An In Vivo Study of the Musculo-Aponeurotic Architecture of Human Masseter Muscle
by
Teodora-Iunia Gheorghe
A thesis submitted in conformity with the requirements for the degree of Master of Science Oral Radiology
Faculty of Dentistry University of Toronto
© Copyright by Teodora-Iunia Gheorghe 2018
ii
An In Vivo Study of the Musculo-Aponeurotic Architecture of
Human Masseter Muscle
Teodora-Iunia Gheorghe
Master of Science Oral Radiology
Faculty of Dentistry
University of Toronto
2018
Abstract
It has been suggested that architectural changes occur in masseter muscle (MM) in conditions of
facial pain. To understand pathological changes, it is necessary to elucidate normal musculo-
aponeurotic architecture. Muscle architecture is characterized by parameters including fiber
bundle length (FBL) and height of aponeuroses. The purpose of this study was to investigate the
architecture of MM volumetrically in 24 asymptomatic participants using ultrasound, in the
relaxed and maximally contracted states. Masseter consisted of superficial (SH) and deep heads
(DH), each arranged in laminae. Fiber bundles extended between superior and inferior
aponeuroses and/or bone. Statistically significant differences were observed in mean FBL and in
the mean height of aponeuroses between the relaxed and contracted states only in superficial
laminae of SH. These results suggest there is differential contraction of the laminae of MM.
Future comparison with pathologic subjects can be made based on the normative database
established.
iii
Acknowledgments
I wish to firstly express my sincere thanks to my amazing supervisor, mentor, and friend: Dr.
Anne Agur. I feel I have been both lucky and privileged to have had the opportunity to work
with you. I am incredibly grateful for the time you sacrificed to teach, guide, and help me at
every step of the way during these past few years.
Many thanks to the members of my Program Advisory Committee: Dr. Ernest Lam, Dr. Susanne
Perschbacher, and Dr. Bernie Liebgott. The wisdom and support you have provided me with is
greatly appreciated.
To Dr. Roger Leekam, your expertise and willingness to impart knowledge have been a great
help. I would like to express my gratitude for the interest you took in this thesis and the
contributions you made to it.
Thank you to my fellow graduate students in Dr. Anne Agur's lab and my co-residents in the
Graduate Program in Oral and Maxillofacial Radiology. This was a wonderful journey, and your
friendship was a large part of that.
I would lastly like to thank my family. To my grandmother: thank you for the confidence you
have always had in me. To my parents: I am exceptionally thankful for your love and tireless
work in helping me get to where I am today.
iv
Table of Contents
Acknowledgments.......................................................................................................................... iii
Table of Contents ........................................................................................................................... iv
List of Tables ............................................................................................................................... viii
List of Figures ..................................................................................................................................x
List of Abbreviations ................................................................................................................... xiii
Chapter 1 ..........................................................................................................................................1
1 Introduction .................................................................................................................................1
Chapter 2 ..........................................................................................................................................2
2 Background .................................................................................................................................2
2.1 Masseter Muscle ..................................................................................................................2
2.1.1 Morphology..............................................................................................................2
2.1.2 Functions ..................................................................................................................3
2.2 Structure of Human Skeletal Muscle ...................................................................................4
2.2.1 Contractile Tissue Elements ....................................................................................4
2.2.2 Connective Tissue Elements ....................................................................................7
2.3 Architectural Parameters ......................................................................................................8
2.3.1 Parameters of Contractile Tissue Elements .............................................................8
2.3.2 Parameters of Connective Tissue Elements .............................................................9
2.4 Cadaveric Masseter Muscle Architecture ............................................................................9
2.4.1 Morphology............................................................................................................10
2.4.1.1 Two-Dimensional Studies..........................................................................10
2.4.1.2 Three-Dimensional Studies........................................................................11
2.4.2 Architectural Parameters ........................................................................................13
2.4.2.1 Two-Dimensional Studies ..........................................................................13
v
2.4.2.2 Three-Dimensional Studies ........................................................................15
2.4.2.2.1 Measurement of Architectural Parameters..................................15
2.4.2.2.2 Quantified Architectural Parameters of Masseter.......................16
2.5 In Vivo Studies of Masseter Muscle...................................................................................18
2.5.1 Electromyography ..................................................................................................18
2.5.2 Computed Tomography .........................................................................................19
2.5.3 Magnetic Resonance Imaging ................................................................................19
2.5.4 Ultrasound ..............................................................................................................20
2.6 Masseter Muscle Pathoses: Clinical Applications .............................................................23
2.7 Gaps in Knowledge and Rationale .....................................................................................24
Chapter 3 ........................................................................................................................................26
3 Research Aims and Hypotheses ................................................................................................26
Chapter 4 ........................................................................................................................................28
4 Materials and Methods ..............................................................................................................28
4.1 Participants .........................................................................................................................28
4.1.1 Sample Size Calculation ........................................................................................28
4.1.2 Screening................................................................................................................28
4.1.2.1 Screening Questionnaire............................................................................28
4.1.2.2 Screening Examination..............................................................................30
4.1.2.3 Preliminary Ultrasound Screening............................................................30
4.1.3 Inclusion and Exclusion Criteria ............................................................................30
4.2 Equipment ..........................................................................................................................31
4.2.1 Ultrasonography .....................................................................................................31
4.2.2 Electromyography ..................................................................................................31
vi
4.3 Participant Positioning .......................................................................................................31
4.4 Ultrasound Protocol ...........................................................................................................31
4.4.1 Anatomic Verification ...........................................................................................32
4.4.2 Ultrasound Image Acquisition ...............................................................................33
4.4.2.1 Image Acquisition in the Relaxed and Maximally Contracted States.......33
4.4.2.2 Ultrasound Images Acquired.....................................................................33
4.4.3 Ultrasound Image Assessment ...............................................................................35
4.4.3.1 Morphology................................................................................................35
4.4.3.2 Architectural Parameters............................................................................35
4.5 Data Analysis .....................................................................................................................37
4.5.1 Morphology............................................................................................................37
4.5.2 Architectural Parameters ........................................................................................38
Chapter 5 ........................................................................................................................................39
5 Results .......................................................................................................................................39
5.1 Gross Morphology .............................................................................................................39
5.2 Fiber Bundle Morphology..................................................................................................41
5.2.1 Laminae of Superficial Head .................................................................................41
5.2.2 Laminae of Deep Head ..........................................................................................43
5.3 Morphology of Aponeuroses .............................................................................................47
5.3.1 Superficial Head.....................................................................................................47
5.3.2 Deep Head ..............................................................................................................49
5.4 Architectural Parameters ....................................................................................................52
5.4.1 Contractile Tissue Parameters................................................................................52
5.4.2 Connective Tissue Parameters ...............................................................................54
Chapter 6 ........................................................................................................................................61
vii
6 Discussion .................................................................................................................................61
6.1 Morphology........................................................................................................................61
6.1.1 Cadaveric Studies of Masseter Muscle ..................................................................61
6.1.2 In Vivo Studies of Masseter Muscle.......................................................................63
6.2 Architectural Parameters ....................................................................................................64
6.2.1 Contractile Tissue Architectural Parameters .........................................................64
6.2.2 Connective Tissue Architectural Parameters .........................................................65
6.3 Contraction of Masseter Muscle ........................................................................................66
Chapter 7 ........................................................................................................................................67
7 Conclusions ...............................................................................................................................67
7.1 Hypotheses .........................................................................................................................67
7.2 Significance........................................................................................................................68
Chapter 8 ........................................................................................................................................70
8 Future Directions .......................................................................................................................70
References ......................................................................................................................................71
Copyright Acknowledgements.......................................................................................................81
viii
List of Tables
Chapter 2 Page
Table 2.1. Musculo-aponeurotic parameters of MM in 2D cadaveric
dissection studies.
14
Table 2.2. Mean FBL of laminae of DH and SH. 16
Table 2.3A. Parameters of aponeuroses of SH. 17
Table 2.3B. Parameters of aponeuroses of DH. 17
Table 2.4. CT studies of normal MM architecture. 19
Table 2.5. MRI Studies of Normal MM FB Architecture. 19
Table 2.6. MRI Studies of Normal MM Aponeurotic Architecture. 20
Table 2.7. US studies of mean contracted lengths of MM in asymptomatic
participants.
20
Table 2.8. US studies of mean CSA of MM in asymptomatic participants. 21
Table 2.9. US studies of mean MV of MM in asymptomatic participants. 21
Table 2.10. US studies of mean MT of MM in asymptomatic participants. 22
Chapter 4
Table 4.1. Screening questionnaire. 29
Chapter 5
Table 5.1. Aponeuroses of SH. 47
Table 5.2. Aponeuroses of DH. 50
ix
Table 5.3. Mean FBLs in SH with three laminae (n=28). 52
Table 5.4. Mean FBLs in SH with four laminae (n=20). 52
Table 5.5. Mean heights of SH aponeuroses where two aponeuroses
are present (n=28).
55
Table 5.6. Mean heights of aponeuroses in SH with three aponeuroses
(n=20).
55
Table 5.7. Mean heights of aponeuroses in DH with two aponeuroses
(n=11).
58
Table 5.8. Mean heights of aponeuroses in DH with three total
aponeuroses, where two aponeuroses attached superiorly
and one attached inferiorly (n=26).
59
Table 5.9. Mean heights of aponeuroses in DH with three total
aponeuroses, where two aponeurosis attached inferiorly and
one attached superiorly (n=4).
59
Table 5.10. Mean heights of aponeuroses in DH with four aponeuroses
(n=4).
59
Table 5.11. Mean heights of aponeuroses in DH with five aponeuroses
(n=3).
60
x
List of Figures
Chapter 2 Page
Figure 2.1. Heads of masseter muscle, lateral views. 2, 3
Figure 2.2. Different types of fiber bundle arrangement in skeletal
muscle.
5
Figure 2.3. Organization of contractile elements of skeletal muscle. 6
Figure 2.4. Light micrograph of skeletal muscle showing sarcomeres. 7
Figure 2.5. Sequential dissection of the laminae of the superficial head
of masseter.
12
Figure 2.6. Three-dimensional digitization of laminae of MM.
Reconstruction of laminae from deep to superficial, lateral
view.
13
Figure 2.7. Incremental summation of digitized points along the length
of a FB.
15
Figure 2.8. Determination of the average tangent vector of an FB. 16
Figure 2.9 US Scan of supraspinatus muscle. 23
Chapter 4
Figure 4.1. Correlation of aponeuroses and FB between a coronally-
sectioned cadaveric specimen and US scan.
32
Figure 4.2. Motion of transducer during panoramic image acquisition. 33
Figure 4.3. Locations of acquisition of US scans of SH. 34
Figure 4.4 Electromyography data 35
xi
Figure 4.5.
Determination of FBL within quadrant 1 of MM in relaxed
state.
36
Figure 4.6. Determination of height of aponeuroses on panoramic
coronal scan in relaxed state.
37
Chapter 5
Figure 5.1. Relaxed panoramic coronal US scans of SH. 40
Figure 5.2. Relaxed panoramic coronal US scans of DH. 41
Figure 5.3. Relaxed panoramic coronal US scan of SH with three
laminae.
42
Figure 5.4. Relaxed panoramic coronal US scan of SH with four laminae. 43
Figure 5.5. Relaxed coronal US scan of DH two laminae. 44
Figure 5.6. Relaxed coronal US scan of DH three laminae. 45
Figure 5.7. Relaxed coronal US scan of DH four laminae. 46
Figure 5.8. Relaxed coronal US scan of DH five laminae. 46
Figure 5.9. Relaxed panoramic coronal US scans of SH. 48
Figure 5.10. SH with two aponeuroses correlated in relaxed panoramic
coronal and relaxed panoramic axial scans.
49
Figure 5.11. Relaxed coronal US scans of DH. 51
Figure 5.12. Coronal US scan of SH showing a longer FBL in the relaxed
state than in the contracted state.
53
xii
Figure 5.13. Relaxed panoramic coronal US scans of L1 and L2 of SH. 54
Figure 5.14. Panoramic coronal US scans of SH showing increase in
height of most superficial aponeurosis upon contraction.
56
Figure 5.15.
Panoramic coronal US scans of SH showing increase in
height of two most superficial aponeuroses upon
contraction.
57
Figure 5.16. Coronal US scans of DH showing no changes in height of
aponeuroses upon contraction.
