Chapter 4

1. Chapter 4 The Response of Biological Tissue to Stress

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. 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

9. 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

10. 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

11. 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

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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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)

22. 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

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

24. 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