Biomech Flashcards
What is injury a result of?
Tissue macro or micro failure under load
What determines whether or not ther will be tissue damage?
Nature of the load and properties of tissue.
How are we injured? (5)
1) High load (single traumatic event)
2) Repetitive loads (end range)
3) Sustained loads (end range) ie: postural syndromes
4) Sudden unguarded movement
5) Normal activity coupled with a failure of segmental coordination (especially in an unstable joint)
Joints are protected by what at mid range and end range?
Mid range- Muscles. End range- Ligaments.
What is Load?
The amount of force on an object.
What are the 3 types of Load?
- Tensile.
- Compressive.
- Shear.
What is tensile or tension?
It is like pulling the head up.
What is stress?
A load divided by the cross sectional areas. Like stepping on someone’s foot in flat shoes vs high heels.
What are the 3 types of stress?
1) Tensile stress
2) Compressive Stress
3) Shear Stress
What type of stress is tissue most susceptible to?
Shear Stress
What is deformation?
A change in length in response to a load.
What happens with tensile deformation?
Stretch an object to be longer.
What happens with compressive deformation?
Compress an object to be smaller.
The amount of deformation is known as what?
Strain.
What is strain?
A change in length divided by the original length. Or the % of change.
What is shear strain?
The angle of deformation in radians.
What is tensile strain?
The % of length change.
What is a muscle strain?
Muscle tear.
Is a biomechanical strain the same as a muscle strain?
No, a muscle strain is a muscle tear whereas a biomechanical strain is the change in legth divided by the original length
What happens when there is a loss of energy in tissue?
A tissue gets weaker and the damage caused can be permanent or transient
What is involved in a sprain?
Ligaments and disks.
What is elasticity?
The ability of a substance to return to its original form following the removal of a deforming load.
What happens to energy with elastic material?
No loss of energy in one cycle of load and unload.
What is plasticity?
The property of a material to permanently deform if loaded beyond its elastic range.
What is an example of plasticity?
Tearing a ligament that causes instability due to permanent deformation
What happens to energy with plasticity?
Loses original form and material loses energy after one cycle of loading or unloading.
What is stiffness?
A measure of resistance offered to external loads by a material as it deforms.
Does a stiffer material need more or less stress to deform?
More stress.
How is stiffness calculated?
the amount of stress/the amount of strain.
What parts in biomechanics need to be stiff?
Bones and spine for stability
What is flexibility?
Measure of compliance offered to external loads by a material as it deforms.
Does a flexible material needs more or less stress to deform?
Less stress.
How is flexibility calculated?
the amount of strain (deformation)/the amount of stress (load).
What is brittleness?
The quality whereby a material exhibits little plastic deformation before failure.
What is failure in regards to brittleness?
< 5% of total elongation of material.
What is ductility?
The quality whereby a material exhibits large plastic deformaitn before failure.
What is failure in regards to ductility?
> 5% of total elongation of material.
What is viscoelasticity of tissue?
The property of a material that links how it will deform to how the load is applied.
What 4 things will change how a viscoelastic material will deform?
rate, speed, quantity and duration of loading.
What is a deformation of a viscoelastic material with time, when the load remains constant?
Creep.
What type of viscoelasticity is when overtime tissue relaxes into new lengths. This results in a decreased load overtime to maintain fixed derfomation?
Relaxation.
What type of viscoelasticity is when loss of energy during a loading cycle despite returining to its original form than its weaker ?
Hysteresis.
What type of viscoelasticity occurs when the process of plastic deformation or failure of material happens from repeated loading?
Fatigue.
What is the resistance to speed where the faster the load, the greater the load is created to deform an object of a given amount?
Damping.
What is the reduction in the viscosity of a fluid following movements?
Thixotropy.
What is the composition of ligaments? (4)
1) Collagen and elastin
2) reticular fibers
3) ground substance
4) Cells (fibroblasts)
What makes up 80-90 % of connective tissue?
COLLAGEN and elastin.
What type of links break first in connective tissue?
Cross links.
With what type of trauma (micro or macro) do tissues tend to fail more?
Microtrauma
What does a micro trauma entail?
Repeated incidence, doesn’t allow body time to heal
What is the first indicator of microtrauma?
Pain
What is the half life of collagen?
Could be as old as the individual, but time can be accelerated in young animals, injured tissues, and immobilized tissues.
A tendon or ligament behaves according to what 3 things?
- properties ( collagen and elastin).
- Proportion of collagen to elastin.
- Fiber orientation.
What does the Strength of tendons or ligaments depends on?
Fiber composition, size, and shape.
What are some ways to increase strength in tendons and ligaments? (3)
1) Increase number of fibers
2) Increase fiber diameter
3) Increased number of fibers parallel to direction of tension
Tendons and ligaments normaly have how much stress on them?
