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EFFECT OF MINISCREW CHARACTERISTICS (LENGTH AND OUTER
DIAMETER) AND BONE PROPERTIES (CORTICAL THICKNESS
AND DENSITY) ON INSERTION TORQUE
AND PULLOUT STRENGTH
Ankit H. Shah, B.D.S., M.D.S.
An Abstract Presented to the Graduate Faculty of
Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2011
1
Abstract
Purpose: To experimentally study the effects of altering
implant length, outer diameter, cortical bone thickness and
cortical bone density on the primary stability of orthodontic
miniscrew implants (MSIs). Methods: Maximum insertion torque
and pullout strength of MSIs were measured in synthetic bone
with different cortical densities (0.64 g/cc or 0.55 g/cc) and
cortical thicknesses (1 mm or 2 mm).
Three different MSIs were
evaluated; they were either 3 mm or 6 mm long, with outer
diameters of either 1.75 mm or 2 mm. Insertion torque was
measured at four different time intervals during insertion. To
test pullout strength, a vertical force was applied at the rate
of 5 mm/min until failure occurred. Results: Statistically
significant differences in insertion torque and pullout
strength were found between the 3 MSIs. Post-hoc tests showed
that the 6 mm MSIs had significantly higher (p< .001) insertion
torque and pullout strength values than the 3 mm MSIs. Post-hoc
tests showed that the 3 mm MSIs with a larger outer diameter
had significantly higher (p< .001) insertion torque and pullout
force values than the 3 mm MSIs with a smaller outer diameter.
The insertion torque and pullout strength values were
significantly (p< .001) greater for the MSIs placed in thicker
and denser cortical bone; 50% and 14% increases in thickness
1
and density produced 9-33% increases in insertion torque and
13-72% increases in pullout strength, respectively. Conclusion:
Both outer diameter and length affect the insertion torque and
pullout strength of the MSIs. Increases in cortical bone
thickness and cortical bone density increase the primary
stability of the MSIs.
2
EFFECT OF MINISCREW CHARACTERISTICS (LENGTH AND OUTER
DIAMETER) AND BONE PROPERTIES (CORTICAL THICKNESS
AND DENSITY) ON INSERTION TORQUE
AND PULLOUT STRENGTH
Ankit H. Shah, B.D.S., M.D.S.
A Thesis Presented to the Graduate Faculty of Saint
Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry
2011
3
COMMITTEE IN CHARGE OF CANDIDACY:
Adjunct Professor Peter H. Buschang,
Chairperson and Advisor
Professor Rolf G. Behrents
Assistant Professor Ki Beom Kim
i
DEDICATION
To, mom and dad, for the sacrifices that you have made
so that my brother and I could pursue further education in
the United States. You have provided unconditional love and
always stood by me, especially, during the difficult times
of my life.
To my lovely wife, Amee, for encouraging me to pursue
my dreams and supporting me in every endeavor of mine. We
will keep moving forward together. I love you more than
words could ever describe.
To my brother, Darshit, for keeping things in
perspective and helping with the thesis.
ii
ACKNOWLEDGEMENTS
It is a pleasure to convey my gratitude and humbly
acknowledge all who assisted with this project:
Dr. Peter H. Buschang for chairing my thesis
committee. You motivated me to think critically during this
project. Thank you for all the guidance in helping me
understand my project more clearly.
Dr. Rolf Behrents. Thank you for your advice,
supervision and time in making sure that the experimental
setup was done properly. Your answers to questions that I
could not answer provided needed insights and results.
Dr. Ki Beom Kim for serving on my thesis commitee.
Thank you for your time and suggestions. Your attention to
detail and thought-provoking guidance is truly appreciated.
Dr. H.M. Kyung and the Dentos® Corporation for
supplying the miniscrew implants used in this experiment.
Don Larabell for designing the apparatus used in the
study.
The Orthodontic and Education Research Foundation for
contributing to the funding of this project.
iii
TABLE OF CONTENTS
List of Tables.......................................... vi
List of Figures........................................ vii
CHAPTER 1: INTRODUCTION.................................
1
CHAPTER 2: REVIEW OF LITERATURE......................... 5
Problems associated with the use of MSIs..... 5
MSI placement sites and its limitations... 5
Failure rates and understanding MSI
failure................................... 8
Miniscrew implant stability.................. 12
Primary stability......................... 13
Secondary stability....................... 15
Testing primary stability ................... 16
Insertion torque.......................... 18
Pullout strength.......................... 20
Importance of testing both insertion torque
and pullout strength...................... 22
Cantilever (Tangential) testing........... 23
Factors determining primary stability........ 23
Insertion modalities...................... 24
Insertion angle...................... 24
Insertion depth and predrilling
diameter............................. 25
Implant factors........................... 26
Inner (Minor/Core) diameter.......... 26
Outer (Major) diameter............... 27
Length............................... 29
Bone factors.............................. 31
Thickness of cortical bone........... 31
Bone mineral density................. 33
Purpose...................................... 36
References................................... 37
CHAPTER 3: JOURNAL ARTICLE..............................
Abstract ....................................
Introduction.................................
Materials and methods........................
Miniscrew implants........................
Synthetic bone ...........................
Testing groups............................
Insertion torque..........................
Pullout strength .........................
iv
45
45
46
49
49
50
51
52
53
Statistical analysis.........................
Results .....................................
Miniscrew characteristics.................
Bone characteristics......................
Intercorrelations ........................
Discussion ..................................
Conclusions .................................
References ..................................
54
54
54
60
65
65
71
73
Vita Auctoris........................................... 77
v
LIST OF TABLES
Table 3.1: Groupings for the evaluation of thickness of
cortical bone (CT) and cortical bone density (CD) for the
3 MSIs.................................................. 52
Table 3.2: Analysis of variance (ANOVA) of differences in
insertion torque (Ncm) measured at 100% of insertion of
screw between MSI # 1, MSI # 2 and MSI # 3 with post-hoc
pair wise comparisons................................... 56
Table 3.3: Analysis of variance (ANOVA) of differences in
pullout strength (kgs.) measured at 100% of insertion of
screw between MSI # 1, MSI # 2 and MSI # 3 with post-hoc
pair wise comparisons................................... 57
Table 3.4: Two-way analysis of variance to evaluate the
effects of cortical bone thickness and density on insertion
torque (at 100% of insertion) for each MSI group........ 61
Table 3.5: Two-way analysis of variance to evaluate the
effects of cortical bone thickness and density on pullout
strength (kgs.) for each MSI group...................... 62
vi
LIST OF FIGURES
Figure 2.1: Simulation of various lengths of miniscrew
implants (6, 8 and 10 mm) and placement angulations (00 and
150).................................................... 7
Figure 3.1: MSI # 1 with an outer diameter of 1.75 mm,
inner diameter of 1.5 mm and length of 6 mm............. 49
Figure 3.2: MSI # 2 with an outer diameter of 1.75 mm,
inner diameter of 1.5 mm and length of 3 mm............. 50
Figure 3.3: MSI # 3 with an outer diameter of 2 mm, inner
diameter of 1.5 mm and length of 3 mm................... 50
Figure 3.4: Changes in insertion torque (Ncm) from initial
engagement (0 mm) to 100% engagement for cortical bone
thickness of 1 mm and density of 0.56 g/cc.............. 58
Figure 3.5: Changes in insertion torque (Ncm) from initial
engagement (0 mm) to 100% engagement for cortical bone
thickness of 2 mm and density of 0.56 g/cc.............. 58
Figure 3.6: Changes in insertion torque (Ncm) from initial
engagement (0 mm) to 100% engagement for cortical bone
thickness of 1 mm and density of 0.64 g/cc.............. 59
Figure 3.7: Changes in insertion torque (Ncm) from initial
engagement (0 mm) to 100% engagement for cortical bone
thickness of 2 mm and density of 0.64 g/cc.............. 59
Figure 3.8: Temporal changes in insertion torque (Ncm) from
the initial engagement of 6 mm MSI (MSI # 1) through 25%,
50%, 75% and 100% of the length ........................ 63
Figure 3.9: Temporal changes in insertion torque (Ncm) from
the initial engagement of 3 mm MSI (MSI # 2) through 25%,
50%, 75% and 100% of the length ........................ 63
vii
Figure 3.10: Temporal changes in insertion torque (Ncm)
from the initial engagement of 3 mm MSI (MSI # 3) through
25%, 50%, 75% and 100% of the length ................... 64
Figure 3.11: Correlations between insertion torque and
pullout strength by miniscrew type...................... 64
viii
CHAPTER 1: INTRODUCTION
The use of miniscrews implants (MSIs) has
revolutionized the specialty of orthodontics. MSIs are now
more commonly being used in clinical practice to enhance
orthodontic anchorage. The surgical procedure for miniscrew
implant placement is easy, enabling rapid healing and
removal without causing irreversible damage.1 MSIs can be
loaded immediately, are easily adapted to routine
orthodontic mechanics,
2
are low-cost devices and reduce
treatment time when compared with conventional implants by
the virtue of early loading.3,
4
These benefits have led to a
rapid rise in the popularity of miniscrew implants.
Implants have been used in dentistry and orthopedics
for various applications since the early 1930s. As early as
1945, Gainsforth and Higley placed a vitallium screw in a
dog to enhance anchorage. The first big stride towards the
use of bone for skeletal anchorage was made by Per-Ingvar
Brånemark5 in his landmark studies in the late 1950s and
early 1960s. Pioneering data from Linkow demonstrated that
implants can be used as anchors in orthodontics.6 However,
it was in 1997 that a miniscrew implants (MSIs)
specifically designed for orthodontic use were first
described by Kanomi.7
1
Most of our knowledge regarding the effects of various
miniscrew design factors on their stability and eventual
success comes from the orthopedic and prosthetic
literature. However, the screws are used to compress and
fixate bones together in these fields. Also, they use
larger screws with totally different loading forces,
patterns and applications as compared to orthodontics.