58
xiii
List of Abbreviations
2D Two-dimensional
3D Three-dimensional
ADP Adenosine disphosphate
ATP Adenosine triphosphate
CSA Cross-sectional area
CT Computed tomography
DC/TMD Diagnostic criteria for temporomandibular disorders
DH Deep head
EMG Electromyography
F Female
FB(s) Fiber bundle(s)
FBL Fiber bundle length
ICC Intraclass correlation coefficient
L Lamina
LOA Line of action
M Male
MM Masseter muscle
MRI Magnetic resonance imaging
MV Muscle volume
MT Muscle thickness
PA Pennation angle
PCSA Physiological cross-sectional area
Pi Phosphate group
SH Superficial head
Sp Specimen
TMD(s) Temporomandibular disorder(s)
TMJ Temporomandibular joint
US Ultrasound
1
Chapter 1
1 Introduction
Temporomandibular disorders (TMDs) are responsible for an annual loss of over 17.8 million
workdays in the United States (Wadhwa et al., 2008). Discerning the etiology of these poorly-
understood conditions is imperative. Masseter muscle (MM) involvement has been reported in
conditions of chronic and acute facial pain, including TMDs (Ariji et al, 2004), and it has been
suggested that architectural changes occur in the contractile and connective tissue elements in
MM in these patients (Ariji et al, 2010).
Muscle architecture, a primary determinant of muscle function, is the arrangement of the
contractile and connective tissue elements in the muscle volume. The contractile elements
include the fiber bundles (FB), whereas the connective tissue elements are composed of
aponeuroses/internal tendons (Li et al., 2015).
Two-dimensional (2D) and three-dimensional (3D) cadaveric studies have determined that MM
has a multilaminar morphology, with FBs extending between alternating superior and inferior
aponeuroses (Ebert, 1939; Schumacher, 1961; Ebrahimi, 2015). Three-dimensional cadaveric
studies have involved the digitization of FBs and aponeuroses volumetrically. It was suggested
that detailed 3D models of musculo-aponeurotic architecture could be used to develop in vivo
ultrasound (US) protocols to study normal and abnormal muscle architecture (Ebrahimi, 2015).
Ultrasound is a non-invasive and inexpensive imaging modality that has high inter-rater
reliability and small measurement error (König et al., 2014). Ultrasound protocols to study the
musculo-aponeurotic architecture of skeletal muscles have been developed based on detailed 3D
analysis of the muscle architecture in cadaveric specimens (Kim et al, 2010). However, no in
vivo US studies could be found that investigated MM architecture at a FB level.
The current thesis will focus on the investigation of the in vivo musculo-aponeurotic architecture
of MM using US to determine morphology and quantify architectural parameters.
2
Chapter 2
2 Background
2.1 Masseter Muscle
2.1.1 Morphology
Masseter muscle is one of the four muscles of mastication. MM is composed of a superficial
head (SH), and a deep head (DH) (Figure 2.1). The SH originates on the zygomatic process of
the maxilla and the inferior border of the anterior two thirds of the zygomatic arch. The muscle
fibers of the SH then travel postero-inferiorly to insert on the inferior portion of the lateral
surface of the ramus and angle of the mandible. The DH originates at the medial surface and
inferior border of the posterior third of the zygomatic arch, and inserts on the superior portion of
the lateral surface of the ramus of the mandible (Liebgott, 2018).
3
Figure 2.1. Heads of masseter muscle, lateral views. A. Superficial head (SH) and deep head (DH). B. Attachments of SH and DH.
2.1.2 Functions
Masseter is integral in moving the mandible relative to the maxilla to facilitate functions such as
chewing, drinking, swallowing, and speaking (Farella et al., 2008). It has been shown that
separate heads of multi-headed muscles, such as MM, can have common and independent
functions. When acting bilaterally, the SH elevates and protrudes the mandible. Unilateral
contraction of the SH results in ipsilateral excursion. Upon bilateral contraction, the DH retrudes
the mandible (Liebgott, 2018). Combinations of these movements ultimately accomplish the
complex functions that MM is involved in.
4
2.2 Structure of Human Skeletal Muscle
A skeletal muscle consists of contractile and connective tissue elements. The contractile
elements include FBs/fascicles, whereas the connective tissue elements are composed of
aponeuroses/internal tendons. Muscle architecture, a primary determinant of muscle function, is
the arrangement of the contractile and connective tissue elements in the muscle volume. The
closely-associated contractile and connective tissue elements of a skeletal muscle are aligned
such that they transmit forces upon contraction (Lieber and Ward, 2011). Superficially, skeletal
muscles may appear similar, however due to their specific internal arrangement of FBs,
aponeuroses, and tendons they can differ significantly in function (Lieber and Friden, 2001).
2.2.1 Contractile Tissue Elements
The shapes of skeletal muscles vary according to the arrangement of FBs (Figure 2.2). The most
common shapes include circular, flat, quadrate, fusiform, and pennate. Circular muscles have
FBs that surround an opening, and on contraction act as sphincters (eg. orbicularis oculi closes
the eye) (Liebgott, 2018; Moore et al., 2011). Flat muscles are large, thin, sheets of parallel FBs
that attach to extensive aponeuroses. Quadrate muscles also have a parallel FB arrangement, but
are rectangular in shape. Some quadrate muscles eg. rectus abdominus have intervening
tendinous intersections between a series of muscle bellies. In contrast, fusiform muscles have a
belly with tapered ends, resembling a cigar-shaped structure (Agur and Dalley, 2012).
Pennate muscles are the most frequently occurring skeletal muscles in the human body, and are
classified as unipennate, bipennate, or multipennate. Unipennate muscles have oblique FBs that
attach on one surface of a septum, thus resembling a half-feather. In contrast, in bipennate
muscles FBs attach to both surfaces of a septum, thus resembling the quill and barbs of a feather
(Grant, 1942). In multipennate muscles, FB extend obliquely between multiple septa (Agur and
Dalley, 2012).
5
Figure 2.2. Different types of fiber bundle arrangement in skeletal muscles. Used with
permission from: Essential Clinical Anatomy. 5ed. Moore, K.L.M., Agur, A.M.R., Dalley, A.F.
Wolters Kluwer Health. 2014.
The contractile tissue elements of skeletal muscles are highly organized in a hierarchical manner
(Figure 2.3). Skeletal muscles as a whole, are surrounded by a sheath of fibro-elastic connective
tissue called the epimysium. Within the muscle volume, individual muscle fibers are grouped
together into FBs. Fiber bundles in turn are covered by another sheath of connective tissue called
the perimysium. Fiber bundles are composed of myocytes or muscle cells, each wrapped in a
connective tissue layer called the endomysium. Myofibrils, the major constituent of the
cytoplasm of muscle cells, consist of actin and myosin myofilaments. The actin and myosin
myofilaments are arranged into sarcomeres, the contractile unit of skeletal muscle (MacIntosh et
al., 2006).
6
Figure 2.3. Organization of contractile elements of skeletal muscle. Used with permission from: Clinically Oriented Anatomy. 8ed. Moore, K.L.M., Dalley, A.F., Agur, A.M.R. Wolters Kluwer Health. 2017.
The mechanism by which skeletal muscle contraction occurs is described by the sliding filament
theory. Myosin filaments move along actin filaments, resulting in greater overlap between actin
and myosin, as well as a decrease in the total length of the sarcomere (Figure 2.4) (Huxley and
Hanson, 1954; Huxley and Niedergerke, 1954). This occurs in a repetitive sequence referred to
as the "cross-bridge cycle" (Spudich, 2001). The cross-bridge cycle decreases the length of each
individual FB upon contraction. Therefore, muscle length is also decreased (Huxley, 2004).
7
Figure 2.4. A. Light micrograph of skeletal muscle showing sarcomeres A. At rest. B.
Contracted. (Z-discs are located at the ends of sarcomeres). Used with permission from:
Histology. 7ed. Ham, A.W.. J.B. Lippincott Company.1974.
2.2.2 Connective Tissue Elements
The connective tissue components of skeletal muscle involved in FB attachment include tendons
and aponeuroses, which function to transmit contractile forces to the bone (Bengamin et al.,
2008; Iwanuma et al., 2011). Tendons and aponeuroses contain collagen fibers and also have a
cellular component (Ham, 1974).
Tendons are comprised of regularly arranged dense connective tissue consisting primarily of
collagen, with sparse tenocytes and tenoblasts (Ham, 1974; Sharma and Maffuli, 2006). The
collagen fibers of tendons are oriented in the same direction and plane (Bengamin et al., 2008).
Since tendons attach muscle to bone, this organization gives tendons high tensile strength (Ham,
1974).
Aponeuroses, sometimes referred to as internal tendons, may be found within the muscle
volume, or on the surface of a muscle. They can occur as flat sheets that occupy a small or large
area, or be in the form of a partition ie. septum. Aponeuroses consist of layers of regularly
arranged connective tissue, where collagen fibers are oriented in orthogonal planes relative to
one another. This gives aponeuroses tensile strength ie. the ability to withstand pull, in all
8
directions. It also allows aponeuroses to stretch when pulled eg. upon muscle contraction (Ham,
1974).
2.3 Architectural Parameters
The architecture of the contractile and connective tissue elements of skeletal muscle can be
characterized by the spatial arrangement of these elements throughout the muscle volume.
Furthermore, there are quantifiable architectural parameters for both the contractile and
connective tissue elements of skeletal muscle. The morphology and architectural parameters can
be used to compare relative functional capabilities between muscles.
2.3.1 Parameters of Contractile Tissue Elements
The contractile tissue element architecture can be described by the attachment sites of the FBs to
aponeuroses and/or bone. Quantifiable parameters include fiber bundle length (FBL), pennation
angle (PA), muscle volume (MV), and physiological cross-sectional area (PCSA) (Lee et al.,
2012; Li et al., 2015).
Fiber bundle length is defined as the distance between the two attachment sites of a FB. For
example, a FB may span between two aponeuroses where the attachment site on each
aponeurosis can be defined specifically. Upon contraction, the sarcomeres of a muscle shorten
(Lieber and Friden, 2001). Muscles that contain a greater number of sarcomeres in series,
therefore also have longer FBs. The longer the FBs, the greater the relative excursion capability
(Brand et al, 1986; Stevens et al, 2014).
The PA is the angle between the FB and the line of action (LOA) of a muscle. The LOA cannot
be visualized, but can be computed using 3D data. The PA influences the relative force
generating capability of a muscle, because only part of the force produced by each FB is
transmitted to the attachment site along the LOA. In pennate muscles, the force generating
capacity decreases according to the cosine of the PA (Lieber and Friden, 2001; Narici, 1999).
However, the arrangement of FBs in a pennate manner also allows for a greater density of
packing of FB within the volume of a muscle. This results in pennate muscles having a greater
relative force generating capacity than muscles of equal volume with parallel FB arrangement
(Gans and de Vree, 1987; Gans and Gaunt, 1991; Zajac, 1992).).
9
MV is the space occupied by a muscle, and in a broad sense, can affect the functional capability
of a muscle. However, the excursion capability and relative force generating capacity of a muscle
are also dependent on the FB arrangement. If two muscles of equivalent MV have different FB
arrangements, they can differ in their excursion and relative force generating capabilities (Zajac,
1989; Zajac, 1992).
The PCSA is an estimate of the relative force generating capability of a muscle. Physiological
cross-sectional area is defined as the total cross-sectional area of the FBs within the muscle
volume (Lee et al, 2012), and is quantified in different ways in 2D and 3D space. The
arrangement of FBs within a muscle will affect the PCSA, and thus its relative force generating
capability (Gans and de Vree, 1987; Gans and Gaunt, 1991).
2.3.2 Parameters of Connective Tissue Elements
Tendons have been described as having cylindrical, ellipsoidal, or flat shapes in cross-section
(Ham, 1974). The architecture of tendons has been characterized by its cross-sectional area,
volume, and height.
Descriptions of muscular aponeuroses are scarce. The architecture of aponeuroses can be
characterized by defining their locations within the muscle volume, and quantified using
parameters such as height, width, volume, and surface area. These parameters influence the force
generating capability and the ability to store elastic energy of a muscle (Oda et al., 2015; Raiteri
et al., 2016).
2.4 Cadaveric Masseter Muscle Architecture
Assessment of the architecture of MM in the past has been carried out using cadaveric dissection.
These earlier studies were not volumetric and consisted of line illustrations, photographs, and
quantification of architectural parameters using manual methods eg. calipers and protractor
(Schumacher, 1961; van Eijden et al., 1997).
Three-dimensional digitization is a contemporary method that has been utilized for the
investigation of cadaveric muscle architecture (Kim et al., 2007; Ravichandiran et al., 2009). A
recent study carried out in our laboratory used this technique to determine the arrangement of the
10
contractile and connective tissue elements of MM volumetrically, and to quantify musculo-
aponeurotic parameters in situ (Ebrahimi, 2015).
This section of the thesis is a review of the previous 2D and 3D literature of MM morphology
and architecture.
