Download AN EVALUATION OF THE INSERTION TORQUE OF

AN EVALUATION OF THE INSERTION TORQUE OF ORTHODONTIC MINISCREW
IMPLANTS IN RELATION TO TOOTH ROOT CONTACT.
Michael B. McEwan, B.S., D.D.S
An Abstract Presented to the Graduate Faculty of
Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry (Research)
2012
Abstract
Introduction: Miniscrew impants (MSIs) are a useful tool when
absolute orthodontic anchorage is required.
While helpful in
many regards, problems can still arise. The purpose of this
study was to quantify how insertion torque changes during
insertion of a self-drilling MSI with tooth root contact.
Methods: MSIs from two manufacturers (3M Unitek™ TAD and
Dentaurum tomas® pin) were inserted in pig cadaver mandibles and
divided into three groups: Control (miss), Glance, and Direct
Hit. Cone-beam computed tomography (CBCT) was used to verify MSI
location. Insertion torque was continuously recorded during MSI
insertion and inspected for fracture or damage upon removal.
Scatterplots of time vs insertion torque were created. Results:
3M Unitek MSIs showed higher insertion torque than Tomas MSIs in
control groups. Self-drilling MSIs were unable to directly
penetrate the roots. Self-drilling MSIs which contact teeth show
a higher insertion torque than control groups with no contact.
Tomas® MSIs showed higher fracture rates at the tip, possibly
due to a weaker thread-cutting design. Conclusions: Contact with
a tooth root with a self-drilling MSI causes an increase in
insertion torque and thus tactile feedback may help the
experienced clinician determine if root contact has occurred.
1
Additionally, thread-cutting tips appear to be more prone to
breakage when a tooth root is contacted.
2
EVALUATION OF INSERTION TORQUE OF ORTHODONTIC MINISCREW IMPLANTS
IN RELATION TO TOOTH ROOT CONTACT.
Michael B. McEwan, B.S., D.D.S
A Thesis Presented to the Graduate Faculty of
Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Master of Science in Dentistry (Research)
2012
COMMITTEE IN CHARGE OF CANDIDACY:
Professor Rolf G. Behrents,
Chairperson and Advisor
Associate Clinical Professor Donald R. Oliver
Professor Eustáquio Afonso Araújo
i
DEDICATION
To my parents, for their unconditional love and support. They
once told me I could do anything I wanted to and I have relied
on that statement many times when the going got tough!
To my wife and best friend, Lea. You are an amazing person and I
don’t tell you that enough! Thanks for the love, patience, and
support. I’m excited for all the new adventures that await us!
To my two girls, Mazy and Evey. Sorry I haven’t been around to
play very much lately – I’ve been busy finishing this thesis!
You two mean the world to me!
ii
ACKNOWLEDGEMENTS
Thanks to…
Dr. Rolf Behrents for his support and guidance throughout this
thesis and my time here at SLU.
Dr. Donald Oliver for his time, attention to detail, and
willingness to serve.
Dr. Eustáquio Araújo for his time and contributions to this
project.
Dr. Heidi Israel for help with the statistical analysis.
Mr. Don Larabell for help in customizing our instrumentation for
this project.
Gary Schwend, Jeanette Deiters, and the employees at Trenton
Processing for accommodating my weird requests and helping me
acquire samples to test.
3M Unitek and Dentaurum for providing the miniscrew implants
used for this project.
Saint Louis University Orthodontic Education and Research
Foundation, for financial support.
iii
TABLE OF CONTENTS
List of Tables..................................................v
List of Figures................................................vi
CHAPTER 1:INTRODUCTION..........................................1
CHAPTER 2:REVIEW OF THE LITERATURE
History and Contemporary Use...........................4
Nomenclature and Miniscrew Implant Design..............5
MSI Failures and Success...............................8
Role of Primary Stability in Success..............9
Insertion Torque..................................11
Tooth Root Contact.....................................15
MSIs and Tooth Contact............................19
Consequences of Tooth Contact.....................20
Tactile Perception................................24
Summary................................................27
Works Cited............................................28
CHAPTER 3:JOURNAL ARTICLE
Abstract...............................................38
Introduction...........................................39
Materials and Methods..................................42
Results................................................46
Insertion Torques vs Time.........................47
MSI Fractures.....................................54
Discussion.............................................55
Conclusions............................................62
Works Cited............................................63
Vita Auctoris....................................................67
iv
LIST OF TABLES
3.1: Mean maximum insertion torques............................54
v
LIST OF FIGURES
3.1: Pig mandible area of interest.............................42
3.2: MSIs tested...............................................43
3.3: Custom insertion torque measuring device..................44
3.4: Close up view of insertion device.........................45
3.5: Control sample image and photo............................47
3.6: CBCT image of glance group................................48
3.7: 3M Unitek control group insertion torque..................49
3.8: Tomas control group insertion torque......................49
3.9: Mean control groups insertion torque .....................50
3.10: Direct hit example.......................................50
3.11: Mean direct hit insertion torque.........................51
3.12: 3M Unitek glance group insertion torque..................52
3.13: Tomas glance group insertion torque......................52
3.14: Mean 3M Unitek glance and control group insertion torque.53
3.15: Mean Tomas glance and control group insertion torque.....53
3.16: Close up of fractured Tomas MSIs.........................55
vi
CHAPTER 1:INTRODUCTION
Miniscrew implants (MSIs) are a treatment adjunct designed
to provide absolute skeletal anchorage in orthodontics. They
have gained in popularity due to their simple placement, low
cost, patient-acceptance, and ability to eliminate patient
compliance issues in treatment.
MSIs are a relatively new offering for most practices. In a
2008 survey of orthodontists, only 7.8% had reported using MSIs
more than five years, and yet 60-91% of orthodontists report an
active case using an MSI.1–3 A recent survey showed 45.5% of
orthodontists reported not placing their own implants.
Potential root contact and damage was cited by 32.8% as the main
reason for delegation to other practitioners.1
Severe, irreversible damage can be caused by MSIs
contacting the roots of teeth.4–7 Fortunately, damage caused by
MSI contact most often results in minor damage that heals
uneventfully.8–10 Root contact should also be of concern to the
practitioner since it has been shown to dramatically increase
MSI failure rates.5,11
Many authorities on MSIs state that a clinician will be
able to feel if he or she contacts a tooth root during
insertion, as there will be a dramatic increase in torque and
resistance.12–16 Only a few studies have evaluated the claim that
insertion torque changes when a tooth root is contacted.8,17,18
1
These studies looked at MSIs which required predrilling a pilot
hole. Newer self-drilling MSI designs, which require no pilot
hole, have been introduced. A self-drilling MSI is preferred by
clinicians due to the simplicity of the procedure.19–21 Only 10.5%
of clinicians report using a pilot hole “mostly” or “always.”1
Insertion torque has been typically measured as the
“maximum insertion torque” and is often derived from MSIs being
placed to almost their final length and then measuring the
insertion torque of the last turn or measuring just the highest
value encountered during insertion.8,17,18 It would be more
valuable to measure insertion torque continuously, which better
emulates what happens clinically and will provide more
information as to what happens when a tooth root is contacted
and the rate of change seen.
