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EVALUATING STABILITY CHANGES OF MINISCREW IMPLANTS USING
RESONANCE FREQUENCY ANALYSIS IN BEAGLE DOGS
Derid S. Ure, B.S., D.D.S.
An Abstract Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Masters of Science in Dentistry
2010
Abstract
Purpose: This study evaluated changes in miniscrew implant
(MSI) stability over eight weeks using resonance frequency
analysis.
The study was designed to evaluate the impact of
pilot holes and placement sites on changes in stability.
Method: Implant Stability Quotient (ISQ) values were measured using the Osstell® Mentor device for 22 MSIs, 1.6 mm
in diameter and 9 mm in length, placed in the maxilla of
adult beagle dogs (20 months old).
Measurements were taken
weekly, starting at the time of placement and ending at
eight weeks.
Using a split mouth design, 1.1 mm wide pilot
holes were randomly selected and drilled to a depth of 3 mm
for half of the MSIs prior to placement.
MSI placement was
also divided between keratinized and non-keratinized tissue.
Results:
Nine of the 22 MSIs failed; all of the
failures were related to having been placed in nonkeratinized tissue.
MSIs that failed showed significantly
(p<0.05) higher decreases in stability during the first
three weeks than the MSIs that remained stable.
MSIs that
remained stable throughout the study also showed decreases
in stability during the first three weeks and increases in
stability between the third and fifth week (p<0.05). Pilot
holes had little or no effect (p>0.05) on MSI stability.
1
Conclusion:
Stability of MSIs decreases from week one to
week three and increases from week three to week five. Pilot holes do not affect the stability of MSIs.
Placement
of MSIs into non-keratinized tissue negatively impacts
their stability and increases the likelihood of failures.
2
EVALUATING STABILITY CHANGES OF MINISCREW IMPLANTS USING
RESONANCE FREQUENCY ANALYSIS IN BEAGLE DOGS
Derid S. Ure, B.S., D.D.S.
A Thesis Presented to the Faculty of the Graduate School
of Saint Louis University in Partial Fulfillment
of the Requirements for the Degree of
Masters of Science in Dentistry
2010
COMMITTEE IN CHARGE OF CANDIDACY:
Adjunct Professor Peter H. Buschang,
Chairperson and Advisor
Assistant Professor Ki Beom Kim
Associate Clinical Professor Donald R. Oliver
i
DEDICATION
This thesis is dedicated to four groups of people:
First, to all my teachers when I was younger, for
their unselfish dedication to my growth and success.
Second, to all my professors at C.A.D.E., for showing
me that there is so much more to the profession then just a
good career.
Third, to my parents, who have provided my personal
foundation by their example of consistent, quiet effort,
always expecting me to do my best.
Fourth, to my wife Rachel and our little family, for
their sacrifice, allowing me to pursue my goals and giving
me joy along the way.
ii
ACKNOWLEDGEMENT
I would like to acknowledge the following individuals
and groups for their help and support of this thesis:
Dr. Peter H. Buschang, for chairing my thesis committee.
Thank you for all the guidance helping me understand
my project more clearly.
Dr. Ki Beom Kim, for serving on my committee.
Thank
you for your support and availability giving me someone
that I could always approach with my questions and concerns.
Dr. Donald Oliver, for serving on my committee.
Thank
you for your attention to detail that allowed me to focus
not only on the big picture but also the finishing touches.
Dr. John Long and the staff at SLU Comparative Medicine, for making every Monday for nine weeks so much fun.
Neodent, for providing all the customized miniscrews
used during this project.
The 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: LITERATURE REVIEW
History of Skeletal Anchorage Devices in Orthodontics... 5
Understanding Miniscrew Implant Stability.............. 10
Primary Stability ................................... 11
Secondary Stability ................................. 23
Healing Curves: Primary vs. Secondary Stability ..... 32
Measures of Stability.................................. 39
Invasive Methods .................................... 40
Non-Invasive Methods ................................ 45
Summary................................................ 61
References............................................. 62
CHAPTER 3: JOURNAL ARTICLE
Abstract............................................... 81
Introduction........................................... 82
Materials and Methods.................................. 86
Animals ............................................. 86
MSI Placement ....................................... 87
Miniscrew Implants .................................. 90
Resonance Frequency Measurements .................... 90
Statistical Analysis ................................ 92
Results................................................ 93
Failures ............................................ 93
Comparison of ISQ Changes of all MSI Over the First
Three Weeks of the Study ............................ 95
Longitudinal Changes in ISQ of MSIs Maintained
Throughout the Study ............................... 100
Discussion............................................ 101
Conclusions........................................... 110
References............................................ 111
Vita Auctoris ........................................... 116
iv
LIST OF TABLES
Table 3.1: Weekly ISQ values from placement to eight
weeks....................................................94
Table 3.2: Descriptive statistics and statistical
comparisons (Wilcoxon signed-rank test) of ISQ values between MSIs that Failed vs. Survived, MSIs that received a
Pilot hole vs. no Pilot hole and MSIs placed in Keratinized
vs. non-Keratinized tissue from surgery to three
weeks....................................................95
Table 3.3: Descriptive statistics and statistical
comparisons (Wilcoxon signed-rank test) of changes in ISQ
from surgery to three weeks..............................97
Table 3.4: Longitudinal statistical comparisons of ISQs
for MSIs that survived the entire study.................100
v
LIST OF FIGURES
Figure 2.1: Primary and secondary stability curves.......34
Figure 2.2: Implant modes of vibration...................51
Figure 2.3: Minimizing implant modes of vibration........52
Figure 2.4: Orientation of Osstell Mentor transducer
during measurements......................................58
Figure 3.1: Primary, secondary and total stability
curves over eight weeks..................................83
Figure 3.2: The effect on total stability of a shift in
secondary stability......................................84
Figure 3.3: MSIs in Dog A and Dog B by location with
pilot hole information...................................88
Figure 3.4: Brassler handpiece...........................89
Figure 3.5: Pilot hole drill.............................89
Figure 3.6: MSI side and top view........................90
Figure 3.7: Smartpeg.....................................90
Figure 3.8: MSI and Smartpeg connected...................90
Figure 3.9: Orientation of Osstell® transducer in
relation to the occlusal plane...........................91
Figure 3.10: Orientation of Osstell® transducer in
relation to the Smartpeg and MSI.........................91
Figure 3.11: Timing of MSI failures by dog, anterior/
posterior location, tissue type and pilot hole presence..94
Figure 3.12: ISQ over the first three weeks of failed
and survived MSIs........................................96
Figure 3.13: ISQ over the first three weeks for pilot
vs. no pilot hole MSIs...................................96
vi
Figure 3.14: ISQ over the first three weeks for
keratinized vs. non-keratinized placement of MSIs........97
Figure 3.15: Change in ISQ over time for MSIs that
failed and survived over the first three weeks...........98
Figure 3.16: Change in ISQ over time for MSIs with
pilot hole vs. no pilot hole over the first three weeks..99
Figure 3.17: Change in ISQ over time for MSIs placed
in keratinized vs. non-keratinized tissue over the
first three weeks........................................99
Figure 3.18: Mean ISQ values of MSIs that survived the
entire study from placement through week eight..........101
Figure 3.19: Stability of dental implants from placement
through week eight......................................105
Figure 3.20: Mean ISQ values of MSIs that survived the
entire study from placement through week eight divided
by predominant stability type...........................105
vii
CHAPTER 1: INTRODUCTION
One of the challenges facing orthodontics is the control of unwanted tooth movement.
The ability to limit un-
wanted tooth movement can often lead to the successful
treatment of complex malocclusions that otherwise would be
nearly impossible to treat.
Anchorage, defined as minimiz-
ing unwanted tooth movement, has long been an important
consideration for practitioners since the beginning of the
specialty.
Recently, skeletal anchorage in the form of miniscrew
implants (MSIs) has grown in popularity in orthodontics as
a means to selectively control tooth movement.
The use of
skeletal anchorage has allowed the improved treatment of
some of the most difficult malocclusions.
The explosion of
the use of MSIs has revealed that one of the problems with
this type of treatment is the rate at which MSIs fail.
The
success rates reported in the literature range from less
than 50% to over 95%.1-4
The unpredictability of MSIs lim-
its their usefulness as a treatment modality.
Research suggests that there are a number of factors
that affect MSI failures including, mobility, excessive
heating of the bone during placement, placement in keratinized vs. non-keratinized soft tissue, host factors such as
1
uncontrolled diabetes, excessive loading and poor oral hygiene.
While all of these factors have been associated
with failure, the actual cause of all MSI failure is the
loss of bone-to-implant contact and consequently, stability.
Stability can be divided into two types, primary sta-
bility, a mechanical interlocking between the implant and
surrounding bone directly after placement, and secondary
stability, a biological phenomenon that begins soon after
placement where bone surrounding the implant is remodeled
and new bone is added.
Secondary stability develops as a
result of the healing process of bone.
The overall stabil-
ity experienced by a MSI is a combination of primary and
secondary stability.
Understanding changes in MSI stabil-
ity over time could lead to the development of clinical
management techniques that could enhance stability and consequently, improve the predictability of the success of
MSIs, enhancing their usefulness to the profession.
Various techniques have been suggested as a way to
study stability of MSIs.
Some examples of measures of sta-
bility include, insertion torque, removal torque, histomorphometric studies and pullout.
The problem with these
measures of stability is that they require the destruction
of the bone-to-implant contact, rendering them not useful
for clinical use.
Other less invasive methods have also
2
been proposed.
The most promising of these methods, reso-
nance frequency, has been used to successfully study the
stability of dental implants over time in clinical situations.
There is a need to better understand the changes in
stability experienced by miniscrew implants in order to develop methods to improve the predictability of implant success.
It is the purpose of this study to determine if
resonance frequency can be used to study changes in
miniscrew implant stability in dogs over an eight-week period.
This study will also evaluate the effect of using a
pilot hole and tissue type on the early stability of MSIs.
In order to better understand the relationship between
miniscrew implants, their success and primary and secondary
stability, the subsequent review of the literature will be
divided into the following sections:
1. The history of
skeletal anchorage will be explored.
This will provide un-
derstanding of how the concept of using bone as a source of
biological anchorage developed.
2. The concepts surround-
ing stability will be explored.
First, the idea of primary
stability will be defined and factors that help determine
this stability will be evaluated with special emphasis on
the effect of pilot holes and tissue type.
Second, the
concept of secondary stability will be reviewed. Factors
3
that help determine this stability will be presented and
examined in detail.
3. A look at the biological process of
healing and how it relates to primary and secondary stability will be considered.
4. Methods to evaluate the stabil-
ity of MSIs will be investigated with particular emphasis
on resonance frequency as a means to evaluate changes in
stability over time.
4
CHAPTER 2: LITERATURE REVIEW
History of Skeletal Anchorage Devices in Orthodontics
The need to enhance anchorage in orthodontics has led
to the development of many methods to control unwanted
tooth movement.
These methods include the use of Tweed,5
segmented,6 bi-dimensional,7 bioprogressive8,9 and other
types of mechanics.
In the bioprogressive technique, Rick-
etts advocated the use of the cortical plate of the alveolar process as a source of anchorage; by placing tooth
roots in close approximation to the cortical bone, it was
thought that their movements could be slowed down compared
to the other teeth in the arch.8,9
The use of devices that
utilize the bone as a direct source of anchorage was envisioned even before Ricketts introduced his concept of cortical anchorage.
Using the bone surrounding the teeth as a
source of anchorage during treatment has long been a goal
of orthodontists.
As early as 1945, Gainsforth and Higley used a vitallium screw placed in the ramus of dogs that was then connected to the canine tooth in an effort to enhance anchorage.
Unfortunately, when orthodontic forces were placed on
the screws they failed.
Interest in the use of bone for
skeletal anchorage began to move forward following the
5
landmark studies by Brånemark in the late 1950s and early
1960s when he noticed that titanium optical chambers placed
in rabbits could not be removed from the bone.