2.4.1 Morphology
2.4.1.1 Two-Dimensional Studies
There are few studies that have investigated the morphology of MM. Ebert (1939), Schumacher
(1961), Lam (1991), and Gaudy et al. (2000) found MM to have a laminar arrangement,
consisting of FBs and aponeuroses. The number of laminae reported in each study varied. Ebert
(1939) reported that MM was divided into anterior, superficial, and deep portions. The
superficial and anterior portions merged and had between two to four layers, whereas the deep
portion had at least one. Schumacher (1961) found five laminae. In contrast, Gaudy et al. (2000)
reported that MM was a pennate muscle composed of three parts each consisting of multiple
laminae: superficial (two laminae), intermediate (one lamina), and deep (four laminae). Lam
(1991), in a fresh cadaveric specimen, confirmed the laminar structure of MM as seen in
magnetic resonance imaging (MRI).
When considering laminar structure, Ebert (1939), Schumacher (1961), Lam (1991), and Gaudy
et al. (2000) describe different morphologies of MM. Ebert (1939) found different regional
arrangements of FBs and aponeuroses in six cadaveric specimens. There were two to four
superior and inferior alternating aponeuroses, which were contiguous between the anterior and
superficial portions of MM. Superior attachment sites were located along the zygomatic arch,
and inferiorly on the lateral surface of the mandible. The deep portion was described as
containing at least one aponeurosis. Fiber bundles in all portions of the muscle extended between
superior and inferior aponeuroses and/or bone. Based on the dissection of 30 specimens,
Schumacher (1961) described MM as having a complex arrangement of aponeuroses that served
as attachment sites for FBs. Five different aponeuroses, which either originated at the zygomatic
arch or on the angle and ramus of the mandible, were reported within the volume of MM.
Irrespective of origin, all of these aponeuroses terminated within the muscle belly. The cadaveric
proof-of-principle carried out by Lam (1991) confirmed that MM contained five “tendinous
11
inscriptions” that originated at the zygomatic arch and angle or ramus of the mandible, and
terminated within the muscle belly. The description of the aponeurotic architecture of MM
presented by Gaudy et al. (2000) differed from Schumacher (1961) and Lam (1991). Gaudy et al.
(2000) reported one tendinous sheet in the superficial part of MM, and a “series of tendinous
slips” in the intermediate part. The tendinous components of the superficial and intermediate
layers had separate superior attachments to the zygomatic process of the maxilla and the
zygomatic bone. However, the tendinous fibers of the superficial and intermediate layers merged
inferiorly at the mandibular ramus and angle. The four laminae of the deep part all had tendinous
components with a complex arrangement of tendinous sheets, “cones”, and/or “bundles.”
2.4.1.2 Three-Dimensional Studies
Recently, a cadaveric study of MM architecture using 3D digitization was conducted in our
laboratory (Ebrahimi, 2015). This involved the dissection and digitization of 9300 FBs and
associated aponeuroses in eight specimens. In all specimens, the SH and DH of MM had a
multilaminar arrangement of FBs and aponeuroses (Figure 2.5). The SH consisted of two
laminae in one specimen, three laminae in five specimens, and four laminae in two specimens.
The DH comprised one lamina in one specimen, two laminae in five specimens, and three
laminae in one specimen.
12
Figure 2.5. Sequential dissection of the laminae of the superficial head of masseter. A. Lamina 1
(L1), most superficial. B. Lamina 2 (L2). C. Lamina L3. Deep head, DH; Mandible, M;
Zygomatic arch, Z.
The individual laminae consisted of an aponeurosis and attached FBs (Figure 2.6). The
attachment sites of aponeuroses alternated between superior sites along the zygomatic arch, and
inferior attachment sites on the lateral surface of the ramus and the angle of the mandible. Fiber
bundles extended between superior and inferior aponeuroses, or between superior aponeuroses
and bone (Ebrahimi, 2015).
13
Figure 2.6. Three-dimensional digitization of laminae of SH. Reconstruction of laminae from
deep to superficial, lateral view. A. Lamina 3 (L3). B. Lamina 2 (L2). C. Lamina 1 (L1). D.
Reconstruction of MM. Aponeurosis, AP; Mandible, M; Zygomatic arch, Z. Used with
permission from Musculo-Aponeurotic Architecture of the Human Masseter Muscle: a Three-
Dimensional Cadaveric Study. Ebrahimi, E.A. University of Toronto. 2015.
2.4.2 Architectural Parameters
2.4.2.1 Two-Dimensional Studies
Review of the literature revealed four cadaveric studies that quantified the architectural
parameters of the contractile elements of MM in two dimensions (Table 2.1). However, no two-
dimensional cadaveric studies were found that quantified connective tissue architectural
parameters.
Schumacher (1961) and Weijs and Hillen (1984) did not specify the regions of MM studied,
whereas Ebert (1939) and van Eijden et al. (1997) used the locations of the attachments of FBs
and aponeuroses to divide MM into multiple parts. Ebert (1939) measured musculo-aponeurotic
parameters in the anterior, superficial, and deep portions of MM. Van Eijden et al. (1997)
divided MM into superficial and deep parts. The superficial part was then further subdivided into
three anteroposterior regions, and the deep part into four anteroposterior regions.
14
Table 2.1. Musculo-aponeurotic parameters of MM in 2D cadaveric dissection studies. Study Region n # FBs
sampled Mean
FBL (mm) Mean PA (°)
Mean MV (cm3)
Mean PCSA (cm2)
Ebert, (1939) Anterior 6 X 21-30 12-14 X X
Superficial 19-26
Deep 16-19
Schumacher, (1961)
MM 30 X 25.8 X 7.99 3.0
Weijs and Hillen, (1984)
MM 6 25 22.2±0.1 X 14.3±5.4 6.6±2.7
van Eijden et al., (1997)
MM 8 70 21.3±2.9 X 24.6±4.4 10.3±1.4
Superficial 30 24.6±4.1 16.5±4.5 18.5±4.1 6.8±1.0 Deep 40 18.0±2.8 6.7±3.2 12.2±1.3 3.5±0.8
Measurements of musculo-aponeurotic parameters were made manually, on cadaveric specimens
in the closed mouth position in all four studies. Fiber bundle length was measured using calipers
and rulers either in situ or after removal of the FBs (Ebert, 1939; Schumacher, 1961; Weijs and
Hillen, 1984; van Eijden et al., 1997). Ebert (1939) reported ranges of FBL in the three parts of
MM that he described: anterior, superficial, and deep. Schumacher (1961) and Weijs and Hillen
(1984) measured FBs on the surface of MM, whereas van Eijden et al. (1997) measured the
lengths of 10 FBs within each of the anteroposterior regions that comprised the superficial and
deep parts. Ebert (1939) reported the range of PAs and van Eijden et al. (1997) the mean PA.
Both studies used a protractor and acetate paper tracings of an unspecified number of tendon
plates and FBs for angle measurement. Measurements of MV involved the dissection and
weighing of contractile tissue. PCSA was calculated as the ratio of MV to mean FBL, limiting
the generalizability of the results as muscle architecture is not homogenous.
Some architectural parameters varied little, while others varied to a greater extent amongst the
three studies. Mean FBL was fairly consistent, and ranged between 21.3 mm and 25.8 mm.
However, van Eijden et al. (1997) noted that FBs in the deep part of MM tended to be shorter
than those in the superficial part. Van Eijden (1997) also found that mean PA was smaller in the
deep part, than in the superficial part of MM. Mean MV ranged between 7.99 cm3 and 24.6 cm3,
and mean PCSA ranged between 3.0 cm2 and 10.3 cm2. Therefore it can be seen that there was
much variation in both MV and PCSA.
15
2.4.2.2 Three-Dimensional Studies
Only one 3D, cadaveric study investigating the musculo-aponeurotic parameters of MM was
found within the literature (Ebrahimi, 2015). This was the same study that digitized 9300 FBs
and associated aponeuroses in eight cadaveric specimens in our laboratory. Musculo-aponeurotic
parameters of both contractile and connective tissue elements were quantified. Mean values of
FBL, PA, MV, and PCSA were reported within each lamina of MM. As well, the mean height,
width, and surface areas were provided for each aponeurosis, based on the origin of the
aponeurosis and the relative medio-lateral position.
2.4.2.2.1 Measurement of Architectural Parameters
All digitization was carried out on specimens that had been stabilized in a maxillo-mandibular
position of maximal interincisal opening (Ebrahimi, 2015). Fiber bundles were digitized at 1-2
mm intervals, throughout the volume of MM. Quantifying FBL involved the summation of the
distances between digitized points (Figure 2.7). This took into account the curvature of FBs.
Figure 2.7. Incremental summation of digitized points along the length of a FB. Distance
between two digitized points, μ.
In order to determine PA, tangent vectors were calculated at each digitized point (Figure 2.8),
and averaged for each FB. The LOA was the mean of the average tangent vectors of all FBs, and
the PA was the angle between the average tangent vector of a FB and the LOA. The mean MV
was computed as a function of FBL and cross-sectional area. Multiple cross-sectional areas were
measured at 1 - 3 mm intervals along an FB, and averaged to obtain a mean cross-sectional area
for each FB. The volume of an FB was calculated as the product of FBL and the mean cross-
sectional area.
16
Figure 2.8. Determination of the average tangent vector of an FB. Tangent vectors at each
digitized point, green lines; Average tangent vector, red line.
The MV for a specific muscle was the sum of the volumes of individual FBs. Physiological
cross-sectional areas of individual FBs were calculated as the product of the cosine of the PA and
the mean cross-sectional area. Individual PCSAs of each FB were added together to determine
the PCSAs of laminae or MM as a whole.
2.4.2.2.2 Quantified Architectural Parameters of Masseter
In a 3D cadaveric study of the SH and DH, Ebrahimi (2015) quantified contractile and
connective tissue architectural parameters throughout the volume of each head of MM. The mean
FBL of the DH (20.1±7.7 mm) was shorter than that of the SH (32.7±12.6 mm). At the laminar
level, mean FBL tended to be longer in more superficial laminae in both the SH and DH in five
specimens. In three specimens, the FBL of L2 of SH was greater than that of L1 (Table 2.2).
Table 2.2. Mean FBL of laminae of DH and SH. (Specimen, Sp). SH (mm) DH (mm)
Sp L1 L2 L3 L4 L1 L2 L3 1 40.4±13.8 33.1±15.9 40.9±12.4 X 25.1±5.9 17.7±5.4 X 2 28.4±8.7 29.4±6.9 22.8±8.9 X 18.1±5.2 10.9±2.2 X 3 33.7±12.7 22.5±5.8 35.0±8.2 X 15.7±4.9 X X 4 28.6±7.7 42.5±11.4 36.3±11.7 23.8±9.5 17.7±5.9 X X 5 26.2±9.4 40.9±17.1 26.8±12.4 23.3± 7.1 10.2±3.0 15.5±4.1 X 6 52.5±8.6 47.7±6.0 X X 33.9±5.3 27.3±6.8 X 7 32.0±3.6 30.1±6.0 28.5±4.5 X 21.4±4.4 24.7±6.8 X 8 29.7±4.4 25.9±3.8 26.4±4.3 X 21.7±2.8 23.1±6.5 22.2±2.5
Used with permission from Musculo-Aponeurotic Architecture of the Human Masseter Muscle: a Three-Dimensional Cadaveric Study. Ebrahimi, E.A. University of Toronto. 2015..
17
Ebrahimi (2015) found that mean PA was greater in the DH (21.1±10.4°) than in the SH
(15±10.1°). In five specimens, mean PA decreased gradually from superficial to deep laminae in
the SH. Whereas in three specimens the mean PA of L3 and/or L4 of SH was greater than more
superficial laminae. Mean PA was greater in deeper laminae than the superficial laminae of DH.
The MV and PCSA of the most superficial laminae were consistently larger than the MV and
PCSA of the deepest laminae in both SH and DH.
The connective tissue parameters quantified by Ebrahimi (2015) are shown in Tables 2.3A. and
B. Aponeuroses are listed based on superior or inferior attachment sites, and numbered medio-
laterally with the most superficial aponeurosis having the lowest number. In both SH and DH,
the width, height, and surface areas of aponeuroses generally decreased from superficial to deep.
Table 2.3A. Parameters of aponeuroses of SH. Aponeurosis Mean Width
(mm) Mean Height
(mm) Mean Surface Area
(cm2) Superior Aponeuroses
1 21.9±7.0 41.3±6.9 8.2±2.3 2 21.7±6.1 32.6±12.1 6.0±2.0 3 18.6±2.9 29.4±7.4 4.4±1.5 4 18.3±8.5 22.9±11.5 3.4±2.4
Inferior Aponeuroses 1 21.6±6.4 29.5±5.4 5.1±0.8 2 18.7±8.0 27.1±7.0 4.0±1.4 3 12.7±5.8 30.4±10.9 3.2±1.9 4 18.5 17.1 2.2
Table 2.3B. Parameters of aponeuroses of DH. Aponeurosis Mean Width
(mm) Mean Height
(mm) Mean Surface Area
(cm2) Superior Aponeuroses
1 18.8±9.4 19.5±4.9 3.2±2.4 2 13.8±1.0 10.1±2.3 1.2±0.6
Inferior Aponeuroses 1 11.4±4.8 21.3±4.4 2.0±0.8 2 7.2±0.9 17.2±1.7 1.1±0.4
Used with permission from Musculo-Aponeurotic Architecture of the Human Masseter Muscle: a Three-Dimensional Cadaveric Study. Ebrahimi, E.A. University of Toronto. 2015.
