Original Article

A Longitudinal Study of Incremental Expansion Using a

Mandibular Lip Bumper
C. Chris Murphy, DDS, MSa; W. Bonham Magness, DDS, MSb; Jeryl D. English, DDS, MSc;
Sylvia A. Frazier-Bowers, DDS, PhDd; Anna Maria Salas, DDS, MSe

Abstract: A retrospective study using models was performed to evaluate the incremental expansion that
occurred during mandibular lip bumper therapy in 44 adolescent patients. The purpose was to determine
whether expansion occurs evenly between appointments or whether it attenuates with treatment time. Dental
cast measurements were made for arch width and arch length. Treatment duration was broken into near-
equal time segments and compared. Results showed that about 50% of the total expansion achieved oc-
curred within about the first 100 days. Forty percent of the total amount of expansion occurred during the
next 200 days, with only about 10% of the total expansion occurring after the first 300 days. It is unnec-
essary to have the appliance in place for longer than 300 days. The percentage of expansion that occurred
at each time segment was not related to whether the patient had concomitant maxillary expansion. (Angle
Orthod 2003;73:396–400.)
Key Words: Expansion; Mandibular; Lip bumper

INTRODUCTION dental arch in both an anterior-posterior and a transverse

Orthodontists have always been faced with the problem direction.3–5 Typically, it is made of 0.0450 stainless steel
of straightening crowded teeth. A variety of treatment mo- wire, and it spans the mandibular dentition from molar to
dalities have been used to accomplish this task, some of molar. The wire is kept away from the facial surfaces of
which include tooth extraction, expansion, interproximal the teeth, usually at the level of the gingival margin, and
enamel reduction, flaring incisors, and uprighting molars. may or may not be covered anteriorly with plastic or acryl-
A recent trend in orthodontics is the reemergence of non- ic. The appliance fits into tubes located on the lower molars
extraction treatment.1 Because many patients have signifi- and has adjustment loops located just mesial to these tubes.
cant crowding, more focus has been placed on developing The lip bumper displaces the facial musculature, preventing
the arches through expansion therapy. The mandibular lip it from coming into contact with the lower teeth, and allows
bumper is an aid to nonextraction therapy because of its the lingual forces of the tongue to remain unbalanced, thus
ability to develop the lower arch.2 The key to using this causing forward and lateral expansion of the mandibular
appliance effectively is in knowing exactly how it works dental arch. This disruption of the equilibrium surrounding
and how the expansion is distributed throughout treatment. the dentition is the fundamental concept on which the lip
The lip bumper allows for expansion of the mandibular bumper is based.
The subsequent expansion has been documented in the
a
Department of Orthodontics, Dental Branch, University of Texas literature as occurring between the molars, premolars, ca-
Health Science Center at Houston. nines, and an anterior flaring of the incisors.6–8 The man-
b
Clinical Professor, Department of Orthodontics, Dental Branch, dibular lip bumper is also used to tip the molars distally
University of Texas Health Science Center at Houston. utilizing the distal force created by the facial musculature
c
Graduate Program Director and Chair, Department of Orthodon-
tics, Dental Branch, University of Texas Health Science Center at
on the appliance itself.2,9,10 Osborn et al7 quantified many
Houston. of the dimensional changes that occur during lip bumper
d
Associate Professor, Department of Orthodontics, Dental Branch, use. In their study of 32 patients, they found that the arch
University of Texas Health Science Center at Houston. width increased 2 mm at the canines, 2.5 mm at the first
e
Clinical Assistant Professor, Department of Orthodontics, Dental premolars, 2.4 mm at the second premolars, and 2 mm at
Branch, University of Texas Health Science Center at Houston.
Corresponding author: Jeryl D. English, DDS, MS, Orthodontic the first molars and that the arch length increased by 1.2
Department, 6516 MD Anderson Blvd, Suite 371, Houston, TX 77030 mm. Other studies had similar results.6,11
(e-mail: jeryl.d.english@uth.tmc.edu). Previous studies attempted to quantify the experimental
Accepted: November 2002. Submitted: August 2002. changes associated with the use of the mandibular lip bum-
q 2003 by The EH Angle Education and Research Foundation, Inc. per. These studies focused primarily on the final treatment

