POST-MORTEM SHEAR TESTING OF IMMEDIATELY LOADED

3 MM AND 6 MM MINISCREW IMPLANTS AT SIX WEEKS

POST INSERTION IN THE BEAGLE DOG

Damen M. Caraway, B.S., D.D.S.

An Abstract Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment

of the Requirements for the Degree of Master of Science in Dentistry

2007

Abstract

Introduction: The use of miniscrew implants for

orthodontic anchorage has raised questions concerning their

limitations. Specifically, the maximum shear force that an

immediately loaded miniscrew can withstand has not been

investigated. The specific aim of this study is to

determine the maximum shear resistance of miniscrew

implants. The effect of immediate loading on the maximum

shear resistance of miniscrew implants will be compared

between implants of two different lengths, and with three

different applied force loads. A comparison of shear force

at failure will also be made according to the depth of the

miniscrews in bone. Methods: The sample was derived from

five skeletally mature beagle dogs that had 60 miniscrews

placed at predetermined locations in the palate and buccal

surface of the mandible. Miniscrews were immediately

loaded with either 0 (control), 600, or 900 g. After six

weeks of continuous force application, 45 of the miniscrews

remained in place. The dogs were then sacrificed, and bone

samples from the maxilla and mandible were dissected such

that each contained one orthodontic miniscrew. The bone

specimens were mounted in dental stone for testing purposes

and secured in a universal vice for mechanical testing.

1

Testing was performed by the application of a shear force

in the same direction as the original force until failure

of the implant. The maximum force sustained by the

implants prior to failure was recorded in Newtons (N).

Results: The mean shear force at failure of 6 mm miniscrew

implants was significantly higher (53.0 N ± 8.3 N)

(Mean ± SE) than that of 3 mm implants (31.9 N ± 4.1 N).

No significant difference in force at failure was noted

between implants that were immediately loaded, and those

that served as controls. A significant difference was

determined to be present between the groups formed by

extent of bony purchase. The groups with 2-3 mm and 3 mm+

of bony purchase showed a significantly higher shear force

at failure than the groups with 0-1 mm or 1-2 mm of

insertion depth. Shear force at failure showed a

moderately strong correlation (r=0.57) with the depth of

the miniscrew in bone. Conclusions: Immediate loading of

miniscrews has no significant effect on maximum shear force

at failure. Complete cortical engagement by miniscrews may

result in significantly higher shear resistance.

Miniscrews as short as 3 mm can withstand shear forces well

beyond levels typically used in orthodontics.

2

POST-MORTEM SHEAR TESTING OF IMMEDIATELY LOADED

3 MM AND 6 MM MINISCREW IMPLANTS AT SIX WEEKS

POST INSERTION IN THE BEAGLE DOG

Damen M. Caraway, B.S., D.D.S.

A Thesis Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment

of the Requirements for the Degree of Master of Science in Dentistry

2007

COMMITTEE IN CHARGE OF CANDIDACY: Professor Rolf G. Behrents, Chairperson and Advisor Assistant Professor Ki Beom Kim Assistant Professor Donald R. Oliver

i

DEDICATION

To my lovely wife, Tiffany, for her unwavering support

during my years of formal (and informal) education; I thank

you for your love, patience, sacrifice, and understanding

in my pursuit for something better. I love you more than

words can describe.

To my wonderful children, Avery, Gavin, and Sydney;

the time not spent with you during my professional training

will ultimately allow me to spend more time with you in the

future; I do this all for you.

To my parents, who inspired me to be a little better

and supported me every step of the way.

To all the teachers in my many years of education; I

express to you my gratitude, and hope you always feel that

your efforts are appreciated.

ii

ACKNOWLEDGEMENTS

I would like to acknowledge the following individuals:

Dr. Rolf Behrents for chairing my thesis committee.

Thank you for your guidance, insights and time. You have

fulfilled your responsibility in teaching me how to think.

Dr. Don Oliver for serving on my thesis committee.

You are a great teacher and mentor. Your love for

orthodontics and teaching are obvious. Thank you for your

time and suggestions.

Dr. Ki Beom Kim for serving on my thesis committee. I

truly appreciate your assistance with the writing of my

thesis.

I would also like to thank the following individuals

for their help and expertise:

Dr. Micah Mortensen for his diligence and assistance

throughout the entire process of developing and completing

this thesis.

Dr. Heidi Israel for her assistance with the

statistics in this project.

The Orthodontic and Education Research Foundation for

contributing to the funding of this project.

iii

TABLE OF CONTENTS

List of Tables............................................v Chapter 1: Introduction...................................1 Chapter 2: Review of the Literature Anchorage in Orthodontics......................4 In Search of Skeletal Orthodontic Anchorage....5 Osseointegrating Titanium Implants.............6 Alternative Forms of Skeletal Anchorage........8 Palatal Implants...........................9 Palatal Onplants..........................10 Miniplates................................11 Orthodontic Miniscrew Implants................11 Design....................................12 Sites for Placement.......................15 Time of Loading...........................16 Applied Force.............................18 Miniscrew Implant Stability...................19 Testing of Bone Screws........................21 Management of Bone Samples................21 Pull-Out Tests............................22 Orthopedic Screws....................23 Maxillofacial Rigid Fixation Screws..23 Orthodontic Miniscrews Implants......24 Shear Tests...............................26 References....................................28 Chapter 3: Journal Article Abstract......................................37 Introduction..................................39 Methods and Materials.........................44 Sample Selection..........................44 Miniscrew Implants........................44 Preparation of Samples for Testing........45 Mechanical Testing........................46 Statistical Analysis......................47 Results.......................................49 Miniscrews Failures.......................49 Maximum Shear Force Measurements..........50 Discussion....................................53 Conclusions...................................61 References....................................63 Vita Auctoris............................................67

iv

LIST OF TABLES

Table 3.1: Type, Location and Load Category of Surviving Miniscrews.........................50 Table 3.2: Maximum Shear Force at Failure in Newtons (N)..................................51 Table 3.3: Mann-Whitney U Test Results of Loaded Versus Control Implants......................52 Table 3.4: Kruskal-Wallis and Scheffe´ Post Hoc Results of Maximum Force at Failure by Depth of Screw in Bone.............................52

v

CHAPTER 1: INTRODUCTION

In orthodontics, malpositioned teeth are moved into

proper alignment by the application of force. This force

originates from wires, elastics and other appliances

attached to the teeth. Often, teeth that are in proper

alignment are used to provide the force to move those that

are not, and are referred to as anchorage teeth. In

accordance with Newton’s third law, there is a reactive or

“equal and opposite force” for every applied orthodontic

force. Unfortunately, these reactive forces often result

in undesirable movements of the teeth serving as anchorage.

Anchorage, broadly defined as the degree of

resistance to displacement, is a critical component to

successful orthodontic treatment. As a result,

orthodontists have historically used a variety of

appliances and strategies to enhance anchorage,

particularly when minimal movement of the teeth providing

the anchorage is desired. This allows the movement of

malaligned teeth while leaving teeth that do not need to be

moved relatively undisturbed.

Anchorage enhancing appliances, such as headgear,

are highly dependent upon patient compliance for success.

1

In an effort to establish anchorage without significant

reliance on patient cooperation, other forms of anchorage

have been investigated. Restorative dental implants,

despite their stability in bone, have limited use in

orthodontics due to cost, an extensive healing period after

surgical placement, and anatomic placement limitations.

Still, the use of these titanium dental implants as a form

of anchorage has provided the potential for absolute,

compliance independent, orthodontic anchorage.

Orthodontic miniscrew implants have been designed to

circumvent the limitations posed by restorative dental

implants. These smaller bone screws are significantly less

expensive, are easily placed and removed, and can be placed

in almost any intra-oral region, including between the

roots of the teeth. Some basic questions remain, however,

concerning the limitations of miniscrews. Specifically,

what is the maximum amount of lateral or shear force that

can be applied to these miniscrews before they fail? How

does a force that is applied immediately after the

miniscrew is placed affect the maximum holding power of the

implant? To what extent does the total length of screw

engaged in bone affect the maximum shear force the implant

can withstand? These are questions that remain unanswered

in the literature.

2

The present study intends to provide information on

the maximum shear force that immediately loaded orthodontic

miniscrew implants can withstand before failure. In a

preliminary study,1 3 and 6 mm orthodontic miniscrews were

placed in the maxilla and mandible of the beagle dog and

immediately loaded. Two levels of force were applied

(600 and 900 grams). After a period of six weeks, the dogs

were sacrificed. These implanted miniscrews were utilized

in the present study to determine the maximum shear force

that can be applied prior to implant failure. Comparisons

of the maximum force at failure will be made according to

implant length (3mm, 6mm), applied force (0g, 600g, 900g),

location (maxilla, mandible) and depth of the screws in

bone.

3

CHAPTER 2: REVIEW OF THE LITERATURE

Anchorage in Orthodontics

The attainment and control of anchorage is

fundamental to the successful practice of orthodontics and

dentofacial orthopedics. According to Newton’s well-known

law of physics, action and reaction forces are equal and

opposite. In orthodontics, anchorage is used to describe

resistance to reaction forces.2 Teeth are the usual source

of anchorage and, in the typical orthodontic biomechanical

situation, are pitted against one another to produce tooth

movement. The teeth serving as the anchorage unit, by

virtue of their number, position, size and number of roots,

intend to offer resistance to movement so as to bring about

the movement of the other teeth. A threshold value of

force to initiate tooth movement has not been identified,3

but appears to be very low.2 For example, tooth movement

has been detected with as little as 4 gm of force.4

Considering this principle, is has been concluded that the

practice of pitting more teeth with a larger root surface

area against fewer teeth with less surface area in intra-

arch mechanics may not be sufficient to prevent movement of

anchor teeth.5-7 Therefore, in order to achieve increased

4

anchorage control, a supplemental form of anchorage is

often required. Traditionally, headgear and intermaxillary

elastics have been used as forms of supplemental anchorage.2

While this form of supplemental mechanics may be effective

in increasing anchorage, effectiveness depends upon the

cooperation of the patient. Consequently, orthodontic

anchorage control has historically been contingent on

patient compliance. Due to the inconsistent nature of such

compliance,8 orthodontists often note the unfavorable

reciprocal movement of the intra-arch and inter-arch

“anchor” teeth.