18
2.5 In Vivo Studies of Masseter Muscle
In vivo studies have been carried out using electromyography (EMG) and different forms of
imaging including computed tomography (CT), MRI, and US.
2.5.1 Electromyography
In EMG, electrodes are used to detect and record changes in the electric potential of a muscle
(Sadikoglu et al., 2017). Because normal skeletal muscle tissue is inactive at rest, EMG can be
used to determine if a contraction has occurred. When a contraction occurs, the EMG unit will
detect action potentials. As this contraction increases in magnitude, an increasing number of
action potentials are produced (Kamen, 2004).
EMG can utilize surface electrodes or intramuscular needle electrodes. Surface electrodes can be
used in superficial muscles to provide an idea of whether muscle activation has occurred.
Intramuscular needle electrodes can be placed within a muscle and can record signals over a
much smaller area, allowing differentiation between action potentials originating in different
parts of muscles (Sadikoglu et al. 2017). MM has been studied in the past using both surface and
intramuscular electromyography.
Intramuscular needle EMG has been used to separate activity in different parts of MM. Using
intramuscular EMG, it was found that there was task-dependent differential activation of the
superficial and deep, and/or anterior and posterior portions of MM (Blanksma and van Eijden,
1995). As well, it was shown that during static contraction there was differential activation of the
parts of MM based on changes in bite-force direction (Blanksma et al, 1992). McMillan and
Hannam (1991) also described layers of "cigar-shaped motor-unit territories" found throughout
MM, that they stated was consistent with Schumacher's (1961) layered anatomical description of
MM.
Surface EMG has been used as a gauge to determine different levels of activity in MM. For
example, it was found that when the cusps and incisal edges of teeth are in minimal contact in
maximum intercuspation, the activity within the masseter muscle was at 5% of maximal
contraction (Roark et al, 2003). This fact has been used to standardize participant positioning in
an MRI study of MM to create a reproducible and stable state that is close to rest (Cioffi et al,
2012).
19
2.5.2 Computed Tomography
Two anatomical studies of MM were found using CT in asymptomatic participants (Table 2.4).
The cross-sectional areas (CSAs) that these two studies report are almost identical.
Table 2.4. CT studies of normal participants Study Participants
CSA (cm2)
Weijs and Hillen (1985) 16 M (28-46 yrs)
5.33±1.49
Weijs and Hillen (1986) 50M (22-46 yrs)
5.33±1.43
Though CT can provide a volumetric image of MM, its soft tissue resolution does not allow for
visualization of intramuscular architecture.
2.5.3 Magnetic Resonance Imaging
Masseter has been investigated using MRI, however these studies are few in number (Tables 2.5
and 2.6). The four studies that investigated CSA reported overlapping results. Raadsheer et al
(1994) was the only study that measured muscle thickness (MT), and therefore comparison to
other MRI studies is not possible. The mean MVs measured by Cioffi et al (2012) and Boom et
al (2008) using MRI were similar. In addition to quantifying MV, Cioffi et al (2012) also
quantified the volume of the aponeuroses and stated that their arrangement was variable and
complex. Lam et al (1991) estimated tendon plane-orientation, and described a pattern of tendons
extending partially into the belly of the muscle and having superior and inferior attachments.
This study also described three compartments, each having different muscle fiber attachment
sites in 3D space. Though imaging of connective tissue architecture is possible using MRI, FB
parameters including FBL cannot be measured consistently using MRI as entire FB are difficult
to image.
Table 2.5. MRI studies of normal MM FB architecture. Study Participants
MT
(mm) CSA (cm2)
MV (cm3)
Boom et al., (2008) 9M/22F X 4.9±1.2 23.8±8.6
Cioffi et al., (2012) 7F/7M X X 28.8±7.7
Goto et al., (2005) 5M/5F X 5.2±1.0 X Hannam and Wood, (1989)
6F/16M X 5.8±1.1 X
Raadsheer et al., (1994) 15M 16.8±2.3 X X Spronsen et al., (1991) 32M X 4.7±0.8 X
20
Table 2.6. MRI studies of normal MM aponeurotic architecture. Study Participants
Aponeurotic
volume (cm3)
Tendon-plane orientation
Number of compartments
Cioffi et al., (2012) 7F/7M 2.0±0.8 X 3 or more
Lam et al., (1991) 3M/2F X Reported per subject and tendon-
plane
3
2.5.4 Ultrasound
Ultrasound can be utilized to describe the morphology of skeletal muscle, as well as to quantify
architectural parameters in vivo. Previous in vivo studies of MM have focussed on quantification
of the muscle as a whole (muscle length, MT, MV, and CSA). No studies were found that
quantified aponeurosis/tendon architectural parameters.
The two studies found that quantified mean muscle length of MM (Table 2.7) measured length in
the contracted state. Benington et al. (1999) measured the length from "origin to insertion" of
MM, and Naser-Ud-Din et al. (2010) measured the length between the "zygomatic tubercle" and
"gonial angle". The antero-posterior position of the transducer was not specified. The mean
contracted length reported by Naser-Ud-Din et al. (2010) was greater than that reported by
Bennington et al. (1999).
Table 2.7. US studies of mean contracted lengths of MM in asymptomatic participants.
Study Participants Mean Contracted Length (mm)
Benington et al., (1999) 6F/4M (15-31yrs)
M: 53.8±5.8 F: 46.5±8.2
Naser-Ud-Din et al., (2010) 8F/3M (22-30yrs)
64.7±6.8
Mean CSA was reported in four studies in relaxed and/or contracted states (Table 2.8).
Benington et al. (1999) calculated mean CSA as the volume of the five middle axial slices of
MM, divided by their thickness. Close et al. (1995) manually traced CSA on one slice in the
middle of the "muscle belly" of MM. Naser-Ud-Din (2010) also manually traced CSA on one
axial slice of the left MM of each participant, however the location of the axial slice is not
specified. Uchida et al. (2011) measured CSA from manual tracings of MM on three repeated
slices parallel to the occlusal plane. The findings of Close et al. (1995) and those of Benington et
21
al. (1999) are similar, despite being acquired in the relaxed and contracted states. Similarly,
Uchida et al. (2011) found little difference between mean CSA in the relaxed and contracted
states (0.8 mm).
Table 2.8. US studies of mean CSA of MM in asymptomatic participants. Study Participants Region Mean CSA (cm2)
Relaxed Contracted Benington et al., (1999) 6F/4M
(15-31yrs) 5 axial slices in
middle of muscle belly
X M: 4.6±9.8 F: 3.1±0.4
Close et al., (1995) 20F/19M (21-47yrs)
Axial slice in middle of muscle belly
M: 4.3±1.5
F: 3.0±1.2
X
Naser-Ud-Din et al., (2010) 8F/3M (22-30yrs)
Left MM X 6.2±1.7
Uchida et al., (2011) 13F/11M (17.8-
41.17yrs)
3 repeated axial slices parallel to occlusal plane
4.1±0.7 4.9±1.0
Naser-Ud-Din et al. (2010) and Benington et al. (1999) quantified mean MV (Table 2.9).
Benington et al. (1999) measured MV by summing the total number of voxels comprising MM,
whereas Naser-Ud-Din et al. (2010) determined the MV by estimating the volumes of two cones
with abutting bases. There is a very large difference between the mean MV reported by
Benington et al. (1999) and that reported by Nasser-Ud-Din et al. (2000), likely due to the
discrepancy in methodologies used.
Table 2.9. US studies of mean MV of MM in asymptomatic participants.
Study Participants Mean MV (cm3) Benington et al., (1999) 6F/4M
(15-31yrs) M: 23.0±7.1 F: 11.3±0.8
Naser-Ud-Din et al., (2010) 8F/3M (22-30yrs)
3.2±1.0
22
Mean MT of MM in the relaxed and/or contracted states was the most frequently investigated
parameter (Table 2.10). When comparing these studies, it can be noted that the sites of scanning
were sometimes related to the middle or inferior third of MM, or to the occlusal plane. In five
studies the scanning was sequential, and in two studies the site was not stated. The sites of
scanning and transducer orientation were not defined relative to landmarks identifiable using US.
The reported mean MT in the relaxed state ranged from 6.8 mm to 14.87 mm, and in the
contracted state from 9.0 mm to 19.07 mm. In addition to the studies listed in Table 2.10, Rani
and Ravi (2010) and Rohila et al. (2012) also related MT to facial morphology and occlusion.
Table 2.10. US studies of mean MT of MM in asymptomatic participants. Study Participants Scanned plane Mean MT (mm)
Relaxed Contracted Bakke et al., (1992)
42F
(20-31yrs) 3 coronal planes within
inferior 1/3 9.8-12.6 X
Benington et al., (1999)
6F/4M (15-31yrs)
Axial plane "mid-belly" X M: 11.1±1.3 X F: 9.5±1.2
Bertram et al., (2003)
32F/10M (18-59yrs)
5 axial planes between zygomatic arch and mandibular angle
6.8±1.8 to 12.9±2.4
9.0±2.5 to 16.1±3.2
Egwu et al., (2012) 30F/30M (19-
30yrs) Axial plane parallel to
occlusal plane M: 14.87±3.42 F: 11.94±1.82
M: 19.07±3.69 F: 15.00±1.66
Emshoff et al., (2003) 30
(19-56yrs)
5 axial planes between zygomatic arch and mandibular angle
7.1±1.8 to 13.5±2.8
X
Giorgiakaki et al. (2007) 52F
(23.7±2.5 yrs) Axial plane parallel to
occlusal plane X 13.9±1.45
Jonasson and Kiliardis (2004) 62F
(40-75yrs) Axial plane parallel to
occlusal plane X 13.1±.9
Kiliardis and Kalebo (1991) 20F/20M (21-
35yrs) Axial plane parallel to
occlusal plane M: 9.7±1.5 F: 8.7±1.6
M: 15.1±1.9 F: 13.0±1.8
Kubo et al., (2006)
5M (25-28yrs)
Serial coronal planes 12.8±1.2 15.7±1.1
Kubota et al., (1998)
80M (21-25yrs)
Middle axial 1/3 15.8±3.0 16.7±2.7
Naser-Ud-Din et al., (2010) 8F/3M (22-30yrs) X X 13.7±2.2
Raadsheer et al., (1994)
15M (25-51yrs)
3 axial planes 11.1±2.8 to
14.3±2.0 13.2±2.8 to
16.3±2.3
Raadsheer et al., (2004) 64F/57M (18-36yrs)
Middle axial 1/3 M: 14.0±1.7 F: 12.2±1.9
X X
Satiroglu et al., (2005) 23F/24M Middle axial 1/3 13.6±1.9 14.6±1.8
Soyoye et al., (2017) 45F/21M
(12-30yrs) Axial plane parallel to
occlusal plane 11.2±2.4 12.8±2.6
Strini et al., (2013) 13F/6M
(24.1±3.6 yrs)
Axial plane halfway between zygomatic arch
and gonial angle 12.1±1.4 14.2±1.7
Tircoveluri et al., (2013) 35F/35M (18-25yrs)
Middle axial plane M: 11.6±0.7 F: 12.5±0.5
M: 13.5±0.9 F: 12.5±0.6
23
Fiber bundles can be visualized and measured using US (Kim et al., 2010; Kim et al., 2013). The
FBs of skeletal muscle appear hypoechoic, however the fibro-adipose tissue surrounding and
separating FBs appears hyperechoic. This allows indirect visualization of FBs as singular,
hyperechoic lines (Figure 2.9). Although FBL has been reported in other skeletal muscles using
US (Kim et al., 2010; Kim et al., 2013), no US study could be found investigating FBL in MM.
Figure 2.9. US scan of supraspinatus muscle. Aponeurosis, yellow line; Fiber bundle, red line.
Used with permission from Supraspinatus Musculotendinous Architecture: a Cadaveric and In
Vitro Ultrasound Investigation of the Normal and Pathological Muscle. Kim, S.Y. University of
Toronto. 2009.
No in vivo US study could be found that quantified the musculo-aponeurotic architectural
parameters of the aponeuroses of MM. Since aponeuroses are fibrous flat sheets of varying
thickness, they appear linear and hyperpechoic on US scans (Figure 2.9). Also, because
aponeuroses are sheet-like, they are contiguous throughout the volume of a muscle in three-
dimensions (Kim et al., 2010; Kim et al., 2013).
