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
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.
Review of Literature
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
Review of Literature
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-
Review of Literature
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
Review of Literature
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
Review of Literature
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
Review of Literature
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.
Review of Literature
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
Review of Literature
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
Review of Literature
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
Review of Literature
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
Review of Literature
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
Review of Literature
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 %).
Review of Literature
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
Review of Literature
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,
Review of Literature
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
Review of Literature
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.
Review of Literature
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
Review of Literature
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.
Review of Literature
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.
Review of Literature
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
Review of Literature
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.
Review of Literature
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.
Review of Literature
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
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
Material and Methods
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
7 mm from the alveolar crest
9 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
Material and Methods
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
Material and Methods
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
Material and Methods
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).
Material and Methods
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.
At 7mm, lowest cortical bone density was found in the U7-U8 region (772.00
± 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.
At 9mm, lowest cortical bone density was found in the U7-U8 region (716.76
± 207.72 HU). The highest cortical bone density was found in U6-U7 region
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.
At 7mm, lowest cortical bone density was found in the U7-U8 region (726.63
± 241.70 HU). The highest cortical bone density was found in U5-U6 region
(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.
At 9mm, lowest cortical bone density was found in the U7-U8 region (721.40
± 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,
SD=178.08) and thickness (M=1.07, SD=0.42) with r=0.63
iii. Pearson’s r at region U6-U7 indicated a statistically
significant positive correlation between density (M=937.32,
SD=158.01) and thickness (M=1.17, SD=0.42) with r=0.60
iv. Pearson’s r at region U7 indicated a statistically significant
positive correlation between density (M=869.53,
SD=194.05) and thickness (M=1.01, SD=0.38) with r=0.58
v. Pearson’s r at region U7-U8 indicated a statistically
significant positive correlation between density (M=770.67,
SD=215.44) and thickness (M=0.81, SD=0.34) with r=0.60
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.
Density Mean ± SD Mean ± SD Mean ± SD
U5-U6 895.91 ± 158.69 911.14 ± 137.91 987.55 ± 161.78 .199 NS U6 901.52 ± 122.89 933.67 ± 186.20 958.17 ± 125.93 .085 NS U6-U7 916.23 ± 127.00 985.41 ± 110.99 1022.9 ± 167.16 .257 NS U7 825.50 ± 178.84 845.73 ± 193.11 844.02 ± 193.55 .942 NS U7-U8 696.91 ± 184.64 772.00 ± 216.18 716.76 ± 207.72 .541 NS
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
MEAN DIFFERENCE BETWEEN SITES (TABLE 4)
U5-U6 U6 U6-U7 U7 U7-U8 P value Sig.
Density Mean ± SD Mean ± SD Mean ± SD Mean ± SD Mean ± SD
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
GRAPH 2a : DIFFERENCE IN CORTICAL BONE DENSITY AT THE 3 VERTICAL
LEVELS FROM THE ALVEOLAR CREST IN CLASS II SKELETAL PATTERN
GRAPH 2b : DIFFERENCE IN CORTICAL BONE THICKNESS AT THE 3 VERTICAL
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
GRAPH 2c : DIFFERENCE IN CORTICAL BONE DENSITY AT THE 5 ANTERO-
POSTERIOR REGIONS CLASS II SKELETAL PATTERN
GRAPH 2d : DIFFERENCE IN CORTICAL BONE THICKNESS AT THE 5 ANTERO-
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.
Density Mean ± SD Mean ± SD Mean ± SD
U5-U6 959.40 ± 103.35 972.76 ± 136.00 1009.4 ± 122.87 .511 NS U6 866.27 ± 159.86 928.76 ± 134.20 1002.8 ± 117.17 .034 NS U6-U7 828.23 ± 185.92 897.40 ± 169.03 897.00 ± 106.84 .392 NS U7 844.80 ± 208.34 877.73 ± 151.10 838.80 ± 205.83 .834 NS U7-U8 667.50 ± 218.46 726.63 ± 241.70 721.40 ± 250.78 .753 NS
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
MEAN DIFFERENCE BETWEEN SITES (TABLE 6)
U5-U6 U6 U6-U7 U7 U7-U8 P value Sig.
Density Mean ± SD Mean ± SD Mean ± SD Mean ± SD Mean ± SD
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
GRAPH 3a : DIFFERENCE IN CORTICAL BONE DENSITY AT THE 3 VERTICAL
LEVELS FROM THE ALVEOLAR CREST IN CLASS III SKELETAL PATTERN
GRAPH 3b : DIFFERENCE IN CORTICAL BONE THICKNESS AT THE 3 VERTICAL
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
GRAPH 3c : DIFFERENCE IN CORTICAL BONE DENSITY AT THE 5 ANTERO-
POSTERIOR REGIONS CLASS III SKELETAL PATTERN
GRAPH 3d : DIFFERENCE IN CORTICAL BONE THICKNESS AT THE 5 ANTERO-
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**
Sig. (2 tailed) 0.000
N 150
U7 Density Pearson Correlation 0.583**
Sig. (2 tailed) 0.000
N 150
U7-U8 Density Pearson Correlation 0.606**
Sig. (2 tailed) 0.000
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.
Density Mean ± SD Mean ± SD Mean ± SD
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
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.
Summary & Conclusion
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.
Summary & Conclusion
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