Download efficiency of the distal screw in the distal movement of maxillary molars

Mauro Cozzani, DMD,
MScD1
Francesco Zallio, MD, DDS2
Luca Lombardo, DDS3
EFFICIENCY OF THE DISTAL SCREW
IN THE DISTAL MOVEMENT
OF MAXILLARY MOLARS
Antonio Gracco, DDS3
Aim: Conventionally, noncompliance distal movement of molars relies
exclusively on intraoral anchorage. The distal screw, a distal jet appliance supplemented by two paramedian mini-implants, is an innovative alternative. The aim of this study was to evaluate the suitability of
this device to move molars bodily and distally. Methods: The effects
of the distal screw were evaluated in a sample of 18 consecutively
treated preadolescent and adolescent individuals (nine females and
nine males; mean age at the start of treatment, 11.2 years). Two conical mini-implants (length 11.0 mm, diameter 1.5 to 2.2 mm) were placed
in the anterior paramedian area of the palate of each patient. The coil
springs of the device were activated to deliver a force of 240 cN per
side. The dental and skeletal effects were investigated on pre- and
posttreatment cephalometric radiographs. Results: The distal screw
produced a Class I occlusion of the first molars by moving them distally
4.7 mm, which is more than conventional appliances can accomplish.
Although this took longer than conventional devices (9.1 months), it
had the advantage of a roughly 2.1-mm premolar distal movement (ie,
no anchorage loss as with traditional techniques). Conclusions: The
distal screw anchored by two palatal mini-implants allows not only
translatory molar distal movement, but also distal movement of the
maxillary first premolars, thereby avoiding characteristic anchorage
loss. World J Orthod 2010;11:341–345.
Key words: maxillary molar distalization, mini-implant, skeletal anchorage
istal molar movement is useful in
resolving a Class II occlusion in
patients with dentoalveolar protrusion
and only slight skeletal discrepancy. 1
While the conventional approach
involves extraoral traction,2 comparable
results have been achieved using fixed,
esthetically acceptable appliances that
rely on intraoral anchorage, thereby eliminating the need for patient compliance.3
In general, these devices exploit a combination of dental (maxillary premolars)
and palatal (Nance button) anchorage.4
This approach leads to an anchorage
D
1Private
2Private
Practice, La Spezia, Italy.
Practice, Sestri Levante (GE),
Italy.
3Research
Assistant, Department of
Orthodontics, University of Ferrara,
Ferrara, Italy.
CORRESPONDENCE
Dr Mauro Cozzani
Via Fontevivo 21 N
La Spezia 19125
Italy
Email: [email protected]
loss (a mesial displacement of the premolars, canines, and incisors),5 a situation that cannot be improved by
bracketing additional teeth.6
One intraorally anchored device is the
distal jet, an intramaxillary appliance
that is effective due to two Ni-Ti coil
springs attached to the bands on the
maxillary first molars.7 If the distal jet is
constructed according to Bolla et al, 1
the force vector passes through the center of resistance of these teeth, which
results in almost complete bodily movement.8 Several authors, including those
341
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Cozzani et al
Fig 2 Intraoral photograph of
a distal screw, which is used
as anchorage to retract the
canines.
Fig 3 A 2-mm titanium screw
expressly designed for the distal screw.
Fig 1 Metallic plate to be inserted in the
resin of the Nance button; note that all
holes are designed to fit the head of the
specific mini-implant (Fig 3).
of the present study, have developed
skeletally anchored alternatives to the
conventional distal jet.4,9–11 The distal
screw uses two palatally inserted miniimplants for skeletal anchorage. Numerous studies have demonstrated that the
optimal sites for mini-implant are not only
the lingual interradicular spaces,12 but
also the paramedian region of the palatal
vault.13,14
The aim of this study was to clinically
evaluate the efficiency of the distal
screw.