These reasons make it difficult to extrapolate their
findings to orthodontics.
There has been some literature published in recent
years that has tried to address the effects of MSI
diameter,8,9 length,10,11 placement method,12,13 insertion
torque,14 and pullout strength.15 However, most of the
orthodontic literature consists of case reports and force
application research. More research with well-controlled
studies needs to be performed to improve miniscrew implant
design and its effect on stability.
New miniscrew implant designs have been introduced
that may allow greater versatility in terms of MSI usage
and lead to more creative anchorage possibilities. For
instance, almost all of the currently available MSIs are at
least 6 mm long. Longer screws are used to minimize the
risk of MSI fracture during insertion. Attempts to maximize
stability while minimizing placement torque has led to the
2
development of smaller MSIs, which could expand the scope
of their clinical use.
However, some important questions regarding the
relationship between MSI design and stability have yet to
be answered before smaller MSIs can be used predictably.
For example, how will the changes in design features
influence placement torque and/or stability? Are there
design features that could be modified to improve on the
basic designs presently available?
The purpose of the present study was to provide a
better understanding of MSI design features and bone
characteristics that influence primary stability. This
information would be helpful in designing better implants,
having fewer clinical failures and improving guidelines for
clinical use. This study is unique in that it evaluates a
specific design feature while keeping all other design
features constant. Until now, there have been only a few
carefully drafted studies designed in this manner to
evaluate the smaller miniscrew implants used in
orthodontics.
The aims of this project are to compare the effects
of MSI length, MSI outer diameter, cortical thickness and
cortical density on pullout strength and insertion torque.
3
The following hypotheses will be tested:
1. There are no differences in maximum insertion
torque and pullout strength between the 3 mm and
6 mm MSIs.
2. There are no differences in maximum insertion
torque and pullout strength between MSIs with
1.75 mm and 2 mm outer diameters.
3. There are no differences in maximum insertion
torque and pullout strength between synthetic
cortical bone that is 1 mm and 2 mm thick.
4. There are no differences in maximum insertion
torque and pullout strength between cortical bone
having a density of 0.64 g/cc (40 pcf) and a
density of 0.56 g/cc (35 pcf).
4
CHAPTER 2: REVIEW OF LITERATURE
Problems associated with the use of MSIs
Although miniscrew implants have had a reasonably high
success rate, they are not devoid of limitations. Some of
the common concerns among clinicians include risk factors
for failure16 and limitations of some placement sites.17
MSI placement sites and space limitations
In recent years, numerous sites have been used by
clinicians for the placement of miniscrews. One of the most
common sites for MSI placement is the posterior buccal
alveolar bone. Interradicular distances between the
posterior teeth are important because they are thought to
be related to both safety and stability of miniscrew
implants. Additionally, the diameter of the miniscrew
implant is restricted by the available interradicular
space.
Park et al. performed a three-dimensional evaluation
of interradicular spaces and cortical bone thickness at
various MSI placement sites in adults. In the maxilla,
average interradicular distances ranged from 1.62 to 3.35
mm. Interradicular spaces tended to increase between the
CEJ and more apical aspects of the teeth.17
5
To ensure optimal periodontal health and implant
stability, it has been emphasized that a minimum of 1 mm
clearance is necessary around the MSI.18
Poggio et al.
suggested interradicular spaces greater than 3.1 mm as
“safe zones” for miniscrew implants with diameters of 1.21.3 mm.18
Longer miniscrew implants have several disadvantages
when placed in regions with limited interradicular spaces.
Clinically, it is almost impossible to place a miniscrew
implant exactly between the roots. Even minor deviations of
as little as 150 from the ideal path can damage the roots
when inserting longer implants (Fig 2.1). Although
reparative process can heal the defect, extensive damage
can be caused by these MSIs.19
6
Figure 2.1 Simulation of various lengths of miniscrew implants (6, 8
and 10 mm) and placement angulations (00 and 150).
Adapted from Park et al.17
Root proximity has also been identified as a major
factor influencing miniscrew failure.
20, 21
Longer
miniscrews are more likely to be in a closer proximity to
the root than shorter miniscrews.
Results by Kuroda et al.
demonstrated a significant positive correlation between
secondary stability and clearance of miniscrew implants.20
It is especially challenging to place miniscrew
implants in narrow interradicular spaces in the maxilla,
especially when they are placed in the attached gingiva. It
7
has been shown that only 1.6 to 1.7 mm of interradicular
space is available 5 to 7 mm from the CEJ.17 In such
instances, it has been recommended that the orthodontists
open up the interradicular space by diverging the root
apices before placing the miniscrew implant.22 However, this
can lead to increased treatment time and unpredictable
control of orthodontic mechanics. As such, it becomes
imperative for the clinicians to evaluate the anatomy at
the desired implant placement sites. Shorter MSIs could
make it possible to place MSIs in sites with limited
interradicular space.
Failure rates and understanding MSI failure
Loss of miniscrew stability limits their usefulness.
The ultimate cause of implant failure is a lack of bone-toimplant contact. A number of factors have been suggested as
possible reasons for implant loss. Application of excessive
forces on the miniscrew implant,23,24 excessively large lever
arms (thick mucosa),23,24 peri-implantitis when inserted in
the unattached mucosa,16 insufficient primary stability,25
and bone damage during insertion due to compression or
over-heating, are just some of the implicated factors.26
Failures can be subdivided into the host factors, the
surgical technique or the management of the miniscrew
8
during treatment. While it is not clear how host factors
affect the long-term stability of MSIs, their effects have
been established for endosseous implants. Ashley et al.
examined the dental implants and determined that
osteoporosis, uncontrolled diabetes, smoking and
parafunctional habits, all interfered with the healing
process and were risk factors for implant failure.27
Surgical factors include overheating during
placement,28,29 excessive trauma to the adjacent bone,30 and
the failure to establish proper initial stability of the
miniscrew.31 Management factors pertain to the lack of
proper miniscrew hygiene maintenance at home, inflammation
or infection, and excessive loading of the miniscrew.30
Various clinical studies have evaluated the factors
that could cause miniscrew failure. In a retrospective
evaluation of clinical cases, Cheng et al. reported
miniscrew success rates of 89%.16 Peri-implant soft tissue
characteristics and anatomic location were identified as
two independent prognostic indicators of the MSI failure.
Lack of keratinized mucosa increased the implants’
susceptibility to plaque induced tissue destruction. An
association was found between peri-implant infection and a
high rate of implant loss. Implants placed in the posterior
mandible also demonstrated greater failure rates, which
9
were thought to be due to lesser amounts of attached gingiva
in the posterior region. Overheating, due to the increased
density of bone in the mandibular posterior region, was
also thought to be a cause of failure rates.
Another retrospective study of treated cases performed
by Park et al. reported an overall success rate of 91.6%.31
Mobility of the miniscrew, miniscrews placed in the
mandible, inflammation of the gingiva around the screw, and
miniscrews placed in the right side were some of the
factors identified as increasing the risk of MSI failure.
They noted that minimally mobile miniscrews can be
maintained when the applied force is light. While mobility
does not represent failure, it does increase the risk of
failure. They further noted that if heavy forces were
applied, the mobility may be increased; increases in
osseous microfracture and bone trauma can occur and lead to
failure when heavy forces are applied.
Miniscrews in the mandible demonstrate greater failure
rates than MSIs placed in the maxilla, possibly due to its
greater density and the increased potential of irritation
during mastication.24 The mandible’s greater density can
lead to more drilling, which could cause overheating. Heat
greater than 47°C may cause bone necrosis.28,29
10
Miyawaki et al. suggested that factors associated with
failure were the implant’s diameter, inflammation of the
peri-implant tissue and the mandibular plane angle.32 They
found that screws with 1.0 mm diameters had success rates
of 0%, but screws with 1.5 mm and 2.3 mm diameters had
success rates of 83.9% and 85%, respectively. They also
showed that patients with high mandibular plane angles
tended to have thinner buccal cortical bone and may lack
sufficient mechanical interdigitation.
Inflammation can increase the risk of miniscrew
failure due to bone damage around the neck of the MSI. Over
time, inflammation may lead to progressive loss of bone.
This could cause the screw to lose its mechanical grip and
fail. Park et al. attributed the greater success of
miniscrews placed on the left than the right side to the
fact that the majority of the patients were right-handed
and might be expected to have better hygiene on the left
side. Better hygiene results in less inflammation and
possibly promotes greater success of miniscrew stability.33
It, thus, becomes imperative to gain an understanding
of the MSI stability and the factors determining it.
11
Miniscrew implant stability
The primary goal of using miniscrew implants in
orthodontics is to achieve “absolute” anchorage. Absolute
anchorage minimizes the possibility of any reciprocal
effects on the anchorage units. It can be achieved by
maximizing the stability of the MSIs. Achieving initial
stability of MSIs and subsequently maintaining this
stability throughout the course of treatment is important
for its success. Stability is necessary for the miniscrew
to act as a successful anchor and be able to resist
orthodontic forces. Based upon timing and bone response,
stability can be divided into two phases: primary and
secondary stability. Primary stability, a mechanical
phenomenon, refers to the initial stability of the MSI and
is a function of the interdigitation of the implant with
the bone. Secondary stability results from the healing of
the bone around the implant surface. Clinical evidence from
dental implantology suggests that primary stability of the
MSIs is one of the most important factors determining its
survival rate and reliability.34
12
Primary stability
Miniscrews need to achieve adequate primary stability
immediately after their insertion so that they can heal
properly and be loaded immediately. Mechanical interlocking
and miniscrew insertion technique are most critical for
achieving the proper amount of primary stability.
