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POST-MORTEM SHEAR TESTING OF IMMEDIATELY LOADED
3 MM AND 6 MM MINISCREW IMPLANTS AT SIX WEEKS
POST INSERTION IN THE BEAGLE DOG
Damen M. Caraway, B.S., D.D.S.
An Abstract Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2007
Abstract
Introduction: The use of miniscrew implants for
orthodontic anchorage has raised questions concerning their
limitations.
Specifically, the maximum shear force that an
immediately loaded miniscrew can withstand has not been
investigated.
The specific aim of this study is to
determine the maximum shear resistance of miniscrew
implants.
The effect of immediate loading on the maximum
shear resistance of miniscrew implants will be compared
between implants of two different lengths, and with three
different applied force loads.
A comparison of shear force
at failure will also be made according to the depth of the
miniscrews in bone.
Methods: The sample was derived from
five skeletally mature beagle dogs that had 60 miniscrews
placed at predetermined locations in the palate and buccal
surface of the mandible.
Miniscrews were immediately
loaded with either 0 (control), 600, or 900 g.
After six
weeks of continuous force application, 45 of the miniscrews
remained in place.
The dogs were then sacrificed, and bone
samples from the maxilla and mandible were dissected such
that each contained one orthodontic miniscrew.
The bone
specimens were mounted in dental stone for testing purposes
and secured in a universal vice for mechanical testing.
1
Testing was performed by the application of a shear force
in the same direction as the original force until failure
of the implant.
The maximum force sustained by the
implants prior to failure was recorded in Newtons (N).
Results: The mean shear force at failure of 6 mm miniscrew
implants was significantly higher (53.0 N ± 8.3 N)
(Mean ± SE) than that of 3 mm implants (31.9 N ± 4.1 N).
No significant difference in force at failure was noted
between implants that were immediately loaded, and those
that served as controls.
A significant difference was
determined to be present between the groups formed by
extent of bony purchase.
The groups with 2-3 mm and 3 mm+
of bony purchase showed a significantly higher shear force
at failure than the groups with 0-1 mm or 1-2 mm of
insertion depth.
Shear force at failure showed a
moderately strong correlation (r=0.57) with the depth of
the miniscrew in bone.
Conclusions: Immediate loading of
miniscrews has no significant effect on maximum shear force
at failure.
Complete cortical engagement by miniscrews may
result in significantly higher shear resistance.
Miniscrews as short as 3 mm can withstand shear forces well
beyond levels typically used in orthodontics.
2
POST-MORTEM SHEAR TESTING OF IMMEDIATELY LOADED
3 MM AND 6 MM MINISCREW IMPLANTS AT SIX WEEKS
POST INSERTION IN THE BEAGLE DOG
Damen M. Caraway, B.S., D.D.S.
A Thesis Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2007
COMMITTEE IN CHARGE OF CANDIDACY:
Professor Rolf G. Behrents,
Chairperson and Advisor
Assistant Professor Ki Beom Kim
Assistant Professor Donald R. Oliver
i
DEDICATION
To my lovely wife, Tiffany, for her unwavering support
during my years of formal (and informal) education; I thank
you for your love, patience, sacrifice, and understanding
in my pursuit for something better.
I love you more than
words can describe.
To my wonderful children, Avery, Gavin, and Sydney;
the time not spent with you during my professional training
will ultimately allow me to spend more time with you in the
future; I do this all for you.
To my parents, who inspired me to be a little better
and supported me every step of the way.
To all the teachers in my many years of education; I
express to you my gratitude, and hope you always feel that
your efforts are appreciated.
ii
ACKNOWLEDGEMENTS
I would like to acknowledge the following individuals:
Dr. Rolf Behrents for chairing my thesis committee.
Thank you for your guidance, insights and time.
You have
fulfilled your responsibility in teaching me how to think.
Dr. Don Oliver for serving on my thesis committee.
You are a great teacher and mentor.
Your love for
orthodontics and teaching are obvious.
Thank you for your
time and suggestions.
Dr. Ki Beom Kim for serving on my thesis committee.
I
truly appreciate your assistance with the writing of my
thesis.
I would also like to thank the following individuals
for their help and expertise:
Dr. Micah Mortensen for his diligence and assistance
throughout the entire process of developing and completing
this thesis.
Dr. Heidi Israel for her assistance with the
statistics in this project.
The Orthodontic and Education Research Foundation for
contributing to the funding of this project.
iii
TABLE OF CONTENTS
List of Tables............................................v
Chapter 1: Introduction...................................1
Chapter 2: Review of the Literature
Anchorage in Orthodontics......................4
In Search of Skeletal Orthodontic Anchorage....5
Osseointegrating Titanium Implants.............6
Alternative Forms of Skeletal Anchorage........8
Palatal Implants...........................9
Palatal Onplants..........................10
Miniplates................................11
Orthodontic Miniscrew Implants................11
Design....................................12
Sites for Placement.......................15
Time of Loading...........................16
Applied Force.............................18
Miniscrew Implant Stability...................19
Testing of Bone Screws........................21
Management of Bone Samples................21
Pull-Out Tests............................22
Orthopedic Screws....................23
Maxillofacial Rigid Fixation Screws..23
Orthodontic Miniscrews Implants......24
Shear Tests...............................26
References....................................28
Chapter 3: Journal Article
Abstract......................................37
Introduction..................................39
Methods and Materials.........................44
Sample Selection..........................44
Miniscrew Implants........................44
Preparation of Samples for Testing........45
Mechanical Testing........................46
Statistical Analysis......................47
Results.......................................49
Miniscrews Failures.......................49
Maximum Shear Force Measurements..........50
Discussion....................................53
Conclusions...................................61
References....................................63
Vita Auctoris............................................67
iv
LIST OF TABLES
Table 3.1:
Type, Location and Load Category of
Surviving Miniscrews.........................50
Table 3.2:
Maximum Shear Force at Failure in
Newtons (N)..................................51
Table 3.3:
Mann-Whitney U Test Results of Loaded
Versus Control Implants......................52
Table 3.4:
Kruskal-Wallis and Scheffe´ Post Hoc Results
of Maximum Force at Failure by Depth
of Screw in Bone.............................52
v
CHAPTER 1: INTRODUCTION
In orthodontics, malpositioned teeth are moved into
proper alignment by the application of force.
This force
originates from wires, elastics and other appliances
attached to the teeth.
Often, teeth that are in proper
alignment are used to provide the force to move those that
are not, and are referred to as anchorage teeth.
In
accordance with Newton’s third law, there is a reactive or
“equal and opposite force” for every applied orthodontic
force.
Unfortunately, these reactive forces often result
in undesirable movements of the teeth serving as anchorage.
Anchorage, broadly defined as the degree of
resistance to displacement, is a critical component to
successful orthodontic treatment.
As a result,
orthodontists have historically used a variety of
appliances and strategies to enhance anchorage,
particularly when minimal movement of the teeth providing
the anchorage is desired.
This allows the movement of
malaligned teeth while leaving teeth that do not need to be
moved relatively undisturbed.
Anchorage enhancing appliances, such as headgear,
are highly dependent upon patient compliance for success.
1
In an effort to establish anchorage without significant
reliance on patient cooperation, other forms of anchorage
have been investigated.
Restorative dental implants,
despite their stability in bone, have limited use in
orthodontics due to cost, an extensive healing period after
surgical placement, and anatomic placement limitations.
Still, the use of these titanium dental implants as a form
of anchorage has provided the potential for absolute,
compliance independent, orthodontic anchorage.
