Biomechanics (tissues) Flashcards
What are the types of forces on the musculoskeletal system?
- Tension
- Compression
- Bending
- Shear
- Torsion
- Combined loading
What is tension?
Forces are from opposite directions
(Pulling two ends apart)
What can overload with ‘tension’ cause?
Sprains, strains, (sometimes) peripheral nerve injury
E.g. hamstring tear
What is compression?
Forces moving in an approximating (similar) direction
(pushing two ends towards e/o)
What can overload with ‘compression’ cause?
fractures
(sometimes) disc damage/nerve compression
E.g. stress fracture of vertebrae, disc herniation
What is bending?
Force that produces tension on one side of the body’s longitudinal axis and compression on the other side
legit just bending
What is shear?
Combination of tension and compression
forces NOT moving in opposite or approximating (similar) directions exclusively
tbh, the forces look like they from opp sides (like upper part has force from side A, lower part has force from side B)
E.g. ACL ruptures
What is torsion?
Force applied in twisting
What is combined loading?
When loading results in more than one type of stress (force)
What is stress?
A physical quantity
An external force
Force per unit area applied to the material
What is strain?
Stresses lead to strain (deformation)
E.g. putting pressure on an object causes it to stretch
Strain is a measure of how much an object is being stretched
How to calculate stress?
Force/cross-sectional area
How to calculate strain?
Elongation/original length
Basically: new length - original length/original length
What is elastic modulus?
calculate = Stress/Strain
indicator of an object’s likelihood to deform when a force is applied
Look at slide 8 of mechanics biological tissues notes (stress-strain curves)
HAVE YOU LOOKED AT IT?
What are the three regions in the stress-strain curve?
- Initial linearly elastic region
- Intermediate region
- Final region
What is initial linearly elastic region?
Where the
slope = elastic modulus
What is the intermediate region?
Exhibit yielding & nonlinear elasto-plastic material behaviour
Strain hardening; entering plastic phase
Basically, being stretched & deformation can become permanent
What is the final region?
Exhibits linear plasticity where slope = strain hardening modulus
Necking
Until failure
What is plastic behaviour?
An object or material has plastic behavior when stress is larger than the elastic limit
Basically, when stress is removed, the object doesn’t return to original (deformation is permanent)
What is uncrimping?
Taking up slack
From slack to straight
E.g. flabby resistance band; you pull it straight BUT not stretched yet
`Look at slide 10 of mechanics biological tissues notes
HAVE YOU LOOKED AT IT?
What happens at the toe phase (0-2%)?
Uncrimping: takiing up the slack (from slack to straight with NO stretch at all)
Macroscopic slack (bc not homogenous)
Needs more force to bring about deformation
What happens during the linear stretch (~2-5%)?
Elasticity (tissues will be stretched according to force applied)
Start of plasticity halfway through (abit)
Therapeutic range (around 4-6%)
Viscoelasticity
Force
Relaxation
Creep
Hysteresis
What happens during the primary failure phase (5-8%)
Part of the therapeutic range (until ~6%)
Viscoelasticity
Plasticity
Force
Creep
Relaxation
1-2 degree injury (aft the end of therapeutic range = injuries!)
What happens during total failure phase (after 8%)?
Influenced to failure
basically, total failure (e.g. ligament tear)
What is viscoelasticity?
Materials that show viscous & elastic characteristics when undergoing deformation (time-dependent)
All connective tissues are viscoelastic materials (basically - fluid-like component in their behaviour)
What is viscosity (viscoelasticity)?
Material’s resistance to flow (a fluid property - there’s fluid phase & solid phase)
High viscosity fluids: flow slowly (e.g. honey)
Lower-viscosity fluids: flow quickly (e.g. water)
↓es w temperature (easier to flow; e.g. use of heat before stretching)
↓es w slowly applied loads
What is elasticity (viscoelasticity)?
Material’s ability to return to its original length/shape aft removal of deforming load
Length changes/deformations proportional to applied forces/loads
When stretched, work is done (force x dist) = energy in stretched material inc.
What does elasticity depend on?
Depends on collagen & elastin amounts
If more elastin = stretch better
if more collagen = more difficult to stretch
What are the time- and rate-dependent properties?
- creep
- stress-relaxation
- strain-rate sensitivity
- hysteresis
What is creep?
Progressive strain (deformation) of a material when under a constant load over time
Basically, deformation increases as (fixed/constant) force increases. When force removed, tissue recovers to original length in nonlinear manner
E.g. gravitational force on body. Stand whole day, gravitational force on vertebral column = intervertebral discs squashed; but sleep (supine) = force is removed (?) = taller
Example of creep
Gravitational force on body.
Stand whole day, gravitational force on vertebral column = intervertebral discs squashed; but sleep (supine) = force is removed (?) = intervertebral discs return to normal position = talller
What is stress relaxation?
Reduction of stress within a material over time as the material is subjected to a constant deformation
Stress generally ↓ w time (therefore relaxation)
BASICALLY, stretch in same position for long period = becomes easier = then can stretch further
Examples of stress relaxation
Stretch until the force drops then stretch some more = achieves the overlap b/w elastic & plastic phase = permanent change
Serial casting = stretch the tissue to max & cast it (at max range) = next day, push more & cast again & so on
What is hysteresis?
