Download The effectiveness and long-term stability of overbite correction with incisor intrusion mechanics

ORIGINAL ARTICLE
The effectiveness and long-term stability of
overbite correction with incisor intrusion
mechanics
Huda Al-Buraiki,a Cyril Sadowsky,b and Bernard Schneiderc
Chicago, Ill
Background: Correction of deep overbite with subsequent achievement of long-term stability is difficult. The
purpose of this study was to investigate the effectiveness and long-term stability of overbite correction with
incisor intrusion mechanics. Methods: The treated group consisted of 25 subjects (13 female, 12 male) with
deep overbite of at least 4 mm (mean overbite, 5.9 mm). The orthodontic treatment was initiated in the late
mixed or early permanent dentition, and all patients were treated nonextraction. All patients had lateral
cephalometric radiographs taken at pretreatment (T1), posttreatment (T2), and postretention (T3). The
treatment included cervical headgear and lever arches to intrude mainly the maxillary incisors and
occasionally the mandibular incisors. Premolars were not included in the fixed appliances during the
treatment. The untreated group consisted of 25 age- and sex-matched subjects from the Bolton Growth
Study. Results: The mechanics used were effective in overbite correction. During the posttreatment period,
overbite increased by 0.7 mm. Conclusions: Although this change was statistically significant, the amount
was small and is considered clinically insignificant, given the severity of the overbite pretreatment.
Furthermore, a net overbite correction (T3-T1) of 3.3 mm and postretention overbite on 2.6 mm is an excellent
clinical outcome. (Am J Orthod Dentofacial Orthop 2005;127:47–55)
D
eep overbite is a common feature of many
malocclusions. One of the objectives of orthodontic treatment is to establish a normal overbite. A review of the literature reveals a great controversy among the proponents of the different treatment
modalities. Some investigators insist that overbite is
best treated by incisor intrusion to a leveled functional
occlusal plane. The smile line has been used to help
determine which incisors to intrude. Others resorted to
“leveling and aligning,” using continuous archwires,
achieving overbite correction by extrusion of the buccal
segments with minimal intrusion and some proclination
of incisors.
Many studies have been performed to examine the
types and amounts of postretention relapse after orthodontic treatment.1-6 All studies showed that overbite
decreased during treatment and had a tendency to
From the Department of Orthodontics, University of Illinois at Chicago.
a
Former resident.
b
Professor.
c
Clinical professor.
Reprint requests to: Dr Cyril Sadowsky, University of Illinois at Chicago,
College of Dentistry, Department of Orthodontics (MS 841), 801 South
Paulina, Chicago, IL 60612; e-mail, [email protected].
Submitted, July 2003; revised and accepted, October 2003.
0889-5406/$30.00
Copyright © 2005 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2003.10.034
increase after treatment. The degree of relapse varied
from one study to another (Table I).
The majority of the stability studies were focused
on quantifying the amount of relapse and identifying
the factors that might have contributed to it, such as
extraction, pretreatment overbite, and intercanine
distance. In addition, overbite was one of several
variables studied rather than the main focus of these
studies. Thus, very few studies have described the
mechanotherapy and its effectiveness in overbite
correction.
The purpose of this study was to investigate the
effectiveness and long-term stability of overbite correction with incisor intrusion mechanics.
MATERIAL AND METHODS
Records of the treated group were obtained from the
office of Dr Andrew Haas, Cuyahoga Falls, Ohio. The
group consisted of 25 subjects (13 female, 12 male)
with deep overbite of at least 50% mandibular incisor
overlap. Twenty-two subjects had Class II, Division 1
malocclusion, and 3 had Class I malocclusion. The
orthodontic treatment was initiated in the late mixed or
early permanent dentition, and all patients were treated
nonextraction. Premolars were not included in the fixed
appliances during treatment, as is often the case in this
practice. All patients had lateral cephalometric radio47
48 Al-Buraiki, Sadowsky, and Schneider
Table I.
