BIOMECH of soft tissues and injuries Flashcards

1
Q

strength of materials

A
  • the ability of the material to withstand forces w/o breaking or failing
  • can depend on:
    –> microstructure
    –> age
    –> temp
    –> fluid content
    –> type
    –> velocity
    –> direction of loading
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2
Q

STRESS

A
  • force applied on any material generates equal resistive force
  • the resistive force generated inside the material per unit area is called stress
  • STRESS = F/A
    (F is the applied force, A is the cross-section area)
  • stress causes strain
    ○ The linear deformation (Change in length) per unit length is called longitudinal strain
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3
Q

Stress and Strain

A
  • Internal resistance of a material to an applied load is called stress
    ○ Axial (compressive or tensive), shear, or torsional
  • Change in shape or deformation of the material is described by the strain ( ε)
  • Normalise it = absolute strain divided by resting length
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4
Q

STRAIN

A
  • change in length / original length = strain
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5
Q

Young’s Modulus: Stiffness of Material (WORDS)

A
  • Stress on vertical, strain on horizontal
  • Up until proportional limit (A) = have a constant value / stiffness = proportional limit = linear response
  • From a to b (elastic limit / yield point) = there is a non-linear response
  • From b to c = have a problem = when you remove the stress, it doesn’t go back to its original length/value = returns to some offset value / permanent set = known as plastic deformation
  • Beyond plastic deformation = material starting to break down beyond their return point
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6
Q

Young’s Modulus: Stiffness of Material (DOT POINTS)

A
  • Point A is the proportional limit.
  • Point B is the elastic limit.
  • Point C is the yield point.
  • Point D represents ultimate strength or ultimate tensile strength.
  • Point E represents the fracture point.
  • The area under the curve O to B represents the elastic region, and the area under the curve B to E is the plastic region.
  • Hooke’s law = means up to the proportional limit = get a linear response
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7
Q

Stress vs Strain for Cortical Bone under Tension

A
  • OA: the body acts perfectly elastic and strictly follows Hooke’s law.
  • AB: the body is still elastic, but Hooke’s law is not maintained strictly.
  • Beyond B – plastic deformation
  • D: Ultimate tensile strength
  • E: the body fractures and breaks down
  • E.g. if a ligament undergoes certain stress/strain up to A = not a big deal = probably designed to handle that = even up to B point = will return back to its original length
  • When get to C + D = when there is a problem = when you get lax joints = joint more susceptible to injury = in those cases surgery is a possibility, on a conservative level do lots of S+C to make sure that joint is stronger than normal = lig under less load in future
  • Don’t want to overdo certain stretching exercises + quick, abrupt loads = get a lot of stress, especially when get a large force over a small cross-sectional area = really pushing the characteristics of this curve
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8
Q

Mechanical Testing of Bone

A
  • 1000 micro strain in compression would shorten a bone by 0.1% of its original length
  • Ultimate Failure Point occurs at 2.5% change in bone length
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9
Q

Viscoelasticity

A
  • Elastic materials immediately return to their original state once the stress is removed
  • Can overstretch elastic materials = losing its stretch = plastic deformation
  • Deformation of elastic material containing fluid causes delay in return of material to its original shape.
  • Viscoelastic materials exhibit time-dependent strain: creep
  • Viscoelastic = properties of solid + liquid
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10
Q

Viscoelastic Response of Bone Loading

A
  • Characteristic of viscoelastic tissues, bone is stronger when loaded fast than it is when loaded slow
  • Stress/strain curves vary with the velocity of loading as is a viscoelastic material
  • Bone is actually stronger when loaded fast = can withstand higher loads, when loaded fast = has that inherent ability
  • Blue = less steep = therefore the stiffness is lower
  • If jumping or receive a quick blow = intelligent material = want that material to be able to withstand the quick forces = have to have this ability to stiffen itself according to that type of loading
  • When loaded slowly = doesn’t need such a response
  • Remember human bones are often loaded under quick, compressive or tensile loads = would like them to be stronger when loaded faster
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11
Q

Soft Tissues

A
  • Tendon
    • Ligament
    • Skin
  • Collagen fibres make up >75% of solid matter in most soft tissues
  • Collagen is a strong, elastic fibre responsible for the strength/ stiffness of these tissues
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12
Q

Arrangement of Collagen

A
  • Collagen fibres are in different orientations in different tissues = means different abilities to withstand strength
  • Tendon has to be very strong + but also have ability to stretch = have a layered, more linear aspect
  • Ligament a bit more mattered
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13
Q

Arrangement of Collagen

A
  • Collagen fibres are in different orientations in different tissues = means different abilities to withstand strength
  • Tendon has to be very strong + but also have ability to stretch = have a layered, more linear aspect
  • Ligament a bit more mattered
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14
Q

Stress vs Strain
LIGAMENTS

A
  • Patella Tendon = much stiffer + stronger structure than the ACL
    ○ Linear up to 10-12%
  • Cruciate Ligaments
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15
Q

Tendon Force vs Length

A
  • For younger population = less stiff = expect younger children to be more flexible
  • MEN greater stiffness / strain than women
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16
Q

Articular Cartilage - INTRO

A
  • Complex behaviour due to movement of water in and out of tissue
  • Cartilage allows large amount of strain
  • ## Red = articular cartilage = can exhibit a lot of strain which is what you want it to do = different properties due to viscoelastic nature
17
Q