Currently, no studies have investigated tooth contact using
newer self-drilling MSI designs. Additionally, torque-limiting
drivers to place MSIs are becoming more commonplace.20 Added
information as to appropriate values to use as the maximum
torque values and whether torque-limitations can be used to
detect tooth contact could prove valuable.
The purpose of this research is to quantify what happens to
insertion torque when a self-drilling MSI is inserted and
contacts a tooth.
Insertion torque will be continuously
recorded so the rate of change can be studied.
2
This study addresses the following questions: Does
insertion torque increase with root contact? How much and how
quickly does it change? Could this change be used to detect root
contact?
3
CHAPTER 2: REVIEW OF THE LITERATURE
Miniscrew implants (MSIs) provide exciting opportunities to
treat orthodontic patients in ways that were not previously
possible and, at the same time, avoid reliance on patient
compliance. As a result, MSIs have attracted great interest,
generated hundreds of articles published in the last few years,
and have become a “standard in the modern orthodontic
practice.”19,20
Miniscrews are a recent addition to the clinician’s
armamentarium, only 7.8% of orthodontists in a recent survey had
been using MSI more than five years.1 Usage of MSIs has increased
dramatically. Various surveys have shown usage rates varying
from 60-91% of orthodontists using miniscrews at least once in
the past year, and the majority are placing their own
miniscrews.1–3
History and Contemporary Use
The history of MSI use in orthodontics is an interesting
story. Brånemark’s work in the late 1950s and early 1960s is
considered the pioneering work establishing the potential for
osseointegration with titanium implants, though it was preceded
by the attempts of others to replace teeth using biocompatible
materials which, due to failure, fell into disuse.12,22 Today,
4
osseointegrated titanium dental implants have become commonplace
and have success rates exceeding 95%.23
The first published report of successful miniscrew use for
orthodontic anchorage appeared in 1983 when Creekmore and Eklund
inserted a vitallium bone screw just below the anterior nasal
spine in a 25 year-old female to intrude and torque the
maxillary incisors.24 It wasn’t until 14 years later in 1997 that
Konami wrote of a small, 1.2mm diameter titanium, modified bone
screw designed specifically for orthodontic use.25
The next year
Costa et al., introduced a screw with a bracket-like head, and
was the first MSI to approximate the designs in use today.26
Rapidly, a number of MSIs were developed and introduced, each
with varying designs. In 2007, there were 13 FDA-approved
miniscrew implants approved for orthodontic use, with each major
orthodontic manufacturer offering MSI options for clinicians.27
When compared to traditional larger endosseous implants, MSIs
are smaller, more versatile, easier to place, less expensive,
can be immediately loaded, and create less discomfort for the
patient.28
Nomenclature and Miniscrew Design
It is important for students, orthodontists, and
researchers to use similar nomenclature and terms when working
with MSIs to facilitate communication. What has been referred to
5
so far as miniscrew implants or MSIs, have also been referred to
as: microimplant, microscrew implant, mini-implant, mini dental
implant, miniscrew, temporary anchorage device (TAD), and
OrthoImplant.12,27 Cope explains that, in his view, micro- is an
inappropriate term, since it is derived from microscopic, or
something so small that it can only be visualized with a
microscope.12 The term mini- is derived from miniature, which is
merely something smaller. Sung et al., disagrees saying that
micro- can be used to emphasize small size such as in the terms
micrognathia, microsomia, microdontia and that micro- should be
used for implants smaller than 1.9mm and mini- for implants
greater than 1.9mm, but still much smaller than traditional
dental implants.29 Additionally, using the word screw is
valuable, since an implant is “any device…surgically placed in
the bone of upper or lower jaw” and MSIs are defined by having
screw components such as a body with threads and a defined
head/end for orthodontic use.12 Again Sung et al., disagrees,
saying that implant is defined by the MDD (European equivalent
of the FDA), as a device left in the body more than 30 days.29
The nomenclature preferred by Cope has been the most frequently
used since 2004 and will be used throughout this study.27
Another point of confusion is the use of the terms, selftapping (or pre-drilling), self-drilling (or drill-free), and
nondrill—free.12,27,29 Many papers refer to self-tapping as screws
6
that require a pilot hole the entire length of the screw and
have a blunted tip.18 Cope feels that the term self-tapping
should be discarded, since self-tapping is simply the ability of
the screw to create its own thread and advance itself, which all
MSIs do.12 In this study the term self-drilling will be used to
refer to a screw design with a very sharp tip, which is inserted
without a pilot hole and pre-drilled, which requires a pilot
hole either through the cortex of bone only or the entire length
of the screw.
It is unclear if there is a difference in clinical success
between self-drilling and pre-drilled MSIs.
Kim and Choi
reported loss rates of 64% for self-drilling and 34% of predrilled MSIs.29 Kim et al., however, favored self-drilling
implants since they showed less mobility and more bone-to-metal
contact.30 Suzuki and Suzuki did not detect a difference in
success rates in self-drilling and pre-drilled MSIs.31 Both selfdrilling and pre-drilled designs can be successful and clinical
success may have more to do with insertion torque encountered
and bone quality and quantity than with screw design.20 A selfdrilling MSI is preferred by clinicians due to the simplicity of
the procedure.19–21 Only 10.5% of clinicians report using a pilot
hole “mostly” or “always.”1
Lastly, there are two types of screws designs that are
common, thread-forming and thread-cutting. Both feature sharp
7
threads for forming the internal threads in the bone when the
screw is advanced, but a thread-cutting screw has a notch
removed parallel to the long axis of the screw at the tip of the
screw. As the screw is advanced, it actually cuts and removes
bone from the advancing threads and aids in removing the swarf,
or the bone debris.12 A disadvantage is that the screw is
weakened by removing the notch and some claim that the already
smaller screw size being reduced weakens the screw too much and
can lead to excessive fracture.12
MSI Failures and Success
Failure rate is one of the biggest concerns of clinicians
using MSIs.1 Failure is defined differently by various
clinicians, varying from any mobility or inflammation to the
most accepted definition of loss or mobility of the screw that
renders the MSI inoperative.12 Failure rates of 10-30% are
frequently published.32–34 As such, causes for failure have been
explored by many researchers.11,27,35–37
Miyawaki et al. found that the diameter of a MSI of 1.0 mm
or less, inflammation of the peri-implant tissue, and a high
mandibular plane angle (i.e., thin cortical bone), were
associated with mobility (i.e., failure).34 Excessive forces
placed on the MSI and a large lever arm (i.e., the result of
incomplete insertion due to such things as thick mucosa) were
8
associated with higher failure rates.38,39 Other authors noted
such factors as patient manipulation, poor hygiene, and periimplant soft tissue character contribute to failure.27,33,40–42 Of
particular interest in overcoming failure has been our
understanding of the importance of primary stability and
insertion torque on clinical success.
Role of Primary Stability in Success
Much of what we know about MSIs is derived from our
experiences with osseointegrated dental implants. It is known
that a dental implant’s prognosis is highly correlated to its
primary stability.43–45 Primary stability refers to a mechanical
stability derived from the bone-to-implant contact which results
in a lack of implant mobility immediately after placement.
Primary stability is crucial for development of secondary
stability.46 Secondary stability refers to the remodeling and
regeneration of the bone/implant interface as the bone that was
damaged by the implant insertion heals and forms a stable
biocompatible interface with the titanium screw.
