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Orthopaedic Biomechanics > Ch. 4 - Soft Tissue Mechanics > Flashcards

Ch. 4 - Soft Tissue Mechanics Flashcards

1
Q

What are the major soft tissues encountered in the MSK system?

A
  1. Tendon
  2. Ligament
  3. Articular cartilage
  4. Intervertebral disc
  5. Muscle
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2
Q

What are the functions of soft tissue?

A
  1. Connect tissues - Ligaments provide connection btw bones. Tendons connect muscles to bone.
  2. Control relative joint motion - Ligaments.
  3. Lubricate joints - Articular cartilage ensures low friction to allow joint mobility.
  4. Actuate skeletal system - Muscles.
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3
Q

What is special about soft tissues as structural elements?

A
  • Must withstand large loads
  • Must provide kinematic restraint
  • Fail at much higher strains than bone
  • Show a non-linear elastic response, with increased stiffness at higher applied strain
  • Show pronounced viscoelastic (rate or time-dependent) behaviour
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4
Q

What is viscoelasticity?

A

Time- or rate-dependent behaviour of stress-strain levels (assuming material is loaded to a level where it does not suffer any irreversible deformation).

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5
Q

What does viscous mean?

A

It means that the applied stress is proportional to the time rate of change (of strain). It is modelled with a dashpot.

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6
Q

What does elastic mean?

A

It means that it shows an instantaneous, reversible response upon loading. Linear elasticity is governed by Hooke’s Law. It is modelled using a spring.

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7
Q

State 2 common effects observed in viscoelastics solids.

A
  1. The material is stiffer at higher applied strain rate. The faster the material is loaded, the more relaxation mechanisms are not able to follow.
  2. When applying cyclic loading on a viscoelastic material, it does not follow the same path in both loading and unloading paths of the stress-strain curve. The paths are out of phase = Hysteretic behaviour. The area within the hysteresis loop is energy dissipated with one loading/unloading cycle.
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8
Q

What are the 2 most important behaviours shown by viscoelastic materials.

A
  1. Creep - Material is loaded with a constant stress. The instantaneous response of the material is elastic followed by a time dependent increase in strain, called creep.
  2. Stress Relaxation - Material is deformed with a strain step and held at a constant strain (deformation). Initial stress is governed by the instantaneous elastic behaviour, after which stress decreases gradually until an equilibrium stress level is reached.
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9
Q

What are the constitutive equations for:

a) an elastic spring
b) a viscous dashpot

A

a) sigma = E x epsilon
(stress is proportional to strain)
b) sigma = mu x time derivative of epsilon
(stress is proportional to strain rate)

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10
Q

Describe a Maxwell linear viscoelastic model.

A

The Maxwell model consists of a spring and dashpot in series. The same stress is displayed by both the spring and dashpot, but they can both deform independently of each other.

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11
Q

Describe the creep response of a Maxwell model.

A

We see instant elastic deformation of the spring, followed by linear increase of the strain in the damper. Upon unloading, the instant deformation of the spring is recovered but the strain of the damper stays.

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12
Q

Describe the stress-relaxation response of a Maxwell model.

A

There is instantaenous stress due to deformation of the spring. Over time, the damper elongates, resulting in reduction of stress over time. The dashpot will elongate until the stress reaches zero.

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13
Q

Describe a Kelvin-Voigt linear viscoelastic model.

A

The Kelvin-Voigt viscoelastic model consists of a spring and dashpot in parallel. Deformation of both elements is the same, but they can each carry different stresses.

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14
Q

Describe the creep response of a Kelvin-Voigt model.

A

There is no instantaneous response, as the load is immediately taken up by both elements. The creep rate decreases with time as more and more of the load is taken up by the spring. Deformation eventually finds an equilibrium where the load is totally take up by the spring, and the dashpot is unloaded. When the load is removed, the creep strain will be recovered completely if enough time is allowed.

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15
Q

Describe the stress-relaxation response of a Kelvin-Voigt model.

A

Initial stress is infinite, because the strain rate in the strain step is infinite, leading to infinite stress in the dashpot. As strain is held constant, the strain rate becomes 0 and stress in damper vanishes. Only spring (instant elastic) contribution to stress is kept.

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16
Q

Describe the Standard Linear Solid model.

A

The SLS model consists of a spring in series with a Kelvin-Voigt element.

