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Impact of Orthodontic Mini-Screw Angulation Relative to Direction of Force Application on Stability, Movement, and the Peri-implant Interface by Dr. Michael Patrick O’Toole A thesis submitted in conformity with the requirements for the degree of Master of Science Department of Graduate Orthodontics Faculty of Dentistry University of Toronto © Copyright by Dr. Michael P. O’Toole, 2011
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Impact of Orthodontic Mini-Screw Angulation Relative to
Direction of Force Application on Stability, Movement,
and the Peri-implant Interface
by
Dr. Michael Patrick O’Toole
A thesis submitted in conformity with the requirements
for the degree of Master of Science
Department of Graduate Orthodontics
Faculty of Dentistry
University of Toronto
© Copyright by Dr. Michael P. O’Toole, 2011
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Impact of Orthodontic Mini-Screw Angulation Relative to
Direction of Force Application on Stability, Movement,
and the Peri-implant Interface
Dr. Michael P. O’Toole
Master’s of Science Degree
Department of Graduate Orthodontics
Faculty of Dentistry
University of Toronto
2011
Abstract:
The purpose of this study was to determine the impact of insertion angle of
orthodontic mini screws on the stability and resistance to movement of the mini screw,
and on the peri-implant interface. Three orthodontic mini screws were placed in each
tibia of six New Zealand white rabbits bilaterally (N=36), with randomized angulation
(65° away, 65° toward, or 90° to the direction of applied force). After two weeks, two
orthodontic mini screws within each tibia were loaded with a 200g Nitinol closed-coil
spring for up to 14 days. No statistically significant differences were found among the
variably angulated loaded and unloaded orthodontic mini screws in the amount of
movement or change in angulation demonstrated over the experimental period. Micro
CT analysis revealed no clinically significant differences in the amount of cortical bone-
to-implant contact. Mini screw placement angulation seems to have minimal impact on
stability and migration of orthodontic mini screws over time.
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Acknowledgements:
I would like to extend my sincere gratitude to all those who helped me with this project.
Drs. B.Tompson, J. Daskalogiannakis, and S.-G. Gong , committee members who
provided continuous support and encouragement throughout my orthodontic education. I
cannot thank you enough for your valuable guidance and insightful comments throughout
the past three years.
Dr. J.E. Davies, external committee member for his advice, particularly with the micro
CT work, and the use of his lab.
Mrs. Susan Carter, for all of her efforts during the animal experimentation portion of this
project.
3M Unitek Canada, for their generous donation of all the orthodontic mini screws and
other necessary materials.
My classmates, Matt, Joanie, and Mandeep, for three wonderful years.
My parents, who have provided significant guidance and help over the years in order that
I may realize my goals.
Last, to my fiancée, and bride to be, Melissa, whose unconditional love, support, and
understanding has allowed me to pursue my dreams. To her, I dedicate my thesis.
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Table of Contents:
Abstract ii
Acknowledgements iii
Table of Contents iv
List of Tables vi
List of Figures vii
List of Acronyms ix
Literature Review
Introduction 1
Failure Rates of Orthodontic Mini Screws 4
Implant Design Relative to Stability of Orthodontic Mini Screws 14
Insertion Technique Relative to Stability of Orthodontic Mini Screws 27
Cortical Bone Thickness Relative to Stability of Orthodontic Mini Screws 37
Osseointegration of Orthodontic Mini Screws 44
Immediate Versus Delayed Loading of Orthodontic Mini Screws 50
Movement of Orthodontic Mini Screws 54
Impact of Angulation on Stability of Orthodontic Mini Screws 61
Purpose of the Study 69
Research Questions 69
Hypotheses 70
Pilot Study 71
Materials and Methods
Animal Model 74
Facilities 75
Study Design 75
Orthodontic Mini Screw Insertion (Initial Surgery) 77
Treatment Regimen 79
Fluorescent Bone Labeling 81
Micro CT scan 81
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Analysis 82
Results
Orthodontic Mini Screw Retention 84
Movement of Orthodontic Mini Screws 84
Micro CT analysis 91
Discussion 95
Conclusion 106
References 108
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List of Tables:
Page
1. Success rates for several different orthodontic mini screws with variable
loading regimens as reported in the orthodontic literature 5
2. Bone-to-implant contact (BIC) values from recently reported studies in
the dental literature examining variably loaded orthodontic mini screws 46
3. Measures of cortical bone thickness along rabbit tibia proximal segment 72
4. Sample sizes for each of the orthodontic mini screw orientations 85
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List of Figures:
Page
1. CT images of rabbit tibia; proximal antero-medial surface encircled 71
2. Dissection of proximal anteromedial surface of rabbit tibia 73
3. Post-insertion orientation of unloaded control and two test mini-screws 73
4. Post-insertion orientation of the three mini-screws 73
5. Orientation of mini screws in relation to applied orthodontic forces 76
6. Initial incision into rabbit tibia with soft tissue reflection of periosteum 77
7. Placement of angulated orthodontic mini screws 77
8. Determination of inter-implant distance to ensure uniform loading 78
9. Placement of stainless steel reference pin 78
10. Setup and positioning for cone beam CT scans 79
11. Exposure of orthodontic mini screws and placement of Ni-Ti spring 79
12. Timeline of experimental protocol and analysis 80
13. Average movements of variably angulated orthodontic mini screws as
measured from the head of the mini screw 86
14. Average movements of variably angulated orthodontic mini screws as
measured from the mini screw body at the cortical bone surface level 86
15. Average movements of variably angulated orthodontic mini screws
as measured from the apex of the mini screw 87
16. Average movements of variably angulated loaded orthodontic
mini screws relative to unloaded controls as measured from the
head of the mini screw 88
17. Average movements of variably angulated loaded orthodontic
mini screws relative to unloaded controls as measured from the
mini screw body at the cortical bone surface level 88
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18. Average movements of variably angulated loaded orthodontic
mini screws relative to unloaded controls as measured from the
head of the mini screw 88
19. Average angulation changes of variably angulated orthodontic
mini screws relative to the cortical bone surface 90
20. Overall mean displacement of the orthodontic mini screws as measured
from the mini screw head, body, and apex 91
21. Mean percent cortical bone-to-implant contact of variably angulated 92
orthodontic mini screws
22. Micro CT image of orthodontic mini screw in association with 92
thickened cortical bone
23. Micro CT image of a longitudinal slice through the threaded portion of
the orthodontic mini screw illustrating the high degree of bone-to-
implant contact 93
24. 3D rendering of an experimental orthodontic mini screw traversing
through the cortical bone. Pink regions denote bone-to-implant contact,
whereas green zones depict areas void of bone 93
25. Micro CT image depicting the presence of a “micro-crack” within the 94
cortical bone adjacent to the orthodontic mini screw
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List of Acronyms:
BIC Bone-to-implant contact
BIL Bone-implant length
BIR Bone-implant contact ratio
BSBA Bracket screw bone anchor
CBCT Cone beam computed tomography
CT Computed tomography
GCTF Gingival connective tissue fibers
IPT Implant placement torque
ISQ Implant stability quotient
Ni-Ti Nickel-titanium
PLGA Poly lactic-co-glycolic acid
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Introduction:
During orthodontic treatment, proper anchorage is often crucial for a successful
outcome. However, the traditional use of dental anchorage typically results in
undesirable movement of the anchor teeth. Attempts to overcome this problem have led
to the creation of multiple extraoral, intraoral, tooth- and/or tissue-borne devices. In
recent years, skeletal anchorage provided by orthodontic mini implants and mini screws
has attracted much attention as an ideal alternative for maintaining anchorage.1 An
orthodontic implant is any implant used during orthodontic treatment as anchorage for
orthodontic tooth movement.2 However, there is no universally agreed upon
nomenclature for orthodontic mini screws, a subset of orthodontic implants, as
dimensions and typology vary, but most consist of a diameter between 1- 2.5 mm and
variable lengths from 6- 15mm.3 These various mini implant and mini screw systems
provide significantly greater anchorage control in comparison to other treatment
modalities, such as headgear.4
Thiruvenkatachari et al. (2006) compared the amount of anchorage loss of first
permanent molars with and without the use of orthodontic mini-screw anchorage during
canine retraction. Ten orthodontic patients underwent therapeutic extraction of first
premolars. Orthodontic mini screws (1.3mm diameter and 9mm length) were randomly
placed in the maxilla and mandible on one side of the arch between the first molar and
second premolar. Nickel-titanium coil springs (100g force) were placed from the implant
to the canine on the implant-anchored side, and between the molar and the canine on the
molar-anchored side for a period of four to six months. Superimposition of pre-treatment
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and post-treatment cephalograms showed no mesial migration of the first molar on the
implant-anchored side. However, the molar-anchored side yielded a mean anchorage loss
of 1.6mm in the maxilla and 1.7mm in the mandible (range: 1mm to 2mm).5 In another
related article, Thiruvenkatachari et al. (2008) also found that canine retraction proceeded
at a faster rate when orthodontic mini screws (1.2mm diameter and 9mm length) were
used for anchorage. Again, cephalometric superimpositions revealed that mean rates of
canine retraction were 0.93mm per month in the maxilla and 0.83mm per month in the
mandible on the implant-anchored side, and 0.81mm per month in the maxilla and
0.76mm per month in the mandible on the molar-anchored side. Orthodontic mini screws
are able to not only maximize anchorage, but may also slightly enhance the rate of tooth
movement. Albeit, the differences are very small.6
A number of published reports highlight successful treatment outcomes with the
use of orthodontic mini implants.7, 8
In a randomized controlled trial of forty patients
exhibiting bialveolar dental protrusion that underwent extraction of all first premolars,
Upadhyay et al. (2008) compared the treatment outcomes for retraction of anterior teeth
by conventional means versus en-masse retraction with pre-drilled orthodontic mini
screws (1.3mm diameter and 8mm length) placed between the first molars and second
premolars in all quadrants. The orthodontic mini screws prevented any anchorage loss,
and permitted intrusion of the first permanent molars. The facial vertical dimension was
also significantly reduced in conjunction with forward rotation of the mandible for the
group treated with orthodontic mini screws. Although the soft-tissue response was
variable, greater positive changes were reported in the orthodontic mini screw treatment
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group. Facial convexity angle, nasolabial angle, and lower lip protrusion all exhibited
greater changes compared to the group treated without orthodontic mini screws.8
Beyond their effectiveness, additional reasons suggested for the increasing use of
orthodontic mini implants and mini screws relate to their versatility in a variety of clinical
applications, minimal surgical invasiveness, independence from patient cooperation, and
relatively low cost.1, 9
Scholz and Baumgaertel (2009) have recently suggested that the
strong body of evidence-based research, involving both the basic sciences and clinical
applications of orthodontic mini-screws, is responsible for their surge in popularity. The
same authors are of the opinion that the use of orthodontic mini screws is not a fad, but
rather a successful treatment adjunct, that is quickly becoming an integral part of post-
graduate orthodontic education and clinical practice.10
As mentioned, the published orthodontic literature contains a significant number
of articles examining various factors associated with orthodontic mini screws. As a
result, the section on the review of the literature is rather extensive and complicated. To
aid the reader, a brief summary containing a critical interpretation of the published
evidence by topic is included at the end of each section (in italics).
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Failure rates of orthodontic mini screws:
For orthodontic mini screws to be a successful alternative for anchorage, some
issues must be resolved to increase their efficacy. Prosthodontic implants generally have
high clinical success rates, though there is variability. Bornstein et al. (2007) and Khayat
et al. (2007) collectively examined 986 prosthodontic implants of variable diameters and
reported cumulative success rates of 98.6% over two years, and 99.3% over three years,
respectively.11, 12
In comparison, failure rates of orthodontic mini screws cited in the
literature are highly variable, with most ranging between 10% and 30% (table 1).3, 13-18
In
perhaps the largest review of published clinical trials, Crismani et al. (2010) examined
the outcomes of fourteen studies involving 452 patients, and a total of 1519 orthodontic
mini screws of various designs (table 1). The mean overall success rate was 83.8% +/-
7.4%, but mini screws with lengths shorter than 8mm and diameters of less than 1.2mm
appeared to compromise success rates even further.13
Antoszewska et al. (2009) reported
an unusually high success rate of 93.43% after retrospectively examining 350 self-
tapping orthodontic mini screws (187 Abso Anchor mini-screws, Dentos, Daegu, South
Korea; and 163 Ortho Easy Pin mini screws, Forestadent, Pforzheim, Germany) placed,
with pre-drilling, in the maxilla and mandible of 130 patients for a variety of orthodontic
purposes.19
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In a preliminary study involving the placement of thirty-six self-tapping
orthodontic mini screws (Jeil Medical Corp., South Korea), in both maxilla and mandible,
and followed to a maximum time of 425 days, Fritz et al. (2004) found a corresponding
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failure rate of 30%. Five additional orthodontic mini screws became mobile, but
continued to meet their anchorage requirements and were not evaluated as failures. In
this study, no statistical correlations were possible due to the small sample size, but the
author noted that three of the failures occurred in patients that were heavy smokers. It
was also found that clinician experience pertaining to insertion techniques of orthodontic
mini screws is an important factor, since failure rates displayed a tendency to decrease
with increasing duration of the study.20
Another retrospective study found that
orthodontic mini screw failures were associated with the specific mini screw type, area of
placement, and patient age.21
Motoyoshi et al. (2007), found that 36.2% of orthodontic
mini screws loaded within the first month of placement in adolescents failed.22
Chen et
al. (2007) postulated that the decreased failure rates demonstrated in adult patients can be
attributed to an age-related increase in bone density and cortical bone thickness providing
greater mechanical retention.21
Motoyoshi et al. (2010) also suggested that age and
cortical bone thickness were significant factors affecting stability of orthodontic mini
screws. However, cortical bone thickness was only correlated with placement torque in
the maxilla (Pearson correlation coefficient, r= 0.392, p< 0.05), and not the mandible (r=
-0.019). The authors suggested that this was due to the exclusive use of a bone drill in
the mandible to perforate the thicker cortical bone. In addition, age was inversely
correlated with placement torque (r= -0.287), and the authors speculated that this was due
to a decrease in bone density with increasing age.23
This contrasts suggestions made by
Chen et al. (2007).21
This study only provides indirect evidence, since placement and
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removal torques were examined, but not specific failure rates of individual orthodontic
mini screws.23
Other investigators have also suggested that application of excessive forces on the
mini screws, a large lever arm, peri-implantitis when inserted in the unattached mucosa,
insufficient primary stability, and bone damage on insertion (frictional necrosis) all
contribute to orthodontic mini screw failures.24
Cho et al. (2010) examined the effects of
rotation moments on the stability of thirty-six immediately loaded (either 1Ncm or 2Ncm
Ni-Ti closed coil springs) orthodontic mini screws (1.45mm diameter; 7mm length; OAS-
1507C, Biomaterials Korea Inc., Seoul, Korea) placed in the mandibular buccal alveolar
bone of six adult male beagle dogs for a period ranging up to twelve weeks. The lever
arms (7mm length) associated with each mini screw were randomly assigned to produce
either a clockwise or a counterclockwise moment. Three of the mini screws undergoing
2Ncm of orthodontic load in a counterclockwise direction failed. In addition, bone-to-
implant contact was significantly less (p< 0.05) for mini screws receiving
counterclockwise moments. The authors suggested that counterclockwise rotations may
impair stability of orthodontic mini screws leading to gradual loosening.25
Furthermore,
Wilmes et al. (2008) have shown that insertion angle of orthodontic mini screws plays an
important role in their primary stability. An insertion angle ranging from 60º to 70º was
optimal and the improved stability was likely due to greater engagement of the more rigid
cortical bone.26
Lim et al. (2009) retrospectively examined 378 orthodontic mini screws of
varying dimensions in 154 patients over a three-year period. One type of mini screw
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(Osteomed screw, Osteomed, Dallas, Tex) with a 1.2mm diameter and variable lengths of
4, 6, 8, 10, and 12mm required pre-drilling. Another mini screw brand (OSAS, Epoch
Medical, Seoul, Korea) of 1.6mm diameter and 6, 8, and 9 mm lengths was placed with a
drill-free method and had a straight profile. The third mini screw (Orlus, Ortholution,
Seoul, Korea) had a diameter of 1.8mm and variable lengths of 6, 7, 8, 10, and 12mm.
This orthodontic mini screw was also self-drilling and had a tapering profile. All mini
screws used in the study were placed with a mucoperiosteal incision prior to insertion.27
The overall success rate for all orthodontic mini screws involved was 83.6%.
Success rates in the maxilla (86.0%) were ten percent higher than in the mandible with
100% of mini screws placed in the palate remaining stable throughout their duration of
use. Also, those mini screws placed in unattached mucosa had success rates (88.0%) that
were only 2.7% lower than those placed in attached gingiva (90.7%).27
This difference
was insignificant compared to that reported by other authors.24
Orthodontic mini screws
that were 8mm in length had the highest overall success rate (87.6%) regardless of
diameter. However, this difference was not statistically significant. In fact, there was no
statistically significant association of any of the risk factors examined in this study as
they relate to stability of orthodontic mini screws.27
Viwattanatipa et al. (2010) undertook a survival analysis of ninety-seven
orthodontic mini screws (1.2mm diameter, 8mm- 12mm lengths) placed with pre-drilling
in the maxilla of forty-nine patients. The loading regimen ranged from immediate to
delayed by up to six months. An orthodontic force of either 175g or 200g was
subsequently applied to all mini screws. Cumulative survival rates were 86% at six
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months, but dropped to 57% by twelve months post-placement. After six months the
failure rate effectively doubled from approximately 2.6 failures per 100 mini screws each
month to 5.2 failures per 100 mini screws each month. Also, the authors found those
mini screws buried subcutaneously requiring a second exposure surgery had a risk-ratio
17.66 times greater than mini screws placed and left exposed to the oral environment. At
one year, only 38% of the mini screws placed in a two-stage procedure were present,
whereas, 84% of mini screws placed by means of a one-stage surgery survived.28
The
authors note that this contrasts conventional placement of prosthodontic dental implants
where a two-stage procedure does not hinder success rates, and may even improve upon
the chances of survival.29
In addition, orthodontic mini screws placed in loose non-
keratinized mucosa had a risk ratio of 8.63, suggesting that their hazard of failure was
763% higher versus mini screws placed in attached keratinized tissue. Tissue
inflammation about the mini screws also appeared to increase the likelihood of failure,
but there was no association between patient age and mini screw failure.28
This contrasts
findings of other studies, but the small sample size of this study may prevent detection of
any differences in mini screw success rates relative to patient age.21, 22
Recently, Asscherickx et al. (2008) examined the success rates of mini screws
relative to their vertical distance from the alveolar crest and proximity to adjacent roots.
Twenty bracket screw bone anchors (BSBAs) with 1.7mm diameter and 6.0mm length
(titanium bone screw: Leibinger-Stryker, BmgH & Co, Freiburg, Germany; titanium
0.018 slot bracket: Ormco, Orange, California, USA) were placed with pre-drilling of a
pilot hole through the cortical bone in five male beagle dogs and subsequently followed
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to a maximum of 25 weeks post-insertion. The BSBAs were either immediately loaded
or delayed (6 weeks) in loading. Eleven BSBAs were inserted within 1.0mm of the
alveolar crest and nine of these failed. Five of six BSBAs placed in direct contact with a
tooth root, as observed histologically, failed. A defect in the tooth root was visible, as
was repair of the cementum lining. Furthermore, all BSBAs placed both in contact with a
root surface and less than 1.0mm away from the alveolar crest failed. Lastly, only one of
five BSBAs placed within 1.0mm of a root surface failed. This single BSBA was also
less than 1.0mm away from the level of the alveolar crest, and this may account for the
failure. Therefore, as long as there was no contact present between the adjacent root
surface and the BSBA and the distance to the bony crestal ridge was more than 1.0mm,
the success rate was 100% in this study. However, the author admits that no firm
conclusions can be drawn from these results due to the small sample examined, but the
trends are very suggestive that both proximity to the alveolar crest and root surface
contact during placement of orthodontic mini screws are additional risk factors.30
Finite
element analysis of bone stress when an orthodontic mini implant is close to the roots of
adjacent teeth corroborates the above findings. The von Mises stress (yielding of
materials under multi-axial loading conditions) increased as the distance between the
implant and the adjacent root surface decreased. However, the stress was significantly
greater only when the implant touched the adjacent root surface. When contact occurred
stress increased to 140 MPa or more, and bone resorption could be predicted. The
stresses generated also varied based on cortical bone thickness.31
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Zheng et al. (2009) investigated the influence of a recent tooth extraction in
proximity to a pre-existing orthodontic mini screw. Ninety-six mini screws with 1.6mm
diameter and 6mm length (Medicon Company, Tuttlingen, Germany) were placed in the
mandible, 6mm below the height of the alveolar crest, of six male beagle dogs. The dogs
were grouped based on allotted healing time of the mini-screws: 1 week, 3 weeks, and 8
weeks. The mini screws of the test group were placed proximal to the third and fourth
premolar, whereas, those in the control group were placed in the interradicular bone of
the second premolar and first molar. The third and fourth premolars were extracted in
each jaw at the time of placement for all orthodontic mini screws.32
Upon histologic examination after week one, an inflammatory reaction, involving
primarily neutrophils and macrophages, was visible at the bone-implant interface in both
test and control groups, but there was a much stronger expression of this reaction along
the mini screws of the test group, nearest the extraction sites. Two mini screws in the test
group were found to be loose during this time period, and those mini screws nearest the
extraction sites had lower maximal removal torques versus control mini screws.
However, by week three, those mini screws that survived in the test group had greater
maximal removal torques, with a larger number of osteoblasts secreting a bony matrix
along the implant surface. This difference disappeared by week eight, as there was no
significant difference in removal torques or bone-implant contact between the two
groups. Therefore, orthodontic mini screws placed in the vicinity of recent extraction
sites are at greatest risk of failing under applied orthodontic loads during the first week.
However, the risk is negated by at least the third week after placement.32
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Of interest, Baek et al. (2008) examined success rates of reinstalled self-drilling
and self-tapping conical orthodontic mini screws (ORTHOplant, Zbiomaterials Korea,
Seoul, South Korea) with 2.0mm collar diameter and 5.0mm length placed
interproximally in the posterior maxilla of fifty-eight patients. An orthodontic force of
less than 200g was applied no sooner than two weeks post-placement of the 109 mini
screws. When failure (defined as loss and or mobility of the orthodontic mini screw in
less than eight months or before treatment was completed) occurred, the new mini screw
was installed either at the same area within four to six weeks, or placed immediately, but
at an adjacent site. There was no statistically significant difference in terms of success
rates between those mini screws initially installed (75.2%) and those reinstalled (66.7%).