2.6 Masseter Muscle Pathoses: Clinical Applications
Although MM has roles in many normal activities, it is also involved in parafunction (eg.
clenching, bruxism), and in pathologic conditions of chronic and acute facial pain (Okeson,
2013; Schiffman et al., 2014; Wieckiewicz et al., 2014). Masseter muscle pathoses are currently
classified under TMDs. Historically, there were many inconsistencies between studies in the
diagnostic criteria used to diagnose TMDs, and different types of data were recorded in
24
published reports (Manfredini et al, 2011). It was for this reason that criteria classifying TMDs
were instituted.
The current system used for the classification of TMDs (including masseter muscle pathoses) is
called the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) for Clinical and
Research Applications (Schiffman et al., 2014).Under the DC/TMD, MM involvement has been
grouped into "pain diagnoses" and "joint diagnoses" (Schiffman et al., 2014). "Pain diagnoses"
include local myalgia, myofascial pain, and myofascial pain with referral. "Joint diagnoses"
include disc displacement, subluxation, and degenerative joint disease. It is recommended that
screening for "pain diagnoses" is carried out using TMD Pain Screener, a six-part questionnaire
that has a 99% sensitivity and 97% specificity for painful TMDs (Gonzalez et al, 2011).
Schiffman et al. (2014) also provides guidelines for questions and examination procedures that
can be used to screen for "joint diagnoses."
Temporomandibular disorders have a bimodal age distribution with one peak in the third and
fourth decades, and another in the sixth decade. The first peak is associated with soft tissue
disturbances such as disc displacement, whereas the second peak is related to degenerative joint
disease (Manfredini et al, 2010).
The etiologies of TMDs are poorly understood. However, it has been suggested that architectural
changes occur in the contractile and connective tissue elements in MM (Ariji et al, 2010). Local
or regional differences in musculo-aponeurotic architecture could therefore be important in the
diagnosis of pathosis involving local or regional pain (Lam, 1991).
2.7 Gaps in Knowledge and Rationale
Two-dimensional cadaveric studies have described the morphology of MM, and also quantified
some architectural parameters from sampled FBs (Ebert, 1939; Schumacher 1961; Weijs and
Hillen, 1984; van Eijden et al. 1997). A more recent 3D digitization study was volumetric and
generated Cartesian coordinate data for both the contractile and connective tissue elements. This
enabled detailed study of the 3D architecture of MM in cadaver. In vivo studies to date have been
based on minimal knowledge of the anatomical morphology of MM resulting in findings that are
broad-based and difficult to apply to clinical practice. Detailed 3D analysis of muscle
25
architecture in cadaveric specimens has been used to develop US protocols for the study of
intramuscular architecture (Kim et al., 2010), however this has yet to be done in MM.
Review of the in vivo literature of MM has revealed several gaps in the knowledge that should be
further explored:
1. The arrangement of aponeuroses has been described in a general fashion, but never
precisely localized in the muscle volume relative to FB attachment. Therefore, the
morphology and relationships of FBs and aponeuroses has not been characterized in MM
using US.
2. Intramuscular contractile tissue architectural parameters (eg. FBL) have not been
quantified in vivo in MM using US.
3. Though the total aponeurotic volume of MM and tendon-plane orientations have been
previously described (Lam et al, 1991; Cioffi et al, 2012), the height of aponeuroses has
not been investigated in vivo in MM using US.
4. Comparisons have not been made between intramuscular architectural parameters at rest
and upon contraction. Visualization and quantification of the architecture of MM at rest
and upon contraction, can provide additional insight into how the muscle works, and
therefore further study is necessary.
It is necessary to elucidate normal musculo-aponeurotic architecture and function in order to
understand pathologic changes. No in vivo studies of MM were found in the literature that
investigated the musculo-aponeurotic architecture throughout the muscle volume, or analyzed
FB and aponeurotic architectural parameters during function. Studying the morphology and
architectural parameters of MM with ultrasound in the relaxed and contracted states provides an
opportunity to bridge form and function, and therefore also sets the foundation for the study of
pathologic change.
26
Chapter 3
3 Research Aims and Hypotheses
The purpose of this study is to investigate the in vivo musculo-aponeurotic architecture
throughout the volume of MM in asymptomatic participants with US in the relaxed and
maximally contracted states.
1. Research Question: Can the laminar morphology of the superficial and deep heads of
masseter be visualized with US, throughout the muscle volume?
Research Aim: To investigate the in vivo morphology of the heads of masseter
volumetrically ie. the attachment sites and arrangement of the FBs and aponeuroses with
US.
H0: The laminar morphology of the heads of masseter cannot be visualized throughout
the muscle volume using US.
H1: The laminar morphology of the heads of masseter can be visualized throughout
the muscle volume using US.
2. Research Question: Are there differences in intramuscular contractile tissue
element parameters (fiber bundle length) within each lamina of masseter when comparing
the relaxed and maximally contracted states?
Research Aim: To image fiber bundles within the laminae of MM to quantify and
compare architectural parameters (fiber bundle length) in the relaxed and maximally
contracted states.
H0: There is no significant difference in fiber bundle length within the different laminae
of MM between the relaxed and maximally contracted states.
H1: There is a significant difference in fiber bundle length within the different laminae of
MM between the relaxed and maximally contracted states.
27
3. Research Question: Are there differences between aponeurotic architectural
parameters in the relaxed and maximally contracted states?
Research Aim: To image laminae of MM to quantify aponeurotic architectural
parameters in the relaxed and maximally contracted states.
H0: There is no significant difference in the height of aponeuroses within the different
laminae of MM between the relaxed and maximally contracted states.
H1: There is a significant difference in the height of aponeuroses within the different
laminae of MM between the relaxed and maximally contracted states.
4. Database Development: The data obtained to accomplish the above research aims will
also result in the establishment of a normative database of the relaxed and contracted
musculo-aponeurotic architecture of MM. This can be used in future research for
comparison to patients with jaw muscle pathoses.
28
Chapter 4
4 Materials and Methods
4.1 Participants
Twenty-four asymptomatic participants (12F/12M) were recruited to take part in this study. The
mean age of participants was 25.8±4.1 years, and the age range was 20-37 years. This is an age
group where TMDs are commonly diagnosed, and this sample of participants can therefore serve
as a control group for comparison in future studies with patients with pathoses. Written and
verbal informed consent was obtained from all participants. Ethics approval was granted by the
Health Sciences Research Ethics Board of the University of Toronto (Protocol #34354).
4.1.1 Sample Size Calculation
The sample size calculation was based on the mean cadaveric FBL of MM from Ebrahimi
(2015), which was 32.7±12.6 mm. With a power of 80% and a type 1 error level of 0.05, in order
to detect a significant difference of 10% between the relaxed and contracted states, the sample
size required was therefore 24 participants.
4.1.2 Screening
Screening to determine eligibility of each potential participant was carried out in three parts:
• A questionnaire.
• An examination.
• Preliminary US scans of MM, bilaterally.
The aim of screening was to determine if potential participants could have "pain diagnoses"
(Gonzalez et al., 2011) or "joint diagnoses" (Schiffman et al., 2014).
4.1.2.1 Screening Questionnaire
The screening questionnaire is detailed below in Table 4.1. TMD Pain Screener (Gonzalez et al.,
2011) consists of three questions, all of which were included (questions 1-3). Questions 4-9
29
were based on the criteria of Schiffman et al. (2014) for joint diagnoses, including subluxation,
disc displacement, and degenerative joint disease.
Table 4.1. Screening questionnaire. 1. In the last 30 days, which of the following best describes any pain in your jaw or temple area on either side? a. No pain b. Pain comes and goes c. Pain is always present 2. In the last 30 days, have you had pain or stiffness in your jaw on awakening? a. No b. Yes 3. In the last 30 days, did the following activities change any pain (that is, make it better or make it worse) in your jaw or temple area on either side? A. Chewing hard or tough food a. No b. Yes B. Opening your mouth or moving your jaw forward or to the side a. No b. Yes C. Jaw habits such as holding teeth together, clenching/grinding, or chewing gum a. No b. Yes D. Other jaw activities such as talking, kissing, or yawning a. No b. Yes 4. In the last 30 days have you had any noise present with jaw movement or function? 5. In the last 30 days, has your jaw locked with limited mouth opening, even for a moment, and then unlocked? 6. Has your jaw ever locked so that your mouth would not open all the way? 7. Do you have any history of limitation in jaw opening severe enough to interfere with your ability to eat? 8. In the last 30 days, has your jaw locked or caught in a wide-open mouth position, even for a moment, so that you could not close? 9. Have you ever been unable to close your mouth from a wide-open position without using your hands to close your mouth?
30
4.1.2.2 Screening Examination
The examination involved palpation of each potential participant's TMJ bilaterally, while asking
them to do each of the following movements three times (Schiffman et al, 2014):
• depress mandible maximally and then elevate
• left and right lateral mandibular excursions
• protrude and retrude the mandible
While performing the examination, close attention was paid to determine if there is any evidence
of clicking, popping, snapping, or crepitation during any of these movements. Each potential
participant was also asked to report if they perceived any TMJ noises during the examination.
4.1.2.3 Preliminary Ultrasound Screening
Masseter was scanned at rest bilaterally in the coronal and axial planes in order to determine if
there is gross pathosis present.
4.1.3 Inclusion and Exclusion Criteria
Potential participants were included in the study if all of the inclusion criteria listed below were
met:
• 20-40 years of age
• Screening questionnaire - negative answers to all questions
• Screening examination - no positive findings
• Preliminary US scan - no evidence of gross pathosis
Potential participants were excluded from the study if during screening it was determined that
there was a possible history of a pain or joint diagnosis (Gonzalez et al. 2011; Schiffman et al.,
2014), or if there was evidence of gross pathosis on preliminary US scan. The exclusion criteria
therefore consisted of:
• Less than 20 or more than 40 years of age
• Screening questionnaire - positive response to any question
• Screening examination - TMJ noise during screening examination perceived by examiner
31
• Screening examination - Report of TMJ noise by potential participant
• Preliminary US scan - evidence of gross pathosis
4.2 Equipment
4.2.1 Ultrasonography
A Logiq e (General Electric Healthcare, Chicago, IL) real-time ultrasound scanner fitted with
two linear probes (L10-22 and L4-12t) was used. A water-soluble gel was used to improve
conduction of the sound waves. Images were analyzed to determine the architectural parameters
in each position/state using ImageJ (National Institutes of Health, Bethesda, MD), an image
processing software used to display and analyze many image formats.
4.2.2 Electromyography
A small, portable microEMG surface electromyography unit (OT Bioelettronica, Torino, Italy)
with a circular 4 cm surface electrode, was used to confirm or rule out MM activity at the time of
image acquisition. To enable scanning, the electrode was placed on the contralateral masseter.
The center of the electrode was positioned in the center of the muscle belly as determined by the
boundaries of MM.
4.3 Participant Positioning
Throughout ultrasonography, participants were seated in a chair with their back supported, feet
on the floor, and hands resting on their thighs. The participant's head was positioned so that the
Frankfort horizontal plane was parallel to the floor.
4.4 Ultrasound Protocol
The US protocol utilized in this study was developed based on the detailed 3D cadaveric study of
MM carried out by Ebrahimi (2015). Knowledge of the laminar morphology of the SH and DH
of MM, as well as of the arrangement of aponeuroses and FBs was used in constructing this
protocol.
32
4.4.1 Anatomic Verification
In order to confirm that FBs and aponeuroses of MM can be visualized using US, an anatomic
verification experiment was carried out in cadaver. The MM was exposed, sectioned coronally,
and scanned with US (Figure 4.1). The location of two aponeuroses and a FB were correlated
between the cadaveric specimen and the US image. Though the US scan corresponds closely to
the image of the cadaveric specimen, exact correlation is not possible since only the dissected
surface of the cadaveric specimen is visible, whereas the US scan is a tomographical slice.
Figure 4.1. Correlation of aponeuroses and FB between a coronally-sectioned cadaveric
specimen and US scan. A. Cadaveric MM, with green box showing region of interest. B.
Cadaveric region of interest. C. US scan. D. Cadaveric region of interest with superimposed
anatomic markings. E. US scan with superimposed anatomic markings. Angle of mandible, A;
Aponeuroses, yellow lines; Fiber bundle, red line; Ramus of mandible, white line. Zygomatic
arch, Z.
