orthodontics

1. EARLY TREATMENT SYMPOSIUM
Treatment of crowding in the
mixed dentition
Anthony A. Gianelly, DMD, PhD, MD
Boston, Mass
I
t is a well-estab-
lished fact that arch
length is lost during
the transition from the
mixed to the permanent
dentition, particularly in
the mandibular arch. One
estimate is that the aver-
age mandibular arch loss
is 1.8 mm.1
Since this
reduction, bilaterally,
represents 3.6 mm of
arch perimeter, the ques-
tion arises: in patients with crowding in the mixed
dentition, can simple arch length maintenance during
the transition period provide adequate space to prevent
crowding in the permanent dentition?
The results of 2 studies2,3
indicate that the answer to
this question is yes. In the first study, 100 mixed
dentition models of the mandibular arch were analyzed.
(Only mandibular arch conditions were evaluated be-
cause they generally dictate the strategy for maxillary
arch treatment.) A conventional tooth size-arch size
assessment was performed to quantify the crowding.
The mesiodistal diameters of the teeth were measured,
and a brass wire was extended from the mesial of a first
molar to its antimere, adapted over the fossae of the
posterior teeth and the cusp tips of the canines, and
idealized in the anterior segment between the canines.
Crowding, present in 85 of the 100 patients, averaged
4.39 Ϯ 3.39 mm (range, 4.46-13.46 mm). The sizes of
the unerupted permanent teeth were derived nonradio-
graphically by using ratios of primary to permanent
teeth to determine the leeway space that would become
available by maintaining arch length.4
The leeway
space was 5.15 Ϯ 0.68 mm (range, 1.6-7.64 mm).
When the leeway space was compared individually to
crowding, there was adequate space to resolve crowd-
ing in 62 (72%) of the patients. Of the remaining 23
patients, 7 had less than 2 mm of crowding, and 16 had
2 mm or more.
The relationship between early loss of a primary
canine and crowding was also evaluated. Nineteen
subjects lost a primary canine prematurely, and 18 of
these patients had crowding. Only 7 of the 18 (39%)
had adequate space to resolve the crowding when the
leeway space was included. In contrast, when crowding
was not associated with the early loss of a primary
canine, the leeway space proved adequate to correct the
crowding in 82% of the patients. As expected, the early
loss of a primary canine combined with crowding
results in a more crowded dentition. This study indi-
cated that the space to resolve crowding in the mixed
dentition could be obtained in most patients simply by
maintaining arch length during the transition from
mixed to permanent dentition, but these results are
theoretical and should be tested clinically.
The second study was a clinical investigation in-
volving the evaluation of the outcomes of arch length
maintenance by means of passive lingual arches during
the transition period in 107 consecutive patients. Tooth
size-arch size discrepancies were measured as previ-
ously described at 2 times—in the mixed dentition and
in the early permanent dentition with the second pre-
molars at least 50% erupted. Total arch length, repre-
senting the combined sum of right and left distances
from the mesial contact points of the first molars to the
contact points between the central incisors, and the
widths between canines, premolars, and molars were
also measured. Even though lingual arches were placed,
total arch length decreased Ϫ0.44 mm (Ϫ0.17 mm on
the right and Ϫ0.27 on the left). Arch length decreased
in 62 subjects, increased in 39, and remained the same
in 6. Arch width increased during the transition from
mixed to permanent dentition. Intercanine width in-
creased by 1.49 mm, interfirst premolar width increased
by 2.27 mm, and intermolar width increased by 0.72
mm. The increase in intercanine width, though larger
than noted in untreated subjects,1
is similar to that
reported by DeBaets and Chiarini5
(1.1 mm) after
Goldman School of Dental Medicine, Department of Orthodontics, Boston
University, Boston, Mass.
Presented at the International Symposium on Early Orthodontic Treatment,
February 8-10, 2002; Phoenix, Ariz.
Am J Orthod Dentofacial Orthop 2002;121:569-71
Copyright © 2002 by the American Association of Orthodontists.
0889-5406/2002/$35.00 ϩ 0 8/1/124172
doi:10.1067/mod.2002.124172
569