American Journal of Orthodontics and Dentofacial Orthopedics
January 2005
Literature survey of overbite treatment outcome
Overbite
Authors, year
(reference)
T1 (mm)
T2-T1
(mm)
⌬ (%)
T3-T2 (mm)
Relapse (%)
Bishara et al,
1973 (1)
Hellekant et al,
1989 (2)
Fidler et al,
1995 (5)
4.7 ⫾ 1.8
⫺1.7
36
0.4
23
5.1 ⫾ 1.4
⫺2.5
49
0.8
32
4.4 ⫾ 1.6
⫺1.5 ⫾ 1.8
34
0.9 ⫾ 1.6
60
Sadowsky et al,
1994 (4)
4.5 ⫾ 1.9
⫺1.9 ⫾ 1.9
42
0.5 ⫾ 1.1
26
Carcara 2001,
(6)
4.8 ⫾ 2.2
⫺2.7 ⫾ 1.1
56
0.8 ⫾ 0.9
30
Table II.
Malocclusion
Class II
Division 1
Class II
Division 1
Class I;
Class II
Division 1;
Class II
Division 2
Class II
Treatment
Edgewise
appliance
Edgewise
appliance
Headgear,
edgewise
appliance
Headgear,
edgewise
appliance
Headgear,
edgewise
appliance
Retention
time
(y)
Postretention
Time
Mean 14.8 mo
ⱖ2 y
Mean 14 y
8.4 ⫾ 3.2
6.3 ⫾ 1.8 y
3.3
Mean 7.4 y
Characteristics of treated group
Time (y)
Characteristic
Mean
SD
Range
Age at T1
Age at T2
Age at T3
Treatment time (T2-T1)
Posttreatment duration (T3-T2)
Retention time
Maxillary
Mandibular
Postretention time
Maxillary
Mandibular
11.3
14.2
26.6
3
12.3
1.7
2
4
1.5
3.9
8.2-15.1
10.8-18.3
20.2-38.4
0.9-4.9
7.7-25.7
5
8
1.3
1.3
2.3-7.7
6.1-12.1
7.3
4.3
4.3
3.9
2-20.3
1-18.3
graphs taken at pretreatment (T1), posttreatment (T2),
and postretention (T3). A member of the auxiliary staff
in the office identified records that satisfied the selection criteria described above. Table II summarizes the
characteristics of the sample.
All subjects were treated by a single operator with
a .018 ⫻ .025-in standard edgewise appliance. In the
mechanotherapy, the principle of the lever arch was
used to intrude incisors or prevent their eruption. The
.017 ⫻ .025-in or .018 ⫻ .025-in mandibular archwire
was stepped down mesial to the molar tube, bypassing
the buccal segments and engaging the incisors anteriorly. The .017 ⫻ .025-in or .018 ⫻ .025-in maxillary
ideal archwire was gabled several millimeters anterior
to the molar tube to engage the incisors anteriorly while
bypassing the canines and premolars. When engaged,
the maxillary archwire had an intrusive and lingual root
torque effect on the incisors. The anteroposterior dis-
crepancy was corrected with cervical headgear, with
the outer bow long and bent up to deliver a distal root
moment to the first molars (counteracting the mesial
root thrust created by the intrusion arch). An intrusion
force (with a slight retraction vector) was delivered to
the maxillary incisors by the elastics (2-4 oz) worn
from canine hooks on the inner bow of the cervical
headgear and engaging the incisal wings of the brackets
of the maxillary incisors. Tandem mechanics was
initially used in 16 of the 25 patients with mandibular
arch length deficiency. Tandem mechanics included a
cervical headgear together with a mandibular .022-in
archwire engaged in the mandibular molar headgear
tube and directly ligated to the mandibular incisors with
a V-shaped bend in the canine region. Sliding hooks
and active open-coil springs were then inserted distal to
the bend and Class III elastics attached to the hooks to
avoid mandibular incisor proclination.
Al-Buraiki, Sadowsky, and Schneider 49
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 127, Number 1
Table III.
Age and sex distribution of treated and untreated groups
T1 age (y)
Sample
Treated
Female
Male
Total
Untreated
Female
Male
Total
T2 age (y)
n
Mean
SD
Mean
SD
13
12
25
11.2
11.3
11.3
1.6
1.8
1.7
14
14.4
14.2
1.8
2.3
2
13
12
25
11.3
11.4
11.3
1.6
1.6
1.6
14.1
14.4
13.3
1.6
2
1.8
At the completion of active treatment, the appliances were removed, and retainers were inserted.