Angle Orthodontist, Vol 73, No 4, 2003 396

INCREMENTAL EXPANSION WITH LIP BUMPER 397

result and did not discuss what actually happens to the den-
tition between each appointment visit. It has not been doc-
umented whether most of the dimensional change occurs at
the beginning of treatment or at the end of treatment, or
whether it occurs slowly throughout the entire treatment
period. A study that analyzes the changes that occur be-
tween each appointment could prove useful to the patient
and the clinician. In this study, we propose to identify when
arch expansion occurs during lip bumper therapy. To
achieve this objective, we analyzed orthodontic study mod-
els of 44 consecutively treated patients who had impres-
sions taken at every appointment during active lip bumper
treatment by a single orthodontic practitioner. This study
will add valuable new information to lip bumper treatment
that may help the clinician maximize treatment efficiency. FIGURE 1. Illustration of measurements performed.
If the clinician knows when most of the expansion occurs
during lip bumper therapy, the patient will not have to
Measurements
spend unnecessary time wearing the appliance, and the cli-
nician can change faster to fixed appliances for definitive The following measurements were made to the nearest
correction of the malocclusion. 10th of a millimeter using digital calipers, each landmark
being marked with a sharp lead pencil (see Figure 1):
MATERIALS AND METHODS
Mandibular arch length—measured by summing both the
In this retrospective study of 44 patients, a single ortho- right and the left distances from the mesial contact
dontic practitioner treated 24 female and 20 male patients points of the first permanent molars to the contact point
with a mandibular lip bumper. The patients’ ages were be- of the central incisors or to the midpoint between the
tween 9 and 17 years, with a mean age of 12 years. The centrals if spaced.
lip bumper treatment time ranged from 7 to 18 months, with Arch widths between mandibular canines—measured by the
an average of 12.5 months. The sample included patients distance between two reproducible landmarks located
in the mixed dentition and the early permanent dentition. near the cusp tips of both canines.
Twenty-seven patients had a class I molar relationship, 13 Arch width between mandibular first/second premolars—
were class II, and four were end-on class II. Appliances measured by the distance between two reproducible
used on the mandibular arch consisted solely of a com- landmarks located near the cusp tips of both first/sec-
mercial, prefabricated lip bumper (American Orthodontics ond premolars.
Corp, Sheboygan, Wis) of 0.045-inch stainless steel wire Arch width between mandibular first permanent molars—
with no acrylic or plastic added to the anterior segment. measured by the distance between two reproducible
Fifteen patients had maxillary expansion in conjunction landmarks located near the central pits of both first
with the lip bumper therapy, whereas 29 patients had only molars.
traditional fixed appliances on the maxillary dentition. Of
To determine the incremental expansion that occurred be-
those patients who had expansion, 11 had Haas-style rapid
tween each appointment, the same landmark on each tooth
palatal expanders, three had the inner bow of a Kloehn
for each successive patient cast was identified. A pilot study
cervical facebow, and one had a quad helix.
was done to determine the possible error in measuring the
incremental differences between appointments. Three pa-
Clinical procedure
tients were selected at random, and each dimension was
In all cases, the lip bumper was fixed in place 24 hours measured on each cast for each patient. Once completed,
per day with ligature wire, elastic power chains, or elastic all marks were removed without damaging the casts, and
separators running from the hooks on the lip bumper to the the same protocol was performed two more times, each on
buccal tubes on the bands of the lower first molars. It was separate days. The results for each of the three trials were
adjusted at the level of the free gingival margin with 1.0– then compared to determine the error.
1.5 mm of advancement relative to the gingival margin. The Measurements for this study were only made on per-
lip bumper was routinely inserted without active expansion manent teeth. Measurements were not analyzed for those
unless it was required to correct lingual torque of the man- patients whose primary teeth exfoliated during the duration
dibular molars. Mandibular alginate impressions were taken of lip bumper treatment because of the tooth mobility that
at each four- to six-week adjustment appointment and exists before exfoliation. There were a limited number of
poured into diagnostic study models immediately. cases where the primary teeth were retained throughout

