anatomical, biomechanical and behavioural characteristics - cIRcle

324 Pages·2009·6.21 MB·English

ANATOMICAL, BIOMECHANICAL AND BEHAVIOURAL CHARACTERISTICS
OF THE HUMAN MASSETER MUSCLE
by
MoNICA DE LORENZO TONNDORF
D.D.S., Federal University of Rio de Janeiro, 1986
A THESIS SUBMHTED IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
in
THE FACULTY OF GRADUATE STUDIES
(Department of Oral Biology)
We accept this thesis as conforming
THE UNIVERSITY OF BRITISH COLUMBIA
October 1993
©Mônica L. Tonndorf, 1993
______________________
In presenting this thesis in partial fulfilment of the requirements for an advanced
degree at the University of British Columbia, I agree that the Library shall make it
freely available for reference and study. I further agree that permission for extensive
copying of this thesis for scholarly purposes may be granted by the head of my
department or by his or her representatives. It is understood that copying or
publication of this thesis for financial gain shall not be allowed without my written
permission.
(Signature)
Department of Oral Biology
The University of British Columbia
Vancouver, Canada
Date November 8, 1993
DE-6 (2/88)
ABSThAC
There is little information on the anatomical and functional organization of
the humanjawmuscles, despite their importance to the masticatory systemand disorders
which affect it. The current studies examined the anatomical, biomechanical and
functional characteristics in the multipennate human masseter muscle as a model for
understanding function in the human jaw.
Masseter anatomy was investigated in fetuses, adult cadavers, and living
subjects by histology, gross anatomical dissection and Magnetic-Resonance (MR)
imaging. Nerve pathways and muscle-fibre arrangements suggested that both fetal and
adult muscles couldbe divided into at leastfour neuromuscular compartments. Although
some fetal specimens had developed peimation, internal aponeuroses were seldom
present. Structural variations were also found in the cadaveric material and living
subjects. As the number of aponeuroses increasedwith development, and their thickness
varied between individuals, individualized contraction strategies are likely to occur
during function.
Movements of masseter insertion sites during jaw function were modelled in
dry skulls. Muscle origins and insertions were measured three-dimensionally at different
gapes and simulated masticatory positions. Their movements varied with craniofacial
dimensions and their locations on the mandible. For some parts of the muscle, the
balancing-side and the incisal-contact positions provided the most advantageous lines of
muscle fibre action, while for others, the working-side task was most favourable.
Separate portions of the muscle are thus uniquely placed to perform specific tasks.
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Movements of four insertion sites were also recorded in four living subjects. Insertion
areas were identified on MR-derived reconstructions, and movements recorded with a
jaw-tracking device. Ranges of insertion displacement were similar to these estimated
for dry skulls, and varied between individuals.
Interference electromyography, and single-motor-unit (MU) techniques were
used to study physiological responses in eight subjects. The behaviour of low-threshold
MUs was investigated relative to changes in bite side and experimental paradigmwhile
subjects were biting on a force transducer. For each unit, the recruitment threshold,
sustained-bite forces, the rate and regularity of sustained firing, and the coefficient of
variation, were measured. Assessments were also made of the accuracy with which the
target rates were attained, and ofthe contribution ofeach MU’s firing rate to bite force.
These measures of behaviour frequently differed between tasks, but not reproducibly.
The highest reproducibility and firing-rate accuracy were achieved when visual and
auditory MU feedbackwas provided. Atendencyfor increased firing rate, and decreased
discharge variability was found when subjects had no feedback. As approximately 50%
of the units did not show reproducible behavioural characteristics, it seems that
differences in intramuscular activation, differential activation of other muscles, and the
inherent variability seen in low-threshold MU studies generally make quantitative
comparisons of focal activity in the masseter implausible.
Finally, a method was developed for locating the positions of moving needle
electrodes relative to internal aponeuroses. It combined scanning electromyography,
optical tracking of electrodes, MR imaging, and three-dimensional reconstruction. The
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territorial sizes of 162 MUs were then assessed in the muscles of four subjects. Their
mean width was 3.7 ± 2.3 mm. Most MU territories were confined between tendons,
although 10% of the units clearly extended across at least one tendon. This focal
dispersion ofmost territories provides a firm anatomical basis for selective activation of
the muscle.
The findings collectively indicate an anatomical, biomechanical and
physiological basis for differential motor control of at least four neuromuscular
compartments in the human masseter muscle. The extent to which the central nervous
system selectively activates these compartments, or coactivates them, remains to be
demonstrated under functional conditions.
