MASTER OF DENTAL SURGERY

EVALUATION OF FACTORS INFLUENCING THE

PLACEMENT OF MINI IMPLANTS IN

INFRAZYGOMATIC CREST REGION

Dissertation submitted to

THE TAMILNADU Dr. M.G.R. MEDICAL UNIVERSITY

In partial fulfillment for the degree of

MASTER OF DENTAL SURGERY

BRANCH V

ORTHODONTICS AND DENTOFACIAL ORTHOPAEDICS

MAY - 2020

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Acknowledgement

ACKNOWLEGEMENT

Firstly, I owe a deep debt of gratitude to Dr. N. R. Krishnaswamy for his

constant guidance and support during my time as a student in the field of Dentistry.

I am humbled by his patience and kindness towards me. His extensive knowledge in

and passion for the subject of Orthodontics have served me in improving my ability

to reason and my rationality. To me, sir has always been awe-inspiring and

impeccable with his word. I am lucky to be his post graduate student.

This thesis would be meaningless without my professor and guide

Dr. Anand M. K. In the position of a guide, he never restricted my freedom of

thought and expression with respect to my thesis subject. Without his guidance and

support I could not have completed this work. As for the writing of the thesis itself,

I am particularly grateful to him for his helpful criticism of early drafts, careful

reading and fact checking. I sincerely thank him for having faith in me at all times.

I really could not have imagined a better guide than him.

I sincerely thank my professors, Dr. Shakeel, Dr. Sriram, Dr. Jayakumar,

Dr. Rekha, Dr.Shobbana, Dr. Kavitha, Dr. Premalatha, Dr. Bharath and

Dr. Divyalakshmi for their enthusiasm, competence and drive that helped me

improve my understanding of the art and science of Orthodontics.

I am extremely thankful to Dr.Rooban Thavarajah, Professor, Dept. of Oral

Pathology, for his willingness to help me with the statistical part of thesis. It was

incredibly kind and generous of him to do so.

I extend my gratitude to my fellow students, both past and present, for being

extremely helpful, friendly and co-operative.

I thank all the support staff of the department for their timely help.

CONTENTS

S .No. TITLE PAGE NO

1. INTRODUCTION 1

2. AIM AND HYPOTHESIS 4

3. REVIEW OF LITERATURE 5

4. MATERIALS & METHODS

6. RESULTS

8. DISCUSSION

9. SUMMARY AND CONCLUSION

10. BIBLIOGRAPHY

11. ANNEXURES -

Introduction

Introduction

1

INTRODUCTION

Usage of mini screw implant as a component of skeletal anchorage has surged

over the past decade. The reason behind this paradigm shift is the versatility of

a mini-implant, helping the clinician achieve various desired orthodontic tooth

movements (intrusion, extrusion, retraction,en-masse distalisation, protraction)

during conventional fixed appliance therapy, as well as orthopaedic jaw

movements during growth modification treatment1. Consequently, the sites of

mini-implant placement are many but have been broadly classified, based on

the area of the jaw bone and in relation to tooth roots, into (i) inter-radicular

and (ii) extra-alveolar2.

Extra alveolar site mini-implants have advantages over inter-radicular ones;

i. They are far removed from the path of orthodontic tooth movement

ii. Relocation of the implant in order to avoid root contact during

treatment, a feature of inter-radicular implant, can be prevented

iii. the extra alveolar sites (infrazygomatic crest, mandibular buccal shelf,

mandibular ramus, palate, etc) represent basal bone, whose bone

quality is usually better that inter alveolar bone sites3

Among the extra alveolar sites, the infrazygomatic crest is a tricky area for

mini-implant placement. This buccal process of maxilla that ascends to meet

the zygomatic bone, usually originates lateral to the roots of the first maxillary

molar. Although deemed an extra alveolar site, the infrazygomatic crest, a

Introduction

2

curved pillar of cortical bone, has insufficient thickness for an infrazygomatic

crest mini-implant that is available in 2 mm diameter and varying lenghts of 8,

10 and12 mm, as the infrazygomatic crest forms the lateral boundary to the

maxillary sinus, and a mini-implant of any of the aforementioned lengths is

sure to penetrate deep into the maxillary sinus. As a result, clinicians routinely

place an infrazygomatic crest implant anywhere between 5 mm to 11 mm from

the crestal bone or marginal gingiva. Considering root length of maxillary

molars to average 12 or 13 mm,4 infrazygomatic crest implant will end up in

an inter radicular area or approximating molar root. In order to avoid root

contact during placement and during tooth movemnet clinicians routinely take

an oblique or angulated approach during insertion of the IZC implant.

Eric J. W. Liou et al5 recommended placing the IZC implant 14 - 16 mm

from the occlusal plane i.e. 5 mm to 7 mm from the alveolar crest, at an angle

of 55° - 70° in relation to the mesiobuccal root of the maxillary first molar. On

the other hand, John Lin and Eugene Roberts6 suggest placing the implant in

relation maxillary second molar.

Regardless, mini-implant failure, characterised by losening or lack of primary

stability, remains a major hindrance to the successful completion of

orthodontic treatment. Like every other implant, the infrazygomatic crest

implant too is subject to various factors that influence or affect its overall

stability. Those factors can be patient related (ex. Quality and quantity of

bone, systemic conditions, oral hygiene, normal anatomical variations),

implant related (ex. Diameter, length, implant material, number of threads,

Introduction

3

pitch, inner and outer diamaters, implant tip, etc) and clinician related

(clinician skill, on which side of the oral cavity the implant is placed, etc).

It can be reasoned that even with a perfectly designed implant and an

experienced clinician, poor bone quality can lead to implant failure. Moreover,

since mini-implants commonly used in orthodontics are not characterised by

osseointegration, their intial and overall stability is dictated by mechanical

retention that depends upon the amount and quality of bone present to contact

the implant surface. Low quality bone will also have difficulty remodelling in

response to stresses and microdamage brought upon it during implant

insertion.

Furthermore, it is well recognised that bone quality is affected by function, all

over the body8. With regard to the jaws and particularly the infrazygomatic

crest, masticatory function can determine or alter bone density. Since

masticatory function is linked to craniofacial morphology,9 it is reasonable to

assume that different sagittal skeletal patterns can show variation in bone

parameters with regard to the infrazygomatic crest region. Consequently, these

differences, if found, can serve to effect the clinician’s decision during mini-

implant placemnt in the infrazygomatic crest region.

Introduction

4

Aim

Therefore, the aim of the present study is to evaluate the cortical bone

thickness and density of the infrazygomatic crest region in Class I, Class II

and Class III skeletal patterns and to elavuate the influence of cortical bone

density on stress distribution in peri-implant bone.

Null hypothesis

There is no difference in cortical bone thickness and density between Class I,

Class II and Class III skeletal patterns with respect to infrazygomatic crest

region.

Objectives

i. To evaluate cortical bone thickness and cortical bone density of the

infrazygomatic crest bone on cone-beam computed tomography

scans of patients with Class I, Class II and Class III skeletal

pattern/malocclusion.

ii. To analyse the influence of cortical bone density on stress

distribution in peri-implant bone via finite element method.

Review of Literature


5

REVIEW OF LITERATURE

With the introduction of mini implants to orthodontics, qualitative and

quantitative assessments of the maxillary and mandibular bones spiked,

aiming to determine those bone related parameters that would positively or

negatively influence overall stability of the implant both at inter-radicular and

extra alveolar sites. Furthermore, finite element analysis studies provide an

insight into bone material properties and their influence on stress distribution

in peri-implant bone.

Creekmore and Eklund12

;1983 , attempted to determine if a metal implant

could withstand a constant force over a long period of time of adequate

magnitude to depress an entire anterior maxillary dentition without becoming

loose, infected, painful, or pathologic. They inserted a surgical vitallium bone

screw anterior nasal spine. They noted that the bone screw did not move

during treatment and was not mobile at the time it was removed.

Costa et al.14

; 1998 reported the use of miniscrews for orthodontic anchorage

in 14 patients. Without a soft tissue flap, a 1.5-mm-diameter pilot hole was

placed under local anesthesia followed by placement of a 2.0-mm miniscrew,

which was immediately loaded orthodontically. Importantly, only 2 of 16

miniscrews loosened and were lost before completion of orthodontic

treatment. They concluded that stability is limited after loading with torsion.


6

Kanomi13

; 1997, used an implant, made from a mini-bone screw used to fix

bone plate for plastic reconstruction. The mini-implant was 1.2 * 6 mm,

introduced between the lower central incisors for intrusion. The treatment was

uneventful and he reported that the mini-implant is too small to cause

irreversible damage. Although the implant used by him was an osseointegrated

implant.

Hugo De Clerck25

; 2002, developed a Zygoma Anchorage System (ZAS) in

which the miniscrews were placed at a safe distance from the roots of the

upper molars. Because of its location and its solid bone structure, the inferior

border of the zygomaticomaxillary buttress, between the first and second

molars, was chosen as the implantsite. He reported that combining three

miniscrews with a titanium miniplate can bring the point of force application

near the center of resistance of the first permanent molar. Using this system he

corrected Class II malocclusion by intruding and retraction the upper

dentition.

Eric. J.W. Liou et al.5; 2007, measured the thickness of the infrazygomatic

(IZ) crest above the maxillary first molar at different angles and positions to

the maxillary occlusal plane, with a view to derive clinical implications and

guidance for inserting miniscrews in the IZ crest without injuring the

mesiobuccal root of the maxillary first molar. Bone thickness of the IZ crest

above the maxillary first molar is 5 to 9 mm, when it is measured at 40° to 75°

to the maxillary occlusal plane and 13 to 17 mm above the maxillary occlusal

https://www.jco-online.com/archive/1997/11/763-mini-implant-for-orthodontic-anchorage/#Footnotes


7

plane. The clinical implication for miniscrew insertion in the IZ crest of adults

is 14 to 16 mm above the maxillary occlusal plane and the maxillary first

molar, and at an angle of 55° to 70° to the maxillary occlusal plane.

Roberts and Lin6; 2017, citing that it was not clear whether IZC 6 or 7 was

the preferred site from an anatomic perspective according to Eric Liou,

suggested that because the alveolar bone is thicker on the buccal surface of the

second molar based on a study by Chen et al; 2008, the IZC 7 site is usually

preferable for TADs. They reported extra radicular placement of TAD is more

predictable above the mesiobuccal or distobuccal roots of the second molar

(U7)

Chen et al.2;2010, reported that in their study, the average bone depths were

around or > 10 mm, except for the IZ crest and midpalatal region; the average

cortical bone thicknesses were around or > 2 mm, except for the incisive fossa,

IZ crest, and midpalatal region.The bone depth of the IZ crest should be at

least 6 mm to adequately sustain a miniscrew throughout treatment. The

average bone depth of the IZ crest in this study was 5.89 mm; the bone depth

of the IZ crest in the male group was longer than 6 mm, but not that in the

female group. It was supposed that the variation in IZ crest thickness might be

due to variations in the maxillary sinus among individuals.

Seipel23

;1948, observed no strict division of trajectories from the premolar

area even if the majority of the architectural fibres from the first premolar turn

towards the canine crest and from the second premolar towards the alveolo-


8

zygomatic crest which is a corresponding landmark in the molar region. The

ascending architecture of the anterior molar region is concentrated into the

alveolo-zygomatic crest, which also receives contributions from the premolar

region. The first molar has been judged as being in a normal position when

placed under the key-ridge of the alveolo-zygomatic crest. Behind the alveolo-

zygomatic crest there are some ascending trajectories from the posterior molar

region, turning towards the inner posterior wall of the zygomatic process and

by way of the post zygomatic fossa reaching the zygomatico-temporal region.

Otherwise the posterior molar region and the tuber maxillae only rarely exhibit

a definite architectural arrangement when tested with the crevice-line method.

Banri Endo30

;1965, In the facial skeleton the direction of the axis of the

principal strain which is identical with the direction of the principal stress

changes with the shift of the load along the dental arch. It might be surmized,

therefore, that the stress trajectories exert any influence over the formation of

the split-line patterns in the facial skeleton, although both the phenomena do

not exactly coincide. The infero-anterior part of the maxilla is relatively weak

among various parts of the facial skeleton. This fact may suggest that the

human facial skeleton is rather adapted to the use of the posterior teeth.

Farnsworth et al45

; 2011, reported that no significant interaction and no

significant sex differences in cortical thickness. There were significant

differences between adolescents and adults; adult cortices were significantly

thicker in all areas except the infrazygomatic crest. In the maxilla, age group


9

differences in cortical thickness were greater at the 5-6 than at the 6-7

sites.Variability between subjects was great, with differences in cortical bone

thicknesses ranging from 0.2 to 1.8 mm in adolescents and 0.5 to 1.8 mm in

adults. The adolescents and adults in their study showed cortical bone

thicknesss of 1.45 ± 0.39 mm and 1.34 ± 0.24 mm in the infrazygomatic crest

area respectively. This measument was done above the mesio buccal root of

the first molar. If the primary determinant of the age-related differences in

cortical bone thickness were changes in functional capacity, then sex

differences in cortical bone thickness might be expected because males have

larger bite forces and masticatory muscles than do females. However, we

found no sex differences in cortical thickness in either the maxilla or the

mandible.