Orthodontic miniscrew implants have been designed to
circumvent the limitations posed by restorative dental
implants.
These smaller bone screws are significantly less
expensive, are easily placed and removed, and can be placed
in almost any intra-oral region, including between the
roots of the teeth.
Some basic questions remain, however,
concerning the limitations of miniscrews.
Specifically,
what is the maximum amount of lateral or shear force that
can be applied to these miniscrews before they fail?
How
does a force that is applied immediately after the
miniscrew is placed affect the maximum holding power of the
implant?
To what extent does the total length of screw
engaged in bone affect the maximum shear force the implant
can withstand?
These are questions that remain unanswered
in the literature.
2
The present study intends to provide information on
the maximum shear force that immediately loaded orthodontic
miniscrew implants can withstand before failure.
In a
preliminary study,1 3 and 6 mm orthodontic miniscrews were
placed in the maxilla and mandible of the beagle dog and
immediately loaded.
(600 and 900 grams).
were sacrificed.
Two levels of force were applied
After a period of six weeks, the dogs
These implanted miniscrews were utilized
in the present study to determine the maximum shear force
that can be applied prior to implant failure.
Comparisons
of the maximum force at failure will be made according to
implant length (3mm, 6mm), applied force (0g, 600g, 900g),
location (maxilla, mandible) and depth of the screws in
bone.
3
CHAPTER 2: REVIEW OF THE LITERATURE
Anchorage in Orthodontics
The attainment and control of anchorage is
fundamental to the successful practice of orthodontics and
dentofacial orthopedics.
According to Newton’s well-known
law of physics, action and reaction forces are equal and
opposite.
In orthodontics, anchorage is used to describe
resistance to reaction forces.2
Teeth are the usual source
of anchorage and, in the typical orthodontic biomechanical
situation, are pitted against one another to produce tooth
movement.
The teeth serving as the anchorage unit, by
virtue of their number, position, size and number of roots,
intend to offer resistance to movement so as to bring about
the movement of the other teeth.
A threshold value of
force to initiate tooth movement has not been identified,3
but appears to be very low.2
For example, tooth movement
has been detected with as little as 4 gm of force.4
Considering this principle, is has been concluded that the
practice of pitting more teeth with a larger root surface
area against fewer teeth with less surface area in intraarch mechanics may not be sufficient to prevent movement of
anchor teeth.5-7
Therefore, in order to achieve increased
4
anchorage control, a supplemental form of anchorage is
often required.
Traditionally, headgear and intermaxillary
elastics have been used as forms of supplemental anchorage.2
While this form of supplemental mechanics may be effective
in increasing anchorage, effectiveness depends upon the
cooperation of the patient.
Consequently, orthodontic
anchorage control has historically been contingent on
patient compliance.
Due to the inconsistent nature of such
compliance,8 orthodontists often note the unfavorable
reciprocal movement of the intra-arch and inter-arch
“anchor” teeth.
In Search of Skeletal Orthodontic Anchorage
Orthodontists have recognized that stability of
reactive anchorage units could be significantly increased
if orthodontic anchorage could be provided by the skeletal
bone itself,9 and in the 1940s began to conduct research on
the subject.
An early study by Bernier and Canby suggested
that surgical vitallium bone screws were inert and stable
in bone.10
However, when Gainsforth and Higley9 attempted
to use these screws as a source of orthodontic anchorage
they were largely unsuccessfully.
5
Subsequently, many other investigators have
attempted to identify a successful form of skeletal
anchorage through the use of a variety of endosseous
implants and bone plates.
In addition to the original
study using surgical vitallium screws, research was
conducted during this early era on each of the following:
blade implants,11 vitreous carbon,12,13 bio-glass coated
aluminum oxided,14 and vitallium implants.15,16
All of these
implant types have exhibited some degree of success in
terms of implant stability when subjected to orthodontic
forces.
However, each implant system exhibited some form
of weakness such that stability was unpredictable.
Consequently, none of these implant systems have gained
widespread clinical acceptance.
It was not until the
development of osseointegrating titanium implants that a
reliable source of skeletal anchorage was established and
found widespread clinical application.
Osseointegrating Titanium Implants
In the 1960s, Brånemark discovered the unique
healing response exhibited by bone when it was exposed to
titanium.
He and his colleagues later described the
6
process by which a titanium fixture could be embedded and
incorporated into bone.17
This phenomenon became known as
osseointegration, and was introduced to restorative
dentistry in 1965.
From this principle came the
development of titanium dental implants, which resemble the
root of a tooth (approximately 4 mm X 9-15 mm) and provide
for the replacement of missing teeth without compromising
adjacent teeth.
Dental implants, having been proven highly
successful,18 have been referred to as “the most influential
change in dentistry during the last half-century.”19
Despite the discovery of osseointegration in the
1960s and rapid development of traditional titanium dental
implants, such devices were not evaluated for use as
orthodontic anchorage until the 1980s.20-25
In an early
study by Roberts and associates, they demonstrated that
implant osseointegration and stability persisted despite
the application of an orthodontic force.20
Consequently,
osseointegrating titanium dental implants were considered
an effective source of skeletal orthodontic anchorage.26-29
Endosseous titanium dental implants have been used
to provide anchorage independent of patient compliance and
without the need to accept unfavorable reciprocal movement
of anchor teeth.
Unfortunately, dental implants are
associated with some distinct and significant disadvantages
7
that limit routine use in orthodontics.
For example, the
size of restorative implants (approximately 4 mm X 9-15 mm)
limits the anatomic sites available for placement (e.g.,
edentulous areas or the retromolar region).
Furthermore,
use of these implants is highly dependent on a precise 2stage surgical protocol and a healing time of 3-6 months
prior to the application of orthodontic force.18,20,22,30
Considering the time required to complete orthodontic
treatment alone, this additional time for healing is
considered a significant deterrent in terms of the use of
dental implants.
Such limitations have motivated a search
for alternative forms of orthodontic anchorage via
implants.
Alternative Forms of Skeletal Anchorage
Based on the need to develop a form of skeletal
anchorage in orthodontic patients, alternate forms of
implant anchorage have been developed.
These devices
include palatal implants, palatal onplants, surgical
miniplates, and miniscrews.
8
Palatal Implants
Designed after a traditional restorative implant,
palatal implants are intended to osseointegrate and provide
a rigid point of attachment for the teeth.
The Straumann
Orthosystem® (Institut Straumann AG, Waldenburg,
Switzerland) is an example of a palatal implant.
This type
of implant consists of three parts: a self-tapping
endosseous body, a smooth cylindrical collar, and an
octagonal head used to connect attachments.31
In terms of placement, after the removal of a small
circular section of palatal mucosa and the preparation of
an appropriate pilot hole, the implant is inserted,
covered, and allowed to heal.32
After a healing period of
at least 3 months32,33 a second surgical procedure is
performed to uncover the implant and place an apparatus
that allows attachment to the teeth.
Despite the established success of palatal implants
in providing anchorage, there are, again, certain drawbacks
to this system that have limited clinical acceptance.
The
size of the implant and the invasive surgical procedures
required for placement, use and removal are likely the most
significant disadvantages.
In addition, the healing time
required prior to loading, and the time and cost associated
with fabrication of custom attachments make this type of
9
implant even less appealing.
Lastly, because these
implants are not intended to be permanent, there are
potential problems associated with their removal from the
palate.
If the degree of osseointegration cannot be
overcome by the use of a hand ratchet, trephination of the
implant and the surrounding bone must be performed.