When force is applied (loaded) and removed (unloaded) to a structure, the resulting load-deformation curves do not follow the same path
NOT all energy gained due to lengthening work (force x dist) is recovered during exchange from energy to shortening work (some energy lost, as heat)
BASICALLY, deformation does not return to original (still stretched) bc energy recovered is less
What is strain-rate sensitivity?
Tissues behave differently if loaded at different rates
*subsequent stress-relaxation will be LARGER if load applied fast
**Creep will take longer to occur if rapid loading
basically how fast/slow you apply stress
What happens to strain-rate sensitivity if load is applied rapidly?
Tissue is stiffer
larger peak force can be applied to tissue
Need more force since stress is lower
Fast = strain not that strong = need more force
What are the biological tissues?
Hard: bone, teeth
*Soft: tendons, ligaments, joint capsules, skin, muscles, articular cartilage
Phases of a bone fracture
Acute phase: Week 1-2
Sub-acute phase: Week 2 - Month 3
Chronic: After month 3
What happens in a fracture?
Just read through and agar agar know:
1. blood pours into injured area to form a clot - aka fracture haematoma (scaffold for migration of cells & source of growth factors)
2. granulation tissue gives rise to the callus = acts like a protective cast
3. endochondral ossification - soft callus converred to hard bony framework
4. Callus remodelled so thaat cartilaginous structure converts to calcified bone matrix & bone is shaped to near-normal shape
Purpose of bones
- supports & protects internal organs
- assists movement: sites for muscle attachment & facilitates muscle actions & body movement
- mineral “bank” : reservoir for calcium deposit = maintain homeostasis of bld calcium
- blood cell production
- adipose tissue (yellow marrow)
Types of bones
- Long bone (e.g. humerus)
- Short bone (e.g. carpal bone)
- Irregular bone (e.g. vertebra)
- sesamoid bone (e.g. patella, sesamoid of thumb)
- Flat bone (e.g. sternum)
Composition of bones
Most to least:
Calcium
Organic compounds (mostly collagen)
Phosphate
Carbonate
Magnesium
Sodium & Potassium
What are the two types of bone tissue?
Cortical (aka compact) bone tissue
Cancellous/trabecular (aka spongy) bone tissue
What are cortical (compact) bone tissues?
Dense material forming outer shell (cortex) of the bones
What are cancellous/trabecular (spongy) bone tissue?
Consists of thin plates (trabeculae) in a loose mesh structure enclosed by cortical bone
What are bones covered by?
Dense fibrous mbn - periosteum
Covers the entire bone except for joint surfaces
What are joint surfaces covered by?
articular cartilage
Spongy bone vs compact bone
Aka: Cancellous tissue vs cortical tissue
Area: spongy makes up inner cavity of bone while compact is the outer covering
Porosity: spongy more porous than compact
Functional unit: functional unit of spongy is trabeculae while compact is osteons
What are the major factors influencing mechanical behaviour of bone?
- Composition of bone
- Mechanical properties of tissues comprising the bone
- Size & geometry of bone
- Direction, magnitude & rate of applied loads
What can bones be characterised as (3 points)?
- Nonhomogenous material: consists of cells, organic & inorganic substances w diff material properties
- Anisotropic (direction dependent): direction of force applied affects behaviour
- Viscoelastic (time & rate dependent): bone can resist rapidly applied load better than slowly applied loads (basically bone is stiffer & stronger at higher strain rates)
What are the effects of anisotrophy?
Stress-strain behaviour dependent on orientation of bone w respect to direction of loading
If force is applied longitudinally (e.g. top down like in walking/standing), stronger & stiffer (larger elastic modulus)
If force applied transversely (e.g. 90 degree to the bone), more brittle & weaker
Note: in soccer/muay thai, there may be transverse force but if it’s fast (e.g. kick), more force is needed
What is the viscoelastic property of bone?
Brittle: material fails before permanent deformation
Ductile: material deforms greatly before failure (to do with stiffness)
What are the factors affecting integrity of bone (4 points)?
- Osteoporosis
- Surgery
- Bone defects
- Screw holes for pins & bone plates
How does osteoporosis affect integrity of bone?
Reduces bone integrity in terms of strength & stiffness by reducing apparent density
How does surgery affect bone integrity?
Alters normal bone geometry
May cause leg length difference = some muscles may be shorter = muscle imbalance
How do bone defects affect integrity of bone?
Usually congenital
e.g. genu valgus/varus
How does screwing holes for pins & bone plates affect integrity of bone?
Causes stress concentrations on bone (abnormal force)
What is osteoporosis characterized by?
Low bone mass
Deterioration of bone micro-architecture
Compromised bone strength
most epidemic bone dz in older populations
What does osteoporosis lead to?