Similar to the experience with endosseous implants, primary
stability and the resulting secondary stability has been also
deemed crucial for long-term MSI success.19,20,28,47–49 Lack of
primary stability would allow the MSI to have micromotion,
9
leading to a fibrous capsule formation, inflammation in the
area, and lack of bone-to-implant contact.46
Ure et al. studied primary stability and the transition to
secondary stability of MSIs using a non-destructive method of
resonance frequency analysis in dogs.50 They showed that the
overall stability of an MSI decreases over the first three weeks
as primary stability decreases and healing and bone remodeling
occur at the bone/screw interface and secondary stability
increases almost to previous levels by from weeks 5-8 weeks. The
MSIs primary stability holds the MSI tight and overcomes the
“stability dip” in the transition from primary to secondary
stability, thus primary stability is an important predictor for
success in MSI placement.51
Primary stability also allows for immediate loading of the
MSI.47,51 Immediate loading is preferred by clinicians for its
simplicity and efficiency.1
Primary stability can be measured by a variety of methods.
Histological assessment allows one to visualize the bone-toimplant contact, but is limited to only looking at the areas
selected for inspection.30 Resonance frequency analysis has been
routinely used in traditional dental implant literature, but
only recently have studies used it to look at MSIs.50,52,53
Unfortunately, these studies used custom methods to analyze the
MSIs and no commercial attachments are available from the vendor
10
for MSIs. Lastly, and most commonly, insertion torque has been a
common measure of primary stability. It has been used since the
1960s for evaluating screws in bone and currently in MSI
studies.33,51,54
Insertion Torque
Primary stability is almost synonymous with insertion
torque.18,20 Insertion torque is the measure of the rotational
force needed to insert the MSI into bone and is reported in most
literature as Newton cm.55 Insertion torque is determined by bone
quality, implant design and size, and insertion methods such as
pilot-hole drilling.51
Bone thickness and density has a positive linear
relationship with insertion torque.51,56 The bone’s outer cortex
thickness and density plays a large role in what the insertion
torque will be.51 Such areas of thick, dense cortex are often
found in the human mandible and implant site preparation, such
as pre-drilling, has been recommended to decrease insertion
torque in this area.20,51
Screw design can have a strong influence on insertion
torque. As expected, larger diameter screws require greater
insertion torques.54 Similar diameter screws can show great
variation in insertion torque.51,56,57 One factor is a conical body
vs a cylindrical body. A conical MSI design shows increased
11
insertion torque compared to cylindrical designs.54,57,58 An
additional benefit of the conical design is the MSI is more
likely to miss contacting teeth, since it is narrower in the
apical portion.12,59
An additional design consideration is a thread-cutting vs.
a thread-forming design. As mentioned previously, a threadcutting design features a notch in the tip which actively cuts
the internal threads of the bone at the apex.12 Thread-cutting
designs are common in osteosythesis screws, used to fixate bone
fractures, and are thought to alleviate bone stress and voids
between the screw and bone.60 Kuhn et al. studied bone
deformation between thread-cutting and thread-forming predrilled osteosythesis screws.61 Less bone deformation and lower
insertion torque was seen in the thread-cutting screws. Though
the fracture rate of thread-cutting MSIs has not been
specifically addressed in any MSI study, they may be more prone
to fracture at the tip.12,62
The last major influence of insertion torque is implant
site preparation, namely pilot hole drilling. Drilling a pilot
hole decreases insertion torque.18,51,56 Self-drilling MSIs tout
the benefit that they require no pilot hole. Kim et al. found
less mobility and more bone/implant contact with self-drilling
MSIs compared to pre-drilled MSIs after 12 weeks in beagle
12
dogs.30 He attributed this difference to the surgical trauma of
pilot-hole drilling and resultant bony damage.
High insertion torque results in high primary stability,
thus higher insertion torque is favorable, up to a point.51 Too
high of an insertion torque might result in fracture of the MSI.
A number of studies using MSIs have experienced screw fracture
with high insertion torques.51,54 Additionally, excessively high
insertion torques can lead to hoop stress from excessive bone
compression, which leads to microcracks, local ischemia, and
bone necrosis.43,63,64 Wawrzinek et al. documented increased
microfractures in overtightened MSIs in pig bone.65 Lee and Baek
also documented increased microfractures in rabbit tibias in
MSIs with larger insertion torques due to conical design or
larger diameters.66
Wilmes et al. advocates predrilling just through the bone
cortex even with self-drilling MSIs in the entire mandible and
in the palate to avoid MSI fracture and bone damage.67 Other
authors have made similar statements.20
Lehnen et al. found that patients have no overall
preference between self-drilling and pre-drilled MSIs. Patients
disliked the sound of the pilot hole drill in the pre-drilled
group and they disliked the increased pressure sensation in the
self-drilling group, but overall there was no difference in
overall preference.68
13
The idea that there is an ideal insertion torque in MSI
placement has not been investigated in depth. Motoyoshi et al.
investigated the relationship between insertion torque and
clinical success of MSIs.69 A total of 124 MSIs (8mm length and
1.6mm diameter, Biodent Co, Tokyo Japan) were placed in 41
patients in both the maxilla and mandible with pilot holes using
a 1.3mm drill. Maximum insertion torque values were recorded and
the implants were immediately loaded. Torque values were used to
divide the groups into three groups, <5 Ncm, 5-10 Ncm, and
>10 Ncm and the success rates of each group were compared. The
group of 5-10 Ncm had a success rate of 96.2% compared to 72.7%
for the <5 Ncm groups and 60.9% for the >10 Ncm group. They
recommended that insertion torque be monitored and changed by
varying the diameter of MSI and using a pilot hole to obtain an
optimal torque value of 5-10
Ncm.
This is in contrast to a study by Chaddad et al., who
tested an experimental surface-roughened MSI vs a commercial
smooth-surfaced MSI and found no difference in success rates
between the two.70 They did, however, find that MSIs with
insertion torques over 15 Ncm had 100% success rates and MSIs
with insertion torques less than 15 Ncm had 69% success rates,
thus showing that the ideal torque value for MSIs is still not
established.
14
Both studies only investigated pre-drilled MSIs, however, a
more recent study did compare a self-drilling vs pre-drilled
MSIs and found that although self-drilling had higher maximum
insertion torque (14.5 Ncm) vs pre-drilled (9.2 Ncm), there was
no statistically significant difference in success rates and
they called into question Motoyoshi’s claim of determining an
ideal placement torque.31
Tooth Root Contact
In a 2008 survey of orthodontists, 45.5% of orthodontists
reported not placing their own implants.
Potential root damage
was cited by 32.8% as the main reason for delegation to other
practitioners, despite the fact that orthodontist-placed MSIs
have higher success rates.1
One complicating factor is that due to tooth anatomy,
locations for MSI placement are limited. Although “safe zones”
have been developed to guide clinicians in their MSI placement
locations, anatomy is different for each patient.71,72 The buccal
area continues to be favored by clinicians, but offers the least
flexibility in placement.71
A recent study by Antoszewska et al. had 35 orthodontists
place MSIs in the maxilla of typodonts.73 Fear level of the
orthodontists was surveyed on a 10-point visual analog scale and
root contacts were counted. Root contacts occurred in 23.5% of
15
the insertions. Fear level before MSI insertion (4.6) was
significantly decreased after insertion (3.2). Clinicians cited
their fear stemmed from the potential of hitting teeth or the
maxillary sinus.