The maxillary retainer consisted of a Hawley appliance with a passive anterior bite plane. The patients
were instructed to wear the retainers for 24 hours per
day for 5 years. In the mandible, a fixed lingual wire
was used. Some patients had a bonded canine-tocanine wire, whereas others had a bonded firstpremolar-to-first-premolar wire. Patients were observed until a decision was made regarding the future
of the third molars. The T3 records were taken, on
average, 7 years 4 months and 4 years 4 months after
the removal of maxillary and mandibular retainers,
respectively (Table II).
The untreated group consisted of 25 age- and
sex-matched subjects (13 female, 12 male) who
exhibited Class II, Division 1 malocclusion with
deep overbite but had not undergone any orthodontic
treatment. The cephalograms for this group were
drawn from copies of the Bolton Growth study,
available in the Department of Orthodontics, University of Illinois at Chicago. Owing to the limited
number of subjects with a T3 record matching the
age of subjects in the treated group, only T1 and T2
records were used. Table III summarizes the age and
sex distribution of the treated and untreated groups.
Both skeletal and dental variables were measured
on the individual cephalogram at T1, T2, and T3. The
skeletal variables included the ANB, mandibular plane
angle, Y-axis, the upper, lower, and total anterior face
heights, and finally, the lower anterior face height ratio
(Fig 1). The dental variables included overjet (the
horizontal distance between the maxillary and mandibular incisor tips measured parallel to the functional
occlusal plane), overbite (the vertical distance between
the incisal tip of the maxillary and mandibular incisor),
and the interincisal angle.
Additional variables were measured with cephalometric superimposition. The overall superimposition
was patterned after Björk and Skieller,7 registering on
Fig 1. Skeletal variables measured on individual cephalograms. P, porion; S, sella; Or, orbitale; A, A-point; B,
B-point; PO, pogonion; Gn, gnathion; Me, menton; N,
nasion. Measurements: 1, ANB angle; 2, mandibular
plane angle; 3, y axis; 4, upper anterior face height; 5
lower anterior face height; 6, total anterior face height.
the anterior inferior outline of sella turcica and the
anterior contour of the middle cranial fossa (De
Coster’s line), including the greater wings of the
sphenoid, cribriform plate, the orbital roof, and the
inner surface of the frontal bone. This superimposition
was used to transfer the Frankfort plane from 1 cephalogram to the other. Maxillary superimposition was
patterned after Doppel et al.8 It is achieved by superimposing on the posterior border of the zygomatic
processes. Vertically, the floor of the orbit was raised
1.5 mm for each millimeter the palatal plane is lowered.
50 Al-Buraiki, Sadowsky, and Schneider
Fig 2. Variables measured on maxillary superimposition
(solid line, T1; broken line, T2). 1, ⌬ U1 angle; 2, ⌬ U1
vertical; 3, ⌬ U6 vertical; 4, maxillary rotation.
Mandibular superimposition was patterned after
Bjork’s structural method, in which the tip of the chin,
the inner cortical plate and trabecular pattern of the
symphysis, the mandibular canal, or the lower contour
of the molar germ of the second or third molar before
root formation are superimposed.9 On the maxillary
and mandibular superimposition, the following variables were measured (Figs 2 and 3):
1. Change in the angulation of incisors: angle formed
between long axes of the 2 maxillary and 2 mandibular incisors. A negative value indicates incisor
uprighting.
2. Change in the vertical position of incisors: vertical
distance between 2 points located 17 mm apical to
the incisal edge of each incisor (approximating the
center of resistance).
3. Change in the vertical position of molars: measured
at the mesiobuccal cusp.
4. Maxillary and mandibular rotation: the angle
formed by the two Frankfort horizontal lines. A
negative value indicates an opening (clockwise)
rotation.