Primary stability is the mechanical stability
occurring immediately after the insertion of miniscrew. The
quality of bone, implant design and the insertion technique
are the essential factors influencing primary stability.34
Adequate primary stability is crucial because it allows the
orthodontist to be able to load the MSI immediately after
placement.1,35 Even more importantly, primary stability plays
a critical role in the development of secondary stability.36
With adequate primary stability (lack of implant mobility),
bone can form and remodel around the implant. This process
leads to an increase in secondary stability of the implant.
Poor primary stability causes shearing stresses at the
bone-implant interface, which contribute to the development
of a fibrous rather than a mineralized interface.37
Minimizing trauma adjacent to the miniscrew during
placement is very important. Trauma can increase the
possibility of necrosis, which reduces the effective
13
stability of the screw. Trauma also necessitates bone
remodeling, healing and the formation of woven bone. It is
thought that woven bone is undesirable for implant
stability due to its poor organization and structure (i.e.
it may not provide adequate strength to support loading
forces).38
Micromotion during healing can also have deleterious
effects on primary stability. Micromotion refers to sliding
movements and gap-opening at a bone-implant interface.39
Greater primary stability may be related to decreased
micromotion, which in turn translates to a better healing
response. Micromotion during healing may result in
microfracture, necrosis, bone resorption and the subsequent
formation of a fibrous capsule around the screw. Without
proper bone support and remodeling, there may be eventual
loosening and failure of the miniscrew.
Upon implant insertion, there is a marked compression
of local bone resulting in circumferential hoop stresses.
Hoop stresses are compressive stresses generated in the
bone around the implant threads. They are important for
enhancing primary stability. However, when hoop stresses
are too high they can produce local ischemia and bone
necrosis.40,41 Alternately, compressive stresses that are too
low provide insufficient primary stability. As such,
14
implants should be placed in cortical bone with compressive
stresses that are neither too high nor too low in order to
achieve maximum implant stability.25
Microfractures associated with compressive forces that
are too high can also be problematic. For example, Nam et
al. studied the cortical bone strains during the placement
of orthodontic miniscrew implants by 3D finite element
analysis. Bone strains greater than 4000 microstrains
(i.e., the reported upper limit for normal bone remodeling)
have been observed in the bone along the entire length of
the MSI. Higher strains were recorded at the bone in the
vicinity of the screw tip. They concluded that bone strains
during MSI insertion might have a negative impact on the
physiological remodeling of bone.
Secondary stability
Regeneration and remodeling of the bone at the
implant-tissue interface following placement is responsible
for increases in stability over time. The initial healing
reaction, predominantly, involves bone remodeling at the
periosteal and endosteal surfaces. A woven bone callus
fills with lamellae via the process of lamellar compaction,
and areas of new bony contact appear around the implant
surface.42
15
This type of remodeled bony contact is termed as
secondary bone contact (secondary stability) and it
predominates at later healing times.43
Testing primary stability
Various techniques have been used to test the primary
stability of endosseous implants. It is highly desirable to
have a quantitative method for establishing primary implant
stability at the time of placement.
When analyzing primary stability, one has to ensure
that no bone remodeling has occurred. As such, tests are
typically performed during or immediately after implant
insertion.15 In situations where non-viable tissues are
being tested, primary stability can be measured at any
time.
The various methods available to test implant
stability can be divided into invasive and non-invasive
methods. The noninvasive methods include percussion
testing, radiographic methods, resonance frequency analysis
and placement torque.40
Various invasive methods are also available to assess
the implant-tissue interface after implant placement.
Invasive methods to measure implant stability are all
destructive in nature and, consequently, can only provide
16
cross-sectional data at one point in time. This limits
their usefulness in understanding the healing process and
in appreciating its relationship with stability.
One invasive method used to evaluate primary stability
measures cutting torque resistance.44 This technique
measures the energy needed to remove bone prior to implant
placement. Friberg et al. showed a positive correlation
between cutting torque resistance and bone density, which
is one of the factors that determines stability.44 The
limitation of this method of measurement is that repeated
measures cannot be made; it is only useful to estimate the
implant stability prior to placement. It is used most
frequently for prosthetic dental implants where the larger
size of the implant necessitates the removal of bone prior
to placement. Bone removal prior to placement of
orthodontic mini-screw implants is often not needed due to
their small size. This factor also limits the importance of
this method for orthodontic applications.
For the analysis of primary stability, insertion
torque and vertical pullout strength are perhaps the best
and most commonly used methods.
17
Insertion torque
Insertion torque is an objective method of measuring
implant stability that was originally introduced by Hughes
and Jordan.45 This is probably the most often used method to
evaluate primary stability. It describes the rotational
force required to insert a screw into bone.45,46 The force
used to insert the MSI is transferred through the screw and
produces a compressive force on the adjacent bone. A
minimal level of insertion torque is required to achieve an
adequate amount of stability.5,17 However, too much torque
during placement may cause damage to the adjacent bone and
eventually result in screw failure. With increasing torque,
microdamage may accumulate in the bone surrounding the
implant, leading to a reduction of bone holding strength.47
Another probable consequence of excess insertion torque is
failure of the miniscrew itself via its bending or
fracture.48
Motoyoshi et al. studied the success-rates of 124
miniscrew implants to determine the placement torque
required to enhance the success rates of miniscrew
implants.25 Maximum implant placement torque (IPT) was
measured by using a torque screwdriver. Based on the
calculations of the risk ratio for failure, they recommend
an IPT within a 5-10 Ncm range for MSIs 1.6 mm in diameter.
18
If the amount of torque during placement is less than 5
Ncm, proper mechanical interlocking of the screw and bone
may not be attained and initial stability may be
compromised. However the study showed a statistically
significant increase in failure in situations with excess
insertion torque (> 10 N cm).
A significant difference in
the IPT between maxilla and mandible was also found.
The
IPT in the mandible was significantly higher in the failure
group than in the success group. They hypothesized that a
stiffer cortical bone or a larger screw diameter will
result in a greater implant placement torque, which if
within the above limits, will in-turn enhance the primary
stability of miniscrew implants.
During implant placement, the torsional forces are low
as the screw threads are first engaged and inserted through
the cortex. The force levels increase and peak once the
entire cortical layer is engaged. Insertion torque
increases rapidly and reaches a maximum value upon screw
head contact. The countersink friction, which is the
contact of the screw head with the bone, creates this peak
in insertion torque.45 After this point, insertion torque
will decrease as the screw or bone fails under shear
stress. The material surrounding the threads becomes
stripped and the screw eventually spins freely in the hole.
19
At this point the holding strength of the screw becomes
severely limited.47
However, insertion torque, as a measure of primary
stability is not free of limitations. It only provides
cross-sectional data because it can only measure stability
at the time of placement. Subsequent measurements to
evaluate primary stability cannot be collected. Studies of
changes in bone surrounding the implant are not possible
with this method.
Pullout strength
The pullout strength test is considered an accurate
method for evaluating the relative strength or “holding
power” of surgically placed bone screws.49,50 Pullout tests
measure this holding power by applying tension along the
longitudinal axis of the screws. Bechtol et al. defined
holding power as, “the maximum uniaxial tensile force
needed to produce failure in the bone”. The holding power
of a screw in a living bone is a function of the weakest
element in the bone-screw composite system (i.e., the bone
adjacent to the screw).51,52 According to Hughes et al.,
pullout force is not a mechanical property of a screw, but
is related to the shearing strength of the material into
which it is inserted.45
20
Since the screw and bone are interdigitated about the
screw threads, two main areas for potential failure exist
when a screw is pulled out of the bone: the screw and the
bone. Failure in pullout usually occurs by shearing of the
bone material around the screw because the materials used
in implants are many times stronger than bone.46
When pullout tests are performed either immediately
after placement or in nonviable tissues no adaptive healing
responses can occur. As such, it is a test of primary
stability. A pullout test directed vertically, with forces
parallel to the long-axis of a screw, is one that tests the
primary force a screw is designed to resist.
Pullout is a popular test of holding power used in
orthopedics, orthodontics, neurosurgery, and maxillofacial
surgery to evaluate the biomechanical performance of
screws.10,40-42 Pullout testing of MSIs in a human model has
yet to be reported in orthodontics. Although human models
provide useful clinical information, they add variation to
testing due to differences in density and cortical
thickness among samples and within samples.54-55 According to
Huja et al.,15 the pullout strength at implant placement
might be the best indication of the maximum holding power
of the screw, even after the remodeling response replaces
the necrotic bone near the implant.
21
Importance of testing both insertion torque and pullout
strength
More than one determinant of stability is often used to
assess implant stability. Insertion torque and pullout
strength tests have been combined in the orthopedic and
surgical literature to evaluate the performance of screws.
A combination of these tests makes it possible to evaluate
various factors and their interrelationship.
Pullout strength cannot be reliably predicted from
insertion torque. However, a linear relationship has been
found between both the insertion torque and ultimate screw
pullout.56,57 Daftari et al. found a positive correlation
between insertion torque and pullout force (r= 0.65) in
synthetic bone.58 Zdeblick et al. tested whether insertion
torque could be used as a method of predicting screw
success after placement in vertebrae. They found that there
was a correlation between the two and suggested that
insertion torque was the best predictor of bone to screw
failure.
