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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_537_Библиотеки_им_академика_М_И_Перельмана
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impact the bone while the tensile strength is much less than that of the
compressive strength [13].
Facial skeleton has predetermined locations called buttresses that
provide local structural support, and these locations are classified based on
directionality. For instance, vertical buttresses are formed by
pterygomaxillary, zygomatic, nasomaxillary, and vertical ramus of the
mandible [39], which are displayed Fig. 2 [23].
Fig. 2 Vertical buttresses marked on skull [39]
Inferior, middle, and superior are the three horizontal buttresses
displayed on the left image of Fig. 3. The inferior horizontal buttress
consists of the arch of the palate and the symphysis region of the mandible.
The frontal bandeau forms the superior horizontal buttress, while the middle
horizontal buttress is formed by the inferior orbital rims and the zygomatic
bones displayed on the right side [39]. The transverse buttress, displayed in
the right image of Fig. 3, consists of the body of the mandible, zygomatic
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arches, medial and lateral walls of the orbit, and the posterior part of the
alveolar process of the maxilla and mandible [39].
Fig. 3 Regional markings of horizontal (left) and transverse (right) buttresses [39]
3 Basics of Mandible Bone Biomechanics
An oversimplification of the mandibular force structure can be outlined as
shear, tensional, and compressional forces acting as the linear forces
applied on the mandibular region, while angular loads are torsional and
bending [38]. Figure 4 demonstrates the different forces and long axis of the
mandible [39].
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Fig. 4 The forces acting on the mandible and its long axis are displayed
Initially, mandible biomechanics was outlined by simplifying the impact
of existing forces, and it was explained by using the existing tensile stresses
on the upper border and compressive stresses on the lower border, which is
known to be the simple beam mechanism. The missing elements in this
consideration included the impact of the musculature, forces attained from
chewing, and bending of mandible under functional stress. Advancements
in biomechanics allowed strain gauge, free body diagram, finite element
analysis (FEA), and photo-elastic resin to be used in mandibular mechanical
advancements [40–42].
Finite element analysis is frequently used in research for analysis of
mandibular biomechanics; it has been shown that simple beam mechanism
holds if occlusal force is applied on the anterior of the fracture site, while
effects of contralateral muscle action come into play when occlusal force is
applied posterior to fracture site [43–48].
The mandible laterally bending during powerful clenching causes
tensile stress generation over lingual cortex and compressive stress
generation at buccal surface [50]. Torsional forces that are seen in the
symphysis/parasymphysis region of the mandible cause compressive stress
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at the superior margin and tensile stress at the inferior margin in the
symphysis region [49].
Ultrasonic wave [6, 7, 9–12] and material testing [4, 5] techniques are
used on cadaveric models in the literature to be able to configure
mandibular bone material properties. Mandibular cortical bone
biomechanics is typically identified in three orthogonal directions
consisting of longitudinal, radial, and tangential. Loading cannot be
performed in the radial direction due to the small cortical thickness within
the region [1].
The cortical bone of the mandible is determined to be anisotropic [1].
The bone is stiffer in the longitudinal direction than it is in the radial and
tangential directions. The mean elastic moduli in the tangential and radial
directions are about 40–70% of the ones along the longitudinal direction.
Stiffer and stronger nature of the mandible in the longitudinal direction
may be because of the orientation of collagen fibers, osteons, and apatite
crystals, which are reported to be primarily oriented in this direction [8].
The mandible is determined to be the most resistant to sagittal bending
moments due to larger longitudinal stiffness and strength.
The least resistance of the mandible to shear is found to be in a plane
constructed of the radial and tangential axes, while the shear moduli
calculations in the literature indicated the mandible to be most resistant to
shear in a plane constructed of the longitudinal and tangential axes [4, 5,
10]. Figure 5 demonstrates different areas of force applications in
longitudinal (L), tangential (T), and radial (R) directions on the mandibular
region of both male and female [3]. Figure 6 demonstrates the torsional
force direction on the mandible, along with the forces applied on the
mandible.
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Fig. 5 Longitudinal (L), tangential (T), and radial (R) directions on the mandibular region of male
and female
Fig. 6 Torsion application on the mandibular section, with forces applied in the region demonstrated
by arrows
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Not surprisingly, there are regional variations measured in the literature
based on elastic moduli and yield strengths. For instance, up to 150%
difference between the minimum and maximum regional values with no
systematic differences are determined upon the analysis of the collected
data [4, 5]. On the contrary, the lower border inferior to the canine is
determined to be the stiffest mandibular corpus in [9]; this region has the
highest resistance to torsion, where torsional stresses are suspected to
change during function.