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17
Q

Describe the creep response of an SLS model.

A

There is instantaneous deformation of the spring E1, followed by inverse exponential increase in strain of KV element. Upon unloading, the deformation of the spring E1 is recovered instantaneously, after which if enough time is allowed creep strain on KV element will be recovered.

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18
Q

Describe the basic role of tendons.

A

Tendons connect muscles to bones and are loaded in series with muscles. They are optimised to store elastic energy and transmit large forces on a regular basis.

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19
Q

Describe the basic role of ligaments.

A

Ligaments connect bones to bones. They guide and restrict joint motion, ensuring joint stability. Functional forces are usually small, as they should not hinder joint motion in the physiological range. Joint motion is restricted when a critical point is reached during an unusual event.

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20
Q

What is the composition of tendons? Why?

A 
Tendons must be stiff and strong to be able to transmit large forces without mechanical loss from muscles to skeleton on a regular basis. Therefore, they need a higher amount of highly aligned collagen fibril.
Composition:
60% water (wet weight)
75-85% collagen type I
1-3% elastin
1-2% proteoglycan
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21
Q

What is the composition of ligaments? Why?

A 
Ligaments need to stretch considerably without failing to help guide large joint motions. Therefore, they feature high quantity of elastin and less aligned collagen fibrils, which both lead to increase in ultimate strain ligaments can reach before failure.
Composition:
60% water (wet weight)
70-80% collagen type I
1-15% elastin
1-3% proteoglycan
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22
Q

State the hierarchical structure of tendon from smallest to largest element.

A
  1. Tropocollagen
  2. Microfibril
  3. Sub-fibril
  4. Fibril
  5. Fascicle
  6. Tendon
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23
Q

When does tendonitis occur? How does the healing process progress?

A

Tendonitis occurs when tendon becomes partially torn or inflamed.
Healing occurs more slowly than in bone since tendons are less vascularised than bones. It progresses over 3 phases:
1. Inflammatory - damages tissue removed
2. Proliferation - new ECM laid down with limited order
3. Remodelling - loosely oriented tissue gets remodelled to regain its original degree of order and therefore its full mechanical integrity

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24
Q

What are bursae?

A

Bursae are small fluid-filled sacs that help cushion the movement of certain tendons over bone.

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25
Q

What is endotendon and what is its purpose?

A

Endotendon contains nerves, blood vessels and lymphatics. It is a loose connective tissue wrapping vesicles. it allows them to slide wrt each other, thus providing more flexibility.

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26
Q

Why is tendon more cartilaginous near bone contact?

A

This enhances compressive strength. It helps better withstand the compressive contact stresses and reduces friction btw neighbouring tissues.

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27
Q

What is the difference between avascular and vascular tendons?

A

Avascular tendons have a synovial sheath for low-friction sliding. Epitenon is a connective tissue btw tendon and the heat which seretes synovial fluid. Blood arrives through the viniculae.
Vascular tendons do not have a synovial sheath. The paratenon is a connective tissue surrounding the tendon and allows blood supply directly into the tendon.

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28
Q

Describe the basic structure of ligaments.

A

Ligament fascicles are connected by endotendon, but unlike in tendon, they are not necessarily aligned with the main structure. Ligaments are tight fibrous bands of dense regular connective tissue in which collagen fibres are arranged in parallel wavy bundles. Ligaments have a scarce blood supply and inside the ligament is a fibrous joint capsule lined by the synovium.

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29
Q

Give an example of 2 tissue ligaments with different orientations. Why is this the case?

A

ACL fascicles are spirally wound around each other, while collateral ligament fascicles lie parallel to the ligament length.
Fascicles can slide relative to each other quite easily, making it possible for the ligament to respond effectively to changing load situation. In the ACL, some fascicles take up most of the load in twisting motions of the knee, while others are dominant for translation. The mechanical properties of the structure thus depend highly on applied loading mode and direction (highly anisotropic and heterogenous).

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30
Q

Describe the 4 stages of the mechanical behaviour of tendon and ligament on a stress-strain curve.

A
  1. Initial non-linear toe region - Characterised by sequential recruitment of collagen fibres with higher loading
  2. Quasi-linear region - Stretching of aligned collagen fibres is dominating
  3. Damage initiation - When overloading, some fibrils will start failing, leading to reduction in the apparent stiffness of ligament or tendon
  4. Failure region - Once the tissue is stretched too far, large defects are developed as more and more fibres fail. This will ultimately lead to a tear in the ligament or tendon, which corresponds to total failure.
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31
Q

What are the differences between the stress-strain behaviour of tendon and ligament?

