that there
were no differences in facial height of hyperdivergent patients with first
premolar extraction treatment when pre- and posttreatment results were
compared.
This study showed a significant increase of linear
measurements after orthodontic treatment in group 1 (Table 5
), corroborating the findings of Staggers17 and
Kocadereli18 but disagreeing with those of Chua et al.20
P1 extraction did not significantly change angular and proportional measurements
(Table 5 ), supporting the results of Kocadereli,18 Cusimano et
al,19 and Chua et al.20
Taner-Sarisoy and Darendeliler31 reported that
treatment with fixed appliances and premolar extractions did not significantly
alter the growth pattern. Yet, LAFH can be significantly influenced by
orthodontic treatment. The net increase of LFHR is due to extrusion of molars by
treatment mechanics and residual vertical growth of the patients. It is possible
that mesial molar movement may help accommodate these effects and work to
maintain LFHR.
Group 2 had more cases with increased LFHR (74.1% vs 51.9%)
and fewer cases with decreased LFHR (14.8% vs 40.7%) than group 1. It has been
shown that molars can be extruded when extraction space is
closed.17,19 Extrusion appears to maintain or even
increase the FVD. Therefore, greater mesial movements can possibly allow for
more molar extrusion due to the chosen mechanics of space closure. If extrusion
of the posterior teeth keeps pace with the increase in anterior facial height,
SN-MP will be maintained and the bite-closing effect of mesial molar movement
will be nullified.19 If the vertical growth of the ramus or posterior
alveolar bone do not compensate extrusion of molars, LFHR can be increased. In
this study, increases of LFHR in group 2 could be due to less compensation for
molar extrusion compared with group 1.
Residual growth has to be considered because it can influence
LAFH. In female individuals the growth is nearly over at 14 years. The average
ages of groups 1 and 2 were 15.6 ± 3.9 years and 16.2 ± 4.0 years, respectively,
so we cannot talk about the influence of residual growth because it is very
limited at these ages. However, in this study all linear measurements increased
after treatment. This result suggests that some residual growth as well as
treatment effects took place. This finding is similar to the studies of
Staggers17 and Kocadereli18 with growing children. Because
the mean age of two groups was similar in this study, the effect of growth on
LAFH between groups 1 and 2 can be expected to be the same. Thus, the effect of
growth on LAFH in this study can be eliminated.
In this study the effects of P1 and P2 extractions on change
of FVD were compared in relation to the concept that mesial molar movement will
close FVD by reducing the wedging effect. However, the results showed that there
were no significant differences in FVD changes between groups 1 and 2 except for
MMA and SN-PP (Table 5
). The reason why SN-PP and MMA showed significant differences might be
due to differences in skeletal characteristics and arch length discrepancy
between groups 1 and 2, even though these met the sample selection criteria such
as orthognathic Class I malocclusions within the same range of vertical and
anteriorposterior measurements.
Garlington and Logan32 observed a significant
decrease in LAFH in the mandibular second premolar enucleation cases due to
forward rotation of the mandible, but they found no significant differences in
total facial height and the MMA. This suggests that there were compensatory
changes in the maxillary vertical growth.
The results indicate that the null hypothesis is invalid and
suggest that the FVD is maintained or even increased regardless of amount of
mesial molar movement. Further studies are required on the biological response
to treatment effects as well as compensatory mechanisms, particularly those
affecting vertical facial dimensions. It would be of interest to study these
patients in the long term to determine how LAFH changes with time.
Regardless of maxillary and mandibular P1 or P2 extraction
treatments, there was no decrease of FVD and no significant difference in FVD
changes in the patients with a Class I malocclusion and hyperdivergent facial
type. Therefore, the wedge effect concept that the bite is closed by extraction
of P2 and forward movement of molars seems invalid. In these patients premolar
extraction decisions could be based on other criteria, such as incisor
retraction, area of crowding, tooth sizes, and condition of teeth, rather than
on a desire to change FVD.
TABLE
1.