33
4.4.2 Ultrasound Image Acquisition
4.4.2.1 Image Acquisition in the Relaxed and Maximally Contracted States
Ultrasound scanning of MM was carried out bilaterally in relaxed and maximally contracted
states. In order to standardize the maxillo-mandibular position of participants in the relaxed state,
an occlusal registration of the molars and premolars was acquired with teeth in first contact. This
position was selected as this is when MM is at 5% of its maximal activity, which is a
reproducible state that approximates rest (Roark et al, 2003). The impression material used was
Blu-Mousse (Parkell Products, Brentwood, NY), a light body polyvinyl siloxane material. This
occlusal registration was placed in the participant's mouth during US scanning in the relaxed
state. For image acquisition in the contracted state, participants were asked to bite in maximum
intercuspation, and contract maximally.
4.4.2.2 Ultrasound Images Acquired
Ultrasound scans were acquired of the SH and DH of MM in order to visualize the FBs and
aponeuroses of each lamina. Static and panoramic images were acquired. A static image is an
image captured with the transducer held in one position. Capturing a panoramic image involves
moving the transducer as the image is being acquired in order to visualize a larger area.
Panoramic images can be acquired in any plane, including the axial and coronal planes (Figure
4.2).
Figure 4.2. Motion of transducer during panoramic image acquisition (dashed arrow) A.
Panoramic coronal scan of superficial head. B. Panoramic axial scan within superficial head.
34
The following scans were acquired from SH in the relaxed and maximally contracted states:
• Panoramic coronal scan from angle of mandible to centre of origin of SH on zygomatic
arch.
• Three panoramic axial scans in the superior, middle, and inferior thirds of SH
(Figure 4.3A.).
• One to three static coronal scans, depending on the number of laminae, within each
quadrant of SH (Figure 4.3B.).
Figure 4.3. Locations of acquisition of US scans of SH. A. Panoramic axial scans. B. Static coronal US scans. Inferior third, Inf 1/3; Middle third, Mid 1/3; Superior third, Sup 1/3; Quadrants 1-4, Q1-4. The following scans were acquired from DH in the relaxed and maximally contracted
states:
• Panoramic coronal scan from the inferior border of the posterior third of the
zygomatic arch to the termination of the muscle on the ramus of the mandible.
• One static axial scan just inferior to the origin of the deep head at the zygomatic
arch.
• Up to two static coronal scans just inferior to the origin of the deep head at the
zygomatic arch.
For images that were acquired in the contracted state, image acquisition required no more than 1-
2 seconds from the time the participant was asked to bite down till the time the participant was
told they can relax.
35
4.4.3 Ultrasound Image Assessment
Ultrasound images were used to elucidate the morphology of MM, and to quantify architectural
parameters. Comparisons were made between US images acquired in the relaxed and maximally
contracted states. EMG data was used to confirm that there was no evidence of active contraction
of MM at the time of acquisition of images representative of the relaxed state, and that there was
muscle activity at the time of acquisition of images in the contracted state (Figure 4.4).
Figure 4.4 Electromyography data showing A. Relaxed state B. Contraction.
4.4.3.1 Morphology
The morphology of the heads of MM was visualized on US scans in the relaxed and contracted
states. Panoramic coronal images were used to determine the arrangement and number of
laminae within each head of MM. As well, the morphology and number of aponeuroses within
SH and DH was characterized using panoramic coronal images. Panoramic axial images were
used to confirm the morphology of MM seen on panoramic coronal images. Static coronal
images of SH and DH were used to elucidate the FB morphology of MM.
4.4.3.2 Architectural Parameters
Architectural parameters were measured on US scans using ImageJ (National Institutes of
Health, Bethesda, MD) in both the relaxed and maximally contracted states. Static coronal scans
of each quadrant of SH were used to determine FBL within each lamina. Fiber bundle length was
36
measured as the distance from origin to insertion of a FB (Figure 4.5). Depending on the size of
the lamina, two to four FBs were measured in each lamina in both the relaxed and contracted
states. The height of aponeuroses was measured from the attachment site of the aponeurosis to
its termination within the muscle belly, on panoramic coronal US scans (Figure 4.6).
Figure 4.5. Determination of FBL within quadrant 1 of MM in relaxed state. A. US scan. B. US
scan with anatomic markings. Aponeuroses, yellow lines; FBL, length of red line; Ramus of
mandible, R; Subcutaneous tissue, S; Zygomatic arch, Z.
37
Figure 4.6. Determination of height of aponeuroses on panoramic coronal scan in relaxed state.
A. US scan. B. US scan with anatomic markings. Aponeuroses, length of yellow lines; Angle of
mandible, A; Ramus, R; Subcutaneous tissue, S; Zygomatic arch, Z.
4.5 Data Analysis
4.5.1 Morphology
Analysis of the relationships between FBs and aponeuroses was carried out in order to determine
the morphology of MM. The number of laminae within the SH and DH were documented. As
well, the number of aponeuroses, and the locations of attachment sites were recorded in order to
determine the aponeurotic morphology of MM.
38
4.5.2 Architectural Parameters
The architectural parameters quantified within each lamina, FBL and the height of aponeuroses,
were input into SPSS (International Business Machines, Armonk, NY). Descriptive statistics
(mean, standard deviation) were used to define architectural parameters in the relaxed and
maximally contracted states within each lamina of SH and DH. The mean FBL and mean height
of aponeuroses in the relaxed and maximally contracted states were compared using paired
t-tests and Wilcoxon paired signed-rank tests. A difference in an architectural parameter between
the relaxed and maximally contracted states was deemed significant if p<0.05. Percent
differences were calculated for mean FBL between the relaxed and maximally contracted states
in each lamina of SH. To assess reliability, FBL and heights of aponeuroses were measured in
every lamina of five MMs, twelve weeks after initial measurements were carried out. Repeat
measurements were made on the same unmarked US scans where measurements had been made
previously. The intraclass correlation coefficient (ICC) was then applied comparing the new data
set to the initial data set, in order to determine intra-rater reliability.
39
Chapter 5
5 Results
The in vivo morphology of MM was characterized. Fiber bundles and aponeuroses were
visualized using US and measured throughout the volume of MM in all participants, in the
relaxed and maximally contracted states. The following sections elaborate on the in vivo laminar
structure, and musculo-aponeurotic parameters.
5.1 Gross Morphology
In all participants, MM was found to have two heads: SH and DH. The SH was greater in size
than the DH. On panoramic coronal US scans of SH, DH could not be visualized (Figure 5.1).
However, in panoramic coronal scans of DH, angled axial cross-sections of SH were seen
(Figure 5.2).
40
Figure 5.1. Relaxed panoramic coronal US scans of SH. A. US scan B. SH highlighted (green).
Angle of mandible, A; Ramus, R., Subcutaneous tissue, S; Zygomatic arch, Z.
41
Figure 5.2. Relaxed panoramic coronal US scans of DH. A. US scan. B. Superficial Head
(green) and deep head (pink) highlighted. Angle of mandible, A; Ramus, R., Subcutaneous
tissue, S; Location of zygomatic arch, Z.
5.2 Fiber Bundle Morphology
In vivo US scans of MM showed that the SH and DH have a laminar arrangement. The laminae
consisted of aponeuroses and FBs. The FBs attached to superior and inferior aponeuroses, and/or
bone.
5.2.1 Laminae of Superficial Head
The laminae of SH were arranged sequentially from superficial to deep. The SH consisted of:
• three laminae in 58.3% (n=28/48) of MMs (Figure 5.3)
• four laminae in 41.7% (n=20/48) of MMs (Figure 5.4)
42
The laminae were flat on coronal scans, with the exception of L1 when four laminae were
present. In this case L1 was identified only in the inferior half of SH, and was curved in shape
(Figure 5.4).
Figure 5.3. Relaxed panoramic coronal US scan of SH with three laminae. A. Coronal scan. B.
Scan showing laminar arrangement (L1-L3). C. Tracing of laminae. Aponeuroses, yellow lines;
Angle of mandible, A; Ramus, R; Subcutaneous tissue, S; Zygomatic arch, Z.
43
Figure 5.4. Relaxed panoramic coronal US scan of SH with four laminae. A. Coronal scan. B.
Scan showing laminar arrangement (L1-L4). C. Tracing of laminae. Aponeuroses, yellow lines;
Angle of mandible, A; Ramus, R; Subcutaneous tissue, S; Zygomatic arch, Z.
5.2.2 Laminae of Deep Head
The laminar morphology of DH was more varied than that of SH. The DH consisted of:
• two laminae in 22.9% (n=11/48) of MMs (Figure 5.5)
• three laminae in 62.5% (n=30/48) of MMs (Figure 5.6)
• four laminae in 8.3% (n=4/48) of MMs (Figure 5.7)
• five laminae in 6.3% (n=3/48) of MMs (Figure 5.8)
Superficial laminae were flat and oriented at an acute angle to the ramus of the mandible. In
contrast, the deepest lamina was triangular in shape regardless of the number of laminae present
(Figures 5.5-5.8).
44
Figure 5.5. Relaxed coronal US scan of DH with two laminae. A. Coronal scan. B. Scan
showing laminar arrangement (L1-L2). C. Tracing of laminae. Aponeuroses, yellow lines;
Ramus, R; Subcutaneous tissue, S; Superficial head, SH; Location of zygomatic arch, Z.
45
Figure 5.6. Relaxed coronal US scan of DH with three laminae. A. Coronal scan. B. Scan
showing laminar arrangement (L1-L3). C. Tracing of laminae. Aponeuroses, yellow lines;
Ramus, R; Subcutaneous tissue, S; Superficial head, SH; Location of zygomatic arch, Z.
46
Figure 5.7. Relaxed coronal US scan of DH with four laminae. A. Coronal scan. B. Scan
showing laminar arrangement (L1-L4). C. Tracing of laminae. Aponeuroses, yellow lines;
Ramus, R; Subcutaneous tissue, S; Superficial head, SH; Location of zygomatic arch, Z.
Figure 5.8. Relaxed coronal US scan of DH with five laminae. A. Coronal scan. B. Scan
showing laminar arrangement (L1-L5). C. Tracing of laminae. Aponeuroses, yellow lines;
Ramus, R; Subcutaneous tissue, S; Superficial head, SH; Location of zygomatic arch, Z.
47
5.3 Morphology of Aponeuroses
Both heads of MM contained aponeuroses that extended longitudinally from superior and
inferior attachment sites to terminate within the muscle belly. These aponeuroses were organized
in a layered manner relative to each other.
5.3.1 Superficial Head
In the majority of participants, the SH contained two aponeuroses. However, three aponeuroses
were also a frequent finding (Table 5.1).
Table 5.1. Aponeuroses of SH. Total number of
aponeuroses n Number of aponeuroses
Superior Inferior 2 28 1 1 3 20 2 1
When two aponeuroses were present, their origins were as follows (Figure 5.9A. and 5.10.):
• superficial aponeurosis from the inferior third of the ramus or angle of the mandible
• deep aponeurosis from the zygomatic arch
When three aponeuroses were present, their origins were as follows (Figure 5.9B.):
• superficial aponeurosis from the zygomatic arch
• middle aponeurosis from the ramus or angle of the mandible
• deep aponeurosis from the zygomatic arch, deep to the superficial aponeurosis
48
Figure 5.9. Relaxed panoramic coronal US scans of SH. A. Two aponeuroses. B. Three
aponeuroses. Aponeuroses, yellow lines; Angle of mandible, A; Ramus, R; Subcutaneous tissue,
S; Zygomatic arch, Z.
49
Figure 5.10. SH with two aponeuroses correlated in relaxed panoramic coronal and relaxed
panoramic axial scans. A. Superior third. B. Middle third C. Inferior third. Aponeuroses, yellow
lines; Angle of mandible, A; Ramus, R; Subcutaneous tissue, S.
5.3.2 Deep Head
The number of aponeuroses within the DH ranged between two and five (Table 5.2). Most
commonly, three aponeuroses were identified, followed by two. Four or five aponeuroses were
rarely found.
50
Table 5.2. Aponeuroses of DH. Total number of
aponeuroses n Number of aponeuroses
Superior Inferior 2 11 1 1 3 26 2 1
4 1 2
4 4 2 2 5 3 3 2
The lateral surface of DH was separated from SH by a superficial aponeurosis (Figure 5.11).
Superior aponeuroses attached to the zygomatic arch at different depths. Inferior aponeuroses
attached to the ramus of the mandible. Deeper inferior aponeuroses had progressively more
superior attachment points along the ramus. The attachment points of the aponeuroses of DH
alternated between superior and inferior sites. In two cases however, the attachment sites did not
alternate perfectly and two adjacent aponeuroses attached inferiorly.
51
Figure 5.11. Relaxed coronal US scans of DH. A. Two aponeuroses. B. Three aponeuroses.
C. Four aponeuroses. D. Five aponeuroses. Aponeuroses, yellow lines; Ramus, R;
Subcutaneous tissue, S; Superficial head, SH; Zygomatic arch, Z.