Further in-
vestigation led him to observe the ingrowth of bone around
the titanium devices resulting in what he termed “osseointegration.”10,11
To Brånemark, osseointegration was the lit-
eral fusion of bone and titanium.
For the purpose of this
study osseointegration will be defined as, direct structural contact between living bone and the surface of an artificial implant at the light microscopic level, commonly
known as bone-to-implant contact.
Soon dentistry came to
understand the principles of osseointegration and new restorative procedures were developed that utilized dental
implants to replace missing teeth.
As dental implants be-
came more predictable and used on a wider scale, orthodontists began to realize their potential for supplementing
orthodontic anchorage.
To retract teeth, Linkow utilized a blade style dental
implant with rubber bands attached.12
The problem with us-
ing this type of anchorage is the dental implant must be
placed before orthodontics commences and given time to osseointegrate.
As teeth move relative to the dental implant
during orthodontic treatment, the location of the implant
may not end up in the ideal position for restorative pur6
poses.
To overcome this limitation, Kokich13 and Smalley
and Blanco14 developed protocols to determine proper placement so that dental implants used for orthodontic tooth
movement were in the proper location for restorative treatment.
These approaches to skeletal anchorage all required
dental restorations in order to justify the placement of
the implants.
In addition to restorative need, they also
required space for placement. This limited the usefulness
of these types of skeletal anchorage devices to a small
percentage of patients.
In response to growing interest in skeletal anchorage
and to overcome the limited usefulness of dental implants
as a source of orthodontic anchorage, new types of skeletal
anchorage systems developed.
These new anchorage systems
are placed specifically for anchorage purposes and removed
after being utilized for orthodontic purposes.
Roberts et al. introduced the first of these orthodontic specific systems.15,16
It consisted of a miniature ver-
sion of the dental endosseous implant that was placed in
the retromolar area of the mandible and was removed after
having been used for orthodontic anchorage.
This system
however, was only useful for anchorage purposes in the man-
7
dible and was often difficult to remove due to osseointegration that occurred during treatment.
Wehrbein et al. used small endosseous implants placed
in the palate for orthodontic purposes.17
This system of
orthodontic anchorage was also limited in its usefulness
since removal required the elimination of bone circumferentially around the implant in a process called trephination.
To overcome this limitation a palatal onplant was
developed.18
Instead of placing the implant within the
bone, this system was placed under the periostial membrane
in direct contact with the cortical portion of the bone.
It required that the bone grow into the base of the onplant
to ensure its success as a source of anchorage.
The
placement and removal surgeries, time required for osseointegration, and difficulty of removal, also made this
anchorage system less than ideal for orthodontic purposes.
The use of titanium plates for rigid fixation of orthognathic surgery patients led to the development of yet
another type of anchorage system.
Bone plates, developed
by Sugawara and Nishimura, were modified titanium rigid
fixation plates that were fixed to the bone and then passed
through the oral mucosa into the oral cavity and used as a
source of anchorage.19
The benefit of this type of system
was that it could be placed in both the maxilla and mandi8
ble.
However, it still required two surgical procedures,
one for placement and one for removal, which limited its
usefulness and acceptance by the profession.
In an attempt to increase the acceptance of skeletal
anchorage by patients and the profession, Creekmore and Eklund introduced the use of surgical screws to intrude maxillary anterior teeth in 1983.20
However, the screw they
used was rather large and the surgical procedure was unspecified.
In 1997, a formalized insertion protocol for
using surgical screws as anchorage was developed by Kanomi.21
The screws he advocated were much smaller that
those used by Creekmore and Eklund, with a diameter of 1.2
mm and length of 6 mm.
These small surgical screws were
the forerunners of the modern miniscrew implants (MSIs).
However, it would not be until Costa et al. simplified the
placement procedure in 1998, that MSIs started gaining
widespread attention in the profession.22
Since 1998 numer-
ous MSI systems for orthodontic anchorage have been developed and popularity of MSIs has grown at an ever-increasing
rate since they were first introduced.
They are now an in-
tegral part of many orthodontists’ treatment procedures.
In order to maximize efficiency and the success of MSIs,
the biological principles surrounding the stability of MSIs
must be explored and appreciated.
9
Ultimately, stability of
a MSI is a function of the biological changes that occur in
the bone surrounding the implant.
The source of this sta-
bility and how it changes over time will now be considered.
Understanding Miniscrew Implant Stability
In order for MSIs to be useful to orthodontics as a
source of anchorage, it is necessary that they be able to
withstand orthodontic forces placed on them and remain in a
stable position in the bone until their use is no longer
required.
Orthodontic forces used for tooth movement usu-
ally fall in the range of 1-3 N (roughly 3.5-11 oz or 100300 gm).23
For the purpose of this research, stability of
an implant will be defined as the implant’s ability to
withstand loading with orthodontic forces.
Stability can be divided into two types that are vital
to the overall success of the MSI, primary stability, a mechanical phenomenon due to initial contact between the implant and bone, and secondary stability, associated with
the remodeling and deposition of new bone around the implant over time.24
Understanding how these types of stabil-
ity relate to each other and to overall MSI stability is
necessary in order to increase the success rates of MSIs.
The concept of primary stability and the factors that af-
10
fect it will now be considered.
Then secondary stability
will be examined.
Primary Stability
According to Wilmes et al., primary stability, a mechanical phenomenon derived from the contact of the implant
with the surrounding bone, occurs immediately after placement of the MSI.25
For MSIs, the cortical bone surrounding
the implant is the major determinant of primary stability.23
Depending on the length of the MSI placed, location and
surgical procedure (depth of placement), primary stability
can be derived from either mono or bi-cortical placement.26
Primary stability is important for two reasons.
First, primary stability is critical from an orthodontic
standpoint because it allows for the MSI to be loaded by
the orthodontist immediately after placement, which increases efficiency.22,27,28
Second, primary stability is
critical for the development of secondary stability.29
The
relationship of adequate primary stability and secondary
stability can best be understood by considering the
stresses surrounding MSIs during placement.
Placement of an MSI into bone causes compression of
bone and generates stresses in the bone surrounding the im-
11
plant.
The nature and severity of these stresses are par-
tially determined by the cortical bone thickness.30,31
The
thicker the cortical bone, the more the stresses produced
by compression of bone concentrate within the cortical
plate.
These stresses play an important role in the move-
ment of the MSI in relation to the surrounding bone.
If these stresses are adequate, the MSI is held in a
rigid position, ensuring an adequate environment in which
to remodel the bone surrounding the implant and enhance
secondary stability.
If the stresses are not sufficient then movement of
the MSI relative to the surrounding bone can lead to failure.
This lack of adequate stability has been identified
as a risk factor for early implant loss due to failure of
osseointegration.29,32
Implant movement may lead to mi-
crofracture, necrosis, bone resorption and eventual formation of a fibrous capsule.
As movement occurs, additional
trauma to the bone in the form of microfractures in bone
surrounding the implant can occur.
When the body removes
the damaged bone during the healing process, the MSI becomes even less stable and movements may become even
larger.
These movements can lead to the formation of fi-
brous tissue instead of bone, as demonstrated with blade
implants.33,34
Small implant movements lead to the recruit12
ment of pluripotent bone cells to the area needing repair.
These cells can differentiate into bone, cartilage and fibrous tissue.35
Similar to fracture healing, if movement in
the repair area is present, then these cells differentiate
into fibrous tissue, which cannot be converted into bone.
Once fibrous tissue forms between the implant and bone,
failure of the MSI can be expected.36
Without adequate pri-
mary stability and bone support, secondary stability could
be limited due to the formation of a fibrous capsule and
the miniscrew could eventually loosen.
If the stresses during the insertion of the MSI are
excessive then failure of the implant to attain secondary
stability can also be anticipated.
The compression of bone
during insertion generates circumferential hoop stresses.37
Hoop stress is defined as a mechanical stress produced by
rotationally-symmetric objects that results from forces
acting circumferentially (perpendicular both to the axis
and to the radius of the object).38
While hoop stresses
that press the bone against the MSI provide primary stability, excessive stresses can be detrimental to the long-term
stability of the implant.
Excessive hoop stresses can pro-
duce local ischemia which leads to bone necrosis and consequently micromotion of the implant.37
As previously de-
scribed, this micromotion may often lead to the formation
13
of a fibrous capsule surrounding the implant and to eventual failure.
However, hoop stresses are not the only
stresses encountered during placement of a MSI.
As a miniscrew is inserted, the threads of the screw
are positioned so that they draw the screw into the bone.
This creates a vertical stress on the bone immediately surrounding the implant known as a shear stress. As more bone
is contacted during placement these shear stresses increase.39
If these stresses become excessive, the bony ma-
terial surrounding the threads becomes stripped and the
screw spins freely.40,41
This leads to a compromise in the
stability of the screw.42
While the importance of understanding the nature of
primary stability cannot be over stated, factors that contribute to this stability are also important to comprehend.
Determinants of Primary Stability
There are three general factors that determine the extent of the primary stability, including bone attributes,
implant design and insertion technique.25
14
Bone Attributes
Bone attributes play an important role in the primary
stability of MSIs.
One characteristic of bone, cortical
thickness, can have a large impact on the stability of
miniscrews.
It has been reported that thicker cortical
bone is considered better for MSI placement.28,43
If primary
stability is derived from bone-to-implant contact, and most
of that contact is found in the cortex, then a thicker cortex will yield a higher primary stability, other factors
being equal.
One measure of primary stability, insertion
torque, has been shown to have a direct relationship with
cortical thickness.1,25,44-46
The thicker the given cortex,
the higher the accompanying insertion torque.
Another
measure used to evaluate primary stability, pullout
strength, also shows a positive relationship with cortical
thickness.42,44,47,48
The thicker the cortex the higher the
force required to pull the MSI out of the bone.
Bone density also plays a key role in primary stability.
Studies evaluating bone density have demonstrated
positive correlations with insertion torque and pullout
strength.25,42,47
It makes intuitive sense that the higher
the density of bone, the greater the primary stability.
This could easily be explained by higher initial bone-toimplant contact.
Recently, Hung, using synthetic bone,
15
demonstrated the relationship of bone density and stability
for MSIs.49
In dental implant literature, this relationship
has been shown to be so strong that rating systems of bone
density have been developed.50,51
Ryken et al. developed a
mathematical model based on bone density and insertion
torque to predict the stability of screws used in spinal
surgery.52 Studies using endosseous implants have shown that
the quality of the bone surrounding the dental implant can
have a significant effect on the stability.39,48,53
Having
adequate bone density to provide primary stability is considered so important for dental implants that a novel insertion method was introduced by Büchter and others.
This
technique attempts to increase the density of the bone by
compacting the bone surrounding the implant during placement; accomplished by inserting successively larger compressive osteotomes into a smaller than normal pilot hole.54
Implant design
The design of the MSI can affect primary stability.
Multiple studies have evaluated the effect of implant design on primary stability.
Length, diameter, shape, and
thread design have all been examined and have been found to
16
be significant factors when evaluating insertion torque and
pullout strength to measure primary stability.55,56
Wilmes et al. evaluated miniscrew diameter and shape
and related it to insertion torque.57
They found that
larger diameter MSIs exhibited higher insertion torque.
They also showed that conical shaped miniscrews had higher
insertion torque values.
Lim and others evaluated the effects of length, diameter and shape and their relation to insertion torque.45
They concluded that all these implant design factors play a
role in determining insertion torque. Longer miniscrews and
larger diameter miniscrews had higher insertion torque values. They also determined that tapered or conical shaped
implants had higher insertion torque values, suggesting
higher primary stability.
Brinley et al. evaluated several miniscrew designs in
synthetic and cadaver bone to determine the effect of
thread design on insertion torque and pullout strength.58
They concluded that decreased thread pitch, the distance
between adjacent threads of a miniscrew, led to an increase
in pullout strength.
The study also evaluated the effect
of fluting on insertion torque and found that it would significantly increase primary stability.