This
procedure leaves a void in the palatal bone which is left
to granulate and heal over time.32
Palatal Onplants
The palatal onplant, developed by Block and Hoffman,
is a unique device consisting of two parts: a dome shaped
disk (7 mm diameter x 3.5 mm thick) and an abutment that
screws into the center of the disk.34
This fixture is
designed to lay against the bone of the palate under the
periosteum.
This is in contrast to palatal implants which
penetrate the bone.
When surgically placed
subperiosteally, bone grows into the hydroxyapatite-coated
surface of the disk resulting in osseointegration.
This
process of osseointegration requires at least 10 weeks of
healing,35,36 after which an incision is made and the
abutment is attached and left protruding through the soft
tissue.
The palatal onplant shares similar disadvantages
with the palatal implant.
The possible locations for
10
placement are limited, and three surgeries and an extended
healing time are required.
Miniplates
Miniplates are derivatives of rigid fixation plates
used in maxillofacial surgery.
They are secured to the
bone with two or three bone screws and have an extension
arm designed to extend through the mucosa into the oral
cavity.
The arm, which measures 10.5-16.5 mm, serves as a
point of attachment for the orthodontic appliance.
Unlike
the previously described palatal implants and onplants,
miniplates can be placed in various locations including the
zygomatic buttress, the periform rim, and the lateral
border of the mandible.37
A surgical flap is required to
place miniplates, and a healing period is reccommended.38
second surgical procedure is required to remove the plates
when they are no longer needed.
Orthodontic Miniscrew Implants
The development and improvement of dental implants
and maxillofacial fixation methods brought about the
evolution of orthodontic miniscrew implants.
11
They have
A
been designed to circumvent the shortcomings of other forms
of skeletal anchorage in the context of orthodontic
anchorage.
The first successful screw shaped implant used
exclusively for orthodontic anchorage was reported in 1983.
In this report maxillary incisor intrusion was accomplished
in a deep-bite patient with a miniscrew for anchorage.15
Since that time many miniscrew designs have been developed,
and there has been a dramatic increase in use and
popularity.
It has been argued, however, that their
utilization has preceded a thorough understanding of the
biology involved and their mechanical potentials.39
Design
In recent years, many different miniscrew implants
have been designed and manufactured for orthodontic use.
Today, the material of choice for miniscrews is titanium.
Titanium allows for the small size and weight of the
miniscrew without compromising strength and
biocompatibility.40
The common shape of these designs is a
threaded cylindrical body with a conical tip.
Variations
center on the basic features of the screw portion in terms
of diameter, length, thread width and pitch, and head
design.
Another important variation involves the screw
being self-tapping and self-drilling.
12
The sharp threads of
a self-tapping screw cut into the bone and advance the
screw as it is turned.
All screws are self-tapping.
screws are not self-drilling, however.
All
Those that are not
require the preparation of a drilled pilot hole prior to
insertion.
Self-drilling screws have a drill-shaped point
and a specialized cutting flute that allows insertion
without prior drilling.
This type of miniscrew, sometimes
called drill-free, has been shown to exhibit more bone-tometal contact and less mobility than miniscrews placed with
a pre-drilled pilot hole.41
The design of a miniscrew can significantly affect
its function and stability.
Increased length, which can
provide for bicortical placement, improves primary, or
initial, stability.42
Numerous studies have reported
successful use of miniscrews 6 mm in length.43-47
When
Deguchi et al. loaded 96 implants (1 mm x 5 mm) with 200300 g of force, 93 of the miniscrews were still stable
after 3 months.48
There are no reports in the literature,
however, of the stability of miniscrews shorter than 4 mm.
The diameter of a miniscrew is another design
feature that seems to play an important role in stability.
Miniscrew diameters vary widely among, and within different
manufacturers.
Miniscrews currently on the market range in
diameter from 1.2 to 2.0 mm.
Various diameters of
13
miniscrews have been shown to be successful in providing
anchorage.
Park et al. reported on 227 miniscrews of four
types with diameters of 1.2 mm and 2.0 mm.49
The overall
success rate was 91% and no difference was noted between
the implants of different diameters.
In a case report
describing the use of miniscrews in non-extraction
treatment, miniscrews measuring 1.2 mm X 6 mm were used
with success to move an entire arch en masse.50
There
appears to be a limit, with regard to the diameter of
miniscrews, below which success is compromised.
In a study
by Miyawaki et al.,51 all 1.0 mm diameter screws failed, but
the 1.5 mm and 2.3 mm diameter screws showed no significant
differences with success rates of 83.9% and 85%,
respectively.
The authors concluded that a diameter of
less than 1.0 mm was a significant criterion associated
with failure.
The advantage of a thinner screw is that it
can be placed in more locations, such as between the roots
of teeth.
The drawback, however, is the greater potential
for screw fracture.52
Cope has stated that the minimum
diameter to avoid metal failure should be 1.5 mm.53
14
Sites for Placement
Numerous studies and clinical reports show a variety
of implant placement sites including the retromolar pad,54
palate,55,56 and the maxillary and mandibular buccal cortex.51
In addition, it has been shown that implants can be
inserted into the anterior nasal spine, the symphysis57 and
are the only type of implant that can be placed between the
roots of the teeth.58
In studies of miniscrew implants that have been
performed on dogs, implants have been placed in the
palate,43,59,60 the lingual cortical plate of the mandible,45
and the maxillary and mandibular buccal cortex.43,44
The literature is conflicting when comparing the
stability of implants placed in the maxilla versus those
placed in the mandible.
Cheng and colleagues found that
miniscrews in the posterior mandible were susceptible to
increased failure rates when compared to the anterior
mandible, anterior maxilla, and posterior maxilla.61
Tseng et al. also found higher failure rates of miniscrews
in the mandible.62
Relating bone contact to stability,
Wehrbein and associates found a 79% bone-to-implant contact
in the maxilla as compared to 68% in the posterior
mandible.63
15
In contrast to the aforementioned studies, two
additional studies suggest a higher degree of stability for
implants placed in the mandible.
Deguchi et al. reported
that implants placed in the mandible were found to have
higher bone-to-implant contact,48 while Bischof et al.
showed that 3 months after placement, implants in the
mandible were more stable than those placed in the
maxilla.64
Time of Loading
Suggested healing times for orthodontic miniscrew
implants cover a broad range.
One of the earliest studies
on these implants recommended a period of 9 months prior to
force application.54
a more recent study.
The lack of consensus is evidenced by
Using the beagle dog as a model,
Ohmae et al. tested the efficacy of miniscrews for
orthodontic intrusion.65
After allowing six weeks for the
4 mm long miniscrews to heal, they were loaded with a 150 g
of force.
At the end of the 18 week loading period, all 36
of the implants were stable in the bone, and 4.5 mm of
intrusion had been achieved.
Despite this success, the
author suggested that the 6 week healing time may still
have been too short.
16
Immediate loading of miniscrews has become more
common, with several reports in the literature to support
the practice.57,66-68
Doi conducted a study in which 48
miniscrews (6 mm) were placed in the jaws of four beagle
dogs.
Immediately after placement, two miniscrews were
connected to each other by nickel titanium coil springs
that produced either 300 or 600 g of force.
Two of the 48
miniscrews were placed near erupting teeth and failed
shortly after placement.
This required their removal, and
the exclusion of the other miniscrews to which they were
connected.
The force remained active for 5 weeks.
At the
end of the testing period 5 out of the 44 remaining
miniscrews demonstrated significant mobility.