2. lingual arch placement. They suggested that the in-
crease occurred as a result of distolateral migration of
the canines into the leeway space.
The average amount of crowding in the mixed
dentition was 4.8 mm Ϯ 2.1 mm. After their permanent
teeth erupted, 65 of the 107 patients (61%) had ade-
quate space to align the incisors. Another 8 demon-
strated less than 0.5 mm of crowding. If total arch
length were maintained, these 8 patients would be
included in the group with adequate space for align-
ment. Thus, in 73 of 107 patients (68%), arch length
preservation from the mixed to the permanent dentition,
combined with spontaneous developmental arch
changes, provided adequate space to resolve crowding.
In 81 patients (76%), crowding after use of the lingual
arch was less than 1 mm, and, in 93 subjects (87%), it
was less than 2 mm.
Arch length preservation liberates space
A comparison of these 2 studies is useful. In the
theoretical part of the first study, the space necessary to
resolve crowding would be available in 72% of the
subjects with crowding in the mixed dentition simply
by preserving arch length. In the clinical study, ade-
quate space was present to align the incisors in 68% of
the patients. This finding is remarkably similar to data
recorded by DeBaets and Chiarini,5
who determined
that sufficient space to resolve crowding was available
after lingual arch therapy in 70% of the patients in their
sample.3
In a more global sense, the concept that arch
length preservation during the transition from mixed to
permanent dentition can provide space for alignment is
accepted because it is well known that arch length
preservation liberates space. For example, Reballato et
al6
noted that approximately 4 mm of arch perimeter is
lost during the transition from mixed to permanent
dentition, but that placement of a lingual arch prevented
this loss. Yet we did not expect that as many as 87% of
the subjects with crowding in the mixed dentition
would have less than 2 mm of crowding if the space
provided by arch length preservation were used. This
finding has profound clinical implications because it
means that arch length preservation in most patients,
and an increase of less than 1 mm per side in others, can
provide enough space to accommodate an aligned
permanent dentition in almost 90% of those who have
crowding in the mixed dentition. The reason for limit-
ing any arch length increase to less than 1 mm per side
is that Little et al7
observed that the largest postreten-
tion irregularity index (6.06 mm) occurred in patients
whose arch lengths were increased more than 1 mm in
the mixed dentition.
One clinical implication is the type of treatment.
Procedures performed in the mixed dentition to “devel-
op” arches to gain space for alignment may be unnec-
essary because arch length preservation with a simple
appliance, such as a lingual arch, may be all that is
required. Also, some of these procedures are not useful.
For example, common wisdom indicates that maxillary
expansion will remove the constraining influences from
the mandibular arch and allow it to “develop” laterally.
This is an area of particular interest in the intercanine
dimension because it provides more space for align-
ment that any other transverse change.8
In fact, studies
that examined this presumed relationship have found
that the mandibular intercanine dimension change is
either nonexistent9,10
or limited to 1 mm.11
A second consideration is the timing of treatment.
The results of the lingual arch studies indicate that the
timing of treatment to resolve crowding could be at the
terminal phase of the mixed dentition because the
changes that appreciably affect arch dimensions nor-
mally occur at this time. One major exception is the
early loss of a primary canine. This requires immediate
intervention to control both arch length and symmetry.
Under these conditions, the opposite canine is removed,
and a lingual arch is placed. If an arch length increase
of up to 1 mm is necessary, a lip bumper can gain this
space within 6 months after the first premolars erupt.12
Two questions are important. First, how stable is
the alignment after lingual arch therapy? Only 1 inves-
tigation in the literature examines this relationship.
Dugoni et al13
reported that mandibular incisor align-
ment in 76% of patients treated successfully with only
a lingual arch in the mixed dentition was considered
stable 9 years postretention. The average irregularity
index in this group was a low 2.65 mm, which is
considered acceptable. This contrasts with the irregu-
larity index of 6.06 mm determined by Little et al7
in
patients who experienced an increase of more than 1
mm in arch length during mixed dentition treatment.
Second, what is the fate of the third molars, when
normal mesial migration during the transition from
mixed to permanent dentition is prevented by a lingual
arch? At present, we have no data to answer this
question. However, there are indications that third
molar impaction rates are higher when comparing
nonextraction and extraction treatments.14
Lingual arch
placement is a nonextraction approach, and it may
result in a higher rate of third molar impactions.
In summary, the leeway space, coupled with devel-
opmental changes that normally occur when arch length
is preserved during the transition from mixed to per-
manent dentition, provides adequate space in most
patients to resolve crowding in the mixed dentition. The
resolution occurs during and after the transition. This
American Journal of Orthodontics and Dentofacial Orthopedics
June 2002
570 Gianelly

3. indicates that an appropriate time to start active treat-
ment to correct crowding, in most instances, is the late
mixed dentition stage of development.
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American Journal of Orthodontics and Dentofacial Orthopedics
Volume 121, Number 6
Gianelly 571