Of those mini screws replaced, nineteen were placed in the same position as the mini
screw that failed. From this batch, only thirteen (68.4%) remained stable. In addition,
fifteen mini screws were placed immediately after failure, but in an adjacent location, and
their success rate was 53.3%. The mean duration of use for those mini screws initially
placed (10.0 months) was significantly longer than for reinstalled mini screws (6.4
months). The author also noted that 77.0% of the mini screws originally placed at the
start of the study failed within the first three months. However, the results of this study
must be judged with caution since the pooled data contained inconsistent methods for
replacement of mini screws, some being re-implanted at the same site and others at an
alternate site. The time of replacement was also variable, ranging from immediate to six
weeks post- failure.33
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Success rates of orthodontic mini screws are highly variable, but generally
remain lower in relation to prosthodontic dental implants. The orthodontic literature
pertaining to survival of orthodontic mini screws can be summarized into five general
parameters: patient selection, site of placement, implant design, insertion technique, and
loading regimen. Orthodontic mini screw failure rates remain higher in adolescent
patients, but cortical bone thickness and bone maturity may be the reason. Cortical bone
thickness, and proximity to the alveolar crest, the adjacent periodontal ligament space,
and regions of recent tooth extraction appear to have a significant influence on success
of orthodontic mini screws. There exists a controversy in the orthodontic literature as to
whether or not placement in attached keratinized gingiva versus loose alveolar mucosa
has a significant effect on survival of orthodontic mini screws. Some authors have
suggested that implant dimensions have an impact on success rates of orthodontic mini
screws. However, the influence of length appears to have a negligible effect in relation
to diameter. Narrow diameter orthodontic mini screws have an increased failure rate.
Placement technique decisions, including angulated placement, ideal insertion torque
values, pre-drilling versus self-drilling, and one versus two stage placement procedures,
are also significant factors. However, practitioner experience may supersede all of these
protocols. Lastly, loading factors such as the use of immediate versus delayed loading,
magnitude of the applied force levels, the use of long lever arms, and application of
clockwise versus counter-clockwise moments all contribute to the potential viability of
orthodontic mini screws.
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Implant design relative to stability of orthodontic mini screws:
Several recent studies have examined the biomechanical properties of various
orthodontic mini screws that contribute to primary stability.24, 34, 35
Wilmes et al. (2006)
examined the parameters affecting the primary stability of several orthodontic mini
screws. The five mini screws included were the Tomas®-pin 08 and 10 (Dentaurum,
Ispringen, Germany; 1.6mm x 8mm, and 1.6mm x 10mm respectively) and three Dual
Top anchor screws of variable length and diameter (Jeil Medical Corporation, Seoul,
Korea; 1.6mm x 8mm, 1.6mm x 10mm, and 2.0mm x 10mm). One-thousand mini screw
insertions were undertaken with variable pre-drilling in the ilium of country pigs and the
insertion and removal torques were measured.34
The Dual Top screws with a diameter of 2mm achieved significantly greater
primary stability with a median relative insertion torque of 158.7 +/- 45.2. The narrow
diameter (1.6mm) Dual Top screws followed, with only a minimal difference in median
relative insertion torques for the 8mm and 10mm lengths (89.0 +/- 33.2 and 91.2 +/-27.6
respectively). The Tomas®-pin types produced much smaller median relative insertion
torques (8mm Tomas®-pin: 24.8 +/- 16.8; 10mm Tomas
®-pin: 29.2 +/- 14.7). The
authors suggest that one apparent reason for the decreased primary stability of the
Tomas®-pin is the cylindrical shape of the intra-osseous portion of the mini screw. Mini
screws with a conical design appear to achieve greater primary stability.34
Mischkowski et al. (2008) found similar results when comparing the primary
stability of four different orthodontic mini screws: Aarhus Anchorage screw (Medicon
eG, Tuttlingen, Germany); FAMI screw (Gebrüder Martin GmbH & Co. KG, Tuttlingen,
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Germany); Dual Top Anchor screw (Jeil Medical Corporation, Seoul, Korea); and Spider
screw (HDC Company, Sarcedo, Italy). Approximately thirty mini screws of each type
were placed with pre-drilling in bovine femur. Insertion torque values were measured and
pull-out tests were performed at angles of 0º, 20 º, and 40 º to the long axis of the mini
screws. The conical screws, Dual Top and FAMI, achieved the highest maximal
insertion torques, with an average value of 40.22 Ncm (+/- 6.51) and 22.67 Ncm (+/-
3.82), respectively. The average maximal insertion torques for the cylindrical screws,
Spider and Aarhus, were significantly lower (19.34 Ncm (+/- 4.14) and 16.07 Ncm (+/-
1.89), respectively). Pull-out tests revealed the influence of thread design on primary
stability and peak load values. The Dual Top and Spider screws, with longer thread
lengths achieved higher values for peak loads during pull-out testing, but this difference
decreased under increasingly angular loads, up to 40º from the long axis of the mini
screws. The authors concluded that longer threads may lead to greater axial peak loads,
but do not provide advantages under angular loading.35
Lim et al. (2008) reported similar results relative to the external (outer) diameter
of orthodontic mini screws in relation to insertion torque. An unknown number of
conical and cylindrical ELI mini screws (Biomaterials) with differing internal and
external diameters, causing variations in the thread depth, were placed in solid rigid
polyurethane foam (Sawbones; Pacific Research Laboratories Inc, Wash). However, E-
Glass-filled epoxy sheets with variable thickness (1.0mm, 1.5mm, and 2.0mm) were
attached, with acrylate bond, over top the artificial bone block to simulate the presence of
cortical bone. Insertion torque values were recorded every tenth of a second. The
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authors only analyzed the changes of external diameter in the cylindrical mini screws.
Nonetheless, it was found that changes in the external diameter, not the internal diameter,
of the orthodontic mini screws had the most profound effect on insertion torque values.36
This study failed to delineate the effects of angular loading on the primary stability of
orthodontic mini screws with variable thread depths (external diameter) as discussed by
Mischkowski et al. (2008).35
However, the results showed differences in insertion
torques during placement between cylindrical and conical mini screws. The former
appear to maintain a relatively high insertion torque throughout placement with only a
slight increase over the last few threads. The conical mini screws initially exhibited low
insertion torque values during placement, but significantly higher insertion torque values
occurred over the final portion of screw threads. Therefore, cylindrical mini screws were
found to have greater overall insertion torques during placement, until near the
completion of insertion when the threads on the parallel portion of the tapered mini
screws engaged the cortical bone, establishing final insertion torque values greater than
that achieved with cylindrical mini screws.36
These findings are comparable to those of
other published studies.34-38
Kim et al. (2008) also compared the stability of cylindrical and conical
orthodontic mini screws with 1.6mm collar diameter and 6mm length (Jeil Medical
Corporation, Seoul, Korea). Maximum insertion torques and maximum removal torques
were measured for twenty mini screws (10 of each type) placed in solid rigid
polyurethane foam. In addition, sixteen mini screws from both test groups were placed in
the maxilla (buccal and palatal) and mandible (buccal only) of two beagle dogs. An
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orthodontic load of 200g to 300g was applied one-week post-insertion for the study
duration of seventeen weeks.38
The conical mini screws yielded significantly higher maximum removal torques
(5.16 Ncm +/- 0.85) compared to the cylindrical mini screws (3.47 Ncm +/- 0.71).
Maximum insertion torques were also higher for the conical group (16.61 Ncm +/- 0.42).
The authors speculate that excessive insertion torque may cause over-compression,
increasing the chance for micro-fractures and ischemia of the surrounding bone, leading
to increased failure rates. However, in this study there were no statistically significant
differences in failure rates between the cylindrical and conical mini screws (18.75% and
25.0% respectively). Also, resonance frequency analysis performed on the mini screws
placed in the beagle dogs revealed no significant difference in stability over the duration
of the study period.38
Florvaag et al. (2010) examined five self-drilling and self-tapping mini-screw
types (FAMI 2, Orlus mini implant, T.I.T.A.N. Pin, Tomas®-pin, and Vector TAS) with
variable diameters ranging from 1.6mm to 2.0mm, and minimum lengths of 8mm.
Overall, one hundred and ninety six mini screws were placed, with and without pilot hole
preparation in thirty bovine femoral heads, utilized for the striking similarity in cortical
bone thickness relative to human maxillary and mandibular alveolar cortices. All mini
screws were inserted perpendicular to the bony surface, but pull-out testing was
performed at three inclinations relative to the long axis of the mini screw: axially, 20°,
and 40°. The three cylindrical mini screw designs (FAMI 2, T.I.T.A.N. Pin, and
Tomas®-pin) placed with drill-free insertion achieved the highest axial pull-out values
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(802.1 N, 763.5 N, 886.5 N respectively). The cylindrical mini screws also exhibited the
greatest mean values for pull-out tests performed at 20° angulations. However, it was the
cylindrical mini screws that showed the most significant decrease in pull-out resistance.
At 40° angulation, the pull-out test results were comparable amongst the different mini
screw types with the exception of the Tomas®-pin, which still maintained higher pull-out
test values (632.0 N). 39
The findings of this study contrast those of other authors.34, 35, 38
However, it is unclear how the authors were able to control for the significant variability
in length and diameter of the various orthodontic mini screws. In addition, this study
only examined the primary stability of orthodontic mini screws.
Morarend et al. (2009) placed titanium orthodontic screws (KLS Martin,
Jacksonville, Florida) in twenty-four hemi-sected maxillae and mandibles from human
cadavers. A total of forty-eight large-diameter (2.5mm diameter; 17mm length) and
twenty-four small-diameter (1.5mm diameter; 15mm length) orthodontic screws were
placed monocortically, with an additional twenty-four small diameter (1.5mm diameter;
17mm length) mini screws placed bicortically between the first and second premolars in a
random distribution. The orthodontic screws were also varied by placement in an apical
or coronal position from the alveolar crest. All screws were placed with prior cortical
pre-drilling at an angle perpendicular to the buccal bone surface. Each orthodontic screw
underwent an applied orthodontic load, perpendicular to the long axis of the screw, with
an Instron diametral materials testing machine at a pre-determined position of 3mm from
the screw-bone interface. The large-diameter screws exhibited significantly greater mean
anchorage force values compared to the small-diameter screws placed monocortically in
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both the maxilla and mandible when a deflection up to 0.6mm was applied. However, the
smaller diameter screws placed bicortically provided resistance similar to, or greater than,
the large-diameter screws placed through a single layer of cortical bone when undergoing
similar deflections.40
Miyawaki et al. (2003) also found similar results after retrospectively comparing
success rates of various diameter orthodontic mini screws with mini-plates in the maxilla
and mandible of fifty-one patients that were subsequently loaded with an applied
orthodontic force of less than 2N. All ten orthodontic mini screws with a 1.0mm
diameter and 6mm length failed in this study, despite the relatively high success rates for
the other test groups. The second group, consisting of one hundred and one orthodontic
mini screws (1.5mm diameter; 11m length), had an 83.9% success rate over the one-year
study period. This was comparable to the twenty-three largest diameter orthodontic mini
screws (2.3mm diameter; 14mm length) utilized, reporting a success rate of 85.0%.41
In another study, Wilmes et al. (2008) investigated various mini screw parameters
amongst twelve different implant types of varying diameters and lengths.24
As
demonstrated in previous studies, conical mini screws performed superiorly to cylindrical
designs.34
Again, the diameter of the orthodontic mini screws also had a significant
impact, whereas the influence of mini screw length was negligible. Insertion torques
dramatically increased with larger intra-osseous shaft diameters. Interestingly, the
ORLUS mini screw showed the greatest insertion torques (median: 183.65 Nmm,
variance: 192.79). The author speculates that this was due to the large inner diameter of
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the threaded part in the region around the implant neck that would engage the cortical
bone, prior to significant tapering toward the tip of the mini screw.24
Brinley et al. (2009) examined insertion torques and pull-out strengths of sixty
self-drilling and self-tapping orthodontic mini screws (1.8mm diameter; 6mm length)
with variable thread designs placed in synthetic and cadaver bone models. The thread
pitch was altered to three different angulations: 0.75mm, 1.0mm, and 1.25mm. In
addition, a portion of the mini screws with 1.0mm pitch had three longitudinal flutes that
spanned the entire length of the threaded portion. Each flute was 0.225mm wide and
their depth extended to the core of the mini screw. These mini screws were thread
cutting since the flutes had cutting surfaces to facilitate placement and removal.42
Orthodontic mini screws with 0.75mm thread pitch exhibited greater primary
stability. Pull-out resistance in the synthetic model (Mean: 22.16N) was significantly
greater than that demonstrated by the other test groups (1.0mm Mean: 10.8N, 1.25mm
Mean: 12.70N). Even though, overall insertion torques and pull-out strengths in the
cadaver bone model were not significantly different amongst the test groups, those mini
screws with 0.75mm thread pitch displayed a consistent tendency for higher insertion
torques and pull-out strengths. The authors recognized that unaccounted variability in
bone quality and quantity (cortical bone thickness), especially within the cadaver bone
model, likely contributed to non-statistical differences amongst the groups.42
Fluted orthodontic mini screws provided significantly greater insertion torques
and pull-out strengths in both synthetic bone (p<0.001 and p<0.001 respectively) and
cadaver bone models (p<0.001 and p<0.027 respectively). The authors speculated that
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fluting causes segmentation of the threads providing greater flexibility and decreased
stiffness, thus more closely mimicking the properties of the surrounding bone. This is
thought to provide more uniform stress distribution, decreasing localized areas of
increased strain with bone.42
The addition of microstructures to orthodontic mini screws was also investigated
in an effort to enhance success rates through enhanced bony interactions and soft tissue
adaptation. Kim et al. (2008) examined the effects of microgrooves (50um pitch and
10um depth) placed along the coronal neck of thirty-two orthodontic mini screws with
1.6mm diameter and 6mm length (Jeil Medical Corporation, Seoul, Korea). The mini
screws were placed without pre-drilling into the dentate areas of the maxilla and
mandible in two beagle dogs. An orthodontic load of 200g to 300g was applied one week
after insertion until the end of the study period (17 weeks).43
A 6.25% overall failure rate was found for mini screws with microgrooves,
whereas 25% of mini screws lacking microgrooves around the gingival collar failed.
However, due to the small sample size of the study, a statistically significant difference in
failure rates could not be reached. In addition, those mini screws lacking microgrooves
displayed significantly less bone-implant contact (23.39% +/- 9.10) on the pressure side
than the tension side (44.37% +/- 23.59). Mini screws with microgrooves did not exhibit
a significant difference in bone implant contact between corresponding pressure and
tension sides (40.08% +/- 16.85 and 41.63 +/- 14.17 respectively). Statistical analysis
also indicated a significant difference when comparing bone-implant contact on the
pressure sides between mini screws with and without microgrooves (p<0.01).43
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The authors also noted a change in the alignment of the gingival connective tissue
fibers (GCTFs) between test groups. GCTFs normally tend to organize parallel to the
smooth mini screw surface. However, the mini screws with microgrooves had GCTFs
oriented perpendicular or oblique to the machined collar of the mini screw. This may be
beneficial in preventing epithelial downgrowth along the threads of orthodontic mini
screws. 44
Therefore, the addition of microgrooves may have some beneficial effects on
the soft tissue and bone adaptation around the collars of orthodontic mini screws.43
Another study examined microthreads with a pitch approximately one-half that of
the regular threads, on an unspecified number of tapered, self-drilling, and self-tapping
orthodontic mini screws (Jeil Medical Corporation, Seoul, Korea) with a diameter of
1.6mm and variable lengths (6mm and 8mm). The study also compared cylindrical
versus tapered designs of orthodontic mini screws, but of importance was the data
obtained from the insertion and removal of the dual-thread orthodontic mini screws in the
solid rigid polyurethane foam (Sawbones, Pacific Research Laboratories Inc., Vashon,
Washington) with density of 30 pcf. This material is commonly used to test the
mechanical properties of dental implants and orthodontic mini screws, but it is not
representative of the clinical scenario. Cortical bone is the denser portion of human
alveolar bone, and is responsible for bearing most of the applied load. Unlike the
homogenous polyurethane blocks, there is an uneven distribution of retentive forces
throughout the adjacent bone tissue, with the thicker, but less dense, layer of underlying
cancellous bone contributing minimally.45
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The standard tapered orthodontic mini screws exhibited the highest maximum
insertion torque (p<0.001) when compared with the cylindrical and dual-thread tapered
mini screw test groups. On the other hand, the dual-thread tapered group showed the
greatest maximum removal torque (p<0.001). This is important because maximum
removal torque is thought to better represent primary stability of orthodontic mini screws
when compared to maximum insertion torque.40
Also, the dual threads prevented a
dramatic increase in insertion torque as the mini screws were increasingly embedded in
the polyurethane blocks. This modification in mini screw design may aid in preventing
excess harm to the surrounding bone and also minimize the risk for implant fracture.
However, the greater number of threads found on dual-thread mini screws increases the
number of rotations, and time required to embed the mini screws within bone, which, in
turn may place greater stress on adjacent bony structures.45
Chaddad et al. (2008) examined the role of surface characteristics on primary
stability and survival rates of orthodontic mini screws. Seventeen machined smooth
titanium Dual-Top (Jeil Medical Corporation, Seoul, Korea) orthodontic mini screws
(1.4mm, 1.6mm, and 2.0mm diameters; 6.0mm, 8.0mm, and 10.0mm lengths) and fifteen
sandblasted, large grit, acid-etched surface treated mini screws (C-implant, Implantium
Inc, Seoul, Korea) with a 2mm polished collar (1.8mm diameter; 8.5mm, 9.5mm, and
10.5mm lengths) were placed in ten patients. Pre-drilling of the cortical bone was done
prior to insertion for all mini screws, and a torque ratchet was used in placement to
determine insertion torque values. Immediate loading of all mini-screws was performed
with a 50- 100g force (NiTi coil-spring or elastic chain), which was increased to 250g of
-24-
applied force after two weeks. There were no statistically significant differences in
primary stability or survival rates over the 150-day study period between those mini
screws with and without surface treatment to enhance osseointegration. However, the
survival rate for mini screws with a surface treatment was 93.5%, compared to 82.5% for
the machined smooth mini screws. The small sample size and variability in dimensions
of the mini screws utilized make it difficult to draw straightforward conclusions, although
it appears that surface characteristics do not significantly influence survival rates of
immediately loaded orthodontic mini screws.46
Ikeda et al. (2011) also compared orthodontic mini screws fabricated with either a
sandblasted, large-grit, and acid-etched surface (n= 21) or a machined smooth surface (n=
21). All forty-two orthodontic mini screws (IMTEC Corporation, Ardmore, Oklahoma),
with 1.8mm diameter and 6mm length, were randomly placed with a split-mouth design
in the interradicular areas of the mandibular first and second molars in seven mature male
foxhound dogs. Within each animal, all six orthodontic mini screws were placed, with
pilot hole preparation, and a digital torque driver aligned perpendicular to the cortical
bone surface. Two orthodontic mini screws per side were immediately loaded with 200g
Ni-Ti closed coil springs for a period of nine weeks. Afterwards, the animals were
euthanized and the orthodontic mini screws were carefully removed with trephination of
the surrounding bone. The orthodontic mini screws were analyzed with microcomputed
tomography scans.47
Surface treated mini screws exhibited a 100% success rate, whereas, the machined
smooth surface mini screws had an 85.7% success rate. However, the only control mini
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screw to fail was found to have been placed into the inferior alveolar canal. The two
loaded mini screws that failed survived until termination of the study, but were clinically
mobile with no greater than 1mm of movement. Microcomputed tomography revealed
that surface treated mini screws maintained three to six percent more cortical bone and
approximately nine percent more non cortical bone along the implant length (p < 0.05).
The authors found that there was no statistically significant difference in placement
torque between the groups. Removal torque was not analyzed, but the authors suggest
that higher torque values are of little apparent consequence.47
Lin et al. (2010) utilized finite element modeling to examine the effects of
changes in orthodontic mini screw design (material, length, diameter, thread shape,
thread depth, head diameter, and head exposure length). The properties that decreased
bone strain or created a more even distribution of von Mises strain in the surrounding
bony tissues were mini screw material type, exposure length, and diameter. Von Mises
strain examines the three dimensional deformation that can occur at a given point within
an object in relation to yield stress (failure). Based on the Taguchi method, it was
determined that material type elicited the greatest contribution (63%) in determining bone
strain. Titanium alloys provided more uniform strain versus biodegradable mini screw
materials, such as poly lactic-co-glycolic acid (PLGA). Also, as the mini screw head
increasingly protruded from the cortical bone surface (exposure length) a greater moment
arm was created upon application of the orthodontic load. Exposure length had the
second greatest contribution to strain distribution (24%). Orthodontic mini screw
diameter also markedly affected strain values in cortical bone (7%). All other
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parameters, including changes in thread design and shape, provided contributions to the
model of no greater than 2%. However, the authors noted that the finite model did not
account for the differences in properties between cortical and underlying cancellous bone,
and thus their results must be taken with caution.48
Orthodontic mini screw design is highly variable, but certain features have a
significantly greater impact on stability than others. Orthodontic mini screws composed
of titanium alloy (Ti-6Al-4V) seem to have significantly improved stability compared to
those made of other materials, such as stainless steel. The diameter of the portion of the
mini screw that traverses the cortical bone layer has a profound impact on the success of
the mini screws, whereas implant length has a negligible effect. Furthermore, increasing
thread depth (difference between external thread diameter and body diameter) has a
greater influence on stability than a similar increase in the body diameter of the
orthodontic mini screw. “Bench top” studies suggest that conical mini screws provide
greater stability than cylindrical mini screws. However, clinical studies have not found a
significant difference in success between these two designs. Shallow thread pitch in the
collar region of the orthodontic mini screw that engages the cortical bone layer has also
proven advantageous. Vertical flutes spanning the threaded portion of the orthodontic
mini screws provide greater flexibility of the threads and improved stress distribution to
the surrounding bone. The addition of a variety of microstructures, such as
microgrooves and microthreads were also shown to enhance stability of the orthodontic
mini screws. This was achieved by enhancing either bone-to-implant contact or by
improving the gingival collar around the neck of the mini screws.