The author
concluded that miniscrews can be loaded immediately with
orthodontic, and even orthopedic, force levels with a
success rate of 100%.43
Another study by Owens was designed to place 56
miniscrews (1.8 mm X 6 mm) into the jaws of 7 beagle dogs.69
Twenty-one of the implants were immediately loaded with
either 25 or 50 g of force.
Even though three of the
immediately loaded miniscrews failed within 21 days of
placement, a comparison of the delayed vs. immediately
loaded miniscrews showed no differences in failure rate.
This data suggests that the success of miniscrew implants
17
is not dependent on timing of implant loading.
There are
no studies in the literature, however, that have described
the affect immediate loading has on the maximum amount of
force an implant can withstand before failing.
Applied Force
The literature supports the view that a wide variety
of force loads can be applied to miniscrew implants without
the implant failing.
In an early study, miniscrews were
loaded with 60, 120 and 180 g of force.16
After 28 days,
the implants showed no significant movement at any of the
force levels described.
Another author described the
placement of 96 miniscrews into the buccal and lingual
cortical plates of 8 mature beagle dogs.45
The implants
were immediately loaded with 25, 50, or 100 g of force
which remained active over 98 days.
One of the 96
miniscrews, which was loaded with 100 g of force, failed
after 50 days, but all others remained stable in the bone.
In another study, after a healing period of 12 weeks, a
group of 20 miniscrews were loaded with 250 to 350 g.70
All
of the miniscrews withstood the force until they were
removed 3 months later.
In another study, Doi immediately
loaded miniscrews with either 300 or 600 g and noted
significant mobility in 5 of the 44 implants.
18
He also
measured the displacement of the miniscrews and reported an
average of less than 0.5 mm per implant loaded with 600 g.
Turley et al. placed implants in the zygomatic buttress of
dogs, and allowed them to heal for 20 weeks.22
The implants
successfully withstood a load of 1000 g for 18 weeks.
Though these studies have demonstrated a range of force
loads that can be applied to miniscrews without significant
failure, there is no research delineating the maximum force
load that can be sustained by orthodontic implants.
Miniscrew Implant Stability
In contrast to osseointegrating dental implants,
miniscrew implants are intended to be temporary.
Thus, the
screws are intentionally not subjected to surface
treatments (e.g., sandblasting, etching, plasma spraying)
designed to increase the percentage of bone-to-implant
contact.60
At the time of miniscrew removal, integration is
overcome by hand with a surgical driver.
Upon placement, the ability of a miniscrew to
provide anchorage depends on the mechanical retention
provided by intimate contact between bone and the surface
of the implant.
This mechanical retention, also known as
19
primary stability, is critically important for orthodontic
anchorage to be successful,71 and is influenced by several
factors.
For instance, implants with greater diameter and
thread depth will have a larger surface area in contact
with bone, and theoretically, more primary stability.
The quality and quantity of the bone into which
miniscrews are placed also influences their primary
stability and subsequent success.72,73
Cortical bone is much
denser than cancellous bone, and provides for more intimate
contact between the bone and the threads of the miniscrew.
A recent study found a weak but significant positive
correlation (r = 0.39) between cortical bone thickness and
miniscrew implant pull-out strength in dog bone.59
Indirectly, Miyawaki and colleagues related cortical bone
thickness to implant failure by noting that patients with
high mandibular plane angles were more likely to experience
implant failure.51
The link was provided by Tsunori et al.
who quantified a thinner cortical plate in patients with
high mandibular plane angles.74
The extent to which miniscrew design and bone
quality influence the stability of miniscrews and the
maximum force they can withstand is not completely known or
described in the literature.
It has been suggested that
orthodontists are still in search of the formula for
20
ultimate miniscrew stability.39
As different variables that
affect miniscrew stability are investigated, such as
miniscrew design, stability can be quantified for
comparison.
Quantifying the stability of miniscrews is
accomplished by performing the same tests on miniscrews
that are used to evaluate orthopedic bone screws.
Testing of Bone Screws
Before World War II, selection of screws for
orthopedic implantation was based primarily on the ease of
insertion.75
Later, stability became the primary selection
factor and tests were designed to determine the differences
between various screws.
The most common test performed on
bone screws of any type is the pull-out test.
An
alternative to the pull-out test is the shear test, which
examines the effects of tangential or lateral forces.
Management of Bone Samples
Many of the tests performed on bone screws utilize
non-living bone.
It is important to note that for accurate
correlation of fresh bone tests to living bone, proper
preservation techniques used during preparation and testing
21
of bony samples are essential.76
Various methods that have
been used to adequately preserve bone have been described
in the literature.
In early tests, bone samples were
wrapped in wet paper towels, enclosed in plastic, and
stored at 7°C for no more than 48 hours.77
More recently,
fresh bone samples have been stored in refrigerated
physiologic saline,78 wrapped in saline soaked gauze and
stored at -15°C,59 or simply sealed in plastic bags and
stored at -25°C prior to testing.79
It has been shown that
the freezing process does not have an adverse effect on the
elastic properties of bone.76,80,81
Pull-Out Tests
The pull-out test is considered an accurate method
of evaluating the relative strength or “holding power” of
surgically placed bone screws.77,82
Holding power is defined
as the maximum uniaxial tensile force needed to produce
failure in the bone.83
Pull-out tests measure holding power
against tension applied along the longitudinal axis of the
screws.
Results of pull-out tests have been reported on
numerous types of bone screws.
22
Orthopedic Screws
Several authors have reported specific results of
pull-out tests on screws used in orthopedic surgery.77,84,85
Koranhi et al. placed large diameter screws in canine and
bovine femurs to test the difference between two thread
types.77
No difference between the screws was detected, but
the results showed a linear relationship between pull-out
strength and cortical bone thickness.
In another study,
screws intended for use on the spine were inserted into
porcine (pig) vertebral bodies.84
Despite differences in
diameter (6.5-7.5 mm), length (25-35 mm) and thread depth
(1-1.8 mm), no significant differences were noted in axial
pull-out strength which measured an average of 268 lbs
(1194 N).
The authors concluded that the shorter test
screws with increased thread depth could provide as much
holding power as the routinely used longer screws.
Maxillofacial Rigid Fixation Screws
The designs of currently available orthodontic
miniscrew implants are similar to rigid fixation bone
screws.
Rigid fixation bone screws, used in orthognathic
surgery to affix rigid plates to the bones of the face,
have been tested by means of pull-out tests.78,82,86-88
Foley
et al. tested five different types of fixation screws in
23
the long bones of a mongrel dog and found a mean pull-out
tension of 44.5 kilograms (436 N).82
In another study, five
additional types of fixation screws were tested using bones
from the skull and mandible of human cadavers.86
The screws
varied in diameter and length and demonstrated pull-out
strengths of up to 620 N (1 N = 102 g).
In pull-out tests
of 2.0 mm diameter screws in porcine rib with a cortical
thickness of between 0.5 and 2.0 mm, Boyle et al. found a
mean pull-out force of 21 kg (205 N).88
In a similar test
by the same author another group of 2.0 mm diameter screws
showed pull-out strengths ranging from 16-25 kg
(157-254 N).78
It should be noted that the results of tests
performed on screws in animal bone may not directly
correlate to human orofacial cortical bone due to
differences in mechanical properties.
Specifically, in the
dog model the alveolar process has a higher density than
the equivalent structure in the human.89
Orthodontic Miniscrews Implants
Recently, three studies have been conducted on the
pull-out strength of orthodontic miniscrew implants.
Pickard placed miniscrews in cadaver mandibles and
evaluated the effects of implant orientation on stability
24
and resistance to failure.90
The study also intended to
identify the maximum holding power of miniscrew implants in
the human mandible.