To test intraoperator reliability for the angular and
linear measurements, 20 radiographs (10 patients) were
selected at random from the original sample. These
radiographs were retraced and remeasured, with the 2
measurements at least 4 weeks apart. The data obtained
from the repeated measurements were statistically com-
American Journal of Orthodontics and Dentofacial Orthopedics
January 2005
Fig 3. Variables measured on mandibular superimposition (solid line, T1; broken line, T2). 1, ⌬ L1 angle; 2, ⌬ L1
vertical; 3, ⌬ L6 vertical; 4, mandibular rotation.
pared with the original measurement with a singlesample paired t test.
An independent t test was used to determine the
differences between the treated and untreated groups at
T1. For the variables measured on the individual
cephalograms (T1, T2, and T3), a paired t test was used
to assess treatment and posttreatment changes. For the
variables derived from the superimposition, changes
were measured rather than absolute values; treatment
and posttreatment changes were tested with a singlesample t test. An independent t test was used to
compare T2-T1 changes between the treated and untreated groups. Difference were considered statistically
significant at P ⬍ .01.
RESULTS
Intraoperator reliability was examined with the paired
t test. There were no statistically significant differences
between the measurements from the original tracings and
those repeated at a later date. (Table IV).
Table V summarizes the pretreatment measurements for the treated group as compared with the
untreated group at T1. The treated group had a mean
ANB angle of 5.9° with an increased overjet and
overbite (6.2 mm and 5.9 mm, respectively). The
mandibular plane, Y-axis, and lower anterior face
height ratio seem to be within normal limits. Independent sample t test revealed no statistically significant
differences between the 2 groups in all variables except
for the anterior face height measurement. Although the
Al-Buraiki, Sadowsky, and Schneider 51
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 127, Number 1
Table IV.
Paired t test of repeated measurements
Variables
ANB (°)
Mandibular plane (°)
Y-Axis (°)
Upper anterior face height (mm)
Lower anterior face height (mm)
Total anterior face height (mm)
Overjet (mm)
Overbite (mm)
Interincisal angle (°)
⌬ L1 angle (°)
⌬ L1 vertical (mm)
⌬ L6 vertical (mm)
Mandibular rotation (°)
⌬ U1 angle (°)
⌬ U1 vertical (mm)
⌬ U6 vertical (mm)
Maxillary rotation (°)
Mean
difference ⫾ SD
P value
0.2 ⫾ 0.5
0.0 ⫾ 1.4
⫺0.3 ⫾ 1.2
0.1 ⫾ 1
0.7 ⫾ 3.3
0.4 ⫾ 1.1
0.1 ⫾ 0.6
0.0 ⫾ 0.5
0.2 ⫾ 2.8
0.5 ⫾ 2.4
0.2 ⫾ 0.7
0.1 ⫾ 0.9
0.3 ⫾ 1.3
0.6 ⫾ 1.5
0.1 ⫾ 0.6
0.2 ⫾ 0.9
0.2 ⫾ 1.2
.3
1.0
.4
.8
.5
.3
.8
1.0
.9
.6
.5
.9
.5
.3
.8
.6
.6
upper, lower, and total anterior face heights were
statistically different, there was no statistically significant difference in lower anterior face height ratio
between the treated and untreated groups.
Treatment changes
The ANB angle was reduced by 2.6°, and the
mandibular plane and Y-axis increased by 2.1° and 2°,
respectively. All anterior face height measurements
increased without a change in the anterior face height
ratio. The overjet and overbite were reduced by 4.1 mm
and 4 mm, respectively, whereas the interincisal angle
remained unchanged. There were no statistically significant changes in the angulation of maxillary and mandibular incisors and the vertical position of maxillary
incisors. Maxillary molars, mandibular molars, and
mandibular incisors moved occlusally. The maxilla and
mandible rotated clockwise 1.4° and 1.6°, respectively
(Table VI).
Mean differences between the treated and
untreated groups
In the treated group, T2-T1 change represents the
net effect of treatment and growth, whereas in the
untreated group it represents growth alone. The
changes reported below are mean differences between
the two groups (Table VI). Treatment reduced the ANB
angle by 2.5° and increased the mandibular plane and
Y-axis angles by 3.1° and 2.1°, respectively. There was
an increase in all anterior face height measurements,
with no change in the lower anterior face height ratio.