Bone fragility
Increased risk of fracture under low loads
Types of soft tissues (6 points)
- Articular cartilage
- tendon
- ligament
- muscle
- joint capsules
- skin
Collagen fibres
Not effective under compression
Individual fibrils of collagen fibers surrounded by gel-like ground substance (mainly water)
Possess two phases: solid-fluid OR viscoelastic material behaviour
What is the composition of soft tissues?
All soft tissues are composite (made up of 2 or more materials) materials
Main structural elements:
Collagen fibres: gives structure & stiffness
Elastin fibres: thinner & cross-linked; can be crimped/uncrimped
What happens to collagen fibres when stretched & released?
When stretched: energy stored in fibre like a spring
When released: releases energy & fibre returns to its unstretched state
Elastin fibres
Fibrous protein whose material properties resemble the properties of rubber
Elastin + microfibrils form highly extensible elastic fibres that are reversible at high strains
Difference in behaviour of elastin fibres & collagen fibres
Elastin fibres possess low-modulus elastic material property (not stiff)
Collagen fibres show a higher modulus viscoelastic behaviour (stiffer)
Collagen vs Elastin
Found in: collagen in skin & protective tissues (e.g. joint capsule); elastin in connective tissue of elastic structures
Abundance: collagen 3rd most abundant protein; elastin less abundant
Colour: collagen white; elastin yellow
Purpose: Collagen gives strength to structures; elastin provides elasticity to structures
Production: collagen produced until ageing process; elastin produced mainly in fetal period
Viscoelastic model comprises:
A spring: models elastic behaviour
A dashpot: models time dependent behaviour
Movement of body segments (skeletal muscles)
Achieved as a result of forces generated by skeletal muscles which convert chemical energy into mechanical work
What are skeletal muscles composed of?
Muscle fibres & myofibrils
What kind of material behaviour does skeletal muscles exchibit?
Viscoelastic material behaviour (bc have both collagen & elastin)
Muscles are viscous in a sense bc no internal resistance to motion
What is muscle contraction?
development of tension in the muscle
concentric, eccentric & isometric contractions
What is flexibility of the human body due to?
Joints
articulations
skeletal system
(therefore, after surgery/injury, must stretch the joints (some force = some deformation); if lie down whole day, tissues shorten = become tight = problem)
What are the two primary functions of joints?
Mobility
Stability
Structural classification of joints (3 points)
FIbrous joints
- dense connective tissue connect bones
- b/w bones in close contact
Cartilaginous joints
- hyaline cartilage/fibrocartilage connect bones
Synovial joints
- most complex
- allow free movement
Functional classification of joints (3 points)
Synarthrotic joints
- considered immovable
amphiarthrotic joints
- slightly movable
diarthrotic joints
- freely movable
Look at slide 54 of mechanics biological tissues notes
HAVE YOU LOOKED AT IT?
Glenohumeral joint
Ball-and-socket joint
Enables arm to move in 3 planes (triaxial motion)
- High level of mobility
- Reduced stability
- Increase vulnerability of joint to injuries (e.g. dislocations)
Humeroulnar joint
Movement only in one plane (uniaxial motion)
More stable
Less prone to injuries than shoulder joint
What are the functions of articular cartilage (4 points)?
- Covers articulating surfaces of bones at diarthrodial (synovial) joints (if X there, wear & tear)
- Provides weight bearing surface w low friction & tear
- facilitates relative movement of articulating bones
- distributes loads over larger contact area (reduces stress applied to bones)
What happens to the articular cartilage under tension?
Cartilage responds by realigning the collagen fibres which carry the tensile loads applied to tissue
What are shear stresses on the articular cartilage due to?
Frictional forces (2 surfaces rubbing on e/o) b/w the relative movement of articulating surfaces
What are the changes of the joints over time (lifespan changes)?
- Joint stiffness in older people
- Fibrous joints change first
- Symphysis joints may change in the vertebral column (water loss from intervertebral discs) = loss of disc height & flexibility
- Synovial joint loses elasticity
What are tendons?
Fibrous connective tissues
Execute joint motion by transmitting mechanical forces from MUSCLES TO BONES
Passive tissues (cannot contract to generate forces)
What are ligaments?
Fibrous connective tissues
Attaches bones to to another bone across a joint
Tendons compared to muscles
Tendons are stiffer, have higher tensile strength & can endure larger stresses
What do tendons enable muscles to do?
Enables muscles to transmit forces to bones without wasting energy to stretch tendons
What is the purpose of ligaments?
Guide & stabilise skeletal joint movement
Prevent excessive motion
Ligament vs tendons (elastic fibres)
Ligaments have a greater proportion of elastic fibres = higher extensibility BUT lower strength & stiffness
Also viscoelastic & exhibits hysteresis
Difference between tendon & ligament
Main function: tendon connect muscle to bone; ligament connect bone to bone at joints
Toughness & elasticity: tendon tougher; ligament more elastic
Injuries: tendon - tenosynovitis, avulsion (w fracture), tendinitis (mostly inflammation); ligament - sprains & torn ligament
Formation: tendon - modification of the white fibrous tissues; ligament - formed with yellow elastic tissue modification along collagen fibres