Pulling on the implants at various
angles, he determined that a direction of force along the
long axis of the implant offered the greatest resistance to
failure.
The results of his axial pull-out tests showed a
maximum force at failure of 342 N ± 80.9 N (Mean ± S.D.).
Huja and colleagues have conducted two studies on the pullout strength of miniscrews.
Both of the studies were
designed to determine the maximum pull-out strength of
miniscrews in the maxilla and mandible of beagle dogs.
In
the first study, the implants were placed and tested
immediately after the dogs were sacraficed.59
Average pull-
out strength of all implants measured 222 N.
The second
study tested the screws in the same manner after they were
allowed to heal, unloaded, for 6 weeks.60
Average pull-out
strength of implants after 6 weeks of unloaded healing was
245 N.
There was no significant difference in the pull-out
strengths between the two time periods.
The authors did,
however, show a weak but positive correlation between pullout strength and cortical plate thickness in both
studies.59,60
25
Shear Tests
Although tests of the stability of bone screws have
been primarily focused on pull-out, it is important to
recognize that these pull-out tests alone are not totally
adequate to measure the anchorage potential of bone screws.
They do not, for example, address shearing forces which are
present in a clinical setting.82
There is a limited number
of reports on shear testing of bone screws or implants in
the literature.84,90
Glatzmaier et al. tested the shear
strength of a bioresorbable polylactide implant in vitro.91
These experimental implants showed a shear force at failure
of 50 N.
Pierce et al, working with instrumentation screws
used in the vertebral bodies of the spine, conducted pullout and shear tests on screws with diameters of between 6.5
and 7.5 mm.84
The results showed an average maximum shear
force at failure of 786 N.
Pickard has conducted the only shear tests on
orthodontic miniscrew implants.90
As described above, he
studied the effect of orientation of miniscrew implants on
resistance to failure.
Placing miniscrews in human cadaver
bone and immediately afterwards performing shear tests, he
determined that the mean shear force required to cause
failure (123 N) is roughly a third of the mean axial pullout force (342 N).
26
There are no reports in the literature of shear
tests on miniscrew implants that have been placed in living
bone and allowed to heal, nor have there been shear tests
performed on miniscrews that have been immediately loaded.
The purpose of this study is to determine the affect of
immediate loading on the maximum shear resistance of
miniscrew implants, and to compare this effect between
implants of two different lengths, and with three different
applied force loads (0, 600 g, 900 g).
This will be
accomplished in a dog model using a sample involving 3 mm
and 6 mm miniscrews that were immediately loaded or
unloaded (control) for a period of six weeks, and then
subjecting both loaded and control implants to shear force
testing.
The goal of shear force testing is to imitate, as
closely as possible, the conditions that exist when an
orthodontic miniscrew is subjected to lateral forces in the
mouth.
Many variables are responsible for the maximum
shear force a miniscrew can withstand.
By studying the
expression of these variables, this study may give
clinicians a better indication of what can be expected of
orthodontic miniscrews in clinical practice.
27
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36
CHAPTER 3: JOURNAL ARTICLE
Abstract
Introduction: The use of miniscrew implants for
orthodontic anchorage has raised questions concerning their
limitations.
Specifically, the maximum shear force that an
immediately loaded miniscrew can withstand has not been
investigated.
The specific aim of this study is to
determine the maximum shear resistance of miniscrew
implants.
The effect of immediate loading on the maximum
shear resistance of miniscrew implants will be compared
between implants of two different lengths, and with three
different applied force loads.
A comparison of shear force
at failure will also be made according to the depth of the
miniscrews in bone.
Methods: The sample was derived from
five skeletally mature beagle dogs that had 60 miniscrews
placed at predetermined locations in the palate and buccal
surface of the mandible.
Miniscrews were immediately
loaded with either 0 (control), 600, or 900 g.
After six
weeks of continuous force application, 45 of the miniscrews
remained in place.
The dogs were then sacrificed, and bone
samples from the maxilla and mandible were dissected such
that each contained one orthodontic miniscrew.
37
The bone
specimens were mounted in dental stone for testing purposes
and secured in a universal vice for mechanical testing.
Testing was performed by the application of a shear force
in the same direction as the original force until failure
of the implant.
The maximum force sustained by the
implants prior to failure was recorded in Newtons (N).
Results: The mean shear force at failure of 6 mm miniscrew
implants was significantly higher (53.0 N ± 8.3 N)
(Mean ± SE) than that of 3 mm implants (31.9 N ± 4.1 N).
No significant difference in force at failure was noted
between implants that were immediately loaded, and those
that served as unloaded controls.
A significant difference
(p<0.05) was determined to be present between the groups
formed by millimetric measurements of bony purchase.
The
groups with 2-3 mm and 3 mm+ of bony purchase showed a
significantly higher shear force at failure than the groups
with 0-1 mm or 1-2 mm of insertion depth.
Shear force at
failure showed a moderately strong correlation (r=0.57)
with the depth of the miniscrew in bone.
Conclusions:
Immediate loading of miniscrews has no significant effect
on maximum shear force at failure.
Complete cortical
engagement by miniscrews may result in significantly higher
shear resistance.
Miniscrews as short as 3 mm can
38
withstand shear forces well beyond levels typically used in
orthodontics.
Introduction
The attainment and control of anchorage is
fundamental to the successful practice of orthodontics and
dentofacial orthopedics.
Teeth are the usual source of
anchorage and, in the typical orthodontic treatment, are
pitted against one another to produce tooth movement.
Teeth, alone, do not provide absolute, or maximum
anchorage.1-3
If maximum anchorage is needed, a
supplemental form of anchorage is usually required.
Headgear serves as an effective form of supplemental
anchorage, but it depends upon the cooperation of the
patient for success.
Due to the inconsistent nature of
such compliance,4 orthodontists often note the unfavorable
reciprocal movement of intra-arch and inter-arch “anchor”
teeth.
Orthodontists have long recognized that stability of
reactive anchorage units could be significantly increased
if orthodontic anchorage were provided from within skeletal
bone itself.5
Early investigators attempted to identify a
39
successful form of skeletal anchorage through the use of a
variety of endosseous implants,6-8 but were largely
unsuccessful.
Since the time that Brånemark introduced the
biologic basis of osseointegration, titanium dental
implants have been used for the replacement of missing
teeth.
Subsequently, dental implants were evaluated for
their use as orthodontic intraoral anchorage in the
1980s.9-14
Restorative implants for orthodontic usage,
however, have significant disadvantages such as cost, an
extensive healing period after surgical placement, and
anatomic placement limitations that preclude routine use.
Miniscrew implants have been developed to enhance
orthodontic anchorage and minimize the need for patient
compliance.
They provide significant advantages over
dental implants due to their versatility of placement, ease
of removal, and lower cost,15 and have seen a dramatic
increase in use and popularity in recent years.
Multiple case reports have documented the successful
use of miniscrews,16-22 but some results have been
conflicting.
Success rates in human subjects, for example,
range from 49% to 100%.3,23
There is also a lack of
consensus concerning ideal miniscrew design, placement
techniques, allowable force levels, and timing of force
40
application.
The lack of consistent results and consensus
may be due to the fact that there are major questions
concerning orthodontic miniscrews that need to be answered
through basic science and clinical trials.
One such question is: what is the maximum force that
orthodontic miniscrews can withstand?
The ability of a
miniscrew to provide anchorage depends on the mechanical
retention provided by intimate contact between bone and the
surface of the implant.
