The Response of Biological Tissue to Stress 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 Biological tissues must demonstrate the ability to withstand excessive or repetitive stresses if musculoskeletal health is to be maintained

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 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 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 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 curve The shape and position of the load-deformation curve depends on a number of factors: Stiffness Viscoelasticity Age Exercise

Levers 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 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 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 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 Damage to articular cartilage may result from microtrauma (degeneration), macrotrauma, or an inflammatory process Degeneration: osteoarthritis Primary and secondary Inflammation: Rheumatoid arthritis

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

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

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