Ono et al.44

;2008, investigated cortical bone thickness in the posterior

alveolar regions of the maxilla and mandible in forty-three orthodontic

patients. Cortical bone thickness was measured at 1.0mm intervals in a plane

parallel to the occlusal plane of each tooth from 1mm to 15mm below the level

of the alveolar crest. Overall, average cortical bone thickness ranged from

1.09mm to 2.12mm in the maxilla, and from 1.59mm to 3.03mm in the

mandible, with maxillary cortical bone thickness significantly thinner than that

observed in the mandible. More specifically, mesial to the first molar, average

cortical bone thickness ranged from 1.09mm to 1.62mm in the maxilla


10

Melsen and Costa24

; 1999, used 2mm diameter and 8mm long titanium-

vanadium mini-implants into the infrazygomatic crest and mandibular

symphysis of monkeys. Their histological examination clearly demonstrated

that the type of bone and the locale into which the screws were inserted

affected their stability.

Borges et al.48

; 2010 assessed maxillary and mandibular alveolar and basal

bone density in Hounsfield units In the maxilla, the greatest bone density was

found between the premolars in the buccal cortical bone of the alveolar region.

The maxillary tuberosity was the region with the lowest bone density..

Chugh et al.21

; 2013, summarised the results of studies relating to bone

density and implant stability. They concluded that knowledge of low density

sites prior to implant placement allows clinician to use longer implant in these

areas to improve retention. In areas of high bone density, use of pre-drilling

method avoids the breakage of implant. Sufficient irrigation should be done to

prevent overheating of bone in that area. Immediate loading of mini-implants

is possible because of higher bone density in all the areas of cortical bone. In

areas of low bone density, it is necessary to augment the anchorage as per

requirement

Peterson et al.11

; 2006, attempted to determine regional variability of material

properties in the dentate maxilla. Cortical samples were removed from 15 sites

of 15 adult dentate fresh-frozen maxillas. Cortical thickness, density, elastic

properties, and the direction of greatest stiffness were obtained. They


11

hypothesized that there are important regional differences within the maxilla

that correspond with variations in function and development. They noted that

the palatal site between the canine and first premolar, the alveolar sites

supporting the teeth tended to be thicker than the other maxillary sites. The

thickest sites were buccally and lingually near the canine ( 2.3 mm; site, 2.4

mm) The thinnest sites were found at the pterygomaxillary process and

alveolar bone above the third molar. Overall, where cortical bone was thin, its

density was high. For instance, the infraorbital sites (12–15) on the body of the

maxilla (Fig. 4) ranged in thickness from 1.1 to 1.5 mm, yet were high in

density (>1.80 g/cm3 ). Overall, the densest site 15 (1.90 g/cm3 ) was at the

zygomaticomaxillary suture, where the high density contrasted with the

thinness of the cortex (1.1 mm;). Cortical bone in the alveolar region tends to

be thicker, less dense, and less stiff. Cortical bone from the body of the

maxilla is thinner, denser, and stiffer. Palatal cortical bone is intermediate in

some features but overall is more similar to cortical bone from the alveolar

region. The principal axes of stiffness varied regionally and were not as

consistent as those in the mandible. Cortical bones near the incisors and

canines has greater thickness than at other maxillary alveolar sites, but its

density and stiffness are intermediate. The area above the second molar and

under the root of the zygomatic process has the densest and stiffest cortical

bone in the alveolar area. This is not surprising, as we might expect higher

loads in this area due to the mechanical advantage of the muscles of

mastication that results in larger occlusal forces in this area.


12

Ohiomoba et al.46

;2017, Cortical bone density and thickness significantly

increased from the coronal (2 mm) to the apical (8 mm) regions of the alveolar

bone . At 8 mm from the alveolar crest, interradicular buccal cortical bone was

thickest (1 mm) and densest (1395 Hounsfield units) between the first and

second molars. Gender was not significantly associated with bone thickness.

Masumoto et al.37

; 2001, evaluated the relationship between different facial

types, molar inclination and thickness of mandibular cortical bone in dry

skulls of 31 Japanese individuals. The results of this study provide evidence

that buccal cortical bone thickness is associated with the facial type. A thicker

buccal cortical bone is associated with a smaller gonial angle and mandibular

plane angle. They suggest that the thickness of the cortical bone seems to be

influenced by masticatory function and mandibular movements.

Chen et al.40

; 2010, In their study of 20 skeletal class II (ANB > 2.0 SD) adult

females (18–42 years old), subdivided into three groups by the FMA with

cephalometric analyses: high FMA group: FMA: 37.5 2.0, 7 cases . Average

FMA group: FMA: 28.8 1.8, 8 cases and Low FMA group: FMA: 20.6 2.5, 5

cases, observed that the cortical bone thickness was not significantly different

in the five measured areas. The upper posterior area and the infrazygomatic

crest area showed no significant difference among different FMA groups

Fulya Ozdemir et al.39

; 2014 quantitatively evaluated the cortical bone

densities of the maxillary and mandibular alveolar processes in adults with

different vertical facial types using cone-beam computed tomography. They


13

concluded that patients with the hyperdivergent facial type tend to have less-

dense buccal cortical bone in the maxillary and mandibular alveolar processes

than those patients with other facial types.

Horner et al.38

; 2012, analysed cortical bone thickness in 30 hypodivergent

and 27 hyperdivergent subjects. Cortical bone thickness, alveolar ridge

thickness and medullary bone thickness were evaluated and compared. They

concluded that cortical bone tends to be thicker in hypodivergent subjects than

in hyperdivergent subjects. Medullary space thickness is largely unaffected by

facial divergence and Cortical bone was 0.08 to 0.64 mm thicker in

hypodivergent than hyperdivergent subjects

NM Al-Jaf et al.36

; 2018, conducted a study to assess buccal cortical bone

thickness of the alveolar process in the maxilla and mandible from CBCT

scans in 94 adult subjects with Class I, II and III sagittal jaw relationships and

normal vertical relationship. Buccal cortical thickness was measured in the

alveolar process of the maxilla and mandible from distal of canine to mesial of

second molar at two different vertical levels (6, and 8mm) from the

cementoenamel junction (CEJ). They concluded that the maxilla shows a

different pattern for each sagittal relation. In Class I subjects, a slightly higher

mean value for cortical thickness was detected posteriorly (between the

molars), but further analysis showed no significant difference between sites.

For Class II, and III, the sites with highest cortical mean values were located


14

more anteriorly and no significant difference between buccal cortical thickness

at 6mm and 8mm vertical level.

Rossi et al.47

; 2017 found no significant difference in cortical bone thickness

between the three skeletal malocclusions (no difference between males and

females nor between adults and adolescents). Although cortical bone density

of maxilla was found to be lower in Class III adult females in comparison to

Class I and Class II skeletal pattern, the authors claimed that they could not

deduce any clinical relevance from their data. there was a linear increase of

cortical bone thickness from crest to base and from anterior to posterior

regions in both alveolar crests; alveolar cortical bone showed a higher density

and thickness in the mandible than in the maxilla; areas showing at least 1

mm thickness for miniplate fixation in the maxilla were found at the

infrazygomatic crest, lateral to the pyriform aperture and also at the alveolar

bone area from the canine to the first molar.

Bakke27

;2006, observed that maximum bite force varies with skeletal

craniofacial morphology, decreasing with increasing vertical facial

relationships, the ratio between anterior and posterior facial height,

mandibular inclination, and gonial angle. It has been proposed that bite force

reflects the geometry of the lever system of the mandible. The number of

occlusal contacts is a stronger determinant of muscle action and bite force than

the number of teeth present. The occlusal contacts have been shown to

determine 10% to 20% of the variation of maximum bite force in adults, and


15

the association between maximum bite force and contacts is higher in the

posterior region than in the anterior region. One way to explain the correlation

between occlusal contacts and bite force is that “good” occlusal support (ie,

force distributed over many teeth) may result in stronger or more active jaw

elevator muscles that can develop higher bite force.

Miralles et al.9;1991, recorded no significant differences in maximal

clenching force between the three sagittal skeletal patterns, although postural

resting force of the muscles in Class III skeletal pattern was greater compared

to Class I and Class II skeletal patterns.

Bae et al.29

;2017, The results of this study indicated that masticatory

efficiency was the highest in patients with Angle’s Class I malocclusion,

followed by those with Angle’s Class II and Angle’s Class III malocclusions.

Moreover, a weak positive correlation was observed between masticatory

efficiency and the occlusal contact area.

Consolaro and Romano3;2014, summarized the prevailing hypotheses for the

failure of min implants. They highlighted the importance of implant

installation or placement sites on the failure of mini implants

Motoyoshi et al.61

; 2006, noted that the success rate for implants with an IPT

of more than 5 N cm and less than 10 N cm was significantly higher than that

for implants with IPT 5 N cm or less, and more than 10 N cm in the maxilla


16

B. Wilmes and Dreschner.59

; 2011, in their study involving 600 mini implant

insertions into pig compacta ranging in thickness from 0.5 to 2.5mm, observed

that cortical bone thickness has a great impact on insertion torque, and

therefore on primary stability of the implant. Owing to the implant fractures

observed at torques above 23 Ncm the authors advised generally to limit

insertion torques to a maximum of 20 Ncm to avoid implant fractures and

excessive bone stresses. Insertion torques for the 2.0 mm * 10 mm screws in

their study reached high torque values in bone with a thick compacta.

Motoyoshi et al.62

(2010). The relationships among placement and removal

torques, placement period, age, sex, and cortical bone thickness measured

using 134 implants showed that placement torque was significantly related to

age and cortical bone thickness in the maxilla, whereas removal torque was

not significantly related to placement period, age, sex, or cortical bone

thickness.

Deguchi et al.43

2012, cortical bone thickness resulted in approximately 1.5

times as much at 30 degrees compared with 90 degrees Significantly more

distance from the intercortical bone surface to the root surface was observed at

the lingual region than at the buccal region mesial to the first molar. At the

distal of the first mandibular molar, significantly more distance was observed

compared to that in the mesial, and also compared with both distal and mesial

in the maxillary first molar. There was significantly more distance in root

proximity in the mesial area than in distal area at the first molar, and


17

significantly more distance was observed at the occlusal level than at the

apical level. The safest location for placing miniscrews might be mesial or

distal to the first molar, and an acceptable size of the miniscrew is less than

approximately 1.5 mm in diameter and approximately 6 to 8 mm in length.

Uribe et al.22

; 2015, investigated the failure rate of mini-implants placed in

the IZ region was the rationale behind the study.Data from a total of 30

consecutive patients (mean age 22.2 ± 11 years) who had 55 IZ mini-implants

placed was collected. All mini-implants were placed at an approximate angle

of 40° to 70° to maxillary occlusal plane in the IZ area by palpating the “key

ridge” above the first permanent molar. The findings of our study show that IZ

mini-implants have slightly lower success rate (78.2 %) than that of the

average mini-implant.This is in contrast to Liou et al.’s findings who reported

100 % success of mini-implants placed in this region.One important variable

for the different success rates of mini-implants is skeletal facial pattern.

Moon et al.10

;2008, found similar success rates (77 %) to those of our study

for mini-implants placed interdentally in patients with high Frankfurt-

mandibular plane angle (FMA). Majority of our patients had average FMA

and mandibular plane angle (MPA) as 31.3° and 39.9°, respectively. This

finding is also in agreement with a study by Miyawaki et al. who also

reported that mini-implants placed in patients with high MPA had lower

success rates (72.7 %).


18

Marquezan et al.49

; 2011 evaluated bone density in two bovine pelvic regions

and verify the primary stability of miniscrews inserted into them. However,

the miniscrew primary stability was not different when varying the bone type.

Insertion torque and pull out strength were not influenced by these differences

in bone density when cortical thickness was about 1 mm thick.

Miyawaki et al17

;2003, found 1-year success rate of screws with 1.0-mm

diameter was significantly less than that of other screws with 1.5-mm or 2.3-

mm diameter or than that of miniplates. A high mandibular plane angle and

inflammation of peri-implant tissue after implantation were risk factors for

mobility of screws. However, they could not detect a significant association

between the success rate and the following variables: screw length, kind of

placement surgery, immediate loading, location of implantation, age, gender,

crowding of teeth, anteroposterior jaw base relationship, controlled

periodontitis, and temporomandibular disorder symptoms. They concluded

that the diameter of a screw of 1.0 mm or less, inflammation of the

peri-implant tissue, and a high mandibular plane angle (ie, thin cortical bone),

were associated with the mobility (ie, failure) of the titanium screw placed into

the buccal alveolar bone of the posterior region for orthodontic anchorage.

The loosening and failure of MSIs are major limitations for their use.