This mechanical retention, also
known as primary stability, is influenced by several
factors.
For instance, implants with greater diameter and
thread depth will have a larger surface area in contact
with bone, and theoretically, will achieve greater primary
stability.
The quality and quantity of the bone into which
miniscrews are placed also influences their stability and
subsequent success.24,25 Cortical bone, for example, is much
denser than cancellous bone, and provides for more intimate
contact between the bone and the threads of the miniscrew.
The extent to which these and other factors
influence the stability of miniscrews and the maximum force
they can withstand is not completely known or described in
the literature.
It has been suggested that orthodontists
are still in search of the formula for ultimate miniscrew
stability.26
41
Recently, three studies have been conducted on the
pull-out strength of orthodontic miniscrew implants.
Pickard placed miniscrews (1.8 mm x 6 mm) in cadaver
mandibles and evaluated the effects of implant orientation
on stability and resistance to failure.27
The results of
his axial pull-out tests showed a maximum force at failure
of 342 N ± 80.9 N (Mean ± S.D.).
Huja and colleagues have
conducted two studies on the pull-out strength of
miniscrews.
Both of the studies were designed to determine
the maximum pull-out strength of miniscrews in the maxilla
and mandible of beagle dogs.
In the first study, the
implants (2 mm x 6 mm) were placed and tested immediately
after the dogs had been killed.28
The second study tested
the screws in the same manner after they were allowed to
heal, unloaded, for 6 weeks.29
Average pull-out strength of
implants after 6 weeks of unloaded healing was 245 N.
In each of these studies, pull-out tests were
performed.
Pull-out tests measure holding power against
tension applied along the longitudinal axis of screws.
Pull-out tests alone are not totally adequate to measure
the anchorage potential of bone screws because they do not
address shearing forces which are present in a clinical
setting.30
Shear tests more closely mimic clinical
42
situations in which a lateral or tangential force is
applied to the head of a miniscrew.
Pickard has conducted the only shear tests on
orthodontic miniscrew implants.27
As described above, he
studied the effect of orientation of miniscrew implants on
resistance to failure.
Placing miniscrews in human cadaver
bone and immediately afterwards performing shear tests, he
determined that the mean shear force required to cause
failure (123 N) is roughly a third of the mean axial pullout force (342 N).
There are no reports in the literature of shear
tests on miniscrew implants that have been placed in living
bone and allowed to heal, nor have there been shear tests
performed on miniscrews that have been immediately loaded.
The specific aim of this study is to determine the maximum
shear resistance of miniscrew implants.
The affect of
immediate loading on the maximum shear resistance of
miniscrew implants will be compared between implants of two
different lengths, and with three different applied force
loads.
A comparison of shear force at failure will also be
made according to the depth of the miniscrews in bone.
43
Methods and Materials
Sample Selection
Five beagle dogs were utilized as the model for this
study.
As part of a previous study31 these dogs were
acquired, maintained and had implant placement surgery
performed in the Comparative Medicine Department at Saint
Louis University School of Medicine.31
Miniscrew Implants
A total of 12 surgical grade titanium implants were
placed in the mouth of each dog.
The miniscrews used for
this study were the AbsoAnchor® system (Dentos, Inc.,
Daegu, Korea).
Two different lengths of miniscrews were
used for the study: 3 mm and 6 mm.
commercially available.
The 6 mm miniscrews are
The 3 mm miniscrews were specially
constructed for this and the preceding project by Dentos,
Inc.
Both implants measured 1.3 mm in diameter, and had a
notched design at the tip that allowed for self-drilling.
Two implants of the same length were organized as a pair,
and a third implant was placed between the pair to serve as
an unloaded control.
The implant pairs were loaded with
either 600 or 900 g of force at the time of placement with
nickel titanium coil springs.
Four sets of 3 implants (2
44
study and 1 control) were placed in each dog, with one set
being placed in each of the following locations: maxillary
right palate, maxillary left palate, mandibular left
buccal, and mandibular right buccal.
One set of 6 mm
miniscrews and 3 sets of 3 mm miniscrews (1 palatal and 2
mandibular buccal) were placed in each dog with the 6 mm
set always being placed in the palate.
Preparation of Samples for Testing
Six weeks following initial miniscrew placement the
dogs were sacrificed with a lethal dose (3-5 ml) of
pentobarbital (Euthanasia-5, Henry Schein, Inc., Port
Washington, NY), and the jaws were immediately removed by
dissection.
The direction of intraoral force application
was precisely marked on each implant with indelible ink,
and the coil springs providing the force were carefully
removed from the implants.
All soft tissue was removed
from the jaws, and they were then sealed in plastic bags
and frozen at -30°C until the time of testing.
On the day of testing, individual bones were allowed
to thaw to room temperature and were dissected into small
segments such that each contained one miniscrew surrounded
by at least 4 mm of bone.
Each segment was radiographed
from various angles to allow detection of broken implants
45
or sheared tips, and to determine whether bicortical
engagement had occurred.
In order to determine the amount of miniscrew
engaged in the bone, repeat measures of the length of screw
protruding from the bone were made with a digital caliper,
and the mean values recorded in millimeters for each
implant.
The amount of implant engaged in bone was then
determined by subtraction from the known length of each
miniscrew.
The last step in preparation for testing was
completed by embedding the bone segments into a small (4 cm
x 4 cm) square receptacle containing freshly mixed, unset
dental stone.
The surface of the bone into which the
miniscrew was inserted was left uncovered.
The stone was
allowed to set for 10 to 15 minutes resulting in a rigid
block that could be secured for shear testing.
Mechanical Testing
The shear testing was completed with an Instron
Machine Model 1011 (Instron Corp, Canton, MA) outfitted
with a 100 lb load cell.
To allow forces to be applied at
right angles to the miniscrews, a variable angle vice was
used to hold each stone block.
The vice was secured to a
custom x-axis and y-axis sliding table which was bolted to
46
the frame of the Instron.
The vice and sliding table could
be locked into place, which allowed the miniscrew to be
oriented and firmly held in the correct position for
testing.
To ensure that the line of action was directly
through the head of the miniscrew and in the same direction
as the initial intraoral force load, a plumb bob was used
to align each implant prior to testing.
The Instron
machine was used to subject the screws to shear forces
until failure.
A predetermined crosshead speed of 1.0 mm
per minute was used.
Forces were applied to the screws by
threading two 0.012 inch stainless steel ligatures through
the head of each miniscrew and tying them to a custom hook
attached to the Instron machine.
The load-displacement
data were recorded, and the peak load at failure was
obtained from the readout and reported in Newtons (N).
All
dissections, bone specimen preparation, testing and data
recording were performed by one operator (DC).
Statistical Analysis
Independent sample t tests were used to compare the
maximum force at failure of 3 mm versus 6 mm miniscrews.
Independent sample t tests were also used to compare the
differences between the maximum force at failure of the
47
3 mm implants placed in the maxilla and the mandible.
Maximum force at failure was compared between the different
force load groups (0, 600, and 900 g) by means of a one-way
analysis of variance (ANOVA).
Loaded (0g) and unloaded
(600g, 900g) implant groups were compared using independent
t tests, and, due to small sample sizes, a non-parametric
equivalent of the t test (Mann-Whitney U) was also
performed.
The results of both tests are reported here.
A measurement of millimeters of bone engaged by each
miniscrew was used to create the following groups: 0-1 mm,
1-2 mm, 2-3 mm and 3 mm+.
The maximum force at failure
between these groups was compared.
