Effect of screw diameter on orthodontic skeletal anchorage

ORIGINAL ARTICLE
<strong>Effect</strong> <strong>of</strong> <strong>screw</strong> <strong>diameter</strong> on orthodontic skeletal
anchorage
Chad Morarend, a Fang Qian, b Steve D. Marshall, c Karin A. Southard, d Nicole M. Grosland, e Teresa A. Morgan, f
Michelle McManus, g and Thomas E. Southard h
Dubuque and Iowa City, Iowa
Introduction: Many case reports have documented the successful use <strong>of</strong> titanium mini<strong>screw</strong>s for orthodontic
anchorage. However, the literature lacks a well-controlled study examining the effect <strong>of</strong> mini<strong>screw</strong> <strong>diameter</strong> on
anchorage force resistance. The purpose <strong>of</strong> this in-vitro study was to compare the force resistance <strong>of</strong> larger<strong>diameter</strong>
monocortical mini<strong>screw</strong>s to smaller-<strong>diameter</strong> monocortical mini<strong>screw</strong>s; and to compare the force
resistance <strong>of</strong> larger-<strong>diameter</strong> monocortical mini<strong>screw</strong>s to smaller-<strong>diameter</strong> bicortical mini<strong>screw</strong>s. Methods:
Ninety-six titanium alloy <strong>screw</strong>s were placed into 24 hemisected maxillary and 24 hemisected mandibular
specimens between the first and second premolars. Specimens were randomly and evenly divided into
2 groups. In the first group, 24 large-<strong>diameter</strong> <strong>screw</strong>s (2.5 3 17 mm) and with 24 small-<strong>diameter</strong> <strong>screw</strong>s
(1.5 3 15 mm) were placed monocortically. In the second group, 24 large-<strong>diameter</strong> <strong>screw</strong>s (2.5 3 17 mm)
were placed monocortically and 24 small-<strong>diameter</strong> <strong>screw</strong>s (1.5 3 15 mm) were placed bicortically. All <strong>screw</strong>s
were subjected to tangential force loading perpendicular to the mini<strong>screw</strong> with lateral displacement <strong>of</strong> 0.6 mm.
Statistical analyses, including the paired-samples t test and the 2-samples t test, were used to quantify <strong>screw</strong>
force-deflection characteristics. One-way analysis <strong>of</strong> variance (ANOVA) with the post-hoc Tukey studentized
range test was used to determine any significant differences, and the order <strong>of</strong> those differences, in force anchorage
values among the 3 <strong>screw</strong> types at maxillary and mandibular sites. Results: Mean mandibular and
maxillary anchorage force values <strong>of</strong> the 2.5-mm monocortical <strong>screw</strong>s were significantly greater than those
<strong>of</strong> the 1.5-mm monocortical <strong>screw</strong>s (P \0.01). No statistically significant differences in mean mandibular anchorage
force values were found between the 2.5-mm monocortical <strong>screw</strong>s and the 1.5-mm bicortical <strong>screw</strong>s.
However, mean maxillary anchorage force values <strong>of</strong> the 1.5-mm bicortical <strong>screw</strong>s were significantly greater
than those <strong>of</strong> the 2.5-mm monocortical <strong>screw</strong>s (P \0.01). Data analyzed with 1-way ANOVA with the posthoc
Tukey studentized range tests indicated that the mean mandibular and maxillary force values <strong>of</strong> the
2.5-mm monocortical <strong>screw</strong>s and the 1.5-mm bicortical <strong>screw</strong>s were significantly greater than those <strong>of</strong> the
1.5-mm monocortical <strong>screw</strong>s (P\0.01). Based on the 2-samples t test, mean anchorage force values at mandibular
sites were significantly greater than at maxillary sites for the 2.5-mm monocortical <strong>screw</strong>s and the 1.5mm
monocortical <strong>screw</strong>s. There were no statistically significant differences in mean anchorage force values
between maxillary and mandibular sites for the 1.5-mm bicortical <strong>screw</strong>s. Conclusions: In vitro, larger-<strong>diameter</strong>
(2.5 mm) monocortical <strong>screw</strong>s provide greater anchorage force resistance than do smaller-<strong>diameter</strong>
(1.5 mm) monocortical <strong>screw</strong>s in both the mandible and the maxilla. Smaller-<strong>diameter</strong> (1.5 mm) bicortical
<strong>screw</strong>s provide anchorage force resistance at least equal to larger-<strong>diameter</strong> (2.5 mm) monocortical <strong>screw</strong>s.