Important risk factors for MSI failure include placement in the mandible,

placement in thin (\1 mm) cortical bone, and placement torque values outside

the 5 to 10 Ncm range. According to Costa et al and Miyawaki et al, cortical


19

bone quality and quantity are major factors associated with primary stability of

MSIs, probably because it is achieved by mechanical retention rather than

osseointegration. Wilmes et al found that cortical bone thickness has a strong

effect on the primary stability of MSIs. Placement torque and pullout strength

of MSIs have also been correlated with cortical bone thickness. Clinically,

MSI failures have been reported to result from thin cortical bone. Miyamoto

et al suggested that cortical bone thickness plays a greater role in determining

stability than implant length. It is unclear at this point which property

(thickness or density) is more relevant to OMI survivability. Ohiomoba et al.;

highlighted this conflict in their study, where average density and thickness

values are not directly correlated; and so, left it to the clinician's discretion to

reconcile this difference when making treatment decisions.

Campos et al.32

;2014, concluded that owing to different configurations of

image acquisition, which may be specific for each CBCT device or altered for

several applications of these examinations in dentistry, the correction methods

of gray values obtained in CBCT still do not generate consistent values which

are independent of the devices and their configurations or of the scanned

objects

Molteni;33

2013 The basis of the HU scale, its correlation with measured

computed tomography (CT) numbers, and the limitations in the accuracy of

such correlation due to artifacts are discussed. Rendering of tissue densities

based on HU values of two CBCT systems [NewTom VGi and Hyperion X9,


20

respectively large and small field of view (FOV)] are measured using a

phantom. Data produced from small FOV CBCT acquisition are generally less

affected by artifacts compared with large FOV CBCT. Artifacts challenge the

accurate conversion of density values into HUs. Care should be taken when

interpreting quantitative density measurements obtained with CBCT. With

more advanced software and methods, it may be possible to improve the

consistency and accuracy of density measurements.

Harold Frost;8 1987, showed that bone strains in or above the 1500-3000

microstrain range cause bone modelling to increase cortical bone mass, while

strains below the 100-300 microstrain range release BMU-based remodeling

which then removes existing cortical-endosteal and trabecular bone. That

arrangement provides a dual system in which bone modeling would adapt

bone mass to gross overloading, while BMU-based remodeling would adapt

bone mass to gross underloading, and the above strain ranges would be the

approximate "setpoints" of those responses. The anatomical distribution of

those mechanical usage effects are well known. If circulating agents or disease

changed the effective setpoints of those responses their bone mass effects

should copy the anatomical distribution of the mechanical usage effects

Motoyoshi et al.55

2007, in a bid to verify the clinical threshold for successful

implantation, analysed the biomechanical influences in the bone around the

mini-implant using the finite element method, and examined the differences in

stress distribution according to differences in the CBT. The maximum stress


21

decreased markedly as CBT increased. The stresses in the models with CBT

values of 0.5 and 0.75 mm were approximately 40 and 28 MPa, respectively,

whereas the stresses were less than 25 MPa in the models with CBT > 1.0 mm.

The authors hypothesized that the reason why the stress was highest in 2 mm

thick cortical bone was related to the buffer function of cancellous bone. To

verify this, they calculated the total stress on the section in the middle of the

implant hole in the cancellous bone for each model, and found that it increased

with thinner cortical bone. When the total bone thickness is fixed, the

cancellous bone becomes thicker as the cortical bone becomes thinner, and the

load in the thicker cancellous bone supporting the implant body increases,

reducing the load in the thin cortical bone. The maximum stress would then be

less in the model of 1 mm cortical bone than in 2 mm bone.

Lakshmikantha et al.63

; 2019, Studied microdamage to cortical bone at

insertion site between self-drilling vs self-tapping miniscrews at different

angles of insertion. In their study, they saw that there is an increase in bone

microdamage following placement of microimplants by the no drill method

and an increase in bone microdamage is seen following placement of

microimplants at an angle to the cortical bone surface. Hence, a better stability

of microimplant can be derived with a microimplant that will be inserted

perpendicular to the cortical bone surface and utilizing a pre-drill before

insertion.


22

Jaffin and Bermin58

; 1991, showed that the quality of bone stands out as the

single greatest determinant in fixture loss. Types I, II, and III bone offer good

strength. Type IV bone has a thin cortex and poor medullary strength with low

trabecular density. Ninety percent of 1,054 implants placed were in Types I, II,

and III bone. Only 3% of these fixtures were lost; of the 10% of the fixtures

placed in Type IV bone, 35% failed. Presurgical determination of Type IV

bone may be one method to decrease implant failure.

Hsu and Chang53

; 2001, summarised that great versatility of an FEM

analysis is contained within a single computer program and the selection of

program type, geometry, boundary conditions, element selection are controlled

by user-prepared input data. The principal difficulty in simulating the

mechanical behavior of dental implants lies in the modeling of human maxilla

and mandible and its response to applied load. Certain assumptions are needed

to make the modeling and solving process possible and these involve many

factors which will potentially influence the accuracy of the FEA results: (1)

detailed geometry of the implant and surrounding bone to be modeled, (2)

boundary conditions, (3) material properties, (4) loading conditions, (5)

interface between bone and implant, (6) convergence test, (7) validation.

Ntolou et al.15

; 2018, analyzed the factors related to the clinical application of

orthodontic mini-implants. Sites of high cortical bone thickness, high

cancellous bone density, sufficient available bone, and thin attached gingiva

are ideal for mini-implant insertion. Extremely thick cortical bone requires


23

attention. In thick cortical bone, shorter mini-implants can be selected. For

sites of low cortical bone thickness and low cancellous bone density, longer

and wider mini-implants are indicated. Very thin cortical bone and very low

cancellous bone density negatively affect the prognosis of mini-implants. Very

narrow implants entail fracture risk. Predrilling is preferred at high bone

quality sites, whereas it is used with caution or even be avoided at low bone

quality sites. Angled placement might be considered to increase bone-to-

implant contact and reduce root injury risk. Loading time depends on insertion

torque. Successful application of mini-implants is based on proper insertion

site and mini-implant characteristics selection, proper insertion, absence of

inflammation, and proper orthodontic loading.

Papageorgiou et al.16

; 2012, reported that from the 4987 miniscrew implants

used in 2281 patients, the overall failure rate was 13.5%. Failures of

miniscrew implants were not associated with patient sex or age and miniscrew

implant insertion side, whereas they were significantly associated with jaw of

insertion.

Chen et al.18

; 2007, reported that for self-tapping mini-implants, the diameter

and the length of the implant should be 0.2 to 0.5 mm larger than the width

and the depth of the bone hole for optimal placement torque. For mini-

implants, healing time is unnecessary. The selection of the tooth-bearing mini-

implant size depends on the bone available.


24

Motoyoshi et al.19

; 2007, reported the success rate was 63.8% in the early-

load group (less than 1-month latent period) of adolescents, 97.2% in the late-

load group (3-month latent period) of adolescents and 91.9% in the adult

group. In measurements of the placement torque in adolescents, the success

rate of the 5-10 N cm group was significantly higher than the other groups

only in the maxilla of the early-load group. Although the optimum torque

could not be defined, a latent period of 3 months before loading is

recommended to improve the success rate of the mini-implant when placed in

the alveolar bone in adolescent patients.

Liou E. J.26

, 2003, Sixteen adult patients with miniscrews (diameter = 2 mm,

length = 17 mm) as the maxillary anchorage were included in this study.

Miniscrews were inserted on the maxillary zygomatic buttress as a direct

anchorage for en masse anterior retraction.. On average, the miniscrews tipped

forward significantly, by 0.4 mm at the screw head. The miniscrews were

extruded and tipped forward (-1.0 to 1.5 mm) in 7 of the 16 patients.

Miniscrews are a stable anchorage but do not remain absolutely stationary

throughout orthodontic loading. They might move according to the

orthodontic loading in some patients. To prevent miniscrews hitting any vital

organs because of displacement, it is recommended that they be placed in a

non-tooth-bearing area that has no foramen, major nerves, or blood vessel

pathways, or in a tooth-bearing area allowing 2 mm of safety clearance

between the miniscrew and dental root.


25

Hagberg. C28

; 1985, Electromyographic (EMG) activity of the superficial

masseter and the anterior temporal muscles versus the bite force was studied in

10 young women. The average bite force between the first molars was 396 N

(Newton). Steeper slopes for the EMG versus force regression curve at high

contraction levels than at low contraction levels for the superficial masseter

muscle may indicate that this muscle has a recruitment pattern that differs

from that of the anterior temporal muscle. There was significantly increased

activity in the descending part of the trapezius muscle mainly during high bite

force levels in half the subjects.

Koc. D et al.31

; 2010, reported that The normal aging process may cause the

loss of muscle force. Indeed, the jaw closing force increases with age and

growth, stays fairly constant from about 20 years to 40 or 50 years of age, and

then declines. In children with permanent dentition between the ages of 6 and

18, bite force has been significantly correlated with age

Razi et al.34

; 2014, noted a strong linear relationship between the gray scale

and HU values in all the systems, which can be attributed to similarity of

effective factors influencing the gray scale and improvement of the image in

the new version of devices.

Hsu et al.35

; 2012, found that dental CBCT provided superior predictions of

cortical bone bending fracture loads than did areal BMD measured using

DXA. Furthermore, strong correlations were found between the BSI

( = vCtBMD×CSMI) and the fracture loads (r = 0.822 and 0.842 for femurs and


26

tibias, respectively). Dental CBCT is a noninvasive method that requires low

radiological dosages to predict bone strength, and might constitute a suitable

alternative to pQCT, especially when frequent radiological examinations must

be conducted within a short time period.

Germec-Cakan et al.41

; 2014, found no difference in cortical bone plate

thickness between Class I, II and III subjects when related to mini-implant

placement sites. As the measurement site moved towards the posterior,

maxillary palatal cortical thickness decreased except in Class III cases, while

mandibular buccal bone thickness increased in all malocclusion groups.

Khumsarn et al.42

; 2016, In both the maxilla and mandible, the mesiodistal

distances, the width of the buccolingual alveolar process, and buccal cortical

bone thickness tended to increase from the CEJ to the apex in both Class I and

Class II skeletal patterns.

Huiskes and Nunamaker51

; 1984, opined that Mechanical stresses, caused by

joint loading, play a key role in the adaption of interface bone and in the

loosening process and loosening and bone resorption is associated with high

peak stresses at the interface in the immediate post-operative stage. In

addition, there appears to be similarity between the local stress patterns and

the bone morphology at the interface if resorption does not occur. Finally, it is

found that implants of high local stiffness generate lower peak stresses in

bone, as compared with low stiffness implants.


27

Suzuki et al.52

; 2011, said that by changing the implantation angle, the ranges

of the maximum stress distribution at the supporting bone were 9.46 to 14.8

MPa in the pin type, and 17.8 to 75.2 MPa in the helical thread type. On the

other hand, the ranges of the maximum stress distribution at the titanium

element were 26.8 to 92.8 MPa in the pin type, and 121 to 382 MPa in the

helical thread type. the maximum stresses observed in all analyzed types and

shapes of miniscrews were under the yield stress of pure titanium and cortical

bone. This indicates that the miniscrews in this study have enough strength to

resist most orthodontic loads.

Ashman et al.54

; 1984, Even though bone is both anisotropic and

heterogeneous, in only a few studies has an attempt been made to either

characterize the degree of anisotropy or to determine the elastic properties of

bone as a function of anatomical position. Perhaps the main reason that this

experimental work has not been done is that the traditional engineering

methods for material property dctcrmination are difficult to apply to bone. Not

only do the overall dimensions of the bone limit the size of the test specimen,

but the heterogeneity of bone requires that the specimens be small in order to

insure that the properties are nearly uniform throughout the test specimen. An

additional problem arises from the anisotropy of bone. This anisotropy

requires that traditional mechanical tests be applied in several different

directions in order to obtain enough information for the calculation ofall of the

independent elastic coefficients.


28

Huang et al.56

; 2002, showed that dental implant installed in type I bone has .

highest resonance frequency. In contrast, the lowest resonance frequency was

found in the type IV model. These results imply that an implant with a lower

interface restriction has a lower resonance frequency.

Li et al.57

; 2007, For a piece of bone under constant uniaxial loading, we can

obtain its density time histories under different levels of loading with a

constant magnitude. Under stresses of 0 and 2 MPa, underload resorption

reduced the bone density, while under stresses of 4, 6, and 8 MPa, the bone

density increased under the stress stimulus. The higher the stress value, the

higher the converged density. Under a stress of 9 MPa, however, the bone

density decreased very quickly because of overload resorption, a result which

cannot be produced by the old model, the bone density under a stress of 4 MPa

changed very little. This is because under that load the density change rate is

near zero. That stress is a critical stress at a density of 1.0 g cm−3. Different

bone densities have different critical stresses.

Trisi et al.60

; 2009, showed that increasing the peak insertion torque reduces

the level of implant micromotion. In addition, micromotion in soft bone was

found to be consistently high, which could lead to the failure of

osseointegration. Thus, immediate functional loading of implants in soft bone

should be considered with caution.

Material and Methods


29

MATERIAL AND METHODS

Cone-beam Computed Tomography (CBCT) scans of 50 patients who had

sought orthodontic treatment at Ragas Dental College and Hospital were

retrospectively selected. The images were procured using Kodak 9500 unit

(Carestream Health, Rochester, NY) with 18.4 * 20.6cm field of view (FOV),

90 kVp, 108 mAs and 0.30 voxel size. The sample was divided into 3 groups:

Class I (n=18), Class II (n=17), Class III (n=15) skeletal patterns based on

ANB angle.