17
Surgical Procedure
Another factor that can affect the primary stability
of miniscrews is the method of placement.
During placement
of a MSI the general goal is to perform the placement with
as little trauma as possible.
A minimally traumatic inser-
tion theoretically gives the miniscrew the best chance to
go through its healing phase quickly and uneventfully.
Trauma necessitates bone remodeling, healing and formation
of woven bone.
Due to its poor organization and structure,
woven bone may provide limited support to withstand orthodontic loading forces.
In order to minimize trauma several
surgical techniques have been developed.
The major tech-
nique advocated to minimize the trauma associated with MSI
placement includes the preparation of the implant site by
drilling a pilot hole.
The rational behind the use of a pilot hole is to
minimize the stresses of the bone by removing a portion of
the bone prior to screw placement.
Excessive stresses can
lead to microfracture of the bone or ischemic necrosis and
eventual failure of the screw.37
Pilot holes were originally used by surgeons to allow
placement of screws into bone.
In 1959, Boucher was one of
the first to use pilot holes when placing large pedicle
screws during spinal fusion surgery.59
18
He advocated the use
of pilot hole placement to help prevent premature screw
failure, which was common at the time.
His goal was to
create a more stable fixation resulting in higher surgical
success rates.
The use of pilot holes became commonplace
in spinal surgery by the 1980s.60
In order to understand
how a pilot hole affects the holding power of a screw in
bone, Daftari et al. evaluated correlations between insertion torque, pullout strength and pilot hole.61
Pilot holes
for spinal surgery were necessary due to the size of the
screws being placed.60
The success of this method in spinal
fusion surgery led to the adoption of pilot holes by other
surgical specialties.
Craniofacial surgeons utilized pilot
holes as part of their surgical protocol, even though they
used much smaller screws, comparable to the size of MSIs.62
Because placement of screws in the medical literature
advocated the use of pilot holes, they were thought to be a
good idea during MSI placement as well.21
However, to sim-
plify placement and to overcome some of the risks of placing a pilot hole, such as, nerve damage, tooth damage and
bone necrosis, some began to advocate the placement of
screws without a pilot hole.63
Screws were designed to be
self-drilling and claimed easier placement due to the simplified procedure.
Following the use of the non-drill in-
sertion procedure, reports of screw breakage during place19
ment began to surface.64-66
High insertion torque values and
screw diameter were given as a reason for screw fracture.65
So limiting insertion torque became a valid reason to place
MSIs with a pilot hole.
Chen et al. showed that MSIs
placed without pilot holes had higher insertion torque values than those that had pilot holes drilled.64
This was
similar to work performed by Oktenoglu and colleges.67 Another factor that can lead to increased insertion torque is
the density of the bone in the area of placement.
The
denser the bone the higher the insertion torque.64,68
Ex-
cessive insertion torque not only increases the risk of
fracture of the screw, but also may lead to failure at a
later time.1
It has become generally well accepted that in
areas of the mouth where there is bone with a higher density, i.e. the posterior mandible, the use of a pilot hole
may be the prudent method of placement.1
It is important to consider the size of the pilot
hole.
Hung, concluded that increasing the pilot hole di-
ameter lowers the insertion torque and pullout strength of
miniscrews in synthetic bone.49
She also showed that the
density of the bone surrounding the implant affected these
measures.
Heidemann and coworkers, who studied the rela-
tionship of pilot hole size and stability, found that a pilot hole up to 85% of the external diameter of the screw
20
could be used without any loss in pullout strength.69
This
was confirmed in work by Gantous and Phillips when they
showed that a pilot hole 85% of the external diameter of
the screw allowed for adequate stability.62
However, if the
bone is of lower quality or density, a smaller diameter pilot hole should be used.70
It is clear that the use of a pilot hole decreases
primary stability, as measured by insertion torque, but
what effect does it have on the secondary, long term, stability?
In their work involving the impact of pilot holes,
Präger et al. showed no difference in bone-to-implant contact after 12 weeks of healing between groups of implants
placed with and without pilot holes, indicating that the
use of pilot holes has no impact on secondary stability.71
In contrast, Heidemann et al. found a very slight decrease
in bone-to-implant contact in the pilot hole group.72
While advocates of pilot holes suggest that their use
minimizes trauma during placement, others contend that
their use can actually cause excessive trauma.
Heidemann
et al. indicated that the use of pilot holes carries its
own set of risks, including damage to the teeth and surrounding structures, as well as drill breakage and thermal
insult to the bone.63
As a living biological system, bone
is sensitive to adverse thermal episodes.
21
It has been
shown that an increase above 47° Celsius can cause necrosis
of the bone.73,74
As previously explained, necrosis may
eventually lead to MSI failure.
Thermal insult can be pre-
cipitated by excessive pressure on the drill during placement,75 a worn out drill,75 lack of irrigation76 and excessive speed of the drill.76
To overcome these negative side effects of using a pilot hole, many practitioners prefer a drill-free placement
protocol.
This type of placement is suggested because it
is quicker, may be less destructive if the root is contacted and generates less heat.
However, placement of an
MSI without a pilot hole may still generate excessive
heat.77
This is due to the friction between the implant and
bone which can be measured by insertion torque.
The higher
the insertion torque the higher the friction between the
bone and screw, producing greater heat upon insertion.66
This may be especially relevant in dense bone that has a
higher insertion torque.
For the placement of an MSI, one
factor that determines the amount of friction is the thickness of the cortical plate.
So anatomical areas, such as
the posterior mandible, that have thicker cortical plate,
may benefit from the surgical protocol of properly placing
a small pilot hole.66
22
The use of pilot holes and other modifications of the
surgical procedure for MSI placement are important because
they ultimately have an effect on the long-term secondary
stability of the screw.
It is secondary stability which
provides the benefit of MSIs to orthodontics by allowing
loading and resistance to movement over a clinically useful
time period.
Consequently, an understanding of secondary
stability is imperative.
Secondary Stability
Secondary stability is produced by biological processes that include the deposition of new bone and the remodeling of immature bone around an implant.23,78
stability is dependent on primary stability.
Secondary
With adequate
primary stability (lack of implant mobility), bone can form
and remodel around the implant.
This process leads to an
increase in secondary stability of the implant.23
It has
been shown in dental implant literature that this increase
in stability takes place approximately 4 weeks after placement of the implant.24,79
While primary stability provides the miniscrew with
sufficient stability to withstand 1-3 Newtons orthodontic
forces initially, secondary stability is needed to allow
23
the implant to resist the orthodontic forces used throughout the duration of treatment.23
This longer-term stability
is provided by the osseointegration of the MSI with the
newly formed bone being laid down.
It is thought that the
deposition of new bone and the remodeling of previously deposited bone occur as a response to stresses experienced by
the bone in the area immediately surrounding the implant.
Bone and implants have a different elastic moduli (a measure of stiffness).
This difference in moduli creates
stresses within the bone when the bone is bent.80,81
During
function, force is applied to the bone, causing the bone to
bend.
Because bone has a lower modulus of elasticity (17.9
GPa)82 than titanium alloy (116 GPa),83 it bends more than an
implant.
This differential bending generates stresses in
the bone surrounding the implant.
These stresses are
thought to promote remodeling and healing of the bone.78
It
is thought that the generation of stresses during mastication accounts for the abnormally high levels of cortical
bone remodeling (100-200%) that occur during the first year
after dental implant placement.23
The normal rate of remod-
eling in the cortex is 2-10% per year.23
Understanding what secondary stability is and the
process of osseointegration provides the opportunity to review what factors can affect this stability.
24
Determinants of Secondary Stability
Insufficient primary stability (i.e. mobility) facilitates the formation of fibrous tissue instead of bone at
the implant surface.29,33
This may lead to excessive mobil-
ity and eventual implant failure.
The following discussion
of determinants of secondary stability presupposes that
adequate primary stability existed.
Host factors
Host factors play an important role in determining
secondary stability.
The body’s ability to heal itself is
paramount to the development of long-term implant stability
because the healing process is essentially what occurs
around an implant after placement.
In examining dental im-
plants, Ashley et al. determined that osteoporosis, uncontrolled diabetes, smoking and parafunctional habits, interfered with the healing process and were risk factors for
implant failure.84
Poor bone density has also been identified as a risk
factor for implant failure.85
It appears that bone must be
of adequate quality to help maintain a certain level of
stability in order to prevent the loosening and failure of
25
implants over time.
This concept is supported by studies
that show different success rates for implants placed in
anatomical areas with differing quality of bone.46,2,86
Simi-
larly, the density of bone has also been sited as a risk
factor for premature failure of miniscrews.1
Along with physiology and anatomy, host behavior has
been shown to have an effect on secondary stability of
MSIs.
The ultimate cause of implant failure is loss of
bone-to-implant contact.
Inflammation around the implant,
leading to mobility and inhibition of osseointegration has
been identified as a risk factor for MSI failure.
Oral hy-
giene has been shown to play an important role in minimizing inflammation around implants.2,3,87
Poor oral hygiene
has been suggested as an important factor relating to MSI
success.2
It appears from work by Park et al. that care
around the implant is the most important aspect of oral hygiene.2
Without the foundation of a biologically sound host
and proper oral hygiene, the development of secondary stability is not predictable.
Implant Design
Similar to primary stability, implant design can play
an important role in the development of secondary stabil-
26
ity.
MSIs that have a good design for primary stability
are likely to have a good chance for secondary stability.
Designs that produce adequate bone-implant contact minimize
the micromotion of the screw and allow for the development
of additional bone-implant contact.
Recent advances in dental implants have shown that
treating the surface of the implant where it contacts bone
can enhance the deposition of secondary bone and, consequently, stability.88,89
Traditionally, MSIs have been pro-
duced with a machined smooth surface.
This surface can be
treated in a number of ways in order to enhance bone-toimplant contact.
In a process termed plasma spraying, im-
plants can be coated with bioactive materials such as hydroxyapetite.90,91
Bioactive proteins, such as melanin, can
also be wiped on the surface of the implant prior to placement.92
The most common treatment is to roughen the surface
of the implant via sand blasting or acid etching, or a combination of both, to create more surface area microscopically for bone to contact.
This process, called SLA (sand
blasted, large grit and acid etched) treatment, has been
used on MSIs in an attempt to increase secondary stability.
Work by Ikeda showed significantly more bone-to-implant
contact for SLA treated MSIs than conventional smooth surfaced ones.93
It may be possible to enhance secondary sta27
bility of MSIs via surface treatment.
The concern with
this approach is the fear of breakage upon removal due to
excessive osseointegration.94
Kim et al. reported higher
total energy required for removal of SLA treated implants,
but showed no difference in maximum removal torque between
SLA treated and conventional MSIs.95
Surface treatment of
MSIs may be a future method to enhance secondary stability.
Surgical Procedure
Another factor that can impact secondary stability is
the procedure used during insertion.
During surgical
placement the goal is to perform the procedure so that
there is an optimal environment for healing to occur.
Therefore, all of the issues discussed previously that affect primary stability, would also affect long-term stability.
There are, however, additional surgical factors that
affect secondary stability.
Location of placement has been shown to affect success rates of MSIs.
It has been proposed that root proxim-
ity is a major factor for screw failure.96
Kang et al.
showed a failure rate of 79.2% for screws that invaded the
roots vs. 8.3% failure rate for those only in alveolar
bone.97
It was suggested that when a miniscrew is in con-
28
tact with the root, micromotion is introduced by the root
during mastication, which leads to inflammation and eventual failure of the screws.
Avoidance of the roots during
placement is important.
Insertion torque has also been shown to affect secondary stability.
A survey conducted by Buschang et al.
showed that practitioners that measure insertion torque
have a significantly lower failure rate than those who do
not measure this important parameter.4
This is in agreement
with work by Motoyoshi et al., who reported a range of insertion torque values that will lead to optimal success
rates.1
Insertion torque values that are too high or too
low can lead to compromises in implant stability.
Loading of MSIs has been a topic of debate for some
time, with timing of load having been especially controversial.