A combination of sagittal bending, corpus rotation, and transverse
bending is determined to occur during biting and the power stroke of
mastication [1], resulting in a complex pattern of stresses and strains in the
mandible. Geometric and material properties of the mandible play a crucial
role on the resisting forces and bending and torsional moments. Moments of
inertia and the distribution of bone tissue are the relevant critical variables.
Overall, the mandible is stiffer in the longitudinal direction than in the
transverse directions. Additionally, the vertical cross-sectional dimension of
the mandible is identified to be larger than its transverse dimension. These
findings indicate the mandible’s ability to resist relatively large vertical
shear forces and bending moments that occur in the sagittal plane [1].
It is important to note that the assumptions made on the points of force
applications with the associated magnitudes and directions of the forces
acting on the jaw resulted in variational results. The use of static situations
is another factor that impacts the results. Another factor is the limitation of
the information received; the data are obtained only about strains in the
superficial bone layer of the jaw. Uncertainties, assumptions, and
simplifications in the research literature can result in variational results that
are also difficult to evaluate with different conclusions.
Mathematical models of the mandible are also used in the literature.
Material complexity of the trabecular bone makes it significantly
heterogeneous; therefore, high variability of the elasticity and strength
across the anatomical regions with the existence of trabecular bone is no
surprise. Additional factors such as aging, disease, and minerals (such as
calcium) existing within the bone impact the biomechanical properties;
therefore, it is hard to identify the biomechanical properties of such bone
within the mandibular region [2]. There are complex mechanical behaviors
such as multiaxial loading, time-dependent failure, and damage
accumulation that have been observed empirically for biomechanical
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observations. A unique tool for studying such behavior is the
microstructural class of finite element models. Bone fracture, bone
remodeling, and design/analysis of bone-implant systems are strongly
impacted by the associated biomechanical findings. Particularly in the case
when a donor bone is used for reconstruction of the mandible,
biomechanical properties of the donor bone and the associated mandible are
expected to have a good match.
4 Basics of Nasal Bone Biomechanics
The nasal apertures are broad; they may have rate high in endomorphic
characteristics and may have a high periosteal activity that can be expressed
by a high factor in “sturdiness” [14].
Nasal reconstruction is commonly applied to nasal septal cartilage,
which is a common source of chondrocytes [15]. The biomechanical
properties of septal cartilage are race dependent; therefore, one of the
factors that impact the biomechanical properties of the nasal bone is race.
For instance, the Asian nose is generally described to have flared nostrils, a
bulbous tip, short columella, wide alar base, acute nasolabial angle, thick
nasal skin, and low dorsum in contrast to the Caucasian nose [16].
Septal extension grafts are used in nose correction surgical procedures
[17]. In Asians, septal extension has been used commonly; however, there
are other graft materials used in the literature along with the stability of the
aesthetic results [16]. Anisotropic behaviors of septal cartilage under
compression is investigated in [16] in relation to the collagen within the
cartilage of the septum within the nose based on its unique structure and
orientation.
Regardless of gender, age, or axial tension, human septal cartilage
tensile properties have been shown to be weaker than articular cartilage
[18]. An implant’s mechanical behavior requires to carefully define the
compressive mode of loading that are impacted by tensile properties and
characteristics [19]. Correction of short nose when septal cartilage is
unavailable because of prior harvesting, trauma, or genetics can be
accomplished by using conchal cartilage as the septal extension graft
material [20]. The costal cartilage is essential for more severe deformities
or graft-depleted revision rhinoplasty, even though septal and conchal
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cartilage are both used to extend the caudal septum when correcting mild
short nose [21].
The correction method is location dependent. Nasal septal deformities
in the caudal–cephalic and dorsal–maxillary planes alone (or combined)
may be a result of generic, developmental, and traumatic factors. The
biomechanics of the cartilage gains importance during the application of
endonasal approach for such corrections. Local and global clinical
assessment of the restructured nose is essential. Cosmetic and functional
outcomes from a nasal surgical procedure depend on nasal bones,
perpendicular and quadrilateral plates, lateral cartilages, and turbinate [22].
5 Biomechanics of Maxillofacial Reconstructions
Protection and support of the brain and masticatory system are
accomplished by the bones in the craniomaxillofacial region. Large bone
defects in the craniomaxillofacial region and how it influences the
designing process of the implant are essential to be observed.
Level of energy generated with the associated forces is a way to identify
the biomechanics on facial structure. Mechanical complexity generated by
the forces is uneasy to explain. For instance, impact energy observed during
an accident is absorbed by facial soft tissues and skeleton, while a part of it
is transmitted to intracranium [63]. This is a simple indicator that the force
of the impact is a significant factor for identifying the severity of the facial
fracture and head injury. Reactional force is another factor that plays a
significant role in maxillofacial injuries. The maxilla is highly important for
functional, physiological, and esthetic reasons when central craniofacial
fractures such as nasal, lacrimal, vomer, maxillary, ethmoidal, and frontal
bones occur. It is also important due to its alliance with other bones in the
central area, which plays a crucial role in its capability to absorb
considerable impact energy and helps to protect the brain from direct
collision [59].