Subjects in Class I group had ANB angle 1°- 4°, in Class II had ANB

angle >5° and Class III had ANB angle < 0°.

CBCT scans were selected based on the following inclusion criteria:

i. Adults between the ages of 20 and 40 years

ii. Permanent dentition

iii. No missing or unerupted teeth

iv. No systemic conditions or bone related pathologies

v. No history of usage of medication altering bone properties

vi. No severe craniofacial syndromes

vii. No periodontal bone loss


30

The CBCT scans were imported into 3D software (version 11.9, Dolphin

Imaging Systems, Chatsworth, Calif ) for analysis in digital imaging and

communications in medicine (DICOM) file format.

Cortical bone thickness and density were measured only on one side/quadrant

as it was shown in a previous study by Moon et al10

that there was no

significant difference in cortical bone thickness between the sides of a jaw.

Infrazygomatic crest cortical bone thickness and density were measured at the

following five antero-posterior regions: (Figure 1)

U5-U6 (between second premolar and first molar)

U6 (mid-root region of first molar)

U6-U7 (between first molar and second molar)

U7 (mid-root region of second molar)

U7-U8 (distal to second molar)

At each of these abovementioned regions, cortical bone thickness and density

were further evaluated at increasing distances to the alveolar bone/CEJ. The

three vertical levels of measurement were: (Figure 4)

5 mm from the alveolar crest



Prior to measurement, each region was oriented in all 3 planes of space. The

axial slice was used to locate each region while the actual measurements for


31

each region, at three different levels from the alveolar crest were performed on

the coronal slice. (Figures 1, 2, 3)

While cortical bone thickness was measured as the distance between the outer

limit and the inner limits of the cortical bone (in millimeters) in a direction

perpendicular to the buccal bone surface, the cortical bone density was

recorded as the mean (Hounsfield units HU) along the line between the outer

and inner points, indicative of cortical thickness. This procedure was repeated

at increasing vertical distances from the alveolar crest (5, 7, 9 mm).

For the vertical measurements, a 1mm unit Grid was used with the first of the

horizontal lines representing the crest of the alveolar bone. After a period of

one month, 15 CBCTs were randomly selected from the study sample and the

measurements were performed by the same operator. (Figure 4, 5, 6)

Once it was determined that the bone densities at all the aforementioned

regions belonged to either one of two different bone types of Misch’s bone

density classification, a Finite Element study was performed to evaluate the

influence of cortical bone density on stress distribution in peri-implant bone.

Finite Element Model

Geometry

For obtaining a detailed geometry of the mini implant and surrounding bone

to be modeled, 3D blue light scanning of a dried human skull and a 2mm


32

diameter and 10mm length IZC mini screw implant (Bio-Ray, Syntec

Scientific Corp., Taipei, Taiwan) was performed. The region of interest

(infrazygomatic crest with molar teeth) was sectioned from the remaining

scanned image of the skull. Keeping these configurations as base, a 3D Finite

Element Model was built so that stress could be evaluated in three axes (x,y

and z). The coordinates were imported into the Hypermesh software (version

11.0, Troy, MI) as key points of the definitive image. One model of the

infrazygomatic crest bone was modeled with the cortical bone representing

high density bone while the second model of the infrazygomatic crest was

modeled with the cortical bone representing low density bone. In both models

the cortical bone thickess was kept constant at 2 mm and the cancellous bone

density was also kept constant. The teeth were modeled for the purpose of

guidance for mini implant insertion. (Figures 7a, 7b)

Material Properties

The material properties of cortical bone were modeled in the FEA as

orthotropic with 9 independent constants. The independent material constants

(Young’s modulus, Shear modulus and Poisson’s ratio) for the high density

cortical bone and low density cortical bone were taken from the study done by

Peterson et al11

; 2006 who had determined elastic moduli from apparent

densities of different regions of dentate maxilla. Since determination of

complex cancellous bone is quite difficult, the cancellous bone in both models

was modeled as linearly isotropic and a homogenous material with only two


33

independent material constants (1Young’s modulus and 1 Poisson’s ratio).

The mini implant was assumed to be made of stainless steel and was modeled

as homogenous, isotropic and linearly elastic. The material properties of

elements were based on previous studies. Each model consisted of

approximately the following nodes and elements:

Bone-implant interface

For simulating insertion of the mini implant into the infrazygomatic crest

cortical bone, a pilot hole of 0.5mm was created in the mesh model. The

coefficient of friction between the implant and bone during insertion was

k=0.3. The insertion simulations were repeated for 90° insertion

(perpendicular to the infrazygomatic crest surface) and 20° insertion (oblique

insertion).


34

INPUT VALUES FOR HIGH DENSITY BONE

E1, E2, E3 – Young’s Moduli G12, G23, G13 – Shear Moduli

V12, V13, V23 – Poisson’s ratios

Young’s modulus Poisson’s ratio

Cancellous bone 3.0 GPa 0.30

Stainless steel 190 GPa 0.25

E1 E2 E3 G12 G31 G23 V12 V13 V23

Cortical

bone

6.9 8.8 10.5 2.8 2.9 4.0 0.38 0.36 0.50

INPUT VALUES FOR LOW DENSITY BONE

E1 E2 E3 G12 G13 G23 V12 V13 V23

Cortical

bone

9.2 14 18.7 3.8 4.3 6.5 0.38 0.28 0.48

Figures

Figures

CBCT ANALYSIS

FIGURE 1 : AXIAL SLICE AS VIEWED IN DOLPHIN IMAGING SOFTWARE

(VERSION 11.9). IMAGE IS ORIENTED IN THE AXIAL PLANE FOR LOCATING

THE REGION OF INTEREST

Figures

FIGURE 2 : ORIENTATION IN THE SAGITTAL PLANE (EXAMPLE: REGION

BETWEEN SECOND PREMOLAR AND FIRST MOLAR U5-U6

Figures

FIGURE 3: ORIENTATION IN THE CORONAL PLANE. CORTICAL BONE

THICKNESS AND DENSITY ARE EVALUATED ON CORONAL SLICE

Figures

FIGURE 4 : CORTICAL BONE THICKNESS MEASURED AT 5MM, 7MM AND

9MM FROM THE ALVEOLAR CREST USING A 1MM SQUARE GRID. YELLOW

LINE REPRESENTS THE ALVEOLAR CREST

CORTICAL BONE THICKNESS MEASURED PERPENDICULAR TO THE BONE

SURFACE.

Figures

FIGURE 5 ; MEASUREMENTS ARE CHECKED ON THE HOUNSFIELD UNIT

LAYOUT TO AVOID INCONSISTENCIES IN IMAGE CONTRAST

Figures

FIGURE 6 : CORTICAL BONE DENSITY MEASURED AS THE MEAN VALUE

OF 3 POINTS ALONG THE LINE OF CORTICAL BONE THICKNESS

MEASURMENT. (HU – HOUNSFIELD UNITS)

Figures

FINITE ELEMENT ANALYSIS

FIGURE 7a : SIMULATION OF INFRAZYGOMATIC CREST WITH A

SIMULATED IZC IMPLANT INSERTED PERPENDICULARLY TO THE BONE

SURFACE (900)

FIGURE 7b : SIMULATION OF INFRAZYGOMATIC CREST WITH A

SIMULATED IZC IMPLANT INSERTED OBLIQUELY TO THE BONE

SURFACE (200)

Figures

VON MISSES STRESS IN PERI-IMPLANT CORTICAL BONE OF

DIFFERENT DENSITIES ON 900 / PERPENDICULAR INSERTION

FIGURE 8a : STRESS IN LOW DENSITY CORTICAL BONE = 4.5 MPa

FIGURE 8b : STRESS IN HIGH DENSITY CORTICAL BONE = 3.2 MPa

Figures

VON MISSES STRESS IN PERI-IMPLANT CORTICAL BONE OF

DIFFERENT DENSITIES ON 200 / OBLIQUE INSERTION

FIGURE 9a : STRESS IN LOW DENSITY CORTICAL BONE = 6.5 MPa

FIGURE 9b : STRESS IN HIGH DENSITY CORTICAL BONE = 5.3 MPa

Results

Results

35

RESULTS

A sample of 50 CBCTs was categorised into Class I (n=18), Class II (n=17)

and Class III (n=15) skeletal groups. The age of the sample was in the range of

20-29 years. The density and thickness of the cortical bone were recorded on

the coronal slices of each sample at five different regions starting from the

distal of the maxillary second premolar and ending distal to the second

maxillary molar on the buccal side including the infrazygomatic crest region.

The procedure was repeated at increasing distances from the crest of the

alveolar bone (5, 7 and 9 mm from the alveolar crest/CEJ).

CLASS I

Mean differences in heights (Table 1, Graphs 1a, 1b)

Within Class I skeletal pattern, cortical bone density and thickness showed no

statistically significant difference when evaluated at different distances (5mm,

7mm and 9mm) to the alveolar crest/CEJ.

Cortical bone density in Class I group ranged from 822.69 ± 194.98 HU to

1006.5 ± 196.44 HU. This minimum density (822.69 ± 194.98 HU) was

recorded at the 5mm level; the maximum density (1006.5 ± 196.44 HU) was

recorded at the 9mm level. But there was no statistically significant difference

between the values.

Results

36

Cortical bone thickness in Class I group ranged from 0.86 ± 0.23 mm to 1.42 ±

0.60 mm. This minimum thickness (0.86 ± 0.23 mm) was recorded at the 5mm

level; the maximum thickness (1.42 ± 0.60 mm) was recorded at the 9mm

level. But there was no statistically significant difference between the values.

(Table 1)

Mean differences in sites (Table 2; Graphs 1c, 1d)

Within Class I skeletal pattern, cortical bone density and thickness showed no

statistically significant difference when evaluated at different sites (between

second premolar and first molar U5-U6, mid-first molar U6, between first

molar and second molar U6-U7, mid-second molar U7, and distal to second

molar U7-U8).

Cortical bone density in Class I group ranged from 822.69 ± 194.98 HU to

1006.5 ± 196.44 HU. This minimum density (822.69 ± 194.98 HU) was

recorded at the U7-U8 region; the maximum density (1006.5 ± 196.44 HU)

was recorded at the U6 region. But there was no statistically significant

difference between the values.

Cortical bone density in Class I group ranged from 0.86 ± 0.23 mm to 1.42 ±

0.60 mm. This minimum thickness (0.86 ± 0.23 mm) was recorded at the U7-

U8 region; the maximum thickness (1.42 ± 0.60 mm) was recorded at the U6

region. But there was no statistically significant difference between the values.

Results

37

Therefore, there was a relative uniformity in cortical bone density and

thickness in the evaluated sites in Class I skeletal pattern. (Table 2)

CLASS II

Mean difference in heights (Table 3; Graphs 2a, 2b)

Within the Class II group, although no statistically significant differences in

cortical bone density were observed between different distances from the

alveolar crest, cortical bone thickness showed statistically significant

differences for regions U5-U6, U6 and U6-U7.

At 9 mm distance from alveolar crest, cortical bone showed highest thickness

for those regions:

U5-U6: 1.23 ± 0.45 mm (P = .006)

U6: 1.25 ± 0.40 mm (P = .000)

U6-U7: 1.32 ± 0.42 mm (P = .007)

The lowest thickness for cortical bone for those regions was observed at 5 mm

level.

U5-U6: 0.87 ± 0.30 mm (P = .006)

U6: 0.78 ± 0.25 mm (P = .000)

U6-U7: 0.91 ± 0.23 mm (P = .007)

Results

38

In U7 and U7-U8 regions, there was no significant difference in bone

thickness values between 5, 7 and 9mm levels from the alveolar crest.

(Table 3)

Mean difference in sites (Table 4; Graphs 2c,2d)

Cortical bone density varied significantly between sites at all three distance

from the alveolar crest.

At 5mm, lowest cortical bone density was found in the U7-U8 region (696.91

± 184.64 HU). The highest cortical bone density was found in U6-U7 region

(916.23 ± 127.00 HU) with a statistically significant P value = 0.000. But

cortical bone thickness showed no statistically significant difference in sites at

this level. Cortical bone thickness did not show any significant variation

among the different regions. It was also <1 mm at all regions.


± 216.18). The highest cortical bone density was found in U6-U7 region

(985.41 ± 110.99 HU) with a statistically significant P value = 0.006. Cortical

bone thickness too varied with density in the same regions. The lowest cortical

bone thickness was found in the U7-U8 region (0.73 ± 0.23 mm), while the

highest thickness was recorded at U6-U7 region (1.10 ± 0.24 mm) with a

statistically significant difference; P= 0.007.



Results

39

(1022.9 ± 167.16 HU) with a statistically significant difference;

P value = 0.000. Cortical bone thickness also showed lowest value in the

U7-U8 region (0.71 ± 0.21mm) and the highest value was recorded at the

U6-U7 region (1.32 ± 0.42 mm) with a statistically significant difference of

P = 0.000.

CLASS III

Mean difference in heights (Table 5, Graphs 3a, 3b)

Within Class III skeletal pattern, cortical bone density did not show any

significant variation when evaluated at different distances to the alveolar crest.