A
  • Tendon is stronger and stiffer than ligament, as fibrils are more aligned with the loading axis (tendon must transmit larger forces)
  • Ligaments can take higher strains before damage initiates (ligament must allow large movements)
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32
Q

Explain why in tendon and ligament, Hookean Law does not apply until all collagen fibrils have been recruited.

A

Collagen fibrils are sequentially activated. To model this we can consider an array of springs to represent the crimped collagen fibrils with different intiail lengths. As unit cell is progressively stretched, more and more fibres are getting stretched. Each time a fibre gets recruited, the apparent stiffness of the unit cell is increased. When all fibres have been recruited, linear elastic behaviour is reached. The fibres that have been recruited earlier are stretched more. Therfore, damage region is also characterised by a progressive degradation of stiffness, as more fibres reach their limit and fail.

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33
Q

How does the in vitro behaviour of tendons and ligaments compare to that in vivo?

A

In vitro - Both tendons and ligaments have initial non-linear toe region.
In vivo - For tendons, the initial slack region does not exist, as they are always held under some tension. For ligaments, the situation is less clear. If a slack region exists, ligaments would allow substantial joint laxity and muscles would play the main role for stabilisation of the joint. If ligaments are pre-loaded, they could play a role for joint stabilisation in a larger part of the ROM.

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34
Q

For mechanical testing of tendons and ligaments, we use bone-tendon-bone or bone-ligament-bone specimens. Why is the soft tissue not isolated?

A

The removal of soft tissue from the bone may damage the tissue. Furthermore by keeping bone at the edges, the sample is easier to clamp.

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35
Q

What is avulsion (vs. midsubstance failure) and when is it observed?

A

Avulsion is when failure occurs at the bone-soft tissue interface. This is undesired. It mostly occurs in younger animals, where the growth plate is not yet closed.

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36
Q

How do the mechanical properties of tendon and ligament change with age?

A
  1. Strength decreases
  2. Stiffness increases significantly
  3. Ultimate load is reduced
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37
Q

What vascular and composition changes take place with increase in age that alter the mechanobiology, healing capacity and function of soft tissue?

A
  1. Maturation of collagen cross-link
  2. Increase in fibre diamter
  3. Increase in collagen content
  4. Reduction in elastin content
  5. Reduction in water content
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38
Q

What are the 3 main types of cartilage?

A
  1. Hyaline cartilage
  2. Fibrocartilage
  3. Elastic cartilage
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39
Q

What are the 3 main components of articular cartilage?

A
  1. Porous matrix
  2. Water
  3. Ions
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40
Q

What are the 2 main functions of articular cartilage?

A
  1. Transmit large normal joint loads to the subchondral bone

2. Allow relative motion of the joint surface with minimal friction and wear

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41
Q

Describe the composition of articular cartilage.

A 
Articular cartilage is built up by cells called chondrocytes.
Composition:
10-20% collagen type II
5-10% proteoglycans
68-85% fluid
other non-collagenous proteins
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42
Q

What are the 3 main properties of articular cartilage?

A
  1. Not innervated/aneural
  2. Not vascularised
  3. Alymphatic
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43
Q

What is the role of the synovial capsule?

A

The synovial capsule surrounds the joint. It is innervated to sense motion, load, vibration and pain. It is vascularised and secretes synovial fluid. It provides much of the cellular activity needed to sustain cartilage.

44
Q

What are the cell types of the synovial capsule (synoviocytes)? What do they each do?

A
  1. Fibroblasts - synthesise hyaluronan and glycoproteins
  2. Macrophages - remove debris in joint space
  3. Mast cells - mediate inflammatory response/reactions
  4. Lymphocytes - trigger immune response when activated by a foreign body
  5. Dendritic cells - activate T-lymphocytes
  6. Adipocytes - cells for fat storage
45
Q

What 2 elements is the ECM of articular cartilage made up of?

A
  1. Collagen

2. Proteoglycans

46
Q

Describe the structure and role of collagen in the ECM of articular cartilage.

