Lecture 4 - Tissue Mechanics: Tendons and Ligaments

Question 1

What do tendons and ligaments have in common?

Answer 1

• Both are composed of dense connective tissue
• Both are biologically active

Question 2

What is the difference between tendons vs ligaments?

Answer 2

• Tendons connect muscles to bones
• Ligaments connect bones to bones

Question 3

What are the functions of tendons?

Answer 3

• Force transmitter
• Macroscopic movements (create torque to rotate joint)
• Alter stress field within bone (compression, bending)
• Compliance
• Proprioception
• Stores elastic energy

Question 4

What are the functions of ligaments?

Answer 4

• Microscopic movement
• Force transmission is a minor function
• Maintains optimal joint alignment
• Provides joint stability
• Compliance
• Proprioception

Question 5

What are ligaments and tendons classified as?

Answer 5

Connective tissue

Question 6

What are the three categories of connective tissue?

Answer 6

1. Proper connective tissue:
   a) Loose: within and between
   muscle sheaths, delicate
   b) Dense: less flexible,
   resistant to stress (tendons
   and ligaments)
2. Supporting connective tissue:
   a) Bone
   b) Cartilage
3. Specialized connective tissues:
   a) Adipose tissue
   b) Hematopoietic tissue

Question 7

What are the components of tendons and ligaments?

Answer 7

• Cells: 20%
• Extracellular Matrix (ECM): 80%

Question 8

What makes up the ECM?

Answer 8

a) Fibers
- Collagen
- Elastic
b) Ground substance
- Glycoproteins
- Protoglycans
- Inorganic components
- H20

Question 9

What is the primary type of cell?

Answer 9

Fibrocyte/tenocytes

Question 10

What are fibroblasts?

Answer 10

What fibrocytes are called when manufacturing proteins

Question 11

What do fibroblasts manufacture?

Answer 11

Components of the ECM

Question 12

What primarily makes up the ECM?

Answer 12

Collagen (gives white colour and provides tensile strength)

Question 13

What do fibroblasts secrete?

Answer 13

Procollagen

Question 14

What does procollagen assemble into?

Answer 14

Tropocollagen

Question 15

What does tropocollagen assemble into?

Answer 15

Tendons and ligaments

Question 16

What is the composition of tendons high in?

Answer 16

Collagen (75-85%)
greater than ligaments

Question 17

What is the composition of ligaments high in?

Answer 17

• Collagen (70-80%)
• Elastin (5-15%)*
  greater than tendons

Question 18

What are the arrangements of collagen fibers in tendons and ligaments?

Answer 18

Tendons: Parallel
Ligaments: Less parallel

Question 19

In composition, what are tendons and ligaments similar in?

Answer 19

• Water (~55-65%)
• Dry matter (30-45%)

Question 20

What does the parallel arrangement of collagen fibers help with?

Answer 20

Withstanding high unidirectional loads

Question 21

What does the less parallel arrangement of collagen fibers help with?

Answer 21

Sustain tensile stresses in one direction and smaller stresses in other

Question 22

What are the two ways to measure tissue properties?

Answer 22

1. Subjecting tissue preparations to uniaxial tensile loads to failure
2. Measurement of in vivo forces

Question 23

What are load deformation curves?

Answer 23

• Represents structural properties of the tissue
• Deformation produced by a load depends on the size of the structure

Question 24

What is tensile stress?

Answer 24

Externally applied tensile load per cross sectional area

Question 25

What is tensile strain?

Answer 25

the elongation/unit length of material in response to tensile load

Question 26

What do mechanical properties depend on?

Answer 26

1. Composition of the tissue 2. Orientation of collagen fibers 3. Number/type of collagen cross links 4. Components of ground substance

Question 27

What do the slopes of the stress-strain curves indicate?

Answer 27

Steep slope: high Young's modulus (stiffness) Gradual slope: low Young's modulus (compliant)

Question 28

What are the different parts of a stress-strain curve?

Answer 28

1. Toe: - Elongation - Uncrimpling of collagen 2. Linear: - Slippage b/w fibers - Return to initial length 3. Plastic: - Microfailures - Collagen structure disrupted 4. Major failure - Visible narrowing 5. Complete failure

Question 29

What are some findings of comparing stress-strain curves?

Answer 29

- Differences between connective tissue types - Cortical bone requires major stress - Cartilage and trabecular bone deform easily

Question 30

What are the characteristics of viscoelasticity?

Answer 30

1. Creep 2. Stress relaxation 3. Recovery 4. Rate effects

Question 31

What tissues are viscoelastic?

Answer 31

Tendons and ligaments

Question 32

What is viscoelasticity?

Answer 32

Time-dependent response of a material subjected to a constant load or deformation

Question 33

What is hysteresis?

Answer 33

The amount of energy lost during loading

Question 34

What makes up a hysteresis graph?

Answer 34

- The loading curve of a viscoelastic material - The unloading curve of a viscoelastic material

Question 35

What are the 3 modes of failure?

Answer 35

1. Rupture (tearing) 2. Failure through the enthesis (attch. point near bone) 3. Avulsion failure (pulling away of bony attachment)

Question 36

What affects the failure modes?

Answer 36

- Varies among different tendons/ligaments - Affected by age, speed, structure, etc

Question 37

What are the effects of rate and duration of force application?

Answer 37

1. Tendons are more sensitive to strain rate 2. Higher strain rates increase stiffness 3. Strain rates may affect the mode of failure - Avulsion: lower strain rate - Substance rupture: higher strain rate 4. Tendons and ligaments are viscoelastic - Rapid loads: stiffer and abrupt rupture - Slow loads: fluid and elongate

Question 38

What are the effects of temperature?

Answer 38

- Heat (>55-60 degrees): produce irreversible shrinkage - Viscoelastic properties increase with increased temperature: increased stress relaxation/creep

Question 39

What are the effects of maturation and aging?

Answer 39

- Maximal tissue strength achieved at skeletal maturity - Very young/old: lower stress/strain and decreased stiffness - Resistance training can reverse or slow effects of aging

Question 40

What are the effects of hormones?

Answer 40

- Cortisol and glucocorticoids: reduce the synthesis of type I collagen - Relaxin: softens and increases extensibility - Growth hormone: increases collagen synthesis

Question 41

What is tissue remodelling?

Answer 41

Adaptation of biological tissues to change in the mechanical stresses they are subjected to daily

Question 42

What is specific adaptation to imposed demand?

Answer 42

Explains remodelling in response to alterations in external loading in soft tissues

Question 43

What effects does joint immobilization have in normal connective tissue?

Answer 43

- Reduces the tensile forces - Reduces the load to failure and stiffness - Decreases the strain/elongation to failure - Frequency of avulsion failures increases

Question 44

What effects does joint remobilization have in normal connective tissue?

Answer 44

- Allows restoration of mechanical properties

Question 45

What happens to healing connective tissue?

Answer 45

- Healing is slow - Early mobilization (within limits) strengthens the unions, speeds healing and reduces scar tissue adhesions

Question 46

What are the 4 stages of healing?

Answer 46

1. Hemorrhagic 2. Inflammatory 3. Proliferative 4. Remodelling and maturation

Question 47

What is the response to stress enhancement?

Answer 47

- Increased loading can lead to tendon hypertrophy and increased strength/stiffness - Exercise may protect against weakening effects of inactivity - Exercise may increase the strength of some tissues - Excessive loads can have harmful effects