Only cortical bone thickness showed significant difference in values in the

U5-U6 and U6-U7 regions. In these two regions, thicker cortical bone was

found at 9 mm level:

U5-U6: 1.43 ± 0.49 mm (P = 0.004)

U6-U7: 1.31 ± 0.49 mm (P = 0.020)

And the lowest thickness was found at the 5 mm level:

U5-U6: 1.01 ± 0.27 mm (P = 0.004)

U6-U7: 0.95 ± 0.23 mm (P = 0.020)

Results

40

Mean difference in sites (Table 6, Graphs 3c,3d)

Cortical bone density and thickness varied significantly with respect to

different sites/regions.

At 5mm, lowest cortical bone density was found in the U7-U8 region

(667.50 ± 218.46 HU). The highest cortical bone density was found in U5-U6

region (959.40 ± 103.35 HU) and U6 region (866.27 ± 159.86 HU) with a

statistically significant P value = 0.001. But cortical bone thickness showed no

statistically significant difference in sites at this level. Cortical bone thickness

did not show any significant variation among the different regions.



(972.76 ± 136.00 HU) and U6 region (928.76 ± 134.20 HU) with a statistically

significant P value = 0.003. Cortical bone thickness too varied with density in

the same regions. The lowest cortical bone thickness was found in the U7-U8

region (0.76 ± 0.30 mm), while the highest thickness was recorded at U5-U6

region (1.04 ± 0.26 mm) and U6 region (1.04 ± 0.30 mm) with a statistically

significant difference; P= 0.008.


± 250.78 HU). The highest cortical bone density was found in U6 region

(1009.4 ± 122.87 HU) and U6 region (1002.8 ± 117.17 HU) with a statistically

significant difference; P value = 0.000. Cortical bone thickness also showed

Results

41

lowest value in the U7-U8 region (0.76 ± 0.38 mm) and the highest value was

recorded at the U5-U6 region (1.43 ± 0.49 mm), U6 region (1.14 ± 0.42 mm)

and U6-U7 region (1.31 ± 0.49 mm) with a statistically significant difference

of P = 0.001. (table 6)

CORTICAL DENSITY AND THICKNESS AT DIFFERENT

DISTANCES FROM THE ALVEOLAR CREST

Cortical bone density and thickness were recorded at 5, 7 and 9mm distance

from the alveolar crest on each sample. The regions premolar-molar (U5-U6)

and mid-first molar (U6) showed highest density at the 9 mm level, whereas

the regions first molar-second molar (U6-U7), mid-second molar (U7) and

second molar-third molar (U7-U8) showed high densities at 7 mm distance

from the alveolar crest. But the values were not statistically significant with

regard to density.

Statistically significant difference in cortical bone thickness was found

between different distances from the alveolar crest. In the U5-U6 region,

cortical bone thickness was high at the 9 mm level (M=1.33, SD=0.52) and the

least at 5 mm level (M=0.99, SD=0.35) and this difference was statistically

significant (P=0.000). There was a statistically significant difference in

thickness between the 7mm and 9mm levels (P=0.001) but not between the

5mm and 7mm levels. In the U6 region, cortical bone thickness was high at

the 9 mm level (M=1.27, SD=0.49) and the least at 5 mm level (M=0.92,

SD=0.32) and this difference was statistically significant (P=0.000).

Results

42

There was a statistically significant difference in thickness between the 7mm

and 9mm levels (P=0.017) but not between the 5mm and 7mm levels. In the

U6-U7 region, there was a statistically significant difference in thickness only

between the 5mm and 9 mm levels(P=0.000). Differences in cortical thickness

in the U7 and U7-U8 regions were not statistically significant.

INTERGROUP COMPARISON OF CORTICAL DENSITY AND

THICKNESS (CLASS I, CLASS II & CLASS III(Table 7; Graphs 4a, 4b)

Results showed statistically significant differences in cortical bone density

between malocclusions only in two regions U6-U7 and U7-U8 only (P=0.004,

0.000).

In the U6-U7 region, Class I showed highest density (974.85 ± 174.31), Class

II intermediate density (953.28 ± 122.49 ) and Class III least density (874.21

±157.49). The difference between Class I and Class III was statistically

significant with a P value of 0.004. The difference between Class II and Class

III was statistically significant with a P value of 0.038. But the difference

between Class I and class II was not statistically significant.

In the U7-U8 region, Class I showed highest density (865.03 ± 180.02), Class

II intermediate density (728.56 ± 201.75) and Class III least density (705.18

±233.49). The difference between Class I and Class II was statistically

significant with a P value of 0.002. The difference between Class I and

Class III was statistically significant with a P value of 0.000. But the

Results

43

difference between Class II and class III was not statistically significant.With

regard to cortical bone thickness, results showed statistically significant

differences between malocclusions only in two regions U7 and U7-U8

(P=0.014 and 0.000).

In the U7 region, Class I showed highest thickness (1.09 ± 0.48), Class III

intermediate thickness (1.06 ± 0.32 ) and Class II least thickness (0.89 ± 0.28).

The difference between Class I and Class II was statistically significant with a

P value of 0.019. The difference between Class I - class III and Class II- Class

III was not statistically significant.

In the U7-U8 region, Class I showed highest thickness (0.97 ± 0.41), Class III

intermediate thickness (0.76 ± 0.32) and Class II least thickness (0.72 ± 0.22).

The difference between Class I and Class II was statistically significant with a

P value of 0.000. The difference between Class I and Class III was statistically

significant with a P value of 0.008. But the difference between Class II and

class III was not statistically significant.

CORRELATION BETWEEN CORTICAL BONE DENSITY AND

THICKNESS (Table 8; Graph 5)

Data gathered from one hundred and fifty CBCTs, evaluated for cortical bone

density and thickness at five different regions, was analysed by Pearson’s test

to determine the correlation co-efficient between the aforementioned

variables. (Table 8)

Results

44

i. Pearson’s r at region U5-U6 indicated a statistically

significant positive correlation between density (M=961.03,

SD=155.92) and thickness (M=1.11, SD=0.43) with r=0.61

ii. Pearson’s r at region U6 indicated a statistically significant

positive correlation between density (M=949.63,


iii. Pearson’s r at region U6-U7 indicated a statistically



iv. Pearson’s r at region U7 indicated a statistically significant

positive correlation between density (M=869.53,


v. Pearson’s r at region U7-U8 indicated a statistically



Overall, a moderate positive correlation was seen between cortical bone

density and thickness.

RESULTS OF FEM STUDY

At 90° insertion, the cortical bone and the curvature supported the mii-

implant with least effect on cancellous bone.

Perpendicular insertion (90°) (Figures 8a, 8b)

Results

45

At 90° insertion, between the low density cortical bone and high density

cortical bone, least stress was observed in the high density cortical bone.

High density cortical bone stress : 3.2 MPa

Low density cortical bone stress : 4.5 MPa

Cancellous bone stress : 1 MPa

Oblique insertion (20°) (Figures 9a, 9b)

At 20° insertion, between the low density cortical bone and high density

cortical bone, least stress was observed in the high density cortical bone.

High density cortical bone stress : 5.3 MPa

Low density cortical bone stress : 6.5 MPa

Cancellous bone stress : 14 MPa

Between the two angulations, the more oblique angulation resulted in

greater stress in cortical bone coupled with 14 times more stress in the

cancellous bone.

Results

46

STATISTICAL ANALYSIS:

Based on Kolmogorov-Smirov and Shapiro-Wilk tests for normality

performed to analyse distribution of data, the data was found to be normally

distributed, 1-way analysis of variants (ANOVA) was used to evaluate

differences between three skeletal patterns. Differences between and within

regions were evaluated using Bonferroni Posthoc tests. Correlation between

cortical bone thickness and density were evaluated with Pearson’s correlation

test. Intra class correlation test was done to evaluate reliability of

measurements. In all the above statistical tools the probability value of <0.05

was considered as significant .

Tables and Graphs

Tables and graphs

CLASS I

MEAN DIFFERENCES BETWEEN HEIGHTS (TABLE 1)

5 7 9 P value Sig.

Density Mean ± SD Mean ± SD Mean ± SD

U5-U6 946.18 ± 162.94 967.05 ± 182.36 958.83 ± 217.51 .579 NS U6 967.61 ± 189.35 988.28 ± 207.19 1006.5 ± 196.44 .987 NS U6-U7 945.47 ± 165.98 956.33 ± 238.81 991.05 ± 151.23 .720 NS U7 895.08 ± 209.69 957.50 ± 192.26 883.47 ± 213.36 .530 NS U7-U8 822.69 ± 194.98 888.61 ± 160.68 865.02 ± 180.02 .523 NS

Thickness

U5-U6 1.10 ± 0.42 1.09 ± 0.42 1.32 ± 0.60 .357 NS U6 1.15 ± 0.39 1.27 ± 0.52 1.42 ± 0.60 .137 NS U6-U7 1.03 ± 0.38 1.13 ± 0.41 1.33 ± 0.59 .279 NS U7 0.93 ± 0.34 1.16 ± 0.46 1.16 ± 0.58 .330 NS U7-U8 0.86 ± 0.23 0.96 ± 0.44 1.06 ± 0.48 .310 NS The mean difference is significant at the level 0.05 NS - Non-significant

MEAN DIFFERENCE BETWEEN SITES (TABLE 2)

U5-U6 U6 U6-U7 U7 U7-U8 P value Sig.

Density Mean ± SD Mean ± SD Mean ± SD Mean ± SD Mean ± SD

5 946.18 ± 162.94 967.61 ± 189.35 945.47 ± 165.98 895.08 ± 209.69 822.69 ± 194.98 .141 NS

7 967.05 ± 182.36 956.33 ± 238.81 988.28 ± 207.19 957.50 ± 192.26 888.61 ± 160.68 .634 NS

9 958.83 ± 217.51 1006.5 ± 196.44 991.05 ± 151.23 883.47 ± 213.36 865.02 ± 180.02 .175 NS

Thickness

5 1.10 ± 0.42 1.15 ± 0.39 1.03 ± 0.38 0.93 ± 0.34 0.86 ± 0.23 .123 NS

7 1.09 ± 0.42 1.27 ± 0.52 1.13 ± 0.41 1.16 ± 0.46 0.96 ± 0.44 .362 NS

9 1.32 ± 0.60 1.42 ± 0.60 1.33 ± 0.59 1.16 ± 0.58 1.06 ± 0.48 .358 NS

The mean difference is significant at the level 0.05 NS - Non-significant

Tables and graphs

GRAPH 1a : DIFFERENCE IN CORTICAL BONE DENSITY AT THE 3 VERTICAL

LEVELS FROM THE ALVEOLAR CREST IN CLASS I SKELETAL PATTERN

GRAPH 1b : DIFFERENCE IN CORTICAL BONE THICKNESS AT THE 3 VERTICAL

LEVELS FROM THE ALVEOLAR CREST IN CLASS I SKELETAL PATTERN

0

200

400

600

800

1000

1200

U5-U6 U6 U6-U7 U7 U7-U8

5 mm

7 mm

9 mm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

U5-U6 U6 U6-U7 U7 U7-U8

5 mm

7 mm

9mm

Tables and graphs

GRAPH 1c : DIFFERENCE IN CORTICAL BONE DENSITY AT THE 5 ANTERO-

POSTERIOR REGIONS CLASS I SKELETAL PATTERN

GRAPH 1d : DIFFERENCE IN CORTICAL BONE THICKNESS AT THE 5 ANTERO-

POSTERIOR REGIONS CLASS I SKELETAL PATTERN

0

200

400

600

800

1000

1200

5 mm 7mm 9mm

U5-U6

U6

U6-U7

U7

U7-U8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

5 mm 7 mm 9 mm

U5-U6

U6

U6-U7

U7

U7-U8

Tables and graphs

CLASS II

MEAN DIFFERENCE BETWEEN HEIGHTS (TABLE 3)

5 7 9 P value Sig.