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Insertion technique relative to stability of orthodontic mini screws:
Thread designs of orthodontic mini screws have continuously changed allowing
for self-tapping placement in a pre-drilled position, and recently self-drilling whereby
pilot holes are no longer required prior to placement.49, 50
The diameter of pre-drilling
sites was crucial as the larger the pilot hole diameter, the lower the primary stability of
the orthodontic mini screw.34, 50
Wilmes et al. (2009) examined the impact of pre-drilling
diameter on primary stability of three hundred Dual Top (Jeil, Korea) orthodontic mini
screws with 1.6mm diameter and 10mm length. The osseous sites in iliac bone segments
of twelve pig cadavers, with variable cortical bone thickness (0.5mm to 3.0mm), were
prepared to a uniform 3mm depth with different pre-drilling diameters of 1.0mm, 1.1mm,
1.2mm, and 1.3mm. The mini screws were also inserted at variable depths of 7.5mm,
8.5mm, and 9.5mm with insertion torques recorded from twenty-five replicates
performed for each combination of pre-drilling diameter and insertion depth.51
Both insertion depth and pre-drilling diameter had a drastic influence on insertion
torque, even when discrepancies in cortical bone thickness were accounted for. The
overall mean insertion torque for the 1.0mm site preparation was 83.50Nmm (+/- 33.56),
and this decreased for the 1.1mm pilot hole to 77.5Nmm (+/- 27.54). The mini screws
placed in the 1.2mm and 1.3mm pilot holes elicited a mean insertion torque of
61.70Nmm (+/-28.46) and 53.10Nmm (+/- 32.18) respectively. This trend was similar
for all insertion depths, with increases in the pre-drilling diameter being associated with a
significant decrease in the insertion torque of the orthodontic mini screws.51
However,
only the larger pre-drilling diameters (1.2mm and 1.3mm) with a shorter insertion depth
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of 7.5mm yielded mean insertion torques (39.10Nmm +/-18.35 and 29.80Nmm +/-19.07
respectively) that were consistently lower than that recommended by the literature.51, 52
Heidemann et al. (1998) examined the relationship of pilot hole (pre-drilling) size
relative to “holding power” of titanium osteosynthesis screws (1.5 mm and 2.0 mm
diameter) placed in discs of polyvinylchloride, wood, and porcine mandibular bone with
variable thicknesses (range: 2- 4 mm). Pilot hole diameters were continually increased
from 66% to 95% of external screw diameter as the screws underwent torque
measurements and pull-out testing. Pooled mean critical pilot hole diameter was
approximately 85% (range: 79%- 91%) of the external screw diameter. Beyond this
point, a rapid decrease in “holding power” was found to occur. Unfortunately, this study
neglected to report the depth of pilot hole preparation.53
Gantous et al. (1995) undertook a similar study comparing the “holding power” of
1.0 mm, 1.5 mm, and 2.0 mm diameter Synthes self-tapping fixation screws placed in
blocks of laminated phenolic resin (Delron, grade DF 105, Formica Limited) of variable
thickness (1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, and 5.0 mm). Ten replicates were arranged
for pull-out testing for each combination of screw diameter, pilot hole diameter, and
Delron block thickness. Again, the depth of pilot hole preparation was not reported. As
expected, the pull out force was found to significantly increase with increasing Delron
block thickness. More importantly, it was found that pilot hole size could be increased to
0.85 mm (85% of external screw diameter) for 1.0 mm diameter screws with no
appreciable decrease in holding power during pull-out testing. However, screw fracture
routinely occurred with the thicker Delron blocks. Therefore, this data was excluded
-29-
from examination and a critical pilot hole diameter could not be found. For 1.5 mm and
2.0 mm screws, there was no significant loss in “holding power” with increasing pilot
hole diameter up to 82% and 83% of external screw diameter, respectively. Beyond the
critical pilot hole diameters described, a decrease in “holding power” resulted with 1.5
mm diameter fixation screws exhibiting a sharper decline versus the 2.0 mm diameter
fixation screws.54
This study presented similar critical pilot hole diameter outcomes,
relative to the external screw diameter, to those demonstrated by Heidemann et al.
(1998).53, 54
Newer drill-free (self-drilling) screws have a pointed tip and some also have a
specially formed cutting flute that enables them to be inserted without any osseous site
preparation. Heidemann et al. (2001) examined the peri-implant interface of self-tapping
and drill-free designs for both orthodontic mini screws and micro screws (typically
1.5mm diameter or less) placed in female Göttingen minipigs. Pilot holes were only
drilled prior to placement of self-tapping screws. Microradiographic and histologic
analysis of the twenty screws placed revealed that drill-free screws elicited the greatest
mean bone-to-metal contact (mini screws: 88.4% +/- 2.9; micro screws: 93.8% +/- 3.0).
For self-tapping micro screws it was 81.0% +/- 5.9, and the five self-tapping mini screws
demonstrated the least bone-to-metal contact with a mean value of 54.9% +/- 14.8.
Fluorescence microscopy revealed that significantly more of the residual bone was found
in the region of the screw threads placed with a drill-free technique (mini screws: mean
71.8% +/- 13.7; micro screws: mean 67.9% +/- 7.0) versus those self-tapping screws
placed in a pre-drilled site (mini screws: mean 33.1% +/- 16.9; micro screws: mean 42.5
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+/- 9.5).49
In a similar study by Kim et al. (2005), thirty-two self-drilling orthodontic
mini screws (1.6mm diameter) were inserted into the jaws of two beagle dogs. Sixteen
mini screws were placed with pre-drilling, whereas the remainder had no osseous site
preparation. Nickel-titanium coil springs (200g to 300g force) were applied after one
week and left active during the eleven-week study period. Orthodontic mini screws that
were self-drilling showed significantly more bone-to-metal contact and overall less
mobility as demonstrated through measurement with the Periotest™ (Siemens AG,
Bensheim, Germany).50
Wu et al. (2008) also evaluated the differences between the pre-drilling and drill-
free methods for mini screw placement. Thirty-six orthodontic mini screws (1.0mm
diameter and 6mm length) were placed in the posterior maxilla of six beagle dogs, with
pre-drilling for only eighteen of the mini screws. The mini screws were left unloaded to
heal for a variable duration (2, 4, or 6 weeks) prior to evaluation. Both qualitative and
quantitative histologic assessments were made in addition to pull-out testing. The mean
pull-out forces after two weeks and four weeks were significantly higher in the drill-free
group (312.85N +/- 89.89 and 380.57N +/- 68.04 respectively) than in the pre-drilling
group (196.41N +/-81.34 and 250.73N +/- 71.71 respectively). After eight weeks there
were no significant differences between the drill-free (457.37N +/- 80.90) and pre-drilled
(392.93N +/- 67.41) mini screws. As with previous studies, the mean amount of bone-
implant contact was significantly different (p<0.05) between the pre-drilled and drill-free
groups. However, after the eight-week healing period the difference was not significant
(drill-free: 70.34% +/- 8.85, pre-drilled: 58.94% +/- 11.59).55
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Another important aspect when placing orthodontic mini screws is insertion
torque. Motoyoshi et al. (2006) examined the implant placement torque (IPT) of 124
orthodontic mini screws (ISA orthodontic implants, BIODENT Co. Ltd, Tokyo, Japan)
with 1.6mm diameter and 8mm length that were placed in the buccal alveolar bone of
forty-one orthodontic patients. Pre-drilling to a depth of 8mm was performed prior to
placement of the mini screws. All orthodontic mini screws were placed with a torque
screwdriver (N2DPSK, Nakamura MFG Co. Ltd) that yielded IPT values with three
percent accuracy according to the manufacturer’s specifications. Each orthodontic mini
screw was subsequently loaded with an applied orthodontic load of less than 2N for a
period lasting up to six-months.52
The authors found that an IPT in the range of 5Ncm to 10Ncm was ideal for this
specific orthodontic mini screw, yielding an overall success rate of 96.2%. However,
when IPT was less than 5Ncm success rates decreased to 72.7% overall. Similarly, when
IPT increased beyond 10Ncm success rates significantly decreased to 60.9%. It was
concluded that a low IPT is suggestive of poor primary stability and eventual failure of
the orthodontic mini screw. Alternately, a very high IPT likely places significant stresses
on the surrounding bone leading to bone degradation or frictional necrosis.52
However,
the validity of IPT as an indirect measure for primary stability is questionable. A study
by Degidi et al. (2009) examined the IPT of seventeen prosthetic dental implants of
variable manufacturers removed from patients for a variety of reasons. Although the
study design was of limited quality, the authors concluded that there was no statistically
significant correlation between IPT and bone-implant contact (p= 0.892). Two possible
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reasons for the lack of correlation were provided: primary stability is not only influenced
by bone volume, but also by density and thickness of the cortical layer; or there is no true
relationship between bone structure and insertion torque values.56
Motoyoshi et al. (2007) also studied the relationship between IPT, cortical bone
thickness, and other relevant factors on stability of orthodontic mini screws. Eighty-
seven orthodontic mini screws (1.6mm diameter, 8mm length) were placed in the
posterior alveolar bone of the maxilla and mandible of thirty-two orthodontic patients
(age range:14.6yrs- 42.8yrs). Mini screws were deemed to be successful in the absence
of pain or clinically detectable mobility after having been subjected to orthodontic force
for a minimum of six months. A significantly higher success rate (100%) resulted when
IPT was maintained within 8 Ncm to 10 Ncm, in comparison to those groups with higher
or lower placement torque ranges.57
Motoyoshi et al. (2010) examined IPT as it relates to removal torque for 109
machine-surfaced orthodontic mini screws (ISA orthodontic implants, Biodent, Tokyo,
Japan; 1.6mm diameter, 8mm length) placed in the buccal alveolar bone distal to either
the second premolar or the second molar of fifty-two orthodontic patients (age range:
13.9- 63.5years) with pilot hole preparation. For those mini screws placed in the
mandible pre-drilling with a larger diameter pilot hole was undertaken. Immediately
after mini screw placement a maximal orthodontic force of 2N was applied for a mean
time period of 23.1 months (SD 6.7). No overall correlation between IPT and removal
torques was obsrved in this study. However, after classifying the mini screws into 3
categories based on IPT (Group 1: 0-5Ncm, Group 2: 5-10Ncm, and Group 3: 10-
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15Ncm), unique relationships between IPT and removal torque became evident. It was
found that there was no statistically significant difference between IPT (3.67Ncm, SD
1.03) and removal torque (4.31Ncm, SD 2.03) for the low torque mini screws (Group 1).
On the other hand, torque values dropped significantly after clinical use from IPT to
removal torque measures in group two (7.89Ncm, SD 1.47 to 4.33Ncm, 2.00) and group
three (11.41Ncm, SD1.03 to 3.83Ncm, SD 2.26). Mini screws placed with an
increasingly higher IPT underwent a greater loss in torque value, though this appeared to
stabilize around 4Ncm. The authors suggested that IPT may be indicative of primary
stability, whereas removal torque may be related to surface properties of orthodontic mini
screws and correlated with long-term (secondary) stability. It was also recommended
that a torque of 4 Ncm was sufficient to maintain clinically acceptable anchorage. It
appears that the article does not entirely support this notion. Rather, this study may
suggest that, in most instances, removal torque values will “normalize,” as long as a
minimum initial IPT is achieved.23, 58
During placement of orthodontic mini screws, knowledge of the proximity to
adjacent root surfaces is of utmost importance. As discussed, Ascherickx et al. (2008)
found that those mini screws placed within 1.0mm of the alveolar crest of interproximal
bone or contacting the root surface and periodontal ligament of nearby teeth had an
increased likelihood of failure. Fortunately, it was also found that there was no evidence
of lasting damage on the contacted root surface.30
Research by Renjen et al. (2009)
supports this notion. Sixty self-drilling and self-tapping mini screws, with 2.0mm
diameter and 10mm length (Rocky Mounntain Orthodontics, Denver, Colorado), were
-34-
placed with the intention of contacting the adjacent teeth in three male beagle dogs. At
twelve weeks the animals were euthanized and twenty of the mini screws with the most
prominent root contact, as assessed radiographically, were selected for histologic
examination. Sixteen sites showed significant root injury with seven mini screws
displaying penetration into the dentinal layer. Five mini screws showed penetration into
the pulpal canal with root fragmentation. Despite the extensive root damage, the
histologic specimens were void of inflammatory infiltrate and no evidence of pulp
necrosis was visible. In fact, reparative cementum was visible along the periphery of
damaged root surfaces and even in areas of the displaced root fragments, though points of
ankylosis were also present in these areas.59
The findings in this study are contradictory
to those produced by Herman et al. (2005). It was found to be impossible to insert mini
screws (Imtec Ortho Implant) directly into the root surface in a bench top setting.60
However, this difference is likely attributed to the ability of the threads of the mini
screws, and not the tip, to cause damage to the nearby root surface.59
Kuroda et al. (2007) found similar results when evaluating three-dimensional
computed tomography images or two-dimensional dental radiographs of 216 orthodontic
mini screws placed in 110 patients. Two different mini screws (both self-tapping and
self-drilling) were used in the study: the AbsoAnchor (Dentos, Daegu, Korea) with
1.3mm diameter and variable lengths ranging from 6mm to 12mm, and the alternative
mini screw (Gebrüder Martin GmbH, Tuttlingen, Germany) with 1.5mm diameter and
9mm length. Loading of the mini screws (50g to 200g) was variable, ranging from
immediate to 12 weeks post-insertion. The mini screws were classified into three
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categories based on the distance between the mini screw and the adjacent tooth root.
Those mini screws in category I had definite separation from the root and associated
lamina dura. Category II included mini screws whose tip appeared to contact the
adjacent lamina dura, and category III mini screws were overlaying the lamina dura.61
The author found the overall success rates to be significantly greater in the
maxilla than in the mandible (p<0.001), so the data were evaluated separately for each
jaw. In the maxilla, 82 mini screws (52.6%) were classified into category I, 35 mini
screws (22.4%) to category II, and the remainder (39 mini-screws or 25.0%) were
grouped into category III. Based on success rates, (after application of orthodontic force
to the mini screws lasting approximately one year or until the completion of orthodontic
treatment), there was a statistically significant difference amongst the categories.
Category I mini screws placed in the maxilla had a 96.3% success rate, compared with
91.4% for those of category II. However, both groups elicited a relatively high success
rate, regardless of the statistical significance. Category III mini screws had an observed
success rate of only 74.4%. This was similar to success rates reported for mini screws
placed in the mandible. Mini screws assigned to category III had a low success rate of
only 62.5%.43
Therefore, based on the above studies it appears that the proximity of
orthodontic mini screws to adjacent root surfaces is a major risk factor for failure, and
some authors suggest selecting mini screws with even smaller diameters and shorter
lengths to prevent contact.30, 61
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With regard to placement technique, most mini screws are self-threading, but only
recently have some self-drilling mini screws been developed. Regardless, when the need
arises, pilot hole preparation diameter and length is crucial. Most studies suggest that
the critical pilot hole diameter is approximately 85% of the external thread diameter of
the orthodontic mini screw. Increasing pilot hole diameters beyond this range
significantly increases the likelihood of failure. Furthermore, orthodontic mini screw
insertion torque at the time of placement has also proven important. The ideal range
seems to be between 5- 10Ncm. Lastly, proximity to adjacent teeth and periodontal
ligament space was also shown to increase the likelihood of orthodontic mini screw
failure.
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Cortical bone thickness relative to stability of orthodontic mini screws:
Several studies have investigated cortical bone thickness throughout the alveolus
of the maxilla and mandible, including other areas such as the hard palate.62-68
This is
due to the importance of cortical bone, since it has a demonstrated ability to bear more
load in clinical situations, relative to the underlying trabecular bone. Cortical bone also
has a higher modulus of elasticity, higher strength, and higher resistant to deformation.
However, much of this evidence is derived from literature pertaining to larger prosthetic
dental implants, although it also has significant applications in the usage of orthodontic
mini-screws.69-71
Ono et al. (2008) investigated cortical bone thickness in the posterior alveolar
regions of the maxilla and mandible in forty-three orthodontic patients (mean age: 24.0
+/- 8.2 years; range: 13.1- 48.0 years) where the treatment plan called for the use of
orthodontic mini screws. Cone beam CT scans (voxel size 0.125mm) were taken of 39
maxillae and 41 mandibles. Cortical bone thickness was measured at 1.0mm intervals in
a plane parallel to the occlusal plane of each tooth (mesiobuccal and mesiolingual cusps)
from 1mm to 15mm below the level of the alveolar crest. Overall, average cortical bone
thickness ranged from 1.09mm to 2.12mm in the maxilla, and from 1.59mm to 3.03mm
in the mandible, with maxillary cortical bone thickness significantly thinner than that
observed in the mandible (p< 0.001). More specifically, mesial to the first molar, average
cortical bone thickness ranged from 1.09mm to 1.62mm in the maxilla, and 1.59mm to
2.66mm in the mandible. Again, mesial to the first molar in the mandible, cortical bone
thickness was significantly thinner in adolescents (p< 0.05). Also, maxillary cortical
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bone thickness mesial to the first molar was thinner in females versus males (p< 0.05).
The authors also examined the distribution of cortical bone with a thickness greater than
1.0mm. Mesial to the first molar, this distribution ranged 56% to 97% for the maxilla
and 90% to 100% for the mandible. A similar trend occurred distal to the first molar.
Overall, there was a tendency for these ranges to increase with greater distance from the
alveolar crest, especially in the mandible. The authors also noted that thinner cortical
bone within the region of maxillary attached ginigiva, especially in female patients, may
be insufficient to support orthodontic mini screws.62
Deguchi et al. (2006) also investigated maxillary and mandibular cortical bone
thickness mesial and distal to the first molars, distal of the second molars, and in the
premaxillary region of ten patients (average age: 22.3yrs). Cone beam CT scans with
slice thickness of 0.5mm were taken in high-resolution mode and measurements of
cortical bone thickness were made at various angles (30°, 45°, and 90°) relative to a line
parallel to the long axis of the adjacent teeth in the maxilla and mandible. Measurements
were made within 3mm to 4mm of the alveolar crest and at a more apical position (6mm
to 7mm). In the premaxillary region, measurements of cortical bone thickness were taken
at A-point and near the anterior nasal spine.63
Ninety degree measurements at the occlusal level, mesial and distal to the first
molar and distal to the second molar, in the maxilla revealed mean cortical bone
thicknesses of 1.8mm +/- 0.6mm, 1.5mm +/- 0.5mm, and 1.3mm +/- 0.5mm,
respectively. At the more apical level mean cortical bone thickness was 1.6mm +/-
0.6mm mesial to the first molar and 1.6mm +/ 0.5mm distal to the first molar. In the
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mandible, occlusal level mean cortical bone thickness measurements mesial and distal to
the first molar, and distal to the second molar were 1.9mm +/- 0.6mm, 2.0mm +/- 0.6mm,
and 1.9mm +/- 0.7mm, respectively. Apically, cortical bone thickness mesial and distal
to the first molar was 1.8mm +/- 0.5mm for both regions. A significant difference
between maxillary and mandibular measurements mesial and distal to the first molar (<
0.05) and distal to the second molar was (p< 0.01) observed. Reported measurements of
lingual cortical bone thickness were similar to those at the corresponding buccal
positions, except at the distopalatal aspect of the second molars where significantly
thicker cortical bone was present (p< 0.01). In the premaxilla, mean cortical bone
thickness at A-point (1.4mm +/- 0.5mm) was significantly less (p< 0.0001) than at the
anterior nasal spine (3.6mm +/- 0.6mm).63
The authors found no significant differences in cortical bone thickness based on
sex or age. Aside from differences between the jaws, there was little difference observed
in cortical bone thickness, especially about the first molars.63
However, this may be
attributed to low power due to the small sample size. These findings contrast the results
reported by Ono et al. 2008.62, 63
Kim et al. (2006) examined cortical bone thickness in the maxillae of twenty-
three Korean cadavers (mean age: 49.5 years). Sites were sectioned to allow visual
measurement of buccal and palatal cortical bone thickness, then subsequently decalcified
for slide preparation.72
Values obtained for cortical bone thickness were similar to those
presented in previous studies.62, 63
However, in most of the maxillae there was a tendency
for buccal cortical bone thickness to be greatest near the alveolar crest, with gradual
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thinning mid-root, and a subsequent increase more apically. The authors also examined
palatal cortical bone from the alveolar crest to the mid palatal suture. In this region
cortical bone thickness was relatively uniform between maxillae and no significant
differences were found between sectioned sites.72
Baumgaertel et al. (2009) examined cone beam CT images of thirty dry skulls to
obtain measurements of cortical bone thickness in the maxillary and mandibular alveolar
process. Measurements were taken in all interdental regions at three defined levels from
the alveolar crest (2mm, 4mm, and 6mm). As with previous studies, they found that
buccal cortical bone thickness was greater in the mandible than maxilla. In addition,
cortical bone thickness of the mandibular and maxillary anterior sextant consistently
increased from alveolar crest to more apical regions. However, in the maxillary buccal
sextants cortical bone decreased slightly in thickness at the 4mm measures before
increasing again apically.65
This was similar to the results by Kim et al. (2006).72
In
another study based on cone beam CT images of thirty dry skulls, Baumgaertel et al.
(2009) examined cortical bone thickness of the palate exclusively. Cortical bone
thickness ranged from 0.1mm to 2.78mm across the entire region. However, cortical
bone thickness tended to decrease as measurements moved laterally from the mid-palatal
suture, and from anterior to posterior.67
This differed from outcomes observed by Kim et
al. (2006), where no difference in palatal cortical bone thickness was observed.72
Motoyoshi et al. (2007) examined the relationship between cortical bone
thickness and implant placement torque for eighty-seven orthodontic mini screws (1.6mm
diameter, 8mm length) placed in the posterior buccal alveolar bone of thirty-two
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orthodontic patients. After a minimum loading period of six months, they found that the
cortical bone thickness was significantly greater in the success group (1.42mm +/-
0.59mm versus 0.97 +/- 0.31mm; p= 0.015). Calculation of an odds ratio revealed that
orthodontic mini screws had a significantly greater likelihood of failure (OR= 6.93, p=
0.047) when cortical bone thickness was less than 1.0mm. In addition to controlling
implant placement torque, the authors suggested that a site for placement of orthodontic
mini screws should have a minimum cortical bone thickness of at least 1.0mm.57
Miyamoto et al. (2005) investigated the interaction of cortical bone
thickness and implant length on primary stability of prosthetic dental implants. A total of
225 implants (Astra Tech, Mölndal, Sweden; 3.5mm diameter; 8mm, 9mm, 11mm,
12mm, 15mm, and 17mm lengths) were placed in the maxilla and mandible of fifty
Japanese patients (mean age: 52.5 years) with some degree of edentulism. Cortical bone
thickness about the implant site was obtained from CT images taken before surgical
placement of all implants. Stability of each implant was analyzed with resonance
frequency analysis. All implants exhibiting clinical mobility were excluded since this
increased the variability of stability measures. Mean maxillary cortical bone thickness
was 1.49mm +/- 0.34mm, whereas mean mandibular cortical bone thickness (1.9mm +/-
0.56mm) was significantly larger (p< 0.0001). The implant stability quotient (ISQ)
obtained from resonance frequency analysis for the maxillary implants was 63.5 +/- 5.2
ISQ, while mandibular implants had a significantly higher (p< 0.0001) value (71.7 +/-
5.23 ISQ). More importantly, Pearson’s correlation coefficient showed a strong
correlation between ISQ values (indicative of primary stability) and cortical bone
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thickness (r= 0.84). Implant length showed no correlation with ISQ values (r= -0.25).
