2011 JCO, Inc. May not be distributed without permission. www.jco-online.

com

Locking the Screw

after Rapid Palatal Expansion:
A Superfluous Procedure?
LUIS TOMAS HUANCA GHISLANZONI, DDS, MS
LORENZO FRANCHI, DDS, PHD
TIZIANO BACCETTI, DDS, PHD

he rapid palatal expander (RPE) is widely used

to correct maxillary constriction. Clinically,
there are only a few differences among the various
expansion protocols, including the number and
frequency of turns (activation rate) of the midline
jackscrew for rapid or slow expansion,1,2 the attach
ment method (banded or bonded acrylic),3 and the
decision whether to use deciduous or permanent
teeth for anchorage.4
The screw of an RPE is commonly blocked
with composite or a stainless steel ligature after
the desired expansion has been achieved, the ob
jective being to prevent relapse due either to the
forces generated by stretched tissues of the en
larged maxillary bone trying to return to their
previous state5 or to back-turning from manipula
tion by the tongue.6 Little research has been pub
lished, however, that might confirm such relapse.
We used a prospective clinical trial and a

Dr. Huanca Ghislanzoni

theoretical approach to investigate whether it is

necessary to lock the screw after active expansion.
Materials and Methods
The prospective clinical trial was performed
in Dr. Huanca Ghislanzonis private practice. Be
cause a statistical power greater than .9 was de
sired, a sample size of at least 45 subjects was
needed. Forty-eight consecutive patients (21 males
and 27 females) presenting with maxillary defi
ciency, as indicated by a unilateral or bilateral
crossbite, were chosen for treatment with rapid
palatal expansion. The mean age at the start of
treatment was 7.8 1.2 years.
A Hyrax expansion screw* coated with a
*Part A0620, Leone S.p.A., Sesto Fiorentino, Florence, Italy;
www.leone.it.

Dr. Franchi

Dr. Baccetti

Dr. Huanca Ghislanzoni is a doctoral student, Department of Human Morphology, University of Milan, and Dr. Franchi is an Assistant Professor,
Department of Orthodontics, University of Florence, Florence, Italy, and a Thomas M. Graber Visiting Scholar, Department of Orthodontics and
Pediatric Dentistry, School of Dentistry, University of Michigan, Ann Arbor; e-mail: lorenzo.franchi@unifi.it. Dr. Baccetti, who was killed in a tragic
accident in November (see The Editors Corner, JCO, November 2011), was a Contributing Editor of the Journal of Clinical Orthodontics, an Assistant
Professor, Department of Orthodontics, University of Florence, and a Thomas M. Graber Visiting Scholar, Department of Orthodontics and Pediatric
Dentistry, School of Dentistry, University of Michigan.

VOLUME XLV NUMBER 12

2011 JCO, Inc.

657

Locking the Screw after Rapid Palatal Expansion: A Superfluous Procedure?

Fig. 1 Rapid maxillary expander cemented to first

molars.

friction agent was cemented to the first molars of

each patient (Fig. 1). A full turn of the screw pro
vided .8mm of expansion. The treatment protocol
was the same for each patient: two quarter-turns
per day (.4mm of expansion), with weekly visits
to note progress. The active expansion phase
lasted a mean 15 3 days. In each case, when the
amount of expansion was judged satisfactory, with
a slight overcorrection, a notch was carved with a
diamond bur into the lingual surface of the screw
spindle (Fig. 2). The notch served as an unam
biguous reference point for any backward move
ment of the screw components, since such
movement would cause a displacement of the
notch. The screws were not blocked with compos
ite or ligatures.
After an average 5.5 months of retention with
passive expanders, the appliances were removed.
At the debanding appointment, each screws notch
position was checked, and the number of reverse
turns needed to deactivate the screw was counted
to verify that none of the screws had reversed by
exactly one or more full turns, which might have
created an illusion of stability.
Results
All 48 patients completed the treatment. An
average 30 quarter-turns were made, resulting in
an average screw opening of 6.1 1.2mm. None
of the notches was found to be displaced, and the

658

Fig. 2 Notch carved in lingual surface of spindle

as reference mark to indicate any relapse of
expansion screw.

number of deactivation turns matched the num

ber of activations in each subject. Since there had
been no relapse in any of the patients, no further
statistical analysis of the results was required.
Discussion
Locking the jackscrew in place after achiev
ing the desired rapid palatal expansion is a univer
sal clinical management tip that actually appears
to have little substantiation. The resistance force
of the maxillary tissues against the expander was
studied by Isaacson and colleagues in five patients,
using a modified RPE with a dynamometer con
necting the expansion screw and the bands on one
side of the mouth to an acrylic plate placed against
the palatal alveolar process of the opposite side.7-9
The expansion screw was activated .8mm per
complete turn, as in many current RPE designs. In
four of the patients, the forces measured by the
dynamometer dropped to zero five to seven weeks
after the end of active expansion. In the fifth
patient, for whom the maximum possible daily
activations were performed in the clinic, a drop to
zero was noted after only five days. This sudden
decrease was attributed to back-turning of the
screw, perhaps caused by masticatory function or
manipulation by the patient. In a more recent study,
Halazonetis and colleagues measured the contribu
tion of the stretched cheeks in resisting maxillary