A
  • Bundled in fibrils
  • Types II, VI, IX, X, XI
  • Provides tensile strength
  • Network of fibrils entrap swollen proteoglycan aggregates - in compression, fibrils buckle easily and do not resist deformation; however, trapped proteoglycans can inhibit flow of fluid and thus provide resistance to compressive loading
47
Q

Describe the structure and role of proteoglycans in the ECM of articular cartilage.

A
  • Protein core with GAG chains (brush-like structure, negatively charged in physiological conditions)
  • Trap and impede water, forming a hydrogel with resilience and stress-distributing properties
48
Q

What is the chondron of articular cartilage?

A

The condron is made up of the pericellular matrix + synovial capsule + chondrocytes.

49
Q

What is the role of chondrocytes in the chondron of articular cartilage?

A

They are <5% of AC total volume. Their role is to replace degraded matrix molecules (i.e. synthesise protein). They originate from MSCs. They use mechanical signals, environmental factors and genetic factors to regulate metabolic activity.

50
Q

What does matrix turnover depend on in the chondron of articular cartilage?

A

Matrix turnover depends on cell ability to detect changes in tissue composition and organization.

51
Q

List the 4 different zone of articular cartilage.

A
  1. Superficial zone
  2. Middle zone
  3. Deep zone
  4. Calcified zone
52
Q

Describe the superficial zone of articular cartilage.

A
  • Located at the surface
  • 10-20% of total tissue thickness
  • Chondrocytes are flattened and discoidal
  • Collagen fibrils lie mainly parallel to surface
  • Highest collagen content
  • Lowest proteoglycan content
53
Q

Describe the middle zone of articular cartilage.

A
  • 40-60% of total tissue thickness
  • chondrocytes are spherical and randomly distributed in the tissue
  • collagen is oriented isotropically
54
Q

Describe the deep zone of articular cartilage.

A
  • 30% of total tissue thickness
  • chondrocytes are spherical but arranged in columns
  • collagen fibrils radially arranged and tightly packed
  • high concentration of proteoglycans
  • low concentration of water
55
Q

Describe the calcified zone of articular cartilage.

A
  • 5-10% of total tissue thickness
  • provides firm attachment to underlying subchondral bone
  • very few chondrocytes
  • high calcium content
56
Q

Describe the mechanical behaviour and material properties of articular cartilage.

A
  1. Anisotropic (mechanical properties depend on direction of loading)
  2. Heterogenous (collagen, proteoglycan and water content vary spatially)
  3. Considered a poroelastic solid (the tissue itself)
57
Q

Articular cartilage is considered a poroelastic solid. Thus, tissue behaviour is determined by the interaction of 2 main components. What are these? What could be considered a 3rd contributing component?

A
  1. Intrinsic material properties of the solid ECM made up of proteoglycans wrapped in a collagen network
  2. Flow of interstitial fluid through the porous ECM, which is resisted by the matrix.
  3. The ionic phase contributes to the mechanical response through its influence on swelling behaviour of the material and contributes to resistance to flow.
58
Q

Looking at the stress-strain curve of cartilage, explain the 4 phases of cartilage’s tensile response.

A
  1. Toe region - fibres in cartilage are not all stretched at once, but first rotated into loading direction. Fibres are being recruited and the model of spring recruitment applies here too.
  2. Linear elasticity - once a majority of fibres is loaded, the behaviour of cartilage is linearly elastic. The whole collage network is stretched during this phase.
  3. Microdamage - as the network is stretched more and more, some of the fibrils will start to reach their critical strain. There is a short region where further deformation can still be endured by the cartilage during which more and more fibrils are damaged and the material gets progressively torn.
  4. Failure - when damage in the material is too high after substantial straining and a flaw has reached a critical size, the cartilage will fail catastrophically.
59
Q

How would you measure the intrinsic compressive properties of articular cartilage?

A

You could measure it by confined compression creep tests (aka. uniaxial strain test), in which the cartilage-bone plug is inserted tightly into a rigid cylindrical chamber, allowing deformation to occur only in the direction of loading. At the top of the specimen is a porous platen through which fluid can escape, and which exerts a constant stress on the specimen. Creep occurs as fluid is pressed out of the cartilage until a steady state is reached.

60
Q

How could you determine the equilibrium compressive modulus of articular cartilage experimentally?

A

By performing a confined compression creep test at different stress levels until steady-state is reached. This would allow us to make a plot of equilibrium stress against equilibrium strain, from which the equilibrium compressive modulus of the tissue can be defined.