The authors concluded that primary stability of implants depends largely on local bone
properties, such as cortical bone thickness, and not implant length. However, the authors
also remarked that resonance frequency analysis may be easily influenced by several
factors.64
Wang et al. (2010) had thirty-two orthodontic mini screws (Aarhus, Medicon,
Tuttlingen, Germany) of 1.6mm diameter and 8mm length placed in the anterior
mandible of eight young immature and eight adult beagle dogs. The animals were
euthanized immediately thereafter and micro CT scans were completed along with pull-
out testing to analyze the relationships between multiple properties of bone (bone density,
relative bone volume, and cortical bone thickness) and pull-out strength. Mean bone
density values for the adult dogs were 781.94 +/- 21.46 mg of HA/ cm3 and for the
younger beagles were 713.61 +/- 13.08 mg of HA/ cm3. Cortical bone thickness for adult
and immature beagle dogs was 1.14mm +/- 0.11mm and 1.07mm +/- 0.86mm,
respectively. In addition, statistically significant differences were found between pull-out
strengths for adult and immature dogs (218.40 N +/- 24.5 N and 130.82 N +/- 2.2 N).
Bone density was demonstrated to have the greatest correlation (r= 0.920) with pull-out
strength, and cortical bone thickness showing the least significant correlation (r=
0.263).73
Cha et al. (2010) reported similar findings after examining placement and
removal torque in combination with bone mineral density and cortical bone thickness
obtained from micro CT imaging for ninety-six orthodontic mini screws of 1.4mm
diameter, 7mm length, and either conical or cylindrical design (Biomaterials Korea,
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Seoul, Korea) placed in six adult beagle dogs. Both bone mineral density and cortical
bone thickness demonstrated positive correlations with insertion torque (r= 0.58 and r=
0.48, respectively). In this study, cortical bone thickness demonstrated a greater
association with primary stability.74
Cortical bone thickness is a crucial factor in determining orthodontic mini screw
success. It appears that cortical bone is generally thinner in adolescent patients, and in
females more so than males. In general, cortical bone thickness in the maxilla is
significantly reduced compared to the mandible. Also, maxillary cortical bone thickness
in the alveolar process decreases from anterior to posterior, whereas the opposite trend
is true in the mandible. Palatal cortical bone thickness decreases from anterior to
posterior and from the mid-palatal suture laterally. There is also a general trend of
increasing cortical bone thickness with increased vertical distance from the alveolar
crest. However, there is some evidence that alveolar cortical bone may be thinner near
the mucogingival junction.
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Osseointegration of orthodontic mini screws:
Until recently, a significant deficit existed in the literature examining the complex
interactions at the peri-implant interface, pertaining specifically to orthodontic mini
screws. Still, much of the supporting research has been extrapolated from studies
examining prosthetic dental implants. Vannet et al (2007) provides some insight into the
extent of osseointegration surrounding orthodontic mini screws. Histomorphometric
evaluation of eight semi-self tapping BSBAs (bracket screw bone anchors), 1.7mm
diameter and 6mm length (titanium bone screw: Leibinger-Stryker GmbH & Co,
Freiburg, Germany; titanium 0.018” slot bracket: Ormco, Orange, California, USA),
placed in the mandibles of five beagle dogs revealed an overall mean osseointegration of
74.48% (+/- 15.33). Initially, twenty mini screws were placed with pre-drilling through
the cortex. Eight BSBAs underwent immediate loading with a 200 cN Nitinol closed coil,
whereas another eight BSBAs were loaded after a period of six or twelve weeks.
However, only eight of the BSBAs remained after the twenty-five week study period.
The authors claim that the degree of osseointegration observed over the twenty-four week
study period did not depend on placement location, nor on whether the screws were
loaded (immediate or delayed) or unloaded.75
Similar results were reported in humans
for orthodontic mini implants, which are approximately twice the diameter of orthodontic
mini screws. A study of twenty short self-tapping orthodontic mini implants
(Orthosystem; 3.3mm diameter and 4mm or 6mm length) placed in eighteen patients
showed an average osseointegration, based on histologic assessment, of 68.22% (SD
14.35) for midpalatal implants and 64.85% (SD 2.89) for retromolar implants. The
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orthodontic mini implants also had a sandblasted and acid-etched surface treatment. Pre-
drilling of the entire mini implant length was performed prior to placement. The
orthodontic mini implants were left unloaded for a period of three months, with
subsequent loading of variable orthodontic forces (2-6 N) for a period ranging from nine
to twenty-two months.76
Woods et al. (2009) examined the extent of osseointegration that occurred around
fifty-six tapered orthodontic mini screws (1.8mm diameter; 6mm length; IMTEC
Corporation, Ardmore, Oklahoma, USA) placed with pre-drilling in the buccal alveolar
bone of seven mature male beagle dogs. Each quadrant received one loaded mini screw
and one unloaded control mini screw. Immediate and delayed loading was performed
with either 25g or 50g of applied force. Histologic analysis was performed at three levels
along the threaded portion of the orthodontic mini screws: coronal, middle, and apical
portion. Overall, the amount of bone-implant contact was highly variable, ranging from
16.6% to 87% in the maxilla and 2.2% to 94.8% in the mandible. There was no
significant difference between the amount of osseointegration between the immediate and
delayed loaded orthodontic mini screws (44.4% and 35.4% respectively) (table 2). It was
previously suggested that a significant amount of bone-implant contact was required, but
this study found that a minimum of 2.2% osseointegration was required to maintain
stability of orthodontic mini screws. Histologic analysis revealed a tendency for
decreased bone-implant contact at the coronal level, but the differences between these
levels were not significantly different. In this study, several orthodontic mini screws
showed more bone-implant contact in the apical third, and the authors speculate that
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perhaps medullary bone may play an equally important role as cortical bone in
maintaining long-term stability.77
In a study by Yano et al. (2006), twenty straight (cylindrical) orthodontic mini
screws (1.2mm diameter and 4mm length) and twenty tapered (conical) mini screws
(1.4mm diameter and 4mm length) were placed with pre-drilling in the tibia of twenty
male Wistar rats. Ten orthodontic mini screws of each type underwent a differential
loading regimen of either immediate loading at the time of insertion or delayed loading
after a healing period of six weeks. All loaded mini screws underwent a traction force of
approximately 2N for a period of two weeks.78
Scanning electron microscopy revealed a difference at the peri-implant interface
between the mini screw types. Straight mini screws exhibited gaps between the threaded
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portion and the adjacent cortical bone. This was not evident along the surface of the
tapered mini screws where cortical bone was in direct contact with the threaded surface.
Furthermore, there was a statistically significant difference in the mean amount of bone-
implant contact between the straight and tapered orthodontic mini screws of the
immediate-loading group (33.3% +/- 11.8 and 82.3% +/- 15.0, respectively). This
difference was also evident in the delayed-loading group (53.7% +/- 13.9 and 88.0% +/-
11.6, respectively). However, there was no significant difference in the amount of
osseointegration for tapered orthodontic mini screws that underwent either immediate or
delayed loading (table 2). Interestingly, the degree of bone-to-implant contact about
tapered mini screws appeared to be independent of any orthodontic loading as the amount
of osseointegration was similar between the loaded test groups and unloaded controls.78
Wu et al. (2009) investigated the effects of variable healing periods (0, 1, 2, 4,
and 8 weeks) on ninety unloaded orthodontic mini screws (Medicon, Tuttlingen,
Germany) with 1.9mm diameter and 6mm length placed with pre-drilling in the mid-
diaphyseal tibia of fifteen New Zealand white rabbits. Thirty mini screws were prepared
for histologic examination, whereas the remainder underwent mechanical tests.
Histologic analysis of the bone-implant interface in those rabbits sacrificed immediately
after mini screw placement displayed an interposed layer of erythrocytes and bony debris.
In those specimens examined after one week of healing an inflammatory response
predominated with macrophages displaying prominent ruffled membranes. Collagen
fibers and granulation tissue were also found at the peri-implant interface. Tiny
trabeculae growing toward the mini screws from areas close to endostea suggested that
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this time frame corresponds with the beginning of new bone growth. An increased
amount of collagen fiber layers and connective tissue was present around mini screws
analyzed after the two-week healing period. Osteoblasts and fibroblasts were observed in
increased numbers highlighting the increase in new bone growth. Only after four weeks
was new woven bone in conjunction with large trabeculae, encompassed in a non-
calcified matrix, found along the bone-implant interface. The clustered osteoblasts in the
area appeared to be forming the woven trabeculae. New regions of mature, compact, and
highly calcified bone were observed about the mini screws after eight weeks of healing.
The newer lamellar bone was highly compact and nearly indistinguishable from that of
the old bone in the area. Osteoblasts were now present in mature lacunae.79
The authors noted that the corresponding mechanical tests (maximum removal
torque and maximal pull-out strength values) were only significantly increased after the
four-week healing period. From this it was suggested that loading of mini screws with
orthodontic forces should be done no sooner than four- weeks post-insertion to allow for
adequate osseointegration. However, the maximal removal torque values and maximal
pull-out strengths continued to increase from the time of insertion in relation to healing
(non-parametric permutation test; r= 0.788, p< 0.0001, and r= 0.811, p< 0.0001,
respectively). In fact, the biomechanical measurements taken immediately after
placement of five mini screws, where there was no healing period, were still likely high
enough to support immediate loading. The maximal removal torques for this time-point
were greater than 20Ncm and the maximal pull-out forces appeared to be no less than
100N (reported in graph form).79
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Duyck et al (2001) examined the marginal bone interactions surrounding tapered
prosthetic dental implants in rabbit tibia that underwent static and dynamic loading
directed axially or transversely to the long axis of the implant. A finite element model
showed that the overall stress created was equivalent between the pull-out and transverse
forces, but the resultant strain distributions were quite different. High cortical bone
strains are found circumferentially at the implant neck when pull-out forces are exerted.
In the case of an applied transverse force, high strain levels in the marginal bone are
limited to the pressure side. These regions of high strain in the marginal bone resulted in
crater defects, but only in those implants that underwent dynamic loading.80
Osseointegration and bone-to-implant contact values of orthodontic mini screws
are derived primarily from animal studies. Most studies incorporate the trabecular bone
in their histologic or micro CT analysis resulting in wide variability and complicating
statistical analysis. Self-drilling orthodontic mini screws placed without pilot hole
preparation were shown to have increased bone-to-implant contact values. Increased
bone-to-implant contact values in the region traversing the cortical bone have been
shown when tapered mini screws are considered versus their cylindrical counterparts.
Immediate versus delayed loading of an orthodontic mini screw seems insignificant with
regard to bone-to-implant contact. Surface treatments of the orthodontic mini screws
such as sand-blasting and acid-etching do enhance osseointegration, but also increase
micromechanical retention making removal more difficult.
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Immediate versus delayed loading of orthodontic mini screws:
Ohashi et al. (2006), in a systematic review of the orthodontic literature pertaining
to orthodontic implants and mini screws, examined the effects of various loading
protocols on stability. Of the eleven articles deemed scientifically acceptable by their
inclusion criteria, five examined the use of orthodontic implants. All of the implants
underwent delayed loading for a minimum of two months, with an average waiting time
of four to six months. Subsequently, only six articles evaluated orthodontic mini screws,
with diameters ranging from 1.8mm to 2.0mm. Loading protocols varied from
immediate loading to a waiting period two to four weeks. The authors suggest that this
period of delay prior to loading provides for tissue healing around the mini screw, but
does not influence osseointegration. Furthermore, immediate loading may increase the
risk of fibrous tissue migration, eventually interposing between the bone and mini screw.
This may be advantageous in providing short-term stability, but may prove counter-
productive in the long term. However, the authors provide no strong histological
evidence to support these hypotheses.81
Zhang et al. (2010) examined the influence of orthodontic loading on fifty-four
self-drilling orthodontic mini screws (Aarhus Microscrew, Medicon Company,
Tuttlingen, Germany) with 1.6mm diameter and 6mm length placed in nine male beagle
dogs. The mini screws underwent immediate loading (0 days), or early loading at either
2 weeks or 4 weeks post-insertion with an orthodontic force of 100g for a duration of
eight weeks. All of the mini screws survived the duration of the experiment. However,
the amount of bone-to-implant contact was significantly different (ANOVA, p< 0.01)
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amongst the three groups. The group of eighteen mini screws that underwent immediate
loading exhibited a mean bone-implant contact of 43.74% (SD: 0.0242), compared to
66.26% (SD: 0.0354), and 73.28% (SD: 0.0189) for the two-week, and four-week loading
periods, respectively. Furthermore, the author reported the presence of crescent-shaped
bony trabeculae with evidence of bone remodeling (osteoclast activity) and a large
amount of collagen fibers along the peri-implant interface in the immediately loaded mini
screw group. Similar histological observations were found in the peri-implant region of
the mini screws that underwent orthodontic loading after two-weeks including collagen
fibers partially surrounding the mini screws. In addition, newly formed woven bone
along the mini screw surface was present. Remodeling of bone trabeculae was prominent
around those mini screws loaded after four-weeks with a linear arrangement of
osteoblasts along these trabeculae and the mini screw surface. The authors recommended
a four-week healing period for mini screws to increase their stability during the course of
orthodontic loading, but recognized that there was no apparent difference in the success
of mini screws regardless of the loading regimen.82
Woods et al. (2009) examined the impact of delayed (26 days) versus immediate
loading on stability of orthodontic mini screws (1.8mm diameter and 6mm length;
IMTEC Corporation, Ardmore, Oklahoma, USA) placed in the maxilla and mandible of
mature male beagle dogs. The mini screws were loaded with an orthodontic force of 25g
or 50g (mandibular mini screws only). The amount of osseointegration occurring
concentrically around the orthodontic mini screws was examined. There was no
statistically significant difference in the average percent bone-implant contact between
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delayed (35.4%) and immediately loaded (44.4%) mini screws. Some of the unloaded
controls did become mobile and the authors suggested that loading may actually increase
the likelihood of bone formation with an alternate decrease in potential mobility.77
Yano
et al. (2006) also showed that the mean amount of bone-implant contact about tapered
orthodontic mini screws (1.4mm diameter and 4mm length) placed in rat tibia was not
significantly altered by loading regimen. However, contrary to the observations related
to the unloaded controls in the previous study (Woods et al., 2009), this study found that
the unloaded controls exhibited very similar amounts of osseointegration as the loaded
mini screws.78
Chen et al. (2009) examined the effects of immediate horizontal loading (200g
force) on sixty orthodontic mini screws (1.2mm diameter and 7mm length; AbsoAnchor
system, Dentos, Daegu, Korea) placed with pre-drilling in the maxillary and mandibular
buccal alveolar bone of four female mongrel dogs over a period of nine weeks. The
success rate for the immediate loading group was 89.58% versus 75.0% in the unloaded
control group. This difference was not statistically significant. However, the authors do
conclude that immediate loading does not inhibit osseointegration, but may rather
stimulate bone adaptation. Yet, immediate loading may increase the amount of initial
displacement through adjacent bone of orthodontic mini screws. Average relative
displacements after the nine-week study period were 0.98mm (+/- 0.57mm) in the
maxilla, and 0.53mm (+/-0.48mm) in the mandible. The degree of displacement tended
to be greater in areas of thinner cortical bone. The authors hypothesized that a short
delay in loading (2 weeks) would allow for sufficient healing and compensate for any
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bone damage during placement, thus negating any migration through bone on initial
loading with orthodontic forces.83
Luzi et al. (2009) also examined the effects of immediate loading on fifty self-
tapping orthodontic mini screws (diameter 2.0mm and 9.6mm length; Aarhus Mini-
implant®
, Medicon, Tuttlingen, Germany) in four adult male Macaca fascicularis
monkeys. Mini screws were inserted at a variable number of days prior to euthanasia
(96d, 70d, 39d, and 7d), but all experimental mini screws were loaded with an applied
force of 50cN. Comparisons of bone-implant contact showed higher percentages for the
loaded mini screws than the unloaded samples, but these differences were small.
Between the first week and one month a trend was seen toward decreasing bone-implant
contact, followed by a subsequent dramatic increase. This differs from other studies
suggesting a continual time dependent increase in the amount of osseointegration post-
placement of orthodontic mini screws. The authors conclusions were similar to others in
that immediate loading of orthodontic mini screws with light forces does not have a
significant negative effect on the surrounding bone.84
Examination of loading protocols for orthodontic mini screws (ranging from
immediate to variably delayed), has proven inconclusive. Some authors suggest that
immediate loading may increase the risk of apical migration of fibrous tissue along the
length of the orthodontic mini screw causing mobility and subsequent failure. Other
authors propose that immediate loading may actually increase the amount of initial bone
formation about the orthodontic mini screw. Currently, there is no clear evidence as to
the superiority of immediate versus delayed loading of an orthodontic mini screw.
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Movement of orthodontic mini screws:
Liou et al. (2004) demonstrated that movement of loaded orthodontic mini screws
may occur over time in a clinical setting. Thirty-two orthodontic mini screws (2mm
diameter; 17mm length) placed in the zygomatic buttress of the maxilla of sixteen adult
patients were followed for a period of nine months. Pilot holes were pre-drilled since the
cortical bone thickness, though variable, is approximately 3mm to 4mm in this region.
The mini screws were loaded two weeks after placement with two nickel-titanium closed-
coil springs on each side. One Ni-Ti coil spring with a 150g force was attached from the
mini screw to the canine. The second Ni-Ti coil spring with a 250g force was also
attached to the mini screw at one end and an archwire hook between the lateral incisor
and canine. Lateral cephalometric radiographs were taken two weeks post-insertion and
at the duration of en masse anterior retraction (9 months). Cephalometric tracings were
superimposed with registration on point sella and a “best fit” amongst the anatomic
structures of the maxilla, cranial base, and cranial vault.85
The orthodontic mini screws were tipped forward significantly at the screw head
(midpoint between the blunt ends of the left and right screws), by 0.4mm (+/- 0.5mm) on
average. In nine of the sixteen patients the orthodontic mini screws remained stationary
in all directions under orthodontic loading. However, for the remaining seven patients
the screw heads of the mini screws showed movement ranging from 0.5mm to 1.5mm in
the direction of the applied force when pre-loading and nine-month post-insertion
radiographs were compared. For these same seven patients the screw bodies (midpoint
between the left and right screw tail and screw head) of the mini screws were extruded
-55-
and tipped forward by a maximum of 1.0mm. The screw tails (midpoint between the
pointed tips of the right and left mini-screws) of the orthodontic mini screws were
extruded and tipped either forward or backward by a maximum of 1.0mm in either
direction. All of the orthodontic mini screws remained clinically stable, but not
absolutely stationary under orthodontic loading. The author suggested that the observed
migration of the orthodontic mini screws was due to an inadequate waiting period prior to
loading, causing a lack of osseointegration and the development of a layer of fibrous
tissue interposed between the mini screws and the surrounding bone. However, the study
provided no histological evidence to support this hypothesis.85
Similar findings were reported in a pilot study by Mortensen et al. (2009) when
comparing the stability of sixty immediately loaded orthodontic mini screws that were
3mm (Dentos; 1.3mm diameter) and 6mm (AbsoAnchor system, Dentos, Daegu, Korea;
1.3mm diameter) in length, placed in the maxilla and mandible of five beagle dogs. The
mini screws were loaded with Ni-Ti coil springs to deliver either 600g or 900g of
orthodontic force. Mobility and displacement through bone were examined. There was
no association between the mobility of orthodontic mini screws (as demonstrated by the
Periotest™) and displacement. All loaded orthodontic mini screws demonstrated
significant decreases in inter-implant distance over the six week study period, as
measured at the head of the mini screws. However, the difference in linear displacement
between the 3mm and 6mm mini-screws was not statistically significant (2.2mm, range:
0.4 to 4.4mm, and 1.8mm, range: 1.0 to 3.4mm, respectively). There was a mean
decrease of approximately 2.0mm in inter-implant distance for the six orthodontic mini-
-56-
screws of 3mm length that underwent loading with 600g of force, versus a mean decrease
in inter-implant distance of 3.1mm when loaded with 900g of force. However, the apices
of the mini-screws were only displaced by about 0.3mm. Regardless, both types of
orthodontic mini-screws experienced significant linear displacements while loaded with
very high orthodontic forces during this study, with the heads of mini-screws of 6mm
length demonstrating individual movement of approximately 0.9mm.86
In a study by El-Beialy et al. (2009), a three-dimensional assessment was
undertaken of forty tapered orthodontic mini screws (1.2mm diameter and 8mm length)
placed with a pilot hole in the cortical bone between the second premolar and first molar
in the maxilla and mandible of twelve patients. A healing period of two weeks was
allowed prior to attachment of Ni-Ti coil springs to produce an orthodontic retraction
force of 150g to 250g for a period of six months. Computed tomography scans of the
maxilla and mandible for each patient were taken prior to loading and at the conclusion
of loading. Points representing the heads and tails of the mini screws, as well as the
original positions of insertion were recorded and compared.87
Seven of the forty mini screws failed and were excluded from analysis. The
majority of the mini screws were displaced, with the screw head tipped toward the
direction of force application, and tails (screw apex) shifted in the opposite direction. On
average the head of the mini screw was displaced by 1.080mm (range: 0.174mm to
4.121mm; SD: 0.787). The tail showed a mean movement of 0.828mm (range: 0.341mm
to 1.796mm; SD: 0.586). A significant correlation for extrusion of the mini screws was
also observed (mean: 0.548mm; range: 0.014mm to 2.557mm; SD: 0.586). The authors
-57-
also noted that five of the mini screws had contacted the adjacent teeth during placement,
but remained successfully intact. However, these mini screws did display a significantly
greater amount of extrusion and movement of the head (p <0.05) when compared with
the remaining mini screws that were void of root contact.87
Liu et al. (2011) studied the movement characteristics of an unknown number of
orthodontic mini screws (1.6mm diameter; 11mm length; Beici Medical, Ningbo, China)
placed in the maxilla of sixty adult patients (aged 19- 27 years) undergoing extraction of
four first premolars and en-masse retraction as part of the treatment plan for bimaxillary
protrusion. Each orthodontic mini screw was placed, at an oblique angle of 30° to 40° to
the long axis of the posterior teeth, between the first molar and second premolar at the
level of the attached gingiva. Each orthodontic mini screw underwent immediate loading
with elastic chains, producing an applied orthodontic load of approximately 150g.