The overjet and overbite decreased by 4.1 mm and 3.9
mm, respectively. The change in the interincisal angle
was not statistically different between the 2 groups.
Maxillary and mandibular superimposition showed that
the mean difference between the treated and untreated
groups was statistically significant for the maxillary and
mandibular rotation and changes in the vertical position
of both the maxillary incisors and molars. The maxilla
and mandible rotated clockwise 1.6° and 2.9°, respectively. Maxillary incisors were relatively intruded 1.5
mm, and maxillary molars were extruded 2.3 mm. It is
interesting to note that the vertical changes of the
mandibular incisors and molars were not statistically
different between the 2 groups.
Posttreatment changes
The ANB and mandibular plane angle decreased by
0.8° ⫾ 1.2° and 2° ⫾ 3°, respectively. All anterior face
height measurements increased, with no change in the
lower anterior face height ratio. There was a statistically significant increase, albeit small, in both overjet
(0.7 ⫾ 0.8 mm) and overbite (0.7 ⫾ 1.1 mm). The
interincisal angle showed no change (Table VII). Maxillary and mandibular superimposition revealed a statistically significant change in mandibular rotation as
well as in the vertical position of incisors and molars,
both maxillary and mandibular (Table VIII). The mandible rotated 2.4° ⫾ 2.3° counterclockwise, maxillary
incisors erupted 1.9 ⫾ 1.5 mm, and maxillary molars
erupted 1.4 ⫾ 1.2 mm. The mandibular incisors erupted
1.6 ⫾ 1.6 mm, whereas the molars erupted 2.1 ⫾ 2.1
mm. Measuring mean differences between the treated
and untreated groups was not feasible, owing to the
lack of T3 data for the untreated group.
DISCUSSION
Study design
This was a retrospective study with inherent limitations. To minimize selection bias, the sample was
identified by a member of the auxiliary staff in the
office to satisfy the selection criteria set by the investigator. The untreated group was age- and sex-matched
to the treated group. In addition, there was no statistically significant difference in pretreatment variables in
the treated and untreated groups except for anterior face
height measurements, which was more likely due to a
difference in magnification (as will be discussed in the
following section). Yet, equivalence between the two
groups cannot be assured because of the retrospective
nature of the study.
Material and method
The treated sample consisted of 25 subjects. The
treatment was initiated in the late mixed or early
permanent dentition just before the pubertal spurt, to
52 Al-Buraiki, Sadowsky, and Schneider
Table V.
American Journal of Orthodontics and Dentofacial Orthopedics
January 2005
Comparison between treated and untreated groups at T1
Variables
ANB (°)
Mandibular plane (°)
Y-axis (°)
Upper anterior face height (mm)
Lower anterior face height (mm)
Total anterior face height (mm)
Lower anterior face height ratio
Overjet (mm)
Overbite (mm)
Interincisal angle (°)
Treated (mean ⫾ SD)
Untreated (mean ⫾ SD)
Mean difference
5.9 ⫾ 1.87
23.1 ⫾ 4.1
57.5 ⫾ 3.0
53.1 ⫾ 2.9
63.8 ⫾ 4.8
117.3 ⫾ 5.8
54.4 ⫾ 2.5
6.2 ⫾ 2.3
5.9 ⫾ 1.2
128.8 ⫾ 11.8
5.7 ⫾ 1.6
23.6 ⫾ 3.2
59.1 ⫾ 2.3
50.6 ⫾ 2.9
59.5 ⫾ 3.5
110.1 ⫾ 5.3
53.9 ⫾ 1.4
5.2 ⫾ 2.1
5.7 ⫾ 1.9
130.4 ⫾ 9.4
0.2
⫺0.5
⫺1.6
2.5*
4.3*
7.2*
0.5
1
0.2
⫺1.6
*P ⬍ .01.
Table VI.