Thickness

U5-U6 0.87 ± 0.30 0.91 ± 0.21 1.23 ± 0.45 .006 * U6 0.78 ± 0.25 0.95 ± 0.36 1.25 ± 0.40 .000 * U6-U7 0.91 ± 0.23 1.10 ± 0.24 1.32 ± 0.42 .007 * U7 0.81 ± 0.22 0.92 ± 0.30 0.91 ± 0.30 .480 NS U7-U8 0.69 ± 0.20 0.73 ± 0.23 0.71 ± 0.21 .865 NS The mean difference is significant at the level 0.05 NS - Non-significant




5 895.91 ± 158.69 901.52 ± 122.89 916.23 ± 127.00 825.50 ± 178.84

696.91 ± 184.64 .000 *

7 911.14 ± 137.91 933.67 ± 186.20 985.41 ± 110.99 845.73 ± 193.11

772.00 ± 216.18 .006 *

9 987.55 ± 161.78 958.17 ± 125.93 1022.9 ± 167.16 844.02 ± 193.55

716.76 ± 207.72 .000 *

Thickness

5 0.87 ± 0.30 0.78 ± 0.25 0.91 ± 0.23 0.81 ± 0.22 0.69 ± 0.20 .106 NS

7 0.91 ± 0.21 0.95 ± 0.36 1.10 ± 0.24 0.92 ± 0.30 0.73 ± 0.23 .007 *

9 1.23 ± 0.45 1.25 ± 0.40 1.32 ± 0.42 0.91 ± 0.30 0.71 ± 0.21 .000 *

The mean difference is significant at the level 0.05 NS - Non-significant

Tables and graphs


LEVELS FROM THE ALVEOLAR CREST IN CLASS II SKELETAL PATTERN


LEVELS FROM THE ALVEOLAR CREST IN CLASS II SKELETAL PATTERN

0

200

400

600

800

1000

1200

U5-U6 U6 U6-U7 U7 U7-U8

5 mm

7 mm

9 mm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

U5-U6 U6 U6-U7 U7 U7-U8

5 mm

7 mm

9 mm

Tables and graphs


POSTERIOR REGIONS CLASS II SKELETAL PATTERN


POSTERIOR REGIONS CLASS II SKELETAL PATTERN

0

200

400

600

800

1000

1200

5 mm 7 mm 9 mm

U5-U6

U6

U6-U7

U7

U7-U8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

5 mm 7 mm 9 mm

U5-U6

U6

U6-U7

U7

U7-U8

Tables and graphs

CLASS III

MEAN DIFFERENCE BETWEEN HEIGHTS (TABLE 5)

5 7 9 P value Sig.



Thickness

U5-U6 1.01 ± 0.27 1.04 ± 0.26 1.43 ± 0.49 .004 * U6 0.92 ± 0.29 1.04 ± 0.30 1.14 ± 0.42 .231 NS U6-U7 0.95 ± 0.23 1.08 ± 0.21 1.31 ± 0.49 .020 * U7 0.98 ± 0.32 1.09 ± 0.29 1.11 ± 0.33 .473 NS U7-U8 0.76 ± 0.30 0.76 ± 0.30 0.76 ± 0.38 .998 NS The mean difference is significant at the level 0.05 NS - Non-significant




5 959.40 ± 103.35 866.27 ± 159.86 828.23 ± 185.92 844.80 ± 208.34

667.50 ± 218.46 .001 *

7 972.76 ± 136.00 928.76 ± 134.20 897.40 ± 169.03 877.73 ± 151.10

726.63 ± 241.70 .003 *

9 1009.4 ± 122.87 1002.8 ± 117.17 897.00 ± 106.84 838.80 ± 205.83

721.40 ± 250.78 .000 *

Thickness

5 1.01 ± 0.27 0.92 ± 0.29 0.95 ± 0.23 0.98 ± 0.32 0.76 ± 0.30 .170 NS

7 1.04 ± 0.26 1.04 ± 0.30 1.08 ± 0.21 1.09 ± 0.29 0.76 ± 0.30 .008 *

9 1.43 ± 0.49 1.14 ± 0.42 1.31 ± 0.49 1.11 ± 0.33 0.76 ± 0.38 .001 *

Tables and graphs


LEVELS FROM THE ALVEOLAR CREST IN CLASS III SKELETAL PATTERN


LEVELS FROM THE ALVEOLAR CREST IN CLASS III SKELETAL PATTERN

0

200

400

600

800

1000

1200

U5-U6 U6 U6-U7 U7 U7-U8

5 mm

7 mm

9 mm

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

U5-U6 U6 U6-U7 U7 U7-U8

5 mm

7 mm

9 mm

Tables and graphs


POSTERIOR REGIONS CLASS III SKELETAL PATTERN


POSTERIOR REGIONS CLASS III SKELETAL PATTERN

0

200

400

600

800

1000

1200

5 mm 7 mm 9 mm

U5-U6

U6

U6-U7

U7

U7-U8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

5 mm 7 mm 9 mm

U5-U6

U6

U6-U7

U7

U7-U8

Tables and graphs

DIFFERENCES IN DENSITY AND THICKNESS AMONG CLASS I, CLASS II, CLASS III

RELATIONSHIP (TABLE 7)

The mean difference is significant at the level 0.05

PEARSON’S CORRELATION (TABLE 8)

5-6 CBTh 6 CBTh 6-7 CBTh 7 CBTh 7-8 CBTh

U5-U6 Density Pearson Correlation .613**

Sig. (2 tailed) .000

N 150

U6 Density Pearson Correlation 0.636**

Sig. (2 tailed) 0.000

N 150

U6-U7 Density Pearson Correlation 0.605**


N 150

U7 Density Pearson Correlation 0.583**


N 150

U7-U8 Density Pearson Correlation 0.606**


N 150

* Correlation is significant at the 0.05 level **Correlation is significant at the 0.01 level

Class I Class II Class III P value Sig.


U5-U6 973.26 ± 179.45 930.87 ± 155.19 980.53 ± 120.61 .231 NS

U6 960.92 ± 212.12 952.71 ± 166.01 932.63 ± 146.35 .728 NS

U6-U7 974.85 ± 174.31 953.28 ± 122.49 874.21 ±157.49 .004 *

U7 912.01 ± 204.06 838.42 ± 185.05 853.78 ± 186.68 .042 *

U7-U8 865.03 ± 180.02 728.56 ± 201.75 705.18 ±233.49 .000 *

Thickness

U5-U6 1.17 ± 0.49 1.01 ± 0.37 1.16 ± 0.40 .096 NS

U6 1.17 ± 0.48 1.02 ± 0.41 1.04 ± 0.35 .143 NS

U6-U7 1.28 ± 0.52 1.09 ± 0.33 1.12 ± 0.36 .061 NS

U7 1.09 ± 0.48 0.89 ± 0.28 1.06 ± 0.32 .014 *

U7-U8 0.97 ± 0.41 0.72 ± 0.22 0.76 ± 0.32 .000 *

Tables and graphs

GRAPH 4a: DIFFERENCES IN CORTICAL BONE DENSITY BETWEEN CLASS I,

CLASS II AND CLASS III SKELETAL TYPES

GRAPH 4b: DIFFERENCES IN CORTICAL BONE THICKNESS BETWEEN CLASS

I, CLASS II AND CLASS III SKELETAL TYPES

0

200

400

600

800

1000

1200

U5-U6 U6 U6-U7 U7 U7-U8

CLASS I

CLASS II

CLASS III

0

0.2

0.4

0.6

0.8

1

1.2

1.4

U5-U6 U6 U6-U7 U7 U7-U8

CLASS I

CLASS II

CLASS III

Tables and graphs

CORRELATION BETWEEN CORTICAL BONE THICKNESS AND DENSITY

GRAPH 5 : HORIZONTAL X-AXIS REPRESENTS DENSITY IN HOUNSFIELD UNITS

VERTICAL Y-AXIS REPRESENTS CORTICAL THICKNESS IN MILLIMETERS

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 200 400 600 800 1000 1200

Discussion

Discussion

47

DISCUSSION

Mini implants were formally introduced to orthodontics in 1983 by

Creekmore and Eklund12

. The implant reportedly used, a vitalium bone

screw, remained immobile or stable during the entire length of treatment. The

stability of the screw became the crucial factor in the success of their

orthodontic treatment. Since then, and until 2005 when mini screws were

commercially available for usage by clinicians, many authors like Kanomi13

,

Melsen and Costa14

, experimented with various mini screw implants to

maximise anchorage during intrusion and retraction procedures. Based on the

results of their experiments, the authors agreed upon the importance of the

quality of bone at the implantation site.

Mini screw implant, unlike an osseo-integrated dental implant, relies on a

mechanical interlock with bone which determines its primary stability7. Since

mini screw implants are almost always immediately loaded, primary stability

will dictate secondary stability and the overall success of an implant. Factors

affecting the primary stability are (i) bone quality and quantity, (ii) implant

design characteristics (iii) placement conditions.15

To be precise, a systematic review and meta-analysis by Papadopoulos et

al.16

revealed that factors influencing mini screw implant stability were:

Discussion

48

i. Patient related : smoking, sagittal skeletal pattern (ANB°), vertical

skeletal pattern, age

ii. Clinician related : experience and skill

iii. Mini screw related : diameter, thread length, overall implant design

iv. Insertion related : insertion torque, insertion angle, cortical bone

thickness, site of implantation, bone quality

v. Treatment related : loading time, type of tooth movement

vi. Complication related : inflammation, proximity to anatomic structures

Miyawaki et al17

encountered more number of failures with mini screw

implants of smaller diameters like 1 mm when compared to 1.5 or 2 mm

coupled with reduced cortical bone thickness. They suggested sufficient

mechanical interdigitation between cortical bone and mini screw implant as a

critical factor for stability. With regard to skeletal pattern, they met with 80%

success in Class I (14 subjects) and Class II (23 subjects) groups, and 100%

success in Class III group (4 subjects). The reason for greater success in Class

III skeletal pattern could be due to the discrepancy in the number of subjects in

each group. Moreover, the authors used titanium mini screw implants which

could have influenced the outcome of their study since titanium is associated

with osseo-integration.

On the other hand, Chen et al18

reported that patients with poor bone density,

in their study, indeed exhibited greater mini screw implant failure.

Additionally, Motoyoshi et al19

, in their clinical trial found significantly

Discussion

49

higher failure rates of mini screw implants in adolescents. They found that

bone density and maturity was low in adolescents and those factors

contributed to the higher loss rate of implants.

Based on previous research, it is reasonable to assume that even with a

perfectly designed implant, an optimum bone-to-implant contact (BIC) can be

achieved only in good quality bone. Surely, a more porous bone can only offer

reduced contact area for mechanical retention of an implant than a denser

bone.

Bone density, has gained various meaning by different authors. Buck and

Wheeler20

,defined bone density as an expression of specific gravity of bone

tissue and also the relative amount of marrow spaces present in a unit of bone

tissue. Chugh et al21

opined that knowledge of bone density aids one in

appreciating the link between adaptive skeletal deformation and its

biomechanical environment. Since, addition of bone mineral occurs in

response to muscle loading forces, this adaptive behavior of bone can be

exploited to increase the stability of a mini screw implant as the amount of

bone in contact with the implant surface increases, offering a superior

biomechanical environment that further improves the stress encountering

potential in bone.

The stress bearing potential is relatively low in porous or low density bone

which, can be explained by the fact that since stress is directly related to strain,

Discussion

50

low density bone will be easily susceptible to microdamage, because force

experienced per unit area increases in porous bone (Stress = Force/Unit Area).

Moreover, much of the past research dealt with inter-radicular miniscrew

implants. But the recent trend towards increased usage of IZC mini screw

implants warrants evaluation of factors influencing implant stability in

infrazygomatic crest region. Uribe et al22

showed that the failure rate of mini

screw implants placed in infrazygomatic region is 21.8% which is higher in

comparison to 13.5% failure rate associated with inter radicular implants.60,61

The important variables influencing failure rates in the infrazygomatic region,

according to Uribe et al, were vertical skeletal pattern, reduced cortical bone

thickness and insertion angle. They also said that it is unknown if reduced

cortical bone thickness is also present in the infrazygomatic region.

The infrazygomatic crest is important for many reasons. It is an extra alveolar

site so, proximity of the implant to roots of teeth can be avoided23

. As a result,

complications associated with inter-radicular implants; for example, PDL

damage, root resorption, hindrance to orthodontic tooth movement particularly

distalisation, can be minimised. Also, Consolaro and Romano3 in their study

explaining the prevailing hypotheses about mini-implant failure, revealed that

because the alveolar processes are subject to deflection under orthodontic

tooth movement forces, a more apical placement of mini-implant would be

favourable as basal bone is less flexible. They further stated that mini screw

implants placed in sites with low cortical bone thickness, low bone density and

Discussion

51

low alveolar bone volume will compromise the mechanical interlocking of

bone and implant, leading to excessive pressure and bone microfractures. They

stressed on the anatomical shape of the placement site as an influence on

implant stability.

Furthermore, infrazygomatic crest is best suited to resist and

dissipate/distribute the stresses/functional loads to the larger part of the

cranium. Based on experiments performed to ascertain the architecture of the

split-lines or crevice lines in that region, usually believed to represent

trajectories of the jaws, Seipel CM23

noted that the ascending split-lines from

the second premolar and first molar were found to concentrate in the

infrazygomatic crest region, making it functionally important. This key-ridge

was observed to migrate during developmental years from being situated

above the second deciduous molar in mixed dentition phase to being located

above the first molar or between the first and second molar in the adult

dentition phase. It has been established that the specialised function and form

of the infrazygomatic crest, characterised by a curved pillar of cortical bone, is

suited for resisting torsional and bending stresses generated during

mastication.