1/3 of their capacitiy.
What type of loads can cause injury to tendons and ligaments? (3)
High loads (trauma), repetitive loads, or sustained loads.
What direction do the fibers run in Tendons and what type of load do they resist?
Parallel and tensile loads.
What direction do the fibers of Ligaments run in?
Parallel and some non parallel
Where is the distrubtion of force in a tendon?
Directly through the tendon
Where is the distrubtion of force in a ligament?
Ligaments can move in more than one direction so the force distribution runs muliple directions
What is stronger skin, tendons, or ligaments?
Tendons are stronger than skin and ligaments.
Collagen will have a slight elongation without much muscle tension then it is followed by what?
A region of stiffness.
What is the strength of collagen like with tension?
One half cortical bone strength.
How much strain can collagen fibers withstand?
6-8%. This makes it ductile. (>5%)
How flexible are elastic fibers?
200 % elongation in elastic range.
What happens after elastic fibers are elongated as long as they can?
They become suddenly stiff and then brittle and then no further deformation before failure.
What is enthesis?
Bone-ligament or bone tendon complex.
A progressive change from ligament to bone protects against injury how?
By allowing better stress absorption and transmission.
What are sharpeys fibers?
Fibers that sink into bone that connect the ligament to bone
What are the structural jobs of ligaments? (3)
stabalize joints, guide joint motion, prevent excessive motion.
What are the functional jobs of ligaments? (2)
Detect rate and vectors of load, detect tissue damage.
What is the Toe region of a stress strain curve for ligaments?
Straightening out the wavey ligament fibers.
What happens with strain when stress is increased on ligaments in the toe region?
The strain remains at zero until the end of the toe region when it starts to go up.
What range comes after the toe region in a stress strain curve for a ligament?
Elastic range.
What is the elastic range like in a stress strain curve for a ligament?
The strain goes up proportionatly to the stress that is placed on the ligaments while in the elastic region.
What happens to tissues that were under elastic strain?
They distort and then go back to orignal form.
What region comes after the elastic range in a stress strain curve for a ligament?
Plastic range.
What is the plastic range like for an stress strain curve of a ligament?
This is wherethe ligament is permanently deformed if loaded beyond the elastic range. Tearing of the ligament and loss of energy.
What type of fibers is the ligamentum flavum made of?
2/3 elastic fibers.
What is the function of the ligamentum flavum?
Absorb shock of flexion and protect nerve roots and spinal cord.
How far can the ligamentum flavum elongate before stiffness increases?
50%.
At what percetage of elongation does the ligamentum flavum fail?
70%.
What is the plastic phase like for the ligamentum flavum?
There’s no real plastic phase so the ligamentum flavum fails abruptly after stiffness.
What is the failure point?
The point of maximum strength of the material.
What happens to energy with a failure point?
Large or complete energy loss.
What is the anterior drawer test?
Pull tibia anterior while patient is supine and knee is bent and this should feel nice and snugg not lose.
Clinical tests of ligaments should be in what region?
Toe region and the first part of the elastic range.
The more fibers that are _______ to the load the stronger the structure?
Parallel.
What region will the beginning of a grade 1 ligament sprain happen in?
At the end of the elastic range.
What are some additional deformations that occur with ligament sprains?
1) Failure of internal structure
2) decrease in strength
3) a sudden relaxation
What is a failure point?
Point of maximum strenght of the material
Which fibers sustain greater force in a ligament?
Fibers that are parallel to the load straighten out first and sustain greater force
What makes a ligament a stronger structure, more parallel or more perpendicular fibers to a load?
More parallel fibers
In the clinical test region of a joint (first region), what occurs in the ligaments?
A small load is enough to cause elongation while the ligament’s wavy fibers straigten out
In the clinical test region of a joint (first region), what joint motion is occurring?
Joint motion what occurs without locking bones together
In the Physiological loading region of a joint (second region), what is it characterized by?
Stiffness, mostly a linear elastic region
In the Physiological loading region of a joint (second region), what is the function?
Stability and limitation of motion
In regards to a joint, when does the beginning of injury occur?
The physiological loading stage
When injury begins in the physiological loading (second region), what is happening microscopically?
Microfailure of collagen fibers
What type of sprain begins in the physiological loading stage (second region) of a joint?
Beginning of grade 1 sprains
How many “regions” is the injury portion of joint motion split into?
3 regions (on the graph it is known as third, fourth and fifth region)
What is the “third region” or beginning of injury in a joint also known as?
Plastic region
What occurs in this “third region” or beginning of injury in a joint?
