Biomechanics 2 Mid Sem
Explain the structure of a tendon and ligament and their difference
- 70-80% collagen fibres: relatively stiff and ductile. collagen is parallel in tendons & almost parallel in ligaments - elastin: which can undergo relatively large strains and is brittle - ground substance: proteoglycans and water form a gel, viscosity decreases with activity. -fibroblasts
ACL rehab steps
- strengthening quads and ham to take the load off the ligament -muscle re-education: balance and coordination.
Characteristics of tendons and ligaments
- viscoelastic structures -primarily sustain tensile loads -poorly vascularized -strength determined by number of crosslinks b/w microfibrils, which is influenced by age, activity and gender
What is a tendon composed of?
-30% collagen: 95% type 1, 5% 3-5 -68% extracellular matrix: water, proteoglycans, glycosaminoglycans and cells -MMPs -2% elastin -tenocytes (fibroblasts)
How can you get acute and chronic inflammation in your ligament
-Long-term exposure of ligaments to static or cyclic loads/movements, long rest to recover -Long-term exposure of ligaments to static or cyclic loads/movements, constantly inflammed
Myofascial force transmission
the force measured at each end of a tendon are not always equal, surgery gets rid of this and the lengthening of one muscle changes the tension in another
sacomere
the functional unit, has a thick filament myosin and thin actin. uses cross bridging for movement.
What are two main factors in the causes of injuries?
the load applied to tissues and the strength of the tissues.
Ligaments as a sensory organ
the sensory organs in ligaments recruit/de-recruit the musculature to participate in maintaining joint stability as needed by the movement type performed.
Muscle-tendon interaction
the tendon shortens and lengthen with muscle to assist in movements, tendon compliance can improve power and efficiancy
sarcomere arrangment
they can shorten approx. 1/2 their resting length due to overlapping.
How do tendons receive water and remove waste?
they have low blood supply, loading drives fluid transport via interstitial fluid flow
How can the muscle belly be shortened but the fascicles not?
they have the same number of fascicles but different CSA
behaviours of viscoelastic material
they have viscous and elastic properties and exhibit hysteresis and time/rate dependent behavior. Viscous elastic material reacts according to how quickly a load is applied. It can handle force at a slower speed but if a material was to hit it at a larger speed/force it can not handle it as it cant move quick enough to respond to the impact. VE materials become stiffer and store more enrgy to failure when loaded at high strain rates
What is the 3 point bending experiment used for?
to test the amount of load a bone can bare
on a load-elongation graph, where does microfailure occur?
towards the end of the linear region
What fracture occurs from tension?
transverse fracture, debonding of cement lines (pulling out of osteons)
for best images measure tendons with
ultrasound, it shows loading as well and change of shape during loading
Define and explain the Stress-strain curve
used to characterise mechanical properties of a material. A straight line upwards indicates no plastic deformation, sloping line indicates plastic deformation.
how does training help tendons
when ^ force/load it has less deformation. at ^ stress it has less strain.
when is ACL most loaded
when the knee is in flexion and the quadriceps are activated
Toe Region
where lots of strain occurs for a small amount of force/stress. This is where the crimp is being stretched out.
list the 6 proteogylcans
decorin, biglycan, fibromodulin, lumican, aggrecan, versican.
what loading is best for tendon healing?
eccentric, 70%effective, heel rises for eg,
machines used in biomechanics
motion capture, dynamometry (for cerebral palsy), ultrasound, MRI, pQCT, biomechanical modeling, fluoroscopy
tendons receive more strain at
muscle attachment
Muscle structure
muscle: fascicle: fibre: myofibril: sacromere
Reflexive control delay
reflexes can be too slow to immediately correct a movement. this is because there is a reflex delay (10ms) and a electromechnical delay (60ms)
elastic modulus
relation between stress and strain, measure of property independent of CSA
Stiffness
represented by k, is the gradient of F-x curve, where k = F/x, measuring resistance to the load.
treating tendonopathy
rest, ice, heel lifts, orthotics, brace, corticosteroid injections, eccentric loading, medical therapies.
Draw a stress-strain graph
see lec notes. Very similar to L-D curve.
Modulus of Elasticity (E)
slope of stress-strain curve, i.e. stress divided by strain, or the stress required to double the length of the material
Ligamento-muscular reflex
strain on ACL causes reflex hamstring activation, con-contraction of hamstrings with quads help unload ACL
Bending boot fracture
foot stuck in boot, anterior compression and posterior tension, fracture begins posteriorly and muscle forces can neutralize tensile stress on posterior tibia.