JCO/DECEMBER 2011

Huanca Ghislanzoni, Franchi, and Baccetti

Ff
F//

0.4mm
1.5mm

Fig. 3 Screw characteristics and resulting forces. A. Screw pitch and thread slope. B. Direction of forces.

expansion; results showed a negligible .6g/cm2 per

millimeter of expansion.10
Other factors that could be considered poten
tial causes of back-turning are vibrations and
lubrication. Vibrations at a particular resonance
frequency can cause a screw to unseat. In the
mouth, the voice can produce vibrations ranging
from 60 to 2,000Hz, with averages of 100Hz for an
adult man, 200Hz for an adult woman, and 400Hz
for a child.11,12 Although no data have been pub
lished on the resonance frequency of an RPE screw
system, it seems unlikely that vocal vibrations
could affect the stability of expansion treatment.
A lubricant reduces the strength and number
of bridges formed between the asperities of sliding
surfaces.13 In studies using artificial saliva, friction
has been variously found to decrease,14 stay the
same,15 or increase16 during orthodontic treatment.
Tselepis and colleagues reported a drop in fric
tional force between stainless steel brackets and
archwires of as much as 60% under lubrication
with artificial saliva.17 Even this much reduction
in static friction would not be enough to allow any
screw to turn back, however, as demonstrated by
the following theoretical discussion.
Geometrical analysis shows that an RPE
screw cannot be unintentionally turned back as
long as the slope of each thread does not exceed a
critical value of 36.5. In fact, the slope of the
threads is the key factor. Our calculations were
based on the specific manufacturing details of the
screw used for the present study, but they may be
applied to virtually any screw of similar thread

VOLUME XLV NUMBER 12

pitch and slope.

The Leone A0620 screw has a mean diam
eter of 1.5mm; a full turn provides .8mm of activa
tion (expansion). Assuming the screw of the RPE
is centered symmetrically between two metal
blocks moving away from each other, this means
that for every full turn, each block moves .4mm
away from the center. That value also represents
the pitch of the screwthe distance between the
centers of two contiguous threads as measured
along the long axis (Fig. 3A). The slope of the
thread (the angle between the thread and a plane
perpendicular to the long axis), can be calculated
using the equation:

pitch
= arctan diameter
(Eq. 1)
By applying this equation to the A0620
screw, the thread-slope angle, , is shown to be
4.9.
The forces from the stretched maxillary tis
sues, acting parallel to the long axis of the screw,
may be broken down into two parts (Fig. 3B): F//
(parallel to the threads) and F (perpendicular to
the threads). F// alone could theoretically cause the
screw to turn back because it acts as a tangential
force, creating a moment around the long axis of
the screw. F is assumed to be the force responsible
for frictional resistance to turning.
The force of static friction is calculated by
multiplying the normal force by the coefficient of
friction, which for stainless steel is about .74.18

659

Locking the Screw after Rapid Palatal Expansion: A Superfluous Procedure?

This force, Ff, acts in the same direction as, but in

opposition to, F//. If F// is greater than Ff, the screw
can turn around its axis; otherwise, it will not
move. An angle of 36.5 (the arctangent of .74) is
the critical angle at which F// is equal to Ff. Under
normal conditions, it is impossible for a shallowerthreaded screw to turn back, because the fric
tional forces will always be greater than the
parallel forces (Fig. 3B).
Projecting the compression force F onto a
coordinate system parallel to the slope of the
threads, F can now be expressed in terms of com
ponents parallel to (F//) and perpendicular to (F)
the threads:

F// = Fsin (a)

F = Fcos (b)
Ff = Fcos (c) (Eq. 2)

where is the slope of the threads and is the

coefficient of friction. The applied load cannot
cause the screw to back out unless the component
of the force parallel to the threads is greater than
the force of friction:
F// > Ff (Eq. 3)
From the identities in Equation 2, it follows
that for the screw to back out, the condition
tan > (Eq. 4)
must be met. For to equal .74, must be greater
than 36.5, which is unlikely with any normal
thread design. Alternatively, for to equal 4.9,
the coefficient of friction would need to be less
than .09 in the static case described here.
Conclusion
Our prospective clinical trial and theoretical
considerations show that locking the expansion
screw of an RPE at the end of active expansion is
an unnecessary precaution in most situations. The
shallow slope of virtually any expansion-screw
threads will prevent relapse of the expansion
mechanism. Although our clinical study used a

660

screw coated with a friction agent, it appears from

our calculations that such coatings, as well as the
ratcheting-type mechanisms incorporated in many
screws, may be superfluous.
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JCO/DECEMBER 2011