Another recent study investigated the surgical site and
clinician experience in MSI/tooth contact using typodonts
mounted realistically in mannequin heads with lips and cheeks.74
The study was divided into a group of eight orthodontists
experienced in MSI placement and another group of 20
inexperienced general dentists. Inexperienced clinicians had a
higher frequency of root contacts (21.3%) than did the
experienced group (13.5%). The inexperienced group had contacts
roughly equal in all quadrants. For the experienced group,
vulnerable sites were the maxillary right and mandibular left
posterior regions, especially the lower left first molar and the
upper right first molar (all but one of the subjects were righthanded). On the left, most contacts were on the mesial of the
root and on the right, most contacts were on the distal.
Experienced clinicians held their posture steady and kept
movements in relation to the mannequin to a minimum.
MSIs can be used in a number of locations in the mouth. The
interradicular spaces between teeth are the most common sites,
however these sites also carry the greatest risk for root
damage.74 Additionally, although MSIs are commonly assumed to be
16
absolutely stable, Liou et al. found that MSIs move up to 1.5 mm
and therefore a clearance of 2.0mm from adjacent teeth is
recommended.75 Maino et al., however, recommend a distance of
only 1mm.76 Although interradicular space increases as one moves
apically on a tooth, this may lead to MSIs being placed in
unattached gingiva, which can lead to soft-tissue inflammation
and overgrowth, leading to screw failure.33
The apprehension about contacting tooth roots has
resulted in a myriad of techniques that have been developed to
avoid tooth root contact, such as using the palate or areas of
the buccal bone with more interradicular space, using panoramic
or periapical radiographs before and after, radiographic stents
or guides, and angled placement.77–82 Placing MSIs requires
practice and experience as there is a tendency for the clinician
to inadvertently change the angle of insertion by pulling the
driver toward the body.4 Miyazawa et al. emphasized that blind
placement of miniscrews is difficult and in his study 53% of
surgical guides fabricated on models required adjustment of
angulations or position after being reviewed with threedimensional radiography (cone-beam computed tomography).83 In a
study of tooth root contact in swine, it was found that although
the researchers thought they had contacted roots, in 26.2% of
cases no actual root contact had occurred.84
17
Fear of iatrogenic damage by MSI contact with tooth roots
is justified. Much information is to be gleaned from the
oromaxillofacial traumatology literature and the use of fixation
screws. The risk of pulpitis or ankylosis is low after contact
with screws. Fabbroni et al. conducted a prospective study
evaluating the placement of 232 intermaxillary fixation screws
in 55 patients.85 These screws are placed after a pilot hole was
drilled. Twenty-six screws (11.2%) had “minor” contact, meaning
that <50% of the hole overlapped the root and 37 screws had
“major” contact, meaning >50% of the hole overlapped the root.
Seventeen teeth tested non-vital, however only 6 showed contact
with the screw. They determined that skilled doctors trying to
avoid roots would still hit them, but fortunately most teeth
appear to do well after the traumatic injury.
In a prospective audit of intermaxillary fixation screws,
Farr and Whear found radiographic evidence of tooth root damage
in 13 of 31 cases (43.3%) and the pulp chamber was entered in 4
of the 13 cases (30.8%).86 The authors emphasized that although
the placement of screws appears to be a simple procedure, root
damage frequently occurs. Additionally, fixation screws are not
typically placed interradicularly, so the likelihood of tooth
contact using oral surgery data would likely be underestimated.
It is recommended that surgeons try and feel for a change in
resistance when the pilot hole drill penetrates the bone cortex.
18
If no change in resistance is felt, then consider that a tooth
is being contacted.
It must be noted that the screws often used in the oral
surgery literature are pre-drilled and require a pilot hole the
entire length of the screw. This is in contrast with
contemporary MSIs which are placed with either a pilot hole
through the bone cortex only or with no pilot hole and placed
directly through the gingiva into the bone.
MSIs and Tooth Contact
In an article describing the Imtec® implant, Herman and
Cope claim that it is “nearly impossible, even with the selftapping property, to place the OrthoImplant directly into the
tooth root.”87 This claim is in direct contrast to Lee et al.,
who warns of the risk of root injuries from MSI placement, and
show a case of a tooth root which had been perforated. Due to
loss of vitality and the presence of vertical root fracture this
tooth had to be extracted.13 Also, Sung et al. states that selfdrilling MSIs can penetrate roots without heavy resistance and
shows photographs of various perforated teeth.29 Other authors
have mentioned the possibility of damaging the tooth’s nerve,
periodontal ligament or root.88,89
Studies were conducted by Lee to develop a “safe” MSI with
a blunted tip to prevent root perforations, however the study
19
showed disappointing results with a 59% failure rate.
They
speculated that blunt tip interferes with final seating of the
MSI and achieving primary stability.13
Consequences of Tooth Contact
Healing typically occurs uneventfully when tooth roots are
damaged experimentally, as long as the area is 2mm x 2mm or
smaller.90,91 Contact with traditional endosseous implants also
appears to heal by forming a cementum layer around the teeth.92
Screw proximity has been linked with increased risk of
failure.5,11 Kang et al. found that 79.2% of MSIs that contacted
teeth in beagle dogs failed, with an average failure time of 16
days.5 One theory is that when a screw contacts a tooth root,
biting forces are transferred to the screw, causing an
intermittent force and interruption of healing and bone
remodeling.93,94
Root damage is one of the most commonly cited potential
complications of MSI placement.1,4,76 Potential serious
complications of root contact with MSIs include ankylosis,
osteosclerosis, and loss of tooth vitality.4,93 A study in beagle
dogs noted root cracking and fracture and ankylosis in roots
that were contacted directly.6
20
The first study looking at MSI contact with teeth and
healing was a study by Asscherickx et al., who described tooth
root repair in beagle dogs after MSI placement.93 A study
examining a new type of miniscrew design was placed between the
roots of the teeth of the beagle dog and removed after 24 weeks.
Three of the twenty MSIs contacted teeth accidentally and
penetrated the dentin of the teeth. Subsequent histology
revealed complete healing of the root with cementum after 12
weeks of healing following removal.
A number of studies used dogs to evaluate sequelae and
healing of teeth contacted with MSIs.8–10,17,84 Similar results were
found in each study, namely that damaged dog teeth healed and
reestablished a periodontal ligament, and bone filled in the
damaged area. Cementum repair occurred on the tooth when teeth
were only partially contacted. Some damage was extensive,
however, resulting in inflammation, tooth necrosis and
ankylosis, particularly when the tooth fractured or the pulp
chamber was compromised.8,84 Healing typically started as early as
week 4 and was complete by week 12. Concave outlines on the root
remained, indicating that osteoblastic activity of bone was
higher than cementoblastic activity.17 No spots of ankylosis was
noted, unless severe damage occurred such as the root
splitting.10 MSIs left in damaged roots for a period of time
consistently showed inflammatory reactions and uneven surface
21
resorption on roots.17 These inflammatory reactions are probably
responsible for MSI loosening and failure after root contact.