An alternative to placing a larger-<strong>diameter</strong> mini<strong>screw</strong> for additional anchorage is a narrower bicortical <strong>screw</strong>.
(Am J Orthod Dent<strong>of</strong>acial Orthop 2009;136:224-9)
Many case reports have documented the successful
use <strong>of</strong> titanium mini<strong>screw</strong>s and variations
<strong>of</strong> surgical <strong>screw</strong>s used for rigid fixation
to provide orthodontic anchorage without patient
a
Private practice, Dubuque, Iowa.
b
Associate research scientist, Department <strong>of</strong> Preventive and Community
Dentistry, College <strong>of</strong> Dentistry, University <strong>of</strong> Iowa, Iowa City.
c
Adjunct associate pr<strong>of</strong>essor, Department <strong>of</strong> Orthodontics, College <strong>of</strong> Dentistry,
University <strong>of</strong> Iowa, Iowa City.
d
Pr<strong>of</strong>essor, Department <strong>of</strong> Orthodontics, College <strong>of</strong> Dentistry, University <strong>of</strong>
Iowa, Iowa City.
e
Assistant pr<strong>of</strong>essor, Department <strong>of</strong> Biomedical Engineering, College <strong>of</strong>
Engineering, University <strong>of</strong> Iowa, Iowa City.
f
Assistant pr<strong>of</strong>essor, Department <strong>of</strong> Oral and Maxill<strong>of</strong>acial Surgery, College <strong>of</strong>
Dentistry, University <strong>of</strong> Iowa, Iowa City.
g
Postgraduate student, College <strong>of</strong> Dentistry, University <strong>of</strong> Iowa, Iowa City.
224
compliance. 1-6 Compared with traditional endosseous
implants, mini<strong>screw</strong>s <strong>of</strong>fer distinct advantages as anchors:
smaller size, flexibility in site placement, ease
<strong>of</strong> placement, less patient trauma, and lower cost. 7,8
h
Pr<strong>of</strong>essor and head, Department <strong>of</strong> Orthodontics, College <strong>of</strong> Dentistry, University
<strong>of</strong> Iowa, Iowa City.
Supported by the Dr. George Andreasen Memorial Fund.
The authors report no commercial, proprietary, or financial interest in the products
or companies described in this article.
Reprint requests to: Thomas E. Southard, Department <strong>of</strong> Orthodontics, College
<strong>of</strong> Dentistry, University <strong>of</strong> Iowa, Iowa City, IA 52242; e-mail, tom-southard@
uiowa.edu.
Submitted, May 2007; revised and accepted, July 2007.
0889-5406/$36.00
Copyright Ó 2009 by the American Association <strong>of</strong> Orthodontists.
doi:10.1016/j.ajodo.2007.07.031

American Journal <strong>of</strong> Orthodontics and Dent<strong>of</strong>acial Orthopedics Morarend et al 225
Volume 136, Number 2
Additionally, because the primary stability <strong>of</strong> mini<strong>screw</strong>s
is believed to result from mechanical interlock,
a waiting period for osseointegration before orthodontic
loading is unnecessary, as is a second surgical procedure
(trephination) for removal. 1,8,9 On the other hand, because
mini<strong>screw</strong>s are not osseointegrated, their anchorage
potential is most likely influenced by the quality
and quantity <strong>of</strong> bone into which they are placed. 10
Case reports have documented problems associated
with mini<strong>screw</strong> anchorage, including peri<strong>screw</strong> inflammation
and <strong>screw</strong> mobility. 11-13 In a prospective study
<strong>of</strong> risk factors associated with mini<strong>screw</strong> failures,
Cheng et al 13 reported that length had no significant
effect on <strong>screw</strong> survival and that loads <strong>of</strong> 100 to 200 g
could be sustained with no significant differences between
successful and failed <strong>screw</strong>s. Although other
researchers identified no major differences in the survival
<strong>of</strong> mini<strong>screw</strong>s placed in keratinized or nonkeratinized
mucosa, 14,15 Cheng et al 13 advised placing them
in keratinized mucosa and using caution when working
in dense bone regions such as the posterior mandible to
avoid bone overheating.
