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injury is the result of:
tissue micro/macro failure under a load
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load:
amount of force on an object
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tensile load
pulling apart
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compressive load
pushing together
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shear load
twisting/sliding
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stress:
- load/crosssectional area
- newton/meters
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deformation
a change in length in response to a load
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strain
deformation/original length
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tensile and compressive strain is measured in:
percents
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shear strain is measured in
radians
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elasticity
no loss of energy, tissue returns to original length
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plasticity
loss of energy, permenent deformation
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stiffness
- amount of stress/amount of strain
- resistance during deformation
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flexibility
- amount of strain/amount of stress
- compliance during deformation
- muscles>ligs>tendons
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brittleness
- failure <5%
- little plastic deformation before failure
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ductility
- failure >5%
- lots of plastic deformation
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viscoelasticity
- materials deformation depends on rate, speed, quality, and duration of loading
- links deformation to load
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creep
deformation of a tissue over time with a constant load
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fatigue
repeated loading causing plastic deformation and failure (bending a paper clip)
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relaxation
less load needed to maintain a deformation (stretching)
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hysteresis
loss of energy from a load even when the tissue returns to its original length
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damping
- resistance to the speed of loading
- fast load= more resistance
- slow load= less resistance
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thixotrophy
reduction in fluid viscosity after movement (ketchup)
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enthesis
bone-tendon/ bone-lig complex
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enthesiopathy
lig pulling on bone (pump bump, Osgood-Schlatters)
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ligs and tendons are made of:
- collagen and elastin fibers
- reticular fibers
- ground substance
- cells
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lig and tendon behavior depends on:
- number of collagen vs number of elastin fibers
- properties of fibers
- fiber orientation
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lig and tendon strength depends on:
- fiber composition
- size and shape
- orientation
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are ligs or tendons stronger? why?
tendons. more parallel collagen fibers
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both tendons and ligs are strong under normal conditions but:
- repetative loads (micro)
- sustained loads (micro)
- heavy loads (macro)
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steps in collagen fiber loading
- 1. take out the slack, straighten out fibers
- 2. resistance/stiffness
- 3. ductile, withstands 6-8% strain
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steps in elastin fiber loading
- 1. flexible (200%)
- 2. suddenly stiff
- 3. brittle= failure!
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function of ligs
- stabilize joints
- guide motion
- prevent excessive motion
- detect weight and direction of a load
- detect tissue damage
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1st region in stress/strain curve
Toe region- taking out slack
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2nd region in stress/strain curve
elastic region- stretch, returns to original length
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3rd region in stress/strain curve
plastic deformation- tearing and deformation
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ultimate failure point:
point of maximum strength, then large or complete loss of energy
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lig flavumm stress/strain curve
- long elastic range then sudden failure
- 2/3 elastin fibers
- absorbs shock
- protective
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ACL stress/strain curve
- toe- straightens out. anterior drawer test
- elastic- beginning of injury, Grade 1 sprain, microfailure
- plastic- grade 1 sprain, pain, instability
- plastic- gross failure (6-8%), partial rupture, joint instability, 50% decrease in strength, severe pain and swelling
- complete failure- grade 3 sprain, complete rupture, instability, dislocation, severe pain, then less pain, then none...weird
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residual effects of severe sprains:
- hypermobility and instability
- joint degeneration
- suscveptability to furthur injury
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loading speed and injury
degree of injury depends on rate and magnatude of loading
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high speed loading causing:
tears 2/3 of the time
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low speed loading causes:
avulsion fracture
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why does the lig give with fast loading?
both bone and lig strengthen but bone strengthens faster so lig fails
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remodeling/healing
- increased stress/movement causes stronger and stiffer lig/tendon...good thing!
- decreased stress/movement causes weaker lig/tendon
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monkey test
8 week imobilization, after 1 year of rehab, still not 100% normal
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effects of aging on ligs
- loss of strength
- loss of stiffness
- loss of energy storage capacity
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sustained loads and ligs
- creep and hysteresis start in 20 minutes
- creep and relaxation set in in first 6-8 hours
- hysteresis lasts 1-2 hours
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repetative stress and ligs
less external load needed to cause tissue failure
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repetative stress causes failure at what percent strain?
4%
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hysteresis curve
- change in length= set
- area under the curve= energy lost
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tendon function
attach muscle to bone
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composition of tendons
almost 100% collagen w/more parallel alignment
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tendonosis
- chronic
- loss of fiber orientation
- local necrosis and NO inflammation
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stress on tendons
depends on size of muscles and tendon and if the muscle is contracting
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will a muscle contraction cause a tendon to rupture??
nope
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why do ligs and tendons heal slowly
poor vascularization
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compression fractures are usually stable or unstable?
stable
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function of bone with sudden compression
- shock absorbers
- end plates and vertical columns bow
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compression fracture
- oblique fracture of osteons in cortex
- 5% loss of height
- structure shortens and widens
- can cause schmorl's nodes
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most common site of compression fracture
TL junction
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Bone behavior under compressive loads
- 1. fracture- compression or end plate
- 2. deformation w/o fracture- osteomalacia
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cause of compression fractures
- fall on butt
- land on heels
- lift heavy load
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spinal percussion is good for:
- tumor or fractures
- would elicit deep, achy pain
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game keepers thumb
- abduction and extension
- sprains ulnar collateral lig
- may cause avulsion
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bone and tension
- failure mechanism: separation of osteons
- ansiotrophic
- lengthens and narrows
- avulsion
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the human femur is strongest with what type of stress:
tensile
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the human femur is weakes with what type of stress
horizontal tensile
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bone and shear stress
- unstable fractures
- always occur with compressive/twisting loads
- the shear strain/deformation= angle created
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how does cortical bone handle different types of stress
compression> tensile> shear
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bone and 3 point bending
fractures on convex side because compressive load in front vs tensile load in back....tensile load gives first!
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