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398 MURPHY, MAGNESS, ENGLISH, FRAZIER-BOWERS, SALAS

TABLE 1. Average Number of Days Included in Each Time Seg- TABLE 2. Total Expansion for Each Measurement
ment for All Patients
Measure- Number of Median Quartile 1 Quartile 3
Time Number of ment Patients Change (mm) (mm) (mm)
Segment Days
3–3 43 2.0 1.1 2.3
1 105.4 4–4 42 3.0 1.8 3.9
2 104.6 5–5 41 3.2 2.3 4.2
3 97.4 6–6 44 3.5 2.4 4.7
4 77.5 AL 44 4.4 3.0 5.7
5 85.8
6 51.7

fourth, and 27% extended through the fifth. Only 7% ex-

treatment. For these specific cases, measurements were an- tended through the sixth time segment and were thus not
alyzed on primary teeth. Data was not analyzed for per- included in the analysis.
manent teeth that erupted near the end of lip bumper treat- Table 2 shows the total expansion in millimeters that oc-
ment or for teeth that were distorted or broken. curred for each dimension measured. The largest median
increases were seen in arch length expansion, and the small-
Data analysis est were observed in width expansion between the canines.
These values were found to be 4.4 and 2.0 mm, respec-
The expansion was analyzed as the percentage of total tively. Because the molars are the only teeth directly at-
expansion attained for each patient during defined time pe- tached to the lip bumper, the expansion seen between them
riods. This was done by dividing the small amount of ex- is active in contrast to the passive expansion that occurred
pansion that occurred between each appointment by the to- across the premolars and the canines. The expansion that
tal expansion achieved to identify what percentage of over- was noted in this study can be attributed to the lip bumper
all expansion occurred between appointments. The per- itself and not to the patient’s growth. This fact is supported
centages of expansion were then grouped according to the in a previous study by Davidovich et al.12 Their study in-
number of appointments required to most closely approxi- cluded an experimental lip bumper group and a control
mate a time segment of 100 days. group that received no treatment at all. In their study of 34
patients, they showed that their control group had a de-
Statistics crease in each dimension measured at the end of the six
The data in this study were not normally distributed, and months as compared with the experimental group, which
the sample sizes were small, thus a nonparametric analysis showed increases.
was performed taking into consideration the amounts of Table 3 shows the percentage of total expansion that oc-
expansion that occurred in each of the five time segments curred in each time segment for each measurement. For
for each patient. Median values were shown to give the each dimension analyzed, the largest percentage of expan-
most representative percentages of expansion. The first and sion occurred in the first time segment, during the first 100
third quartiles also were reported to give the ranges of the days of treatment. As the treatment continued, the percent-
amounts of expansion for 50% of the patients. Mann-Whit- age of total expansion progressively decreased from one
ney tests were done to check for significant differences in time segment to the next, with minimal expansion occurring
expansion between the 15 patients undergoing concomitant in the last two time segments.
maxillary expansion and those who did not. The greatest percentage of expansion, 59.5%, occurred
during the first time segment between the second premolars.
The smallest percentage of expansion, 5%, occurred be-
RESULTS
tween the canines during the last two time segments com-
The appointments were pooled in groups of two or three bined.
to establish approximately equal time segments for each The percentages of total expansion per time segment can
patient. The total sum of the days for those two or three be seen in Figure 2. For each of the tooth types, there is a
appointments approached 100. Table 1 gives the average notable decrease in the amount of expansion that occurred
number of days that were included in each time segment as time progressed.
for all patients. Because this study was retrospective, it was Mann-Whitney tests were performed to compare the
not possible to control the amount of time between appoint- amounts of mandibular expansion that occurred during lip
ments for each patient. bumper therapy in patients who had simultaneous maxillary
One hundred percent of the patients had a treatment pe- expansion vs those who did not. These tests showed no
riod that extended at least through the first two time seg- significant differences in the amounts of expansion that oc-
ments. Eighty-six percent of the treatment periods extended curred for each tooth type.
through the third time segment, 64% extended through the In determining the amount of error in measuring the in-

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INCREMENTAL EXPANSION WITH LIP BUMPER 399