It was suggested that the type <strong>of</strong> force system used
can lead to <strong>screw</strong> failure. 1 Huja et al 10 cautioned that
torsional loading <strong>of</strong> <strong>screw</strong>s might debond any mechanical
or chemical integration between the <strong>screw</strong> and bone
interface. Using maxillae and mandibular cadaver specimens,
Brettin et al 16 tested the hypothesis that bicortical
mini<strong>screw</strong> placement (across the entire width <strong>of</strong> the
alveolus) could provide a force system with superior
force resistance and stability (anchorage) compared
with monocortical placement. Deflection force values
were significantly greater for bicortical than for monocortical
<strong>screw</strong>s. The authors concluded that bicortical
mini<strong>screw</strong> placement provides superior anchorage resistance,
reduced cortical bone stress, and superior
stability than monocortical placement.
In a retrospective study, Miyawaki et al 17 correlated
several variables with the success <strong>of</strong> 134 monocortical
<strong>screw</strong>s. Success rates <strong>of</strong> 0.0%, 83.9%, and 85% were
found with <strong>screw</strong> <strong>diameter</strong>s <strong>of</strong> 1.0, 1.5, and 2.3 mm,
respectively. A high mandibular plane angle was also
found to be associated with a significantly lower success
rate, but no significant correlation was found between
success rate and <strong>screw</strong> length, type <strong>of</strong> placement surgery,
immediate loading, age, sex, crowding, anteroposterior
jaw base relationship, controlled periodontitis, or
temporomandibular joint disorder symptoms.
The wide range <strong>of</strong> reported mini<strong>screw</strong> success rates
and the absence <strong>of</strong> accepted clinical protocols suggest
the need for further research <strong>of</strong> these temporary anchorage
devices. The literature lacks a well-controlled study
examining the effect <strong>of</strong> mini<strong>screw</strong> <strong>diameter</strong> on anchor-
age force resistance. The objective <strong>of</strong> this in-vitro study
was to examine this effect. Specifically, the force resistance
<strong>of</strong> a larger-<strong>diameter</strong> monocortical mini<strong>screw</strong> was
compared with that <strong>of</strong> a smaller-<strong>diameter</strong> monocortical
mini<strong>screw</strong>, and the force resistance <strong>of</strong> a larger-<strong>diameter</strong>
monocortical mini<strong>screw</strong> was compared with that <strong>of</strong>
a smaller-<strong>diameter</strong> bicortical mini<strong>screw</strong>.
MATERIAL AND METHODS
The maxillae and mandibles <strong>of</strong> human cadavers were
obtained from the Department <strong>of</strong> Anatomy and Cell Biology
Deeded Body Program at the University <strong>of</strong> Iowa.
Fully dentate and partially dentate specimens were considered
acceptable. Fully edentulous or partially dentate
specimens with visible, severely atrophic alveolar ridges
were excluded. Maxillary specimens were dissected superior
to the maxillary sinus to avoid damage to the maxillary
alveolar bone and tooth roots and extended distally
to the maxillary tuberosity. Mandibular specimens were
dissected approximately halfway up the ascending ramus.
All maxillae and mandibles were then hemisected,
s<strong>of</strong>t tissue was carefully removed, and they were stored
in 10% buffered formalin solution.
Periapical radiographs <strong>of</strong> each specimen were made
to verify adequate bone and proper root divergence between
adjacent teeth for placement <strong>of</strong> the mini<strong>screw</strong>s.
The site for placement was between the first and second
premolars in both jaws. Twenty-four maxillae from different
cadavers and 24 mandibles from different cadavers
were used. If either the first or second premolar
was missing, the adjacent first or second premolar was
used as a reference for <strong>screw</strong> location based on average
tooth size. After radiographic examination, 12 right and
12 left hemi-maxillae from different cadavers, and
12 right and 12 left hemi-mandibles from different
cadavers, were deemed acceptable.