MSIs.98
Immediate loading may promote the stability of
Animal studies have also reported that immediate
loading of MSIs can be successful.99,100
Others suggest a
healing period of four weeks before loading the implants.101
Motoyoshi et al. found that failure rates for early loaded
(average 2.6 weeks after placement) MSIs were significantly
higher, especially in areas of less dense bone in adolescents than for MSIs loaded after 12 weeks.102
The general
consensus at this time seems to be that immediate loading
29
is considered appropriate.
However, most proponents of
early loading are also advocates of using light forces initially with MSIs.
Magnitude of loading may effect secondary stability.
Excessive loading has been shown to be a risk factor in MSI
failure.103-105
Büchter et al. showed loosening of implants
subjected to forces of 9 N (approximately 900 grams).103
Timing of loading and magnitude of force application may
not be the only force related factors that contribute to
long-term stability.
Costa and co-workers showed that a
torsional force directed in the unscrewing direction could
cause the implant to fail.106
It appears that loading can
play a significant role in the long-term stability of MSIs,
but many questions remain unanswered.
The soft tissue surrounding the MSI can also affect secondary stability.
A relationship between the characteris-
tics of the soft tissue and success rates of MSIs has been
proposed.2,3,107
There are two types of oral tissue into
which implants can be placed, attached gingiva (keratinized) and movable mucosa (non-keratinized).
The concept
that tissue type can affect the success of an implant
originates in the dental implant literature.
For success
of dental implants it has been recommended that they should
only be placed in attached gingiva.108,109
30
This recommenda-
tion is based on the prevention of peri-implantitis that
forms around dental implants and is ultimately responsible
for their failure.
Without an adequate amount of attached
gingiva, inflammation around the implant can persist, leading to bone loss and eventual failure.108-110 This is thought
to be so important, that soft tissue grafting for dental
implants is often performed prior to implant placement in
order to provide an adequate zone of attached gingiva.111
It is also thought that placement tissue is a factor in
MSI success.23
In a comprehensive review of the literature,
Reynders and co-workers suggest placement of MSIs in the
attached gingiva in order to avoid inflammation around the
implant.86
However, this recommendation was based on only
three articles, two of which were not actual studies.3,112,113
Antoszewska et al. also reported significantly higher success rates for MSIs in attached gingiva compared to movable
mucosa.114
This could be explained in several ways.
First,
movable mucosa may cause micromotion of the implant if it
does not possess adequate primary stability leading to implant failure.
this concept.
There are, however, no studies to support
Second, movable mucosa does not provide an
adequate seal around the neck of the implant leading to
possible microbial invasion and inflammation, which can
lead to implant failure.2,3 Park and coworkers showed that
31
once inflammation was present in areas of non-keratinized
soft tissue, it persisted.2
In contrast, Chaddad et al. re-
ported no difference in the success rate of MSIs based on
soft tissue environment.107
Lim et al. also found that soft
tissue was not a factor associated with the success of
miniscrews.115
Currently, the relationship that soft tissue
type has with secondary stability is unclear.
With an understanding of primary and secondary stability
established, it is important to explore the changes that
occur during each type of stability, giving each its unique
attributes and role in determining overall success of MSIs.
One way to understand these changes is to examine the healing curves for primary and secondary stability.
Healing Curves: Primary vs. Secondary Stability
The purpose of stability of MSIs in orthodontics is to
obtain adequate rigidity to resist orthodontic forces for
the duration of treatment.
Consequently, it is important
to understand how stability changes after insertion.
It is
well understood that after insertion of a dental implant
there is a primary mechanical stability due to the tight
fit of the implant with the bone (bone-to-implant contact).116
Over time however, the bone-to-implant contact
32
changes as the body remodels the bone immediately surrounding the implant.
Bone-to-implant contact continues to
change as additional bone is laid down around the implant.
This new bone increases the stability, of the implant until
the stability plateaus and remains relatively constant for
the remaining life of the implant.117
The dynamic nature of
the bone surrounding the implants infers that bone-toimplant contact is in a state of change as the healing
process takes place.
Since bone-to-implant contact deter-
mines stability, it would be reasonable to expect that the
stability of an implant could also fluctuate.
It is well
understood in the dental implant literature that the stability of implants changes over time.118
This concept was
described in a graphical representation during a review of
the dental implant literature by Raghavendra et al. (see
Figure 2.1).118
33
Figure 2.1: Primary and Secondary Stability Curves.
Adapted from Raghavendra et al.118
They suggest stability can be broken down into two
types, primary stability and secondary stability.
The com-
bination of these two sources of stability yields the total
stability of the dental implant.
It is important to note
that in the dental literature we evaluate and study the total stability curve and have limited techniques to separate
primary from secondary stability.
While the primary and
secondary stability curves make intuitive sense they are
merely used to describe the shape of the total stability
curve.
Another challenge associated with evaluating these
curves is that we can look at them statically but have limited methods to examine them dynamically.
34
Primary Stability Healing Curve
Primary stability can be measured by various means including insertion torque, histology, radiology, cutting
torque, pulsed oscillation, impact hammer method, and resonance frequency analysis.
All these measures will be exam-
ined in more detail in the following section.
Much re-
search has been performed to evaluate primary stability for
dental implants, but there is limited understanding of how
primary stability changes over the first weeks after MSI
placement.
Once insertion takes place, the body begins its healing process to remove damaged bone from around the implant.
This early part of the healing process is characterized by
has increased osteoclastic activity.118
Consequently, sta-
bility begins to decrease as bone is removed from around
the implant.
The transition from primary to secondary sta-
bility during the first weeks of healing may also include
the replacement of lamellar bone with softer woven bone,
further decreasing the stability of the implant.116
Accord-
ing to Cochrane et al., this process may contribute to the
loss of primary bone contact and consequently can compromise stability.116
Decreased mechanical stability due to
35
lower bone-to-implant contact explains the shape of the
primary stability curve. Primary stability is the highest
immediately after insertion and declines thereafter.118
This phenomenon was reported by Luzi and et al. when they
noted a decrease in bone-to-implant contact between week
one and week four for loaded MSIs in monkeys.119
Others
have used resonance frequency to evaluate the change in
stability of dental implants over time.120,121
Ersanli et al.
took measurements at surgery, three weeks, six weeks and
six months.120
They found a significant decrease from sur-
gery to three weeks.
Balshi et al. Followed the stability
of 276 dental implants from placement to ninety days.122
Measurements were taken at surgery, 30 days, 60 days and 90
days.
Their results showed a statistically significant de-
crease from 0-30days.
In another study, Barewal et al.
evaluated the changes in stability from zero to ten weeks
according to bone type.123
They found a decrease in stabil-
ity from placement to week three in bone that was considered type 2,3 or 4 (according to the index proposed by Lekholm and Zarb).50 After initial loss of bone-to-implant contact, if for some reason (i.e. infection, fibrous tissue
formation or lack of adequate primary stability) new bone
does not form around the implant, secondary stability never
develops leading to screw failure.
36
Secondary Stability Healing Curve
Secondary stability is a result of the process of bone
deposition and remodeling that occurs following implant
placement.
It pertains to the healing process that re-
places damaged bone surrounding the implant and deposits of
new bone, leading to an increase of the secondary stability
curve.118
This process of osseointegration has been de-
scribed in the dental implant literature.
Schwartz and Boyan described the histological events
of osseointegration.124
Initially, serum proteins attach to
the implant surface immediately after placement.
Then dur-
ing the first three days of healing, mesenchymal cells are
attracted, attach and proliferate.
produces osteoid.
By day six these cells
The calcification of this osteoid is
complete by the second week and remodeling of this new bony
material commences by week three.
Berglundh et al. described similar events.125
Within
two hours of implant placement, erythrocytes, neutrophils,
and macrophages coalesce in a fibrin network.
and mesenchymal cells appear by day four.
Osteoclasts
Woven bone is
produced by day seven and newly formed bone connecting the
woven to the parent bone can be seen by the second week.
There is marked formation of woven and lamellar bone by
week four and extensive remodeling occurs from week 8-12.
37
Other studies have also demonstrated the increase in
osseointegration related to secondary stability.
Melson
and Lang showed an increase in bone-to-implant contact and
bone density for MSIs over the first six months after
placement.126
In agreement with this, Moringa et al. evalu-
ated bone formation around titanium coated plastic implants
that were similar in size to MSIs.127
The implants used in
their study were 1.6 mm in diameter and 7 mm in length.
They evaluated the quantity and quality of the bone over
time using light microscopy, transmission electron microscopy and micro computed tomography.127 They demonstrated
bone formation begins a small distance from the implant and
then by 28 days nearly covers the implant.
They also con-
firmed that, over time, the density of the bone surrounding
the implant increases.
Using resonance frequency analy-
sis, others have also shown an increase in the stiffness in
the bone surrounding an implant as a function of
time.120,121,128
Boronat López et al. described an increase of
stability for implants that began to take place the fourth
week following dental implant placement.79
al.
Stadlinger et
reported an increase in implant stability from one
month to two months post placement.129
They noted in their
study that implant stability values were lower after two
months than at the time of placement.
38
Taking measurements
every thirty days, Balshi et al. reported a statistically
significant increase in stability from 30 to 60 days but no
difference from 60 to 90 days.122
Barewal et al. also re-
ported an increase in implant stability from week three to
week six but then showed no change in implant stability
from week six to week ten.123
It is important to note that while these studies appear to support the idea of the healing curve for secondary
stability, they are ultimately evaluating the total stability and indirectly infer the shape and magnitude of the
secondary stability curve.
With an understanding of the healing process for implants and its relation to stability, it is now important
to understand how stability is measured clinically.
Measures of Stability
The stability of a miniscrew implant is determined by
its ability to resist loading and is a function of the implant’s contact with surrounding bone.
The osseointegra-
tion of the implant changes overtime.
Its quantification
provides important information about healing around the implant.
There are many methods that have been used to
evaluate stability.130
These methods can be categorized as
39
invasive and non-invasive based on their interference with
the osseointegration process.
Invasive Methods
Methods to evaluate implant stability are considered
invasive if they disrupt the processes of osseointegration.
Several methods of measurement can be included in this
grouping.
Insertion torque is probably the most common measure
of an implant’s primary stability.
This technique was de-
scribed by Hughes and Jordan as a method to estimate initial stability of surgical screws.39
Insertion torque meas-
ures the magnitude of rotational force required to insert a
screw into bone and is reported in Newton/cm.39,40
As a
screw is inserted, the bone compresses around the implant.
This compression leads to friction between the implant and
bone and is measured as insertion torque.
This torsional
force is low as the screw is first placed into the cortex.
It increases until the entire cortical layer is engaged.
The maximum value is attained when the head of the screw
makes contact with the cortical plate.39
If the screw is
inserted past this point then stripping of the surrounding
bone occurs and the holding power of the implant is lim-
40
ited.42
Insertion torque is a measure of the bone/tissue
interface and has been shown to correlate with bone density
and implant stabiliy.37,131,132
A minimal amount of insertion
torque is necessary for primary stabiliy.37,133
Chaddad et
al. reported this minimal value to be about 15 N/cm for human subjects.107
This, however, may be due to the gross
method of measurement of insertion torque used in the
study.
In contrast Chen and co-workers reported that MSIs
with insertion torques of 3.5 to 5.6 N/cm in the maxilla of
dogs and 7.4 to 8.7 N/cm in the mandible experienced high
success rates.64
It appears that there is also a maximum
insertion torque that corresponds to increased implant
failure.1
Motoyoshi et al. recommended a range of insertion
torques from 5 to 10 N/cm that correspond with high levels
of success for MSIs with a diameter of 1.6 mm.1
Torque lev-
els outside of this range may lead to implant failure or
breakage.
Insertion torque has its limitations as a measure of
primary stability.
It is only useful for measuring stabil-
ity at the time of placement.
Longitudinal data cannot be
collected and changes in bone surrounding the implant cannot be evaluated with this method.
Another invasive method to evaluate primary stability
is the use of cutting torque resistance.