In Search of Skeletal Orthodontic Anchorage

Orthodontists have recognized that stability of

reactive anchorage units could be significantly increased

if orthodontic anchorage could be provided by the skeletal

bone itself,9 and in the 1940s began to conduct research on

the subject. An early study by Bernier and Canby suggested

that surgical vitallium bone screws were inert and stable

in bone.10 However, when Gainsforth and Higley9 attempted

to use these screws as a source of orthodontic anchorage

they were largely unsuccessfully.

5

Subsequently, many other investigators have

attempted to identify a successful form of skeletal

anchorage through the use of a variety of endosseous

implants and bone plates. In addition to the original

study using surgical vitallium screws, research was

conducted during this early era on each of the following:

blade implants,11 vitreous carbon,12,13 bio-glass coated

aluminum oxided,14 and vitallium implants.15,16 All of these

implant types have exhibited some degree of success in

terms of implant stability when subjected to orthodontic

forces. However, each implant system exhibited some form

of weakness such that stability was unpredictable.

Consequently, none of these implant systems have gained

widespread clinical acceptance. It was not until the

development of osseointegrating titanium implants that a

reliable source of skeletal anchorage was established and

found widespread clinical application.

Osseointegrating Titanium Implants

In the 1960s, Brånemark discovered the unique

healing response exhibited by bone when it was exposed to

titanium. He and his colleagues later described the

6

process by which a titanium fixture could be embedded and

incorporated into bone.17 This phenomenon became known as

osseointegration, and was introduced to restorative

dentistry in 1965. From this principle came the

development of titanium dental implants, which resemble the

root of a tooth (approximately 4 mm X 9-15 mm) and provide

for the replacement of missing teeth without compromising

adjacent teeth. Dental implants, having been proven highly

successful,18 have been referred to as “the most influential

change in dentistry during the last half-century.”19

Despite the discovery of osseointegration in the

1960s and rapid development of traditional titanium dental

implants, such devices were not evaluated for use as

orthodontic anchorage until the 1980s.20-25 In an early

study by Roberts and associates, they demonstrated that

implant osseointegration and stability persisted despite

the application of an orthodontic force.20 Consequently,

osseointegrating titanium dental implants were considered

an effective source of skeletal orthodontic anchorage.26-29

Endosseous titanium dental implants have been used

to provide anchorage independent of patient compliance and

without the need to accept unfavorable reciprocal movement

of anchor teeth. Unfortunately, dental implants are

associated with some distinct and significant disadvantages

7

that limit routine use in orthodontics. For example, the

size of restorative implants (approximately 4 mm X 9-15 mm)

limits the anatomic sites available for placement (e.g.,

edentulous areas or the retromolar region). Furthermore,

use of these implants is highly dependent on a precise 2-

stage surgical protocol and a healing time of 3-6 months

prior to the application of orthodontic force.18,20,22,30

Considering the time required to complete orthodontic

treatment alone, this additional time for healing is

considered a significant deterrent in terms of the use of

dental implants. Such limitations have motivated a search

for alternative forms of orthodontic anchorage via

implants.

Alternative Forms of Skeletal Anchorage

Based on the need to develop a form of skeletal

anchorage in orthodontic patients, alternate forms of

implant anchorage have been developed. These devices

include palatal implants, palatal onplants, surgical

miniplates, and miniscrews.

8

Palatal Implants

Designed after a traditional restorative implant,

palatal implants are intended to osseointegrate and provide

a rigid point of attachment for the teeth. The Straumann

Orthosystem® (Institut Straumann AG, Waldenburg,

Switzerland) is an example of a palatal implant. This type

of implant consists of three parts: a self-tapping

endosseous body, a smooth cylindrical collar, and an

octagonal head used to connect attachments.31

In terms of placement, after the removal of a small

circular section of palatal mucosa and the preparation of

an appropriate pilot hole, the implant is inserted,

covered, and allowed to heal.32 After a healing period of

at least 3 months32,33 a second surgical procedure is

performed to uncover the implant and place an apparatus

that allows attachment to the teeth.

Despite the established success of palatal implants

in providing anchorage, there are, again, certain drawbacks

to this system that have limited clinical acceptance. The

size of the implant and the invasive surgical procedures

required for placement, use and removal are likely the most

significant disadvantages. In addition, the healing time

required prior to loading, and the time and cost associated

with fabrication of custom attachments make this type of

9

implant even less appealing. Lastly, because these

implants are not intended to be permanent, there are

potential problems associated with their removal from the

palate. If the degree of osseointegration cannot be

overcome by the use of a hand ratchet, trephination of the

implant and the surrounding bone must be performed. This

procedure leaves a void in the palatal bone which is left

to granulate and heal over time.32

Palatal Onplants

The palatal onplant, developed by Block and Hoffman,

is a unique device consisting of two parts: a dome shaped

disk (7 mm diameter x 3.5 mm thick) and an abutment that

screws into the center of the disk.34 This fixture is

designed to lay against the bone of the palate under the

periosteum. This is in contrast to palatal implants which

penetrate the bone. When surgically placed

subperiosteally, bone grows into the hydroxyapatite-coated

surface of the disk resulting in osseointegration. This

process of osseointegration requires at least 10 weeks of

healing,35,36 after which an incision is made and the

abutment is attached and left protruding through the soft

tissue. The palatal onplant shares similar disadvantages

with the palatal implant. The possible locations for

10

placement are limited, and three surgeries and an extended

healing time are required.

Miniplates

Miniplates are derivatives of rigid fixation plates

used in maxillofacial surgery. They are secured to the

bone with two or three bone screws and have an extension

arm designed to extend through the mucosa into the oral

cavity. The arm, which measures 10.5-16.5 mm, serves as a

point of attachment for the orthodontic appliance. Unlike

the previously described palatal implants and onplants,

miniplates can be placed in various locations including the

zygomatic buttress, the periform rim, and the lateral

border of the mandible.37 A surgical flap is required to

place miniplates, and a healing period is reccommended.38 A

second surgical procedure is required to remove the plates

when they are no longer needed.

Orthodontic Miniscrew Implants

The development and improvement of dental implants

and maxillofacial fixation methods brought about the

evolution of orthodontic miniscrew implants. They have

11

been designed to circumvent the shortcomings of other forms

of skeletal anchorage in the context of orthodontic

anchorage. The first successful screw shaped implant used

exclusively for orthodontic anchorage was reported in 1983.

In this report maxillary incisor intrusion was accomplished

in a deep-bite patient with a miniscrew for anchorage.15

Since that time many miniscrew designs have been developed,

and there has been a dramatic increase in use and

popularity. It has been argued, however, that their

utilization has preceded a thorough understanding of the

biology involved and their mechanical potentials.39

Design

In recent years, many different miniscrew implants

have been designed and manufactured for orthodontic use.

Today, the material of choice for miniscrews is titanium.

Titanium allows for the small size and weight of the

miniscrew without compromising strength and

biocompatibility.40 The common shape of these designs is a

threaded cylindrical body with a conical tip. Variations

center on the basic features of the screw portion in terms

of diameter, length, thread width and pitch, and head

design. Another important variation involves the screw

being self-tapping and self-drilling. The sharp threads of

12

a self-tapping screw cut into the bone and advance the

screw as it is turned. All screws are self-tapping. All

screws are not self-drilling, however. Those that are not

require the preparation of a drilled pilot hole prior to

insertion. Self-drilling screws have a drill-shaped point

and a specialized cutting flute that allows insertion

without prior drilling. This type of miniscrew, sometimes

called drill-free, has been shown to exhibit more bone-to-

metal contact and less mobility than miniscrews placed with

a pre-drilled pilot hole.41

The design of a miniscrew can significantly affect

its function and stability. Increased length, which can

provide for bicortical placement, improves primary, or

initial, stability.42 Numerous studies have reported

successful use of miniscrews 6 mm in length.43-47 When

Deguchi et al. loaded 96 implants (1 mm x 5 mm) with 200-

300 g of force, 93 of the miniscrews were still stable

after 3 months.48 There are no reports in the literature,

however, of the stability of miniscrews shorter than 4 mm.

The diameter of a miniscrew is another design

feature that seems to play an important role in stability.

