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Chapter 4. The Response of Biological Tissue to Stress. Overview. A wide range of external and internal forces are either generated or resisted by the human body during the course of daily activities

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Chapter 4

Chapter 4

The Response of Biological Tissue to Stress


Overview
Overview

  • 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
Stress

  • 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
Terminology

  • 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


Terminology1
Terminology

  • 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
Load-deformation curve

  • The load-deformation curve, or stress-strain 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 curve1
Load-deformation curve

  • The shape and position of the load-deformation curve depends on a number of factors:

    • Stiffness

    • Viscoelasticity

    • Age

    • Exercise


Musculoskeletal stress
Musculoskeletal stress

  • 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
Collagen

  • 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
Articular cartilage

  • 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 cartilage1
Articular cartilage

  • Damage to articular cartilage may result from microtrauma (degeneration), macrotrauma, or an inflammatory process

    • Degeneration: osteoarthritis

      • Primary and secondary

    • Inflammation: Rheumatoid arthritis


Ligament
Ligament

  • 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


Sprains
Sprains

  • Ligament injuries may be graded by severity:

    • Grade I - painful, but do not have swelling or instability

    • Grade II - marked swelling, and pain. Mild ligament laxity and joint instability

    • Grade III - complete disruption of the ligament with gross instability and laxity


Tendon
Tendon

  • 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
Tendinitis

  • 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
Tenosynovitis

  • 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
Tendinosis

  • 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

  • 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

  • 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

  • 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
Muscle tissue

  • Muscle injury can result from:

    • Excessive strain

    • Excessive tension

    • Contusions

    • Lacerations

    • Thermal stress

    • Myotoxic agents (local anesthetics, excessive use of corticosteroids, snake and bee venoms)


Hematoma
Hematoma

  • 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


Muscle strains
Muscle strains

  • Often graded according to severity:

    • I: involves a tear of a few muscle fibers with minor swelling and discomfort. Associated with no or minimal loss of strength and restriction of movement

    • II: Greater damage of the muscle and clear loss of strength and some loss of function

    • III: Involves a tear extending across the whole muscle belly. Characterized by severe pain or loss of function


Immobilization
Immobilization

  • Continuous immobilization of connective and skeletal muscle tissues can cause some undesirable consequences to the tissues of the musculoskeletal system


Immobilization1
Immobilization

  • 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


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