Comparison of Arch Length Discrepancy and Anterior Bolton Tooth Ratio
in the Pretreatment Measurements Between Groups 1 and 2a
TABLE
2.Comparison of Incisor Angulation in the Pre- and Posttreatment
Measurements Between Groups 1 and 2
TABLE
3.Comparison of the Amount of Incisor Retraction and Mo lar Mesial
Movements and Incisor Angulation Change According to Treatment Between Groups 1
and 2
TABLE
4.Comparison of Pretreatment Measurements in Facial Vertical Dimension
Between Groups 1 and 2
TABLE
5.Comparison of Pre- and Posttreatment Measurements in Facial Vertical
Dimension of Groups 1 and 2 and Changes in Facial Vertical dimension Between
Groups 1 and 2
FIGURE
1. Measurement of the amount of incisor and molar movement in the maxilla.
To measure the amount of incisor and molar movement, the maxilla was
superimposed at ANS with palatal plane and distance change such as retraction of
maxillary central incisor (U1 loss) and mesial movement of maxillary first molar
(U6 loss) were measured on the posttreatment occlusal plane from the projection
point of the maxillary central incisor edge (U1E) and the mesial contact point
of the maxillary first molars (UM) from pre- and posttreatment lateral
cephalograms
FIGURE
2. Measurement of the amount of incisor and molar movement in the mandible.
To measure the amount of incisor and molar movement, the mandible was
superimposed by structural method, and distance change such as retraction of
mandibular central incisor (L1 loss) and mesial movement of mandibular first
molar (L6 loss) were measured on the posttreatment occlusal plane from
projection point of mandibular central incisor edge (L1E) and mesial contact
point of mandibular first molars (LM) from pre- and posttreatment lateral
cephalograms
FIGURE
3. Angular measurements. 1, SN to palatal plane angle (SN-PP, °); 2, FH to
palatal plane angle (FH-PP, °); 3, gonial angle (°); 4, Björk sum (°); 5, SN to
mandibular plane angle (SN-MP, °); 6, FH to mandibular plane angle (FMA, °); 7,
y-axis angle (°); 8, maxillomandibular plane angle (MMA, °); 9, facial axis
angle (FAX, °); 10, AB to mandibular plane angle (AB-MP, °); 11, ODI (2 + 10)
(°); 12, U1 to FH plane (°); 13, U1 to SN plane (°); and 14, IMPA (°)
FIGURE
4. Linear and proportional measurements. 1, Anterior facial height (AFH;
N-Me, mm); 2, posterior facial height (PFH; S-Go, mm); 3, upper anterior facial
height (UAFH; N′-ANS′, mm) (N′ means the projection of N on N-perpendicular
line, and ANS′ means the projection of ANS on N-perpendicular line); 4, lower
anterior facial height (LAFH; ANS′-Me′, mm) (Me′ means the projection of Me on
the N-perpendicular line); 5, total facial height (TFH; N′-Me′, mm) (3 + 4); 6,
anteroposterior facial height ratio (APFHR; S-Go/N-Me, %) (2:1); and 7, lower
facial height ratio (LFHR; ANS′-Me′/N′-Me′, %) (4:5)
aPostgraduate Student, Department of Orthodontics,
College of Dentistry, Seoul National University, Seoul, South Korea
bDirector, Smile Future Dental Clinic, Seoul, South Korea
cAssociate Professor, Division of Craniofacial Sciences and
Therapeutics, School of Dentistry, University of Southern California, Los
Angeles, Calif
dProfessor Emeritus, Department of Orthodontics, College of
Dentistry, Seoul National University, Seoul, South Korea
eAssistant Professor, Department of Orthodontics, College of
Dentistry, Seoul National University, Seoul, South Korea
Corresponding author: Seung-Hak Baek, DDS, MSD, PhD, Department of
Orthodontics, College of Dentistry, Seoul National University, Yeonkun-dong 28,
Jongro-ku, Seoul 110-768, South Korea (E-mail: drwhite@unitel.co.kr)