Miniscrew diameters vary widely among, and within different

manufacturers. Miniscrews currently on the market range in

diameter from 1.2 to 2.0 mm. Various diameters of

13

miniscrews have been shown to be successful in providing

anchorage. Park et al. reported on 227 miniscrews of four

types with diameters of 1.2 mm and 2.0 mm.49 The overall

success rate was 91% and no difference was noted between

the implants of different diameters. In a case report

describing the use of miniscrews in non-extraction

treatment, miniscrews measuring 1.2 mm X 6 mm were used

with success to move an entire arch en masse.50 There

appears to be a limit, with regard to the diameter of

miniscrews, below which success is compromised. In a study

by Miyawaki et al.,51 all 1.0 mm diameter screws failed, but

the 1.5 mm and 2.3 mm diameter screws showed no significant

differences with success rates of 83.9% and 85%,

respectively. The authors concluded that a diameter of

less than 1.0 mm was a significant criterion associated

with failure. The advantage of a thinner screw is that it

can be placed in more locations, such as between the roots

of teeth. The drawback, however, is the greater potential

for screw fracture.52 Cope has stated that the minimum

diameter to avoid metal failure should be 1.5 mm.53

14

Sites for Placement

Numerous studies and clinical reports show a variety

of implant placement sites including the retromolar pad,54

palate,55,56 and the maxillary and mandibular buccal cortex.51

In addition, it has been shown that implants can be

inserted into the anterior nasal spine, the symphysis57 and

are the only type of implant that can be placed between the

roots of the teeth.58

In studies of miniscrew implants that have been

performed on dogs, implants have been placed in the

palate,43,59,60 the lingual cortical plate of the mandible,45

and the maxillary and mandibular buccal cortex.43,44

The literature is conflicting when comparing the

stability of implants placed in the maxilla versus those

placed in the mandible. Cheng and colleagues found that

miniscrews in the posterior mandible were susceptible to

increased failure rates when compared to the anterior

mandible, anterior maxilla, and posterior maxilla.61

Tseng et al. also found higher failure rates of miniscrews

in the mandible.62 Relating bone contact to stability,

Wehrbein and associates found a 79% bone-to-implant contact

in the maxilla as compared to 68% in the posterior

mandible.63

15

In contrast to the aforementioned studies, two

additional studies suggest a higher degree of stability for

implants placed in the mandible. Deguchi et al. reported

that implants placed in the mandible were found to have

higher bone-to-implant contact,48 while Bischof et al.

showed that 3 months after placement, implants in the

mandible were more stable than those placed in the

maxilla.64

Time of Loading

Suggested healing times for orthodontic miniscrew

implants cover a broad range. One of the earliest studies

on these implants recommended a period of 9 months prior to

force application.54 The lack of consensus is evidenced by

a more recent study. Using the beagle dog as a model,

Ohmae et al. tested the efficacy of miniscrews for

orthodontic intrusion.65 After allowing six weeks for the

4 mm long miniscrews to heal, they were loaded with a 150 g

of force. At the end of the 18 week loading period, all 36

of the implants were stable in the bone, and 4.5 mm of

intrusion had been achieved. Despite this success, the

author suggested that the 6 week healing time may still

have been too short.

16

Immediate loading of miniscrews has become more

common, with several reports in the literature to support

the practice.57,66-68 Doi conducted a study in which 48

miniscrews (6 mm) were placed in the jaws of four beagle

dogs. Immediately after placement, two miniscrews were

connected to each other by nickel titanium coil springs

that produced either 300 or 600 g of force. Two of the 48

miniscrews were placed near erupting teeth and failed

shortly after placement. This required their removal, and

the exclusion of the other miniscrews to which they were

connected. The force remained active for 5 weeks. At the

end of the testing period 5 out of the 44 remaining

miniscrews demonstrated significant mobility. The author

concluded that miniscrews can be loaded immediately with

orthodontic, and even orthopedic, force levels with a

success rate of 100%.43

Another study by Owens was designed to place 56

miniscrews (1.8 mm X 6 mm) into the jaws of 7 beagle dogs.69

Twenty-one of the implants were immediately loaded with

either 25 or 50 g of force. Even though three of the

immediately loaded miniscrews failed within 21 days of

placement, a comparison of the delayed vs. immediately

loaded miniscrews showed no differences in failure rate.

This data suggests that the success of miniscrew implants

17

is not dependent on timing of implant loading. There are

no studies in the literature, however, that have described

the affect immediate loading has on the maximum amount of

force an implant can withstand before failing.

Applied Force

The literature supports the view that a wide variety

of force loads can be applied to miniscrew implants without

the implant failing. In an early study, miniscrews were

loaded with 60, 120 and 180 g of force.16 After 28 days,

the implants showed no significant movement at any of the

force levels described. Another author described the

placement of 96 miniscrews into the buccal and lingual

cortical plates of 8 mature beagle dogs.45 The implants

were immediately loaded with 25, 50, or 100 g of force

which remained active over 98 days. One of the 96

miniscrews, which was loaded with 100 g of force, failed

after 50 days, but all others remained stable in the bone.

In another study, after a healing period of 12 weeks, a

group of 20 miniscrews were loaded with 250 to 350 g.70 All

of the miniscrews withstood the force until they were

removed 3 months later. In another study, Doi immediately

loaded miniscrews with either 300 or 600 g and noted

significant mobility in 5 of the 44 implants. He also

18

measured the displacement of the miniscrews and reported an

average of less than 0.5 mm per implant loaded with 600 g.

Turley et al. placed implants in the zygomatic buttress of

dogs, and allowed them to heal for 20 weeks.22 The implants

successfully withstood a load of 1000 g for 18 weeks.

Though these studies have demonstrated a range of force

loads that can be applied to miniscrews without significant

failure, there is no research delineating the maximum force

load that can be sustained by orthodontic implants.

Miniscrew Implant Stability

In contrast to osseointegrating dental implants,

miniscrew implants are intended to be temporary. Thus, the

screws are intentionally not subjected to surface

treatments (e.g., sandblasting, etching, plasma spraying)

designed to increase the percentage of bone-to-implant

contact.60 At the time of miniscrew removal, integration is

overcome by hand with a surgical driver.

Upon placement, the ability of a miniscrew to

provide anchorage depends on the mechanical retention

provided by intimate contact between bone and the surface

of the implant. This mechanical retention, also known as

19

primary stability, is critically important for orthodontic

anchorage to be successful,71 and is influenced by several

factors. For instance, implants with greater diameter and

thread depth will have a larger surface area in contact

with bone, and theoretically, more primary stability.

The quality and quantity of the bone into which

miniscrews are placed also influences their primary

stability and subsequent success.72,73 Cortical bone is much

denser than cancellous bone, and provides for more intimate

contact between the bone and the threads of the miniscrew.

A recent study found a weak but significant positive

correlation (r = 0.39) between cortical bone thickness and

miniscrew implant pull-out strength in dog bone.59

Indirectly, Miyawaki and colleagues related cortical bone

thickness to implant failure by noting that patients with

high mandibular plane angles were more likely to experience

implant failure.51 The link was provided by Tsunori et al.

who quantified a thinner cortical plate in patients with

high mandibular plane angles.74

The extent to which miniscrew design and bone

quality influence the stability of miniscrews and the

maximum force they can withstand is not completely known or

described in the literature. It has been suggested that

orthodontists are still in search of the formula for

20

ultimate miniscrew stability.39 As different variables that

affect miniscrew stability are investigated, such as

miniscrew design, stability can be quantified for

comparison. Quantifying the stability of miniscrews is

accomplished by performing the same tests on miniscrews

that are used to evaluate orthopedic bone screws.

Testing of Bone Screws

Before World War II, selection of screws for

orthopedic implantation was based primarily on the ease of

insertion.75 Later, stability became the primary selection

factor and tests were designed to determine the differences

between various screws. The most common test performed on

bone screws of any type is the pull-out test. An

alternative to the pull-out test is the shear test, which

examines the effects of tangential or lateral forces.

Management of Bone Samples

Many of the tests performed on bone screws utilize

non-living bone. It is important to note that for accurate

correlation of fresh bone tests to living bone, proper

preservation techniques used during preparation and testing

21

of bony samples are essential.76 Various methods that have

been used to adequately preserve bone have been described

in the literature. In early tests, bone samples were

wrapped in wet paper towels, enclosed in plastic, and

stored at 7°C for no more than 48 hours.77 More recently,

fresh bone samples have been stored in refrigerated

physiologic saline,78 wrapped in saline soaked gauze and

stored at -15°C,59 or simply sealed in plastic bags and

stored at -25°C prior to testing.79 It has been shown that

the freezing process does not have an adverse effect on the

elastic properties of bone.76,80,81

Pull-Out Tests

The pull-out test is considered an accurate method

of evaluating the relative strength or “holding power” of

surgically placed bone screws.77,82 Holding power is defined

as the maximum uniaxial tensile force needed to produce

failure in the bone.83 Pull-out tests measure holding power

against tension applied along the longitudinal axis of the

screws. Results of pull-out tests have been reported on

numerous types of bone screws.

22

Orthopedic Screws

Several authors have reported specific results of

pull-out tests on screws used in orthopedic surgery.77,84,85

Koranhi et al. placed large diameter screws in canine and

bovine femurs to test the difference between two thread

types.77 No difference between the screws was detected, but

the results showed a linear relationship between pull-out

strength and cortical bone thickness. In another study,

screws intended for use on the spine were inserted into

porcine (pig) vertebral bodies.84 Despite differences in

diameter (6.5-7.5 mm), length (25-35 mm) and thread depth

(1-1.8 mm), no significant differences were noted in axial

pull-out strength which measured an average of 268 lbs

(1194 N). The authors concluded that the shorter test

screws with increased thread depth could provide as much

holding power as the routinely used longer screws.

Maxillofacial Rigid Fixation Screws

The designs of currently available orthodontic

miniscrew implants are similar to rigid fixation bone

screws. Rigid fixation bone screws, used in orthognathic

surgery to affix rigid plates to the bones of the face,

have been tested by means of pull-out tests.78,82,86-88 Foley

et al. tested five different types of fixation screws in

23

the long bones of a mongrel dog and found a mean pull-out

tension of 44.5 kilograms (436 N).82 In another study, five

additional types of fixation screws were tested using bones

from the skull and mandible of human cadavers.86 The screws

varied in diameter and length and demonstrated pull-out

strengths of up to 620 N (1 N = 102 g). In pull-out tests

of 2.0 mm diameter screws in porcine rib with a cortical

thickness of between 0.5 and 2.0 mm, Boyle et al. found a

mean pull-out force of 21 kg (205 N).88 In a similar test

by the same author another group of 2.0 mm diameter screws

showed pull-out strengths ranging from 16-25 kg

(157-254 N).78

It should be noted that the results of tests

performed on screws in animal bone may not directly

correlate to human orofacial cortical bone due to

differences in mechanical properties. Specifically, in the

dog model the alveolar process has a higher density than

the equivalent structure in the human.89

Orthodontic Miniscrews Implants

Recently, three studies have been conducted on the

pull-out strength of orthodontic miniscrew implants.