41
This technique
measures the energy needed to remove bone prior to implant
placement.134
Friberg et al. showed a positive correlation
between cutting torque resistance and bone density, which
is one of the factors that determine stability.131
The is-
sue with this method of measurement is that it is only useful to estimate the implant stability prior to placement.
Consequently, repeated measures cannot be made.
It is also
only used for dental implants where the larger size of the
implant necessitates the removal of bone prior to placement.
Bone removal prior to placement for MSIs is often
not needed, due to their small size, and sometimes not even
desirable.
This limits the usefulness of this measure for
MSIs.
The gold standard to evaluate secondary implant stability is histomorphometric measurement.
This method exam-
ines the bone-implant interface at a microscopic level to
determine stability.
Bone-to-implant contact, usually re-
ported as a percentage of the total implant surface area,
is the common way that implants are evaluated histologically.125,135,136
An increase in the percentage of bone-to-
implant contact indicates a corresponding increase in stability.
Melson and Lang showed an increase in bone-to-
implant contact six months after placement of miniscrew implants.126
Other studies have also evaluated the dynamics
42
of bone-to-implant contact around orthodontic
miniscrews.97,137-139 Another way to evaluate stability histologically is to count the number of osteocytes in the
bone surrounding the screw. The problem with using histologic evaluation to assess implant stability is that it
cannot be used clinically.
Removal of the implant is re-
quired along with a section of the surrounding bone.
This
is not possible on human subjects.
Another method used to measure secondary stability is
removal torque.
Removal torque measures the critical
threshold when bone-to-implant contact is broken.
indirect method to evaluate secondary stability.
It is an
Johansson
and co-workers reported on this method when they showed
that longer healing times could lead to greater bone-toimplant contact and higher removal torque values.134,135
This relationship was also demonstrated by Kim et al. who
used surface treated implants to show that the longer the
implant remained in place, the higher the removal torque
value.140
Using 1.2 mm diameter implants and pilot holes of
1.0 mm and 1.2 mm, Okazaki et al. showed that removal
torque was independent of pilot hole size over a 12-week
period.141
There are also limitations to this method of
measurement.
It is destructive of the bone-to-implant in-
43
terface and thus, cannot be used for longitudinal measures.
It has a limited role in a clinical setting.
The final invasive method for evaluating secondary
stability is the pullout test.
A pullout test measures the
force required to pull an implant out of bone.
applied parallel to the long axis of the screw.
A force is
This force
is increased until the screw looses its holding power and
comes out of the bone.
This common test has been used in
orthopedics,142,143 otolaryngology,144 dental implants145 and
orthodontics.47
Using MSIs Huja et al. showed that pullout
forces are different for different areas of the mouth, indicating that bone quality is not homogeneous.47
Salmória
and co-workers concluded from their work that pullout is a
more efficient measure of secondary stability than insertion torque because insertion torque could not be used to
predict MSI success.44
without limitations.
This measurement technique is not
It is destructive of the implant tis-
sue interface and thus, cannot give longitudinal measurements.
It cannot be used clinically and is limited to lab
experiments.
While all these invasive methods to measure implant
stability yield valuable information, they are all destructive and consequently, for a given implant, can provide one
44
measurement.
This limits their ability to yield informa-
tion about the healing and stability of a single implant.
Non-Invasive Methods
Non-invasive methods to measure implant stability differ from invasive methods by virtue of the fact that the
use of these measurements does not disturb the bone/implant
interface.
Consequently, they can be used to study the
changes in stability of individual implants over time.
Radiographic x-rays were the first non-invasive technique used to evaluate implant stability.
Hermann et al.
described how the technique of taking successive bitewing
radiographs could be used to evaluate the height of crestal
bone around dental implants.146
Goodson and co-workers re-
ported that radiographs could be used to determine changes
in bone density surrounding dental implants.147
However,
they found that the technique can only be used to detect
changes if the decrease in mineral density is 40% or
greater.147
These radiographic techniques may not be useful
to evaluate MSIs.
Miniscrews are rarely oriented in the
same direction as dental implants, so bitewings cannot be
used to evaluate bone level.
To evaluate changes over
45
time, radiographs must be standardized or they can become
distorted and are not useful.
Another method to evaluate implant stability in a noninvasive manner is the use of finite element analysis models.
Finite element analysis utilizes computer-generated
simulations that are employed to evaluate properties of
bone and its stresses.
It is a theoretical analysis based
on the properties of the material being studied, including
Young’s modulus of elasticity, Poisson ratio, bone density
and PDL properties.
Each of these variables is estimated
by the investigator and used by the computer to determine
the behavior of the material in question.
Consequently, if
the value of these variables is not correct the model may
yield an incorrect or misleading result.
Finite element
analysis has been used to evaluate stresses and strains in
bone surrounding implants in different situations.
For ex-
ample, by altering the characteristics of bone surrounding
the implant, finite element modeling can assess stresses
and strains that would theoretically be present.148,149
method has been used to study MSIs.
This
Dalstra et al. studied
the effect of cortical bone density on stress distribution
around MSIs.31
They showed that during tipping, stresses
are concentrated in the cortical region of the bone and
that implant length and diameter are important in determin46
ing stresses felt by the bone.
Miyajima and colleagues
studied the effect of MSI size and direction of applied
force on stress distribution.30
They found that large and
small screws had similar stress/strain patterns but stress
and strain were higher for the smaller screw when the same
magnitude of force was applied.30
They also discovered that
a more horizontal force produced greater stress in the
bone, while vertical force application distributed the
stress over a larger area.30
As with the other methods, fi-
nite element analysis has some limitations.
It is purely a
theoretical and static evaluation and thus, may be limited
in its clinical usefulness.
It also often assumes a homo-
geneous bone quality surrounding the implant, which likely
is not the case in biological subjects.
Its mathematical
modeling is based on several assumptions.
Consequently, if
those assumptions are incorrect, the model will be wrong
and potentially misleading.
Percussion testing is a method of determining implant
stability that is based on the sound the implant makes when
percussed clinically.
A solid object, usually a mirror
handle, is used to strike the implant and the sound elicited is subjectively evaluated by the practitioner to get a
feel for the stability of the implant.
This method is sub-
jective and nearly impossible to standardize and so has
47
little usefulness as a reliable measure of implant stability.
The impact hammer method of evaluating implant stability is simply an improved percussion test.
It attempts to
limit the subjectivity of the parameters of the percussion
test by using microphones, accelerometers and computer
processing of the detected signal.
This method operates on
a principle similar to a tuning fork.
mer strikes an object (an implant).
at its natural frequency.
quency.
A small impact hamThat object vibrates
A microphone detects the fre-
This information is converted into useful informa-
tion, such as, stability measurements.
The Dental Checker,
an impact hammer method of stability evaluation, developed
by Aoki and Hirakawa, was used by
Elias et al. to detect
tooth mobility by converting tooth rigidity into an acoustic signal.150
The Periotest was developed as a more elabo-
rate version of the Dental Checker.
It uses an electromag-
netically controlled tapping rod with an accelerometer to
produce and evaluate implant stability.
It has been shown
to be a reliable method of determining implant stability.151,152
However, several reports indicate that, while ef-
fective for use with natural teeth, it may lack sensitivity
to adequately determine changes in implant stability.152,153
Natural teeth have a large range of mobility, with Perio48
test readings ranging from -8 to +50.151
much more narrow range (-5 to +5).151
Implants have a
Clinically, osseoin-
tegrated implants have an even smaller range (-4 to
+2).154,155
Consequently, the sensitivity of the Periotest
for implants has been called into question.
This technique
has been shown to be very sensitive to position and orientation.156
Meredith et al. have shown that Periotest meas-
urements can vary by 1.5 units for every millimeter from
the marginal bone the striking point changes.153
However,
Lachmann and co-workers showed that, when used appropriately, this method could identify peri-implant bone loss in
millimeter increments.157
The Periotest has been used to
evaluate the stability of miniscrews.
Maria and colleagues
used the Periotest to show that neither length nor diameter
influences primary stability.158
The limitations of the Pe-
riotest, lack of sensitivity and susceptibility to operator
error, have been criticized in work performed by Salvi and
Lang.159
Another non-invasive method to evaluate implant stability using vibration is the pulsed oscillation technique.
It is a system introduced by Koneko et al. that uses an
electric pulse generator connected to a piezoelectric element that causes a needle to vibrate.160
when the needle
touches the implant it causes the implant to vibrate.
49
A
second needle attached to a piezoelectric element records
resonance vibrations generated from the bone/implant interface.
This converts the bone/implant vibration into an
electric signal that is then evaluated by an oscilloscope.
The sensitivity of this device is dependent on the orientation of the needle, as well as its load and position.
This
method has been shown to have a low sensitivity for the assessment of implant rigidity.161
Resonance Frequency Analysis
Resonance frequency analysis is a method used to determine implant stability based on vibrations of the implant within bone.
According to resonance frequency the-
ory, any object has a tendency to oscillate at larger amplitudes for certain frequencies.
Resonance frequency
analysis uses this concept to excite a dental implant or
MSI by some mechanical means and then measures the oscillation pattern of the implant/bone complex in order to determine the stability of the implant in bone.
For any physi-
cal system there can be multiple modes of vibration.
For a
dental or orthodontic implant this means that it can vibrate in three different ways.
These include rotational,
horizontal and vertical vibration (Figure 2.2).
50
Figure 2.2:Implant modes of vibration
The actual overall vibration is always a combination
or mixture of the three modes.
However, they may not all
be excited to the same degree.
For example, if a force is
applied in a direction perpendicular to the long axis of
the implant, then the horizontal mode of vibration will be
the predominant type.
In this way the variability of the
system can be minimized.
Minimizing two of the modes of
vibration by controlling the direction of force application, allowing the measurement of the third predominant
type, can yield valuable information about the stability of
the implant (Figure 2.3).
51
Figure 2.3: Minimizing implant modes of vibration
For the measurement of implant stability in bone, a
horizontal force that is perpendicular to the long axis of
the implant is used.
This direction is employed because it
is the easiest and most predictable direction to apply a
force to an implant clinically.
Consequently, it is also
the most common direction that a MSI is loaded for orthodontic use.
By minimizing implant vibration in the rota-
tional and vertical direction, the system of implant and
bone can be treated as a cantilever beam.
The following
formula is used to determine the resonance frequency of the
implant in the horizontal direction.162
52
Where Rf is the resonance frequency, E=Young’s Modulus
of elasticity, I=Moment of inertia of the beam (determined
by the shape of the beam),163 l=length of the beam and
m=mass of the beam.
Each of these variables, in any given
system, is a constant defined by the implant and the surrounding bone except Young’s modulus.
It is this variable
that allow the use of resonance frequency to evaluate the
stability of the implant.
Theoretically as the modulus of
elasticity of the bone, i.e. stiffness, increases, the
resonance frequency of the implant increases.
The two
share a direct relationship.
Resonance frequency devices utilize the property that
the vibration of any system, in this case implants, creates
sound waves.
Measurements of resonance frequency are actu-
ally measurements of sound waves that are generated by the
vibrating implant.164
Because there are so many variables
that determine the resonance frequency of a system, single
measurements yield limited clinical information about osseointegration.
In order to evaluate osseointegration, se-
quential measurements are required.165
53
Resonance frequency has been used to evaluate various
properties of the bone/implant interface.
Huang et al.
used a device called the Iplomates to evaluate the relationship between the height of and force of clamping of an
implant and its resonance frequency.166
They found that
resonance frequency levels correlated with the clamping
level and the magnitude of the clamping force.
As clamping
level increased (more implant out of the device), resonance
frequency decreased.
As the force used to clamp the im-
plant increased, resonance frequency increased.
In another
study, Huang and co-workers used finite element analysis to
evaluate the effect of marginal bone density, type and
level on the resonance frequency of an implant.167
They
showed that resonance frequency is determined, at least
theoretically, by each of these factors.
Others have also used the Iplomates device to evaluate
implants.