52
5.4 Architectural Parameters
The following section focuses on the musculo-aponeurotic parameters that were quantified.
5.4.1 Contractile Tissue Parameters
Regardless of the number of laminae present, mean FBL decreased in the laminae of SH from
superficial to deep (Tables 5.3 and 5.4). However, the mean FBL of L3 was greater than that of
L2 when three laminae were identified, and the mean FBL of L4 was greater than that of L3
when four laminae were present. The ICC for FBL was 0.90, with 95% confidence interval (CI)
(0.85, 0.93).
Table 5.3. Mean FBLs in SH with three laminae (n=28).
Lamina (L)
Relaxed Mean FBL (mm)
Contracted Mean FBL (mm)
Percent change
(%)
L1 20.5 ± 6.6a 17.7± 6.5a 13.7
L2 13.2 ± 4.7 12.4 ± 4.7 6.1
L3 16.6 ± 4.7 16.0 ± 4.8 3.6
a statistically significant difference (p<0.05) between relaxed and contracted mean FBL
Table 5.4. Mean FBLs in SH with four laminae (n=20).
Lamina (L)
Relaxed Mean FBL (mm)
Contracted Mean FBL (mm)
Percent change
(%)
L1 21.1 ± 7.7a 17.9 ± 7.2a 15.2
L2 17.3 ± 6.2b 13.9 ± 4.7b 19.7
L3 12.1 ± 6.2 11.2 ± 4.8 7.4
L4 14.9 ± 4.6 15.0 ± 5.6 0.7
a,b statistically significant difference (p<0.05) between relaxed and contracted mean FBL
The relaxed mean FBL was greater than the contracted mean FBL in each lamina (Tables 5.3-5.4
and Figure 5.12). However, this was statistically significant (p<0.05) only in the superficial
laminae: L1 when SH had three laminae, and L1 and L2 when SH had four laminae.
53
Figure 5.12. Coronal US scan of SH showing a longer FBL in the relaxed state than in the
contracted state. A. Relaxed state. B. Contracted state. Aponeuroses, yellow lines; Fiber bundle
length, red line; Ramus, R; Subcutaneous tissue, S; Zygomatic arch, Z.
It should be noted that when three aponeuroses were present, L1 was in the same location and
occupied the same area within the muscle belly as L1 and L2 when four laminae were present
(Figure 5.13).
54
Figure 5.13. Relaxed panoramic coronal US scans of L1 and L2 of SH. A. and B. Scan and
tracing of L1 (blue) in SH with three laminae. C. and D. Scan and tracing of L1 and L2 (blue) in
SH with four laminae. Aponeuroses, yellow lines; Angle of mandible, A; Ramus, R;
Subcutaneous tissue, S; Zygomatic arch, Z.
Fiber bundles extending between the aponeuroses of the laminae of DH could be identified
within the laminar volume, but not quantified on a 2D image due to their 3D spatial orientation.
5.4.2 Connective Tissue Parameters
The mean height of aponeuroses within the SH progressively decreased from superficial to deep
(Tables 5.5 and 5.6). In the relaxed state, the mean height ranged between 34.9 mm and 26.1
mm, and in the contracted state from 39.4 mm to 23.4 mm. The ICC for mean height of
aponeuroses was 0.96, with 95% CI (0.92, 0.98).
55
Table 5.5. Mean heights of SH aponeuroses where two aponeuroses were present
(n=28). (Laminae, L1-L3).
Aponeurosis
Located
Between
Relaxed mean height of aponeurosis (mm)
Contracted mean height of aponeurosis (mm)
1 L1-L2 33.4 ± 7.2a 37.1 ± 6.9a
2 L2-L3 31.4 ±11.3 29.4 ± 10.4
a statistically significant difference (p<0.05) between relaxed and contracted mean heights of
aponeuroses
Table 5.6. Mean heights of aponeuroses in SH with three aponeuroses (n=20).
(Laminae, L1-L4).
Aponeurosis Located
Between
Relaxed mean height of aponeurosis (mm)
Contracted mean height of aponeurosis (mm)
1 L1-L2 34.9 ± 13.3a 39.4 ± 12.6a
2 L2-L3 30.4 ± 6.9b 35.6 ± 10.1b
3 L3-L4 26.1 ± 10.4 23.4 ± 10.0
a,b statistically significant difference (p<0.05) between relaxed and contracted mean heights of
aponeuroses
Statistically significant differences (p<0.05) between the relaxed and contracted mean heights of
the aponeuroses of SH were seen only in aponeurosis 1 when two aponeuroses were present, and
aponeuroses 1 and 2 when three aponeuroses were present. In these cases, there was an increase
in the mean height of aponeuroses from the relaxed to the maximally contracted state (Figure
5.14 and 5.15). It should be noted that these aponeuroses are located at the peripheries of the
laminae where statistically significant decreases were seen in mean FBL upon contraction. This
suggests that when the FBs of superficial laminae contract, they create a pull on the aponeuroses
and cause them to stretch.
56
Figure 5.14. Panoramic coronal US scans of SH showing increase in height of most superficial
aponeurosis upon contraction. A. Relaxed state. B. Contracted state. Most superficial
aponeurosis, yellow line; Angle of mandible, A; Ramus, R; Subcutaneous tissue, S; Location of
zygomatic arch, Z.
57
Figure 5.15. Panoramic coronal US scans of SH showing increase in height of two most
superficial aponeuroses upon contraction. A. Relaxed state. B. Contracted state. Superficial
aponeuroses, yellow lines; Angle of mandible, A; Ramus, R; Subcutaneous tissue, S; Location of
zygomatic arch, Z.
Irrespective of the number of aponeuroses within DH, the mean heights of aponeuroses
decreased from the most superficial aponeurosis to the deepest (Tables 5.7-5.11). No statistically
significant differences were found between the mean heights of aponeuroses in the relaxed and
maximally contracted states in DH (Figure 5.16).
58
Figure 5.16. Coronal US scans of DH showing no changes in height of aponeuroses upon contraction. A. Relaxed state. B. Contracted state. Aponeuroses, yellow lines; Angle of mandible, A; Ramus, R; Subcutaneous tissue, S; Location of zygomatic arch, Z.
Table 5.7. Mean heights of aponeuroses in DH with two aponeuroses (n=11).
(Laminae, L1-L2; Superficial head, SH).
Aponeurosis Located
Between
Relaxed mean height of aponeurosis (mm)
Contracted mean height of aponeurosis (mm)
1 SH-L1 26.4±4.3 24.4±5.2
2 L1-L2 18.5±8.3 17.6±5.9
59
Table 5.8. Mean heights of aponeuroses in DH with three total aponeuroses, where
two aponeuroses attached superiorly and one attached inferiorly (n=26). (Laminae,
L1-L3; Superficial head, SH).
Aponeurosis Located
Between
Relaxed mean height of aponeurosis (mm)
Contracted mean height of aponeurosis
(mm) 1 SH-L1 30.6±7.1 30.7±6.7
2 L1-L2 20.5±3.8 21.4±4.4
3 L2-L3 18.1±6.6 17.7±5.3
Table 5.9. Mean heights of aponeuroses in DH with three total aponeuroses, where two
aponeurosis attached inferiorly and one attached superiorly (n=4). (Laminae, L1-L3;
Superficial head, SH).
Aponeurosis Located Between Relaxed mean height
of aponeurosis (mm)
Contracted mean height
of aponeurosis (mm)
1 SH-L1 23.3±10.4 25.9±5.7
2 L1-L2 21.1±13.0 19.0±4.3
3 L2-L3 12.3±2.9 16.9±2.6
Table 5.10. Mean heights of aponeuroses in DH with four aponeuroses (n=4).
(Laminae, L1-L4; Superficial head, SH).
Aponeurosis Located Between Relaxed mean height
of aponeurosis (mm)
Contracted mean height
of aponeurosis (mm)
1 SH-L1 24.6±6.0 27.6±5.9
2 L1-L2 16.1±4.8 17.7±7.5
3 L2-L3 12.1±7.7 13.6±7.0
4 L3-L4 9.2±2.6 9.1±2.9
60
Table 5.11. Mean heights of aponeuroses in DH with five aponeuroses (n=3). (Laminae,
L1-L5; Superficial head, SH).
Aponeurosis Located Between Relaxed mean height
of aponeurosis (mm)
Contracted mean height
of aponeurosis (mm)
1 SH-L1 29.6±9.5 25.4±2.1
2 L1-L2 29.4±3.7 24.7±3.0
3 L2-L3 17.0±2.8 19.7±0.6
4 L3-L4 14.8±0.6 14.3±5.8
5 L4-L5 11.6±5.7 14.7±1.0
In conclusion, the SH and DH have a multilaminar structure with multiple alternating
aponeuroses. The morphology of DH was more varied than that of SH. Upon maximal
contraction in maximum intercuspation, there were differential changes in the heights of
aponeuroses and lengths of FBs in activated laminae of SH. This suggests there is differential
contraction of the laminae of MM.
61
Chapter 6
6 Discussion
6.1 Morphology
The morphology of MM has been studied rarely throughout the last century. The studies
conducted in the 1900s were published with large intervening intervals eg. 1939, 1961, 1991,
2000 (Ebert, 1939; Schumacher, 1961; Lam, 1991; Gaudy et al., 2000).
Both heads of MM were found to have a laminar arrangement in the current study. The SH had
either three (n=28) or four (n=20) laminae, whereas the DH had two (n=11), three (n=30), four
(n=4), or five laminae (n=3). The laminae of the SH were oriented parallel to the long axis of the
mandibular ramus, whereas the laminae of the DH were oriented at an acute angle to the
mandibular ramus. The varying orientations of the laminae of SH and DH suggests that the heads
of MM may have different functions. The laminar morphology of SH of most participants was
more consistent and less varied than that of DH. The more diverse laminar morphology of DH
may allow for more complex functional partitioning relative to SH.
6.1.1 Cadaveric Studies of Masseter Muscle
Past two-dimensional cadaveric studies divided MM into combinations of anterior, superficial,
intermediate, and/or deep parts. Ebert (1939), Lam (1991), and Gaudy et al. (2000) all included
superficial and deep regions of MM in their descriptions. However, Ebert (1939) also described
an anterior part, contiguous with the superficial part, and Gaudy et al. (2000) described an
intermediate region located between the superficial and deep. Therefore, the finding reported in
the current study that MM was divided into superficial and deep heads is consistent with Lam
(1991), and varies from the reports presented by Ebert (1939) and Gaudy et al. (2000) as they
defined additional anterior and intermediate regions.
The number of laminae of MM reported in these 2D cadaveric studies ranged from two to seven,
whereas in the current study SH was found to consist of three or four laminae, and DH of two to
five. Ebert (1939) reported three to four layers within the contiguous anterior and superficial
regions, and at least one layer in the deep region, whereas Schumacher described five laminae
62
throughout the volume of MM (1961). Gaudy et al. (2000) described two laminae in the
superficial region, one in the intermediate region, and four in the deep. Thus there is overlap
between the total number of laminae identified in MM in the current study and in previous
literature, however direct comparison is not possible as the regions in which laminae were
identified are inconsistent between studies.
The laminar architecture described in the current study consisted of FBs extending between
alternating superior and inferior aponeuroses and/or bone, concurring with the findings of
previous 2D cadaveric studies. In all studies, superior aponeuroses were reported to attach to the
zygomatic arch, and inferior aponeuroses to the angle and/or ramus of the mandible. Ebert
(1939), Schumacher (1961), Lam (1991), and the current study all found that these aponeuroses
terminated within the muscle belly. In contrast, Gaudy et al. (2000) described a complex pattern
of superior and inferior tendinous sheets, cones, and/or bundles that occasionally merged within
the muscle belly.
The number of aponeuroses reported in previous 2D cadaveric literature varied from three to
five, while the current study found two or three aponeuroses in SH, and two to five aponeuroses
in DH. Ebert (1939) reported two to four aponeuroses in the anterior and superficial regions, and
one aponeurosis in the deep region, whereas Schumacher (1961) and Lam (1991) found five
aponeuroses in MM. The numbers of aponeuroses found in the current study are similar to those
in these earlier studies, however direct comparison is impossible as the regions in which
aponeuroses were reported differ.
Three-dimensional studies enable reconstruction of the muscle volume as in situ. The 3D
cadaveric study carried out by Ebrahimi (2015) reported that MM consisted of superficial and
deep heads, consistent with the findings of the current study. Ebrahimi (2015) found that the
heads of MM had a multilaminar arrangement of aponeuroses and FBs, where SH was comprised
of two to four laminae, and DH of one to three laminae. The current study found three or four
laminae in the SH, and two to five laminae in the DH. Therefore, the numbers of laminae in the
current study and those reported by Ebrahimi (2015) are similar.