Pickard placed miniscrews in cadaver mandibles and

evaluated the effects of implant orientation on stability

24

and resistance to failure.90 The study also intended to

identify the maximum holding power of miniscrew implants in

the human mandible. Pulling on the implants at various

angles, he determined that a direction of force along the

long axis of the implant offered the greatest resistance to

failure. The results of his axial pull-out tests showed a

maximum force at failure of 342 N ± 80.9 N (Mean ± S.D.).

Huja and colleagues have conducted two studies on the pull-

out strength of miniscrews. Both of the studies were

designed to determine the maximum pull-out strength of

miniscrews in the maxilla and mandible of beagle dogs. In

the first study, the implants were placed and tested

immediately after the dogs were sacraficed.59 Average pull-

out strength of all implants measured 222 N. The second

study tested the screws in the same manner after they were

allowed to heal, unloaded, for 6 weeks.60 Average pull-out

strength of implants after 6 weeks of unloaded healing was

245 N. There was no significant difference in the pull-out

strengths between the two time periods. The authors did,

however, show a weak but positive correlation between pull-

out strength and cortical plate thickness in both

studies.59,60

25

Shear Tests

Although tests of the stability of bone screws have

been primarily focused on pull-out, it is important to

recognize that these pull-out tests alone are not totally

adequate to measure the anchorage potential of bone screws.

They do not, for example, address shearing forces which are

present in a clinical setting.82 There is a limited number

of reports on shear testing of bone screws or implants in

the literature.84,90 Glatzmaier et al. tested the shear

strength of a bioresorbable polylactide implant in vitro.91

These experimental implants showed a shear force at failure

of 50 N. Pierce et al, working with instrumentation screws

used in the vertebral bodies of the spine, conducted pull-

out and shear tests on screws with diameters of between 6.5

and 7.5 mm.84 The results showed an average maximum shear

force at failure of 786 N.

Pickard has conducted the only shear tests on

orthodontic miniscrew implants.90 As described above, he

studied the effect of orientation of miniscrew implants on

resistance to failure. Placing miniscrews in human cadaver

bone and immediately afterwards performing shear tests, he

determined that the mean shear force required to cause

failure (123 N) is roughly a third of the mean axial pull-

out force (342 N).

26

There are no reports in the literature of shear

tests on miniscrew implants that have been placed in living

bone and allowed to heal, nor have there been shear tests

performed on miniscrews that have been immediately loaded.

The purpose of this study is to determine the affect of

immediate loading on the maximum shear resistance of

miniscrew implants, and to compare this effect between

implants of two different lengths, and with three different

applied force loads (0, 600 g, 900 g). This will be

accomplished in a dog model using a sample involving 3 mm

and 6 mm miniscrews that were immediately loaded or

unloaded (control) for a period of six weeks, and then

subjecting both loaded and control implants to shear force

testing. The goal of shear force testing is to imitate, as

closely as possible, the conditions that exist when an

orthodontic miniscrew is subjected to lateral forces in the

mouth. Many variables are responsible for the maximum

shear force a miniscrew can withstand. By studying the

expression of these variables, this study may give

clinicians a better indication of what can be expected of

orthodontic miniscrews in clinical practice.

27

References

1. Mortensen MG. A comparison of stability of 3 and 6mm miniscrew implants immediately-loaded with two different force levels in the beagle dog. Master's Thesis. Center for Advanced Dental Education. Saint Louis University. St. Louis, MO. 2007. 2. Proffit W, Fields H. Contemporary Orthodontics. Saint Louis, MO: CV Mosby; 2003. 3. Melsen B, Bosch C. Different approaches to anchorage: a survey and an evaluation. Angle Orthod 1997;67:23-30. 4. Weinstein S, Haack DC, Morris LY, Snyder BB, H.E. A. On an equilibrium theory of tooth position. Angle Orthod 1963;33:1-26. 5. Brodie AG, Bercea MN, Gromme EJ, Neff CW. The application of the principles of the edgewise arch in the treatment of Class II, Division 1. Angle Orthod 1937;7:1-14. 6. Dincer M, Iscan HN. The effects of different sectional arches in canine retraction. Eur J Orthod 1994;16:317-323. 7. Thiruvenkatachari B, Pavithranand A, Rajasigamani K, Kyung HM. Comparison and measurement of the amount of anchorage loss of the molars with and without the use of implant anchorage during canine retraction. Am J Orthod Dentofacial Orthop 2006;129:551-554. 8. Sinha PK, Nanda RS. Improving patient compliance in orthodontic practice. Semin Orthod 2000;6:237-241. 9. Gainsforth B, Higley L. A study of orthodontic anchorage possibilities in basal bone. Am J Orthod Oral Surg 1945;31:406-416. 10. Bernier JL, Canby CP. Histologic studies on the reaction of alveolar bone to vitallium implants. J Am Dent Assoc 1943;30:188. 11. Linkow LI. The endosseous blade implant and its use in orthodontics. Int J Orthod 1969;7:149-154.

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12. Sherman AJ. Bone reaction to orthodontic forces on vitreous carbon dental implants. Am J Orthod 1978;74:79-87. 13. Oliver S, Mendez-Villamil C, Evans C, Schnitman P, Shulman L. Change in position of vitreous carbon implants subjected to orthodontic forces [abstract]. J Dent Res 1980;59:280. 14. Lubberts R, Turley P. Force application to bioglass coated alumina implants of various sizes. J Dent Res 1982;61:339. 15. Creekmore TD, Eklund MK. The possibility of skeletal anchorage. J Clin Orthod 1983;17:266-269. 16. Gray JB, Steen ME, King GJ, Clark AE. Studies on the efficacy of implants as orthodontic anchorage. Am J Orthod 1983;83:311-317. 17. Brånemark PI, Adell R, Breine U, Hansson BO, Lindstrom J, Ohlsson A. Intra-osseous anchorage of dental prostheses. I. Experimental studies. Scand J Plast Reconstr Surg 1969;3:81-100. 18. Adell R, Lekholm U, Rockler B, Brånemark PI. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10:387-416. 19. Christensen GJ. The 'mini'-implant has arrived. J Am Dent Assoc 2006;137:387-390. 20. Roberts WE, Smith RK, Zilberman Y, Mozsary PG, Smith RS. Osseous adaptation to continuous loading of rigid endosseous implants. Am J Orthod 1984;86:95-111. 21. Roberts WE, Helm FR, Marshall KJ, Gongloff RK. Rigid endosseous implants for orthodontic and orthopedic anchorage. Angle Orthod 1989;59:247-256. 22. Turley PK, Kean C, Schur J, Stefanac J, Gray J, Hennes J et al. Orthodontic force application to titanium endosseous implants. Angle Orthod 1988;58:151-162. 23. Douglass JB, Killiany DM. Dental implants used as orthodontic anchorage. J Oral Implantol 1987;13:28-38.

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24. Odman J, Lekholm U, Jemt T, Brånemark PI, Thilander B. Osseointegrated titanium implants--a new approach in orthodontic treatment. Eur J Orthod 1988;10:98-105. 25. Higuchi KW, Slack JM. The use of titanium fixtures for intraoral anchorage to facilitate orthodontic tooth movement. Int J Oral Maxillofac Implants 1991;6:338-344. 26. Rasmussen R. A new dimension--implant-assisted orthodontics. Dent Implantol Update 1991;2:24-26. 27. Thilander B, Odman J, Grondahl K, Friberg B. Osseointegrated implants in adolescents. An alternative to replacing teeth? Eur J Orthod 1994;16:84-95. 28. Prosterman B, Prosterman L, Fisher R, Gornitsky M. The use of implants for orthodontic correction of an open bite. Am J Orthod Dentofacial Orthop 1995;107:245-250. 29. Spear FM, Mathews DM, Kokich VG. Interdisciplinary management of single-tooth implants. Semin Orthod 1997;3:45-72. 30. Smalley WM, Shapiro PA, Hohl TH, Kokich VG, Brånemark PI. Osseointegrated titanium implants for maxillofacial protraction in monkeys. Am J Orthod Dentofacial Orthop 1988;94:285-295. 31. Cousley R. Critical aspects in the use of orthodontic palatal implants. Am J Orthod Dentofacial Orthop 2005;127:723-729. 32. Crismani AG, Bernhart T, Bantleon HP, Cope JB. Palatal implants: The Straumann Orthosystem. Semin Orthod 2005;11:16-23. 33. Huang LH, Shotwell JL, Wang HL. Dental implants for orthodontic anchorage. Am J Orthod Dentofacial Orthop 2005;127:713-722. 34. Block MS, Hoffman DR. A new device for absolute anchorage for orthodontics. Am J Orthod Dentofacial Orthop 1995;107:251-258. 35. Janssens F, Swennen G, Dujardin T, Glineur R, Malevez C. Use of an onplant as orthodontic anchorage. Am J Orthod Dentofacial Orthop 2002;122:566-570.