Computed tomography scans were taken at two weeks post placement of the mini screws
and at closure of the extraction spaces (approximately 6 month duration).
Superimposition of the 3D images was performed with Interactive Medical Image
Control System (MIMICS, version 10.01, Materialise, Leuven, Belgium). The head of
the mini screws drifted 0.23mm (+/- 0.08mm) mesially, while the apex also drifted
mesially 0.23mm (+/- 0.07mm). There was only negligible movement of the orthodontic
mini screws in the buccopalatal and vertical directions.88
The degree of movement of
orthodontic mini screws reported by the authors in this study is minimal compared to that
reported by El-Beialy et al. (2009), Liou et al. (2004), and Mortenen et al. (2009).85-88
-58-
Hsieh et al. (2008) found that prosthetic dental implants also progressively moved
when loaded with continuous heavy transverse forces (500g) for a period greater than two
months. Four pairs of titanium implants (ITI Straumann SLA standard threaded type,
4.1mm diameter; 12mm length) were placed in the post-extraction edentulous ridges of a
beagle dog. Three months post-insertion, variable orthodontic forces (100g, 200g, and
500g) were applied between three of the implant pairs using Ni-Ti coil springs for a
period of six months. There was no movement detected for any of the implant pairs after
two months duration. However, after three months duration the pair of implants loaded
continuously with 500g of force exhibited significant movement toward one another
reducing the inter-abutment distance by 2.2mm. The distance between the abutments
further decreased by 0.7mm in the subsequent three months. The body and apical
portions of the implants showed similar changes over the six-month study period.
Despite the amount of migration observed for the implant pair that was subjected to 500g
of force there were no signs of bone loss or mobility on clinical and radiographic
examination. The authors theorized that both the amount of force and duration of loading
play an important role in the potential migration of prosthetic dental implants. Shorter
intervals may allow the occurrence of a modeling response fostering static positioning of
the implant fixture, whereas longer durations could result in remodeling of the
surrounding bone. The accumulation of microdamage stimulates remodeling and may
lead to a net loss of bone and void formation neighboring the implant. Therefore, it is
possible that the displacement of implants under a high stress for a long period of time
-59-
results from the accumulation of trabecular microfractures and adaptive remodeling
during the orthodontic force application.89
The notion that tipping of orthodontic mini screws is due to active bone
remodeling on the pressure side of loaded orthodontic mini screws was also evident in the
findings from fluorescent labeling in a study by Kim et al. (2008) examining the effects
of microgrooves on stability. Three fluorescent dyes were administered at four time
points after orthodontic loading of the mini screws in two beagle dogs: immediate
(tetracycline), 4 weeks (calcein), 8 weeks (alizarin), and 12 weeks (tetracycline). More
bone remodeling was visible on the pressure side than on the tension side of the
orthodontic mini screws. This finding tended to reverse as examination proceeded
toward the tip of the screw where pressure and tension sides were interchanged. Not only
were the fluoroscopic bands broader in these regions of pressure, with increased uptake
of the dyes indicating extensive remodeling, but migration through the bone by the mini
screws was also observed as denoted by the loss of continuity of the colored bands
created by administration of the initial dyes on the pressure sides.43
Early research based on cephalometric superimposition demonstrated that
significant migration of orthodontic mini screws occurs while under orthodontic loading.
In contrast, recent evidence from a cone beam CT study showed that mini screws under
orthodontic load for an average of 6 months experienced clinically insignificant
movement. Studies have also examined the use of orthopedic forces, (often several orders
of magnitude greater than applied orthodontic forces) on mini screws. Their findings
-60-
suggest that orthodontic mini screws may not be expected to remain stationary under
such high loads.
-61-
Impact of angulation on stability of orthodontic mini screws:
Unlike osseointegrating prosthetic dental implants that are meant to be loaded
axially, orthodontic mini implants and mini screws must be able to withstand forces
applied approximately perpendicular to their long axis. Therefore, employing differential
orthodontic mini screw insertion techniques, such as “tent-pegging,” may have a positive
impact on long-term mini screw stability. “Tent-pegging” signifies implant insertion at an
angle where the implant head is oriented away from the direction of force application.
This could provide an additional mechanical advantage to retention of the orthodontic
mini screw, limiting movement over time, mobility, and potential failure.
Wilmes et al. (2008) investigated the impact of insertion angle during placement
on the primary stability of orthodontic mini screws. Two different mini screw sizes
(Dual-Top Screw (Jeil Medical Corporation, Seoul Korea); 1.6mm x 8mm and 2.0mm x
10mm) were placed in twenty-eight pig iliac bone segments that were embedded in resin
(Probase, Ivoclar Vivadent, Schaan Liechtenstein). Pre-drilling to a depth of three
millimeters was performed prior to manual placement of all orthodontic mini screws at
seven different angulations (30°, 40°, 50°, 60°, 70°, 80°, and 90°). Final screwing by
another 0.2mm was performed by a robotic measurement system to determine the
insertion torque for the various scenarios. After analyzing the 616 torque measurements
it was found that orthodontic mini screw angulation influenced the measured insertion
torques. The narrow diameter (1.6mm) Dual-Top Screw had the highest mean insertion
torque value (101 Nmm +/- 31) when placed with an insertion angle of seventy degrees.
The lowest insertion torque (78 Nmm +/- 33) was found for the very oblique insertion
-62-
angle of thirty degrees. The larger diameter (2.0mm) Dual-Top Screw also demonstrated
a similar trend with the highest mean value (167 Nmm +/-62) for insertion angles of
seventy degrees and much lower insertion torques for the very oblique angulations.
Regression analysis revealed maximum insertion torques for the 1.6mm and 2.0mm
diameter Dual-Top Screws at 63.8° and 66.7° respectively.26
The authors suggest that despite a decreased insertion depth of angulated
orthodontic mini screws the slightly longer distance traveled through cortical bone may
provide greater primary stability over those mini screws placed in an upright position.26
This is especially advantageous in regions with reduced bone quality. However,
significantly increased insertion torques beyond 100 Nmm (10Ncm) may lead to greater
risk of compression and necrosis as suggested by Motoyoshi et al. (2006).52
Deguchi et al. (2008) demonstrated that there is indeed a significant difference in
the amount of cortical bone traversed when orthodontic mini screws are placed at an
angle relative to the surface of the cortical bone. Angulated measurements (30°, 45°, and
90° relative to the long axis of the adjacent teeth) obtained from cone beam CT scans of
ten orthodontic patients found a mean cortical bone thickness in the maxillary buccal
region of 2.0mm +/- 0.8mm, 1.5mm +/- 0.6mm, and 1.2mm +/- 0.5mm, respectively.
The amounts of cortical bone thickness observed were significantly more abundant at 30°
than at 45° or 90° (p< 0.0001), as well as at 45° when compared to measures made at 90°
(p<0.01). The authors found that this relationship held true for all areas examined in the
maxilla and mandible. It was concluded that the best location for mini screw placement,
in terms of abundance of cortical bone, is mesial and distal to the first molar. The authors
-63-
also noted that, due to root proximity, the best angulation for orthodontic mini-screw
placement was at 30° relative to the long axis of the adjacent teeth.63
This assertion was
also recommended by Lee et al. (2009), especially in the region of the first and second
molars where oblique placement of orthodontic mini screws permits safe placement
without damage to the adjacent root surfaces and periodontal ligament.90
As previously discussed in relation to movement of orthodontic mini screws, El-
Beialy et al. (2009) conducted a three-dimensional imaging study that examined the
failure rates and movement of forty orthodontic mini screws placed in the maxilla and
mandible of twelve patients and loaded at two weeks post-insertion (force= 150g to
250g), to retract the canines, for a duration of six months. The vertical angulation of the
mini screws immediately after placement was measured and compared between those that
failed prematurely (n= 7) and those that remained present for the duration of the study
(n= 33). The mean vertical angulation for the successful group of mini screws was
34.336º (SD= 9.043) and this was not significantly different (p= 0.757) from the failed
group of mini screws (mean: 35.629º; SD= 13.951). However, the failed group of mini
screws consisted of a very small sample. This makes it difficult to draw strong
conclusions from this study with respect to the influence of vertical angulation on the
stability of orthodontic mini screws.87
Inaba et al. (2009) evaluated the stability of thirty custom-made orthodontic mini
screws (1.4mm diameter; 4.0mm length) placed with pre-drilling at varying inclinations
(60º, 90º, and 120º) relative to the direction of the applied orthodontic load in the nasal
bones of fifteen adult male Japanese white rabbits. A Ni-Ti closed coil spring (Tomy
-64-
International Co. Ltd., Tokyo, Japan) was immediately applied between the test mini
screw and another control mini screw placed 20mm- 25mm further down the root of the
nose, so that a force of approximately 2N was generated. After two-weeks all rabbits
were euthanized to examine the bone-implant interface. In addition, mobility of the mini
screws was assessed immediately post-placement and after the two week duration with
the Periotest™.91
After examination of the surrounding bony tissues with field emission scanning
electron microscopy eight of the mini screws were actually found to be in contact with
the underlying intranasal cortical bone. Therefore, those mini screws with bicortical
anchorage were excluded from all analyses. In addition, microscopy elicited a
quantification of average cortical bone thickness of 1.00mm (+/- 0.23). Bone-implant
length (BIL) was also determined based on the amount of cortical bone in direct contact
with the implant surface in that area. BIL was 3.58mm (+/- 0.91) for those mini screws
angulated 60º toward the orthodontic load, and this was significantly different (p<0.05)
from the BIL of mini screws placed perpendicular (2.27mm +/- 0.42). However, the BIL
for 120º angulated mini screws was only 2.76mm (+/- 0.51). Furthermore, the bone-
implant contact ratio (BIR) was calculated from the following equation:
BIR= (BIL/ length of implant surface in cortical bone) x 100
The BIRs for the 60 º, 90 º, and 120 º angulated orthodontic mini screws were 64.23%
(+/- 11.93), 56.30% (+/- 9.81), and 54.48% +/- 11.74) respectively. There was no
significant differences (p<0.05) amongst the three groups. As mentioned, the author also
used the Periotest™ to examine mobility of the various mini screws including
-65-
differentiation between the traction and non-traction sides. It was found that the
angulated mini screws, independent of direction, tended to exhibit lower Periotest™
values, indicating greater stability.91
However, Zix et al. (2008) have suggested that the
Periotest™ has a lower measurement precision when compared to resonance frequency
analysis and is more susceptible to clinical measurement parameters.92
Furthermore, the
prognostic value of the Periotest™ in terms of predicting loss of implant stability remains
unproven in prospective clinical studies.93
Therefore, the conclusions drawn from this
study are of limited value.
Woodall et al. (2011) created three-dimensional finite element models of
cylindrical orthodontic mini screws with 1.5mm diameter. The mini screws were
embedded at angulations of 30°, 60°, and 90° in a bony matrix, with a cortical bone
thickness of 1.79mm. The mini screws were displaced 0.6mm with a theoretical
orthodontic force applied along a vector parallel to and at a point 2.0mm coronal to the
bone surface. The maximum anchorage resistance forces for the mini screws placed at
30°, 60°, and 90° were 678, 2663, and 3700N, respectively. Furthermore, cortical bone
stress was greatest for mini screws placed at 30°, and least for mini screws placed
perpendicular to the bone surface.94
In the second part of this study, the authors placed, with pilot hole preparation,
ninety-six orthodontic mini screws (KLS Martin, Jacksonville, Florida), with 1.5mm
diameter and 11mm length, in the region of the first and second premolars of forty-eight
resected cadaveric maxillae and mandibles. The orthodontic mini screws were again
placed at angulations of 30°, 60°, and 90° to correspond with the finite element analysis.
-66-
The mini screws underwent tangential force application, at a point 2mm coronal to the
bone surface, parallel to the occlusal plane, as if mimicking retraction of anterior teeth,
until displacement of 0.6mm occurred. One-way analysis of variance with post-hoc
Bonferroni test revealed significantly greater resistance to displacement for the
orthodontic mini screws placed at 90° when compared only to the mini screws of 30°
angulation (p< 0.05). The authors speculated that this was due to an increase in the
distance between force application and the bone surface, with a longer lever arm for mini
screws placed at greater angulations from perpendicular.94
Pickard et al. (2010) examined the effect of orthodontic mini screw orientation on
the resistance to failure occurring at the implant-bone interface in human cadaver
mandibles. Ninety orthodontic mini screws (IMTEC Corporation, Ardmore, Oklahoma;
1.8mm diameter and 6mm length) were placed, with pre-drilling of the outer cortical
bone, in the buccal cortex of nine cadaver mandibles at angles of either 45° or 90° to the
buccal surface. These mini screws were subjected to pull-out tests and shear tests relative
to the direction of maximum and minimum bone stiffness.95
Pull-out tests of the orthodontic mini screws aligned at 90° to the cortical surface
exhibited a significantly higher maximum force at failure (342 +/- 80.9 N, p<0.001) than
all other test groups. However, during shear testing the maximum forces at failure
occurred in association with those orthodontic mini screws angled 45° toward the line of
force for both maximum bone stiffness (253 +/- 74.1 N, p< 0.001) and minimum bone
stiffness (264 +/- 21.0 N, p<0.001). The mini screws placed at 45° angulations opposing
the direction of force (“tent-pegged”) provided the least resistance to failure during shear
-67-
testing. Failure of these “tent-pegged” mini screws occurred in a bimodal fashion with
primary failure resulting from rotation and uprighting to a position perpendicular to the
cortical surface, and then continued rotation toward the direction of applied force until
final failure. Those mini screws placed perpendicular to the cortical surface also
underwent significant rotation toward the direction of applied shear force prior to final
failure. A point of primary failure prior to absolute failure was not detected in the
orthodontic mini screws that were aligned 45° to the cortical surface in the direction of
the shear line of force.95
This study suggests that orthodontic mini screws loaded along their long axis have
the greatest stability and resistance to failure. Therefore, stability and resistance to
failure are expected to increase, as the long axis of a mini screw is more coincident with
the approximate direction of applied force. Any discrepancy between the orientation of
the mini screw and the direction of applied force results in decreased uniformity of the
load distribution on the screw threads and disproportionate loading of the surrounding
bone. Orthodontic mini screws that are “tent-pegged” provide the least stability and
resistance to initial failure, but may be able to support a small applied load after loss of
primary stability since an increase in force magnitude was required beyond primary
failure to cause absolute failure.95
These results on primary stability disprove the assumed mechanical advantage of
“tent-pegging.”95
The study had some limitations because it only examined primary
stability, and oblique mini screw orientation was 45 degrees to the bony surface. This
steep angulation is beyond the range recommended by Wilmes et al. (2008), and in most
-68-
circumstances is difficult to achieve clinically.26
In addition, Huja et al. (2005)
performed axial pull-out tests on fifty-six self-drilling orthodontic mini screws (2mm
diameter, 6mm length; Synthes USA, Monument, Colorado) placed perpendicular to the
cortical bone surface in the maxilla and mandible of four beagle dogs that were
immediately sacrificed. Based on this experience the authors suggest that it is impossible
to perform reliable cantilever (tangential) pull-out tests because of large variations in the
resultant pull-out strengths due to bone bending and mini screw impingement on
surrounding structures (i.e. tooth roots and opposing cortical surfaces).96
Beyond primary
and short-term (two weeks) stability, there exists no published research as to the
relevance of insertion angle on long-term stability of orthodontic mini screws.
-69-
Purpose of the Study:
The intent of this research was to determine whether insertion angle of
orthodontic mini screws in relation to the direction of the applied orthodontic force
contributes to the resistance to movement and stability of the mini screw, and how this
affects the peri-implant interface and surrounding bony tissues.
Research Questions:
Question #1:
Does insertion angle of an orthodontic mini screw relative to the direction of
applied force alter retention of the mini screw?
Question #2:
Does insertion angle of an orthodontic mini screw relative to the direction of
applied force affect movement of the mini screw over time?
Question #3:
Does insertion angle of an orthodontic mini screw influence the quantity (volume
and density) of bone present at the peri-implant interface?
-70-
Hypotheses:
To address the above research questions the following null hypotheses and
alternate hypotheses can be formulated:
Null Hypothesis 1: There exists no difference in the success rates of orthodontic mini
screws regardless of whether the insertion angle of the mini screw is directed toward or
away from the direction of the applied orthodontic force.
Alternate Hypothesis 1: There exists a significant difference between the success rates
of orthodontic mini screws when the insertion angle of the mini screw is directed toward
or away from the direction of the applied orthodontic force.
Null Hypothesis 2: There exists no difference in the amount of movement of an
orthodontic mini screw regardless of whether the long axis of the mini screw is directed
toward, versus away from the direction of the applied orthodontic force.
Alternate Hypothesis 2: There exists a significant difference between the amount of
movement an orthodontic mini screw undergoes when its long axis is directed toward,
versus away from the direction of the applied orthodontic force.
Null Hypothesis 3: There exists no difference in the quantity of bone at the peri-implant
interface regardless of the insertion angle of the mini screw relative to the direction of the
applied orthodontic force.
Alternate Hypothesis 3: There exists a significant difference between the quantity of
bone at the peri-implant interface dependent upon the insertion angle of the mini screw
relative to the direction of the applied orthodontic force.
-71-
Fig. 1: CT images of rabbit tibia; proximal antero-
medial surface encircled
Pilot Study:
The tibiae of three mature male New Zealand white rabbits were obtained from
another unrelated investigation to evaluate cortical bone thickness and to assess
uniformity along the antero-medial surface of the proximal segment. The tibia is the
larger of the two bones of the lower leg, lying medial to the fibula, and fused with the
latter for more than one-half of its length to its distal end. The proximal end is triangular
in cross-section, with anterolateral, anteromedial, and posterior surfaces (fig. one).97
Cone beam CT scans (Hitachi MercuRay cone beam computed tomography system;
Hitachi Medical Systems, Tokyo, Japan)
were performed on the six tibiae, and the
cortical thickness was measured at 6mm
intervals using the CT imaging software
E-film Workstation 2.1 (Merge E-Med)
(table 3). The average overall cortical
bone thickness along the antero-medial surface of the proximal tibial segment was
1.27mm (+/- 0.1mm). These values are comparable to the cortical bone thickness of
human maxillary and mandibular bone as determined by Miyamoto et al. (2005),
Deguchi et al. (2006), and Ono et al. (2008).62-65
CT scans of 225 implant insertion sites
in fifty patients revealed an average cortical bone thickness of 1.49mm (+/-0.34mm) in
the maxilla and 2.22mm (+/-0.47mm) in the mandible.64
Baumgaertel et al. (2009)
analyzed CBCT scans of thirty adult dry skulls and their estimates of cortical bone
thickness were slightly more conservative than that demonstrated by other authors.62-65
-72-
They also demonstrated that cortical bone thickness slightly increased as incremental
measurements descended apically from the alveolar crest toward the underlying basal
bone.62
The cortical bone thickness of rabbit tibia appears to closely correspond to that of
the human maxilla. Also, the cortical bone exhibited uniform thickness along the entire
examined length of the proximal segment. One-way analysis of variance comparing
cortical bone thickness at each of the ten measurement points showed no significant
difference (F(9, 50)= 0.955, p= 0.487) amongst the groups (SPSS v16.0). The above
findings confirm the appropriateness of the rabbit tibia model for use in the present study.
Table 3. Measures of cortical bone thickness at 6mm intervals along rabbit tibia proximal segment
(Total of 6 tibiae from 3 mature white New Zealand rabbits)
Test Tibia
0
0mm
6
6mm
12mm
1
18mm
24mm
30mm
36mm
42mm
48mm
54mm
Average
Thickness†
Std Dev
1
1.4
1.2
1.3
1
1.3
1.4
1.4
1.4
1.3
1.2
1.3 1.32
0.08
2
1.4
1
.2
1
1.3
1
1.3
1.3
1.3
1.3
1.2
1.2
1.2 1.27
0.07
3
1.4
1
.3
1
1.2
1
1.2
1.3
1.2
1.3
1.2
1.3
1.3 1.27
0.07
4
1
1.4
1
.3
1
1.2
1
1.1
1.1
1.1
1.1
1.2
1.2
1.2 1.19
0.10
5
1
1.5
1
.5
1
1.3
1
1.4
1.4
1.4
1.3
1.4
1.4
1.3 1.39
0.07
6
1
1.1
1
.1
1
1.1
1
1.2
1.3
1.3
1.2
1.1
1.2
1.3 1.19
0.09
Average
Thickness††
1
1.37
1
1.27
1
1.23
1
1.25
1.30
1.28
1
1.27
1
1.23
1
1.25
1
1.27 Mean Thickness
1.27
Standard
Deviation
00.14
00.14
00.08
00.10
0.11
0.12
00.10
00.10
00.08
00.05
Mean Std.
Deviation
0.10
† Average uniformity of cortical bone thickness along the length of the rabbit tibia
†† Average cortical bone thickness at specific locations along the length of the rabbit tibia
-73-
Fig. 4: Post-insertion orientation of the three mini-
screws displaying minimal protrusion beyond the
underlying skin
Fig. 2: Dissection of shaved proximal anteromedial
surface of rabbit tibia; the region is void of thick
tissue and ligamentous or muscular attachements
Fig. 3: Post-insertion orientation of unloaded control
(a) and two test mini-screws (b and c) loaded with
NiTi closed-coil; the embellished hole demarcated by
the arrows was created to demonstrate the thin nature
of the tissue overlying the bone
One additional male New Zealand white rabbit cadaver was obtained and utilized
for both exploration of the anatomical site and a mock run through of the experimental
setup. As shown in figure two, the antero-
medial surface of the proximal tibia
provides an approximate 3cm x 2cm area of
direct access to the underlying bone that is
not hindered by thick tissue or ligamentous
and muscular attachments. The thin
movable skin overlying the bone and
periosteum in this region is only 1mm to
2mm thick. First, the orientation of the
three mini screws was marked relative to
the orientation of the underlying bone, then
a 1.0mm diameter tissue punch was used to
perforate the soft tissue layer. Each of the
mini screws was inserted up to its collar
with a manual screwdriver. A nickel-
titanium closed-coil spring was placed
between the two test mini screws and
ligated into position with stainless steel
ligatures (fig. 3). Overall, the procedure
a b
c
-74-
was straightforward and minimally invasive. The post-placement protrusion of the mini
screw heads was minimal (fig. 4).
