Transcript Response of Biological Tissue

Chapter 4
The Response of Biological Tissue
to Stress
Overview
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A wide range of external and internal
forces are either generated or resisted
by the human body during the course
of daily activities
Biological tissues must demonstrate
the ability to withstand excessive or
repetitive stresses if musculoskeletal
health is to be maintained
Stress
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The capacity of a tissue to withstand stress
is dependent on a number of factors:
– Age
– The proteoglycan and collagen content of the
tissue
– The ability of the tissue to undergo adaptive
change
– The speed at which the adaptive change must
occur
Terminology
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Kinetics - the study of forces that arise as
motions change
Mass - the quantity of matter composing a
body
Inertia - the resistance to action or to
change
Force - a vector quantity, with magnitude,
direction and point of application to a body
Terminology
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Load - the type of force applied
Stress - the force per unit area that
develops on the cross section of a structure
in response to an externally applied load
Strain - the deformation that develops
within a structure in response to externally
applied loads
Hysteresis - the difference in the behavior
of a tissue when it is being loaded versus
unloaded
Load-deformation curve
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The load-deformation curve, or stressstrain curve, of a structure depicts the
relationship between the amount of
force applied to a structure and the
structure’s response in terms of
deformation or acceleration
Load-deformation curve
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The shape and position of the loaddeformation curve depends on a
number of factors:
– Stiffness
– Viscoelasticity
– Age
– Exercise
Levers
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First Class: occurs when two forces are applied on
either side of an axis in the fulcrum lies between
the effort and the load. E.g. a seesaw
Second-class: occurs when the load is applied
between the fulcrum and the point where the effort
is exerted. E.g. the wheelbarrow
Third class: occurs when the load is located at the
end of the lever. E.g. flexion at the elbow
Musculoskeletal stress
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Macrotrauma - an acute stress
(loading) that occurs when a single
force is large enough to cause injury
of biological tissues
Microtrauma - a repetitive stress that
in of itself is insufficient to damage the
tissue, causes injury when repeated
over a period of time
Collagen
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Collagen fibers have a wavy or folded
appearance at rest (slack)
When a force lengthens the collagen fibers
this slack is taken up
This slack is called the tissue’s crimp
Crimp is different for each type of
connective tissue and this provides each of
these tissues with different viscoelastic
properties
Articular cartilage
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Articular cartilage is a viscoelastic
structure with a very high tensile
strength and is resistant to
compressive and shearing forces
Articular cartilage has the ability to
undergo large deformations while still
being able to return to its original
shape and dimension
Articular cartilage
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Damage to articular cartilage may
result from microtrauma
(degeneration), macrotrauma, or an
inflammatory process
– Degeneration: osteoarthritis
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Primary and secondary
– Inflammation: Rheumatoid arthritis
Ligament
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Fibrous bands of dense connective
tissue that connect bone to bone and
which behave as a viscoelastic
structures when exposed to stress
Ligament injuries are called sprains
Tendon
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Connects muscle to bone
The causes of a tendon injury center
around microtrauma to the tendon
tissue due to repetitive mechanical
loading from external factors, or
macrotrauma
Tendinitis
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The term tendinitis implies an
inflammatory reaction to a tendon
injury - a microscopic tearing and
inflammation of the tendon tissue,
commonly resulting from tissue fatigue
rather than direct trauma
Tenosynovitis
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Tenosynovitis/tenovaginitis, peritendinitis,
and paratenonitis, indicate an
inflammatory disorder of tissues
surrounding the tendon such as the tendon
sheath – usually the result of a repetitive
friction of the tendon and its sheath
Tendinosis
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The term tendinosis refers to a
degenerative process of the tendon.
Characterized by the presence of
dense populations of fibroblasts,
vascular hyperplasia, and disorganized
collagen
Bone
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Bone is a solid with elastic properties
Bone is stiffer and stronger than other
tissues at higher strain levels
Bone is better able to withstand
compressive forces than tensile or
torsional forces
Bone
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Wolff’s law - forces applied to bone,
including muscle contractions and
weight bearing can alter bone the
internal and external configuration of
bone through adaptation to these
stresses
Bone
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If the adaptations of bone to stress do not
occur fast enough, the bone is resorbed
faster than it is replaced, and bone strength
is compromised
Causes of decreased adaptation include:
– An increase in the applied load
– An increase in the number of applied stresses
– A decrease in the size of the surface area over
which the load is applied
Muscle tissue
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Muscle injury can result from:
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Excessive strain
Excessive tension
Contusions
Lacerations
Thermal stress
Myotoxic agents (local anesthetics, excessive use
of corticosteroids, snake and bee venoms)
Hematoma
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Contusion to a muscle belly
Two types:
– Intramuscular: associated with a muscle
strain or bruise. The size of the
hematoma is limited by the muscle fascia
– Intermuscular. This type of hematoma
develops if the muscle fascia is ruptured
and the extravasated blood spreads into
the interfascial and interstitial spaces
Immobilization
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Continuous immobilization of
connective and skeletal muscle tissues
can cause some undesirable
consequences to the tissues of the
musculoskeletal system
Immobilization
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The undesirable consequences
include:
– Cartilage degeneration
– A decrease in the mechanical and
structural properties of ligaments
– A decrease in bone density
– Weakness or atrophy of muscles