Chang et al. evaluated resonance frequency of
dental implants.
They found that the technique could yield
information about bone-implant union during the healing
process.168
This two-part study first evaluated the reso-
nance frequencies taken over 12 weeks after implant placement and then compared them to in vitro values obtained in
the lab by increasing clamping torque of the same type of
implant.
They concluded that resonance frequency might be
54
helpful in determining the healing status of an implant.
The Iplomates devices is not without its disadvantages.
The major disadvantage of the technique is when an implant
is excited by tapping with a hammer, which is the case with
the Iplomates device, the implant oscillates in all three
modes of vibration.
The magnitude of each mode is deter-
mined by the nature of the impact, i.e. direction and point
of impact.
It is important to restrict the oscillation to
the first mode, which is the mode that is perpendicular to
the long axis of the implant because this will yield the
most applicable clinical results.169
The Osstell Mentor device has also been used to evaluate stability in dental implants.164,170
Instead of using
hammer impaction to excite the implant, a sinusoidal electromagnetic signal is used.
This is the traditional method
of measuring natural frequencies and their corresponding
dampening factors.169
The Osstell device uses a transducer,
located in a clinical handpiece, that is attached to a computer.
This transducer produces the electromagnetic sig-
nal that causes the implant to vibrate.
This vibration is
produced when a metal peg with a small magnet attached to
its top is screwed into the implant and excited by the
electromagnetic signal.
The sinusoidal signal is produced
in a range from 1 kHz to 15 kHz frequencies. The vibration
55
that produces the largest amplitude is the implants natural
resonance frequency.
This resonance vibration of the im-
plant is sensed by a second transducer, located in the same
hand piece.
Its frequency is reported and used to deter-
mine the stability of the implant.
Due to the method of
implant vibration, the Osstell device actually measures the
frequency of the vibration in hertz.
The nature of the
electromagnetic signal used to elicit implant vibration may
actually cause vibration in two horizontal directions simultaneously, which are approximately perpendicular to each
other.
Consequently, for each measurement event two reso-
nance frequencies may be detected and recorded, each yielding information about the nature of the bone-to-implant interface in their respective direction.
The third genera-
tion Osstell Mentor device reports all measurements in
units termed implant stability quotient (ISQ) and not
hertz, which is the units of actual measurement. The ISQ is
based on the underlying resonance frequency and ranges from
1 (lowest stability) to 100 (highest stability).
The Os-
stell device is pre-calibrated so that the ISQ values are
valid for only a single type of implant.171
Consequently,
if the transducer has not been pre-calibrated for a particular type of implant, then comparisons to other types of
implants cannot be made.171
One clinically important factor
56
that can influence the resonance frequency measurements
taken by the Osstell device is the transducers orientation
in relation to the implant.
The manufacturer suggests that
the orientation should be perpendicular to the long axis of
the implant (Figure 2.4).
Because the orientation can af-
fect the measurement of resonance frequency, it is important to standardize the orientation when taking sequential
measurements.164,171,172
When the orientation is standardized,
reliability of the measurements has been shown to be excellent in a clinical environment.173
The Osstell device has
been shown to be more precise than the Periotest device for
evaluating implant stability in the clinical (intraclass
coefficients for implant stability were 0.99 for the Osstell Mentor and 0.88 for the Periotest)173 and laboratory
(intraclass coefficients for implant stability were 0.99
for the Osstell Mentor and 0.86 for the Periotest)174 environment.
This is due to the sensitivity of the Periotest
to operator error during use.
57
Figure 2.4: Orientation of Osstell Mentor transducer during
measurements. Adapted from Osstell Mentor picture library.175
Several studies illustrate the effect that the bone
surrounding the implant has on the resonance frequency.
Ito et al. suggested in an in vitro study that the marginal
or cortical bone plays the most important role in implant
stability.176
This is in agreement with work done by Rod-
rigo and co-workers, who reported higher resonance frequencies in specimens where the cortical bone had been preserved than in specimens when it was eliminated.177
Finite
element models developed by Deng et al. also predicted
higher resonance frequencies when more coronal bone was deposited during osseointegration.178
Other studies have shown correlations between resonance frequency analysis and other measures of implant sta58
bility.
Boronat López et al. demonstrated a significant
correlation between resonance frequency and insertion
torque.179
This was in agreement with work by Turkyilmaz
and colleagues.180
They also showed positive correlations
with resonance frequency and bone density.
This is in con-
trast to a study where no correlations were found between
resonance frequency, bone density and insertion torque.181
Relationships between histomorphometric measurements and
resonance frequency have also been examined.
Zhou et al.
were able to show that increased bone-to-implant contact
was correlated to a higher implant stability quotient.182
This was also shown in work performed by Scarano and coworkers.183
Using different types of implants, Kim et al.
found ISQ and bone-to-implant contact increased during the
first eight weeks after placement for all groups tested.184
Researchers have also studied the use of resonance frequency to predict implant failure and other aspects of implant stability.
Scarano et al. demonstrated that an ISQ
below 36 could be used to identify failed dental implants.185
Others have used resonance frequency to deter-
mine the best time to load implants.186
Some have used
resonance frequency to identify good sites for implant
placement based on initially stability.187
59
A number of studies have brought into question the
usefulness of resonance frequency in determining implant
stability due to the failure to demonstrate correlations
between resonance frequency, cutting torque, marginal bone
levels, timing of loading, bone-to-implant contact and
changes of stability over time.121,172,181,188-190
However, sub-
stantial literature exists that supports this method for
evaluating dental implant stability but only limited literature demonstrating its efficacy in measuring miniscrew
implant stability is available.
Using a third generation Osstell Mentor device,
Katsavrias, showed that the device was reliable when measuring miniscrew implants with a length of 11 mm and an external diameter of 1.6 mm placed in synthetic bone.130
He
showed intraclass correlation for multiple measurements
ranging from 0.953 to 0.992, and single measure correlation
ranging from 0.870 to 0.977.
He also demonstrated signifi-
cant differences between different densities of synthetic
bone.
In an in vitro study, Veltri et al. showed that
resonance frequency could be used for different types of
miniscrew implants.191
They concluded that it was an ac-
ceptable technique to assess primary stability of MSIs in
bone and that the three different MSI systems in the study
showed similar results.
60
Summary
The use of miniscrews for skeletal anchorage in orthodontics has the potential to improve the treatment of certain types of malocclusions.
However, MSI failures will
greatly influence the efficiency and efficacy of treatment.
Having a better understanding of the healing process that
occurs around MSIs will provide valuable information that
could enhance the predictability of their use.
The non-
invasive measurement technique, resonance frequency, holds
great promise for the clinical evaluation of MSI stability.
It may be used to evaluate the transition from primary to
secondary stability producing a better understanding of
time periods that are high risk for screw failure.
The
technique also provides a method to determine the effect
that a modification of the placement protocol of MSIs might
have on the transition from primary to secondary stability.
On that basis, this project will evaluate the usefulness of
resonance frequency to evaluate miniscrews longitudinally
in vivo and the effect of using pilot holes and soft tissue
on the healing process.
61
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80
CHAPTER 3: JOURNAL ARTICLE
Abstract
Purpose: This study evaluated changes in miniscrew implant
(MSI) stability over eight weeks using resonance frequency
analysis.
The study was designed to evaluate the impact of
pilot holes and placement sites on changes in stability.
Method: Implant Stability Quotient (ISQ) values were measured using the Osstell® Mentor device for 22 MSIs, 1.6 mm
in diameter and 9 mm in length, placed in the maxilla of
adult beagle dogs (20 months old).
Measurements were taken
weekly, starting at the time of placement and ending at
eight weeks.
Using a split mouth design, 1.1 mm wide pilot
holes were randomly selected and drilled to a depth of 3 mm
for half of the MSIs prior to placement.
MSI placement was
also divided between keratinized and non-keratinized tissue.
Results:
Nine of the 22 MSIs failed; all of the
failures were related to having been placed in nonkeratinized tissue.
MSIs that failed showed significantly
(p<0.05) higher decreases in stability during the first
three weeks than the MSIs that remained stable.
MSIs that
remained stable throughout the study also showed decreases
in stability during the first three weeks and increases in
stability between the third and fifth week (p<0.05). Pilot
81
holes had little or no effect (p>0.05) on MSI stability.
Conclusion:
Stability of MSIs decreases from week one to
week three and increases from week three to week five. Pilot holes do not affect the stability of MSIs.
Placement
of MSIs into non-keratinized tissue negatively impacts
their stability and increases the likelihood of failures.
Introduction
One of the challenges facing orthodontics is the control of unwanted tooth movement.
Minimizing unwanted tooth
movement has been an important objective of orthodontists
since the beginning of the specialty.
Recently, miniscrew
implants (MSIs) have become a popular means of skeletal anchorage to selectively control tooth movement.
However,
the increased use of MSIs has revealed a substantial number
of failures.
Reported success rates range from less than
50% to over 95%.1-5
The unpredictability of MSIs limits
their usefulness as a treatment modality.
There are a number of factors that may cause MSI failure including, mobility,6-8 excessive heating of the bone
during placement,9-11 placement in keratinized vs. nonkeratinized soft tissue,1,2 host factors such as uncontrolled diabetes,12 excessive loading13,14 and poor oral hy-
82
giene.1
The primary cause of MSI failures is the loss of
bone-to-implant contact or stability.
Stability includes
primary stability, which is a mechanical interlocking between the implant and surrounding bone directly after
placement, and secondary stability, which pertains to remodeling and healing of bone surrounding the implant.15
The
overall or total stability of a MSI represents a combination of primary and secondary stability (Figure 3.1).
Figure 3.1: Primary, secondary and total stability curves
over eight weeks. Adapted from Raghavendra et al.15
Understanding changes in MSI stability over time could
lead to the development of clinical management techniques
that promote faster healing improving primary or secondary
83
stability (Figure 3.2). Our understanding of MSI stability
is limited by our lack of ability to measure stability in
vivo.
Figure 3.2: The effect on total stability of a shift in
secondary stability. Adapted from Raghavendra et al.15
The stability of MSIs has been previously measured using insertion torque,3,16 removal torque,17,18 histological
studies19,20 and pullout tests.21
Because these measures re-
quire the destruction of the bone-to-implant interface,
they are not useful for clinical application.
The most
promising non-invasive method to measure implant stability
is resonance frequency, which has been successfully used to
study the stability over time of dental implants used in
clinical situations.22-24
84
Resonance frequency analysis determines implant stability by measuring vibrations of the implant within
bone.25,26
The stiffer the bone surrounding the implant, the
higher the frequency of the measured vibration.
Because
this measurement is dependent on the quality of the bone
surrounding the implant, it is used as a proxy for the stability of the implant.
Since this method does not disturb
implant/bone interface, it can be used to evaluate changes
in stability of an implant over time.
Changes in stability during the first few weeks after
placement have been evaluated in the dental implant literature using resonance frequencies. Based on 122 dental implants placed in humans, Ersanli et al. showed decreases in
stability during the first three postsurgical weeks and increases in stability between three and six weeks and six
weeks and six months.24
Balshi et al. used monthly evalua-
tions of 276 dental implants to show a decrease in stability during the first month and increases during the second
and third months.22
Barewal et al., who evaluated changes
in dental implant stability according to bone type, found a
decrease in stability during the first three weeks and an
increase from week three to week six.27
While the pattern
of an initial decrease in stability followed by an increase
85
has been established for dental implants, the pattern of
stability for MSIs remains unknown.
The purpose of this study was to determine the longitudinal changes in MSI stability that occur over the first
eight weeks after placement.
Secondary aims of the study
were to evaluate the effect of pilot holes and placement
site (keratinized vs. non-keratinized tissue) on early MSI
stability.
Materials and Methods
Animals
The sample included two healthy male, 20 month-old
beagle dogs (approximate weight 15 kg). Starting 1 week
prior to MSI placement, the dogs were maintained on a soft
diet (Canidae® Lamb and Rice canned food, Canidea Corporation, San Luis Obispo, CA; 5006 Canine Diet [Hard kibble
mixed with warm water], PMI Nutrition International, LLC.,
Brentwood, MO).