Dentition occlusal forces formed the bony conduction trajectories along
the thickened buttresses of the maxillofacial bones consisting of the
following buttresses [64]:
Horizontal buttresses bilateral prefrontal, zygomatic and maxillary
buttresses
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Vertical buttresses bilateral nasomaxillary, zygomaticomaxillary,
pterygomaxillary buttresses, and the median sagittal buttress)
The main stress trajectories existing in the facial region are determined
to be the nasomaxillary, zygomaticomaxillary and pterygomaxillary
trajectories based on the photo-elastic technique in [65]. Mechanics of
muscles, bones, and dentition on the facial structure generate the
mechanical equilibrium of the functional system. Displacement of maxilla
could occur due to maxillary buttress defects, which could cause instable
maxillary segments [67]. Cleft of the unilateral complete cleft lip and palate
interrupt the physiological occlusal stress transmission by abolishing the
integrity of maxillofacial bone structures; the mechanical balance would be
lost, and the stability of the maxillofacial structures would be affected
noting that it would have a negative impact on the growth and development
of the maxillofacial region. The stress distribution between the cleft and the
noncleft side of the congenital unilateral maxilla cleft was determined to be
asymmetrical and uneven by using the finite element method [68–70].
There are other additional maxillofacial structural considerations with
testing of screws and other local biomechanical analyses conducted in the
literature that we leave to the readers [71–76].
6 Biomechanics of Mandibular Reconstructions
There are typical displacement patterns in mandibular fractures. The action
of the strap muscles, such as the geniohyoid with the proximal, causes the
distal, or anterior, segment to rotate downward, and the temporalis muscle
causes the condylar segment rotating upwards. Compressive strain occurs at
the inferior border, while tensile strain occurs at the upper border of a
mandibular fracture. Sub-condylar mandibular fractures will deviate to the
side of the fracture upon opening of the mouth due to the protrusive action
of the lateral pterygoid inserting into the mandibular fovea [94].
Conventional manual-bending reconstruction plate is the conventional
method that was used for mandibular reconstructions. Advancement of
additive manufacturing allowed personalized mandibular plates’ printing to
be able to precisely contour the mandible’s lateral surface, flush with the
inferior and posterior borders, and avoid injuring the inferior alveolar nerve
[84, 85]. This personalized approach through additive manufacturing also
reduces the possibility of plate fracture through reduction of residual
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stresses typically generated through repeated bending of the metal plate
during surgery and possible microcracks that impact the mean stress in
fatigue loading [86]. In addition, structural optimization by determining an
optimal design that is capable of load-carrying mechanical structures also
advanced the mandibular reconstruction via topologically optimized shape
and size [87]. Postoperative outcomes of the conventional manual bending
of the plates included crack of the plates at the mandibular angle area
several years after surgeries [88]. Biomechanical performance is enhanced
through personalized mandibular reconstructive plates using additive
manufacturing. Patient-specific mandibular reconstruction plate is also
facilitated as part of interdisciplinary research of mandibular reconstruction
and oral and axilla-facial surgery [88, 89].
One of the major applications of biomechanics is based on
reconstruction of the mandibular area after resection of a tumor. It is
possible to bridge mandibular continuity defect by using a reconstruction
plate for patients that cannot undertake reconstructive surgery or
postoperative monitoring when tumor recurrence is needed [79–81]. The
failure of the plates designed for mandibular reconstruction applications
was one of the reasons for researchers to invest their time to determine
optimal designs for such applications. The differentiability of the anatomy
naturally forced researchers to design patient-specific mandibular
reconstruction plates. One area of investigation is to determine the
difference between the original mandibular plate design and the structurally
optimized plate design. After tumor resection, mandibular reconstruction
plates that are structurally optimized for individualized treatment were
created to improve the biomechanical performance of bridging segmental
bony defect in the mandibular reconstruction after tumor resection in [77].
The muscles along with the fixation screws placed in this region also had
important roles in the biomechanical specifications of the mandibular
reconstructions. Optimizing the structure and mechanical stability of
patient-specific mandibular reconstruction plates with all the associated
tools to be used (such as screws) along with physiological conditions makes
a difference in applications. Noting the high number of elements taking
place in reporting biomechanical outcomes, researchers attempted to
simplify the analysis to specific methods. Finite element analysis (FEA) is
the most common method used by the researchers (as previously pointed
out), and different calculation methods are used, with von Mises stress
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