T2-T1 changes in treated and untreated groups
Variables
Treated T2-T1
(mean ⫾ SD)
Untreated T2-T1
(mean ⫾ SD)
Mean difference
ANB (°)
Mandibular plane (°)
Y-axis (°)
Upper anterior face height (mm)
Lower anterior face height (mm)
Total anterior face height (mm)
Lower anterior face ratio (mm)
Overjet (mm)
Overbite (mm)
Interincisal angle (°)
⌬ L1 angle (°)
⌬ L1 vertical (mm)
⌬ L6 vertical (mm)
Mandibular rotation (°)
⌬ U1 angle (°)
⌬ U1 vertical (mm)
⌬ U6 vertical (mm)
Maxillary rotation (°)
⫺2.6 ⫾ 1.8*
2.1 ⫾ 2.4*
2.0 ⫾ 2.0*
4.0 ⫾ 2.6*
5.5 ⫾ 2.7*
9.1 ⫾ 4.7*
0.4 ⫾ 1.1
⫺4.1 ⫾ 3.1*
⫺4.0 ⫾ 1.2*
⫺0.5 ⫾ 14.1
⫺1.4 ⫾ 6.4
1.5 ⫾ 1.6*
1.6 ⫾ 1.1*
⫺1.6 ⫾ 2.3*
1.0 ⫾ 10.2
0.1 ⫾ 1.5
4.1 ⫾ 1.9*
⫺1.4 ⫾ 1.4*
⫺0.1 ⫾ 1.0
⫺1 ⫾ 2.1
⫺0.1 ⫾ 1.4
2.2 ⫾ 2.1
3.1 ⫾ 3.1
5.3 ⫾ 3.8
0.3 ⫾ 1.8
0 ⫾ 0.9
⫺0.1 ⫾ 1.4
⫺1.4 ⫾ 6.0
1.4 ⫾ 2.6
1.9 ⫾ 1.3
1.9 ⫾ 1.4
1.3 ⫾ 1.8
0.1 ⫾ 2.3
1.6 ⫾ 0.9
1.8 ⫾ 1.0
0.2 ⫾ 1.0
⫺2.5*
3.1*
2.1*
1.9*
2.4*
3.8*
0.1
⫺4.1*
⫺3.9*
0.9
⫺2.8
⫺0.3
⫺0.3
⫺2.9*
0.9
⫺1.5*
2.3*
⫺1.6*
*P ⬍ .01.
Table VII.
Paired t test of posttreatment changes
Variables
ANB (°)
Mandibular plane (°)
Y-axis (°)
Upper anterior face height (mm)
Lower anterior face height (mm)
Total anterior face height (mm)
Lower anterior face height ratio
Overjet (mm)
Overbite (mm)
Interincisal angle (°)
*P ⬍ .01.
T2
(mean ⫾ SD)
T3
(mean ⫾ SD)
3.3 ⫾ 1.4
25.2 ⫾ 4.5
59.5 ⫾ 3.0
57.1 ⫾ 2.7
69.2 ⫾ 5.3
126.4 ⫾ 6.6
54.7 ⫾ 1.9
2.2 ⫾ 0.6
1.9 ⫾ 0.8
128.3 ⫾ 8.6
2.5 ⫾ 1.9
23.2 ⫾ 4.5
59.0 ⫾ 3.8
58.6 ⫾ 3.0
72.1 ⫾ 6.6
130.7 ⫾ 7.5
55.1 ⫾ 2.4
2.9 ⫾ 0.8
2.6 ⫾ 1.3
127.4 ⫾ 7.0
Difference
⫺0.8 ⫾ 1.2*
⫺2 ⫾ 3.0*
⫺0.5 ⫾ 2.5
1.5 ⫾ 1.4*
2.9 ⫾ 3.2*
4.3 ⫾ 3.7*
0.3 ⫾ 1.1
0.7 ⫾ 0.8*
0.7 ⫾ 1.1*
⫺0.9 ⫾ 7.8
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 127, Number 1
Table VIII.