The animals were housed separately in the
Comparative Medicine Department of Saint Louis University
Medical School.
All procedures were approved by the Saint
Louis University Animal Care Committee (Authorization
#2010).
86
MSI Placement
The dogs were administered 25 mg/kg of Enrofloxacin
(Baytril, Bayer Health Care, LLC., Animal Health Division,
Shawnee Mission, KS) intravenously during implant placement
as a prophylactic measure and intramuscularly for two days
post-surgery.
Carprofen (Rimadyl, Pfizer Animal Health,
Exton, PA) was administered at the rate of 4mg/kg subcutaneously as an analgesic.
Induction of general anesthesia
was accomplished by the intravenous administration of Propofol (Propofol, Abbot Animal Health, North Chicago, IL)
7mg/ml.
Maintenance was achieved by 2-3% isoflurane (Aer-
rane, Baxter Healthcare Corporation, Deerfield, IL).
An IV
drip of 0.9% sodium chloride (Hospira, Inc., Lake Forest,
IL) administered at a rate of 12ml/kg/hr, was given to
maintain hydration.
Postoperatively, the animals were
given Acepromazine Maleate (VEDCO, Inc., St. Joseph, Mo)
0.2ml IV to calm post anesthetic excitement.
Before MSI placement, lateral head films were taken to
determine the best sites for MSI placement.
Sites were
chosen based on the availability of comparable sites in the
same location on the opposite side of the maxilla.
For the
first dog, 10 insertion sites were identified; 12 insertion
sites were identified for the second dog.
One screw of
each matched right and left side pair was randomly selected
87
to receive a pilot hole prior to MSI placement.
All MSIs
were placed in the maxillary premolar region apical to the
furcations (Figure 3.3).
Dog A
Right
Left
= No pilot hole
= Pilot hole
Dog B
= non-Keratinized
= Keratinized
Right
Left
Figure 3.3: MSIs in Dog A and Dog B by location with pilot
hole and soft tissue information.
Ten MSIs (five per side) were placed apical to the root
tips in non-keratinized tissue; the MSIs occlusal to the
root tips were placed in keratinized gingiva.
Placement
sites were swabbed with 0.12% chlorhexidine gluconate (Acclean®, Henry Schein, Inc., Melville, NY).
To aid in visu-
alizing the placement site and prevent soft tissue compli-
88
cations, a 3.0 mm tissue punch (Premier Medical Products,
Plymouth Meeting, PA) was performed.
MSIs that had been
randomly chosen to receive a pilot hole were drilled at
1000 r.p.m., using a Brasseler lab hand piece (uP501 and
UG33, Brassler USA®, Savannah, GA) (Figure 3.4) and constant irrigation with sterile saline solution (Braun Medical Inc., Irvine, CA).
Pilot holes were drilled using a
1.1 mm diameter drill (Sendax Spiral Drill, Imtec Corporation, Ardmore, OK) with an endo stop placed at a distance
of 3 mm from the tip, held in place with flowable composite
(Henry Schein®, Melville, NY) (Figure 3.5).
Figure 3.4: Brassler handpiece.
Figure 3.5: Pilot hole drill.
MSIs were placed using a hand driver (Ancoragem Ortodontica,
Neodent®, Curitiba, Brazil) until the head of the screw was
within 0.5 mm of the cortical bone.
After placement, lat-
eral head films were taken to verify that the roots had not
been contacted.
89
Miniscrew Implants
MSIs used in this study were Ancoragem Ortodontica
screws manufactured by Neodent® (Curitiba, Brazil).
These
cylindrical shaped screws were 9 mm long, with an external
diameter of 1.6 mm, an internal diameter of 1.1 mm, and a
pitch of 0.7 mm.
The head of the screw had been modified
to include a 1.1 mm internal thread that accepts the Osstell® Mentor Smartpeg type A3 (Figure 3.6, 3.7 and 3.8).
Figure 3.6: MSI side
and top view.
Figure 3.7: Smartpeg. Figure 3.8:
MSI and Smartpeg connected.
Resonance Frequency Measurements
After MSI placement, the Osstell® Smartpeg type A3 (Osstell®, Göteborg, Sweden) was placed into the head of the
screw and tightened with finger pressure according to the
manufacturer’s instructions.
Using the Osstell® Mentor
transducer, two sets of measurements were taken for each
90
screw.
The first set of measurements was made parallel to
the maxillary occlusal plane.
The second set of measurements
was made perpendicular to the maxillary occlusal plane (Figure 3.9).
All resonance frequencies were taken perpendicular
to the long axis of the screw and attached Smartpeg (Figure
3.10), and were reported as implant stability quotients
(ISQs).
Figure 3.9: Orientation of Osstell® transducer in relation to
the occlusal plane.
Figure 3.10: Orientation of Osstell® transducer in relation
to the Smartpeg and MSI. Adapted from Osstell® Mentor picture library.28
91
Using the same methodology, resonance frequency measurements were taken at weekly intervals for eight weeks after
placement of the implants.
The previously described proce-
dures were used to place the dogs under general anesthesia
for each of the measurements occasions.
Statistical Analysis
Success for this study was defined as lack of MSI
pullout. Due to the number of failures that occurred after
three weeks, two sets data were evaluated, including 1) all
the MSIs available during the first three weeks, and 2)
only those MSIs that remained intact throughout the eight
weeks of the study.
Wilcoxon signed-rank tests were used to evaluate differences in ISQ between screws that 1) received a pilot
hole and those that did not, 2) were placed in keratinized
gingiva and those placed in non-keratinized gingival, and
3) failed and those that did not.
Wilcoxon signed-rank
tests were also used to evaluate changes in average ISQ between each successive time point from surgery to eight
weeks post insertion.
All Statistical testing was calcu-
lated using SPSS Version 17.0 (SPSS Incorporated, Chicago,
IL).
92
Results
Failures
A large number of failures occurred during the study.
Of the 22 MSIs that were placed, nine (41%) failed at some
point during the study (Figure 3.11).
MSIs failed at week
three (four failures), week five (two failures), week six
(two failures), and week eight (one failure).
All of the
MSIs that failed had been placed into non-keratinized tissue.
ISQ readings could not be taken on one MSI after ini-
tial measurement due to a mechanical failure of the internal threading of the head.
All of the failures occurred
while attempting to unscrew the Smartpeg from the MSI; the
MSI unscrewed from the bone before the Smartpeg loosened.
ISQ values and descriptive statistics for all screws
present are found in Table 3.1.
93
Figure 3.11: Timing of MSI
posterior location, tissue
†This screw had mechanical
week one.
*This screw failed in week
Table 3.1.
Number
Mean
Std. Dev.
Min-Max
Number
Mean
Std. Dev.
Min-Max
failures by dog, anteriortype and pilot hole presence.
failure of internal threading in
eight.
Weekly ISQ values from placement to eight weeks.
T0
22
32.3
7.9
18-45
T1
21
30.2
8.0
17-42
All MSIs until failure
T2
T3
T4
T5
21
21
17
17
26.7
25.0
26.4
26.2
7.7
8.3
5.4
6.2
12-39
9-42 14-36 15-34
T6
15
26.3
4.3
16-32
T7
13
27.0
4.6
16-32
T8
13
27.7
4.7
16-33
T0
12
30.9
9.4
18-45
MSIs surviving the entire study
T1
T2
T3
T4
T5
12
12
12
12
12
30.3
27.8
26.0
26.7
27.7
9.0
6.9
5.7
5.3
5.0
17-42 15-38 14-32 14-33 15-33
T6
12
26.7
4.2
16-32
T7
12
26.8
4.8
16-32
T8
12
27.4
4.8
16-33
94
Comparison of ISQ Changes of all MSI Over the First Three
Weeks of the Study
Descriptive statistics and statistical comparisons for
the first three weeks and adjusted to baseline are found in
Table 3.2 and 3.3.
Table 3.2: Descriptive statistics and statistical comparisons
(Wilcoxon signed-rank test) of ISQ values between MSIs that
Failed vs. Survived, MSIs that receieved a Pilot hole vs. no
Pilot hole and MSIs placed in Keratinized vs. non-Keratinized
tissue from surgery (T0) to three weeks (T3).
Group
T0
T1
T2
T3
Failed
Mean
34.2
30.3
25.6
24
Std. Dev.
5.7
7.0
8.7
10.8
Survived
Mean
31.2
30.4
28.1
26.1
Std. Dev.
9.4
9.0
7.0
5.7
p-value
0.100
0.455
0.944
0.701
Pilot hole
Mean
Std. Dev.
No Pilot hole
Mean
Std. Dev.
p-value
32.6
7.9
30.2
8.0
27.9
8.2
25.7
9.8
32.5
8.3
0.674
30.6
8.3
0.373
26.0
7.5
0.443
24.6
6.8
0.160
27.2
6.6
25.6
5.6
26.7
9.0
0.807
24.7
10.5
0.675
Keratinized
Mean
30.2
29.8
Std. Dev.
9.3
9.2
non Keratinized
Mean
34.8
30.9
Std. Dev.
5.8
6.9
p-value
0.100
0.455
*significant at the !=0.05 level
There were no significant differences in ISQs at any
time point during the first three weeks between the MSIs
that eventually failed and those that did not.
There also
were no significant difference during the first three weeks
95
between those that had pilot holes and those that did not,
or between those that were place in keratinized or nonkeratinized tissues (Figure 3.12, 3.13 and 3.14).
Figure 3.12: ISQ over the first three weeks of failed and
survived MSIs. No time points statistically significant at
the p-value 0.05 level.
Figure 3.13: ISQ over the first three weeks for pilot vs.
no pilot hole MSIs. No time points were statistically significant at the p-value 0.05 level.
96
Figure 3.14: ISQ over the first three weeks for keratinized
vs. non-keratinized placement of MSIs. No time points were
statistically significant at the p-value 0.05 level.
Table 3.3. Descriptive statistics and statistical
comparisons (Wilcoxon signed-rank test) of changes
in ISQ from surgery (T0) to three weeks (T3).
Group
T0-T1
T0-T2
T0-T3
Failed
Mean
-3.9
-8.6
-10.2
Std. Dev.
2.6
6.5
9.0
Survived
Mean
-0.8
-3.1
-5.1
Std.Dev.
1.6
3.7
4.6
p-value
0.001*
0.009*
0.032*
Pilot hole
Mean
Std. Dev.
No Pilot hole
Mean
Std.Dev.
p-value
-1.9
2.5
-6.5
5.9
-7.9
6.1
-2.5
2.9
0.084
-4.7
5.8
0.201
-6.9
8.6
0.168
Keratinized
Mean
-0.5
-3.1
Std. Dev.
1.3
3.8
non-Keratinized
Mean
-3.9
-8.1
Std.Dev.
2.5
6.4
p-value
0.001*
0.013*
*significant change at the !=0.05 level
97
-4.7
4.7
-10.1
8.6
0.037*
The changes in ISQ values that occurred over the first
three weeks were significantly different between the MSIs
that failed and those that did not.
After the first week,
the MSI that eventually failed showed significantly greater
decreases in ISQ than those that did not.
The differences
between the MSI that failed and those that did not increased significantly, from 0.8 to 5.1, over the first
three weeks.
While the changes were also greater for MSI
with pilot holes than for those without pilot holes, the
differences were not statistically significant.
In con-
trast, MSIs placed in non-keratinized tissue showed significantly greater decreases in ISQ values over the first
three weeks than MSI place in keratinized tissue
(Figure
3.15, 3.16 and 3.17).
Figure 3.15: Change in ISQ over time for MSIs that failed
and survived over the first three weeks. Changes at T1, T2
& T3 were statistically significant.
98
Figure 3.16: Change in ISQ over time for MSIs with pilot
hole and no pilot hole over the first three weeks. No significant changes were found.