In the first experiment (larger-<strong>diameter</strong> vs smaller<strong>diameter</strong>
monocortical anchorage), 48 commercially
available titanium <strong>screw</strong>s (KLS Martin, Jacksonville,
Fla) were placed in 12 hemisected maxillae and 12 hemisected
mandibles. Twenty-four large-<strong>diameter</strong> <strong>screw</strong>s
(2.5 3 17 mm) (#25-674-17-1, KLS Martin) and 24
small-<strong>diameter</strong> <strong>screw</strong>s (1.5 3 15 mm) (#25-675-15, KLS
Martin)wereplacedmonocortically.Inthesecondexperiment
(larger-<strong>diameter</strong> monocortical vs smaller-<strong>diameter</strong>
bicortical anchorage), 48 <strong>screw</strong>s were likewise placed in
12 hemisected maxillae and 12 hemisected mandibles.
Twenty-four large-<strong>diameter</strong> <strong>screw</strong>s (2.5 3 17 mm) were
placed monocortically, and 24 small-<strong>diameter</strong> <strong>screw</strong>s
(1.5315mm)wereplacedbicortically.
For each bone specimen, 2 <strong>screw</strong>s were placed
between the first and second premolars with 1 more

226 Morarend et al American Journal <strong>of</strong> Orthodontics and Dent<strong>of</strong>acial Orthopedics
August 2009
Fig 1. Mini<strong>screw</strong> placement distances from crestal
bone: 1.5-mm <strong>diameter</strong> <strong>screw</strong> in coronal position and
2.5-mm <strong>diameter</strong> <strong>screw</strong> in apical position.
coronally positioned and the other more apically positioned
(Fig 1). The coronally positioned <strong>screw</strong> was
placed 5 mm apical to the maximum height <strong>of</strong> the interproximal
crestal bone. The apically positioned <strong>screw</strong>
was placed 4.5 mm apical to the coronally positioned
<strong>screw</strong>. These distances were selected to ensure that all
<strong>screw</strong>s would have adequate bone and proper root divergence
without root contact, would not be placed in the
mental foramen or the maxillary sinus areas, and would
have adequate separation. Based on average bone thickness
values, the monocortical <strong>screw</strong>s were placed to
a depth <strong>of</strong> 4 mm to ensure complete penetration <strong>of</strong> the
buccal plate without engaging the lingual cortical
plate. 16 Bicortical placement was verified by visual observation
<strong>of</strong> the <strong>screw</strong> as it began to protrude through the
external surface <strong>of</strong> the lingual cortical bone. Figure 2
shows monocortical and bicortical <strong>screw</strong> placements
and force applications from an orthogonal view.
For each experiment, bone specimen assignments
were made to ensure an even and random distribution
<strong>of</strong> <strong>screw</strong> <strong>diameter</strong>s between maxillary and mandibular
bone specimens, right and left bone specimens, and coronal
and apical bone sites. All <strong>screw</strong>s were placed by 1
operator (C.M.) with a hand driver (blade #25-483-97,
handle #25-402-99, KLS Martin) after pilot hole drilling
with the manufacturer’s recommended nontapered,
1.1-mm <strong>diameter</strong> twist drill for the 1.5-mm <strong>screw</strong>s (#25-
452-15, KLS Martin) and 1.9-mm <strong>diameter</strong> twist drill
for the 2.5-mm <strong>screw</strong>s (#25-460-19, KLS Martin). A
nontapered drill was used to ensure no differences in
hole sizes for the bicortical samples, since all <strong>screw</strong>s
had a uniform <strong>diameter</strong> throughout. All <strong>screw</strong>s were
placed perpendicular to the buccal bone surface and parallel
to the occlusal plane.
Fig 2. Monocortical and bicortical mini<strong>screw</strong> placements,
orthogonal view.
After <strong>screw</strong> placement, periapical radiographs were
taken <strong>of</strong> all specimens to ensure that the <strong>screw</strong>s had not
violated the mental foramen, the maxillary sinus, and
the roots <strong>of</strong> any teeth. After verification <strong>of</strong> satisfactory
<strong>screw</strong> placement, the most distal portion <strong>of</strong> each bony
specimen was embedded in buff laboratory stone to
a depth <strong>of</strong> 1.0 in and allowed to harden for 24 hours
in preparation for force application.
To mimic orthodontic force, each mini<strong>screw</strong> was subjected
to a tangential force oriented perpendicular to the
mini<strong>screw</strong> (Fig 2).Acustomizedx-y-z tablewith a mounting
device (Brettin et al 16 ) was fabricated to rigidly fixate
andorienteachspecimenembeddedinastoneblock
during testing. Furthermore, a customized grip was designed
and machined from stainless steel to fit between
the threads <strong>of</strong> each mini<strong>screw</strong> and apply a tangential
load at a distance <strong>of</strong> 3 mm from the <strong>screw</strong>-bone interface.