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36. Hong H, Ngan P, Han G, Qi LG, Wei SH. Use of onplants as stable anchorage for facemask treatment: A case report. Angle Orthod 2005;75:453-460. 37. Sugawara J, Daimaruya T, Umemori M, Nagasaka H, Takahashi I, Kawamura H et al. Distal movement of mandibular molars in adult patients with the skeletal anchorage system. Am J Orthod Dentofacial Orthop 2004;125:130-138. 38. Daimaruya T, Takahashi I, Nagasaka H, Umemori M, Sugawara J, Mitani H. Effects of maxillary molar intrusion on the nasal floor and tooth root using the skeletal anchorage system in dogs. Angle Orthod 2003;73:158-166. 39. Cope JB. Temporary anchorage devices in orthodontics: A paradigm shift. Semin Orthod 2005;11:3-9. 40. Favero L, Brollo P, Bressan E. Orthodontic anchorage with specific fixtures: Related study analysis. Am J Orthod Dentofacial Orthop 2002;122:84-94. 41. Kim JW, Ahn SJ, Chang YI. Histomorphometric and mechanical analyses of the drill-free screw as orthodontic anchorage. Am J Orthod Dentofacial Orthop 2005;128:190-194. 42. Pierrisnard L, Renouard F, Renault P, Barquins M. Influence of implant length and bicortical anchorage on implant stress distribution. Clin Implant Dent Relat Res 2003;5:254-262. 43. Doi PAK. A comparison of stability of immediately loaded mini-implants with two different force levels in the beagle dog. Master's Thesis. Center for Advanced Dental Education. Saint Louis University. St. Louis, MO. 2006. 44. Owens SE. Clinical and biological effects of the mini implant for orthodontic anchorage: An experimental study in the beagle dog. Tex Dent J 2005;122:672. 45. Carillo R. Intrusion and root resorption of multiradicular teeth using mini-screw implants as anchorage. Dallas, TX: Baylor College of Dentistry; 2004. 46. Park HS, Lee SK, Kwon OW. Group distal movement of teeth using microscrew implant anchorage. Angle Orthod 2005;75:602-609.

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47. Park HS, Kwon TG, Kwon OW. Treatment of open bite with microscrew implant anchorage. Am J Orthod Dentofacial Orthop 2004;126:627-636. 48. Deguchi T, Takano-Yamamoto T, Kanomi R, Hartsfield JK, Jr., Roberts WE, Garetto LP. The use of small titanium screws for orthodontic anchorage. J Dent Res 2003;82:377-381. 49. Park HS, Jeong SH, Kwon OW. Factors affecting the clinical success of screw implants used as orthodontic anchorage. Am J Orthod Dentofacial Orthop 2006;130:18-25. 50. Park HS, Kwon TG, Sung JH. Nonextraction treatment with microscrew implants. Angle Orthod 2004;74:539-549. 51. Miyawaki S, Koyama I, Inoue M, Mishima K, Sugahara T, Takano-Yamamoto T. Factors associated with the stability of titanium screws placed in the posterior region for orthodontic anchorage. Am J Orthod Dentofacial Orthop 2003;124:373-378. 52. Melsen B. Mini-implants: Where are we? J Clin Orthod 2005;39:539-547; quiz 531-532. 53. Cope J. Temporary anchorage devices in orthodontics: A paradigm shift. Semin Orthod 2005:3-9. 54. Roberts WE, Marshall KJ, Mozsary PG. Rigid endosseous implant utilized as anchorage to protract molars and close an atrophic extraction site. Angle Orthod 1990;60:135-152. 55. Kyung SH, Hong SG, Park YC. Distalization of maxillary molars with a midpalatal miniscrew. J Clin Orthod 2003;37:22-26. 56. Park HS, Jang BK, Kyung HM. Maxillary molar intrusion with micro-implant anchorage (MIA). Aust Orthod J 2005;21:129-135. 57. Costa A, Raffainl M, Melsen B. Miniscrews as orthodontic anchorage: A preliminary report. Int J Adult Orthodon Orthognath Surg 1998;13:201-209. 58. Kanomi R. Mini-implant for orthodontic anchorage. J Clin Orthod 1997;31:763-767.

32

59. Huja SS, Litsky AS, Beck FM, Johnson KA, Larsen PE. Pull-out strength of monocortical screws placed in the maxillae and mandibles of dogs. Am J Orthod Dentofacial Orthop 2005;127:307-313. 60. Huja SS, Rao J, Struckhoff JA, Beck FM, Litsky AS. Biomechanical and histomorphometric analyses of monocortical screws at placement and 6 weeks postinsertion. J Oral Implantol 2006;32:110-116. 61. Cheng SJ, Tseng IY, Lee JJ, Kok SH. A prospective study of the risk factors associated with failure of mini-implants used for orthodontic anchorage. Int J Oral Maxillofac Implants 2004;19:100-106. 62. Tseng YC, Hsieh CH, Chen CH, Shen YS, Huang IY, Chen CM. The application of mini-implants for orthodontic anchorage. Int J Oral Maxillofac Surg 2006;35:704-707. 63. Wehrbein H, Merz BR, Hammerle CH, Lang NP. Bone-to-implant contact of orthodontic implants in humans subjected to horizontal loading. Clin Oral Implants Res 1998;9:348-353. 64. Bischof M, Nedir R, Szmukler-Moncler S, Bernard JP, Samson J. Implant stability measurement of delayed and immediately loaded implants during healing. Clin Oral Implants Res 2004;15:529-539. 65. Ohmae M, Saito S, Morohashi T, Seki K, Qu H, Kanomi R et al. A clinical and histological evaluation of titanium mini-implants as anchors for orthodontic intrusion in the beagle dog. Am J Orthod Dentofacial Orthop 2001;119:489-497. 66. Freudenthaler JW, Haas R, Bantleon HP. Bicortical titanium screws for critical orthodontic anchorage in the mandible: A preliminary report on clinical applications. Clin Oral Implants Res 2001;12:358-363. 67. Park HS, Bae SM, Kyung HM, Sung JH. Micro-implant anchorage for treatment of skeletal Class I bialveolar protrusion. J Clin Orthod 2001;35:417-422. 68. Takano-Yamamoto T, Miyawaki S, Koyama I. Can implant orthodontics change the conventional orthodontic treatment? Dental Diamond 2002;27:26-47.

33

69. Owens SE. Experimental evaluation of tooth movement in the beagle dog utilizing the mini-implant for orthodontic anchorage. Dallas, TX: Baylor College of Dentistry; 2004. 70. Asikainen P, Klemetti E, Vuillemin T, Sutter F, Rainio V, Kotilainen R. Titanium implants and lateral forces. An experimental study with sheep. Clin Oral Implants Res 1997;8:465-468. 71. Huja SS. Biologic parameters that determine success of screws used in orthodontics to supplement anchorage. In: McNamara JJ, editor. Implant Anchorage in Orthodontics. 31st Annual Moyers Symposium. Ann Arbor, MI: In press; 2005. 72. Kido H, Schulz EE, Kumar A, Lozada J, Saha S. Implant diameter and bone density: Effect on initial stability and pull-out resistance. J Oral Implantol 1997;23:163-169. 73. Schwimmer A, Greenberg AM, Kummer F, Kaynar A. The effect of screw size and insertion technique on the stability of the mandibular sagittal split osteotomy. J Oral Maxillofac Surg 1994;52:45-48. 74. Tsunori M, Mashita M, Kasai K. Relationship between facial types and tooth and bone characteristics of the mandible obtained by CT scanning. Angle Orthod 1998;68:557-562. 75. Lyon WF, Cochran JR, Smith L. Actual holding power of various screws in bone. Ann Surg 1941;114:367. 76. Evans FG. Preservation effects. In: Mechanical Properties of Bone. Springfield, IL: Charles C Thomas; 1973. 77. Koranyi E, Bowman CE, Knecht CD, Janssen M. Holding power of orthopedic screws in bone. Clin Orthop Relat Res 1970;72:283-286. 78. Boyle JM, 3rd, Frost DE, Foley WL, Grady JJ. Torque and pullout analysis of six currently available self-tapping and "emergency" screws. J Oral Maxillofac Surg 1993;51:45-50.

34

79. Haher TR, Yeung AW, Caruso SA, Merola AA, Shin T, Zipnick RI et al. Occipital screw pullout strength. A biomechanical investigation of occipital morphology. Spine 1999;24:5-9. 80. Dechow PC, Huynh T. Elastic properties and biomechanics of the baboon mandible (abstract). Am J Phys Anthropol 1994;22:94-95. 81. Zioupos P, Smith CW, An YH. Factors affecting mechanical properties of bone. In: Mechanical Testing of Bone and the Bone-implant Interface. New York, NY: CRC Pres.; 2000, p 65-85. 82. Foley WL, Frost DE, Paulin WB, Jr., Tucker MR. Uniaxial pullout evaluation of internal screw fixation. J Oral Maxillofac Surg 1989;47:277-280. 83. Cantwell M. Comparison of holding power of bone screws. T. A. M. Report, unpublished 1968;294. 84. Pierce W, Sucato D, Young S, Picetti G, Morgan D. Axial and tangential pullout strength of uni-cortical and bi-cortical anterior instrumentation screws. Proceedings of the 49th Annual Meeting of the Orthopedic Research Society; February 2-5, 2003. New Orleans, LA: Rosemont (Ill): Orthopedic Research Society; 2003. 85. Schatzker J, Sanderson R, Mrunaghar JP. The holding power of orthopedic screws in vivo. Clin Orthop 1975;108:115. 86. Saka B. Mechanical and biomechanical measurements of five currently available osteosynthesis systems of self-tapping screws. Br J Oral Maxillofac Surg 2000;38:70-75. 87. Ellis JS, Laskin DM. Analysis of seating and fracturing torque of bicortical screws. J Oral Maxillofac Surg 1994;52:483-486. 88. Boyle JM, 3rd, Frost DE, Foley WL, Grady JJ. Comparison between uniaxial pull-out tests and torque measurement of 2.0-mm self-tapping screws. Int J Adult Orthodon Orthognath Surg 1993;8:129-133.