Pullout strength was used to demonstrate that
insertion torque provides information about the holding
power of the screw. A linear correlation between insertion
torque and pullout strength was found.57
22
Cantilever (Tangential) testing
An alternative to the pullout test is the shear test,
which examines the effects of tangential or lateral forces.
Directional loading might more closely mimics clinical
orthodontic loading situations. However, it is not possible
to obtain standardized and reproducible results in
cantilever (tangential) bending. Tangential tests tend to
introduce confounding variables. Cantilever tests can
result in large variations in pullout strengths, because
factors such as bending of bone and impingement of the
screw tip on surrounding structures, such as tooth roots,
make it difficult to accurately measure the primary
stability of MSIs.15
Pierce et al. studied the axial and tangential pullout
strength of uni-cortical and bi-cortical anterior
instrumentation screws. Screws tested with the axial pullout method had 34% higher pullout force than the same
screws tested with a tangential pull-out method.59
Factors determining primary stability
Primary stability depends on insertion modalities,12,13
implant design,60 and bone factors.61-62 These three factors
are interrelated. It is important to understand their
relations to stability and to each other.
23
Insertion modalities
Insertion angle
Wilmes et al. analyzed the impact of the insertion
angle on the primary stability of MSIs.13 Two MSIs differing
in size were inserted at seven different angles (300, 400,
500, 600, 700, 800, and 900) and the insertion torque was
recorded to assess primary stability. It was shown that the
angle of MSI insertion had a significant impact on primary
stability. The highest insertion torque values were
measured at angles between 600 and 700. Very oblique
insertion angles (300) resulted in reduced primary
stability. Based on the above finding, the authors
hypothesized that oblique insertion of mini-implants might
be advantageous in regions with reduced bone quality.
Pickard et al.63 studied the effects of miniscrew
orientation on implant stability and resistance to failure.
MSIs placed in human cadaver mandibles were oriented at
either 90 degrees or 45 degrees to the bone surface.
Results showed that the implants aligned at 90 degrees had
the highest force at failure of all the groups (342 ± 80.9
N; P< .001). In the shear tests, the implants that were
angled in the same direction as the line of force were the
most stable and had the highest force at failure (253 ±
74.05 N; P< .001). MSIs angled away from the direction of
24
force were the least stable and had the lowest force (87 ±
27.2 N) at failure.
Insertion depth and predrilling diameter
The impact of insertion depth and predrilling diameter
on primary stability of MSIs was studied by Wilmes and
Drescher.12 After implant site preparation with different
predrilling diameters (1.0, 1.1, 1.2, and 1.3 mm), Dual Top
screws (1.6 X 10 mm) were inserted at three different
insertion depths (7.5, 8.5, and 9.5 mm). Insertion torque
was recorded to assess primary stability. Both insertion
depth and predrilling diameter influenced the insertion
torques: the mean insertion torques for the insertion
depths of 7.5 mm, 8.5 mm and 9.5 mm were 51.62 Nmm, 65.53
Nmm and 94.38 Nmm, respectively. The mean insertion torque
for the predrilled 1.0 mm, 1.1 mm, 1.2 mm and 1.3 mm holes
were 83.5 Nmm, 77.5 Nmm, 61.7 Nmm and 53.1 Nmm,
respectively. They concluded that higher insertion depths
results in higher insertion torque and better primary
stability. Larger predrilling diameter results in lower
insertion torque.
25
Implant factors
Implant factors are under the control of the orthodontist
because they can choose the MSIs they use. It has been
shown that various implant factors such as screw
diameter,8,11 screw length,64,65 pitch and flutes,66 are all
important determinants of holding power.
Inner (Minor/Core) diameter
Minor diameter refers to the inner (or core) diameter
of MSIs which can range anywhere from 1.2-1.6 mm. Inner
diameter has been reported to be one of the important
factors determining pullout strength because the maximum
torsional shear strength of the screw is related to the
cube of its diameter; tensile strength corresponds to the
square of its diameter. Minor diameter is also important
because the strength of the screw is directly related to
it.45
Decoster et al. showed that minor diameter had a
negative effect on pullout force, with an increase in minor
diameter leading to a decrease in pullout force. Increasing
the minor diameter from 4 mm to 5 mm decreased the mean
pullout force from 277.8 lbs to 247.8 lbs.54
Carano et al. studied the mechanical properties of
three commercially available self-tapping MSIs used in
26
orthodontic treatment. They suggested that a minor diameter
reduction of as little as 0.2 mm can reduce the resistance
to breakage of the miniscrew implants by 50%. An overall
minor diameter of less than 1.5 mm was not recommended for
orthodontic applications because humans can apply enough
torsional forces to break smaller screws. However, if
placement torque could be reduced through the addition of
other design features, it is theoretically possible to
further reduce screw size.67
Outer (Major) diameter
The orthodontic literature does not contain much
information on the effect of outer diameter of miniscrew
implant on primary stability. However, the orthopedic
literature shows that outer implant diameter is one of the
most important variables in mechanical strength.
Implants
with greater outer diameter show greater primary stability
due to greater surface area in contact with the bone.
Hughes et al. recommend using screws with a larger outer
diameter when greater holding power is desired.45
The major diameter is the diameter as determined by
the outer diameter of the threads. Outer diameters vary
widely among and within different manufacturers. Miniscrews
currently available in the market have outer diameters
27
ranging between 1.2 mm and 2 mm. Various diameters of
miniscrews have been reported to be successful in providing
anchorage.
There is indirect evidence indicating that outer
diameter is important for stability. In the study by
Miyawaki et al,32 all 1.0 mm outer diameter screws failed,
while the 1.5 mm and 2.3 mm diameter screws showed 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.
DeCoster et al. used a synthetic bone model to
determine the maximum bone-screw pullout force of
orthopedic screws with various outer diameters. As the
major diameter was increased, within a range of 3-6 mm, the
mean pullout force also increased in a roughly linearly
fashion from 105.4 lbs to 305.8 lbs. Increasing the
outer/inner diameter ratio, while holding the other
parameters constant resulted in a small, but significant,
increase in pullout force.54
Wilmes et al. studied various parameters affecting the
primary stability of orthodontic MSIs.30 Outer diameter was
28
one of the parameters determined to have an influence on
primary stability. Insertion torques of five different
mini-implant types, tomas®-pin [Dentaurum, Ispringen,
Germany] 08 and 10 mm, and Dual Top® [Jeil Medical
Corporation, Seoul, Korea] 1.6 × 8 and 10 mm plus 2 × 10
mm, were measured to determine their primary stability. The
Dual Top screws with a diameter of 2 mm achieved the
greatest primary stability followed by the Dual Top screws
with a smaller diameter of 1.6 mm.
Length
The relationship between miniscrew implant length and
primary stability has yet to be critically examined in
orthodontics. Other disciplines have found that length is
one of the most important determinants of mechanical
strength; it influences both insertion torque and pullout
strength.10
Hitchon et al. examined the effects of screw length
(12 mm, 14 mm and 16 mm) by testing 201 screw-type implants
in fresh human cadaver specimens.10 Length was shown to have
a statistically significant effect on pullout strength,
with longer screws having a higher resistance to
displacement. This might be expected because holding power
29
is directly proportional to the amount of thread
engagement.68
Chen et al. studied, retrospectively, the relationship
between miniscrew implant length and the retention rate.69
Fifty-nine MSIs, either 8 mm or 6 mm in length, with a
diameter of 1.2 mm, were placed in 29 patients for
orthodontic anchorage. A statistically significant
difference was found between the two groups. The success
rates of the 8 mm MSIs and 6 mm MSIs were 90.2% and 72.2%,
respectively. However, several studies have also shown
higher success rates by increasing the length of the
miniscrews with the same diameter, but the differences were
not statistically significant.64,70
Lim et al. examined the effects of miniscrew length,
diameter and shape on insertion torque.71 Cylindrical and
taper type miniscrews with different lengths, diameters,
and pitches were tested by placing them in synthetic bone.
Their results showed that increasing miniscrew length
resulted in greater insertion torque, suggesting that
greater stability could be achieved.
30
Bone factors
Thickness of cortical bone
Cortical thickness is one of the main factors
influencing insertion torque and, consequently, primary
stability.72 More screw threads are able to engage into
thicker cortical bone which, in turn, translates into
greater primary stability.
In orthopedics, Cleek et al. studied the effects of
cortical bone thickness on pullout strength. Their data
showed that pullout strength was significantly correlated
with cortical thickness (r = 0.56, p = .002).47
Dalstra et al. showed that the maximum stress occurs
at the cortical bone level when an implant is loaded.73
Using a finite element model, they showed that increasing
cortical bone thickness drastically reduced the peak strain
development in the peri-implant bone tissue. This inverse
relationship between cortical bone thickness and peak
strain development suggests that cortical bone thickness is
a key determinant of initial stability.
Motoyoshi et al. recommend that the prepared site
should have a cortical bone that is more than 1.0 mm
thick.74 They found that individuals with greater MSI
success had significantly higher cortical bone thickness.
31
Cortical bone thickness and insertion torque were
significantly greater in the mandible than in the maxilla.25
Others have also found significant correlations
between cortical bone thickness and vertical pullout
strength. Huja et al. performed pullout tests by placing 56
MSIs in the maxillas and mandibles of beagle dogs. They
found a positive correlation between cortical bone
thickness and the maximum force at pullout (Fmax).15 Fmax was
reported to be 134.5 N in the anterior mandible and 388.3 N
in the posterior regions of the mandible. They also showed
that the posterior regions of the jaws had thicker cortical
plates and greater pullout values. In another study, Huja et
al., found peak pullout strength to be directly related
with cortical bone thickness at 6 weeks post-insertion in a
canine model.61
Salmoria et al. found that cortical bone thickness had
a direct effect on pullout strength.72 They measured pullout
strength and cortical bone thickness at the time of
placement and 60 days after placement. After 60 days, both
the thickness of the cortical bone and the pullout strength
had decreased. Bone had resorbed around the neck of the
MSI. They concluded that there was a correlation between
axial pullout strength and cortical bone thickness.