This distance represented a reasonable space that would
be typically occupied by s<strong>of</strong>t tissue in vivo.
Each mini<strong>screw</strong> was subjected to a tangential force
load perpendicular to the <strong>screw</strong> but parallel to the occlusal
plane by using an Instron diametral materials testing
machine (model 1445, Zwick GmbH, Ulm, Germany).
The crosshead speed was set at 0.05 mm per second. 10,16
During loading, displacement <strong>of</strong> each <strong>screw</strong> was measured
up to 0.6 mm, which, from pilot studies, represented
adequate displacement without slippage at the
<strong>screw</strong>-thread/grip interface.
Each <strong>screw</strong> was removed, and the bony specimens
were sectioned at the mesial and distal aspects <strong>of</strong> the
mini<strong>screw</strong> placement site to allow bone thickness

American Journal <strong>of</strong> Orthodontics and Dent<strong>of</strong>acial Orthopedics Morarend et al 227
Volume 136, Number 2
Fig 3. Mean anchorage force values (in newtons) vs
<strong>screw</strong> deflection for 2.5-mm monocortical (mono)
<strong>screw</strong>s and 1.5-mm monocortical <strong>screw</strong>s in the mandible
(Md) and the maxilla (Mx).
visualization and measurement. Buccal cortical plate,
lingual cortical plate, and total alveolar bone width
were measured by 1 observer (C.M.) with a fine-tip digital
caliper. Each measurement was made twice at each
<strong>screw</strong> placement site, and the average <strong>of</strong> the measurements
was recorded.
Statistical analyis
A paired-samples t test was used to determine
whether there were significant differences in force anchorage
at each deflection value for both mandibular
and maxillary sites: between the 2 <strong>diameter</strong>s <strong>of</strong> monocortical
<strong>screw</strong>s (2.5 and 1.5 mm) and between the
larger-<strong>diameter</strong> monocortical <strong>screw</strong> (2.5 mm) and the
smaller-<strong>diameter</strong> bicortical <strong>screw</strong> (1.5 mm). A 2-samples
t test was used to determine whether there were significant
differences in force anchorage values between
maxillary and mandibular, right and left, and apical
and coronal sites for the 2.5-mm monocortical <strong>screw</strong>s,
the 1.5-mm monocortical <strong>screw</strong>s, and the 1.5-mm
bicortical <strong>screw</strong>s. One-way analysis <strong>of</strong> variance (AN-
OVA) with the post-hoc Tukey studentized range test
was used to determine whether there were significant
differences and the order <strong>of</strong> those differences in force
anchorage values among the 3 <strong>screw</strong> types at the maxillary
and mandibular sites. Means and standard deviations
were calculated for bone thicknesses. All data
analysis was conducted by using SAS s<strong>of</strong>tware (version
9.1, SAS, Cary, NC) at either the 0.05 or 0.01 level <strong>of</strong>
statistical significance.
RESULTS
Data analysis showed (Figs 3 and 4) that the mean
mandibular anchorage force values <strong>of</strong> the 2.5-mm
monocortical <strong>screw</strong>s were significantly greater than
Fig 4. Mean anchorage force values vs <strong>screw</strong> deflection
for 2.5-mm monocortical <strong>screw</strong>s and 1.5-mm bicortical
(bi) <strong>screw</strong>s.
those <strong>of</strong> the 1.5-mm monocortical <strong>screw</strong>s when the deflection
increased from 0.04 to 0.6 mm (P \0.01).
Moreover, the mean maxillary anchorage force values
<strong>of</strong> the 2.5-mm monocortical <strong>screw</strong>s were significantly
greater than those for the 1.5-mm monocortical <strong>screw</strong>s
when the deflection increased from 0.14 to 0.6 mm
(P \0.01).
No statistically significant differences in mean mandibular
anchorage force values were found between the
2.5 mm monocortical <strong>screw</strong>s and the 1.5-mm bicortical
<strong>screw</strong>s. However, the mean maxillary anchorage force
values <strong>of</strong> the 1.5 mm bicortical <strong>screw</strong>s were significantly
greater than those <strong>of</strong> the 2.5-mm monocortical
<strong>screw</strong>s when the deflection increased from 0.11 to
0.6 mm (P \0.01).