METHOD AND MATERIALS
Eighteen consecutive patients (nine
males, nine females; mean age at beginning of treatment 11.2 years) with a bilateral dentoalveolar distal occlusion were
treated solely with a distal screw. In six of
these patients, the maxillary second
molars had fully erupted, while they had
erupted partially in four. They had not
erupted in the remaining eight patients.
No patients dropped out during the trial.
For all patients, intra- and extraoral
photographs, impressions, panoramic
radiographs, and lateral cephalographs
were obtained at the beginning and end
of the first molars’ distal movement.
The appliance was a modified distal
jet in which the metallic arms normally
used for dental anchorage were eliminated and the Nance button altered to
enclose a moldable metal plaque fixed by
two mini-implants (Figs 1 and 2). The two
mini-implants were placed in the paramedian region of the anterior palatal vault
along a line connecting the two synergetic premolars. They were inserted by
predrilling and using a manual screwdriver after the patients had rinsed with a
0.1% chlorhexidine gluconate solution.
Local anesthesia with an adrenalin-free
analgetic was performed.
The insertion site was selected on the
basis of various studies demonstrating
its safety, thereby eliminating the need
for any fur ther radiographic evaluation.13,14 Also, according to Ardekian et
al, nasal floor perforations of less than 2
mm tend to heal spontaneously.15
The mini-implants employed were
made of titanium, measured 11.0 mm
long, and were shaped like a truncated
cone with a diameter of 1.5 mm at the
tip and 2.2 mm at the neck. The shank
was 1.0 mm in diameter, the threaded
part had a length of 8.0 mm, and the
head featured a hexagonal slot to house
the head of the screwdriver or contraangle handpiece (Fig 3).
342
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VOLUME 11, NUMBER 4, 2010
Fig 4 Method suggested by
Ghosh and Nanda 16 to quantify dental movements using
cephalometric superimpositions.
Cozzani et al
43 2
SN
SN
5
1
PTM
PP
4 3
PTV
7
8
2
6
9
1
A
5
B
MnPI
Table 1 Mean, standard deviation (SD), minimum, and maximum of all cephalometric
parameters between T1 (beginning) and T2 (end of molar distal movement)
T1–T2 (months)
PTV–U6 (mm)
PTV–U4 (mm)
SN–U6 (degrees)
SN–U4 (degrees)
SN–U1 (degrees)
PP–U6 (mm)
PP–U4 (mm)
PP–U1 (mm)
PTV–A (mm)
Mean
SD
Minimum
Maximum
9.1
–4.7
–2.1
–2.6
–2.0
0.3
0.7
1.3
0.4
0.4
2.7
1.6
1.8
2.3
3.1
2.9
1.9
1.5
0.8
0.8
4.0
–8.2
–6.7
–5.8
–8.4
–4.8
–3.2
–1.9
–1.2
–0.5
13.0
–2.7
–0.1
4.2
4.9
6.8
6.4
3.9
2.4
2.4
The superelastic springs were compressed by adjusting the attachment
screws until a force of 240 cN could be
measured; reactivation was carried out at
4-week intervals.
To quantify the distal movement
achieved, any premolar or canine displacement was evaluated according to
the methodology suggested by Ghosh
and Nanda16 (Fig 4). This method was
chosen to compare the results of this
study with those of other studies.1,17
Statistical analysis
Mean, standard deviation, and range of
each continuous variable were calculated
before (T1) and after (T2) distal movement. Also, the absolute and relative frequencies of the categoric variables were
determined.
RESULTS
The data from the cephalometric analyses
are listed in Table 1. It also shows that the
average time required to achieve a Class I
molar relationship was 9.1 ± 2.7 months.
The mean distal movement of the maxillary molars (PTV–U6) was –4.7 ± 1.6 mm.