Chen et al. recommends immediate removal of MSIs contacting
teeth to prevent additional resorptive damage.17
A more recent paper using a swine model and self-drilling
MSIs investigating root-surface healing was done by Kim and
Kim.95 Results were similar to the previous canine studies,
however histologic analysis revealed evidence of root resorption
caused indirectly by the MSI even when the MSI was up to 1mm
away from the tooth root. Another study in beagle dogs noted
resorption when MSIs were 0.6mm away from the root.6 This was
attributed to compression of bone which was transmitted to the
PDL. During healing, damaged tissues are removed by osteoclasts
and macrophages. Not only are necrotic tissues removed, but also
adjacent bone and cementum. Additionally, when dentin was
contacted, the teeth would quickly deposit tertiary dentin. In
all instances when MSIs were immediately removed, healing
occurred with cementum or dentin deposition and reestablishment
of the PDL.
Maino et al. investigated MSI contact in vivo using
premolars treatment planned for extraction.76 First they placed
MSIs mesial to the tooth root and pushed the teeth into the MSI
for 2 months and extracted the teeth. There was a resorptive
crater in the tooth root and little repair. In another tooth
22
root, they pushed for two months and then stopped pressure for 2
months. A slight residual defect was seen, but cellular cementum
had been deposited to fill the crater. In another tooth that had
been contacted with a drill and then contacted with an MSI and
removed, there was incomplete repair after 2 months with
cellular cementum, but some of the original contour was present.
Inflammatory cells were seen in the periodontal ligament.
Another similar study using human premolars treatment planned
for extraction showed similar results.96
There are reports of root perforations due to MSI
placement, subsequent necrosis, and the need for endodontic
treatment. In one case report, a mandibular incisor root was
perforated by an MSI during orthodontic treatment, and after 8
months a large periapical lesion was discovered via a routine
radiograph.7 The patient was asymptomatic. Conventional root
canal treatment was attempted, but was unsuccessful and surgical
repair was initiated which resolved the lesion.
Roncone recommends diverging the adjacent roots at the
start of treatment if the use of an MSI is planned.97 Other
authors recommend a myriad of techniques to avoid tooth root
contact, such as using the palate or areas of the buccal bone
with more interradicular space, using panoramic or periapical
radiographs before and after, radiographic stents or guides, and
angled placement.77–82
23
A number of clinicians advocate avoiding profound
anesthesia, so as to elicit a patient reaction if a tooth is
contacted.12,98 Usually the patient will feel a dull pain if the
periodontal ligament is contacted.97 Patient reactions can serve
as false positives however as patients may confuse pressure from
MSI placement as pain.
Tactile Perception
Many authorities on MSIs state that a clinician will be
able to feel if he or she contacts a tooth root, as there will
be a dramatic increase in torque and resistance.8,12–16 These
statements have mostly been made when referring to pre-drilled
MSIs, however. In contrast, Sung et al. states that selfdrilling MSIs can be inserted and contact teeth without the
clinician feeling any increased resistance.29 He states that a
self-drilling MSI has a sharper point and is more likely to
damage a root. A pre-drilled MSI is less likely to penetrate a
root.29 All these claims are not evidence-based and merely
anecdotal.
Three studies, however, did measure insertion torque
changes when an MSI contacts teeth. Wilmes et al. placed predrilled MSIs in pig mandibles.18 In this study the researchers
pre-drilled the entire length of the screw, due to difficulty
with screw fracture in pilot tests. They measured resistance to
24
pilot hold drilling and found an increase in resistance when a
tooth was contacted. Additionally, they placed MSIs by hand and
then had a torque-measuring robot perform the final 80 degree
turn and measured the peak insertion torque.
Torque values were
higher in MSIs that had contacted teeth. A similar study by Chen
et al. found that maximum insertion torque was higher in predrilled MSIs that contacted teeth vs. no contact.17
Briceno et al. placed 56 pre-drilled MSIs in the mandibles
of beagle dogs with intentional root contact and evaluated the
short and long-term effects of root contact.8 Predrilling was
done through the cortex and the mean maximum insertion torque
without root contact was 23.8 Ncm vs. 50.7 Ncm with root
contact.
A simultaneous study using the maxilla of the same
dogs was also performed.84 This study did not report insertion
torques, but noted that clinically, more resistance was felt
when teeth were contacted.
The author noted that tactile
resistance wasn’t always accurate as 26.2% of the time the
clinician felt the MSI had hit the root, but histologically
there showed no contact. Using only tactile feedback, there was
tendency to have false-positives.
The operator could not tell
the difference between root fracture and merely hitting the
cementum. They also warn that self-drilling MSIs with sharpened
tips to facilitate placement may not show such a change in
25
resistance.84 At present there are no studies investigating selfdrilling MSI torque changes when contacting a tooth.
Roncone cautioned against an overreliance on tactile
feedback pointing out that bone density in identical sites often
varies widely among different individuals, thus what may feel
like root impingement in an individual may only be particularly
dense bone.97
It may be wise for clinicians to use torque-measuring or
torque-limiting drivers when placing MSIs. Torque-limiting
driver use is associated with higher success rates.1 Baumgaertel
advocates using these instruments to avoid excessive bone damage
and potential MSI breakage, especially in the mandible.20 Wilmes
also recommends that clinicians monitor torque values, modify
MSI diameters, and use pilot holes as necessary to limit
excessive torque.54 McManus et al. proposed that a torque-driver
with an audible click be produced when 5 Ncm and 10 Ncm were
reached to help clinicians know if they are in the ideal
insertion torque range according to Motoyoshi et al.69,99 A study
by Schätzle investigated the accuracy of commercially available
MSI torque-limiting drivers and found that after 100
sterilization cycles, significant variations were observed from
the true torque values.100
26
Summary
As MSIs become commonplace in modern orthodontics,
practitioners desire evidence-based recommendations when
providing treatment.101 Currently, no studies have investigated
tooth contact using newer self-drilling MSI designs.
All
previous studies have drilled pilot holes, most of which drilled
the entire length of the MSI, which is almost never the case in
clinical practice. These data will be useful because it will
evaluate the claim that one can “feel” if a tooth is
encountered.
Additionally, torque-limited drivers to place MSIs
are becoming more commonplace. Information as to appropriate
values to use as the maximum torque values and whether torquelimitations can be used to detect tooth contact could prove
valuable. Lastly, studies that continuously measure insertion
torque as the MSI is inserted, as opposed to just the final turn
of the screw, better emulates clinical practice and will provide
more information as to what happens when a tooth root is
contacted.
27
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healing after root contact or approximation during
placement of mini-implants. Am J Orthod and Dentofacial
Orthop. 2011 Jun;139(6):752–60.
96.
Kadioglu O, Büyükyilmaz T, Zachrisson BU, Maino BG. Contact
damage to root surfaces of premolars touching miniscrews
during orthodontic treatment. Am J Orthod Dentofacial
Orthop. 2008 Sep;134(3):353–60.
36
97.
Roncone CE. Complications encountered in temporary
orthodontic anchorage device therapy. Sem Orthod. 2011
Jun;17(2):168–79.
98.
Costello BJ, Ruiz RL, Petrone J, Sohn J. Temporary skeletal
anchorage devices for orthodontics. Oral Maxillofac Surg
Clin North Am. 2010 Feb;22(1):91–105.
99.
McManus MM, Qian F, Grosland NM, Marshall SD, Southard TE.
Effect of miniscrew placement torque on resistance to
miniscrew movement under load. Am J Orthod and Dentofacial
Orthop. 2011 Sep;140(3):e93–8.