Data analyzed with 1-way ANOVA with the posthoc
Tukey studentized range tests indicated that the
mean force values <strong>of</strong> the 2.5-mm monocortical <strong>screw</strong>s
and the 1.5-mm bicortical <strong>screw</strong>s were significantly
greater than those <strong>of</strong> the 1.5-mm monocortical <strong>screw</strong>s
when mandibular deflection increased from 0.04 to
0.6 mm, or maxillary deflection increased from 0.07
to 0.60 mm (P \0.01). There were no significant
differences between the 2.5-mm monocortical and the
1.5-mm bicortical <strong>screw</strong>s.
Mean anchorage force values at mandibular sites
were significantly greater than at maxillary sites for
the 2.5-mm monocortical <strong>screw</strong>s when the deflection increased
from 0.11 to 0.60 mm (P\0.05). Mean anchorage
force values at mandibular sites were significantly
greater than at maxillary sites for the 1.5-mm monocortical
<strong>screw</strong>s when the deflection increased from 0.24 to
0.60 mm (P \0.05). There were no statistically significant
differences in mean anchorage force values between
maxillary and mandibular sites for the 1.5-mm
bicortical <strong>screw</strong>s. There were no significant differences
in mean anchorage force values between the right and

228 Morarend et al American Journal <strong>of</strong> Orthodontics and Dent<strong>of</strong>acial Orthopedics
August 2009
left sides for any <strong>screw</strong> type. Moreover, there were no
significant differences in mean anchorage force values
between coronal and apical positions for any <strong>screw</strong>
type. Bone thickness means (SD) for maxillary labial,
maxillary lingual, mandibular labial, and mandibular
lingual cortical bone were 2.11 (0.39), 2.01 (0.70),
2.15 (0.62), and 2.97 (0.69) mm, respectively.
DISCUSSION
Two principal findings resulted from this in-vitro
study. The first was that larger-<strong>diameter</strong> monocortical
<strong>screw</strong>s provide increased anchorage force resistance
compared with smaller-<strong>diameter</strong> monocortical <strong>screw</strong>s.
As illustrated in Figure 3, the mean anchorage force
values <strong>of</strong> the 2.5-mm monocortical <strong>screw</strong>s exceeded
those <strong>of</strong> the 1.5-mm monocortical <strong>screw</strong>s in both the mandible
and the maxilla (P\0.01). The second finding was
that smaller-<strong>diameter</strong> bicortical <strong>screw</strong>s can provide anchorage
resistance equal to, or even greater than, larger<strong>diameter</strong>
monocortical <strong>screw</strong>s. As shown in Figure 4,
the mean anchorage force values <strong>of</strong> the 1.5-mm bicortical
<strong>screw</strong>s were comparable with the values for the 2.5-mm
monocortical <strong>screw</strong>s in the mandible but exceeded the
values for the 2.5-mm monocortical <strong>screw</strong>s (P \0.01)
in the maxilla.
Additionally, as shown in these figures, mandibular
bone sites <strong>of</strong>fered significantly greater mean force resistance
than did maxillary bone sites (P \0.05) for both
1.5- and 2.5-mm monocortical <strong>screw</strong>s. This finding
agrees with that <strong>of</strong> Brettin et al 16 but is surprising because
<strong>of</strong> the similar mandibular and maxillary labial
cortical thicknesses measured in this sample. However,
this similarity in thickness could help to explain the
nonsignificant difference between maxillary and mandibular
bone sites for the 1.5-mm bicortical <strong>screw</strong>s.
The nonsignificance between the right and left sites
was expected. However, the nonsignificance between
coronal and apical sites, as also reported by Brettin et
al, 16 is <strong>of</strong> interest and might be valuable to practicing orthodontists.
That is, since vertical placement, within the
limits here, does not impact mean anchorage force resistance,
placement <strong>of</strong> either a monocortical or a bicortical
<strong>screw</strong> in a more coronal position (in attached gingiva)
makes sense to limit the possibility <strong>of</strong> peri-implant inflammation.
8,17 In addition to providing an environment
<strong>of</strong> reduced inflammation, the coronal placement <strong>of</strong>
<strong>screw</strong>s might be biomechanically advantageous because
they are closer to the center <strong>of</strong> resistance <strong>of</strong> the teeth.