32
The significance of cortical bone thickness in
relation to primary stability of miniscrews has been
demonstrated in numerous studies. However, the interaction
between different cortical thickness and the effects of
varying the density of cortical bone on primary stability
of miniscrews has yet to be established.
Bone mineral density
The current knowledge about the correlation between
bone mineral density properties and implant stability comes
primarily from the orthopedic and prosthodontic literature.9
Researchers in both the fields have found that bone density
is significantly correlated with both torque and pullout
strength.75,76 Pullout force increases in a linear fashion as
synthetic bone density increases.54
Insertion torque values are significantly correlated
with bone density, making torque another factor that
influences implant stability.44 The effect of bone mineral
density on insertion torque of bone screws was studied by
Koistinen et al.77 They concluded that the insertion torque
of the wide (3.5 mm) cortical bone screws increased as the
bone mineral density (BMD) increased. Friberg et al. found
a statistically significant correlation between the bone
density values of the prepared site and the resistance
33
during implant placement. They concluded that placement
torque provides reliable information about bone quality.44
In the prosthodontic implant literature, both
insertion torque and pullout strength have shown strong
correlations with bone density. These associations have led
to the development of equations, classification systems and
techniques to determine stability based on bone density.
Bone mineral density was used to develop the mathematical
holding index for accurate prediction of the holding
strength of screws placed in the cervical spine.78
Bone mineral density shows a significant positive
correlation with vertical pullout strength. Many studies
have demonstrated that the quality of the bone surrounding
the implant can have a significant effect on the initial
stability of the implant.23,25,45
Hung et al. studied the effects of pilot hole size and
bone density on miniscrew implants’ stability. Using 120
MSIs divided equally into six groups, they evaluated the
effects of synthetic bone density (0.64 g/cc vs. 0.8 g/cc
cortices) on maximum insertion torque and pullout strength.
They found that the insertion torque and pullout strength
values were significantly (p < .05) greater for the MSIs
placed in high-density than in low-density cortical bone.79
34
Wang et al. analyzed the correlation between the
pullout strengths and the peri-miniscrew bone structure,
using microcomputed tomographic analysis.80 Regression
analyses were used to study the relationship between
pullout strength and bone density, relative bone volume,
and cortical bone thickness. All pairs of pullout force and
bone structural parameters showed significant correlation
coefficients. Pullout force demonstrated the strongest
correlation (r2 = 0.92) with bone density, and the weakest
correlation (r2 = 0.263) with cortical bone thickness.
35
PURPOSE
The purpose of the present study is to isolate
miniscrew characteristics (length and outer diameter) and
bone properties (cortical thickness and density) in order
to determine their effects on primary stability. One design
characteristic will be altered at a time and insertion
torque and pullout tests will be performed to assess the
characteristic’s influence on MSI stability. Such an
experiment will lead to a better understanding of the role
of host factors and implant design factors towards primary
stability of MSIs. The clinical usefulness of this
experiment derives from the fact that orthodontic
miniscrews are often loaded immediately which makes the role
of primary stability very vital. Even more importantly,
primary stability plays a critical role in the development
of secondary stability.36
On that basis, this project will
help evaluate miniscrews and bone characteristics and lead
to a better understanding of the role played by them in the
success rate of MSIs.
36
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miniscrews for skeletal anchorage. Prog Orthod.
2005;6(1):82-97.
68. Lyon WF, Cochran JR, Smith L. Actual holding power of
various screws in bone. Ann. Surg. 1941;114(3):376-384.
69. Chen C, Chang C, Hsieh C, et al. The use of
microimplants in orthodontic anchorage. J. Oral Maxillofac.
Surg. 2006;64(8):1209-1213.
70. Kuroda S, Sugawara Y, Deguchi T, Kyung H, TakanoYamamoto T. Clinical use of miniscrew implants as
orthodontic anchorage: success rates and postoperative
discomfort. Am J Orthod Dentofacial Orthop. 2007;131(1):915.
71. Lim S, Cha J, Hwang C. Insertion torque of orthodontic
miniscrews according to changes in shape, diameter and
length. Angle Orthod. 2008;78(2):234-240.
72. Salmória KK, Tanaka OM, Guariza-Filho O, et al.
Insertional torque and axial pull-out strength of miniimplants in mandibles of dogs. Am J Orthod Dentofacial
Orthop. 2008;133(6):790.e15-22.
73. Dalstra M, Cattaneo PM, Melsen B. D. Load Transfer of
Miniscrews for Orthodontic Anchorage. Journal of
Orthodontics. 1(2004):53-62.
74. Motoyoshi M, Matsuoka M, Shimizu N. Application of
orthodontic mini-implants in adolescents. Int J Oral
Maxillofac Surg. 2007;36(8):695-699.
43
75. Halvorson TL, Kelley LA, Thomas KA, Whitecloud TS, Cook
SD. Effects of bone mineral density on pedicle screw
fixation. Spine. 1994;19(21):2415-2420.
76. Hadjipavlou AG, Nicodemus CL, al-Hamdan FA, Simmons JW,
Pope MH. Correlation of bone equivalent mineral density to
pull-out resistance of triangulated pedicle screw
construct. J Spinal Disord. 1997;10(1):12-19.
77. Koistinen A, Santavirta SS, Kröger H, Lappalainen R.
Effect of bone mineral density and amorphous diamond
coatings on insertion torque of bone screws. Biomaterials.
2005;26(28):5687-5694.
78. Ryken TC, Clausen JD, Traynelis VC, Goel VK.
Biomechanical analysis of bone mineral density, insertion
technique, screw torque, and holding strength of anterior
cervical plate screws. J. Neurosurg. 1995;83(2):325-329.
79. Hung E, Oliver D, Kim KB, Kyung H, Buschang PH. Effects
of Pilot Hole Size and Bone Density on Miniscrew Implants'
Stability. Clin Implant Dent Relat Res. 2010. Available at:
http://www.ncbi.nlm.nih.gov.ezp.slu.edu/pubmed/20345986
[Accessed August 29, 2010].
80. Wang Z, Zhao Z, Xue J, et al. Pullout strength of
miniscrews placed in anterior mandibles of adult and
adolescent dogs: a microcomputed tomographic analysis. Am J
Orthod Dentofacial Orthop. 2010;137(1):100-107.
44
CHAPTER 3: JOURNAL ARTICLE
Abstract
Purpose: To experimentally study the effects of altering
implant length, outer diameter, cortical bone thickness and
cortical bone density on the primary stability of
orthodontic miniscrew implants (MSIs). Methods: Maximum
insertion torque and pullout strength of MSIs were measured
in synthetic bone with different cortical densities (0.64
g/cc or 0.55 g/cc) and cortical thicknesses (1 mm or 2 mm).
Three different MSIs were evaluated: 6 mm long/1.75 mm
outer diameter; 3 mm long/1.75 outer diameter; 3 mm
long/2.0 mm outer diameter. Insertion torque was measured
at four different time intervals during insertion. To test
pullout strength, a vertical force was applied at the rate
of 5 mm/min until failure occurred. Results: Statistically
significant differences in insertion torque and pullout
strength were found between the 3 MSIs. Post-hoc tests
showed that the 6 mm MSIs had significantly higher (p<
.001) insertion torque and pullout strength values than the
3 mm MSIs. The 3 mm MSIs with a larger outer diameter had
significantly higher (p< .001) insertion torque and pullout
force values than the 3 mm MSIs with a smaller outer
45
diameter. The insertion torque and pullout strengths were
significantly (p< .001) greater for the MSIs placed in
thicker and denser cortical bone; 50% and 14% increases in
thickness and density produced 9-33% increases in insertion
torque and 13-72% increases in pullout strength,
respectively. Conclusion: Both outer diameter and length
affect the insertion torque and pullout strength of the
MSIs. Increases in cortical bone thickness and cortical
bone density increase the primary stability of the MSIs.
Introduction
Orthodontic miniscrew implants (MSIs) have become an
important tool in the orthodontists’ armamentarium for
correcting various orthodontic problems. Their many
advantages include affordability, ease of placement and
removal, potential to provide absolute anchorage, and
placement versatility.1-3 In order to minimize the failure
rates of MSIs, their initial stability must be ensured.
Both screw and the host factors affect the initial
stability of MSIs. The screw factors are related to the
screws’ design, including, but not limited to, their outer
diameter and length.2 The host factors are related to the
46
quantity (cortical thickness) and quality (cortical
density) of the bone into which the screws are placed.3
The orthopedic literature has shown that implant
diameter is one of the most important factors for
maximizing pullout strength.4,5 Screws with a larger outer
diameter have been recommended to increase the holding
power.4 However, there are no well-controlled studies in
orthodontics that examine the effects of outer diameter on
the primary stability of miniscrews.
Length of orthopedic screws has also been shown to
have significant effects on pullout strength, with longer
screws requiring higher forces.6 There is also indirect
evidence that longer MSIs are more stable than shorter
MSIs. Chen et al. increased their success rates from 72% to
90% by using 8 mm instead of 6 mm long screws.7 Other
studies have also reported higher success rates for longer
miniscrews, but were unable to demonstrate statistically
significant differences.8,9 The relationship between MSI
length and primary stability has also not yet been
experimentally examined.