The 3D digitization study conducted by Ebrahimi (2015) reported that laminae consist of FBs
that attach to aponeuroses and/or bone, in agreement with the current study. Ebrahimi (2015)
also found that aponeuroses alternated between superior attachment sites on the zygomatic arch
63
and zygomatic process of the maxilla, and inferior attachment sites on the ramus or angle of the
mandible. This is also consistent with the findings of the current study.
The numbers of aponeuroses of MM reported by Ebrahimi (2015) were slightly different from
those reported in the current study. Ebrahimi (2015) found three or four superior aponeuroses
and two to four inferior aponeuroses in the SH, while the current study found two or three
aponeuroses in SH in total. Ebrahimi (2015) also reported that the DH had one or two superior
and/or inferior aponeuroses, whereas the current study found between two and five. In the
current in vivo study, fewer aponeuroses may have been found in the SH due to differences
between direct and US visualization. When using 3D digitization all aponeuroses are serially
dissected and digitized, whereas it may not be possible to resolve all aponeuroses using US. For
example, aponeuroses located on the surface of the SH on US scans cannot be differentiated
from overlying subcutaneous tissue, and as a result cannot be measured. Similarly, if an
aponeurosis was located too close to the mandibular ramus, then it also cannot be resolved
separate from the ramus. These two factors are more likely to affect the SH than the DH. The
most superficial aponeurosis of the DH can be visualized as fibers from the SH separate a large
portion of it from the subcutaneous tissue. Because the laminae of the DH are angled towards the
mandibular ramus, whereas those of the SH are located flat against the ramus, it is also easier to
resolve aponeuroses on the deep surface of the DH than the SH, separate from the mandibular
ramus. The greater variety in the aponeurotic architecture of the DH found in the current study
may also be due to the evaluation of a larger sample size (n=48) than in the study carried out by
Ebrahimi (2015) (n=8).
6.1.2 In Vivo Studies of Masseter Muscle
Cadaveric studies are useful in setting the foundation for in vivo studies of muscle architecture.
Unlike in vivo studies, cadaveric studies also provide information based on direct 3D
visualization. However, in vivo imaging studies have a distinct advantage in that they are
dynamic and can be used to correlate function and associated anatomic changes. Aponeuroses
are clearly visible in magnetic resonance images throughout the volume of MM (Lam et al.,
1991), however FBs are not. Ultrasound can be used to visualize and measure both aponeuroses
and FBs in skeletal muscle (Kim et al., 2010; Kim et al., 2013).
64
In vivo MRI studies have described the morphology of MM as being compartmentalized or
layered (Lam et al., 1991; Cioffi et al, 2012). Lam et al. (1991) described MM as having a
multilayered structure with three internal compartments: superficial, intermediate, and deep,
however Cioffi et al. (2012) reported a complex pattern of at least three compartments that could
not be clearly characterized. The variable number of laminae found in the current study
compared to Lam et al. (1991) could be attributed to a larger sample size or differences in
imaging techniques.
The number and attachment sites of aponeuroses demarcating the laminae has only been
investigated with MRI in a few studies (Lam et al., 1991; Cioffi et al., 2012). Lam et al. (1991)
described a total of four tendon planes. Three tendon planes were attached to the zygomatic arch
or ramus of the mandible in an alternating manner, and the fourth was located on the deep
surface of MM. The current study also found alternating superior and inferior aponeuroses.
However, visualization of a distinct tendon plane on the deep surface of MM with US, separate
from the ramus of the mandible, was not possible. In contrast, Cioffi et al. (2012) stated that the
aponeuroses of MM “generally assumed a fan-shape distribution originating from the zygomatic
arch.”
No previous in vivo US studies were found that investigated the laminar structure of MM. As a
result, comparison to previous US studies is not possible.
6.2 Architectural Parameters
Generalized architectural parameters of MM including MV, MT, and CSA have been quantified
in cadaveric, and in static and dynamic in vivo studies using CT, MRI, and US. More specific
parameters such as FBL and the height of aponeuroses have previously been measured in 3D
and/or 2D cadaveric studies of MM. The only possible comparison of the results of the current in
vivo study are to parameters that have been quantified from the cadaveric studies, as no in vivo
studies investigating FBL and the height of aponeuroses were found.
6.2.1 Contractile Tissue Architectural Parameters
In both 2D and 3D cadaveric studies, mean FBL in MM was found to decrease from superficial
to deep. However, the regions investigated in different studies were not always the same. Ebert
(1939) reported a range of FBL that was greater in the more superficial part of MM than in the
65
deep, while van Eijden et al. (1997) reported that mean FBL throughout three superficial regions
of MM was greater than in four deep regions. Ebrahimi (2015) found a gradual decrease in the
mean FBL from superficial laminae to deep in both SH and DH (Table 2.2). In the current study,
it was also observed that mean FBL in the laminae of SH decreased from superficial to deep
(Tables 5.3 and 5.4).
When comparing mean FBL found in the current study to past reports, it should be noted that the
current thesis is a 2D study carried out with teeth in first contact in the relaxed state, and in
maximum intercuspation upon maximal contraction. Mean FBL in 2D cadaveric studies has been
quantified in the closed mouth position (Ebert, 1939; Schumacher, 1961; Weijs and Hillen, 1984;
van Eijden et al., 1997), whereas 3D cadaveric studies involved measuring FBL in maximal
inter-incisal opening (Ebrahimi, 2015). Thus, the positions used to quantify FBL in the current
study are more similar to those used in past 2D cadaveric studies. Therefore mean FBL found in
2D cadaveric studies is comparable to the mean FBL of the relaxed state found in the current
study, and slightly greater than mean FBL quantified in the maximally contracted state (Tables
2.1, 5.3, and 5.4). The mean FBL quantified in each lamina of SH in the 3D cadaveric
digitization study (Ebrahimi, 2015) was greater than that reported in each lamina of SH in the
current study. However, Ebrahimi (2015) was able to directly visualize and quantify the full 3D
length of each FB, whereas the current US study was two-dimensional.
6.2.2 Connective Tissue Architectural Parameters
No 2D cadaveric study was found that quantified the mean height of aponeuroses of MM.
Therefore, no comparison can be made between the results of the current study and past 2D
cadaveric studies in terms of this architectural parameter.
Ebrahimi (2015) reported mean heights of aponeuroses as part of the 3D cadaveric digitization
study of MM (Tables 2.3 A. and B.), and found results that were similar to those of the current
study (Tables 5.5-5.11). While there is overlap between the mean heights of aponeuroses
reported by Ebrahimi (2015) and the current study, the current study found a greater range likely
due to the greater sample size evaluated. As well, both Ebrahimi (2015) and the current study
observed that the heights of aponeuroses decreased as depth within MM increased.
66
6.3 Contraction of Masseter Muscle
Electromyography studies since the 1990s have suggested that MM is functionally partitioned
(McMillan and Hannam, 1991; Blanksma et al., 1992; Blanksma and van Eijden, 1995),
however, no imaging studies were found that investigated regional changes in MM with
differences in function.
The results of the current study suggest that there is differential contraction of the laminae of
MM. When comparing the relaxed and maximally contracted states of MM, a statistically
significant (p<0.05) decrease was observed in the mean FBL of superficial laminae of SH
(Tables 5.3 and 5.4). It was also found that there was a statistically significant increase in the
mean height of aponeuroses that were located at the peripheries of these superficial laminae upon
contraction (Tables 5.5 and 5.6). In contrast, no significant difference was observed in the mean
heights of any of the aponeuroses of DH between the relaxed and maximally contracted states.
This implies that there is activation only of the superficial laminae of SH when transitioning
from a relaxed state to a maximally contracted state, and therefore that there is differential
activation of the laminae of SH. These findings are supported by Blanksma et al. (1992) and
Blanksma and van Eijden (1995) who suggested there was differential activation of the regions
of MM based on the direction of bite and the task attempted. The concept of differential
activation of the laminae of MM also concurs with McMillan and Hannam's (1991) suggestion
that individual laminae of MM were associated with "cigar-shaped motor-unit territories," which
did not cross aponeuroses.
The current study had several limitations. Only one motion of the mandible was investigated. In
order to confirm that there is differential activation of the laminae of MM, more mandibular
movements need to be studied. Additionally, in the normal relaxed state the teeth are slightly
apart. In the current study, to ensure consistency in position of the dentition, an occlusal
registration maintained the teeth in minimal contact. For participant comfort, the contracted state
can only be maintained for a short period of time. Therefore, image acquisition was rapid,
ranging between 1-2 seconds. As well, the contracted state was visible on US scans, and any
relaxation could be observed.
67
Chapter 7
7 Conclusions
7.1 Hypotheses
The following can be concluded based on the results presented in the current study:
1. Regarding the visualization of the in vivo musculo-aponeurotic morphology of the heads of
MM:
H0 rejected: The laminar morphology of the heads of masseter cannot be visualized throughout
the muscle volume using US.
H1 accepted: The laminar morphology of the heads of masseter can be visualized throughout the
muscle volume using US.
2. Regarding whether there are differences in intramuscular contractile tissue element parameters
(fiber bundle length) within the laminae of MM when comparing the relaxed and maximally
contracted states:
H0 rejected: There is no significant difference in fiber bundle length within the different laminae
of MM between the relaxed and maximally contracted states.
H1 accepted: There is a significant difference in fiber bundle length within the different laminae
of MM between the relaxed and maximally contracted states.
3. Regarding whether there are significant differences between aponeurotic architectural
parameters in the relaxed and maximally contracted states:
H0 rejected: There is no significant difference in the height of aponeuroses within the different
laminae of MM between the relaxed and maximally contracted states.
H1 accepted: There is a significant difference in the height of aponeuroses within the different
laminae of MM between the relaxed and maximally contracted states.
68
7.2 Significance
The contributions of the current study include:
• the development of an in vivo US protocol to study MM based on 3D digitized data.
• the determination of the in vivo laminar musculo-aponeurotic morphology of MM using
US.
• the establishment of the beginning of a normative database of musculo-aponeurotic
architectural parameters of MM in each lamina.
• the proposal of a functional rationale to explain the laminar morphology of MM.
Each of these contributions is discussed below.
The in vivo US protocol developed in the current study is novel and enables, for the first time,
study of the musculo-aponeurotic architecture of MM at the level of FBs and aponeuroses. This
protocol was applied to asymptomatic participants in the current study, however it could also be
used to investigate potential pathologic changes in the contractile and connective tissue
architecture using a non-invasive and inexpensive imaging modality.
The current in vivo US study has determined the laminar morphology of MM in the largest
sample size reported to date. Mean FBL and mean height of aponeuroses in vivo have been
established for the laminae of MM, allowing comparison between regions. This has not been
previously done in vivo.
The results of the current, dynamic, in vivo study suggest that there is activation only of
superficial laminae of the SH upon transition from a relaxed to a maximally contracted state in
maximum intercuspation. It is therefore proposed that there is differential contraction of the
laminae of MM, which had not been reported previously reported in the imaging literature. Thus,
the current study is the first in vivo US study of MM to suggest a rationale for the complex
laminar morphology of MM.
Masseter involvement in TMDs has been documented, however it is unclear how the musculo-
aponeurotic architecture of MM differs between health and pathosis at a laminar level. Regional
69
changes in MM activation may be responsible for localized pain in TMDs. A further dynamic
study of MM in pathologic participants would allow for the localization of changes at the FB
level, which could be responsible for symptoms experienced. This could then be correlated with
habits or parafunction carried out by the patients, and used to develop individualized treatment.
The current study has begun to establish a normative database of musculo-aponeurotic
parameters of MM, which can be used for comparison with age-appropriate pathologic
participants. This can serve as a foundation for the investigation of pathologic changes in MM.
70
Chapter 8
8 Future Directions
The current study has many possible future applications. Some of these are discussed below.
The US protocol devised in the current study can be used to determine if there is activation of
non-superficial laminae of MM with different maxillo-mandibular motions. This can be
accomplished by acquiring US images in bite positions other than maximum intercuspation (eg.
incisal bite). As well, scans can also be acquired in different maxillo-mandibular positions (eg.
protrusion, retrusion, ipsilateral excursion).
Study of patients with pathoses can also be carried out using the current protocol to determine if
changes in musculo-aponeurotic architecture can be localized within the volume of MM. Patients
with diagnoses of the following conditions could be considered for inclusion into future studies:
• Masticatory muscle myalgias
• TMJ disc displacement
• History of TMJ subluxation
• Degenerative joint disease
Age-related changes in the musculo-aponeurotic architecture of MM can also be investigated by
applying the current protocol to different age groups. Since some forms of pathosis are found
more often in certain age groups (eg. degenerative joint disease in the sixth decade). This could
be used to differentiate age-related changes from pathologic changes.
71
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