100. Schätzle M, Golland D, Roos M, Stawarczyk B. Accuracy of
mechanical torque‐limiting gauges for mini‐screw placement.
Clin Oral Implants Res. 2010 Aug 1;21(8):781–8.
101. Madhavji A, Araujo EA, Kim KB, Buschang PH. Attitudes,
awareness, and barriers toward evidence-based practice in
orthodontics. Am J Orthod and Dentofacial Orthop. 2011
Sep;140:309–16.e2.
37
CHAPTER 3: JOURNAL ARTICLE
Abstract
Introduction: Miniscrew impants (MSIs) are a useful tool when
absolute orthodontic anchorage is required.
While helpful in
many regards, problems can still arise. The purpose of this
study was to quantify how insertion torque changes during
insertion of a self-drilling MSI with tooth root contact.
Methods: MSIs from two manufacturers (3M Unitek™ TAD and
Dentaurum tomas® pin) were inserted in pig cadaver mandibles and
divided into three groups: Control (miss), Glance, and Direct
Hit. Cone-beam computed tomography (CBCT) was used to verify MSI
location. Insertion torque was continuously recorded during MSI
insertion and inspected for fracture or damage upon removal.
Scatterplots of time vs insertion torque were created. Results:
3M Unitek MSIs showed higher insertion torque than Tomas MSIs in
control groups. Self-drilling MSIs were unable to directly
penetrate the roots. Self-drilling MSIs which contact teeth show
a higher insertion torque than control groups with no contact.
Tomas® MSIs showed higher fracture rates at the tip, possibly
due to a weaker thread-cutting design. Conclusions: Contact with
a tooth root with a self-drilling MSI causes an increase in
insertion torque and thus tactile feedback may help the
experienced clinician determine if root contact has occurred.
38
Additionally, thread-cutting tips appear to be more prone to
breakage when a tooth root is contacted.
Introduction
Miniscrew implants (MSIs) provide exciting opportunities to
treat orthodontic patients in ways that were not previously
possible and, at the same time, avoid reliance on patient
compliance. As a result, MSIs have attracted great interest,
generated hundreds of articles published in the last few years,
and have become a “standard in the modern orthodontic
practice.”1,2
Miniscrews are a recent addition to the clinician’s
armamentarium, only 7.8% of orthodontists in a recent survey had
been using MSI more than five years.3 Usage of MSIs has increased
dramatically. Various surveys have shown usage rates varying
from 60-91% of orthodontists using miniscrews at least once in
the past years.3–5
Severe, irreversible damage can be caused by MSIs
contacting teeth.6–8 Fortunately, root damage caused by MSI
contact most often results in minor damage that heals
uneventfully.9–13 Root contact should also be of concern to the
practitioner since apart from damage, it has been shown to
dramatically increase failure rates.6,7
39
In a 2008 survey of orthodontists, 45.5% of orthodontists
reported not placing their own implants.
Potential root damage
was cited by 32.8% as the main reason for delegation to other
practitioners, despite the fact that orthodontist-placed MSIs
have higher success rates.3 A recent study by Antoszewska had 35
orthodontists place MSIs in the maxilla of typodonts.14
Fear
level of the orthodontists was surveyed on a 10-point visual
analog scale and root contacts were counted. Root contacts
occurred in 23.5% of the insertions.
Fear level before MSI
insertion (4.6) was significantly decreased after insertion
(3.2). Clinicians indicated that their fears stemmed from the
risk of injury to the teeth and maxillary sinus.
Many authorities on MSIs state that a clinician will be
able to feel if he or she contacts a tooth root, as there will
be a dramatic increase in torque and resistance.12,15–19 Only a few
studies have evaluated the claims that insertion torque changes
when a tooth root is contacted.10,12,20 These studies looked at
MSIs which required predrilling a pilot hole. Newer selfdrilling MSI designs, which require no pilot hole, have been
introduced. A self-drilling MSI is preferred by clinicians due
to the simplicity of the procedure.1,2,21 Only 10.5% of clinicians
report using a pilot hole “mostly” or “always.”3
Insertion torque has been typically measured as the
“maximum insertion torque” and is often derived from MSIs being
40
placed to almost the final length and then measuring the
insertion torque of the last turn or measuring just the highest
value encountered during insertion.10,12,20 More valuable would be
a system that measures insertion torque continuously, which
better simulates what occurs clinically and will provide more
information as to what happens when a tooth root is contacted
and the rate of change seen.
Currently, no studies have investigated tooth contact using
newer self-drilling MSI designs. Additionally, torque-limited
drivers to place MSIs are becoming more commonplace. Added
information as to the appropriate values to use as the maximum
torque values and whether torque-limitations can be used to
detect tooth contact could prove valuable.
The aim of this research is to quantify the insertion
torque when an MSI is inserted and contacts a tooth.
Insertion
torque will be continuously recorded so the rate of change can
be studied.
This study addresses the following questions: Does
insertion torque increase with root contact? How much and how
quickly does it change? Could this change be used to detect root
contact?
41
Materials and Methods
Resected, frozen pig mandibles were prepared to include the
posterior premolar teeth (Figure 3.1). Samples were thawed and
Figure 3.1 CBCT-derived image depicting pig mandible area used in this study (left)
and diagrammatic representation of swine dental arcade with area of interest outlined
(right).
two commercially-available, self-drilling miniscrew implants
were selected to be placed without predrilling or incision of
soft tissue. The two MSIs in this study were a 1.8mm diameter,
6mm long 3M Unitek™ TAD (Monrovia, CA, USA) and a 1.6mm
diameter, 6mm long Dentaurum tomas® pin (Ispringen,
Germany)(Figure 3.2). Torque measurements were continuously
measured as the MSI was inserted until the collar of the
miniscrew approximates the gingiva. Torque measurements were
made using a custom insertion torque measuring device. This
42
Figure 3.2 MSIs tested in this study. 3M Unitek™ TAD (left) and Dentaurum tomas® pin
(right)
method reduces human error in testing, allows for continuous
torque measurement during insertion, and has been proven a
reliable method to measure insertion torque of miniscrew
implants in previous studies.22,23 The device consists of an
aluminum jig that securely holds the sample and is rotated at 9
revolutions per minute by a high torque motor (Model 016-2240108, Display Devices Inc., Arvada, CO, USA). A torque sensor
(Mecmesin Ltd., West Sussex, UK) is securely mounted above and
is fitted with the proprietary driver bit for the MSI. It is
lowered using a using a modified drill press (Model 220-01,
Dremel, Robert Bosch Tool Corp., Racine, WI, USA) into the
revolving sample (Figure 3.3 and Figure 3.4). It was determined
in pilot testing that approximately five and one-half pounds
(5.48 lbs, 2.49 kg) was needed to adequately advance the MSIs.