35

89. Reitan K, Kvam E. Comparative behavior of human and animal tissue during experimental tooth movement. Angle Orthod 1971;41:1-14. 90. Pickard MB. Effect of mini-screw orthodontic implant orientation on implant stability and resistance to failure at the bone-implant interface. Master's Thesis. Baylor College of Dentistry. Dallas, TX. 2004. 91. Glatzmaier J, Wehrbein H, Diedrich P. Biodegradable implants for orthodontic anchorage. A preliminary biomechanical study. Eur J Orthod 1996;18:465-469.

36

CHAPTER 3: JOURNAL ARTICLE

Abstract

Introduction: The use of miniscrew implants for

orthodontic anchorage has raised questions concerning their

limitations. Specifically, the maximum shear force that an

immediately loaded miniscrew can withstand has not been

investigated. The specific aim of this study is to

determine the maximum shear resistance of miniscrew

implants. The effect of immediate loading on the maximum

shear resistance of miniscrew implants will be compared

between implants of two different lengths, and with three

different applied force loads. A comparison of shear force

at failure will also be made according to the depth of the

miniscrews in bone. Methods: The sample was derived from

five skeletally mature beagle dogs that had 60 miniscrews

placed at predetermined locations in the palate and buccal

surface of the mandible. Miniscrews were immediately

loaded with either 0 (control), 600, or 900 g. After six

weeks of continuous force application, 45 of the miniscrews

remained in place. The dogs were then sacrificed, and bone

samples from the maxilla and mandible were dissected such

that each contained one orthodontic miniscrew. The bone

37

specimens were mounted in dental stone for testing purposes

and secured in a universal vice for mechanical testing.

Testing was performed by the application of a shear force

in the same direction as the original force until failure

of the implant. The maximum force sustained by the

implants prior to failure was recorded in Newtons (N).

Results: The mean shear force at failure of 6 mm miniscrew

implants was significantly higher (53.0 N ± 8.3 N)

(Mean ± SE) than that of 3 mm implants (31.9 N ± 4.1 N).

No significant difference in force at failure was noted

between implants that were immediately loaded, and those

that served as unloaded controls. A significant difference

(p<0.05) was determined to be present between the groups

formed by millimetric measurements of bony purchase. The

groups with 2-3 mm and 3 mm+ of bony purchase showed a

significantly higher shear force at failure than the groups

with 0-1 mm or 1-2 mm of insertion depth. Shear force at

failure showed a moderately strong correlation (r=0.57)

with the depth of the miniscrew in bone. Conclusions:

Immediate loading of miniscrews has no significant effect

on maximum shear force at failure. Complete cortical

engagement by miniscrews may result in significantly higher

shear resistance. Miniscrews as short as 3 mm can

38

withstand shear forces well beyond levels typically used in

orthodontics.

Introduction

The attainment and control of anchorage is

fundamental to the successful practice of orthodontics and

dentofacial orthopedics. Teeth are the usual source of

anchorage and, in the typical orthodontic treatment, are

pitted against one another to produce tooth movement.

Teeth, alone, do not provide absolute, or maximum

anchorage.1-3 If maximum anchorage is needed, a

supplemental form of anchorage is usually required.

Headgear serves as an effective form of supplemental

anchorage, but it depends upon the cooperation of the

patient for success. Due to the inconsistent nature of

such compliance,4 orthodontists often note the unfavorable

reciprocal movement of intra-arch and inter-arch “anchor”

teeth.

Orthodontists have long recognized that stability of

reactive anchorage units could be significantly increased

if orthodontic anchorage were provided from within skeletal

bone itself.5 Early investigators attempted to identify a

39

successful form of skeletal anchorage through the use of a

variety of endosseous implants,6-8 but were largely

unsuccessful.

Since the time that Brånemark introduced the

biologic basis of osseointegration, titanium dental

implants have been used for the replacement of missing

teeth. Subsequently, dental implants were evaluated for

their use as orthodontic intraoral anchorage in the

1980s.9-14 Restorative implants for orthodontic usage,

however, have significant disadvantages such as cost, an

extensive healing period after surgical placement, and

anatomic placement limitations that preclude routine use.

Miniscrew implants have been developed to enhance

orthodontic anchorage and minimize the need for patient

compliance. They provide significant advantages over

dental implants due to their versatility of placement, ease

of removal, and lower cost,15 and have seen a dramatic

increase in use and popularity in recent years.

Multiple case reports have documented the successful

use of miniscrews,16-22 but some results have been

conflicting. Success rates in human subjects, for example,

range from 49% to 100%.3,23 There is also a lack of

consensus concerning ideal miniscrew design, placement

techniques, allowable force levels, and timing of force

40

application. The lack of consistent results and consensus

may be due to the fact that there are major questions

concerning orthodontic miniscrews that need to be answered

through basic science and clinical trials.

One such question is: what is the maximum force that

orthodontic miniscrews can withstand? The ability of a

miniscrew to provide anchorage depends on the mechanical

retention provided by intimate contact between bone and the

surface of the implant. This mechanical retention, also

known as primary stability, is influenced by several

factors. For instance, implants with greater diameter and

thread depth will have a larger surface area in contact

with bone, and theoretically, will achieve greater primary

stability. The quality and quantity of the bone into which

miniscrews are placed also influences their stability and

subsequent success.24,25 Cortical bone, for example, is much

denser than cancellous bone, and provides for more intimate

contact between the bone and the threads of the miniscrew.

The extent to which these and other factors

influence the stability of miniscrews and the maximum force

they can withstand is not completely known or described in

the literature. It has been suggested that orthodontists

are still in search of the formula for ultimate miniscrew

stability.26

41

Recently, three studies have been conducted on the

pull-out strength of orthodontic miniscrew implants.

Pickard placed miniscrews (1.8 mm x 6 mm) in cadaver

mandibles and evaluated the effects of implant orientation

on stability and resistance to failure.27 The results of

his axial pull-out tests showed a maximum force at failure

of 342 N ± 80.9 N (Mean ± S.D.). Huja and colleagues have

conducted two studies on the pull-out strength of

miniscrews. Both of the studies were designed to determine

the maximum pull-out strength of miniscrews in the maxilla

and mandible of beagle dogs. In the first study, the

implants (2 mm x 6 mm) were placed and tested immediately

after the dogs had been killed.28 The second study tested

the screws in the same manner after they were allowed to

heal, unloaded, for 6 weeks.29 Average pull-out strength of

implants after 6 weeks of unloaded healing was 245 N.

In each of these studies, pull-out tests were

performed. Pull-out tests measure holding power against

tension applied along the longitudinal axis of screws.

Pull-out tests alone are not totally adequate to measure

the anchorage potential of bone screws because they do not

address shearing forces which are present in a clinical

setting.30 Shear tests more closely mimic clinical

42

situations in which a lateral or tangential force is

applied to the head of a miniscrew.

Pickard has conducted the only shear tests on

orthodontic miniscrew implants.27 As described above, he

studied the effect of orientation of miniscrew implants on

resistance to failure. Placing miniscrews in human cadaver

bone and immediately afterwards performing shear tests, he

determined that the mean shear force required to cause

failure (123 N) is roughly a third of the mean axial pull-

out force (342 N).

There are no reports in the literature of shear

tests on miniscrew implants that have been placed in living

bone and allowed to heal, nor have there been shear tests

performed on miniscrews that have been immediately loaded.

The specific aim of this study is to determine the maximum

shear resistance of miniscrew implants. The affect of

immediate loading on the maximum shear resistance of

miniscrew implants will be compared between implants of two

different lengths, and with three different applied force

loads. A comparison of shear force at failure will also be

made according to the depth of the miniscrews in bone.

43

Methods and Materials

Sample Selection

Five beagle dogs were utilized as the model for this

study. As part of a previous study31 these dogs were

acquired, maintained and had implant placement surgery

performed in the Comparative Medicine Department at Saint

Louis University School of Medicine.31

Miniscrew Implants

A total of 12 surgical grade titanium implants were

placed in the mouth of each dog. The miniscrews used for

this study were the AbsoAnchor® system (Dentos, Inc.,

Daegu, Korea). Two different lengths of miniscrews were

used for the study: 3 mm and 6 mm. The 6 mm miniscrews are

commercially available. The 3 mm miniscrews were specially

constructed for this and the preceding project by Dentos,

Inc. Both implants measured 1.3 mm in diameter, and had a

notched design at the tip that allowed for self-drilling.

Two implants of the same length were organized as a pair,

and a third implant was placed between the pair to serve as

an unloaded control. The implant pairs were loaded with

either 600 or 900 g of force at the time of placement with

nickel titanium coil springs. Four sets of 3 implants (2

44

study and 1 control) were placed in each dog, with one set

being placed in each of the following locations: maxillary

right palate, maxillary left palate, mandibular left

buccal, and mandibular right buccal. One set of 6 mm

miniscrews and 3 sets of 3 mm miniscrews (1 palatal and 2

mandibular buccal) were placed in each dog with the 6 mm

set always being placed in the palate.

Preparation of Samples for Testing

Six weeks following initial miniscrew placement the

dogs were sacrificed with a lethal dose (3-5 ml) of

pentobarbital (Euthanasia-5, Henry Schein, Inc., Port

Washington, NY), and the jaws were immediately removed by

dissection. The direction of intraoral force application

was precisely marked on each implant with indelible ink,

and the coil springs providing the force were carefully

removed from the implants. All soft tissue was removed

from the jaws, and they were then sealed in plastic bags

and frozen at -30°C until the time of testing.