The rabbit model has been used routinely in bone and implant research. Duyck et
al. (2007) also demonstrated that test rabbits easily tolerated the presence of large, long-
standing percutaneous abutments attached to prosthetic dental implants placed in their
hind limbs.80
In a larger study involving twelve New Zealand white rabbits, Slaets et al.
(2009) demonstrated similar tolerances to exposed and even more protrusive implants
and respective attachments.98
Furthermore, the results of our pilot study corroborate the
suitability of this animal model showing similar cortical bone thickness between that
found in the maxilla, and potentially in the mandible, of humans and rabbit tibia. Finally,
the region less than 1mm from the implant surface in the rabbit tibia has been shown to
exhibit a very high remodeling rate. This is a similar peri-implant bone response seen in a
multitude of species, including rabbits and humans.99
Materials and Methods:
Animal Model:
Six mature, male New Zealand white rabbits (mean weight: 3.91 kg) were
obtained for this investigation. All treatment was approved by the University of Toronto
Animal Care Committee (protocol no. 20008348). Upon arrival, the animals were given
a minimum one-week acclimatization period at the Animal Care Facility, Faculty of
Dentistry.
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Facilities:
The animals were maintained in the Animal Care Facility, Faculty of Dentistry.
All cone beam CT scanning was performed within the Radiology Department, Faculty of
Dentistry. The post-treatment micro CT scans were completed on specimens in the
laboratory of Dr. J. Davies, at the Institute of Biomaterials and Biomedical Engineering,
University of Toronto
Study Design:
Combined pairs of loaded angulated orthodontic mini screws, and a single
unloaded control, were randomly allocated to each rabbit tibia. The type and placement
of orthodontic mini screws were as follows:
1) A total of 36 orthodontic mini screws (IMTEC Corporation, Ardmore, Oklahoma,
USA) with 1.8mm diameter and 6mm length (product #: IMTECORTH6);
2) Orthodontic mini screws were placed at one of three optional angles (Fig. 5):
a. Angled 60- 70º from the cortical surface of the rabbit tibia in the direction of the
applied orthodontic load;
b. Angled 60-70º from the cortical surface of the rabbit tibia away from the applied
orthodontic load;
c. Angled perpendicular to the cortical surface of the rabbit tibia and to the applied
orthodontic load;
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3) Experimental orthodontic mini screws were allocated as pairs to each rabbit tibia,
the angulations randomly assigned, so that an equal number of each angulation
was included within the study (fig. 5);
4) A third, unloaded orthodontic mini screw with randomly assigned angulation was
designated as a control in each tibia (fig. 5);
5) The two loaded orthodontic mini screws were placed 20mm apart, measured from
the screw heads, in the anteromedial bony surface of the tibia and loaded after
two weeks with a continuous traction force of 150g generated by a 6mm nickel-
titanium closed-coil spring (Nitinol, 3M Unitek; product #: LCC6M-10) (fig. 5);
6) The Ni-Ti closed-coil springs were tested using a calibrated force gauge to ensure
relatively consistent loading across all experimental mini screws; and
7) Two stainless steel reference markers were inserted into the cortical bone between
the loaded orthodontic mini screws.
Figure 5: orientation of mini screws in relation to applied orthodontic forces
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Fig. 7: Placement of angulated orthodontic mini screws
using fabricated metal stent
Fig. 6: Initial incision into rabbit tibia with soft tissue
reflection of periosteum to expose cortical bone surface
Orthodontic Mini Screw Insertion (Initial Surgery):
At the time of initial surgery, animals were anesthetized by induction with
Ketamine HCL and Xylazine, and then
maintained with Isoflurane via
inhalation. The proximal aspects of both
left and right tibiae were carefully
shaved and disinfected. Local
anesthetic (Lidocaine) was injected at
the surgical site (0.25ml per tibia). Two linear incisions, approximately 15 mm in length,
were made parallel to the long axis of the tibia. The first incision was immediately distal
to the proximal metaphysis, while the second was proximal to the midline of the
diaphysis. The overlying tissues were reflected at the two sites to expose the cortical
bone surface (fig. 6).
All orthodontic mini screws were
placed by the principal investigator
using the Imtec O-Driver. To facilitate
placement of the orthodontic mini
screws, the exposed cortical bone
surface was penetrated up to 0.5mm
with a #2 round bur in a slow-speed
handpiece. Each orthodontic mini screw was placed with an insertion angle dependent
upon the allocated group, as noted previously. A metal guide with two arms fabricated of
Fig. 5: orientation of mini-screws in relation to applied orthodontic forces
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Fig. 8: Determination of inter-implant distance to
ensure uniform loading with Ni-Ti closed-coil springs
Fig. 9: Placement of stainless steel reference pin
(demarcated by single arrow)
0.5mm diameter stainless steel wire directed at angulations of 60° and 70° from the base
aided in placement of the orthodontic mini screws (fig. 7). An inter-implant distance of
20mm was measured between the heads of
the two experimental orthodontic mini
screws (fig. 8). Furthermore, the implant
insertion torque was measured with a
torque driver (Brånemark System®
manual
torque wrench), labeled at 5Ncm
increments and retrofitted with a
modified adapter (Imtec, product #
LT035), to ensure the torque values of the
individual orthodontic mini screws
exceeded the minimum value of the
optimal range of 5 Ncm to 10 Ncm as
determined by Motoyoshi et al. (2006).52
Two fabricated surgical grade
stainless steel pins (0.41mm diameter; 2.5mm length), that were sharpened at one end,
were inserted into the cortical bone by manual pressure using a concave center punch
between the two experimental orthodontic mini screws (fig. 9).100, 101
The surgical sites
were sutured with 4-0 Vicryl leaving the heads of the orthodontic mini screws exposed
transcutaneously. Appropriate antibiotic (Baytril) and analgesic (Buprenorphine HCl)
coverage was given post-operatively.
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Fig. 11: Exposure of orthodontic mini screws and
placement of Ni-Ti closed-coil spring.
Fig. 10: Setup and positioning for cone beam CT
scans within the Hitachi MercuRay unit.
Treatment Regimen:
After fourteen days, the animals were anesthetized with a Ketamine HCL and
Metetomidine combination. The rabbits were individually transported to the cone beam
CT unit (Hitachi MercuRay cone beam computed tomography system; Hitachi Medical
Systems, Tokyo, Japan) within the
Department of Radiology, Faculty of
Dentistry. The rabbits were placed on a
specially designed holder and a baseline
cone beam CT scan (P-mode: 22cm field
of view at 100kvp and 10ma) was taken of
both left and right tibia (fig. 10).
Once transported back to the Animal Care Facility, the rabbits were carefully
monitored while their tibias were again shaved and disinfected. Digital pressure was
applied over the heads of the orthodontic
mini screws where tissue overgrowth
occurred and small 5mm incisions were
made with a scalpel blade for exposure of
the orthodontic mini screw heads. The
6mm Ni-Ti closed-coil springs were
attached to the two experimental orthodontic mini screws and fixated with stainless steel
ligatures (fig. 11). Again, appropriate antibiotic (Baytril) and analgesic (Buprenorphine)
coverage was provided.
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Figure 12: Timeline of experimental protocol and analysis
After a period of no greater than fifteen days from the time of initial loading of
the orthodontic mini screws the rabbits were euthanized by T-61 injection into the ear
vein (Hoechst, Regina, Saskatchewan, Canada) (fig. 12). Specimen collection involving
dissection of the left and right tibial segments was undertaken immediately and all
specimens were stored in formaldehyde for four weeks prior to undergoing alcohol
dehydration of increasing gradients. Cone beam CT scans were performed on all tibia
specimens using identical settings to those described previously.
According to most published reports on the topic of orthodontic mini screws,
failure is clinically evident and is often described as “loosening with mobility”.
Therefore, a successful orthodontic mini screw is considered immobile only if it remains
so throughout the treatment duration.21, 30
From the onset of this experiment, each of the
loaded orthodontic mini screws within an individual rabbit tibia, in addition to the
unloaded control, were inspected daily until either one of the experimental orthodontic
mini screws failed, or the treatment duration was complete. When failure of an
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orthodontic mini screw occurred, it was to be recorded, immediately replaced, and the Ni-
Ti closed-coil spring re-attached so that the other loaded orthodontic mini screw may
continue to be observed over the duration of the study. After termination, each
orthodontic mini screw was again inspected for mobility. A Ni-Ti closed-coil spring,
identical to those used in the study, was re-attached and retracted 5mm to replicate the
experimental loading regimen. Any mobility of the orthodontic mini screws was
recorded.
Fluorescent Bone Labeling:
Two fluorescent bone labels, calcein green and xylenol orange, were administered
to the rabbits at different time points during the experiment. All fluorescent dyes were
freshly prepared and sterile filtered (0.22um pore size) with brief storage at 4°C in a dark
environment. A subcutaneous dose of 15mg/ kg calcein green (Sigma-Aldrich, Ontario,
Canada) was administered as a 2% solution buffered in 2% sodium bicarbonate (Sigma-
Aldrich, Ontario, Canada) at the time of orthodontic mini screw insertion. Xylenol
orange (Sigma-Aldrich, Ontario, Canada) was given at 90mg/ kg as a 3% solution
buffered in 2% sodium bicarbonate (Sigma-Aldrich, Ontario, Canada) on day fourteen.
This coincided with the initiation of loading of the orthodontic mini screws (fig. 12).
Micro CT Scans:
Micro CT scans of each sample were conducted with a micro-tomography system
(MicroCT40, Scanco Medical, Basserdorf, Switzerland). All trimmed tibial samples were
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placed in a poly-methyl-methacrylate (PMMA) holder with distilled water and scanned at
70kVp and 70μA. The specimens were scanned in high-resolution mode with an X, Y,
and Z resolution of 15μm, and acquisition files were obtained at 1000 projections with
2,048 samples each (per 180º of rotation), 0º angle increment, 300ms of integration time,
and 1 frame averaging. The scanning time for each specimen was approximately 1.8
hours.
The final 3D images were composed of 500-700 axial-cut slices, each one being
15μm in thickness. After scanning and reconstruction, a region of interest (ROI)
comprising the peri-implant region was drawn at different sites and depths of the 3D
dataset, so that the final drawings could be morphed and a 3D ROI rendered. Within the
newly designed ROI, bone percentage quantification was possible through threshold
segmentation, which was determined by analyzing the gray-level distribution. Threshold
values were kept the same for all samples. The scans were also inspected for the
presence and location of marginal bone defects and other anomalies around the
orthodontic mini screw collar.
Analysis:
The radiology software CB Works (version 2.0, CyberMed, Seoul, Korea) was
used to obtain all measurements and statistical analyses were performed using SPSS
software (version16.0 for Windows, SPSS, Chicago, Ill). Stability of the stainless steel
reference markers was assessed by comparing the distance between the pair of stainless
steel pins from the initial and post-treatment cone beam CT scans. All measurements
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were repeated three times by the same examiner, and made along the external cortical
bone surface from the mid-point of each of the reference pins.
Failure rates of the orthodontic mini screws, including loss due to infection,
significant mobility, or retention, were compared between the different angulation groups
relative to the control groups. Any significant differences in movement over time,
determined by the change in linear distance from the head, body, and apex of the
orthodontic mini screws with respect to the stainless steel reference markers, were
assessed from the cone beam CT scans using one-way ANOVA between the three
different loaded angulation groups and control (unloaded) groups. Changes in angulation
of the orthodontic mini screws, as measured from the angle formed by the long axis of
the orthodontic mini screw and the cortical bone surface, and relative to the direction of
applied orthodontic force where applicable, were also obtained from the cone beam CT
scans. All measures were repeated three times by the same examiner, with a minimum of
one week between repetitions, and the respective averages used for analysis.
Volumetric analysis of bone percentage at the peri-implant interface was
calculated from micro CT scans along the length of the orthodontic mini screw within the
confines of the cortical bone. Comparisons of bone-to-implant contact were made
between the three different angulation groups and control groups using one-way
ANOVA. Post-hoc analysis was undertaken with Tukey test.
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Results:
Orthodontic mini screw retention:
All thirty-six orthodontic mini screws survived the entire duration of the study
period. However, four of the experimental orthodontic mini screws were not subjected to
the loading regimen due to superficial infections that occurred during the initial two week
healing period. Culture swabs later identified these infections as predominantly of E. coli
origin. Regardless, one hundred percent (loaded n= 20, control n= 12) of the orthodontic
mini screws, including those not loaded with adjacent superficial infections (n= 4),
resisted loading by ex vivo reattachment of a nickel-titanium closed-coil spring,
demonstrating no signs of mobility.
Movement of orthodontic mini screws:
The stainless steel reference markers remained stable throughout the experimental
period. An overall inter-distance change of -0.18mm (SD= 0.3125mm) was found when
the measurements for all twenty-four reference markers were combined.
The measurements obtained to detect movement and angulation changes of the
orthodontic mini screws were inspected for normal distribution. Both Kolmogorov-
Smirnov and Shapiro-Wilk tests confirmed the normality of the data and the use of
parametric testing. In addition, for reasons not directly related to the orthodontic mini
screws, two of the New Zealand white rabbits were euthanized at only one week post-
loading with the Ni-Ti closed-coil springs. However, the linear measurements for each of
the loaded sub-groups (one week and two weeks) were analyzed using two sample t-
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Table 4. Sample sizes for each of the orthodontic mini screw orientations with respect to duration of
loading (one week vs. two weeks). The results of two sample T-tests (p-values) based on linear
amounts of movement of the orthodontic mini screws support combining the data irrespective of the
duration of loading.
tests. None of the associated sub-groups was found to significantly differ from the other,
irrespective of the loading regimen (table 4). Graphical representation of the linear
measures for movement of the head, body, and apex of the orthodontic mini screws in
figures 13, 14, and 15 demonstrates the similarity of the variably loaded sub-groups,
relative to their combined measures, and a lack of any detectable trend. All sub-groups
presented average movement values of less than 0.5mm, with all but two sub-groups
(apex measures of orthodontic mini screws loaded for only one week and placed either
perpendicular or angulated away from the applied force) exhibiting displacement values
less than 0.2mm (fig. 15). Therefore, movement measures for all loaded orthodontic mini
screws were combined prior to assessing whether or not loaded orthodontic mini screws
of variable placement angulations migrated through bone relative to unloaded (control)
orthodontic mini screws.
Orthodontic Mini Screw Orientation
Duration of Loading p-values (significance p<0.05)
One Week Two Weeks Combined Head Body Apex
Control (Unloaded)
Perpendicular 2 4 6 0.819 0.195 0.621
Angulated 2 4 6 0.543 0.644 0.546
Experimental (Loaded)
Perpendicular 3 4 7 0.854 0.187 0.306
Angulated Toward 2 4 6 0.301 0.211 0.854
Angulated Away 1 6 7 0.949 0.187 0.234
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One-way ANOVA was used to compare the migration measurements of loaded
orthodontic mini screws placed at the three different angulations versus perpendicular
and angulated unloaded controls. There were no statistically significant differences in the
measures taken at the head of the orthodontic mini screws between the five
aforementioned groups at the p< 0.05 significance level [F (4, 27)= 2.0149, p> 0.05] (fig.
16). There were also no significant differences detected amongst loaded and unloaded
(control) orthodontic mini screws of variable angulations when movement at either the
mid-point of the body where it traverses the cortical bone surface [F (4, 27)= 0.4904, p>
0.05] or apex [F (4, 27)= 0.2743, p> 0.05] were examined (figures 16 and 17). Again, the
average amounts of orthodontic mini screw migration over the maximum two week
loading period was no greater than 0.2mm for all groups assessed.
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Fig. 16:
Fig. 17:
Fig. 18:
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Changes in the angulations of the orthodontic mini screws were also assessed.
Measurements obtained from cone beam CT scans confirmed that the actual placement
angulations of the orthodontic mini screws prior to loading corresponded to the
experimental protocol. Perpendicular and angulated control (unloaded) orthodontic mini
screws had average placement angulations of 87.87° (SD= 3.80) and 69.59° (SD= 3.20),
respectively. In the experimental (loaded) groups, the perpendicular orthodontic mini
screws had an initial average angulation of 91.65° (SD= 4.46). The orthodontic mini
screws angulated either toward or away from the applied orthodontic force, had average
placement angulations of 68.51° (SD= 3.39) and 67.22° (SD= 1.20), respectively.
Similar to the linear measures, two sample t-tests were used to investigate
potential differences in angulation changes among those orthodontic mini screws loaded
for either one week or two weeks. No statistically significant differences were found
between the variably loaded groups for orthodontic mini screws placed perpendicular to
(p= 0.441), angulated toward (p= 0.996), or angulated away from (p= 0.868) the direction
of applied orthodontic force (fig. 19). All groups demonstrated an angulation change of
less than three degrees. One-way ANOVA was used to assess differences amongst the
loaded and unloaded angulation groups. There were no statistically significant
differences in the amount of change in angulation seen in any of the five groups of
variably angulated (loaded and unloaded) orthodontic mini screws [F (4, 27)= 1.075, p>
0.05]. Overall, the loaded orthodontic mini screws demonstrated an average change in
angulation of 2.08° (SD= 1.86), in the direction of the applied orthodontic load. As
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mentioned, this change in angulation was similar to that elicited by the unloaded
orthodontic mini screws, and was independent of placement angulation.
Since there were no statistically, nor clinically, significant differences in the
amount of movement detected among the variably angulated loaded orthodontic mini
screws all measurements at each of the three reference points (head, body, and apex)
were pooled to provide an overall estimate of the degree of orthodontic mini screw
migration (fig. 20). The head of the orthodontic mini screws demonstrated an overall
non-significant average displacement of 0.119mm (SD= 0.108) in the direction of the
applied orthodontic force. The body of the orthodontic mini screw, at the juncture of the
cortical bone surface, also migrated in the direction of the applied orthodontic force
(mean= 0.117mm, SD= 0.097). However, as shown in figure 20, the apex of the
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orthodontic mini screws moved in a direction opposite that of the applied orthodontic
force by an average of 0.16mm (SD= 0.175).
Micro CT analysis:
The rabbit tibia consists entirely of cortical bone, and was void of any
trabeculations. Therefore, quantitative analysis of bone-to-implant contact was used to
assess the degree of osseointegration among the groups of variably angulated orthodontic
mini screws as they traversed through the cortical bone layer. There were no statistically
significant differences detected when two sample t-tests were used to compare one week
and two weeks loading regimens (p> 0.05). One-way ANOVA revealed a significant
difference in the amount of cortical bone-to-implant contact between the different groups
of orthodontic mini screws [F (4, 25)= 4.3023, p= 0.0087]. Post hoc comparisons using
Fig. 20:
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Fig. 22: Micro CT image of orthodontic mini
screw in association with thickened cortical
bone. X-ray scatter about the orthodontic mini
screw causing visible halation.
the Tukey test indicated that the difference in mean bone-to-implant contact for the
perpendicular control group of orthodontic mini screws (mean= 98.5%, SD= 0.015)
versus the loaded group of orthodontic mini
screws placed with an angulation opposing
the direction of applied orthodontic force
(mean= 93.4%, SD= 0.034) was statistically
significant (fig. 21). There were no
statistically significant differences between
the variably angulated groups of orthodontic
mini screws that underwent delayed loading.
Inspection of the peri-implant interface
based on micro CT analysis revealed a thickening of the cortical bone in the immediate
vicinity for the majority of the orthodontic mini screws (figures 22 and 23). There was
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Fig. 23: Micro CT image of a longitudinal slice through the threaded portion of the orthodontic mini
screw illustrating the high degree of bone-to-implant contact
Fig. 24: 3D rendering of an experimental orthodontic mini screw traversing through the cortical bone.
Pink regions denote bone-to-implant contact, whereas green zones depict areas void of bone
no correlation of increased cortical bone thickness with either loading regimen or
placement angulation of the orthodontic mini screws. Longitudinal slices through the
threads of the orthodontic mini screws demonstrated significant osseointegration within
the original cortical bone layers, and also in the regions of cortical thickening (figure 23).
In addition, there was no
evidence of any effect of placement angulation of the orthodontic mini screws on the
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Fig. 25: Micro CT image depicting the presence
of a “micro-crack” within the cortical bone
adjacent to the orthodontic mini screw (denoted
by red arrows)
presence of cortical bone defects. Figure 24 illustrates the high degree of bone-to-
implant contact detected using threshold segmentation techniques on the micro CT data.
As shown, there are scant regions along the exterior of the orthodontic mini screws as
they traverse the cortical layer that are not in direct contact with bone.
Of interest, several of the orthodontic
mini screws demonstrated the presence of
“micro-cracks” in the immediate cortical bone.
All of the “micro-cracks” discovered on the
micro CT images consisted of a depth of no
greater than 75 um. Again, there were no
correlations for the presence and location of the
“micro-cracks” with either loading regimen or
placement angulation of the orthodontic mini
screws.
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Discussion:
The initial experimental protocol intended to have the orthodontic mini screws
undergo immediate loading for a total duration of three months, throughout which the
loaded orthodontic mini screws underwent continuously applied orthodontic forces from
the Ni-Ti closed-coil springs. This time period was based on a ratio related to the
average duration of use for orthodontic mini-screws in humans and the bony remodeling
cycle differences between humans and rabbits. The conglomerate of cells, including
osteoblasts and osteoclasts, involved in remodeling of bone is known as a basic
multicellular unit (BMU). Once each individual BMU has completed a remodeling cycle,
the end result is termed a bone structural unit (BSU).102
The total time that any particular
BMU remains active to resorb and re-deposit a unit amount of bone is denoted as
“sigma”. In general, sigma is directly proportional to animal size. For instance, rabbits
have a sigma of approximately 6 weeks and humans have a sigma of 16- 20 weeks.102, 103
In humans, orthodontic mini screws are mainly used under orthodontic loading for a
period of 8- 10 months or less before removal. The intended three month duration of this
study was approximately one-third this time period, which corresponds to the relative
sigma ratio between rabbits and humans. However, the actual experimental loading
period of a maximum of two weeks approximately corresponds with a loading duration of
1.5 months for humans.102-104
The orthodontic mini screws in the present study underwent a delayed loading
regimen, with an initial healing period of two weeks. However, the study was repeated
twice. The original study design planned for immediate loading of the experimental
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orthodontic mini screws. When the initial animal trials were undertaken, two of the New
Zealand white rabbits developed unilateral leg fractures simultaneously within two days
of post-op placement of the orthodontic mini screws. Even though any surgical
intervention involving placement of orthodontic mini screws will ultimately weaken the
load bearing ability of the tibia, the circumstances surrounding the unilateral leg fractures
cannot be ignored. All of the animals, despite being separated into individual cages, were
“stampeding” for several minutes at the time of inspection and analgesic administration.