Although Melsen and Costa24

used osseointegrated titanium mini implants in

the infrazygomatic crest region in beagles, the infrazygomatic crest as a site

for implant placement was popularised after Hugo De Clerck25

in 2002

described the ‘Zygoma Anchorage System’ wherein he harnessed the stress

Discussion

52

bearing potential of the zygomatic buttress for mini-plate placement for

retraction in Class II malocclusion. Once mini-implants became commercially

available in 2005,1 IZC implants were designed with following dimensions:

i. 2mm diameter and 10 mm length

ii. 2mm diameter and 12mm length

iii. 2mm diameter and 14 mm length

With regard to the precise placement location of IZC mini-screw implant, Eric

J.W. Liou et al5, measured the thickness of the infrazygomatic (IZ) crest

above the maxillary first molar at different angles and positions to the

maxillary occlusal plane, as guidance for inserting mini screw implants in the

IZ crest without injuring the mesiobuccal root of the maxillary first molar. As

a result they recommended the IZ crest at 14 to 16 mm above the maxillary

occlusal plane and the maxillary first molar, and at an angle of 55° to 70° to

the maxillary occlusal plane.

Prior to recommending the above mentioned location for IZC implant

placement, Eric Liou et al25

, in 2004, inserted 32 mini screw implants into the

thickest area of the zygomatic buttress, above the junction (turning

point)between the alveolar process and the zygomatic process, citing that the

thickness of the cortical bone in that region is approximately 3 – 4mm thick

but, is subject to variation based on the pneumatisation of sinus. The authors

met with 100 percent success with respect to the stability of the 2mm diameter

Discussion

53

and 17 mm long implants. It remains unclear if the thickness of the cortical

bone or the length of the screw was the reason behind the successs.

Later, John Lin and Eugene Roberts6, citing that thickness of bone was

greater on the mesio-buccal and disto-buccal roots of the second maxillary

molar, suggested the site between maxillary first and second molar as the

favourable region for IZC mini screw implant placement. It can be inferred

from Liou’s and Lin’s implant placement guidelines that the authors are

referring to the available bone depth in the IZC region.

Also, as previously discussed, if skeletal pattern is an important variable

influencing cortical bone and mini screw implant stability, then only vertical

facial types have been evaluated frequently. There is no study till date

evaluating the influence of sagittal skeletal morphology on implant

stability in relation to cortical bone thickness and cortical bone density in

the infrazygomatic crest region.

And so, the null hypothesis of this study runs thus:

There is no difference in cortical bone density and thickness in the

infrazygomatic crest region with regard to different sagittal skeletal patterns

i.e. Class I, Class II and Class III.

Discussion

54

So, in our study five different regions (U5-U6: between second premolar

and first molar, U6: mid-root region of first molar, U6-U7: between first

molar and second molar, U7: mid-root region of second molar, and

U7-U8: distal of second molar) were evaluated for cortical bone thickness

and density, at increasing distances to the crest bone (5, 7 and 9mm from

alveolar crest/CEJ) in each of three skeletal facial types.

Since, function plays a major role in development and maintenance of external

form and internal architecture of bone (H.M. Frost26

), maxillary alveolar and

basal bones too are subject to functional loads in the form of bite force/ jaw

closing force/ muscle force which affect the mean occlusal contact area

between the upper and lower dentition ( Bakke et al.27

). In adults, bite force

was found to be at its maximum value ranging from 300-600 Newtons

(Hagberg C.28

).

According to Wolff’s law and Frost’s Mechanostat theory, these functional

forces are responsible for the maintainence of alveolar bone in maxilla too and

the reason behind the characteristic form and function of the infrazygomatic

crest, usually a corresponding landmark to the fist maxillary molar.23

In other

words muscle loading forces influence bone formation and bone density.

Bae et al29

reported that occlusal contact areas and bite forces were

significantly different in Class I, Class II and Class III malocclusions.

Discussion

55

These differences in loading of dental arches, according to Banri Endo30

,

could lead to shift in the direction of the axis of the principal strain ending up

in variable trajectories of stress in the alveolar bone and infrazygomatic crest

regions that may not exactly coincide with the fixed split-lines.

In our study only adults between the ages of 20 and 40 years were

considered so that the influence of growth on bone can be minimised. Koc

D et al31

reported that the jaw closing forces increased with age but stabilise

and remain constant from 20 to 40 years followed by a decline in the closing

force. It has been reported that during the first decades of life (growth and

development), modelling drift characterised by increased bone formation than

bone resorption is the predominant mechanism of bone remodelling. After 40

years, remodelling is characterised by increased bone resorption than bone

formation. Whereas, between 20 and 40 years, bone tissue, in any normal

healthy adult, responds to stress and microdamage by initial bone resorption,

followed by an equal amount of bone formation. Likewise, the age of the

sample in this study ranged from 20 -29 years.

For measuring cortical bone thickness and assessing cortical bone density

in our study, we used the pre treatment CBCT images of patients. The

CBCT images were obtained using Kodak 9500 available with scanning

parameters of 18.4cm*20.6 cm FOV, 90kVp. 108mAs, 0.3 mm voxel size.

The scans were imported into Dolphin Imaging Software (version 11.9,

Chatsworth, Calif) in DICOM file format. The Dolphin Imaging Software

Discussion

56

allows the operator to measure cortical bone thickness and density

simultaneously which contributes to reliability of measurements. Although

the effectiveness of CBCT in assessing bone density has been questioned,

reduced radiation dosage, low cost, high resolution images of high contrast

structures of the craniofacial complex and its common usage in dentistry were

the reasons for choosing this imaging modality.

Objective classification of bone density proposed by Misch and Kircos was

based on CT Hounsfield units21

. These Hounsfield units are quantifications of

gray values obtained from CT image representing the X-ray attenuation co-

efficient of a material relative to water (0 HU) or relative to air (-1000 HU).

D1 bone : >1250 HU;

D2 bone : 850–1250 HU;

D3 bone : 350–850 HU;

D4 bone : 150–350 HU

Da Silva Campos et al32

and Molteni33

have questioned the co-relation

between gray values derived from CBCT and density. They say numerical

values can differ on CBCT scans due to issues like artefacts, beam hardening,

limited field of view, beam divergence, image noise, etc.

Nevertheless, studies have shown a linear correlation between CBCT derived

Houndsfield units and CT derived Hounsfield unit values (Razi et al 34

). Hsu

Discussion

57

et al35

have shown that CBCT derived Hounsfield unit values are in fact quite

superior to dual energy X-ray absorptiometry values.

Considering limitations imposed on frequent use of even CBCT scans via the

SEDENTEXCT guidelines and through the ALARA principle, this study can

serve to effect a clinician’s choice of favourable sites for mini screw implant

placement.

Collection of data was followed by satististical analysis. Our study data was

tested for normality, after which parametric tests; one-way Anova and

Bonferroni Post Hoc, were applied to generate results. P value was kept at

0.05 and the confidence interval at 95%

Statistical analysis showed that cortical bone density in CLASS I GROUP

ranged from 822.69 ± 194.98 HU to 1006.5 ± 196.44 HU. This minimum

density (822.69 ± 194.98 HU) was recorded at the 5mm level; the

maximum density (1006.5 ± 196.44 HU) was recorded at the 9mm level

without any statistically significant difference. Cortical bone thickness in

Class I group ranged from 0.86 ± 0.23 mm to 1.42 ± 0.60 mm. There was

no statistically significant difference in bone density and thickness

between different heights or different sites, indicating that thickness and

density were relatively homogenous, although mean trend showed an

increase toward the U6 region.

Discussion

58

In the CLASS II GROUP, cortical bone density ranged from. This range

indicated a greater variation in density values in comparison to class I

group. Cortical bone thickness ranged from 0.69 ± 0.20 mm to 1.32 ± 0.42

mm. The lowest thickness with the lowest bone density was in U7-U8

region at the 5mm distance to the alveolar bone, while maximum

thickness and density were observed at the 9mm level in U6-U7 regions.

In CLASS III GROUP, cortical bone density ranged from 667.50 ± 218.46

HU to 959.40 ± 103.35 HU. Cortical bone thickness ranged from 0.76 ±

0.30 mm to 1.43 ± 0.49 mm. The lowest thickness with the lowest bone

density and thickness was in U7-U8 region at the 5mm distance to the

alveolar bone, while maximum thickness and density were observed at the

9mm level in U5-U6 regions.

The differences in cortical bone thickness between Class I, Class II and

Class III skeletal patterns assumed statistical significance only in second

molar and distal to second molar regions. Class I pattern was generally

associated with greater cortical thickness and density compared to Class II and

Class III patterns. And distal to second molar, representing the tuberosity

region, the cortical bone was significantly less than 1mm in all three skeletal

patterns.

These results agreed with the results of a study conducted by Al-Jaf et al36

where the mean cortical bone thickness was highest in class I skeletal pattern

Discussion

59

between first and second molar. The other regions were not evaluated in their

study and hence were unavailable for comparison. Class III showed highest

mean value between premolar and first molar, agreeing with results from Al-

Jaf’s study. Results were found to be similar despite the differences in

mandibular plane angle pattern between both studies. Al-jaf et al36

considered

only patients with normal MPA of 27°-37°. This homogeneity was difficult to

achieve due to reduced number of available samples in our study. But results

were found to coincide.

Although many authors showed that differences in cortical bone thickness

existed between different vertical facial groups,37,38,39

a study by Chen et al40

in class II individuals with different FMA revealed that there was no

significant difference in cortical bone thickness with regard to FMA.

Assessment of cortical bone thickness at increasing distances to the

alveolar crest showed highest mean value at 9 mm from the alvealor crest.

This trend was found to be similar in all three skeletal patterns. These values

were also statistically significant in between the premolar and first molar

region and the region between the first and second molar, indicating that

cortical bone thickness increased with increasing distance to alveolar crest or

as one proceeds toward the basal bone. At 4mm distance to the alveolar crest,

Germec-cakan et al41

, did not find any significant difference in cortical bone

thickness between regions.

Discussion

60

As for cortical thickness mesial to first molar, the results of our study

were different from the results of Khumsarn et al42

. In their study at 8mm

distance to CEJ, the cortical bone thickness was greater in class II skeletal

pattern than class I. This difference could be attributed to the age differences

between the samples of both studies. Age of subjects in their study ranged

from 13-29 years. These differences could be due to the influence of growth

and rate of growth in different children.

Studies have found consistently less cortical bone distal to second molars

(Deguchi et al43

) and thickness to increase with increasing distances to the

CEJ. In fact Ono et al44

found cortical bone thickness to be 2.4 mm thick at

15 mm mark from the alveolar crest.

On the whole, the cortical bone thickness of the infrazygomatic crest

region in our study ranged from 0.8 mm to 1.4mm in Class I skeletal pattern,

while for Class II and Class III groups cortical bone thickness ranged from 0.7

to 1.1 mm. these results concur with the results of Farnsworth et al.45

study,

who found mean cortical bone thickness in the infrazygomatic region to be

1.34 mm. More specifically, Ono et al44

found cortical bone thickness mesial

to first molar to range from 1.09 – 1.62 mm.

With regard to cortical bone density, results in our study differed from those

of Ohiomoba et al46

, where cortical bone density increased from coronal to

apical regions. In our study although the mean values showed an increase

Discussion

61

towards the apical regions, the differences were not statistically significant.

Cortical bone thickness and density were generally high in U5-U6, U6 and

U6-U7 regions for all skeletal patterns when compared to U7 and U7-U8

regions. Although Rossi et al47

found no significant differences between the

three skeletal patterns even with regard to age in their study, Borges et al48

found highest bone density between the premolar region and the lowest in the

maxillary tuberosity region and maximum density towards the basal bone.

In order to assess the influence of density on stress distribution in peri-

implant bone, a finite element bone model was constructed and analysed

by simulating 2mm diameter and 10mm long IZC implant into bone.

Density assessment assumes importance because authors have revealed a

positive relationship between density and implant stability (Marquezan et

al49

). In our study we stressed the importance of cortical bone rather than

trabecular bone due to the mechanical and protective functions of cortical

bone influencing primary stability and mechanical retention. This is not

to say that trabecular bone does not contribute to implant stability but

despite its role in resisting compressive stresses, its primary role is that of

metabolic homeostasis of serum calcium. (Marks and Odgren50

)

Remodelling of trabecular bone occurs at a rapid rate thereby it is subject to

greater changes due to its thin trabeculae and rapid surface resorption. This is

a result of its close contact with bone marrow and its circulation. This relative

Discussion

62

reduction in vascularity through the compacted, lamellated cortical bone

makes it a more stable structure to offer mechanical retention of the mini

implant as it is associated with slower changes when compared to trabecular

bone. Nevertheless, adequate blood supply is required for damage repair

through remodeling.

The bone resorption process in the peri-implant bone has been identified as

the reason behind reduced primary stability and eventual loosening of mini

implant According to Frost’s mechanostat theory8, increased bone resorption

will occur either due to underloading (disuse atrophy) or overloading

(pathologic overload) of bone.

In the case of mini implant insertions, high stresses generated at the implant-

bone interface could lead to microstrains in bone, reaching the pathologic

overload window. Since bone is an organic structure, damage repair will occur

automatically to a certain extent but if implant loading is associated with high

stress it could lead to the production of microfractures, where the automatic

repair mechanism of bone is outpaced by bone resorption (Huiskes and

Nunamaker51

). The consequence of this is mini-implant failure.

Many authors have experimentally shown that bone density influences the

stress experienced by bone at the implant-bone interface (Suzuki et al52

). In

our study, to objectively evaluate the influence of different densities on stress

distribution in peri- implant bone, bone and mini screw implant models were

Discussion

63

simulated and analysed via finite element method. The reliability of finite

element analysis is dependant on user input data, mesh geometry, element

type, boundary conditions ,interface between bone and implant, material

properties ,etc. (Hsu and Chang53

).