61
Q

In articular cartilage, what affects tensile equilibrium properties that most?

A

Not water or PG content, but collagen content. This is because resistance to tensile loading is provided mainly by the supramolecular collagen network structure.

62
Q

Why is dynamic viscoelastic shear testing useful to characterise the mechanical properties of articular cartilage?

A

Because it induces no change in volume and thus no fluid flow. This allows us to test viscoelastic properties of the solid phase.

63
Q

What occurs during a dynamic viscoelastic pure shear test?

A

Two opposing sides of the specimen are moved sideways in opposing directions. The resulting shear deformation is resisted by the 3D molecular network and the covalent cross-links.

64
Q

What is the mechanical response determined by for a cartilage sample in a dynamic viscoelastic shear test?

A
  1. No change in volume
  2. No interstitial fluid flow
  3. Resistance of the 3D molecular network
65
Q

Increasing collagen content has no effect on aggregate modulus. Where is the resistance to compression coming from?

A

The compressive equilibrium aggregate modulus is not influenced by collagen content as the thin collage fibrils buckle easily under compressive loading and offer little resistance. Therefore, bulk of compressive response stems from repulsive negative charges of the PG molecules.

66
Q

Increasing collagen content does affect intrinsic behaviour. How?

A

The collagen network still plays an important role in the compressive response of cartilage tissue by constraining the separation and free motion of PG. The collagen network constrains such the swelling of the PGs and is therefore loaded under tension.

67
Q

How is the aggregate modulus of articular cartilage affected by water content and why?

A

The compressive equilibrium agreggate modulus is found to decrease with water content. The reason for this is that if there is more water there is less space for PGs for a constant collage content.

68
Q

When loading cartilage dynamically, its stiffness is considerably higher than intrinsic equilibrium properties. Why is this the case?

A

This is because of interstitial fluid flow through the tissue and the resistance against it offered by the solid matrix. Therefore, cartilage is modelled as a biphasic material consisting of water and solid (PG + collagen).

69
Q

What does Darcy’s Law tell us?

A

We can use Darcy’s Law to relate the pressure gradient and rate of fluid flow through a material with finite permeability. It states that for a cylindrical specimen with a pressure gradient along its long axis, the volumetric fluid flow per unit time is proportional to the pressure gradient.

70
Q

What does the low permeability of cartilage imply?

A

That there is large frictional drag btw the fluid and solid phases, so that large pressure gradients are necessary to force fluid through the tissue.

71
Q

Describe how the permeability of cartilage changes with strain.

A

If the specimen is compressed and a pressure gradient is applied, the permeability decreases due to pore closure (a geometrical effect) as well as an increase in charge density (effect of the ionic phase). As permeability changes with applied strain, it is not a material constant. The fact that it decreases under strain means that if the specimen is compressed, the permeability decreases, which keeps further fluid from exiting the specimen. Therefore the cartilage gets stiffer and reaches a stress equilibrium without a strong loss of height.

72
Q

What are the most important effects of ageing on the structural components of articular cartilage?

A
  1. Loosening of collagen network
  2. Swelling of tissue
  3. Decrease in stiffness
  4. Increase in permeability (reduced resistance against compressive loading)
73
Q

Consider the ionic phase of articular cartilage. It supports the external stresses by development of fluid pressure by swelling. This is caused by 2 mechanisms. Explain them.

A
  1. Chemical expansion stress - Negative charges of GAG chains on PGs lead to repulsive electrostatic forces. As they are confined in the collagen network, a fluid pressure is generated called chemical expansion stress.
  2. Donen osmotic pressure - Positive ions are attracted into the cartilage tissue to neutralise the negative charges of the GAG chain. This leads to a higher ion concentration in cartilage compared to surrounding fluid. This attracts more fluid to enter the cartilage to lower the concentration back to a normal level.
74
Q

What are intevertebral discs?

A

They consist of an outer layer of fibrocartilage and a gel-like core. They connect adjacent vertebral bodies in the spine and are important components ensuring spinal flexibility. They transfer most of the compressive loads transmitted through the spine and allow limited motion btw the vertebrae.

75
Q

The IV joint is classified as an amphiarthrodial joint. What does this mean?

