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
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
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
4
Q
STRAIN
A
- change in length / original length = strain
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
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
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
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
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
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
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
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
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
14
Q
Stress vs Strain
LIGAMENTS
A
- Patella Tendon = much stiffer + stronger structure than the ACL
○ Linear up to 10-12% - Cruciate Ligaments
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
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
Q
Overuse Injuries - summarised?
A
- 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
Q
Intrinsic vs Extrinsic factors
A
- 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
Q
Intrinsic vs Extrinsic factors - ankle sprains
A
- 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
Q
Intrinsic vs Extrinsic Factors: Sports Injuries
A
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
Q
Moment Arms: Normal vs Bow-legged Gait
A
- 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
Q
Injury Mechanisms and Risk Evaluation
A
- 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