Due to the small sample
size of individual groups, a non-parametric test analogous
to ANOVA (Kruskal-Wallis) was used in this comparison.
A
manual calculation using a Scheffé like test was then
performed.
This post hoc test allows intergroup
differences to be identified by ranking group means against
a calculated chi squared statistic.
A critical value for
significance was determined by calculation.
Two group
comparisons were evaluated against the critical value and
deemed significant if in excess of it.
In order to further investigate the relationship
between maximum force at failure and the depth of the
miniscrews in bone, a Pearson Correlation test was
48
completed using raw millimetric measurements.
statistics were performed with α = 0.05.
All
Descriptive
statistics, independent sample t tests, ANOVA, Mann-Whitney
U, Kruskal Wallis, and Pearson correlation tests were
executed with the SPSS statistical program, version 14.0
(SPSS Incorporated, Chicago, IL).
Results
Miniscrew Failures
During the six week course of force application, ten
3 mm experimental, and five 3 mm control miniscrews failed.
Twelve of the miniscrews that failed (80%) had been placed
in the mandible.
Two of the 6 mm miniscrews were
compromised during dissection of the bony segments and were
rendered unsuitable for testing.
These losses resulted in
the exclusion of 17 of the originally placed miniscrews.
The 43 implants remaining in the mouths of the five dogs
served as the sample for this study.
Thirty 3 mm
miniscrews and thirteen 6 mm miniscrews were subjected to
shear force testing.
All of the 6 mm miniscrews and 12 of
the 3 mm miniscrews were located in the palate, while 18 of
the remaining 3 mm implants were in the mandible.
49
A
summary of the type and location of the remaining implants
is presented in Table 3.1.
Table 3.1: Type, Location and Load Category of Remaining
Miniscrews
Type of
Implant
Maxilla
Mandible
Loaded Control
Loaded Control
Total
3 mm
8
4
12
6
30
6 mm
9
4
0
0
13
Total
17
8
12
6
43
During shear testing, thirteen miniscrews fractured
at the bone level.
Two 3 mm miniscrew (10%) and eleven 6
mm miniscrews (85%) broke.
Only two of the thirteen 6 mm
implants subjected to shear testing survived the process
without fracturing.
The miniscrews fractured at shear
force levels ranging from 26 to 126 N.
All the miniscrews
that fractured during testing were included in the
statistical analysis, as failure of either the bone or the
screw constituted the maximum shear force tolerance.
Maximum Shear Force Measurements
The maximum shear force at failure of 6 mm miniscrew
implants was significantly higher than that of the 3 mm
50
implants (Table 3.2) (p<0.05).
When comparing 3 mm
implants in the maxilla versus those in the mandible, there
was no significant difference.
A comparison of the
implants loaded with 0, 600 and 900g of force revealed that
these differing force loads resulted in no significant
difference in maximum force at failure.
Table 3.2: Maximum Shear Force at Failure in Newtons (N)
Type of
Implant
Group
N
Maximum Force at Failure
Mean
S.E.
Min
Max
(N)
(N)
(N)
(N)
3 mm
All
Loaded
Control
Maxilla
Mandible
30
20
10
12
18
31.90
37.33
21.06
39.70
26.71
6 mm
All
Loaded
Control
13
9
4
53.00*
55.47
47.44*
3mm and 6mm
All
Loaded
Control
43
29
14
38.28
42.96
28.60
4.08
5.49
3.82
8.79
3.18
5.00
5.00
7.00
5.00
7.00
99.00
99.00
36.25
99.00
58.00
8.33
11.59
9.08
26.00
26.00
28.00
126.00
126.00
67.12
4.03
5.34
4.85
5.00
5.00
7.00
126.00
126.00
67.12
(* Maximum force at failure significantly higher than similar groups)
According to the results of both the independent t
test and the non-parametric Mann-Whitney U test,
immediately loaded and control implants showed no
51
significant difference in force at failure (Table 3.3).
Further, when loaded and unloaded miniscrews of the same
length were compared, there were no significant differences
in the force required to cause failure.
Table 3.3: Mann-Whitney U Test Results of Loaded versus
Control Implants
Loaded
Maximum
Force
at Failure
Unloaded
Mann-Whitney
N
Mean
S.D.
N
Mean
S.D.
Z
P
29
42.96
28.75
14
28.60
18.16
-1.50
0.13
Table 3.4: Kruskal-Wallis and Scheffe´ Post Hoc Results of
Maximum Force at Failure by Depth of Screw in Bone
KW
Group
N
Mean
Median
Range
0.0-1.0 mm
1.0-2.0 mm
2.0-3.0 mm
3.0 mm+
4
20
7
12
9.31
31.23
46.18
55.08
7.25
29.48
58.00*
47.31*
12.75
91.25
62.25
100.00
Post hoc
Scheffe´ test:
Critical Value: 24.1
Group
0-1 mm
1-2 mm
2-3 mm
3 mm+
12
13.5
25.5*
35*
*p<0.05
52
Χ2
p
14.70
0.002
A significant difference was determined to be
present between the groups formed by millimetric
measurements of bony purchase.
The groups with 2-3 mm and
3 mm+ of bony purchase showed a significantly higher shear
force at failure than the other 2 groups (Table 3.4).
There was, however, no significant difference between the
2-3 mm group and the 3 mm+ group.
Shear force at failure
showed a moderately strong correlation (r=0.57) with the
depth of the miniscrew in bone.
Discussion
Although axial pull-out tests are a standard method
for testing screws, tangential or shear loading more
closely mimics clinical orthodontic loading situations.
Shear tests introduce additional variables not encountered
in direct axial pull-out tests.
Bone, for example, has
been shown to be anisotropic; it exhibits different
mechanical properties when loaded on different axes.32
It
cannot be ensured, therefore, that when a shear force is
applied to a miniscrew that the bone will flex or bend in
the same manner or to the same degree with each test.
When
miniscrews begin to lean as a result of a lateral force, it
53
is possible that the apex of the screw embedded in the bone
could contact surrounding anatomical structures such as the
opposite cortical plate, or the root of a tooth.
These
variables, which are difficult to control, may lead to
variations in the measured shear force loads required to
cause failure.
The variability of shear testing can be looked at
from two contrasting points of view.
This method of
testing can be seen as a weakness of the study because it
introduces variables that are difficult to standardize or
reproduce.
In contrast, performing this type of test can
be seen as a significant strength due to its clinical
applicability.
The bone flexure and anatomic variations
encountered in testing are representative of the actual
situations that exist in clinical practice.
For this
reason, the decision was made to perform shear rather than
axial pull-out tests.
One specific aim of this study was to quantify the
maximum shear resistance of miniscrew implants.
The
miniscrews in this study were of two different lengths,
were placed in different locations in the jaws, and were
subjected to three differing force loads.
As a result of
testing these miniscrews, a range of mean shear forces at
failure (21.1-55.5 N) was determined.
54
Even though studies
performed on animals may not directly correlate to human
studies, the desire was to provide clinicians with an
estimate of the lateral forces that miniscrews can
withstand prior to failure.
Forces required for
orthodontic tooth movement, depending on the type of
movement desired, can range from 0.3 to 4.0 N.33
Orthopedic
appliances such as headgear and palatal expanders exert
forces ranging from 4 to 17 N.34,35
The shear forces
withstood by the miniscrews in this study are higher than
the forces typically used to produce orthodontic and even
orthopedic effects.
The average shear force at failure of 3 mm and 6 mm
miniscrews in this study was 31.9 N and 53.0 N,
respectively.
Only one other study has reported results of
shear force testing to which these values can be compared.