Since bone is a complex organic structure which is heterogenous and

anisotropic its response to forces or loads applied in different directions is not

the same54

. It means that the bone has different elastic moduli or varying

degrees of stiffness along different directions. If stresses and strains in bone

need to be evaluated the simulated bone model should mimic these anisotropic

properties of bone. Although many authors have evaluated stress distributions

in FEA bone model, the material properties were commonly modeled as

linearly isotropic or transversely isotropic and homogenous51,55.

Two separate models with two different orthotropic material properties

for cortical bone were simulated keeping the cortical bone thickness

constant at 2mm for both models and the cancellous bone constant for the

two models. The derived orthotropic material properties for the cirtical

bones were associated with 1.6gms/cm3 and 1.9gms/cm

3. These values

were derived from a study done by Peterson et al.11

Since we aimed at evaluating the influence of only cortical bone density on

implant stability we kept cancellous bone properties constant between

both models. Also the cancellous bone model was modelled as a linearly

Discussion

64

isotropic and a homogenous structure as deriving orthotropic properties

for cancellous bone has been quite difficult in literature. The elastic

modulus of 0.3gm/cm3 for cancellous bone was derived from the previous

studies.

In order to capture the curved geometry of the infrazygomatic area ,a

quadratic 3D tetrahedral mess was generated. Huang et al56

proved that

implant stability was related to density by combining resonance frequency

analysis and FEA models. They found that as the bone quality /density

decreased, the resonance frequency analysis showed a low value indicating

less implant stability. To add to that, their study found an inverse relationship

between bone density and stresses in peri-implant bone.

The results in our FEM study agreed with the conclusions of many

authors. When a 2mm implant was inserted into the low density cortical

bone model (1.6 gms/cm³.) the mechanical stress in the cortical bone was

equal to 4.5 MPa and 1MPa in cancellous bone. In the high density

cortical bone model (1.9 gms/cm3) the cortical bone stress was 3.2 MPa

and 1MPa in cancellous bone indicating that as the bone density increased

the stresses in surrounding bone decreased.

Li et al57

established values based on mathematical model for stresses that

associated with overload and underload bone resorption. According to their

stress curves the threshold for overload bone resorption increases with bone

Discussion

65

density. Overload resorption was observed when von misses stresses exceeded

28MPa for a corresponding cortical bone density of 1.8gms/cm3. With regard

to cancellous bone overload resorption was observed when von misses streses

exceeded 6MPa for a corresponding cancellous bone density of 0.8gms/cm3.

In our study although an inverse relationship was seen between bone

density and mechanical stress. The von misses stress values could not have

crossed the overload threshold values of 28MPa and 6MPa for cortical

and cancellous bone respectively as cortical bone was modelled with 2mm

thickness.

An inverse relationship between bone thickness and mechanical stress was

established by Motoyoshi et al55

. In their study a cortical bone thickness of

less than 1mm was associated with maximum von misses stresses. The new

mathematical bone remodelling model developed by Li et al57

shows that

negative density change can occur at high load levels while under stresses of

4, 6 and 8MPa bone density increased under stress stimulus.

As long as the mechanical stresses were within the critical threshold levels for

underload and overload resorption there would be a positive increase in

density. It will be reasonable to say that stresses generated within this

physiologic load window will be responsible for overall stability and success

of mini-implant.

Discussion

66

In a 5 year analysis the success rates of branemark implants documented by

Jafin and Bermin58

. They noted the quality of bone or bone density was the

largest determinant of implant loss. Out of type 1 ,type 2, type 3 and type 4

bone types, they noted 35% failure of implants in type 4 bone compared to a

combined 3% failure in other bone types suggesting that thin cortex coupled

with reduced bone density are associated with implant failure.

In our study the effect of insertion angle of mini-implant was additionally

evaluated. When the implant was inserted perpendicular to the bone

surface the aforementioned stresses were noticed in the cortical bone.

When the implant was inserted at a more oblique angle of 20 degrees the

von misses stress in the high density cortical bone was 5.3MPa and a

corresponding 14MPa stress in the cancellous bone. In the low density

cortical bone the von misses stresses were 6.4MPa and 14MPa in

cancellous bone. In comparison to a perpendicular insertion of mini-

implant the oblique/angled insertion generated higher stresses in bone in

both the low and high density bone models. Interesting to note was at an

angled insertion the overall stress was 3 to 4 times greater with much of

the stress being transferred to cancellous bone.the stress in the cancellous

bone was 14MPa in both low and high density models typically crossing

the overload threshold of 6MPa for cancellous bone

According to Li’s mathematical bone remodelling model this should cause

overload resorption in cancellous bone and greater stresses in cortical bone.

Discussion

67

This coupled with low bone density will generate much higher stresses. These

results concurred with other with the results of other studies.

Benedict Wilmes59

supported a less oblique angle of insertion ( 60 degrees to

70 degress) for achieving primary stability as this range was associated with

highest insertion torque values while a very oblique insertion angle (30

degrees) resulted in reduced primary stability. At the same time extremely

high insertion torques lead to increased risk of micromotion of mini-implants

and reduced primary stability particularly in soft bone or low density bone as

stated by Trisi et al60

.

Motoyoshi et al61

; 2005 recommended placement torque to be within the

range from 5 to 10 Ncm for increasing the success rate of 1.6mm diameter

mini-implants. In another study by Motoyoshi et al62

in 2010, the authors

assessed the removal torque in relation to different placemet torques (low, 0-5;

intermediate, 5-10; and high, 10-15 N cm) and removal torque did not change

in the low-torque group, but it decreased significantly in the intermediate- and

high torque groups; almost from 8 Ncm to 4 Ncm. They concluded that

immediately after placement, the implant-bone interface is affected by bone

stiffness at the prepared site, screw design and diameter of the mini-implant.

Several months after placement, the increased compressive stress on the bone

surrounding the mini-implant might disappear with accompanying bone

metabolism, thus reducing the torque.

Discussion

68

Since, infrazygomatic crest implants are extra alveolar implants ranging from

2 mm in diameter, insertion torque values greater than 10Ncm can be expected

during their placement. In fact Wilmes et al59

observed insertion torque values

for 2mm diameter implants to reach 10.8 Ncm.

Lakshmikantha et al63

, in their study using optical coherence tomography,

hypothesized that accumulation of stress in the cortical bone leads to

formation of microdamage of the cortical bone as a method to relieve the

stresses accumulating around the microimplant. They define microdamage as

the combination of microcracks, micro elevation and bone debris formation,

collectively effecting the structural integrity of the cortical bone around the

microimplant. They stated that large microcracks can develop into areas of

weak bone structure or poor bone quality and compromise the balance at the

bone-implant interface, leading to failure of microimplants and this

phenomenon is more pronounced at oblique insertions owing to the greater

cortical bone encountered during oblique insertion.

So, it can be inferred that in the case of infrazygomatic crest mini screw

implants that are usually 2mm in diameter, with a tendency for higher

insertion torque values, cortical bone with good density (D1> D2>D3>D4)

becomes a prerequite for enhanced primary and secondary stability.

Stronger cortical bone can limit the spread or propogation of

microcracks, ensuring effective remodeling repair around the mini screw

Discussion

69

implant. This will help the mini screw implant achieving optimum

secondary stability by the end of 3 weeks.

Although correlation between insertion torque values and Hounsfield units

were between 0.76 – 0.85 (r value) were derived from studies reviewed by

Marquezan et al49

bone density value can be a better indictor of overall

stability because insertion torque values represent stability only at the time of

insertion. On the other hand RFA can be used to measure implant stability at

anytime during the life of a mini screw implant in bone. RFA value in turn

depends on optimum bone density and repair mechanisms.

As for the correlation between cortical bone thickness and cortical bone

density, in our study, we found a moderate positive correlation between the

two variables ( r= 0.604, P=0.000). This result agreed with the results of Li et

al57

who found positive correlation between cortical bone thickness and

density that were assessed using both CT and CBCT scans. Their r values

were r=0.924 and r=0.928 on CT and CBCT scans respectively concluding

that increase in cortical bone thickness correlated with increase in cortical

bone density and primary implant stability. They also suggested that in low

bone density regions increasing the insertion depth of mini-implant may

compensate for the reduced primary stability usually associated with these low

density regions. But, Peterson et al11

found a negative correlation between

bone thickness and density in their study. These differences can be attributed

to the methodological variations between the studies. Peterson et al. took ash

Discussion

70

weight to derive apparent density of different regions of bone. Marquezan et

al49

. reasoned that with increase in cortical bone thickness, there will be an

increase in the area of mineralized tissue, therefore, cortical and trabecular

bone densities could be affected by cortical bone thickness.

On the whole the null hypothesis can be rejected as significant differences

were found between Class I, Class II and Class III facial patterns. The

results of the fem study showed that low cortical bone density is

susceptible to a greater magnitude of stress at the implant-bone when

compared to high density cortical bone.

CLINICAL IMPLICATIONS

Acknowledging bone quality and quantity, particularly bone density and

cortical bone thickness will aid a clinician in ensuring stability of IZC mini

screw implant. Because, the bone parameters can help a clinician decide on the

appropriate implant design. Facial skeletal pattern and associated bite forces

can alter the stress trajectories along the infrazygomatic crest thereby,

influencing the density and thickness of cortical bone. A mean increase in

density toward the mesial of first molar region in the Class III skeletal pattern,

an increase in density trend between the first and second molars in the Class II

skeletal pattern may be due to the mesially and distally displaced occlusal

forces and their associated stress trajectories in Class III and Class II groups

respectively. Class I skeletal pattern showed increased bone density lateral to

Discussion

71

the first molar. Furthermore, attention to the extent of sinus floor can help the

clinician avoid penetration into it during IZC mini screw implant placement,

as it can be seen at even 9 -10 mm level from the alveolar crest in some

patients.

This study had the following limitations:

i. The sample was undersized and heterogenous.

ii. Homogeneity can be achieved by limiting the variations in MPA

iii. The use of CBCT for assessing bone density, although acceptable to

many authors, still remains controversial due lack of conversion

algorithms.

iv. The derived density values from CBCT images could not be

adequately correlated to the apparent density values chosen as input

data for finite element analysis.

Summary & Conclusion


72

SUMMARY AND CONC LUSION

The present study was conducted with an aim to evaluate the cortical bone

thickness and cortical bone density of the infrazygomatic crest region in

different sagittal skeletal patterns – Class I, Class II and Class III, with a view

to derive clinical implications for the placement of infrazygomatic crest mini-

implants. It is a retrospective study in which evaluation of the above

mentioned cortical bone parameters were evaluated on CBCT scans of 50

patients. Categorization of the sample into the three sagittal skeletal patterns

was based on ANB angle. The scanned images were imported into Dolphin

Imaging Software (version 11.9, Chatsworth, Calif) in the DICOM file format

for performing the cortical bone thickness and density measurements. Results

were derived using statistical parametric tests. Finally, to objectively evaluate

the effect of cortical bone density on stress distribution in peri-implant bone, a

finite element analysis was run.

Based on the results of the present study, the following conclusions can be

drawn:

i. Cortical bone thickness and density increase at increasing distances to

the alveolar crest or with progression toward basal bone in all three

skeletal patterns.


73

ii. The region distal to second molar is associated with low cortical bone

density. Cortical bone thickness is less than 1mm in all three skeletal

patterns, distal to second molar

iii. Class III skeletal pattern shows tendency for a denser cortical bone

between the second premolar and first molar while Class II skeletal pattern

shows tendency for denser cortical bone between the first and second

molars.

iv. Class I skeletal pattern shows greater cortical bone density in the

mid-root region of the first molar.

v. Significant differences exist between the three sagittal facial patterns with

cortical bone parameters.

vi. Overall cortical density and thickness is higher in Class I facial pattern.

Class II and Class III patterns show relatively lesser overall cortical bone

density and thickness.

vii. Cortical bone thickness in the infrazygomatic crest region for Class I

skeletal pattern is in the range of 0.9 – 1.4 mm. Cortical bone thickness in

the infrazygomatic crest region for Class II and Class III skeletal patterns

is in the range of 0.7 – 1.1 mm.

viii. Cortical bone density has an inverse relationship to stresses generated at

implant-bone interface.

ix. Extremely oblique angle of insertion of mini screw implant relative to

bone surface and anatomical shape is associated with increased stress

generation in bone.

x. Cortical bone thickness and density have a moderate positive correlation.


74

FUTURE DIRECTION

The evaluation of position and extent of the maxillary sinus floor and its

influence on mini screw implant placement in the infrazygomatic region with

regard to different different facial types might add depth and dimension for a

more comprehensive understanding of the infrazygomatic crest as an

implantation site. Furthermore, the effect of bicortical anchorage for IZC

implant may serve to bring an understanding of the relative importance of

cortical and cancellous bone.

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Annexures

Annexures

ANNEXURE – I

Annexures

ANNEXURE – II

Documents

MASTER OF DENTAL SURGERY