Cortical thickness is one of the primary host factors
influencing insertion torque and, consequently, primary
stability.10 Finite element studies have shown that the
maximum stress occurs at the cortical bone level when an
47
implant is loaded.11 On that basis, Motoyoshi et al. have
recommended that cortical bone at MSI insertion sites
should be at least 1.0 mm thick.12 Greater cortical bone
thickness has also been associated with significantly
greater MSI success.13
It is also well established that bone mineral density
is important for ensuring the stability of endosseous
implants.14 It has been recently shown that bone density is
positively correlated with both insertion torque and
pullout strength, suggesting that bone density could affect
a miniscrew implant’s stability.15,16
While the importances of cortical thickness and
cortical density have been evaluated, how they interact to
influence the primary stability of MSIs still remains
unclear. Moreover, very few studies evaluating MSIs have
evaluated both insertion torque and pullout strength. It is
important to evaluate both because they provide different
information pertaining to primary stability. The ideal MSI
would require minimal insertion torque (less potential bone
damage) and have maximal pullout strength (greater holding
power).
The primary aim of this project was to evaluate the
effects of length and outer diameter on insertion torque
and pullout strength. The secondary aim was to study the
48
effects of cortical bone thickness and density on the
stability of miniscrews.
Materials and methods
Miniscrew implants
Three different miniscrew implants were specifically
fabricated by Dentos® (Daegu, Korea). They were either 3 mm
or 6 mm long; the 6 mm MSIs had an outer diameter of 1.75
mm and the 3 mm MSIs had outer diameters of either 1.75 mm
or 2 mm. The inner diameter of all the MSIs was 1.5 mm with
a 0.5 mm pitch. The apical 1.5 mm portion of both the 6 mm
MSIs and the 3 mm MSIs was tapered. The threaded portion of
the 6 mm and the 3 mm MSIs was 5.5 mm and 2.5 mm,
respectively. All the MSIs were self-drilling and selftapping.
Figure 3.1 MSI # 1 with an outer diameter of 1.75 mm, inner diameter of
1.5 mm and length of 6 mm.
49
Figure 3.2 MSI # 2 with an outer diameter of 1.75 mm, inner diameter of
1.5 mm and length of 3 mm.
Figure 3.3 MSI # 3 with an outer diameter of 2 mm, inner diameter of
1.5 mm and length of 3 mm.
Synthetic bone
Cadaver and animal bone densities vary widely, both
within and between specimens.17 Variation is a widely
50
recognized problem in biomechanical testing. Due to the
uniform material properties of synthetic bone, it has
become the standard for evaluating the primary stability of
both endosseous implants and MSIs because it controls the
variability of bone properties that could affect the
evaluation of the screw’s stability.5,18
Manufactured blocks of synthetic polyurethane
cancellous bone (Sawbones®, Vashon, WA) were used to test
the insertion torque and pullout strength of each MSI. The
blocks were composed of two superimposed layers of
synthetic bone. The density of the cortical layer was
either 0.64 g/cc (40 pcf) or 0.55 g/cc (35 pcf). The
cortical densities were selected to represent the human
bone; the density of the cortex of the mandible has been
reported to be 0.64 g/cc.19 Density of the cancellous layer
was 0.48 g/cc for all of the specimens. The cortical
thickness was either 1 mm or 2 mm. The larger bone blocks
were cut into 11 mm cubes for testing purposes.
Testing groups
The three different types of MSIs were inserted into
four different types of bone (2 densities and 2 cortical
thicknesses), resulting in a total of 12 test groups. There
51
were a total of 216 MSIs, with 18 MSIs randomly allocated
to each group (Table 3.1).
Table 3.1 Groupings for the evaluation of thickness of cortical bone
(CT) and cortical bone density (CD) for the 3 MSIs.
Group 1
Group 2
Group 3
Group 4
MSI # 1
CT = 1 mm
CD = 0.56 g/cc
Group 5
MSI # 1
CT = 1 mm
CD = 0.64 g/cc
Group 6
MSI # 1
CT = 2 mm
CD = 0.56 g/cc
Group 7
MSI # 1
CT = 2 mm
CD = 0.64 g/cc
Group 8
MSI # 2
CT = 1 mm
CD = 0.56 g/cc
Group 9
MSI # 2
CT = 1 mm
CD = 0.64 g/cc
Group 10
MSI # 2
CT = 2 mm
CD = 0.56 g/cc
Group 11
MSI # 2
CT = 2 mm
CD = 0.64 g/cc
Group 12
MSI # 3
CT = 1 mm
CD = 0.56 g/cc
MSI # 3
CT = 1 mm
CD = 0.64 g/cc
MSI # 3
CT = 2 mm
CD = 0.56 g/cc
MSI # 3
CT = 2 mm
CD = 0.64 g/cc
Insertion torque
The synthetic bone cubes were placed in a base so that
all the sides except one were secured. The remaining side
was secured with a jig attached to the base. The jig also
served as a guide for the insertion of the MSIs into the
center of the synthetic bone cubes. The jig was attached to
a motor, which rotated the bone cubes at a constant speed
of nine revolutions per minute.
A drill press was modified to secure the Mecmesin®
Advanced Force and Torque Indicator (Mecmesin, Ltd, West
Sussex, UK) which was used to measure insertion torque. The
52
MSI was lowered on to the rotating bone cube with a 3 pound
weight attached to the arm of the drill press. A video
camera was used to record insertion torque; the video was
later reviewed to determine insertion torque of each MSI at
the point of their initial engagement, and when, 25%, 50%,
75% and 100% of the MSI length had been inserted. The MSI
was considered to be 100% inserted when its entire threaded
portion was in the synthetic bone.
Pullout strength
In order to evaluate the pullout strength, each bone
block, along with an embedded MSI, was placed in a custom
metal base of approximately the same dimensions as the bone
blocks and secured with a lid. Pullout was performed by
attaching an adapter that was made to fit the miniscrew
head. The other end of the adapter was secured to an
Instron machine model 1011(Instron Corp, Canton, MA) and a
vertical force at the rate of 5 mm/min was exerted parallel
to the long axis of the MSI until failure occurred. Peak
load at failure of the MSI was recorded from the Instron
machine in kilograms (kg). The pullout strength of the MSIs
was tested in a random order.
53
Statistical analysis
Skewness and kurtosis statistics showed that the
insertion torque and pullout strength were normally
distributed. Analyses of variance (ANOVA) were performed to
evaluate the effects of cortical thickness, cortical
density, MSI diameter, and MSI length. Due to significant
interaction between cortical thickness and density,
separate analyses of variance (ANOVA) were performed to
compare the three MSIs, followed by Bonferroni post hoc
tests. Separate analyses were also performed for each MSI
to evaluate the effects of cortical thickness and density.
Results
Analyses of variance showed that there were
statistically significant differences in insertion torque
and pullout strength between the 3 MSIs evaluated.
Miniscrew characteristics
Insertion torque of MSIs ranged from 4.64 Ncm to 11.26
Ncm. ANOVA showed that the 6 mm MSIs (MSI # 1) had
significantly higher (p < .001) insertion torque values
than either of the 3 mm MSIs (MSI # 2 or MSI # 3). Compared
with the 6 mm MSIs, the shorter 3 mm MSIs with an outer
diameter of 1.75 mm had insertion torque values that were
54
26-33% lower, depending on the cortical thickness and the
density (Table 3.2). The insertion torques of the 3 mm MSIs
with a 2 mm outer diameter were 15-30% less than the 6 mm
MSIs.
Insertion torque for all three MSIs demonstrated an
approximately curvilinear relationship with the amount of
MSI inserted into bone (Figures 3.4-3.7). The slopes from
the point of initial MSI insertion to 0.75 mm were
approximately the same for all the MSIs. After 0.75 mm of
insertion, torque of the 3 mm MSIs increased substantially
more than the torque of the 6 mm MSIs. The increases of
torque for the larger 3 mm MSIs were greater than the
increases of the smaller 3 mm MSIs for all the bone groups.
Depending on the MSI and material properties of the
bone, pullout forces at failure ranged from 6.7-34.1 kgs.
Pullout force at failure was significantly higher (p <
.001) for the 6 mm MSIs than the 3 mm MSIs (Table 3.3). The
3 mm MSIs with an outer diameter of 1.75 mm had pullout
strengths that were 69-72% lower than the 6 mm MSIs.
Pullout strengths of MSI # 3 were 62-65% less than MSI #1.
Except for the group with a cortical thickness of 1 mm
and a density of 0.64 g/cc, the 3 mm MSIs (MSI # 3) with a
2 mm outer diameter had significantly higher (p < .001)
insertion torque values than the 3 mm MSIs with the 1.75 mm
55
Table 3.2: Analysis of variance (ANOVA) of differences in insertion torque (Ncm) measured at 100% of
insertion of screw between MSI # 1 (6 mm length, 1.75 mm outer diameter), MSI # 2 (3 mm length, 1.75 mm
outer diameter) and MSI # 3 (3 mm length, 2 mm outer diameter) with post-hoc pair wise comparisons
ANOVA
(Prob.)
MSI # 1
MSI # 2
MSI # 3
Mean ±S.D
Mean ±S.D
Mean ±S.D
F value/Prob.