Figure 3.17: Change in ISQ over time for MSIs placed in
keratinized and non-keratinized tissue over the first three
weeks. Changes at T1, T2 & T3 were statistically significant.
99
Longitudinal Changes in ISQ of MSIs Maintained Throughout
the Study
While the ISQ values decreased during the first week,
the changes were small and not statistically significant
(Table 3.4).
Statistically significant decreases in ISQ
were found during the second (T1-2) and third weeks (T2-3).
Significant increases occurred during the fourth (T3-4) and
fifth (T4-5) weeks.
The decrease in ISQ values that oc-
curred during the sixth week, as well as the increases that
occurred during the seventh, eighth and ninth weeks were
not statistically significant (Figure 3.18).
ISQ values at
week eight were lower than ISQ values at MSI placement
(T0), but the differences were not statistically significant (p=0.120).
Table 3.4. Longitudinal statistical comparisons of ISQs for MSIs that
survived the entire study.
T0
T1
T2
T3
T4
T5
T6
T7
T8
Mean
30.9
30.3
27.8
26.0
26.7
27.7
26.7
26.8
27.4
Std. Dev.
9.4
9.0
6.9
5.7
5.3
5.0
4.2
4.8
4.8
T0-1
T1-2
T2-3
T3-4
T4-5
T5-6
T6-7
T7-8
p-value
0.120 0.017* 0.006* 0.025* 0.008* 0.059 0.928 0.119
*significant at the !=0.05 level
T0-1=placement to week one, T1-2=week one to week two, etc.
100
Figure 3.18: Mean ISQ values of MSIs that survived the entire study from placement (T0) through week eight (T8).
Significant decreases in ISQ were demonstrated for T1-T2
and T2-T3. Significant increases in ISQ were demonstrated
for T3-T4 and T4-T5.
Discussion
The high number of MSI failures (41%) that occurred
were directly related to their placement in non-keratinized
mucosa.
Of the screws that were placed in non-keratinized
mucosa only one survived the entire duration of the study
(i.e. 90% of them failed).
In contrast, none out of 12
MSIs placed in keratinized tissue failed.
Success rates
previously reported for MSIs range from less than 50% to
over 95%.1-5
However, Most studies report success rates
greater 80%.1,5
Chaddad et al. reported a 100% (11 out of
11 survived) success rate for MSIs placed in keratinized
tissue;29
Cheng et al. also had a high success rate (92%,
101
103 out of 112 survived) for MSIs placed in keratinized
tissue2;
Lim et al. also showed a high success rate of
90.7% (224 out of 247) for MSIs placed in keratinized gingiva.30
Thus, the success rate of MSIs placed in kerati-
nized tissue in the present study is in line with those reported elsewhere, emphasizing the importance of not placing
MSIs in non-keratinized tissues.
It has been previously suggested that the type of soft
tissue into which MSIs are place is a risk factor for failure.1,2
The failures in this study strongly support de-
creases in stability for MSIs placed in non-keratinized
gingival tissue.
The methodology of screwing and unscrew-
ing the Smartpeg device to the head of the implant may have
contributed to the high failure rate.
All of the failures
occurred while attempting to unscrew the Smartpeg, with the
MSI unscrewing from the bone rather than the Smartpeg.
Consequently, screws that would still be considered clinically usable due to lack of mobility, were considered failures, which could artificially inflate the failure rate.
The greater decrease in ISQ experienced by MSIs placed
in non-keratinized tissue indicates a greater loss of bone
surrounding the implant, which is the ultimate cause of MSI
failure.
It has been suggested that a lack of keratinized
tissue around MSIs leads to peri-implant inflammation and
102
eventual failure.2
With inflammation, catabolic processes,
such as the resorption of bone traumatized during MSI insertion are favored, while anabolic processes, such as bone
deposition and mineralization, are inhibited.
This could
lead to less bone surrounding an implant experiencing periimplant inflammation compared to an implant undergoing the
healing process without excessive inflammation.
MSIs that survive maintain a critical level of stability, while the stability of those that fail falls below
threshold levels.
Upon placement, the MSIs in the present
study exhibited adequate primary stability.
It was the
loss of stability that determined whether or not MSIs would
maintain adequate stability or fail.
Our results show that
the MSIs that failed exhibited an accelerated pattern of
stability loss.
During the first three weeks, they exhib-
ited greater losses of stability than the MSIs that survived.
While no previous studies have compared MSI stabil-
ity changes during the first few weeks post placement,
changes in stability have been used to identify dental implants at risk.
Glauser et al. showed a larger decreases
in stability among failed implants compared to those that
survived.31
Friberg et al. and Huwiler et al. showed a
similar pattern for dental implant failures.32,33
Sennerby
and Meredith identify resonance frequency as a method to
103
predict failure of dental implants by evaluating changes in
stability.26
Similarly, changes in stability could be used
clinically to identify MSIs that are at the greatest risk
of failing.
To date, no other method has been shown to be
an adequate predictor of MSI failure.
However, this is an
area that requires further research.
The overall stability of any MSI is due to the combined effects of primary and secondary stability.15
While
the primary and secondary stability curves make intuitive
sense, it is impossible to separate them clinically when
measuring stability.
The best that can be done currently
is to make inferences about the primary and secondary stability at any given point in time based on overall stability and an understanding of the physiological process of
bone healing.
The present study was designed to determine
whether it was possible to identify when decreases in the
primary stability of MSIs occur (i.e. the duration of time
during which the overall stability curve decreases), and
when secondary stability becomes more important than primary stability (i.e. when the overall stability curve
starts to increase).
Based on the results in the present
study, the point of transition from primary to secondary
stability appears to occur at approximately three weeks,
104
which compares closely to the point of transition identified for dental implants.22,23,33,34
(Figure 3.19 and 3.20).
Figure 3.19: Stability of dental implants from placement
(T0) through week eight (T8). Adapted from Huwiler et al.33
Figure 3.20: Mean ISQ values of MSIs that survived the entire study from placement (T0) through week eight (T8) divided by predominant stability type (P=primary or
S=secondary).
105
Primary stability decreases immediately following MSI
placement.
The present study showed significant decreases
in ISQ values during the first three weeks of the study.
This supports the idea that primary stability is highest
immediately after MSI placement, and then decreases over
time, as previously demonstrated for dental implants.
Bo-
ronat et al. reported a decrease in ISQ during the first
four weeks after dental implant placement in the maxilla
and mandible;23
Ersanli et al., who measured stability at
surgery, at three weeks, at six weeks and again at six
months, reported significant decreases in ISQ values
through three weeks;24
Balshi et al., who took monthly
measures of ISQ showed statistically significant decreases
in stability over the first month;22
Barewal et al. re-
ported decreases in stability from placement to week three
in bone that was considered type 2,3 or 4 (according to the
index proposed by Lekholm and Zarb35).27
The decrease in
primary stability of MSIs during the first three weeks can
be explained by the physiological processes occurring
around the implant.
Within two hours of implant placement,
erythrocytes, neutrophils, and macrophages coalesce in a
fibrin network; osteoclasts and mesenchymal cells, which
appear by day four, begin removal of bone damaged during
MSI placement.19
This leads to the decreases in primary
106
stability observed in the present study and holds important
implications for the management of MSIs.
Strategies to re-
duce trauma upon insertion or speed up the transition from
primary to secondary stability should result in higher levels of MSI stability.
Secondary stability, which is associated with healing
and increases in total MSI stability, first becomes evident
three weeks after miniscrew placement.
The results indi-
cate that secondary stability continues to increase through
the fifth week, and then appears to level off.
Boronat
López et al. described an increase of stability for dental
implants beginning the fourth week after placement.23
Stad-
linger et al. reported increases in implant stability between the first and second months after dental implant
placement; their ISQ values were lower after two months
than at the time of placement.36
Barewal et al. also re-
ported an increase in implant stability between week three
and six.
This increase of overall stability demonstrated
between weeks three and six can be explained by the formation of bone surrounding a dental implant, which begins to
occur at approximately three weeks post placement in dogs.34
Resonance frequency could be used to test whether surface
treatments to MSIs, or other factors thought to accelerate
secondary stability, enhance secondary stability.
107
Secondary stability is expected to continue to increase until complete healing around the MSI takes place.
Stability in this study leveled off after week five and did
not change significantly from week five to week eight.
This pattern of healing does not agree with most of the
dental implant literature, which reports increases in stability until much later.23,27,33
This difference can be ex-
plained by circumstances surrounding the dogs.
Rimadyl®, a
non-steroidal anti-inflammatory (NSAID), was administered
by the veterinarian to both dogs during this time period
for pain control.
It has been shown that NSAIDs may in-
hibit bone formation and enhance bone resorption.37
This
would explain the pattern of stability during this time period.
The overall stability of MSIs eight weeks after placement was lower than the primary stability measured at
placement.
Even though the differences were not statisti-
cally significant, the trend indicates a lower stability at
eight weeks.
According to Roberts, there are four stages
of healing (activation, resorption, quiescence, formation)
around dental implants, which take approximately 12 weeks
in dogs.34
Since the present study lasted only eight weeks,
approximately four weeks of bone formation remain, which
108
could account for the apparent lower stability observed at
the eighth week.
Whether or not the MSIs are placed with pilot holes
appeared to have no appreciable effect on ISQ values.
This suggests that, when placing MSIs in bone that is similar in density to the bone used in this study, pilot holes
do not affect the MSI stability.
These findings are at
odds with the existing literature, which shows that pilot
holes decrease insertion torque and pullout strength.38-40 It
is possible that resonance frequency may not be sensitive
enough to detect differences in stability between pilot
holes and no pilot holes for MSIs.
A more plausible expla-
nation relates to the three factors that determine the
resonance frequency of an implant, including its stiffness
within the surrounding bone, the transducer that measures
the vibration, and the length of the implant that extends
out of the bone.26
Since the transducer and the length of
the implant out of bone were held constant throughout the
study, only the stiffness of the bone could explain differences in stability. Because the stiffness of the bone is a
function of its physical composition, it might be expected
to remain the same whether or not a pilot hole is placed.
Since there were no differences in ISQ values after
MSI placement, it does not appear that pilot holes enhance
109
the healing process.
The results do not support the notion
that pilot holes minimize trauma during MSI insertion,
which should facilitate the healing process and improve
stability.
It may simply be that the placement of a pilot
hole causes as much trauma to the bone as does the placement of a MSI with out a pilot hole.
This may not be the
case, however, for bone that is more dense than that used
in the present study. Other methods may have to be used in
order to enhance secondary stability, such as the surface
treatment of the implant.41
Conclusions
• Because the type of tissue has a significant effect on
stability, it may be reasonable to assume that MSIs
placed in non-keratinized tissue may be prone to
higher failure rates.
• MSIs that fail show significantly greater decreases in
ISQ values over time than MSIs that remain stable.
• Resonance frequency can be used to measure the changes
in stability of MSIs over time. Changes in this study
indicated that MSI stability decreases during the
first three weeks after placement, increases between
110
weeks three and five, and shows little or no change
between weeks five and eight.
• Pilot holes in maxillary bone of dogs does not affect
the stability of MSIs.
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VITA AUCTORIS
Derid S. Ure was born on December 19, 1977 in Driggs,
Idaho to Shane and Kaylin Ure.
from 1996 through 1997.
He attended Ricks College
He interrupted his education to
spend two years serving a religious mission for the Church
of Jesus Christ of Latter-day Saints in Alabama from 1997
to 1999.
Afterward, Derid resumed his studies at Ricks
College where he received with an Associates of Arts in
Economics in 2001.
He received his Bachelor of Arts in
Economics from Idaho State University in 2004.
From 2004
to 2007, he attended the University of the Pacific Arthur
A. Dugoni School of Dentistry in San Francisco, CA, where
he received his Doctorate of Dental Surgery in 2007.
It is
anticipated that in January 2010 Derid will Graduate from
Saint Louis University with a Master of Science degree in
Dentistry with an emphasis in Orthodontics and will enter
private practice in Texas.
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