Though debatable, this is an unusual phenomenon when the severity of the behavior is
considered. During and afterwards, it was noticed that the two of the rabbits had
unilateral tibial fractures. As a result, a decision was made to halt the study prior to
placing orthodontic mini screws in the remaining New Zealand white rabbits. Numerous
studies reported in the dental literature have outlined the use of rabbit tibia specifically as
an experimental site for examination of prosthetic dental implants.105-111
Several articles
have also used the same medial surface of the rabbit tibia for experimentation with
orthodontic mini implants and mini screws.112-114
None of these articles reported any
fractures within the tibia as a result of implant placement. Furthermore, not all of the
oblique fractures observed in the present study traversed through an implant site.
The orthodontic mini screws (IMTEC Corporation, Ardmore, Oklahoma, USA)
used in the present study met the recommended standards in terms of implant dimensions
for placement in a rabbit model. Pearce et al. (2007) outlined a series of standards for
various animal models used in implant experimentation. Implants with a diameter of no
greater than two millimeters, and a thread length of six millimeters should only be used,
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with a maximum of six implants placed in any single rabbit.115
Furthermore, other
studies have exceeded this range with success (i.e. no reported fractures). For example,
MacGregor et al. (2004) placed three larger implants subcutaneously in the mid-diaphysis
of only one tibia per rabbit, with subsequent loading of two of the implants (210g applied
orthodontic force) after a twelve day healing period. There were no reported fractures in
any of the twenty-four experimental rabbits.112
Mo et al. (2010) also placed a series of
eight implants (four per tibia), in a series of 44 New Zealand white rabbits. These
implants had a similar supraperiosteal profile, but a much longer threaded profile
(7.5mm) in comparison to the ones used in the present study. Two of the four implants in
each tibia were immediately loaded, and the rabbits were followed for a variable amount
of time, the longest being ten weeks. Again, there were no reported tibial fractures
associated with the larger diameter implants in any of the study rabbits.114
The reason for the significantly decreased study length, as alluded to in describing
the success rates of the orthodontic mini screws, was due to superficial infection. As
suggested by the literature, the New Zealand white rabbits easily tolerated the presence of
the percutaneous orthodontic mini screws.80, 98
However, the presence of multiple
infections at the surgical site was a new revelation, not reported in the literature. The
presence of E. coli, most likely fecal contamination from the external environment,
resulted in localized infections along the scarred regions from the previous surgical
incisions, and ultimately near the orthodontic mini screws. Daily injections of Baytril
were initiated, but the localized infections were deemed non-responsive by the University
veterinarian. In order to maintain a uniform sample it was necessary to terminate the
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study much sooner than anticipated. Therefore, two of the New Zealand white rabbits
were euthanized at one week post-loading of the orthodontic mini screws, with the entire
study terminated after a maximum of two weeks of orthodontic force application.
Regardless of these shortcomings, the present study is unique and provides useful
insight. Despite the presence of superficial infections, one-hundred percent of the
transcutaneous orthodontic mini screws remained after a maximum study period of
twenty-eight days. Furthermore, all the orthodontic mini screws were clinically
immobile. Some studies have used less stringent criteria for success, such as the ability
to remain load bearing, regardless of mobility.21, 30
The reported failure rates of
orthodontic mini screws are highly variable.15-18
However, Fritz et al. (2004) suggests
that clinician experience and adherence to strict placement protocol is crucial in
determining the success of orthodontic mini screws.20
Significant effort was undertaken
during the pilot study leading up to the present experiment to ensure consistent placement
of the orthodontic mini screws. Aside from assessing the surgical site and investigating
the similarities of cortical bone thickness between rabbit tibia and human maxilla and
mandible, much time was spent placing orthodontic mini screws in tibiae from rabbit
cadaver specimens. Also, during the present study a torque driver was used during the
placement of every orthodontic mini screw to establish insertion torque values within the
recommended range. This is seldom done in a clinical setting, and has not been
incorporated into any of the studies reported in the literature, unless insertion torque
values were a measured outcome of the study.52
All of the above may account for the
consistently high success rates of orthodontic mini screws reported in this study.
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However, under the experimental conditions of the present study angulated placement of
orthodontic mini screws does not appear to be a critical factor for determining, or
enhancing, the success rates of orthodontic mini screws.
The use of stainless steel pins, serving as stable reference markers, permitted
accurate measurement of the potential migration characteristics of orthodontic mini
screws. In the present study, the degree of measurement error of the rigid stainless steel
pins (mean migration= 0.18mm, SD= 0.3125), based from the cone beam CT data, was
found to lie within an acceptable range, as reported in the dental literature.116-118
Therefore, it is unlikely that any movement of the reference pins occurred over the study
period. The previously described Hitachi MercuRay cone beam CT system settings
produced a maximum voxel size of 0.4mm. It was shown that increasing the voxel size
beyond this resolution does not yield greater accuracy of measurements. Although, it is
impossible to ensure reliability of measurement errors of less than the voxel size.119
Furthermore, orientation of the right and left tibia, where the orthodontic mini screws
were placed, during image acquisition also has no significant effect on cone beam CT
measurements.118, 120
Nonetheless, it was very challenging and time consuming obtaining
reliable measurements from 3D data.
The findings of this study contradict some of the cephalometric based findings of
Liou et al. (2004). Seven of the sixteen patients that underwent “en-masse” retraction of
the anterior maxillary segment demonstrated significant movement of the orthodontic
mini screws, by as much as 1.0mm.85
This difference is the result of two possible
reasons. The use of cephalometric superimpositions is not an accurate enough means to
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predict actual movement of the orthodontic mini screws, resulting in significant distortion
from patient positioning, magnification error, visualization in only two dimensions, and
imprecise landmark identification.121
Also, patient growth adds an additional layer of
complexity to superimpositions. An alternative explanation is that the study was of a
significantly longer duration (nine months), thus increasing the potential for orthodontic
mini screw migration. Mortensen et al. (2009) also demonstrated significant movement
of variably loaded orthodontic mini screws placed in five beagle dogs. However, the
applied forces (600g and 900g) were several times higher and representative of
orthopedic forces rather than orthodontic forces.86
In another study, El-Beialy et al.
(2009) found similar migrations of the orthodontic mini screws, but with smaller
continuous orthodontic forces (150g to 250g). Measurements were obtained from more
accurate cone beam CT scans. Furthermore, the authors noted that the apex of the
orthodontic mini screws migrated opposite to the direction of the applied orthodontic
forces.87
This trend was also noted in the present study, but to a much smaller degree.
Liu et al. (2011) demonstrated a lack of significant movement of orthodontic mini
screws over an average six month study period. The measurements were also based on
cone beam CT superimpositions. The predominant difference of this study design was
the use of elastomeric chains, delivering an average initial orthodontic force of 150g.88
However, elastomeric chains do not provide a uniform continuous force, but rather
demonstrate significant decay of the applied forces over a very short period of time.122
As in the present study, the authors found that the orthodontic mini screws did not move
to any appreciable extent. However, it was found that the entire orthodontic mini screw
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translated, though only slightly, through the maxillary bone.88
This differs from the
observations of El-Beialy et al. (2009) and the present study, where the apex of the
orthodontic mini screws migrated in the opposite direction. The orthodontic mini screws
in the latter two studies tended to fulcrum at the approximate mid-point of the orthodontic
mini screws near the boundary of cortical bone, with the head and apex moving in
opposite directions.87
This notion is bolstered by the fluorescent bone labeling study of
Kim et al. (2008) involving loaded orthodontic mini screws. The authors found that the
regions of pressure and tension in the surrounding bone adjacent to the orthodontic mini
screws became interchanged on progression to the apex.43
In addition, the tibia of the
New Zealand white rabbits is void of any trabeculations. This would increase the
reliance for support solely from the surrounding cortical bone. However, this may also
account for the small observed changes in movement of the orthodontic mini screw
apices in the opposing direction relative to the applied orthodontic force. If significant
trabeculations were present the fulcruming of the orthodontic mini screws noted in the
present study may not be observed. Therefore, the quality of the surrounding cortical and
cancellous bone may have a significant role in the type of migration, albeit small,
exhibited by orthodontic mini screws.
Studies by Wilmes et al. (2008) and Pickard et al. (2010) suggested that angulated
placement of orthodontic mini screws would enhance their stability, and ultimately
retention. Both studies, the former using pig cadaver iliac crest while the latter utilized
human cadaver mandibles, only examined primary stability phenomena.26, 95
Wilmes et
al. (2008) examined insertion torque values and found that those orthodontic mini screws
-102-
placed at 60° to 70° angulations exhibited the greatest torque values.26
Pickard et al.
(2010) took this a step further by examining pull-out forces for various angulations of
orthodontic mini screws, relative to the direction of applied force. The results of shear
tests found that pull-out forces were greatest for those orthodontic mini screws angulated
toward the direction of applied force, whereas, those placed at an angle opposing the
applied force exhibited the least. However, all angulations of orthodontic mini screw
placement, regardless of the relative direction of applied force, demonstrated very high
pull-out values that were well beyond any acceptable clinical range for orthodontic and
orthopedic purposes.95
In essence, the findings by Pickard et al. (2010) corroborate those
of the present study, in that orthodontic mini screw placement angulation does not appear
to have a significant impact on stability when clinically relevant forces are applied.
Woodall et al. (2011) undertook a similar study, placing ninety-six orthodontic
mini screws, at variable angulations (30°, 60°, and 90°), in multiple cadaveric maxillae
and mandibles. This was done in conjunction with finite element modeling. Analysis of
variance and post-hoc testing demonstrated significantly less resistance to initial
displacement for orthodontic mini screws placed at 30° angulations.94
This is similar to
the findings of Wilmes et al. (2008), where those orthodontic mini screws placed at
excessively acute angulations (45°) demonstrated the weakest torque values.26
Finite
element analysis corroborated these findings, showing an increase in resistance as
orthodontic mini screw angulation progressed to perpendicular placement. These
simulations also suggested that cortical bone stress increased as placement angulation
deviated from perpendicular.94
The findings of the present study do not wholly support
-103-
these claims. Again, it appears that there is no significant influence of placement
angulation (up to approximately 65° relative to the cortical bone surface) when stability
of orthodontic mini screws is examined over time. However, acute placement
angulations (less than 45° relative to the cortical bone surface) of orthodontic mini screws
were not examined in the present study.
Inaba et al. (2009) examined orthodontic mini screw angulation with a continuous
loading regimen over a two week period. Periotest™ was the predominant means of
analysis, aside from histology, to assess stability. However, it was found that over
twenty-five percent of the orthodontic mini screws were secured with bicortical
anchorage in the nasal bone of the New Zealand white rabbits.91
This significantly
diminished the size of the study sample. The advantage of using cone beam CT scans in
the present study was that all orthodontic mini screws were evaluated to ensure the
presence of only monocortical anchorage.
Micro CT scans are a precise, non-destructive technique often used to examine
the properties of bone. In the present study, this permitted a detailed and reliable
inspection of the peri-implant interface about the orthodontic mini screws that is
comparable to that obtained from histologic analysis. However, the titanium alloy (Ti-
6Al-4V) exhibits much stronger X-ray absorption than bone. During micro CT scanning,
as titanium absorbs and scatters X-ray energy at various rates, it often causes inherent
halation artifacts (fig. 22). These partial volume effects will influence micro CT imaging
and parameters associated with calculating bone density about an implant surface.
Therefore, there is a tendency to overestimate the degree of osseointegration present.
-104-
Nonetheless this technique is superior to histologic based analysis of bone-to-implant
contact.123
Most studies examining the degree of osseointegration of orthodontic mini screws
typically incorporate bone-to-implant contact along the entire length of the implants. 75-78,
82 However, in the present study the New Zealand white rabbit tibia was comprised
solely of cortical bone. Therefore, only the region of the orthodontic mini screws that
traversed the cortical bone layer was inspected. Doing so inflated the overall mean bone-
to-implant contact values, making comparison with other studies difficult. However, the
benefit was a significantly decreased standard deviation allowing for improved inter-
group analysis to detect any potential differences. As discussed, the loaded orthodontic
mini screws with placement angulation opposing the direction of force application
exhibited a statistically significant decreased bone-to-implant contact relative to the
orthodontic mini screw control group (unloaded) with perpendicular placement
angulation. However, this small difference is unlikely to be of any clinical significance.
Of greater interest, the cortical bone underwent significant increases in thickness
in the area immediately adjacent to the orthodontic mini screws. In some instances the
cortical bone near the peri-implant interface, as shown in the micro CT images, was twice
the thickness of the surrounding bone. There are a few reasons for the observed
response. First, the surgical procedure to place the orthodontic mini screws involved
raising flaps to expose the underlying cortical bone. It is likely that the periosteum
remained slightly elevated around the orthodontic mini screws. This would permit bone
to form on the cortical bone surface. However, most of the observed cortical thickening
-105-
was due to internal increases. An alternative explanation involves drill-free placement of
the orthodontic mini screws. Most research based applications have involved pre-drilling
of orthodontic mini screws, resulting in removal of most bone fragments in the pilot hole.
Perhaps when self-drilling orthodontic mini screws are used, the threads of the mini
screws displaced bone material into the internal marrow cavity of the tibia. Last,
threshold segmentation showed a high degree of osseointegration in these thickened
regions. The titanium alloy Imtec orthodontic mini screws may have provided a scaffold
to which the bone fragments could osseointegrate. Since orthodontic mini screws are
only required for a temporary period of time, not all mini screws are composed of
titanium alloys, with several brands manufactured from stainless steel. Eulenberger et al.
(1990) studied both stainless steel and titanium alloy mini screws with regards to peri-
implant bone dynamics and removal torque. It was found that after twelve weeks, the
titanium mini screws had significantly improved bone-to-implant contact values and
higher removal torque values.124
Lee et al. (2010) examined the effects of altered orthodontic mini screw diameter
and shape on the surrounding bone. The presence of vertical “micro-cracks” along the
peri-implant interface was discovered. Furthermore, there was no evidence of these small
irregularities propagating into complete fractures of the adjacent bone. The authors
speculated that this was a result of the stresses placed on the bone surrounding the
orthodontic mini screws at the time of implant placement, and is likely a normal sequelae
of implant placement.125
As illustrated in figure 25, similar vertical “micro-cracks” were
discovered around several of the orthodontic mini screws in the present study. Again,
-106-
micro CT images demonstrated that all of the “micro-cracks” spanned a finite distance of
less than 75um. In the present study, a radiologist was recruited to review all cone beam
CT volumes for the presence of any potential tibia fractures. There were no macroscopic
bone fractures to report.
To date, there are still no long-term clinical studies examining the effects of
orthodontic mini screw angulation on retention and movement characteristics. Future
research on this subject should address whether reported differences, currently based on
primary stability and short-term data for orthodontic mini screws placed at variable
angulations, do in fact have clinically significant impacts over the long-term.
Conclusions:
Despite the rigid experimental conditions, the orthodontic mini screws used in this
study (Imtec Ortho Implant; 1.8mm diameter and 6mm length) demonstrated a one-
hundred percent success rate after a maximum twenty-eight day observation period (no
greater than two weeks of loading). Orthodontic mini screw angulation does not appear
to influence success rates over the short term.
There were no statistically significant differences in the amount of movement, or
change in angulation, demonstrated by orthodontic mini screws that underwent one week
or two weeks of loading with an applied orthodontic force (approximately 150g).
There were no statistically significant differences in the change in angulation of
loaded and unloaded orthodontic mini screws, irrespective of placement angulation
(mean change= 2.08°, SD= 1.86).
-107-
There were also no statistically significant differences in the amount of movement
demonstrated by loaded and unloaded orthodontic mini screws of variable angulations at
their head, body, and apex. The loaded orthodontic mini screws demonstrated a non-
significant amount of movement at their head (mean=0.119mm, SD= 0.108), body
(mean= 0.117mm, SD= 0.097), and apex (mean= -0.16mm, SD= 0.175).
Analysis of percent osseointegration values for the various groups of orthodontic
mini screws revealed no clinically significant differences among the groups. However,
an enlargement of the cortical bone layer in the immediate vicinity of the orthodontic
mini screws was observed for all groups.
It appears that orthodontic mini screws do not exhibit any appreciable degree of
movement in the short-term when loaded with a continuously applied orthodontic force,
and that orthodontic mini screw angulation has an insignificant impact on mitigating any
potential migration.
-108-
References:
1. Costa A, Raffaini M, Melsen B. Miniscrews as orthodontic anchorage: a preliminary
report. Int J Adult Orthodon Orthognath Surg. 1998;13(3):201-9.
2. Daskalogiannakis J. 0 Glossary of orthodontic terms. Anonymous Chicago, Ill.;
London: Quintessence Publ.; 2000.
3. Reynders R, Ronchi L, Bipat S. Mini-implants in orthodontics: a systematic review
of the literature. Am J Orthod Dentofacial Orthop. 2009 May;135(5):564.e1,19;
discussion 564-5.
4. Lai EH, Yao CC, Chang JZ, Chen I, Chen YJ. Three-dimensional dental model
analysis of treatment outcomes for protrusive maxillary dentition: comparison of
headgear, miniscrew, and miniplate skeletal anchorage. Am J Orthod Dentofacial
Orthop. 2008 Nov;134(5):636-45.
5. Thiruvenkatachari B, Pavithranand A, Rajasigamani K, Kyung HM. Comparison and
measurement of the amount of anchorage loss of the molars with and without the use of
implant anchorage during canine retraction. Am J Orthod Dentofacial Orthop. 2006
Apr;129(4):551-4.
6. Thiruvenkatachari B, Ammayappan P, Kandaswamy R. Comparison of rate of canine
retraction with conventional molar anchorage and titanium implant anchorage. Am J
Orthod Dentofacial Orthop. 2008 Jul;134(1):30-5.
7. Garfinkle JS, Cunningham LL,Jr, Beeman CS, Kluemper GT, Hicks EP, Kim MO.
Evaluation of orthodontic mini-implant anchorage in premolar extraction therapy in
adolescents. Am J Orthod Dentofacial Orthop. 2008 May;133(5):642-53.
-109-
8. Upadhyay M, Yadav S, Nagaraj K, Patil S. Treatment effects of mini-implants for
en-masse retraction of anterior teeth in bialveolar dental protrusion patients: a
randomized controlled trial. Am J Orthod Dentofacial Orthop. 2008
Jul;134(1):18,29.e1.
9. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod. 1997
Nov;31(11):763-7.
10. Baumgaertel S. State of the art of miniscrew implants: an interview with Sebastian
Baumgaertel. Interviewed by Robert P. Scholz. Am J Orthod Dentofacial Orthop. 2009
Aug;136(2):277-81.
11. Bornstein MM, Harnisch H, Lussi A, Buser D. Clinical performance of wide-body
implants with a sandblasted and acid-etched (SLA) surface: results of a 3-year follow-
up study in a referral clinic. Int J Oral Maxillofac Implants. 2007 Jul-Aug;22(4):631-8.
12. Khayat PG, Milliez SN. Prospective clinical evaluation of 835 multithreaded
tapered screw-vent implants: results after two years of functional loading. J Oral
Implantol. 2007;33(4):225-31.
13. Crismani AG, Bertl MH, Celar AG, Bantleon HP, Burstone CJ. Miniscrews in
orthodontic treatment: review and analysis of published clinical trials. Am J Orthod
Dentofacial Orthop. 2010 Jan;137(1):108-13.
14. Schatzle M, Mannchen R, Zwahlen M, Lang NP. Survival and failure rates of
orthodontic temporary anchorage devices: a systematic review. Clin Oral Implants Res.
2009 Dec;20(12):1351-9.
-110-
15. Berens A, Wiechmann D, Dempf R. Mini- and micro-screws for temporary skeletal
anchorage in orthodontic therapy. J Orofac Orthop. 2006 Nov;67(6):450-8.
16. Cheng SJ, Tseng IY, Lee JJ, Kok SH. A prospective study of the risk factors
associated with failure of mini-implants used for orthodontic anchorage. Int J Oral
Maxillofac Implants. 2004 Jan-Feb;19(1):100-6.
17. Chen Y, Kyung HM, Zhao WT, Yu WJ. Critical factors for the success of
orthodontic mini-implants: a systematic review. Am J Orthod Dentofacial Orthop. 2009
Mar;135(3):284-91.
18. Tsaousidis G, Bauss O. Influence of insertion site on the failure rates of orthodontic
miniscrews. J Orofac Orthop. 2008 Sep;69(5):349-56.
19. Antoszewska J, Papadopoulos MA, Park HS, Ludwig B. Five-year experience with
orthodontic miniscrew implants: a retrospective investigation of factors influencing
success rates. Am J Orthod Dentofacial Orthop. 2009 Aug;136(2):158.e1,10; discussion
158-9.
20. Fritz U, Ehmer A, Diedrich P. Clinical suitability of titanium microscrews for
orthodontic anchorage-preliminary experiences. J Orofac Orthop. 2004 Sep;65(5):410-
8.
21. Chen YJ, Chang HH, Huang CY, Hung HC, Lai EH, Yao CC. A retrospective
analysis of the failure rate of three different orthodontic skeletal anchorage systems.
Clin Oral Implants Res. 2007 Dec;18(6):768-75.
22. Motoyoshi M, Matsuoka M, Shimizu N. Application of orthodontic mini-implants
in adolescents. Int J Oral Maxillofac Surg. 2007 Aug;36(8):695-9.
-111-
23. Motoyoshi M, Uemura M, Ono A, Okazaki K, Shigeeda T, Shimizu N. Factors
affecting the long-term stability of orthodontic mini-implants. Am J Orthod Dentofacial
Orthop. 2010 May;137(5):588.e1,588.e5.
24. Wilmes B, Ottenstreuer S, Su YY, Drescher D. Impact of implant design on
primary stability of orthodontic mini-implants. J Orofac Orthop. 2008 Jan;69(1):42-50.
25. Cho Y, Cha J, Hwang C. The effect of rotation moment on the stability of
immediately loaded orthodontic miniscrews: a pilot study. The European Journal of
Orthodontics. 2010 2010;32(6):614-9.
26. Wilmes B, Su YY, Drescher D. Insertion angle impact on primary stability of
orthodontic mini-implants. Angle Orthod. 2008 Nov;78(6):1065-70.
27. Lim HJ, Eun CS, Cho JH, Lee KH, Hwang HS. Factors associated with initial
stability of miniscrews for orthodontic treatment. Am J Orthod Dentofacial Orthop.