Post-hoc probabilities and
relative changes in %
MSI #
1 vs. 2
MSI #
2 vs. 3
MSI #
1 vs. 3
56
Density
Thickness
0.56
g/cc
<.001;
.001;
<.001;
1 mm
6.43 ± 0.66
4.64 ± 0.54
5.44 ± 0.72
34.80/< .001
↓27.84%
↓14.71%
↓15.4%
0.56
g/cc
<.001;
.001;
<.001;
2 mm
10.19±0.75
7.22 ± 0.66
8.24 ± 0.88
69.91/< .001
↓29.15%
↓12.38%
↓19.14%
Density
Thickness
0.64
g/cc
<.001;
.437;
<.001;
1 mm
9.21 ± 1.07
6.14 ± 0.36
6.46 ± 0.66
90.10/< .001
↓33.33%
↓4.95%
↓29.86%
39.38/< .001
<.001;
↓25.67%
.048;
↓8.82%
<.001;
↓18.47%
0.64
g/cc
2 mm
11.26± 0.98
8.37 ± 0.62
9.18 ± 1.3
Table 3.3: Analysis of variance (ANOVA) of differences in pullout strength (kgs.) measured at 100% of
insertion of screw between MSI # 1 (6 mm length, 1.75 mm outer diameter), MSI # 2 (3 mm length, 1.75 mm
outer diameter) and MSI # 3 (3 mm length, 2 mm outer diameter) with post-hoc pair wise comparisons
Density
Thickness
0.56
g/cc
1 mm
0.56
g/cc
2 mm
57
Density
0.64
g/cc
0.64
g/cc
MSI # 1
MSI # 2
MSI # 3
ANOVA
(Prob.)
Post-hoc probabilities and
relative changes in %
Mean ±S.D
Mean ±S.D
Mean ±S.D
F value/Prob.
MSI #
1 vs. 2
23.79 ± 1.45
6.67 ± 0.79
8.47 ± 1.07
1234.15/
< .001
<.001;
<.001;
<.001;
↓71.96%
↓21.25%
↓64.40%
27.43 ± 1.36
8.61 ± 0.61
9.94 ± 0.86
2017.361/
< .001
<.001;
.001;
<.001;
↓68.61%
↓13.38%
↓63.76%
26.02 ± 1.78
8.15 ± 0.76
10.01 ± 0.78
1142.05/
< .001
<.001;
<.001;
<.001;
↓68.68%
↓18.58%
↓61.53%
34.11 ± 0.82
9.78 ± 0.61
11.91 ± 0.81
5780.34/
< .001
<.001;
<.001;
<.001;
↓71.33%
↓17.88%
↓65.08%
MSI #
2 vs. 3
MSI #
1 vs. 3
Thickness
1 mm
2 mm
Insertion Torque (Ncm)
7
6
5
4
3
Miniscrew # 1
2
Miniscrew # 2
Miniscrew # 3
1
0
0
0.75
1.5
2.25
3
3.75
4.5
5.25
6
6.75
Amount of MSI inserted into bone (mm)
Figure 3.4 Changes in insertion torque (Ncm) from initial
engagement (0 mm) to 100% engagement for cortical bone thickness of
1 mm and density of 0.56 g/cc.
Insertion Torque (Ncm)
12
10
8
6
4
Miniscrew # 1
Miniscrew # 2
2
Miniiscrew # 3
0
0
0.75
1.5
2.25
3
3.75
4.5
5.25
6
6.75
Amount of MSI inserted into bone (mm)
Figure 3.5 Changes in insertion torque (Ncm) from initial
engagement (0 mm) to 100% engagement for cortical bone thickness
of 2 mm and density of 0.56 g/cc.
58
10
Insertion Torque (Ncm)
9
8
7
6
5
4
3
Miniscrew # 1
2
Miniscrew # 2
Miniscrew # 3
1
0
0
0.75
1.5
2.25
3
3.75
4.5
5.25
6
6.75
Amount of MSI inserted into bone (mm)
Figure 3.6 Changes in insertion torque (Ncm) from initial
engagement (0 mm) to 100% engagement for cortical bone thickness of
1 mm and density of 0.64 g/cc.
Insertion Torque (Ncm)
12
10
8
6
4
Miniscrew # 1
Miniscrew # 2
2
Miniscrew # 3
0
0
0.75
1.5
2.25
3
3.75
4.5
5.25
6
6.75
Amount of MSI inserted into bone (mm)
Figure 3.7 Changes in insertion torque (Ncm) from initial
engagement (0 mm) to 100% engagement for cortical bone thickness of
2 mm and density of 0.64 g/cc.
59
outer diameter (MSI # 2). Differences ranged from 0.80-1.02
Ncm or 9-15% (Table 3.2). The maximum pullout force at
failure was also significantly higher (p≤ .001) for the
larger 3 mm MSIs than the smaller 3 mm MSIs (Table 3.3).
The smaller 3 mm MSIs had pullout strengths that were 1321% lower than the larger 3 mm MSIs.
Bone characteristics
The insertion torque of MSIs placed in synthetic bone
with 2 mm cortical thickness was significantly (p< .001)
greater than the torque of MSIs placed in 1 mm thick
cortical bone (Table 3.4). Differences in insertion torques
between 2 mm and 1 mm thick cortical bone (↓50%) were 2.91
Ncm (↓27.10%), 2.40 Ncm (↓30.82%) and 2.76 Ncm (↓31.73%)
for MSIs #1, #2 and #3, respectively.
MSIs placed in synthetic bone with a density of 0.64
g/cc showed significantly (p< .001) higher insertion torque
values than the same MSIs placed in bone with a density of
0.56 g/cc. Differences related to density (↓14%) for MSIs
#1, #2 and #3 were 1.92 Ncm (↓18.78%), 1.33 Ncm (↓18.32%)
and 0.98 Ncm (↓12.53%) less, respectively (Table 3.4).
Changes in insertion torque of the three MSIs over
time
showed
a
curvilinear
pattern,
with
the
greatest
increases in torque occurring later (Figures 3.8-3.10).
60
Table 3.4 Two-way analysis of variance to evaluate the effects of cortical bone thickness and density on
insertion torque (at 100% of insertion) for each MSI group
Comparing the differences in insertion torque for MSI # 1 (6 mm length, 1.75 mm outer diameter)
Effect
F-value
Probability
Diff. in insertion torque (in Ncm)
Thickness
158.54
< .001
1 mm has 2.91 Ncm (↓27.10%) less IT than 2 mm
Density
69.39
< .001
0.56 g/cc has 1.92 Ncm (↓18.78%)less IT than 0.64 g/cc
Comparing the differences in insertion torque for MSI # 2 (3 mm length, 1.75 mm outer diameter)
61
Effect
F-value
Probability
Diff. in insertion torque (in Ncm)
Thickness
330.01
< .001
1 mm has 2.40 Ncm (↓30.82%) less IT than 2 mm
Density
100.37
< .001
0.56 g/cc has 1.33 Ncm (↓18.32%) less IT than 0.64 g/cc
Comparing the differences in insertion torque for MSI # 3 (3 mm length, 2 mm outer diameter)
Effect
F-value
Probability
Diff. in insertion torque (in Ncm)
Thickness
163.64
< .001
1 mm has 2.76 Ncm (↓31.73%) less IT than 2 mm
Density
20.60
< .001
0.56 g/cc has 0.98 Ncm (↓12.53%) less IT than 0.64 g/cc
Table 3.5 Two-way analysis of variance to evaluate the effects of cortical bone thickness and density on
pullout strength (kgs.) for each MSI group
Comparing the differences in pullout strength for MSI # 1 (6 mm length, 1.75 mm outer diameter)
Effect
F-value
Probability
Diff. in pullout strength (in kgs.)
Thickness
190.23
< .001
1 mm has 5.87 kgs. (↓19.07%) less POS than 2 mm
Density
109.82
< .001
0.56 g/cc has 4.46 kgs. (↓14.82%) less POS than 0.64 g/cc
Comparing the differences in pullout strength for MSI # 2 (3 mm length, 1.75 mm outer diameter)
62
Effect
F-value
Probability
Diff. in pullout strength (in kgs.)
Thickness
117.15
< .001
1 mm has 1.78 kgs. (↓19.40%) less POS than 2 mm
Density
64.76
< .001
0.56 g/cc has 1.33 kgs. (↓14.83%) less POS than 0.64 g/cc
Comparing the differences in pullout strength for MSI # 3 (3 mm length, 2 mm outer diameter)
Effect
F-value
Probability
Diff. in pullout strength (in kgs.)
Thickness
65.08
< .001
1 mm has 1.69 kgs. (↓15.43%) less POS than 2 mm
Density
70.78
< .001
0.56 g/cc has 1.76 kgs. (↓16.06%) less POS than 0.64 g/cc
50%
25%
12
75%
100%
Insertion Torque (Ncm)
Thickness 1 mm; Density 0.56 g/cc
10
Thickness 2 mm; Density 0.56 g/cc
Thickness 1 mm; Density 0.64 g/cc
Thickness 2 mm; Density 0.64 g/cc
8
6
4
2
0
0
8
16
24
32
40
48
56
64
72
Time (seconds)
Figure 3.8 Temporal changes in insertion torque (Ncm) from the
initial engagement of 6 mm MSI (MSI # 1) through 25%, 50%, 75%
and 100% of the length.
25%
9
50%
75%
100%
Insertion Torque (Ncm)
8
Thickness 1 mm; Density 0.56 g/cc
Thickness 2 mm; Density 0.56 g/cc
7
Thickness 1 mm; Density 0.64 g/cc
6
Thickness 2 mm; Density 0.64 g/cc
5
4
3
2
1
0
0
4
8
12
16
20
24
28
32
Time (seconds)
Figure 3.9 Temporal changes in insertion torque (Ncm) from the
initial engagement of 3 mm MSI (MSI # 2) through 25%, 50%, 75%
and 100% of the length.