On the day of testing, individual bones were allowed

to thaw to room temperature and were dissected into small

segments such that each contained one miniscrew surrounded

by at least 4 mm of bone. Each segment was radiographed

from various angles to allow detection of broken implants

45

or sheared tips, and to determine whether bicortical

engagement had occurred.

In order to determine the amount of miniscrew

engaged in the bone, repeat measures of the length of screw

protruding from the bone were made with a digital caliper,

and the mean values recorded in millimeters for each

implant. The amount of implant engaged in bone was then

determined by subtraction from the known length of each

miniscrew.

The last step in preparation for testing was

completed by embedding the bone segments into a small (4 cm

x 4 cm) square receptacle containing freshly mixed, unset

dental stone. The surface of the bone into which the

miniscrew was inserted was left uncovered. The stone was

allowed to set for 10 to 15 minutes resulting in a rigid

block that could be secured for shear testing.

Mechanical Testing

The shear testing was completed with an Instron

Machine Model 1011 (Instron Corp, Canton, MA) outfitted

with a 100 lb load cell. To allow forces to be applied at

right angles to the miniscrews, a variable angle vice was

used to hold each stone block. The vice was secured to a

custom x-axis and y-axis sliding table which was bolted to

46

the frame of the Instron. The vice and sliding table could

be locked into place, which allowed the miniscrew to be

oriented and firmly held in the correct position for

testing. To ensure that the line of action was directly

through the head of the miniscrew and in the same direction

as the initial intraoral force load, a plumb bob was used

to align each implant prior to testing. The Instron

machine was used to subject the screws to shear forces

until failure. A predetermined crosshead speed of 1.0 mm

per minute was used. Forces were applied to the screws by

threading two 0.012 inch stainless steel ligatures through

the head of each miniscrew and tying them to a custom hook

attached to the Instron machine. The load-displacement

data were recorded, and the peak load at failure was

obtained from the readout and reported in Newtons (N). All

dissections, bone specimen preparation, testing and data

recording were performed by one operator (DC).

Statistical Analysis

Independent sample t tests were used to compare the

maximum force at failure of 3 mm versus 6 mm miniscrews.

Independent sample t tests were also used to compare the

differences between the maximum force at failure of the

47

3 mm implants placed in the maxilla and the mandible.

Maximum force at failure was compared between the different

force load groups (0, 600, and 900 g) by means of a one-way

analysis of variance (ANOVA). Loaded (0g) and unloaded

(600g, 900g) implant groups were compared using independent

t tests, and, due to small sample sizes, a non-parametric

equivalent of the t test (Mann-Whitney U) was also

performed. The results of both tests are reported here.

A measurement of millimeters of bone engaged by each

miniscrew was used to create the following groups: 0-1 mm,

1-2 mm, 2-3 mm and 3 mm+. The maximum force at failure

between these groups was compared. Due to the small sample

size of individual groups, a non-parametric test analogous

to ANOVA (Kruskal-Wallis) was used in this comparison. A

manual calculation using a Scheffé like test was then

performed. This post hoc test allows intergroup

differences to be identified by ranking group means against

a calculated chi squared statistic. A critical value for

significance was determined by calculation. Two group

comparisons were evaluated against the critical value and

deemed significant if in excess of it.

In order to further investigate the relationship

between maximum force at failure and the depth of the

miniscrews in bone, a Pearson Correlation test was

48

completed using raw millimetric measurements. All

statistics were performed with α = 0.05. Descriptive

statistics, independent sample t tests, ANOVA, Mann-Whitney

U, Kruskal Wallis, and Pearson correlation tests were

executed with the SPSS statistical program, version 14.0

(SPSS Incorporated, Chicago, IL).

Results

Miniscrew Failures

During the six week course of force application, ten

3 mm experimental, and five 3 mm control miniscrews failed.

Twelve of the miniscrews that failed (80%) had been placed

in the mandible. Two of the 6 mm miniscrews were

compromised during dissection of the bony segments and were

rendered unsuitable for testing. These losses resulted in

the exclusion of 17 of the originally placed miniscrews.

The 43 implants remaining in the mouths of the five dogs

served as the sample for this study. Thirty 3 mm

miniscrews and thirteen 6 mm miniscrews were subjected to

shear force testing. All of the 6 mm miniscrews and 12 of

the 3 mm miniscrews were located in the palate, while 18 of

the remaining 3 mm implants were in the mandible. A

49

summary of the type and location of the remaining implants

is presented in Table 3.1.

Table 3.1: Type, Location and Load Category of Remaining Miniscrews Maxilla Mandible Type of Implant Loaded Control

Loaded Control Total

3 mm 8 4 12 6 30 6 mm 9 4 0 0 13 Total 17 8 12 6 43

During shear testing, thirteen miniscrews fractured

at the bone level. Two 3 mm miniscrew (10%) and eleven 6

mm miniscrews (85%) broke. Only two of the thirteen 6 mm

implants subjected to shear testing survived the process

without fracturing. The miniscrews fractured at shear

force levels ranging from 26 to 126 N. All the miniscrews

that fractured during testing were included in the

statistical analysis, as failure of either the bone or the

screw constituted the maximum shear force tolerance.

Maximum Shear Force Measurements

The maximum shear force at failure of 6 mm miniscrew

implants was significantly higher than that of the 3 mm

50

implants (Table 3.2) (p<0.05). When comparing 3 mm

implants in the maxilla versus those in the mandible, there

was no significant difference. A comparison of the

implants loaded with 0, 600 and 900g of force revealed that

these differing force loads resulted in no significant

difference in maximum force at failure.

Table 3.2: Maximum Shear Force at Failure in Newtons (N)

Type of Implant Group N Maximum Force at Failure

Mean S.E. Min Max (N) (N) (N) (N) All 30 31.90 4.08 5.00 99.00 Loaded 20 37.33 5.49 5.00 99.00

3 mm Control 10 21.06 3.82 7.00 36.25 Maxilla 12 39.70 8.79 5.00 99.00 Mandible 18 26.71 3.18 7.00 58.00 All 13 53.00* 8.33 26.00 126.00

6 mm Loaded 9 55.47 11.59 26.00 126.00 Control 4 47.44* 9.08 28.00 67.12 All 43 38.28 4.03 5.00 126.003mm and 6mm Loaded 29 42.96 5.34 5.00 126.00

Control 14 28.60 4.85 7.00 67.12

(* Maximum force at failure significantly higher than similar groups)

According to the results of both the independent t

test and the non-parametric Mann-Whitney U test,

immediately loaded and control implants showed no

51

significant difference in force at failure (Table 3.3).

Further, when loaded and unloaded miniscrews of the same

length were compared, there were no significant differences

in the force required to cause failure.

Table 3.3: Mann-Whitney U Test Results of Loaded versus Control Implants

Loaded Unloaded Mann-Whitney

N Mean S.D.

N Mean S.D.

Z P

29 42.96 28.75 14 28.60 18.16 -1.50 0.13 Maximum Force

at Failure

Table 3.4: Kruskal-Wallis and Scheffe´ Post Hoc Results of Maximum Force at Failure by Depth of Screw in Bone

KW Group N Mean Median Range

Χ2 p

0.0-1.0 mm 4 9.31 7.25 12.75 1.0-2.0 mm 20 31.23 29.48 91.25 2.0-3.0 mm 7 46.18 58.00* 62.25

14.70 0.002

3.0 mm+ 12 55.08 47.31* 100.00

Post hoc Scheffe´ test:

Critical Value: 24.1 Group 0-1 mm 1-2 mm 2-3 mm 3 mm+

12 13.5 25.5* 35*

*p<0.05

52

A significant difference was determined to be

present between the groups formed by millimetric

measurements of bony purchase. The groups with 2-3 mm and

3 mm+ of bony purchase showed a significantly higher shear

force at failure than the other 2 groups (Table 3.4).

There was, however, no significant difference between the

2-3 mm group and the 3 mm+ group. Shear force at failure

showed a moderately strong correlation (r=0.57) with the

depth of the miniscrew in bone.

Discussion

Although axial pull-out tests are a standard method

for testing screws, tangential or shear loading more

closely mimics clinical orthodontic loading situations.

Shear tests introduce additional variables not encountered

in direct axial pull-out tests. Bone, for example, has

been shown to be anisotropic; it exhibits different

mechanical properties when loaded on different axes.32 It

cannot be ensured, therefore, that when a shear force is

applied to a miniscrew that the bone will flex or bend in

the same manner or to the same degree with each test. When

miniscrews begin to lean as a result of a lateral force, it

53

is possible that the apex of the screw embedded in the bone

could contact surrounding anatomical structures such as the

opposite cortical plate, or the root of a tooth. These

variables, which are difficult to control, may lead to

variations in the measured shear force loads required to

cause failure.

The variability of shear testing can be looked at

from two contrasting points of view. This method of

testing can be seen as a weakness of the study because it

introduces variables that are difficult to standardize or

reproduce. In contrast, performing this type of test can

be seen as a significant strength due to its clinical

applicability. The bone flexure and anatomic variations

encountered in testing are representative of the actual

situations that exist in clinical practice. For this

reason, the decision was made to perform shear rather than

axial pull-out tests.