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TABLE OF CONTENTS
PAGE
ABSTRACT ii
TABLE OF CONTENTS v
LISTOFFIGURES xi
LIST OF TABLES xvi
ACKNOWLEDGEMENTS xvili
1. INTRODUCTION 1
1.0 Introduction to the Thesis 1
Review of the Literature
1.1 Skeletal Muscle Development 2
1.2 Muscle Architecture and Mechanics 10
1.3 Motor-Unit Organization 18
1.3.1 Motor-Unit Arrangement 18
1.3.2 Fibre Characteristics 19
1.4 Motor-Unit Activity 20
1.4.1 Recruitment 21
1.4.2 Firing Rate 21
1.4.3 Behaviour 22
1.4.4 Summary 25
1.5 Partitioning 25
V
1.6 Jaw Muscle Organization 28
.
1.6.1 Structure 29
1.6.2 Fibre Composition 33
1.6.3 Jaw Biomechanics 35
1.6.4 Summary 36
1.7 Anatomical Design of the Masseter. 38
1.7.1 Muscle Organization 39
1.7.2 Innervation Pattern 43
1.7.3 Muscle Fibre Characteristics 45
1.7.4 Spindle Distribution 46
1.7.5 Summary 47
1.8 Functional Design of the Masseter 47
1.8.1 Motor-unit Territory 47
1.8.2 Functional Differentiation 49
1.8.3 Motor-unit Activity 52
1.8.3 Conclusion 54
2. STATEMENT OF THE PROBLEM 56
3. STUDIES 60
3.1 Masseter Morphology 61
3.1.1 Exploratory Experiments on Fetal Masseter Anatomy 61
3.1.1.1 Materials and Methods 62
3.1.1.1.1 Nerve Staining 65
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3.1.1.1.2 Connective-Tissue and Muscle-Fibre Staining. 68
3.1.1.1.3 Muscle-Fibre Staining and Orientation
Assessment 68
3.1.1.1.4 Muscle Reconstruction 68
3.1.1.2 Results 74
3.1.1.2.1 Nerve Distribution 74
3.1.1.2.2 Connective-Tissue Development 76
3.1.1.2.3 Muscle-Fibre Orientation 76
3.1.1.3 Discussion 83
3.1.2 Adult Masseter Anatomy 87
3.1.2.1 Materials and Methods 88
3.1.2.1.1 Gross Anatomical Dissection 88
3.1.2.1.2 Chemical Dissection 89
3.1.2.2 Results 90
3.1.2.3 Discussion 98
3.1.3 Morphological Reconstruction in Living Subjects 101
3.1.3.1 Methods 101
3.1.3.1.1 Magnetic-Resonance Imaging 101
3.1.3.1.2 Imaging 103
3.1.3.1.3 Reconstruction 107
3.1.3.1.4 Error of Method 111
3.1.3.2 Results 111
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3.1.3.3 Discussion 117
.
3.2 Movement of Masseter Insertions at different Jaw Positions 125
3.2.1 Simulated Function in Dry Skulls 126
3.2.1.1 Methods 126
3.2.1.2 Results 133
3.2.1.2.1 Cephalometric Analysis of the Skull Sample.. 133
3.2.1.2.2 Sample Variation in Insertion Site Location
during Dental Intercuspation 136
3.2.1.2.3 Effect of Jaw Position on Insertion Site
Location 136
3.2.1.2.4 Differences within Putative Muscle Layers 141
...
3.2.1.2.5 Differences between Putative Muscle Layers 143
.
3.2.1.2.6 Orientation of Masseter Insertion relative to
Origin 145
3.2.1.2.7 Discussion 147
3.2.2 Function in Living Subjects 157
3.2.2.1 Methods 158
3.2.2.2 Results 163
3.2.2.3 Discussion 169
3.3 Evaluation of the Masseter’s Functional Performance 175
3.3.1 Electromyographic Recording Techniques 175
3.3.2 Preliminary Experiments using Single-Wire EMO Recording
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Technique 176
3.3.2.1 Materials and Methods 177
3.3.2.2 Results 183
3.3.2.3 Discussion 187
3.3.3 Motor-Unit Behaviour 192
3.3.3.1 General Methods 192
3.3.3.1.1 Motor-Unit Recording 192
3.3.3.1.2 Bite-Force Measurement 196
3.3.3.1.3 Sampling and Data Analysis 201
3.3.3.2 Effect of Bite Side on MU Behaviour 202
3.3.3.2.1 Methods 204
3.3.3.2.2 Results 208
3.3.3.2.3 Discussion 217
3.3.3.3 Effect of Experimental Paradigm 222
3.3.3.3.1 Methods 222
3.3.3.3.2 Results 224
3.3.3.3.3 Discussion 227
3.4 Motor-Unit Territory Relative to the Masseter’s Internal Architecture 230
.
3.4.1 Methods 232
3.4.1.1 Stereotactic Location of EMG Needle Electrode Scans 232
3.4.1.1.1 Morphologic Reconstruction 232
3.4.1.1.2 Motor-Unit Recording 233
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