Articular cartilage = depth

A
  • Cartilage-synovial fluid system provides low friction interface.
  • Cartilage has little or no blood supply
  • Synovial fluid provides nutrients for the cartilage and removes waste products from the cartilage.
  • Fluid flow in and out of cartilage causes its extreme strain responses
  • This response is greatly influenced by the amount of time for which a force is applied
18
Q

Load Duration

A
  • Load Duration greatly affects stress/strain of Articular Cartilage
  • Constant Stress does not produce constant level of strain
  • This is known as CREEP
  • Stress applied it constant = get a non-linear response (strain)
  • If maintain stress, starts to creep to a higher level
19
Q

Creep

A
  • Increasing deformation under constant load.
  • Creep is particularly evident in articular cartilage as fluid exits the cartilage i.e. intervertebral disks
  • Is present in all soft tissue: viscoelastic materials exhibit time-dependent strain
    - Stretching
20
Q

Ligament Creep: Think about MA

A
  • As move the head forward, the MA is increasing
  • More you lean forward = the worse this situation is going to become because MA has increased = start to load the ligaments of the neck = stretch them = under constant stress = maybe stretched to a point where they can’t return to its original length
21
Q

Creep: Lateral and medial knee cartilage

A
  • Compressive strain, increasing with weight-bearing time
22
Q

Exercise and Injury Implications

A
  • Cyclically loading + unloading exercise (e.g. walking + running) are considered healthy for cartilage tissue
  • Static loading of joint (e.g. standing) is unhealthy for cartilage tissue as the synovial fluid is forced out of the cartilage but not allowed to flow back into the cartilage
  • Cartilage + synovial fluid system acts as dynamic force attenuators = meaning to reduce (viscoelastic system)
  • Under low rates of loading it is not stiff, + it passes the load on to the bone tissue which then causes strain to the bone tissue
  • Under high rates of loading the cartilage is stiff + it protects the bone from harmful high frequency forces = adv of viscoelastic material
23
Q

Injury Mechanisms

A
  • Overuse Injuries
  • Often called “Fatigue Failure”
  • Repetitive cycles of stress can cause injuries at levels of stress below that required from single force application
24
Q

Overuse Injuries - INTRO

A
  • Failure Region
  • Fatigue Limit
    ○ Stresses below this limit can be endured for “infinite” number of cycles
    ○ But as get too fatigue limit = starting to get to failure region = operating at this point, still get a number of cycles = can stop before this so don’t get an injury = the higher the force, the less cycles you can have
  • Static Failure:
    ○ Stress required to fracture from a single application of force
25
Overuse Injuries - summarised?
- Failure region - Repair region = want to work within this region (area under curve, above non-failure region) - Changes in strength = shift the curve up (non-failure region becomes bigger) - Changes in exercise duration - Changes in exercise intensity - Concept can be applied to soft tissues as well as bone ○ Bone does adapt
26
Intrinsic vs Extrinsic factors
- Large external forces cause injuries (extrinsic) - Internal muscle actions can modify these forces (intrinsic) - Normal loading on hip has tension on superior surface + compression on inferior surface - Bone stronger under compression, so more likely to fail in tension - Muscle action can relieve some of this tensile force = intrinsic factors
27
Intrinsic vs Extrinsic factors - ankle sprains
- Muscle actions important for protection against ankle sprains - Witchalls et al. 2011 found that unstable ankles were associated with: ○ Lower inversion proprioception ○ Lower eccentric eversion strength at slower speeds ○ Higher concentric plantar flexion strength at faster speeds
28
Intrinsic vs Extrinsic Factors: Sports Injuries
Intrinsic Factors * Age (maturation, ageing) * Sex * Body comp (e.g. body weight, fat mass, BMI, anthropometry) * Fitness level (e.g. muscle strength/ power, VO2 max, joint ROM) * Health (previous injury, joint instability) * Anatomy (alignment, intercondylar notch width) * Skill level (e.g. sports-specific technique, postural stability) * Psychological factors (e.g. competitiveness, motivation, perception of risk) Extrinsic Factors * Human factors (e.g. teammates, opponents) * Sports factors (e.g. coaching, rules, referees) * Protective equipment (e.g. helmet, mouth guard, shin guards) * Sports equipment (e.g. shoes, ski's, racquets) * Environmental factors (e.g. weather, snow + ice conditions, floor + turf type, maintenance of playing surface)
29
Moment Arms: Normal vs Bow-legged Gait
- Bow legs = if constantly standing bow-legged = constantly getting this moment arm = constantly loading the medial knee, medial osteoarthritis - Use bracing to reduce the varus - Wear a lateral foot wedge = bring force closer to joint = MA reduced - Can use toe-out gait - Load the knee less = walk or run less e.g. water/aqua training instead - NEED TO Reduce KAM = knee adduction moment FOOT ANGLE: - Some use either toe-in or toe-out gait to reduce the MA = Preference is toe-out gait - If MA is longer = going to get more compressive forces on the medial side of the knee VALGUS BRACING - Create a valgus moment to overcome the varus moment
30
Injury Mechanisms and Risk Evaluation
- Injury occurs when mechanical load is in excess of the tissue's load tolerance. - Depends on type of tissue, type of load, external loading moment arm, loading rate, the frequency of load + the magnitude of the load. - Consider both intrinsic + extrinsic factors in evaluating injury risk