One specific aim of this study was to quantify the

maximum shear resistance of miniscrew implants. The

miniscrews in this study were of two different lengths,

were placed in different locations in the jaws, and were

subjected to three differing force loads. As a result of

testing these miniscrews, a range of mean shear forces at

failure (21.1-55.5 N) was determined. Even though studies

54

performed on animals may not directly correlate to human

studies, the desire was to provide clinicians with an

estimate of the lateral forces that miniscrews can

withstand prior to failure. Forces required for

orthodontic tooth movement, depending on the type of

movement desired, can range from 0.3 to 4.0 N.33 Orthopedic

appliances such as headgear and palatal expanders exert

forces ranging from 4 to 17 N.34,35 The shear forces

withstood by the miniscrews in this study are higher than

the forces typically used to produce orthodontic and even

orthopedic effects.

The average shear force at failure of 3 mm and 6 mm

miniscrews in this study was 31.9 N and 53.0 N,

respectively. Only one other study has reported results of

shear force testing to which these values can be compared.

Pickard placed miniscrews in human cadaver mandibles and

determined the mean shear force at failure to be 138 N.27

When comparing the results of the current study with those

reported by Pickard, there is a difference of roughly 90 N.

There are several possible reasons for this difference.

The depth of miniscrew insertion likely varied

considerably between the studies. The miniscrews in this

study were placed through the soft tissue with no reflected

flap, and upon dissection of the soft tissue revealed

55

average insertion depths of 1.6 mm and 3.9 mm for 3 mm and

6 mm miniscrews, respectively. Pickard placed implants

directly into bone that had been denuded of all soft

tissue. This likely allowed improved visibility and

complete insertion of the threaded portion of the

miniscrews. Increasing the depth of insertion can affect

shear force resistance by increasing the surface area of

the screw in contact with bone. Theoretically, additional

surface area in contact with cortical and medullary bone

leads to increased retention of the miniscrew. Had flaps

been laid to ensure that the miniscrews in this study were

placed completely into bone, the maximum force at failure

would likely be higher.

Another factor that may have contributed to the

differing results relates to bicortical engagement.

Pickard reported that the majority of the miniscrews placed

in the mandible contacted the lingual cortical plate on

placement. Embedding the apex of a miniscrew in cortical

bone may add significant resistance to the lateral

displacement of the miniscrew by restricting the apex from

swinging free and allowing earlier failure. It is likely

that this type of bicortical engagement could lead to a

dramatic increase in the force required to cause failure.

No miniscrews in the current study were found (by

56

radiographic and visual examination of sectioned implant

sites) to have engaged two cortical plates.

All but two of the 6 mm miniscrews subjected to

shear testing fractured at the bone level prior to leaning

or pulling free from the bone. It is likely that the mean

shear force resistance for this group of implants would

have been higher had they not fractured. The present study

demonstrated that the limiting parameter for miniscrews

measuring 6 mm in length and 1.3 mm in diameter was not the

ability of the bone to withstand the applied force, but the

strength of the screw itself.

Another possibility for the differences noted in

shear force values is the healing response that takes place

around implants. Because Pickard placed screws in non-

living bone, no such response occurred prior to testing.

In the current study, however, the biologic response to

implant placement was allowed to follow its course for the

duration of the six week testing period. Specifically

related to implants with no applied force, previous

histological studies have reported increased bone

remodeling at the implant-bone interface.36,37 It has been

demonstrated that bone within 1 mm of an implant surface

undergoes a sustained elevated remodeling rate resulting in

a lower microhardness.38 This elevated remodeling rate

57

prevents the surrounding bone from fully mineralizing, and

can lead to a decrease in stability. Due to the fact that

this process is not complete within 6 weeks of implant

insertion,29 this feature may have resulted in a lowered

resistance to shear forces, particularly in the control

implants.

Though the difference was not statistically

significant, the miniscrews in this study that were

immediately loaded had a higher shear force at failure than

did the control miniscrews. This could be due to the

difference in the bone healing response around implants

with and without a constant laterally applied force.

Histological studies have shown a higher rate of

cortication, and an overall higher percentage of bony

contact on loaded implants.39,40 Wehrbein et al. have shown

that after a 10 week healing period, an orthodontic force

applied to dental implants induces marginal bone apposition

adjacent to the implants41 and can increase implant

stability.42 The increased force required to cause failure

in the loaded implants is likely the result of the higher

density of the bone surrounding the implant and the

increased osseous contact with the miniscrews. The results

of this comparison support the conclusions made by previous

58

authors concerning the increased stability of miniscrews

subjected to an orthodontic force load.

The results of this study support the notion that

the cortical plate plays a critical role in providing

stability to miniscrews. Maximum shear force resistance

was significantly enhanced in those implants that fully

engaged the cortical plate. This is evidenced by the

results of the comparison of failure force by insertion

depth. The average cortical plate thickness for all sites

in all of the dogs used in this study was 2.1 mm. The mean

force required to cause failure of all miniscrews engaging

less than 2 mm of bone was 27 N. The miniscrews that

engaged one additional millimeter of cortical plate for a

total of 2-3 mm of insertion depth showed a mean increase

of 15 N of resistance. This is the most likely explanation

of why 6 mm miniscrews in this study showed a significantly

higher resistance to shear forces than the 3 mm

miniscrews. Soft tissue thickness prevented full

engagement of 3 mm implants resulting in an average

insertion depth of only 1.6 mm. On average, the 3 mm

implants did not engage the complete thickness of the

cortical bone, and thus did not receive the full benefit of

the stability it provides. Although the 3 mm miniscrews

showed a lower force at failure than the 6 mm miniscrews,

59

it is important to note that they withstood forces well

beyond those typically used in orthodontics.

This study is unique in that it was designed to

investigate shear testing on miniscrews that have been

placed in living bone and allowed to heal. In contrast to

studies in which miniscrews have been placed into non-

living bone, the design of this study made it possible to

consider the biologic response of cortical bone to implant

placement. In addition, this study is also unique in that

it compared the maximum shear resistance of immediately

loaded and control miniscrews. Because orthodontic

miniscrews are primarily loaded in shear, there is a need

for research exploring the variables affecting implant

stability and resistance to failure when a shear force is

applied. The findings of this study indicate that an

immediately applied force is not detrimental to the maximum

shear resistance of miniscrews. This adds to the existing

evidence in the literature that a period of healing is not

essential in order for a miniscrew to provide sufficient

anchorage for orthodontic tooth movement.

60

Conclusions

This study evaluated the maximum shear force that

orthodontic miniscrew implants can withstand, and compared

the affect of immediate loading on the maximum shear

resistance of miniscrew implants of two different lengths,

and with three different applied force loads. A comparison

of shear force at failure was also made according to the

depth of the miniscrews in bone. The following conclusions

can be made from this study:

1) Miniscrews in this study were able to resist shear

forces prior to failure that are higher than the forces

typically used to produce orthodontic and even

orthopedic effects.

2) Miniscrews that have been immediately loaded with either

600 g or 900 g of force show no statistically

significant difference in shear force at failure when

compared to unloaded control miniscrews. The modest

differences that were noted between these two groups are

likely a result of the different healing processes that

occur around implants with and without a constantly

applied force load.

61

3) Miniscrews measuring 6 mm in length showed a

significantly higher shear force at failure than did

miniscrews measuring 3 mm. This difference is likely

due to the increased insertion depth of the 6 mm

miniscrews.

4) The maximum shear force at failure of miniscrew implants

is significantly increased when the entire thickness of

the cortical plate is engaged by the miniscrew.

62

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34. Graber TM, Chung DDB, Aoba JT. Dentofacial orthopedics versus orthodontics. J Am Dent Assoc 1967;75:1145-1157. 35. Isaacson R, Wood J, Ingram A. Forces produced by rapid maxillary expansion. I and II. Angle Orthod 1964;34:256-270. 36. Deguchi T, Takano-Yamamoto T, Kanomi R, Hartsfield JK, Jr., Roberts WE, Garetto LP. The use of small titanium screws for orthodontic anchorage. J Dent Res 2003;82:377-381. 37. Garetto LP, Chen J, Parr JA, Roberts WE. Remodeling dynamics of bone supporting rigidly fixed titanium implants: A histomorphometric comparison in four species including humans. Implant Dent 1995;4:235-243. 38. Huja SS, Roberts WE. Mechanism of osseointegration: Characterization of supporting bone with indentation testing and backscattered imaging. Semin Orthod 2004;10:162-173. 39. Akin-Nergiz N, Nergiz I, Schulz A, Arpak N, Niedermeier W. Reactions of peri-implant tissues to continuous loading of osseointegrated implants. Am J Orthod Dentofacial Orthop 1998;114:292-298. 40. Melsen B, Lang N. Biological reactions of alveolar bone to orthodontic loading of oral implants. Clin Oral Implants Res 2001;12:144-152. 41. Wehrbein H, Diedrich P. Endosseous titanium implants during and after orthodontic load--an experimental study in the dog. Clin Oral Implants Res 1993;4:76-82. 42. Wehrbein H, Yildirim M, Diedrich P. Osteodynamics around orthodontically loaded short maxillary implants. J Orofac Orthop 1999;60:409-415.

66

VITA AUCTORIS

Damen Matthew Caraway was born on September 11, 1975

in Fort Rucker, Alabama to Donald Monroe and Joyce Marie

Caraway. He graduated from Brigham Young University in

Provo, Utah in 2000 with a Bachelor of Science degree in

Zoology with an emphasis in Human Biology. From 2000 to

2004, he attended the University of Texas Health Science

Center at San Antonio Dental School in San Antonio, Texas.

Graduating Suma Cum Laude and with Research Honors, he was

awarded a Doctorate of Dental Surgery in 2004. It is

anticipated that in January of 2007 Damen will graduate

from Saint Louis University with a Master of Science degree

in Dentistry with an emphasis in Orthodontics, and enter

private practice in Colorado.

In May of 1998, Damen married Tiffany Nicole Liddle.

They are the parents of three children; Avery, Gavin and

Sydney.

67