TABLE 3. Percentage of Total Expansion Per Time Segment for TABLE 4. Summary and Averages of the Median Percentage of
Each Measurement Total Expansion Per Time Segment for Each Measurement
Measure- Time Number Quartile 1 Quartile 3 3–3 4–4 5–5 6–6 AL Average
ment Segment of Points Median (%) (%) (%) (%) (%) (%) (%) (%) (%)
3–3 1 43 55.0 34.5 70.6 Time Segment 1 55.0 48.1 59.5 40.3 51.9 51.0
2 43 25.0 10.0 40.0 Time Segment 2 25.0 26.7 26.3 36.7 33.0 29.5
3 36 20.9 5.5 37.4 Time Segment 3 20.9 17.1 17.2 13.1 15.7 16.8
4 26 0.0 0.0 12.3 Time Segment 4 0.0 5.4 3.6 6.6 8.3 4.8
5 11 5.0 0.0 12.5 Time Segment 5 5.0 9.6 3.3 5.0 4.5 5.5
4–4 1 43 48.1 35.1 69.9
2 43 26.7 13.5 45.8
3 35 17.1 5.7 27.8 Other studies have demonstrated similar results using lip
4 25 5.4 0.3 20.6 bumpers but have not focused on the attenuation in expan-
5 10 9.6 5.5 14.6
5–5 1 41 59.5 33.3 71.7 sion that occurs during treatment. A major concern for or-
2 40 26.3 18.5 37.4 thodontists is treatment effectiveness and efficiency. It
3 31 17.2 0.0 25.8 would be beneficial to know the specific amount of time
4 19 3.6 0.0 23.8 required to achieve sufficient expansion using lip bumper
5 7 3.3 0.0 14.3
therapy. This information would benefit the clinician by
6–6 1 44 40.3 14.5 75.9
2 44 36.7 17.1 55.1 eliminating unnecessary lip bumper wear. This would ob-
3 38 13.1 28.7 27.0 viously be desirable for the patients as well.
4 28 6.6 22.3 23.8 The expansion achieved during lip bumper therapy is
5 12 5.0 0.0 37.0 evenly distributed during treatment. However, the results
AL 1 44 51.9 33.2 80.3
clearly demonstrate that the expansion occurred unevenly
2 44 33.0 17.1 43.1
3 38 15.7 20.4 29.9 and actually decreased with time. Table 4 shows that for
4 27 8.3 21.1 16.7 each of the measurements, the greatest amount of expan-
5 12 4.5 0.0 15.9 sion, about 50% of the total, occurred within about the first
100 days. During the second and third time periods, about
40% of the total amount of expansion will have occurred.
During the last two time segments, the percentage of total
expansion achieved is only about 10%. Therefore, about
90% of the expansion was completed in these cases within
the first 300 days, and treatment effectiveness after this
point yielded only about 10% of the total expansion.
Knowledge of how the lip bumper alters the equilibrium
between the lingual and the vestibular forces surrounding
the teeth may explain the results of this study. We speculate
that upon insertion of the appliance, a new system of forces
is set up around the teeth. With time, the dentition will
adapt to the new force system and reestablish equilibrium.
The gross movements in this process occur early in treat-
ment, and as the teeth approach their new equilibrium po-
sition, their movement tapers down. Although we observed
most of the expansion occurring at the start of treatment,
FIGURE 2. Graphical representation showing percentage of total ex- we did observe some patients who had a higher than normal
pansion per time segment for each measurement. percentage of tooth movement toward the end of treatment.
We speculate that this could be related to whether the ap-
pliance was adjusted toward the end of lip bumper therapy.
cremental differences between appointments, the results of More specifically, most cases in our study had minimal ad-
the previously described pilot study revealed the standard justments to the lip bumper toward the end of treatment.
error of the mean to be 0.0399 and a coefficient of variation However, for those cases that required such adjustments,
equal to 0.0021.
the force system was still undergoing changes, thus not ap-
proaching equilibrium.
DISCUSSION
Maxillary expansion can create secondary expansion in
A critical aspect of nonextraction therapy is the creation the mandibular dentition. Because no differences were
of space. The results from this study show that lip bumper found between those patients who received expansion and
therapy is an effective means of accomplishing this task. those who did not, we propose that the already expanding

Angle Orthodontist, Vol 73, No 4, 2003

400 MURPHY, MAGNESS, ENGLISH, FRAZIER-BOWERS, SALAS

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