A

It means the joint is continuous, slightly movable and connected by small flattened discs of fibrocartilage adhering the ends of each adjacent bone. (This is in contrast to diarthrodial synovial joints like hip or knee which feature joint space filled with synovial fluid and allow large relative motions bc of articular cartilage covering both joint surfaces).

76
Q

The IV disc is a heterogenous organ consists of which 3 elements?

A
  1. Nucleus pulposus
  2. Annulus fibrosis
  3. Cartilage endplates
77
Q

Describe the composition and microstructure of the nucleus pulposus.

A
  • located in the middle of the IVD
  • takes up ~50% volume
  • consists of a highly viscous gel
  • composed of 70-90% water, PGs, and isotropic collagen type II network
78
Q

Describe the composition and microstructure of the annulus fibrosis.

A
  • 25 concentric lamellae of type I collagen with alternating collagen orientation
  • similar to lamellar bone but not mineralised
  • with increasing distance from nucleus, water content in AF decreases while collagen I content increases up to 70%
79
Q

Why does the boundary zone btw NP and AF in an IVD disappear with increasing age?

A

The NP gets more solid-like with age, such that the distinction between the 2 zones disappears.

80
Q

Describe the composition and microstructure of the cartilaginous endplates of an IVD.

A

It is a thin layer of hyaline cartilage that serves as an interface btw the vertebral body and the disc. It is permeable, allowing fluid to be transported in and out of the disc during loading.

81
Q

Cartilaginous endplates are avascular and aneural. What does this imply?

A
  • Cell nutrition is provided by fluid transport
  • No direct mechanism by which damage to disc can be detected
  • Less cell activity due to no direct blood supply means the disc cannot regenerate effectively
  • IV disc must be loaded dynamically to maintain fluid exchange (regular exercise important for spinal health)
82
Q

What type of loading is an IVD mostly subjected to and how does it resist this loading?

A

The main loading is compressive. This loading is resisted by the IVD similarly to how an inflated tire supports the weight of a bicycle or car. The fluid-like NP is pressurized by axial loading and contained by tensile axial and hoop or tangential stresses in the AF. Due to tensile stresses in the AF, the periphery of the adjacent vertebral endplates is also loaded in tension.

83
Q

Explain the mechanism of pressure generation in a healthy IVD.

A

The negative charges of the PGs in the NP attract water that gets transported there through the cartilaginous endplates. Due to resistance of surrounding AF to swelling, an osmotic pressure is generated in the NP.

84
Q

Anterior bending is a common loading scenario for an IVD. Give an example of such loading and explain how it occurs.

A

Anterior bending scenario occurs with activities such as lifting heavy objects or bending forward.
The height of the disk and therefore AF is decreased anteriorly and increased posteriorly. This leads to a decrease in axial tension in the AF in the front and an increase in the back.

85
Q

Why does the AF not collapse during anterior bending in the front?

A

Due to development of hydrostatic pressure in the NP, the net tension of the annulus is maintained also in the front. As a consequence, the stress distribution on the endplates is actually similar to that of the pure compressive situation. This ensure the vertebral bodies are always loaded approximately the same way.

86
Q

Describe the changes in composition and microstructure of IVDs that occur with ageing and how this affects loading of the IVD in elderly patients.

A
  • NP loses water over time and changes from fluid-like to highly viscous gel-like behaviour
  • NP is not able to develop large hydrostatic pressures
  • AF is thus directly loaded by compressive stresses under axial load
  • Adaptive response sets in to thicken AF gradually (may even mineralise), which then encrouches the nucleus
  • ROM reduces significantly
87
Q

What is a disc prolapse?

A

A disc prolapse occurs when the AF gets damaged, allowing the NP to escape. This can occur if internal pressure of the AF or pathological compressive stresses on aged discs are too large.

88
Q

How would you experimentally analyse the clinical case of a disc prolapse?

A

Tensile experimental testing on bone-disc-bone specimens.

89
Q

How would you mechanically model the stress-strain response of an IVD?

A

Using an SLS spring-dashpot model.

90
Q

Modelling an IVD with an SLS spring-dashpot configuration, describe the creep response to static loading.

A

When a step load is applied to an IVD there is an instantaneous elastic response followed by a creep deformation that gradually approaches equilibrium. When the load is removed, there is again an instantenous decrease of elastic deformation, followed by a non-linear creep recovery. The equilibrium configuration that is reached at the end is not equal to the initial configuration. The disc loses height during such a loading cycle due to loss of fluid.

