BIOMECH of soft tissues and injuries

Question 1

strength of materials

Answer 1

• 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

Question 2

STRESS

Answer 2

• 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

Question 3

Stress and Strain

Answer 3

• 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

Question 4

STRAIN

Answer 4

• change in length / original length = strain

Question 5

Young’s Modulus: Stiffness of Material (WORDS)

Answer 5

• 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

Question 6

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

Answer 6

• 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

Question 7

Stress vs Strain for Cortical Bone under Tension

Answer 7

• 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

Question 8

Mechanical Testing of Bone

Answer 8

• 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

Question 9

Viscoelasticity

Answer 9

• 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

Question 10

Viscoelastic Response of Bone Loading

Answer 10

• 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

Question 11

Soft Tissues

Answer 11

• 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

Question 12

Arrangement of Collagen

Answer 12

• 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

Question 13

Arrangement of Collagen

Answer 13

• 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

Question 14

Stress vs Strain
LIGAMENTS

Answer 14

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

Question 15

Tendon Force vs Length

Answer 15

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

Question 16

Articular Cartilage - INTRO

Answer 16

• 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

Question 17

Articular cartilage = depth

Answer 17

• 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

Question 18

Load Duration

Answer 18

• 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

Question 19

Creep

Answer 19

• 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

Question 20

Ligament Creep: Think about MA

Answer 20

• 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

Question 21

Creep: Lateral and medial knee cartilage

Answer 21

• Compressive strain, increasing with weight-bearing time

Question 22

Exercise and Injury Implications

Answer 22

• 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

Question 23

Injury Mechanisms

Answer 23

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

Question 24

Overuse Injuries - INTRO

Answer 24

• 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

Question 25

Overuse Injuries - summarised?

Answer 25

• 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

Question 26

Intrinsic vs Extrinsic factors

Answer 26

• 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

Question 27

Intrinsic vs Extrinsic factors - ankle sprains

Answer 27

• 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

Question 28

Intrinsic vs Extrinsic Factors: Sports Injuries

Answer 28

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)

Question 29

Moment Arms: Normal vs Bow-legged Gait

Answer 29

• 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

Question 30

Injury Mechanisms and Risk Evaluation

Answer 30

• 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