Pickard placed miniscrews in human cadaver mandibles and
determined the mean shear force at failure to be 138 N.27
When comparing the results of the current study with those
reported by Pickard, there is a difference of roughly 90 N.
There are several possible reasons for this difference.
The depth of miniscrew insertion likely varied
considerably between the studies.
The miniscrews in this
study were placed through the soft tissue with no reflected
flap, and upon dissection of the soft tissue revealed
55
average insertion depths of 1.6 mm and 3.9 mm for 3 mm and
6 mm miniscrews, respectively.
Pickard placed implants
directly into bone that had been denuded of all soft
tissue.
This likely allowed improved visibility and
complete insertion of the threaded portion of the
miniscrews.
Increasing the depth of insertion can affect
shear force resistance by increasing the surface area of
the screw in contact with bone.
Theoretically, additional
surface area in contact with cortical and medullary bone
leads to increased retention of the miniscrew.
Had flaps
been laid to ensure that the miniscrews in this study were
placed completely into bone, the maximum force at failure
would likely be higher.
Another factor that may have contributed to the
differing results relates to bicortical engagement.
Pickard reported that the majority of the miniscrews placed
in the mandible contacted the lingual cortical plate on
placement.
Embedding the apex of a miniscrew in cortical
bone may add significant resistance to the lateral
displacement of the miniscrew by restricting the apex from
swinging free and allowing earlier failure.
It is likely
that this type of bicortical engagement could lead to a
dramatic increase in the force required to cause failure.
No miniscrews in the current study were found (by
56
radiographic and visual examination of sectioned implant
sites) to have engaged two cortical plates.
All but two of the 6 mm miniscrews subjected to
shear testing fractured at the bone level prior to leaning
or pulling free from the bone.
It is likely that the mean
shear force resistance for this group of implants would
have been higher had they not fractured.
The present study
demonstrated that the limiting parameter for miniscrews
measuring 6 mm in length and 1.3 mm in diameter was not the
ability of the bone to withstand the applied force, but the
strength of the screw itself.
Another possibility for the differences noted in
shear force values is the healing response that takes place
around implants.
Because Pickard placed screws in non-
living bone, no such response occurred prior to testing.
In the current study, however, the biologic response to
implant placement was allowed to follow its course for the
duration of the six week testing period.
Specifically
related to implants with no applied force, previous
histological studies have reported increased bone
remodeling at the implant-bone interface.36,37
It has been
demonstrated that bone within 1 mm of an implant surface
undergoes a sustained elevated remodeling rate resulting in
a lower microhardness.38
This elevated remodeling rate
57
prevents the surrounding bone from fully mineralizing, and
can lead to a decrease in stability.
Due to the fact that
this process is not complete within 6 weeks of implant
insertion,29 this feature may have resulted in a lowered
resistance to shear forces, particularly in the control
implants.
Though the difference was not statistically
significant, the miniscrews in this study that were
immediately loaded had a higher shear force at failure than
did the control miniscrews.
This could be due to the
difference in the bone healing response around implants
with and without a constant laterally applied force.
Histological studies have shown a higher rate of
cortication, and an overall higher percentage of bony
contact on loaded implants.39,40
Wehrbein et al. have shown
that after a 10 week healing period, an orthodontic force
applied to dental implants induces marginal bone apposition
adjacent to the implants41 and can increase implant
stability.42
The increased force required to cause failure
in the loaded implants is likely the result of the higher
density of the bone surrounding the implant and the
increased osseous contact with the miniscrews.
The results
of this comparison support the conclusions made by previous
58
authors concerning the increased stability of miniscrews
subjected to an orthodontic force load.
The results of this study support the notion that
the cortical plate plays a critical role in providing
stability to miniscrews.
Maximum shear force resistance
was significantly enhanced in those implants that fully
engaged the cortical plate.
This is evidenced by the
results of the comparison of failure force by insertion
depth.
The average cortical plate thickness for all sites
in all of the dogs used in this study was 2.1 mm.
The mean
force required to cause failure of all miniscrews engaging
less than 2 mm of bone was 27 N.
The miniscrews that
engaged one additional millimeter of cortical plate for a
total of 2-3 mm of insertion depth showed a mean increase
of 15 N of resistance.
This is the most likely explanation
of why 6 mm miniscrews in this study showed a significantly
higher resistance to shear forces than the
miniscrews.
3 mm
Soft tissue thickness prevented full
engagement of 3 mm implants resulting in an average
insertion depth of only 1.6 mm.
On average, the 3 mm
implants did not engage the complete thickness of the
cortical bone, and thus did not receive the full benefit of
the stability it provides.
Although the 3 mm miniscrews
showed a lower force at failure than the 6 mm miniscrews,
59
it is important to note that they withstood forces well
beyond those typically used in orthodontics.
This study is unique in that it was designed to
investigate shear testing on miniscrews that have been
placed in living bone and allowed to heal.
In contrast to
studies in which miniscrews have been placed into nonliving bone, the design of this study made it possible to
consider the biologic response of cortical bone to implant
placement.
In addition, this study is also unique in that
it compared the maximum shear resistance of immediately
loaded and control miniscrews.
Because orthodontic
miniscrews are primarily loaded in shear, there is a need
for research exploring the variables affecting implant
stability and resistance to failure when a shear force is
applied.
The findings of this study indicate that an
immediately applied force is not detrimental to the maximum
shear resistance of miniscrews.
This adds to the existing
evidence in the literature that a period of healing is not
essential in order for a miniscrew to provide sufficient
anchorage for orthodontic tooth movement.
60
Conclusions
This study evaluated the maximum shear force that
orthodontic miniscrew implants can withstand, and compared
the affect of immediate loading on the maximum shear
resistance of miniscrew implants of two different lengths,
and with three different applied force loads.
A comparison
of shear force at failure was also made according to the
depth of the miniscrews in bone.
The following conclusions
can be made from this study:
1) Miniscrews in this study were able to resist shear
forces prior to failure that are higher than the forces
typically used to produce orthodontic and even
orthopedic effects.
2) Miniscrews that have been immediately loaded with either
600 g or 900 g of force show no statistically
significant difference in shear force at failure when
compared to unloaded control miniscrews.
The modest
differences that were noted between these two groups are
likely a result of the different healing processes that
occur around implants with and without a constantly
applied force load.
61
3) Miniscrews measuring 6 mm in length showed a
significantly higher shear force at failure than did
miniscrews measuring 3 mm.
This difference is likely
due to the increased insertion depth of the 6 mm
miniscrews.
4) The maximum shear force at failure of miniscrew implants
is significantly increased when the entire thickness of
the cortical plate is engaged by the miniscrew.
62
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VITA AUCTORIS
Damen Matthew Caraway was born on September 11, 1975
in Fort Rucker, Alabama to Donald Monroe and Joyce Marie
Caraway.
He graduated from Brigham Young University in
Provo, Utah in 2000 with a Bachelor of Science degree in
Zoology with an emphasis in Human Biology.
From 2000 to
2004, he attended the University of Texas Health Science
Center at San Antonio Dental School in San Antonio, Texas.
Graduating Suma Cum Laude and with Research Honors, he was
awarded a Doctorate of Dental Surgery in 2004.
It is
anticipated that in January of 2007 Damen will graduate
from Saint Louis University with a Master of Science degree
in Dentistry with an emphasis in Orthodontics, and enter
private practice in Colorado.
In May of 1998, Damen married Tiffany Nicole Liddle.
They are the parents of three children; Avery, Gavin and
Sydney.
67