One-sample t test of posttreatment changes
(superimposition)
Variables
⌬ L1 angle (°)
⌬ L1 vertical (mm)
⌬ L6 vertical (mm)
Mandibular rotation (°)
⌬ U1 angle (°)
⌬ U1 vertical (mm)
⌬ U6 vertical (mm)
Maxillary rotation (°)
T3-T2
(mean ⫾ SD)
1.4 ⫾ 3.9
1.6 ⫾ 1.6*
2.1 ⫾ 2.1*
2.4 ⫾ 2.3*
1.5 ⫾ 6.0
1.9 ⫾ 1.5*
1.4 ⫾ 1.2*
0.5 ⫾ 1.4
*P ⬍ .01.
take advantage of growth. The limited sample size was
the result of 3 factors. The first was that the practitioner
retains for a very long period of time, which makes
obtaining a T3 cephalogram several years postretention
very difficult. Only 3 patients had T3 cephalograms 1
year after mandibular retainer removal (maxillary retention is usually discontinued several years before
mandibular); the others ranged between 1.7 and 18.3
years (the majority in the 2-5-year range). The second
factor was that the T2 cephalogram must have been
taken no later than 3 months from appliance removal.
The third factor was that patients for whom premolars
were bonded, even if it was done toward the end of
treatment, were excluded from the study to ensure that
active premolar extrusion was not part of the leveling
process.
The untreated group was sex- and age-matched at
both T1 and T2 (Table III). There was no statistically
significant difference between the two groups in all
pretreatment variables except for anterior face height
measurements, which were slightly greater in the
treated than in the untreated group. The lower
anterior face height ratio was the same in both
groups, and thus the differences in anterior face
height measurements were more likely than not due
to difference in magnification. This, however, should
have little or no effect on the findings of this study.
The linear measurements examined in this study
were changes in anterior face height measurements
(rather than the actual value at T1, T2, and T3),
overjet, and overbite. Because these measurements
were small in magnitude, the effect of magnification
was considered negligible. For example, a 10%
difference in magnification would theoretically affect changes in overbite and overjet by 0.4 mm
(mean changes were 4 mm in the untreated group).
Al-Buraiki, Sadowsky, and Schneider 53
Effectiveness of treatment mechanics
The results of this study show that incisor intrusion
mechanics used in the treated sample are effective in
the correction of overbite. The mean correction was 4
mm, 68% of the pretreatment overbite. Samples from
different studies exhibit different pretreatment overbite.
As a result, it seems reasonable to examine the percentage of the correction (amount of correction relative to
the pretreatment overbite) rather than the actual value.
The amount of overbite correction reported in this study
seems greater than reported in previous studies.4-6 In
those studies, the amount of correction ranged from 1.5
mm to 2.7 mm, and the correction percentage ranged
from 34% to 58% (Table I). However, the difference
could be due to the deeper pretreatment overbite found
in our sample. The closest posttreatment overbite reported in the literature is 2.1 mm, with a pretreatment
overbite of 4.8 mm (compared with 5.9 mm reported in
this study).
The treatment resulted in both skeletal and dental
changes. The skeletal changes included opening rotation of the mandible, evident in the superimposition as
well as the increase in Y-axis and mandibular plane
angle, all of a similar magnitude. It also included an
increase in all anterior face height measurements. There
was a proportionate increase in upper and lower anterior face heights, thus maintaining the lower anterior
face height ratio.
In the treated group, both mandibular incisors
and molars moved occlusally a similar amount (1.5
mm and 1.6 mm, respectively). The mean differences
between the treated and untreated groups in the
present study were not statistically significant; that is
to say, the treatment had no effect on the mandibular
dentition. The absence of mandibular incisor intrusion could be partly explained by the use of class III
elastics—16 patients that were initially treated with
tandem mechanics—which might have counteracted
the effect of the lever arches on the mandibular
incisors. Other studies have shown more mandibular
molar movement than that reported in this study,
with similar amounts of change for the mandibular
incisors.4,10,11 The reports in the literature suggest
that some leveling took place by extrusion of the
mandibular buccal segments contrary to the findings
in the present study. In the treated group, maxillary
incisors maintained their vertical position while
erupting 1.6 mm in the untreated group (relative
intrusion), and the maxillary molars extruded 4.1 mm
while erupting 1.8 mm in the untreated group.
Previous studies reported similar changes in maxillary incisors and molars during treatment.4,5,10,11
54 Al-Buraiki, Sadowsky, and Schneider
The relative intrusion of maxillary incisors coupled with the vertical occlusal movement of the
molars decreases the amount of incisor overlap and
thus was responsible for the overbite correction. The
extrusion of the molars (maxillary molars moved
more occlusally in the treated than in the untreated
group) can result in 1 of 2 effects, opening rotation of
the mandible or increasing the vertical development
of the mandible. In spite of its small magnitude,
opening rotation of the mandible is evident from the
results of this study. In absence of posterior face
height measurement, the effect of molar extrusion on
posterior vertical development of the mandible cannot be examined.