Mechanobiology
how physical forces and changes to cell or tissue mechanics contribute to development, physiology and disease
sweet spot
ideal training zone for a tendon, which is at the top end of the elastic region so 5-6% strain.
Anticipatory Control
supraspinal/spinal program under control of afferent sensory information
Linear region on the stress/strain graph
is where the elastic properties are taking place, when this line begins to bend is when it is entering the plastic region.
why is running good for your tendons?
it ^ cross-sectional area of tendon and also stiffness ^ strength
How does the VE properties of a ligament effect it?
it decreases their effect as a joint stabilizer and sensory organ, exposing it to injury
tendons at high strain rates are
less deformable with a high degree of stiffness and are more effective in moving large loads
why are tendons VE
likely results from collagen, water, and interactions between collagenous proteins and non-collagenous proteins (e.g., proteoglycans)
Elastrography
tests tissue compressibility, sends waves to generate stress/deformation.
Plastic Deformation
the ability of a material to stay in a shape once bent
What is Mb?
the bending moment Mb = (f/2)l where f = force and l = length
tendinopathy
overuse injury (tendinopathy) is believed to result from small, repetitive strains which are below the failure threshold of the tendon that cause tendon micro-injuries and subsequent inflammation
time dependant behaviour
load relaxation: when something is stressed and held over time, the force stressed upon it will slowly decrease. Also, when you hold the force constant the length of the material will grow overtime
tendons response is influenced by
magnitude and direction of tendon loading -tensile load: ^ use of collagen 1 -compres: ^ collagen 2 -none: dec cell production
what does loading a tendon do?
makes it longer and it bulges a bit from front to back but also becomes narrower from side to side, it also cause it to rotate to the side
Explain each region of the load-deformation curve
A-B: Elastic region, no residual deformation following unloading. B: Yield point/Elastic Limit B-C: Plastic Region, residual deformation following unloading causing permanent structural changes. C: Ultimate failure point, complete rupture. The area under the L-D curve is the energy stored/absorbed.
how do you Calculate bending stress? what about on a longer bone?
BS = Mb x y/I where y = the distance from the neutral axis, I = bending resistance. = (F/2 x Ly)/L where L = load moment arm
fatigue failure
Bone fatigues when the frequency of loading precludes the remodelling necessary to prevent failure and Results in growth of defect (crack propagation).
Rate dependency
Bone is more brittle at high strain rates and 30% stronger in brisk slow walking. If a weight is slowly loaded you can withstand more compared to it being fastly loaded. A bone is more sensitive to this then a ligament.
Spastic cerebal palsy
CP inability to move properly with altered gait. treat spasticity to try and improve it
Strain
Change in length divided by resting length, units = dimensions or %. e.g. a strain of 50% means it has been stretched 50% of its length
titin
Muscle protein, spans the gap b/w the thick filaments and z-lines, restores sarcomere length when cross-bridge stops.
collagen crimp
bend-like pattern in the collagen fibres, it allows tendons to have some flexibility as well as a low compressive stiffness
Muscoskeletal Tissue
bone, ligament, muscle, menisci, nerves, tendons, cartilage
List several differences between compact an trabecular bone
compact: - dense -outer layer of bone -undergoes minor plastic deformation trabecular: -porous -inside bone -undergoes large amounts of plastic deformation -shape and configuration indicates the direction in which loads have been applied to the bone
Sonoelastography
computes relative strain values based on temporal variation of RF signal
Tendon structural orginisation
-collagen fibril -collagen fibre (tenocytes) -endotenon: collagen fibre, fascicle bundle, (blood, lymph and nerve) -epitenon: fascicle bundle (b,l and n) -paratenon: teriary fibre bundle
What factors affect a ligaments strength? and an ACL injury?
-immobilization decreases ligament strength and rates of healing -once over 20, age increase decreases ultimate load - ACL more common in women then men -strong association between menstrual cycle and ACL -high risk in ovulatory phase -moderate exercise increases stiffness and tensile strength
what mechanical factors cause tendinopathy
-instrinsic: alignment -biomechanical: whip lash phenomenon with excessive pronation
ACL injury leads to..