91
Q

How does the creep response of a healthy disc compare to that of a more degenerated one?

A

A healthy disc creeps less and more slowly than a degenerated one, and reaches equilibrium after a longer period of time. The stiffness and viscoelastic time constants are reduced due to disc degeneration.

92
Q

Describe 3 mechanisms through which disc prolapse can occur.

A
  1. Development of large hoop stresses in the annulus due to high loads during traumatic injury.
  2. Collapse of the annulus which can occur due to changed mechanical loading situation in the degenerated IVD, where the annulus is loaded directly in compression.
  3. As there is no blood supply to the disc, there is little cell activity and tears or damage to the annulus cannot be repaired and may accumulate over time. Several microprolapses may accumulate and allow a full prolapse to occur even under physiological conditions.
93
Q

Describe IVD instability and why it occurs.

A

Instability means the disc deforms too much under normal physiological load. It is caused by a loss of ability to maintain sufficient hydrostatic pressure in the NP. Disc height decreases and the surrounding ligaments become slack. The entire motion segment (the disc + 2 adjacent vertebrae) loses its ability to remain upright under applied loading. Movement of the segment can lead to an impingement of the bone and disc elements against the surrounding nerves or even spinal cord.

94
Q

Describe the difference between parallel and pennate muscles.

A

Parallel - muscle fibres are oriented parallel to muscle axis. They allow large movement of the muscle as fibres are long and can contract significantly.
Pennate - muscle fibres are at an angle to the line of action of the muscles. This configuration allows the generation of larger forces and more muscle fibres can be recruited.

95
Q

Why is skeletal muscle called striated voluntary muscle?

A

They have a grain and can be controlled by voluntary contractions.

96
Q

Elaborate on the hierarchical structure of muscle.

A

Muscles consist of fascicles, which consist of individual muscle fibres. Muscle fibres contain myofibrils, which in turn contain think and thick myofilaments of actin and myosin, respectively.

97
Q

Describe the composition and microstructure of a muscle fibre.

A

The thick and thin filaments are parallel and offset in units called sarcomeres. The zone where only actin is present is called A band. The zone where myosin is present is called I band. During contraction, overlap increases. While I band shortens, the length of the A band remains the same.

98
Q

Describe the biological process through which a muscle contracts.

A

Muscle cells produce ATP. This is stimulated by acetylcholine, which is released by motor neurons. once stimulated, the sarcoplasmic reticulum of muscle cells release ionic calcium which interacts with the myofibrils to produce muscular contraction via the sliding filament mechanism.

99
Q

Explain the sliding filament mechanism.

A

Myosin acts like a ratched. Chains of actin transmit the force generated by myosin to the ends of the muscle. Each myosin ratchets along an actin filament repeatedly binding, ratcheting and letting go, sliding the thick filament over the thin filament.

100
Q

Explain the “all-or-none” principle that muscle fibre activation follows.

A

If stimulated beyond a certain threshold, all muscle fibres in a motor unit will contract completely. When stimulated below the threshold, no contraction takes place. However, the generated force is not always the same, as this depends on other factors such as oxygen supply and ATP levels. Generated force is controlled by the CNS by varying the number of activated motor units as well as their frequency of activation.

101
Q

What is meant by a muscle twitch response?

A

This refers to the transient force which quickly falls off produced by the muscle following a short latency period.

102
Q

How could we experimentally obtain a force-length curve for a muscle specimen?

A

Adjust the length of the muscle and use electrical impulses to stimulate it, recording the generated force using a load cell. (“Isometric” test because the length of the muscle isheld constant). This test can be repeated for several muscle lengths to get the force-length curve.

103
Q

Muscles are said to consist of both active and passive components. Elaborate on this idea.

A

The passive components are the viscoelastic response of the tissue. When inactivated, the muscle deforms similarly to tendon or ligament.
When muscle is fully stimulated to a fused tetanus, it generates and additional force in which the generated force depends on the length of the muscle.

104
Q

What type of muscle contraction generates maximum force?

A

Isometric contraction (muscle length stays constant, such that contraction velocity = 0)

105
Q

What is the mechanical power of a muscle equal to? At what level of velocity is maximum power produced?

A

Force * velocity

Maximum power is produced at about 1/3 of the max. velocity.

Orthopaedic Biomechanics (10 decks)

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