Our finding that a 2.5-mm monocortical <strong>screw</strong> provides
superior anchorage resistance compared with
a 1.5-mm monocortical <strong>screw</strong> contrasts with the findings
<strong>of</strong> Miyawaki et al, 17 who reported no difference
in the 1-year success rates <strong>of</strong> 2.3 and 1.5 mm <strong>screw</strong>s
(85% and 84% successes, respectively). But they also
reported a success rate <strong>of</strong> 0.0% for the 1.0-mm <strong>diameter</strong>
<strong>screw</strong>s. Thus, <strong>diameter</strong> does appear to play a role clinically,
but the effect might be less pronounced at lower
loading with larger-<strong>diameter</strong> <strong>screw</strong>s.
The biologic changes on osseous loading could not
be examined in this in-vitro study. However, unlike
traditional endosseous implants, which need a waiting
period for bone healing and osseointegration, the primary
stability <strong>of</strong> mini<strong>screw</strong>s is believed to result from
mechanical retention <strong>of</strong> the <strong>screw</strong> in the bone. 1,9 In-vitro
studies <strong>of</strong> mini<strong>screw</strong>s should, therefore, more
closely replicate their immediate loading response in
situ after placement.
Recently, Huja et al 10 reported the pull-out strengths
<strong>of</strong> monocortical <strong>screw</strong>s placed in the maxillae and mandibles
<strong>of</strong> dogs. Pull-out forces ranged from 134 to 388
N; these forces are somewhat greater than, but similar
to, the force levels we reported here. Although these
force levels are greater than those normally used to
move teeth, well-controlled in-vitro studies such as
these provide valuable insights into anchorage. Furthermore,
greater forces are <strong>of</strong>ten used by orthodontists to
treat patients orthopedically. For instance, after placement
<strong>of</strong> miniplates on the lateral nasal wall for anchorage,
Kircelli et al 18 applied facemask protraction to an
11-year-old girl with severe maxillary hypoplasia. The
forces applied to the plates were 150 to 350 g.
Clinically, the advantage <strong>of</strong> a larger (monocortical)
<strong>screw</strong> is its ability to distribute applied forces over
greater areas <strong>of</strong> bone with less bone stress. For this reason,
when smaller monocortical <strong>screw</strong>s loosen in thin
cortical bone and cannot be retightened, they are typically
replaced with larger <strong>screw</strong>s. The drawback <strong>of</strong>
a larger <strong>screw</strong> is the surrounding anatomic limitations.
The results <strong>of</strong> this study indicate that another alternative
is a narrower bicortical <strong>screw</strong>. A bicortical small-<strong>diameter</strong>
<strong>screw</strong> is particularly useful when there is insufficient
room interproximally to place a larger-<strong>diameter</strong> monocortical
<strong>screw</strong>. Especially in patients with thin maxillary
cortical bone, a bicortical <strong>screw</strong> might be the mini<strong>screw</strong>
<strong>of</strong> choice. The advantage is the same, or better, anchorage
resistance as a larger-<strong>diameter</strong> <strong>screw</strong>. The disadvantage
is that the pilot hole must be drilled through both
cortices; this might be difficult in the posterior areas,
where cheek retraction is a limitation.
CONCLUSIONS
1. In vitro, larger-<strong>diameter</strong> (2.5 mm) monocortical
<strong>screw</strong>s provide increased anchorage force resistance
compared with smaller-<strong>diameter</strong> (1.5 mm)

American Journal <strong>of</strong> Orthodontics and Dent<strong>of</strong>acial Orthopedics Morarend et al 229
Volume 136, Number 2
monocortical <strong>screw</strong>s in both the mandible and the
maxilla.
2. Smaller-<strong>diameter</strong> (1.5 mm) bicortical <strong>screw</strong>s provide
anchorage force resistance at least equal to
larger-<strong>diameter</strong> (2.5 mm) monocortical <strong>screw</strong>s.
3. Mini<strong>screw</strong> anchorage force resistance is independent
<strong>of</strong> the side <strong>of</strong> either the maxilla or the mandible.
Force resistance is independent <strong>of</strong> apical or
coronal site placement, within the limits <strong>of</strong> this
study, for either the maxilla or the mandible.
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