-mechanical deficit (^ anterior translation of tibia on femur) - sensory deficit which increases risk of 2ndry injury -de-synchronization of quad-ham activity -quadriceps atrophy
tendon pathology in injury
-periodic collagen disorganization -hyper-cellularity (^ tenocytes) -tendon thickening -^ vascularity -heterogeneous structure (calcifies)
function of a tenocyte
-produce collagen -produce proteoglycans & elastin - produce MMP's
limitations of sonoelastography
-strain in beam direction not tendon load direction - costrained to conditions of low deformation -relative strain -poor repeatability
What is the function of a ligament
-to attach to articulating bones -to guide joint movement -to maintain joint congruency -to act as a strain sensor
Function of a tendon
-to transmit muscle force to bone -to store and return elastic energy -to monitor muscle tension (Golgi organs)
regions of the stress/strain graph
-toe 2% -linear 2-6% -plastic 6-8% -failure 8-10%
What are the 7 loading modes
1. Unloaded 2. Tension: pulled from both ends 3. Compression: compressed from both ends 4. Bending 5. Shear: top half and bottom pushed in opposite directions 6. Torsion: top half and bottom half twisted 7. Combined loading: compression + torsion
Matrix Metalloproteinases MMPs
20 types, degrade most components of the ECM and are regulated by tissue-inhibiting metalloproteinases TIMPs
Microscopic failure in a tendon occurs at _% and macroscopic failure at _%
4% and > 8%
what is the optimum strain level for tendons
6%
At what % of strain does complete rupture of bone/tendons occur?
8-10%
Define and draw the Load - Deformation Curve
A graph used to characterise the mechanical properties of a structure
Maturation
A ligament substance matures quicker than the bone-ligament junction, but a bone-ligament junction is stronger in maturity
What fracture occurs during torsion?
A spiral fracture as it puts tension along spiral stress lines with maximal stress at the outer surfaces
Achillies Tendon morphology
consists of 3 fascicles (GM,LM and Soleus), has a spiral arrangement, the distal end is free and proximal end has aponeurosis
Elastic Behaviour
Elastic Materials deform when subjected to stress and resume their original shape when un loaded. Stress is a function of strain only and all energy stored in loading is released upon unloading E = 1/2 Fx
Why is it good to exercise our bones?
Exercise is osteogenic, some types are better then others, intensity being more important then duration. Increased bone strength in response to activities that involve high impact and strain rates.W
Stress
Force intensity (pressure), it is force divided by cross sectional area, units = N/m^2 = Pa
What is the area moment of inertia for a cyclinder and rectangular prism and what can you tell from these equations?
I = piR^4/4 I = bh^3/12 The radius of a cylinder has a major influence on how good that material is at bending. The height of a rectangular prism has a massive effect on the materials ability to bend, the lower I is the greater the resistance to bending.
What does is mean when you say 'bone is anisotropic'?
It has different mechanical properties depending on the way in which force is applied. It works best in compression, tension and then shear.
What does hollowing a bone do?
It makes a bone lighter without sacrificing much strength as there is only a slight change in the radius. The bending resistance slightly decreases with the stress increasing but only slightly.
Hysteresis
Load-elongation curve follows different path in loading versus unloading. It is measured by difference in the area under the loading versus unloading curves. Hysteresis area is area bounded by hysteresis loop. It represents energy lost due to the materials internal friction.
What fracture occurs from compression?
Oblique fracture, cracking of osteons, common in vertebrae and can be caused by strong muscle contractions
distance vs sprinters in fascicle length
SPR have longer fascicle lengths as this favours shortening velocity which is required for greater running speeds
3 component model of the MTU
Series Elastic Component SEC (tendon and aponeurosis), contractile component CC and Parallel Elastic Component PEC.
other factors affecting fatigue failure
Surface finish surfaces experience the greatest stress in bending, surface defects allows cracks to initiate, smoother surfaces have a longer fatigue life - eg. hip joint replacements need to be smooth and also the environment such as body fluids as saline is corrosive.
how do you calculate energy from a stress/strain graph?
The area of a triangle = 1/2 base x height which = 1/2 load x deformation, this is used to calculate energy, you can also use 1/2 kx^2
When bending on a bone occurs, what part fails first and what fracture occurs?
The bone will fail at the bent side where tension is occurring as it is undergoing more stress then the compressed side. A transverse failure at the tensile area and oblique failure at the compression side will happen causing a butterfly fracture.
What is the neutral axis?
The medial line in a bone which experiences no net stress due to the counteracting of stresses on each sides
Mechanotransduction
The molecular mechanism by which cells sense and respond to mechanical signals. It is the transformation of mechanical forces acting on a tissue, into a cellular and genetic response.
muscle contraction speed
^ loads mean slow contraction speed, dec load means high contraction speed
muscle contracture
a persistent and involuntary shortening of muscle reducing flexibility, occurs in muscle and tendon,
Muscle spasticity
a velocity dependent resistance to stretch due to an increase in gain of stretch reflex. Occurs due to stroke.
tendons gender and age
age decreases tendon stiffness and so does gender
tendons at low strain rates
are more deformable and hence store more energy, but are less effective in transferring loads