43
Figure 3.3 Custom insertion torque measuring device
As this constant weight was applied at the lever arm and the
motor rotated the sample, insertion torque was recorded via a
computer with appropriate software (Emperor ™ lite, ver. 1.17015, Mecmesin Ltd., West Sussex, UK)
Imaging was performed with a 3-dimensional cone-beam
computed tomography (CBCT) i-CAT scanner (Imaging Sciences
44
A
B
C
Figure 3.4 Close-up view of torque sensor (A), MSI driver and MSI (B), and aluminum
jig with sample held securely with washers and bolts (C)
International, Hatfield, Pa) and viewed with Dolphin 3-D
(Dolphin Imaging Version 11.0, Premium Chatsworth, CA). Lead
markers were placed on the bone sections and were used to select
locations for MSI insertion. MSIs were inserted first without
root contact into the mandibular sections between the roots of
the first, second, and third premolars. After imaging and
removal, MSIs were inserted directly into the tooth roots. On
the contralateral side, MSIs were placed closer to the teeth to
45
produce a “glance” where contact occurred and less than half the
diameter of the MSI was in the root. CBCT was again used to
verify MSI placement and root contact. MSIs were removed and
examined for fractures; fracture incidence and location was
recorded and photographed.
Pilot tests with pig mandibles showed that the MSIs could
be placed accurately. Power analysis using pilot data showed
that a sample size of at least 10 miniscrews per group would
give an 80% probability of detecting a real difference between
the groups at a statistically significant level of 5%.
Scatterplots of time vs insertion torque measurements were
made using Microsoft Excel® 2007 (Microsoft Corporation,
Redmond, WA) and statistical analysis was conducted using SPSS
software (SPSS for Windows, Version 15.0, Chicago, IL).
Due to
the small sample size and possibility of skewed data, nonparametric analysis using the Mann-Whitney U test was used to
determine significant differences (p<.05) among the mean torque
values of the control group vs. the experimental group at
different times.
Results
Cone-beam computed tomography (CBCT) was able to accurately
locate the position of the MSI in relation to the tooth. Figure
46
Figure 3.5 CBCT-derived image representing a control (miss) sample (above) and a
photograph of the same sample (below)
3.5 is an example of the three dimensional view of the mandible
in the control group. Additionally, slices can be made to verify
the position of the MSI such as Figure 3.6, which shows an
example of a glance group coronal slice.
Insertion torques vs time
Both of the control (miss) groups showed a positive linear
relationship with increasing insertion torque as the MSI
47
Figure 3.6 CBCT-derived images. Above is a coronal slice from the 3-dimensional image
below. The above image confirms partial root contact with the MSI
advanced (Figure 3.7 and Figure 3.8). The average insertion
torque of the 3M Unitek MSIs was higher than the Tomas MSIs at
similar timepoints (Figure 3.9).
The MSIs assigned to the direct hit group were unable to
penetrate the tooth (Figure 3.10 and 3.11). They would enter the
bone, and as expected torque would increase, but then the MSI
would stop advancing and just spin and strip the bone. The
insertion torque at this point would plateau at a low value,
48
Figure 3.7 Insertion torque values for the control (miss) group from time = 0 to full
engagement for 3M Unitek MSIs
Figure 3.8 Insertion torque values for the control (miss) group from time = 0 to full
engagement for Dentaurum Tomas MSIs
49
Figure 3.9 Mean insertion torque from time = 0 to full engagement for both control
groups
Figure 3.10 MSIs attempted to penetrate directly into the tooth root were unable to
penetrate the root and would just partially insert and then spin as seen above
50
Figure 3.11 Mean insertion torque from time = 0 to full engagement for direct hit
group
thus only five MSIs of each brand were attempted and the data
wasn’t included in statistical analysis.
The data from the glance groups showed a deviation from the
tight linear relationship see in the control group (Figure 3.12
and Figure 3.13). Figure 3.14 and 3.15 illustrates the mean
insertion torque for the 3M Unitek and Tomas MSIs glance groups
respectively. Although the lines start out similar to the
control group, one can appreciate that the lines exhibit an
inflection point where the rate of increase (slope of the line)
suddenly increases and the insertion torque become higher than
the control group. There was a statistically significant
51
Figure 3.12 Insertion torque values for the glance group from time = 0 to full
engagement for 3M Unitek MSIs
Figure 3.13 Insertion torque values for the glance group from time = 0 to full
engagement for Tomas MSIs
52
Figure 3.14 Mean insertion torque from time = 0 to full engagement for the 3M Unitek
control and glance groups
Figure 3.15 Mean insertion torque from time = 0 to full engagement for the Tomas
control and glance groups
53
Table 3.1 Mean maximum insertion torques every 0.1 mins (6 secs)
3M Unitek
Miss (control)
Glance
p-value
Tomas
Miss (control)
Glance
p-value
N
T=0.1 min
Mean Maximum Insertion Torque (Ncm)
T=0.2 min
T=0.3 min
T=0.4 min
T=0.5 min
10
10
3.08
3.44
5.45
6.12
7.20
10.23
9.42
14.81
11.71
17.00
0.324
0.211
0.002
0.001
0.000
1.88
2.43
3.56
4.57
5.22
7.91
6.91
11.67
8.76
12.86
0.555
0.009
0.001
0.001
0.001
10
7
difference between the glance group and control group at time =
0.3 min (18 seconds) and greater for the 3M Unitek MSI and for
the Tomas MSI at time = 0.2 min (12 seconds) and greater (Table
3.1).
MSI Fractures
Three of the 10 Tomas MSIs advanced partially and then
stopped advancing, only achieving approximately 7-9 Ncm of
torque. Upon removal it was found that the tip of the three
MSIs had fractured and were not included in the analysis (Figure
3.16).
54
Figure 3.16 Close-up view of a control and two of the three Tomas screws whose tips
fractured during insertion
Discussion
Few studies have measured how insertion torque changes with
tooth root contact. A study by Wilmes et al. used pig mandibles
and randomly placed 320 pilot holes drilled the entire length of
the MSI in 11 bones and placed MSIs by hand until the last
0.2mm, which was then done by a robot which measured the
insertion torque.20 The control group and the partially contacted
roots group had insertion torque of 16.1 Ncm and 18.5 Ncm
respectively. This difference was small, but statistically
significant and the authors note that clinicians may not be able
to detect this small a difference clinically. A similar in-vivo
study by Chen et al. again drilled the entire length of the MSI
in mongrel dogs and placed miniscrews by hand and then measured
the last few turns with a torque meter.10 They also found
55
slightly higher insertion torque in root contact vs no contact
MSIs (20.3 vs 17.4 Ncm respectively).
A weakness of these studies is that they pre-drilled, not
just through the cortex, but the entire length of the MSI. This
technique is not used clinically since a pilot hole the entire
length of the screw is unnecessary, creates greater trauma to
the bone, and leaves a void between the MSI tip and bone.24
Because it was the pilot drill which contacted the teeth first
and removed part of the tooth before MSI placement, this may
have diminished the differences seen. The difference may be
greater using the more conventional pilot-hole through the
cortex only or with the newer self-drilling MSIs.
Another
shortcoming in these studies is they measured only the last turn
of the MSI and called it the maximum insertion torque. It would
be better to continuously measure the insertion since the true
maximum insertion torque may occur before the final turn,
especially in studies looking at tooth contact. Also, the last
turn involves the miniscrew body/neck interface. The
transmucosal neck has no threads and can give a torque increase
simply because the miniscrew can no longer advance.
An animal study by Brisceno et al. also looked at insertion
torque when tooth roots are contacted.12 They predrilled through
the bone cortex only due to very high insertion torques which
were encountered and caused MSI breakage. The maximum insertion
56
torque encountered was recorded and was higher in contacted
teeth (50.7 Ncm) than non-contacted teeth (23.8 Ncm). This study
was an improvement, since only the cortex was drilled, but
doesn’t provide a continuous picture of how the torque changes
during insertion, just that the overall value was higher.