Stability of overbite correction
Although statistically significant, the increase in
overbite during the posttreatment period was small (0.7
mm at 17%) and is clinically insignificant. The posttreatment increase in overbite could be relapse or part
of the normal development. Unfortunately, a matching
group for the period T3-T2 was not available to help
answer this question. Driscoll-Gilliland et al12 evaluated growth and stability in untreated and treated
subjects. The untreated group had a similar age distribution (T2-T1) to that of the untreated group in this
study (T3-T2). In their untreated sample, the overbite
remained unchanged, as it did in the T2-T1 period in
the untreated sample in our study. This suggests that the
0.7 mm of posttreatment increase in overbite reported
in our treated sample is relapse rather than normal
development.
The amount of relapse reported in this study is
similar to amounts in previous reports.1,2,4-6 However,
the percentage of relapse (17%) seems to be slightly
lower compared with previous reports (23%-60%). The
difference was small, and statistical testing was not
feasible. When the net correction (T3-T1) is evaluated,
the difference becomes more apparent. The mean net
change in overbite reported in this study was 3.3 mm,
compared with a range of 0.6 mm to 1.9 mm reported
in previous studies.
There was no change in the Y-axis in the posttreatment period. This indicates that the opening rotation of
the mandible that occurred during treatment was not
recovered in the posttreatment period. The reduction in
mandibular plane angle and counterclockwise rotation
of the mandible (2° and 2.4°, respectively) could be due
to either an increase in the posterior vertical development of the mandible or significant bone apposition on
the posterior part of the inferior border of the mandible
(remodeling). It is interesting to note that the untreated
American Journal of Orthodontics and Dentofacial Orthopedics
January 2005
group in the Driscoll-Gilliland study12 showed no
change in mandibular rotation.
During the posttreatment period, there was also
slightly more eruption of the maxillary incisors than of
the maxillary molars. The eruption of the maxillary
incisors might have contributed to the small amount of
overbite increase reported in this study. In the mandible, there was slightly more eruption of the molars than
the incisors. The amount of mandibular incisor eruption
was similar to that reported by Driscoll-Gilliland et
al.12 The differential eruption between mandibular
incisors and molars could be due to the incorporation of
anterior bite plane in the maxillary retainer, holding the
incisors while allowing some eruptions of mandibular
molars.
The postretention time is an important variable in
any study that involves the stability of orthodontic
correction. One might expect an increase in the amount
of relapse with the increase in the postretention duration. However, the postretention period reported in this
study (7.3 ⫾ 4.3 years after maxillary retention and 4.3
⫾ 3.9 years after the discontinuation of mandibular
retention) is long enough to show most if not all the
relapse expected to occur posttreatment, given the long
retention duration, with growth being completed long
before the T3 records were taken.
The relative stability of overbite correction (small
magnitude of change) reported in this sample can be
attributed to several factors. The long retention period
and the retainer design are important. The relative
stability of overjet reported in this study might have
played a role in overbite stability as well. Also, the
effectiveness of the mechanics allowed overcorrection,
which might or might not have contributed to the
relative stability of overbite correction. The reader
should be reminded that the current study excluded
subjects with Class II, Division 2 malocclusions, who
might exhibit a higher overbite relapse.
Whether incisor intrusion mechanics produce a
more stable overbite correction than the other methods
of leveling is a question yet to be answered. It will
require a comparison between matching samples
treated with different mechanics as compared with the
mechanics used in this study.
CONCLUSIONS
Results from this study revealed that the mechanics implemented were effective in overbite correction. Posttreatment changes revealed a small increase
in overbite and overjet that was considered clinically
insignificant. A postretention overbite of 2.6 mm is
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 127, Number 1
an excellent clinical achievement, and a net overbite
correction (T3-T1) of 3.3 mm seems to be greater
than in previous reports.
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