63
36
25%
10
Insertion Torque (Ncm)
9
50%
75%
100%
Thickness 1 mm; Density 0.56 g/cc
8
Thickness 2 mm; Density 0.56 g/cc
Thickness 1 mm; Density 0.64 g/cc
7
Thickness 2 mm; Density 0.64 g/cc
6
5
4
3
2
1
0
0
4
8
12
16
20
24
28
32
36
Time (seconds)
Figure 3.10 Temporal changes in insertion torque (Ncm) from the
initial engagement of 3 mm MSI (MSI # 3) through 25%, 50%, 75%
and 100% of the length.
0.9
Correlation coefficient
0.8
p< .001
0.7
p< .001
0.6
p< .001
0.5
0.4
0.3
0.2
0.1
0
MSI # 1
MSI # 2
MSI # 3
Type of miniscrew
Figure 3.11 Correlations between
strength by miniscrew type
64
insertion
torque
and
pullout
Changes in insertion torque of the three MSIs were similar
during initial insertion.
Compared to a 2 mm cortical thickness, the pullout
strengths of the MSIs #1, #2 and #3 placed into the 1 mm
thick cortex were 5.87 kgs (↓19.07%), 1.78 kgs (↓19.40%) and
1.69
kgs
(↓15.43%) lower, respectively
(Table
3.5).
The
differences were all statistically significant (p< .001).
Pullout
strength
was
also
significantly
(p<
.001)
greater in high density than low density synthetic bone.
The mean differences in pullout strengths between 0.64 g/cc
and 0.56 g/cc densities were 4.46 kgs (↓14.82%), 1.33 kgs
(↓14.83%) and 1.76 kgs (↓16.06%) for MSIs #1, #2 and #3,
respectively (Table 3.5).
Intercorrelations
Insertion torque and pullout strength were
significantly and positively intercorrelated for MSI # 1 (r
=.730; p< .001), for MSI # 2 (r=.809; p< .001) and for MSI
# 3 (r=.649; p< .001) (Figure 3.11).
Discussion
The 3 mm MSIs with an outer diameter of 2 mm provided
greater primary stability than the 3 mm MSIs with an outer
diameter of 1.75 mm. For bone with a cortical density of
65
0.56 g/cc, the 3 mm MSIs that were narrower had insertion
torques that were 12.4-14.7% lower than the wider 3 mm
MSIs. These findings are in agreement with the other
studies in the literature citing the importance of outer
diameter for primary stability of MSIs.5,20,21 Lim et al.
found that the change in the outer diameter had a greater
effect on insertion torque than the length and shape of the
MSI.21 The wider 2 mm outer diameter MSIs have a greater
surface area than the 1.75 mm outer diameter MSIs, and
therefore have to displace more bone during insertion. Such
an increase in diameter might be expected to increase
friction at the bone-screw interface leading to greater
insertion torques.
However, the same screws showed much smaller
differences (5-8.8%) in insertion torque with the higher
density cortical bone. Thus, increases in purchase power
associated with increases in outer diameter of MSIs have
less of an effect on denser bone.
This is important
because the differences in pullout resistance related to
MSI diameter were greater than the differences in insertion
torque, especially in the more dense cortical bone. This
suggests that the pullout resistance is more a function of
the amount of the bone packed between the threads of the
MSIs than insertion torque.
Thus the shorter, wider, MSIs
66
appear to be particularly well suited in denser cortical
bone; they provide substantially greater resistance to
pullout without sacrificing insertion torque.
This again
emphasizes the importance of measuring both insertion
torque and pullout in experiments for optimizing MSI
designs.
Although the insertion torque values of the 3 mm MSIs
were less than the 6 mm MSIs, they are well above the
recommended value of 4 Ncm needed to provide sufficient
anchorage for miniscrew implants.22 More importantly, the
pullout forces of the 3 mm MSIs were also well above the
range of orthodontic forces typically applied. In
orthodontics, the forces required for tooth movement and
skeletal changes can range from approximately 0.3-4N (0.030.40 kgs) and 4.9-9.8N (0.5-1 kgs), respectively.23,24 For
orthodontic applications, it has been suggested that
pullout strength of screws tested in the axial mode should
be reduced by 34% to more closely approximate the pullout
strengths of the same screws tested in the tangential
(cantilever) mode.25 Even after decreasing pullout strength
by 34%, the 3 mm MSIs used in the present study can still
be expected to withstand orthodontic loads. Short 3 mm
mandibular MSIs loaded with 900 or 600 grams of force in
dogs showed success rates of 60%.26 Using rabbits, Liu et
67
al. loaded the 3 mm MSIs with forces ranging from 50 to 200
gm and had an overall success rate of 88%.27
The results of this study indicate that the wider 3 mm
MSIs provide a feasible alternative to the 6 mm MSIs.
Clinically, the advantage of a wider screw is its ability
to distribute applied forces over greater amounts of bone.
The drawbacks of longer screws pertain to the surrounding
anatomic limitations.28,29 A small increase in the outer
diameter of a 3 mm MSI significantly compensates for the
reduction in its length; the short MSI has significantly
lower insertion torque without compromising its pullout
strength.
The 3 mm MSIs with an outer diameter of 1.75 mm had
pullout strength values that were 68-72% less than the 6 mm
MSIs whereas the pullout strengths of the wider 3 mm MSIs
were 61-65% less than the 6 mm MSIs. The substantial
differences in the pullout strength between the 3 mm and
the 6 mm MSIs can be primarily attributed to the cancellous
bone. A greater portion of the longer MSI is anchored in
the cancellous bone. Although less dense than cortical
bone, anchorage in cancellous bone further increases the
purchase power of these miniscrew implants.30
Significantly greater insertion torque and pullout
strength were found with an increase in cortical thickness.
68
Differences in cortical thickness had a greater effect on
insertion torque than pullout strength. Decreasing cortical
thickness by 50% (2 mm vs. 1 mm) produced insertion torque
values that were 27-32% lower and pullout strength values
that were 15-19% lower. This corroborates the findings of
Motoyoshi et al.13 and Huja et al.3 who showed decreases in
insertion torque and pullout strength with decreases in
cortical thickness. Initial stability following MSI
insertion is therefore enhanced by greater cortical bone
thickness. Since the effects of cortical thickness were
relatively greater on insertion torque than pullout
strength, care must be taken when MSIs are placed in
excessively thick cortical bone. For example, it has been
shown that the cortical bone in the mandibular posterior
region can be up to 3 mm thick31; this might be expected to
increase strains and microfractures during insertion, which
could affect healing and compromise secondary stability.
Insertion torque of MSIs placed in high density
cortical bone was 13-19% greater than the torque of screws
placed in low density cortical bone; pullout strength was
15-16% greater. Increases in insertion torque with greater
bone density has been previously described in the
prosthodontic and orthodontic literature.15,16,32,33 Greater
bone density implies greater bone quantity, which requires
69
higher torsional forces to advance the MSIs during
insertion.18 Greater amounts of bone also increase the
amount of bone-to-implant contact and greater engagement of
bone by MSI threads, both of which contribute to increases
in the pullout strength.32
After 0.75 mm of MSI insertion, substantial increases
in the torque of the 3 mm MSIs as compared to the 6 mm MSIs
created a curvilinear pattern. Similar curvilinear patterns
were found by Lim et al. when evaluating the changes in
insertion torque due to changes in length and outer
diameter.21 This curvilinear pattern is created due to the
wider portion of the MSIs being inserted later into the
bone as compared to the tapered initial part. Significant
increases in the torque of the 3 mm MSIs as compared to the
6 mm MSIs can be explained by the differences in the length
of the lever arm. A force applied at a right angle to a
lever multiplied by its distance from the lever's fulcrum
(i.e. the length of the lever arm) is its torque. It can be
hypothesized that this difference in the length of the
lever arm is equivalent to the difference in the length of
the 3 mm and the 6 mm MSIs.
Based on the above findings, the 3 mm MSIs are
recommended at sites having narrow interradicular spaces
and in situations when teeth are moving through the bone,
70
as in the mixed dentition stages.28,31 While both of the 3 mm
MSIs show distinct advantages when place in denser bone,
the wider MSI might provide the extra holding power
necessary for thinner cortical bone.
Conclusions
1. There is a significant difference in the maximum
insertion torque and pullout strength between the 3 mm and
6 mm MSIs. The smaller 3 mm MSIs had insertion torque and
pullout strength values that were 26-33% and 69-72% lower,
respectively, as compared to the 6 mm MSIs.
2. An increase in outer diameter plays a significant
role in increasing the primary stability of miniscrews. The
smaller 3 mm MSIs had insertion torque and pullout strength
values that were 12-14% and 13-21% lower, respectively, as
compared to the larger 3 mm MSIs.
3. Insertion torque and pullout strength increased
significantly as cortical thickness increased. Decreasing
cortical thickness by 50% produced insertion torque values
that were 27-32% lower and pullout strength values that
were 15-19% lower.
4. Insertion torque and pullout strength also
increased significantly as bone density increased.
Decreasing cortical density from 0.64 g/cc to 0.56 g/cc
71
produced insertion torque values that were 12-19% lower and
pullout strength values that were 15-16% lower.
72
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76
VITA AUCTORIS
Ankit H. Shah was born on May 27, 1980 in Ahmedabad,
India to Mina and Hasmukh Shah. He attended Government
Dental College & Hospital, A’bad from 1998 through 2003
where he received his Bachelor of Dental Surgery degree.
From 2003 to 2006, he continued his studies at the same
dental school by pursuing further education in the
specialty of orthodontics. In 2007, he received his Master
of Dental Surgery degree in orthodontics. Afterward, he
decided to move to the United States for higher education.
It is anticipated that in January 2011 Ankit will graduate
from Saint Louis University with a Master of Science degree
in Dentistry with an emphasis in orthodontics and will
enter private practice in Texas.
77