Simultaneously, the first premolar
(PTV–U4) moved distally on average –2.1 ±
1.8 mm. Distal tipping of the first molars
(SN–U6/U4/U1) amounted to –2.6 ± 2.3
degrees and –2.0 ± 3.1 degrees for the
first premolars. The incisors tipped labially
0.3 ± 2.9 degrees. Extrusion with respect
to the bispinal plane of the first molars
(PP–U6/U4/U1) was 0.7 ± 1.9 mm, 1.3 ±
1.5 mm for the first premolars, and 0.4 ±
0.8 mm for the incisors. The distance
PTV–A increased by 0.4 ± 0.8 mm.
343
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WORLD JOURNAL OF ORTHODONTICS
Cozzani et al
DISCUSSION
The introduction of skeletal anchorage to
orthodontics has permitted not only the
simplification of many procedures conventionally employed to control anchorage, but also reduced the undesirable
effects of many appliances. 18 Initial
attempts were based on osseointegrated
implants,19,20 but the related costs, invasiveness, and delay of loading prompted
clinicians to seek alternatives.21,22 Thus,
mini-implants were used based on their
low cost, reduced invasiveness, and versatility.23,24 Clinical and laboratory studies have demonstrated the usefulness of
mini-implants for or thodontic purposes.25,26
Compared to Bolla et al,1 who treated
a similar number and type of patients
using a distal jet, the amount of molar
distal movement was greater with the distal screw (3.2 vs 4.7 mm). Moreover, the
patients treated with the distal screw did
not experience any anchorage loss of the
first premolar in contrast to those treated
with the distal jet (mean loss 1.3 mm).
Actually, the first premolars moved distally, too (2.1 mm). The fact that a Class I
occlusion was achieved in 5 to 6 months
in the study by Bolla et al1 as compared
to 9.1 months in this study could be
explained by the fact that the distal
occlusion might have been more severe
in the patients treated with the distal
screw. In any case, the distal movement
of the first molars was nearly the same in
both studies (0.5 vs 0.6 mm).
From the cephalometric perspective,
the distal screw conserves the positive
characteristics of the distal jet but overcomes its negative aspect: the medioanterior anchorage loss.
Finally, the distal screw seems to
behave clinically differently than conventional and other skeletally anchored distal movement devices. Authors who used
a pendulum with skeletal anchorage
achieved a greater distal movement in a
shorter time (5.4 mm in 6.5 months27),
which was, however, accompanied by
severe first molar (5.6 to 12.2 degrees)
and first premolar tipping (3.8 to 7.9
degrees). 22,27,28 In contrast, this appliance produced a tipping of only 2.6
(molars) and 1.9 degrees (premolars).
Similar results for molar distal movement were documented in a study of 10
adolescent patients treated with a distal
jet anchored to the first premolars by two
palatal mini-implants. 2 It was also
reported that the second premolar moved
1.9 mm distally with about 3.0 degrees of
tipping.2 In contrast to the results of this
study, Kinzinger et al3 described a mesial
displacement of 0.7 mm for the first premolar, which can be explained by the different anchorage setup.
Overall, the distal screw has numerous advantages with respect to both conventional appliances and other skeletally
anchored molar distal movement
devices. In fact, the distal screw not only
overcomes anchorage loss, but also simplifies the treatment because premolar
banding is rendered unnecessary and
the same appliance, once inactive, can
further be employed for final premolar
and canine retraction. As these screws
are positioned in the palate, they do not
interfere with the distal movement of the
posterior teeth. All advantages of the distal screw are obtained without taking
additional radiographs.
CONCLUSIONS
The distal screw allows an almost completely bodily distal movement of the
maxillary first molars and a spontaneous
distal drift of the premolars. In comparison to the distal jet, the distal screw simplifies the clinical procedure without any
special radiographic evaluation.
The longer time needed to achieve a
Class I relationship is compensated by
the simpler subsequent distal movement
of the remaining teeth because the premolars need less distal movement.
344
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VOLUME 11, NUMBER 4, 2010
Cozzani et al
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