Another study involving tooth root contact and healing using the
maxilla of these same dogs was done simultaneously. The authors
reported that there was higher insertion torque using only
tactile feedback, but 26.2% of the time there was a falsepositive where the clinician felt a contact, but none had
occurred histologically.9
The use of hand drivers introduces inconsistency due to the
operator and how hard he or she pushes on the MSI and variably
rotates the driver. The method used in this study is superior
since it eliminates the “human” variability, uses current
placement techniques, and offers continuous data for the entire
placement.
In this study two commercially-available orthodontic
miniscrew implants were compared in terms of their insertion
torque when placed normally between roots and when contacting
teeth. Both control (miss) groups performed similarly with a
positive, linear relationship to time. The 3M Unitek MSIs had
higher torque values when compared to Tomas MSIs in the control
groups at the same timepoints. One reason for this is the 3M
57
Unitek MSI is 1.8mm diameter compared to the 1.6mm for the Tomas
MSI. Larger diameters have been correlated with higher insertion
torques.25,26 Barros et al. found diameter changes of as little as
0.1 mm caused significant insertion torque differences in MSI
placement.27 Another reason for the higher insertion torque seen
in the 3M Unitek MSI may be due to the design of the miniscrew,
as the 3M Unitek MSI has a different taper than the Tomas MSI.
Conical designs have a higher insertion torque due to the upper
part of the tapered body having a larger diameter than the lower
part and this factor creates a tighter contact with the
bone.25,26,28 Additionally, the pitch and thread count is different
between the screws. Wilmes et al. showed that for similar sized
MSIs, there was a wide variation in insertion torque due to the
various design differences such as thread pitch and shape.25,29
Perhaps one should choose various screw designs for different
locations in the mouth to vary the insertion torque.
Directly drilling into a tooth with a self-drilling MSI was
not successful with either brand MSI. Perforation is one of the
most serious potential complications.30 A clinician might prefer
a self-drilling MSI, since he or she knows that it is highly
unlikely to perforate the tooth without the use of a drill.
In each case where a tooth was contacted, there is a
deviation from linearity; the slope of the line suddenly
increases and the value at similar timepoints to the control
58
group is higher. This can only be attributed to contact with the
tooth root, since other variables are controlled. This confirms
quantitatively what clinicians have claimed anecdotally, namely
that insertion torque does increase with tooth root contact.
This finding also agrees with the previously mentioned studies
using pre-drilled MSIs and pilot holes who found torque
increased with tooth contact.10,12,20
When the mean insertion torque is compared to control there
was a statistically significant difference after .2 min (12
seconds) for the 3M Unitek MSI and at .3 min (18 seconds) for
the Tomas MSI. This may be due to the difference in pitch of the
MSIs, which would affect the rate at which they entered the
bone.
Torque-limited or torque-measuring drivers are available
for many of the MSI kits. Some researchers advocate their use to
avoid excessively high insertion torque, which could lead to MSI
fracture and bone damage.1,31,32 The use of a torque-limited driver
could also help prevent complications due to tooth root contact.
MSI contact with tooth roots has been shown to dramatically
decrease success rates.6,7 If only partially contacted, most
roots heal normally, but in some cases severe damage can occur
which can lead to necrosis, ankylosis, or root fractures that
may require endodontic therapy or tooth removal.8,30
These data
show that there is a higher insertion torque caused by tooth
59
root contact. If an operator notices higher than normal torque
values and is unsure of his position, it may be worthwhile to
adjust angulation or location.
Additional research using human cadaver bone and in vivo
studies could better determine the safe ranges for insertion
torque. Ideal insertion torque values have not been established.
Motoyoshi et al. claimed that insertion torque values between 510 Ncm are ideal for a 6 mm long 1.6 mm diameter MSI, but
Chaddad et al. claims insertion torques over 15 Ncm are more
successful in similar screws.33,34 Both of these studies used the
older predrilled MSIs and a recent study by Suzuki and Suzuki
called into question the previous claims of ideal insertion
torque ranges since they found no difference in success rates of
higher torque self-drilling MSIs (14.5 Ncm) and lower torque
pre-drilled MSIs (9.2 Ncm).35
If ideal insertion torques were established, then torquelimited drivers could be set to alert the clinician when this
value has been exceeded, which could avoid damage to bone,
possible MSI fractures, and possible tooth root contact. An
experienced clinician could determine normal torque values
encountered in his office, and if the MSI suddenly increased in
torque he or she could detect root contact and remove the MSI
and choose a better location.
In pilot testing using hand
60
drivers, the researcher in this study felt he was able to
determine via tactile feedback when a tooth root was contacted.
MSIs and their proximity to tooth roots were easily visible
on the CBCT images, and higher insertion torques were
consistently found in the samples that showed tooth root
contact. Clinicians with access to CBCT imaging might find it
useful to check position of MSIs if they are unsure of root
proximity.
One finding was the thread-cutting Tomas tip was more prone
to fracture when contacting a tooth. The fractures seemed to
happen when a glance was occurring. The MSI would contact the
root and then stop advancing and not pass the root. By creating
a cutting tip, metal is removed, which weakens the tip.15,36 On
the other hand, thread-cutting designs are claimed to cause less
compression and alleviate bone stress, thus allowing for a more
less traumatic insertion.37,38 There have been no studies
specifically comparing thread-forming to thread-cutting MSIs
used in orthodontics. This study was not designed to compare
thread-forming to thread-cutting MSI design on insertion torque,
since the miniscrew’s design varied in other ways which could
also influence results.
More studies should be done to
investigate what the benefits, if any, of a thread-cutting
design are, since they potentially have the weakness of
increased fracture.
61
Conclusions
1. 3M Unitek MSIs showed higher insertion torque than Tomas
MSIs in control groups.
2. Self-drilling MSIs which contact teeth show a higher
insertion torque than control groups with no contact.
3. It may be possible to detect tooth root contact by using a
torque-sensitive driver.
4. Tomas MSIs showed higher fracture rates at the tip when a
tooth was contacted, possibly due to the weaker threadcutting design.
62
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Vita Auctoris
Michael Berry McEwan was born February 28th, 1980 in Provo,
UT to Robert and Sandra McEwan.
He grew up in Henderson, IL and
graduated from Galesburg High School in 1998.
He attended
Brigham Young University from 1998-1999. He interrupted his
education to spend two years serving a religious mission for The
Church of Jesus Christ of Latter-day Saints in Mexico from 19992001.
Afterward, he resumed his studies at Brigham Young
University where he received a Bachelor of Science in Food
Science in 2004.
From 2005 to 2009 he attended the University
of Iowa College of Dentistry, where he received his Doctorate of
Dental Surgery in 2009.
It is anticipated that Michael will
graduate from Saint Louis University’s Center for Advanced
Dental Education in January 2012 with a certificate of specialty
in orthodontics and a Master of Science in Dentistry (Research).
Dr. McEwan married Lea Janine McEwan in 2004 and they are
the proud parents of Mazy and Evey. Upon graduation, Dr. McEwan
will enter private orthodontic practice in Colorado.
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