2009 Aug;136(2):236-42.
28. Viwattanatipa N, Thanakitcharu S, Uttraravichien A, Pitiphat W. Survival analyses
of surgical miniscrews as orthodontic anchorage. American Journal of Orthodontics &
Dentofacial Orthopedics. 2009 200907;136(1):29-36.
29. Esposito M, Grusovin MG, Chew YS, Coulthard P, Worthington HV. One-stage
versus two-stage implant placement. A Cochrane systematic review of randomised
controlled clinical trials. Eur J Oral Implantol. 2009 Summer;2(2):91-9.
30. Asscherickx K, Vande Vannet B, Wehrbein H, Sabzevar MM. Success rate of
miniscrews relative to their position to adjacent roots. Eur J Orthod. 2008
Aug;30(4):330-5.
-112-
31. Motoyoshi M, Ueno S, Okazaki K, Shimizu N. Bone stress for a mini-implant close
to the roots of adjacent teeth--3D finite element analysis. Int J Oral Maxillofac Surg.
2009 Apr;38(4):363-8.
32. Zheng L, Tang T, Deng F, Zhao Z. The influence of extraction on the stability of
implanted titanium microscrews: a biomechanical and histomorphometric study. Int J
Oral Maxillofac Implants. 2009 Mar-Apr;24(2):267-74.
33. Baek SH, Kim BM, Kyung SH, Lim JK, Kim YH. Success rate and risk factors
associated with mini-implants reinstalled in the maxilla. Angle Orthod. 2008
Sep;78(5):895-901.
34. Wilmes B, Rademacher C, Olthoff G, Drescher D. Parameters affecting primary
stability of orthodontic mini-implants. J Orofac Orthop. 2006 May;67(3):162-74.
35. Mischkowski RA, Kneuertz P, Florvaag B, Lazar F, Koebke J, Zoller JE.
Biomechanical comparison of four different miniscrew types for skeletal anchorage in
the mandibulo-maxillary area. Int J Oral Maxillofac Surg. 2008 Oct;37(10):948-54.
36. Lim SA, Cha JY, Hwang CJ. Insertion torque of orthodontic miniscrews according
to changes in shape, diameter and length. Angle Orthod. 2008 Mar;78(2):234-40.
37. Song YY, Cha JY, Hwang CJ. Mechanical characteristics of various orthodontic
mini-screws in relation to artificial cortical bone thickness. Angle Orthod. 2007
Nov;77(6):979-85.
38. Kim JW, Baek SH, Kim TW, Chang YI. Comparison of stability between
cylindrical and conical type mini-implants. Mechanical and histological properties.
Angle Orthod. 2008 Jul;78(4):692-8.
-113-
39. Florvaag B, Kneuertz P, Lazar F, Koebke J, Zoller JE, Braumann B, et al.
Biomechanical properties of orthodontic miniscrews. An in-vitro study. J Orofac
Orthop. 2010 Jan;71(1):53-67.
40. Morarend C, Qian F, Marshall SD, Southard KA, Grosland NM, Morgan TA, et al.
Effect of screw diameter on orthodontic skeletal anchorage. Am J Orthod Dentofacial
Orthop. 2009 Aug;136(2):224-9.
41. Miyawaki S, Koyama I, Inoue M, Mishima K, Sugahara T, Takano-Yamamoto T.
Factors associated with the stability of titanium screws placed in the posterior region
for orthodontic anchorage. Am J Orthod Dentofacial Orthop. 2003 Oct;124(4):373-8.
42. Brinley CL, Behrents R, Kim KB, Condoor S, Kyung HM, Buschang PH. Pitch and
longitudinal fluting effects on the primary stability of miniscrew implants. Angle
Orthod. 2009 Nov;79(6):1156-61.
43. Kim TW, Baek SH, Kim JW, Chang YI. Effects of microgrooves on the success
rate and soft tissue adaptation of orthodontic miniscrews. Angle Orthod. 2008
Nov;78(6):1057-64.
44. Chehroudi B, Gould TR, Brunette DM. A light and electron microscopic study of
the effects of surface topography on the behavior of cells attached to titanium-coated
percutaneous implants. J Biomed Mater Res. 1991 Mar;25(3):387-405.
45. Kim YK, Kim YJ, Yun PY, Kim JW. Effects of the taper shape, dual-thread, and
length on the mechanical properties of mini-implants. Angle Orthod. 2009
Sep;79(5):908-14.
-114-
46. Chaddad K, Ferreira AF, Geurs N, Reddy MS. Influence of surface characteristics
on survival rates of mini-implants. Angle Orthod. 2008 Jan;78(1):107-13.
47. Ikeda H, Rossouw PE, Campbell PM, Kontogirogos E, Buschang PH. Three-
dimensional analysis of peri-bone–implant contact of rough-surface miniscrew
implants. American Journal of Orthodontics & Dentofacial Orthopedics. 2011
201102;139(2):e153-63.
48. Lin CL, Yu JH, Liu HL, Lin CH, Lin YS. Evaluation of contributions of
orthodontic mini-screw design factors based on FE analysis and the Taguchi method. J
Biomech. 2010 Aug 10;43(11):2174-81.
49. Heidemann W, Terheyden H, Gerlach KL. Analysis of the osseous/metal interface
of drill free screws and self-tapping screws. J Craniomaxillofac Surg. 2001
Apr;29(2):69-74.
50. Kim JW, Ahn SJ, Chang YI. Histomorphometric and mechanical analyses of the
drill-free screw as orthodontic anchorage. Am J Orthod Dentofacial Orthop. 2005
Aug;128(2):190-4.
51. Wilmes B, Drescher D. Impact of insertion depth and predrilling diameter on
primary stability of orthodontic mini-implants. Angle Orthod. 2009 Jul;79(4):609-14.
52. Motoyoshi M, Hirabayashi M, Uemura M, Shimizu N. Recommended placement
torque when tightening an orthodontic mini-implant. Clin Oral Implants Res. 2006
Feb;17(1):109-14.
-115-
53. Heidemann W, Gerlach KL, Grobel KH, Kollner HG. Influence of different pilot
hole sizes on torque measurements and pullout analysis of osteosynthesis screws. J
Craniomaxillofac Surg. 1998 Feb;26(1):50-5.
54. Gantous A, Phillips JH. The effects of varying pilot hole size on the holding power
of miniscrews and microscrews. Plast Reconstr Surg. 1995 Jun;95(7):1165-9.
55. Wu X, Deng F, Wang Z, Zhao Z, Wang J. Biomechanical and histomorphometric
analyses of the osseointegration of microscrews with different surgical techniques in
beagle dogs. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2008
Nov;106(5):644-50.
56. Degidi M, Perrotti V, Strocchi R, Piattelli A, Iezzi G. Is insertion torque correlated
to bone-implant contact percentage in the early healing period? A histological and
histomorphometrical evaluation of 17 human-retrieved dental implants. Clin Oral
Implants Res. 2009 Aug;20(8):778-81.
57. Motoyoshi M, Yoshida T, Ono A, Shimizu N. Effect of cortical bone thickness and
implant placement torque on stability of orthodontic mini-implants. Int J Oral
Maxillofac Implants. 2007 Sep-Oct;22(5):779-84.
58. Motoyoshi M, Uemura M, Ono A, Okazaki K, Shigeeda T, Shimizu N. Factors
affecting the long-term stability of orthodontic mini-implants. Am J Orthod Dentofacial
Orthop. 2010 May;137(5):588-9.
59. Renjen R, Maganzini AL, Rohrer MD, Prasad HS, Kraut RA. Root and pulp
response after intentional injury from miniscrew placement. Am J Orthod Dentofacial
Orthop. 2009 Nov;136(5):708-14.
-116-
60. Herman R, Cope JB. Miniscrew implants: IMTEC Mini Ortho Implants. Semin
Orthod. 2005;11(1):32-9.
61. Kuroda S, Yamada K, Deguchi T, Hashimoto T, Kyung HM, Takano-Yamamoto T.
Root proximity is a major factor for screw failure in orthodontic anchorage. Am J
Orthod Dentofacial Orthop. 2007 Apr;131(4 Suppl):S68-73.
62. Ono A, Motoyoshi M, Shimizu N. Cortical bone thickness in the buccal posterior
region for orthodontic mini-implants. Int J Oral Maxillofac Surg. 2008 Apr;37(4):334-
40.
63. Deguchi T, Nasu M, Murakami K, Yabuuchi T, Kamioka H, Takano-Yamamoto T.
Quantitative evaluation of cortical bone thickness with computed tomographic scanning
for orthodontic implants. Am J Orthod Dentofacial Orthop. 2006
Jun;129(6):721.e7,721.12.
64. Miyamoto I, Tsuboi Y, Wada E, Suwa H, Iizuka T. Influence of cortical bone
thickness and implant length on implant stability at the time of surgery--clinical,
prospective, biomechanical, and imaging study. Bone. 2005 Dec;37(6):776-80.
65. Baumgaertel S, Hans MG. Buccal cortical bone thickness for mini-implant
placement. Am J Orthod Dentofacial Orthop. 2009 Aug;136(2):230-5.
66. Gracco A, Lombardo L, Cozzani M, Siciliani G. Quantitative cone-beam computed
tomography evaluation of palatal bone thickness for orthodontic miniscrew placement.
American Journal of Orthodontics & Dentofacial Orthopedics. 2008
200809;134(3):361-9.
-117-
67. Baumgaertel S. Quantitative investigation of palatal bone depth and cortical bone
thickness for mini-implant placement in adults. Am J Orthod Dentofacial Orthop. 2009
Jul;136(1):104-8.
68. Fayed MM, Pazera P, Katsaros C. Optimal sites for orthodontic mini-implant
placement assessed by cone beam computed tomography. Angle Orthod. 2010
Sep;80(5):939-51.
69. Holmes DC, Loftus JT. Influence of bone quality on stress distribution for
endosseous implants. J Oral Implantol. 1997;23(3):104-11.
70. Okumura N, Stegaroiu R, Kitamura E, Kurokawa K, Nomura S. Influence of
maxillary cortical bone thickness, implant design and implant diameter on stress around
implants: a three-dimensional finite element analysis. J Prosthodont Res. 2010
Jul;54(3):133-42.
71. Roze J, Babu S, Saffarzadeh A, Gayet-Delacroix M, Hoornaert A, Layrolle P.
Correlating implant stability to bone structure. Clin Oral Implants Res. 2009
Oct;20(10):1140-5.
72. Kim HJ, Yun HS, Park HD, Kim DH, Park YC. Soft-tissue and cortical-bone
thickness at orthodontic implant sites. American Journal of Orthodontics & Dentofacial
Orthopedics. 2006;130(2):177-82.
73. Wang Z, Zhao Z, Xue J, Song J, Deng F, Yang P. Pullout strength of miniscrews
placed in anterior mandibles of adult and adolescent dogs: a microcomputed
tomographic analysis. Am J Orthod Dentofacial Orthop. 2010 Jan;137(1):100-7.
-118-
74. Cha JY, Kil JK, Yoon TM, Hwang CJ. Miniscrew stability evaluated with
computerized tomography scanning. Am J Orthod Dentofacial Orthop. 2010
Jan;137(1):73-9.
75. Vande Vannet B, Sabzevar MM, Wehrbein H, Asscherickx K. Osseointegration of
miniscrews: a histomorphometric evaluation. Eur J Orthod. 2007 Oct;29(5):437-42.
76. Jung BA, Yildizhan F, Wehrbein H. Bone-to-implant contact of orthodontic
implants in humans--a histomorphometric investigation. Eur J Orthod. 2008
Dec;30(6):552-7.
77. Woods PW, Buschang PH, Owens SE, Rossouw PE, Opperman LA. The effect of
force, timing, and location on bone-to-implant contact of miniscrew implants. Eur J
Orthod. 2009 Jun;31(3):232-40.
78. Yano S, Motoyoshi M, Uemura M, Ono A, Shimizu N. Tapered orthodontic
miniscrews induce bone-screw cohesion following immediate loading. Eur J Orthod.
2006 Dec;28(6):541-6.
79. Wu J, Bai YX, Wang BK. Biomechanical and histomorphometric characterizations
of osseointegration during mini-screw healing in rabbit tibiae. Angle Orthod. 2009
May;79(3):558-63.
80. Duyck J, Ronold HJ, Van Oosterwyck H, Naert I, Vander Sloten J, Ellingsen JE.
The influence of static and dynamic loading on marginal bone reactions around
osseointegrated implants: an animal experimental study. Clin Oral Implants Res. 2001
Jun;12(3):207-18.
-119-
81. Ohashi E, Pecho OE, Moron M, Lagravere MO. Implant vs screw loading protocols
in orthodontics. Angle Orthod. 2006 Jul;76(4):721-7.
82. Zhang L, Zhao Z, Li Y, Wu J, Zheng L, Tang T. Osseointegration of orthodontic
micro-screws after immediate and early loading. Angle Orthod. 2010 Mar;80(2):354-
60.
83. Chen Y, Kang ST, Bae SM, Kyung HM. Clinical and histologic analysis of the
stability of microimplants with immediate orthodontic loading in dogs. Am J Orthod
Dentofacial Orthop. 2009 Aug;136(2):260-7.
84. Luzi C, Verna C, Melsen B. Immediate loading of orthodontic mini-implants: a
histomorphometric evaluation of tissue reaction. Eur J Orthod. 2009 Feb;31(1):21-9.
85. Liou EJ, Pai BC, Lin JC. Do miniscrews remain stationary under orthodontic
forces? Am J Orthod Dentofacial Orthop. 2004 Jul;126(1):42-7.
86. Mortensen MG, Buschang PH, Oliver DR, Kyung HM, Behrents RG. Stability of
immediately loaded 3- and 6-mm miniscrew implants in beagle dogs--a pilot study. Am
J Orthod Dentofacial Orthop. 2009 Aug;136(2):251-9.
87. El-Beialy AR, Abou-El-Ezz AM, Attia KH, El-Bialy AM, Mostafa YA. Loss of
anchorage of miniscrews: a 3-dimensional assessment. Am J Orthod Dentofacial
Orthop. 2009 Nov;136(5):700-7.
88. Liu H, Lv T, Wang N, Zhao F, Wang K, Liu D. Drift characteristics of miniscrews
and molars for anchorage under orthodontic force: 3-dimensional computed
tomography registration evaluation. American Journal of Orthodontics & Dentofacial
Orthopedics. 2011 201101;139(1):e83-9.
-120-
89. Hsieh YD, Su CM, Yang YH, Fu E, Chen HL, Kung S. Evaluation on the
movement of endosseous titanium implants under continuous orthodontic forces: an
experimental study in the dog. Clin Oral Implants Res. 2008 Jun;19(6):618-23.
90. Lee K, Joo E, Kim K, Lee J, Park Y, Yu H. Computed tomographic analysis of
tooth-bearing alveolar bone for orthodontic miniscrew placement. American Journal of
Orthodontics & Dentofacial Orthopedics. 2009 200904;135(4):486-94.
91. Inaba M. Evaluation of primary stability of inclined orthodontic mini-implants. J
Oral Sci. 2009 Sep;51(3):347-53.
92. Zix J, Hug S, Kessler-Liechti G, Mericske-Stern R. Measurement of dental implant
stability by resonance frequency analysis and damping capacity assessment:
comparison of both techniques in a clinical trial. Int J Oral Maxillofac Implants. 2008
May-Jun;23(3):525-30.
93. Aparicio C, Lang NP, Rangert B. Validity and clinical significance of
biomechanical testing of implant/bone interface. Clin Oral Implants Res. 2006 Oct;17
Suppl 2:2-7.
94. Woodall N, Tadepalli SC, Qian F, Grosland NM, Marshall SD, Southard TE. Effect
of miniscrew angulation on anchorage resistance. American Journal of Orthodontics &
Dentofacial Orthopedics. 2011 201102;139(2):e147-52.
95. Pickard MB, Dechow P, Rossouw PE, Buschang PH. Effects of miniscrew
orientation on implant stability and resistance to failure. Am J Orthod Dentofacial
Orthop. 2010 Jan;137(1):91-9.
-121-
96. Huja SS, Litsky AS, Beck FM, Johnson KA, Larsen PE. Pull-out strength of
monocortical screws placed in the maxillae and mandibles of dogs. Am J Orthod
Dentofacial Orthop. 2005 Mar;127(3):307-13.
97. Bensley B. Practical anatomy of the rabbit; an elementary laboratory textbook in
mammalian anatomy. 2nd edition ed. Toronto: University of Toronto Press; 1918.
p.107-8.
98. Slaets E, Naert I, Carmeliet G, Duyck J. Early cortical bone healing around loaded
titanium implants: a histological study in the rabbit. Clin Oral Implants Res. 2009
Feb;20(2):126-34.
99. Garetto LP, Chen J, Parr JA, Roberts WE. Remodeling dynamics of bone
supporting rigidly fixed titanium implants: a histomorphometric comparison in four
species including humans. Implant Dent. 1995 Winter;4(4):235-43.
100. BJORK A. Facial growth in man, studied with the aid of metallic implants. Acta
Odontol Scand. 1955 Jun;13(1):9-34.
101. Metaxas A. Primate experiments in bone remodelling in the temporomandibular
joint and facial complex using the herbst appliance. 1983.
102. Eriksen E, Axelrod D, Melsen F. Bone Histomorphometry. Reprint ed.
Anonymous New York: Raven Press; 1994.
103. Roberts WE, Turley PK, Brezniak N, Fielder PJ. Implants: Bone physiology and
metabolism. CDA J. 1987 Oct;15(10):54-61.
-122-
104. Roberts WE, Smith RK, Zilberman Y, Mozsary PG, Smith RS. Osseous adaptation
to continuous loading of rigid endosseous implants. Am J Orthod. 1984 Aug;86(2):95-
111.
105. Jansen JA, van der Waerden JP, de Groot K. Tissue reaction to bone-anchored
percutaneous implants in rabbits. J Invest Surg. 1992 Jan-Mar;5(1):35-44.
106. Zaffe D, Rodriguez Y Baena R, Rizzo S, Brusotti C, Soncini M, Pietrabissa R, et
al. Behavior of the bone-titanium interface after push-in testing: a morphological study.
J Biomed Mater Res A. 2003 Feb 1;64(2):365-71.
107. Ahn SJ, Leesungbok R, Lee SW. Histomorphometric analysis and removal torque
of small diameter implants with alternative surface treatments and different designs. J
Oral Implantol. 2010;36(4):263-72.
108. Sisti K, Guastaldi A, Brochado Antoniolli A, Queiroz T, Tavares H, Souza F, et al.
Surface and Biomechanical Study of Titanium Implants Modified by Laser with and
without Hydroxyapatite Coating, in Rabbits. J Oral Implantol. 2010 Aug 6.
109. Yoon HI, Yeo IS, Yang JH. Effect of a macroscopic groove on bone response and
implant stability. Clin Oral Implants Res. 2010 Dec;21(12):1379-85.
110. Guo Z, Zhou L, Rong M, Zhu A, Geng H. Bone response to a pure titanium
implant surface modified by laser etching and microarc oxidation. Int J Oral Maxillofac
Implants. 2010 Jan-Feb;25(1):130-6.
111. Rebollal J, Soares Gde A, Vidigal GM,Jr. Histomorphometric analysis of
hydroxyapatite-coated implants in rabbit cortical bone using longitudinal and transverse
histologic sections. Implant Dent. 2010 Apr;19(2):137-44.
-123-
112. MacGregor S. The effect of micro-implant surface geometry on peri-implant bone
adaptation under orthodontic loading in the rabbit tibia. 2004.
113. Lee N, Baek S. Effects of the diameter and shape of orthodontic mini-implants on
microdamage to the cortical bone. American Journal of Orthodontics & Dentofacial
Orthopedics. 2010 201007;138(1):8.e1,8.e8.
114. Mo SS, Kim SH, Kook YA, Jeong DM, Chung KR, Nelson G. Resistance to
immediate orthodontic loading of surface-treated mini-implants. Angle Orthod. 2010
Jan;80(1):123-9.
115. Pearce AI, Richards RG, Milz S, Schneider E, Pearce SG. Animal models for
implant biomaterial research in bone: a review. Eur Cell Mater. 2007 Mar 2;13:1-10.
116. Baumgaertel S, Palomo JM, Palomo L, Hans MG. Reliability and accuracy of
cone-beam computed tomography dental measurements. Am J Orthod Dentofacial
Orthop. 2009 Jul;136(1):19,25; discussion 25-8.
117. Lagravere MO, Carey J, Toogood RW, Major PW. Three-dimensional accuracy of
measurements made with software on cone-beam computed tomography images. Am J
Orthod Dentofacial Orthop. 2008 Jul;134(1):112-6.
118. Ludlow JB, Laster WS, See M, Bailey LJ, Hershey HG. Accuracy of
measurements of mandibular anatomy in cone beam computed tomography images.
Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 2007 Apr;103(4):534-42.
119. Damstra J, Fourie Z, Huddleston Slater JJ, Ren Y. Accuracy of linear
measurements from cone-beam computed tomography-derived surface models of
-124-
different voxel sizes. Am J Orthod Dentofacial Orthop. 2010 Jan;137(1):16.e1,6;
discussion 16-7.
120. Berco M, Rigali PH,Jr, Miner RM, DeLuca S, Anderson NK, Will LA. Accuracy
and reliability of linear cephalometric measurements from cone-beam computed
tomography scans of a dry human skull. Am J Orthod Dentofacial Orthop. 2009
Jul;136(1):17.e1,9; discussion 17-8.
121. Gliddon MJ, Xia JJ, Gateno J, Wong HT, Lasky RE, Teichgraeber JF, et al. The
accuracy of cephalometric tracing superimposition. J Oral Maxillofac Surg. 2006
Feb;64(2):194-202.
122. De Genova DC, McInnes-Ledoux P, Weinberg R, Shaye R. Force degradation of
orthodontic elastomeric chains--a product comparison study. Am J Orthod. 1985
May;87(5):377-84.
123. Butz F, Ogawa T, Chang TL, Nishimura I. Three-dimensional bone-implant
integration profiling using micro-computed tomography. Int J Oral Maxillofac
Implants. 2006 Sep-Oct;21(5):687-95.
124. Eulenberger J, Steinemann SG. Removal torques of small screws of steel and
titanium with different surfaces. Unfallchirurg. 1990 Mar;93(3):96-9.
125. Lee NK, Baek SH. Effects of the diameter and shape of orthodontic mini-implants
on microdamage to the cortical bone. Am J Orthod Dentofacial Orthop. 2010
Jul;138(1):8.e1,8; discussion 8-9.