Progressive failure of collagen fiber bundles with no change in gross appearance
What type of sprain occurs in this “third region” or beginning of injury in a joint?
A grade 1 sprain where microfailure, slight pain, minimal signs and no instability occurs
What is occuring in the ligament during the “third region” or beginning of injury in a joint?
The ligament is stretched and the joint moves more than it should
What occurs “fourth region” in joint motion? (3)
1) Gross failure with ultimate loading (ultimate strain is 6-8 %)
2) tensile failure of collagen fibers
3) shear failure between fibers
What extent of injury occurs in the “fourth region” of injury in a joint?
A Grade 2 Sprain: Partial rupture, joint instability, 50% decrease of stregth and severe pain and swelling
What grade of sprain does instability occur?
Grade 2 sprain
What occurs “fifth region” in joint motion?
Complete failure of the ligaments
What injury occurs “fifth region” in joint motion?
Grade 3 sprain : Complete rupture, complete instability and dislocation with severe pain at injury followed by decreased pain/no pain afterwards
Where will a grade 1,2,3 sprain happen at?
1- in the end of the elastic range, and start of the plastic range.
2-the end of the plastic range.
3- after the plastic range with failure.
What is pain like with a grade 3 sprain or failure of a ligament?
severe pain at injury followed by decreased pain at injury followed by decreased pain and sometimes no pain.
What is a residual effect of a significant sprain?
Hypermobility and instability (empty feel on endplay, palpatory pain, repeated dislocation, excessive motion)
Hypermobility and instability following a significant sprian can lead to what?
Degeneration of joints, and susceptibility to further the injury.
How will loading speed affect the degree injury and what are the 2 factors it is dependent on?
With viscoelastic material, the degree of injury depends on the magnitude of the load and the rate that the load is applied.
How much will the strength of tissue increase with a 4 times speed increase of a load?
50% in strength, 30% in energy storage capacity and 30% increase in strain to failure.
With high speed loads what tears first?
Ligaments (2/3 times and bone 1/3 times)
With low speed loads what tears first?
Bone avulsion-pull a chunck of bone off with the ligament.
What gets stronger with increased speed of a load bones or ligaments?
both.
Since both bones and ligaments get stronger with increased speed of a load which one gets stronger?
Bone strength is increased more than ligaments with speed so the ligament will fail first with a fast load.
Why will slower loading make bone fail first?
Slow loading makes bone more brittle.
After slouching what load is created?
Flexion at the Lumbosacral junction.
What happens in about 20-30 minutes when ligaments and the posterior disc is placed under sustained flexion (like with bad posture from sitting)?
Creep and Hysteresis.
What is creep ?
Creep- a deformation of viscoelastic material with time when load remains CONSTANT.
What is hysteriesis?
Distortion over time but without loss of energy, tissue is temporarily weaker
What type of tissues can creep occur in?
Almost any type.
What is relaxation?
Over time the tissue relaxes into its new length. So this results in a decrease of load over time to maintain a fixed deformation.
Creep/relaxation take how long to occur?
Some take place within a few minutes, but most take place in the first 6-8 hours.
Creep/relaxation continues for how long?
At a slow rate for months.
What happens to a structure after hysteresis?
when unloaded a structure regains shape at a different rate compared to when it was deformed.
With hysteresis what makes the spine more susceptible to injury?
Ligaments are a little longer and they have lost some energy and they no longer act as shock absorbers.
Hysteresis on the spine lasts for how long?
1-2 hours.
What is “set” from a stress strain graph for hysteresis?
The amount of tissue length change after the load is removed. Hysteresis will lengthen the tissues and the set should be a positive number.
After hysteresis and even after the material returns to its original length what happens to the energy?
There is still a loss of energy.
What type of loads even within the physiological range can eventually cause microfailure due to hysteresis?
Repetitive or sustained.
What is fatigue?
The process of plastic deformation or failure of a material from repeated loading.
Less external load is necessary to cause tissue failure with what type of stress?
Prolonged cycles of repetitive stress.
What will repeated loading even in the elastic range cause?
Fatigue cracks, general weakening of material and ultimate load capacity decreases.
Low back can have a micro failure with what % of strain (elongation)?
4%.
4% strain on the low back can cause a micro failure so the low back is what?
Brittle.
What is more commonly seen in practice repetitive microtraumas or a single traumatic event?
Repetitive microtraumas.
How often should breaks be taken in order to reduce the exposure to repetitive or sustained overloading?
20 minutes.
What will help a recovering ligament/tendon become stronger and stiffer?
Increased stress.
What happens to ligaments and tendons with less stress?
They become less stiff and weaker.
What can cross friction massage theoreticaly do?
Promotes paralleled collagen deposition and coss links.
How many weeks will a broken bone be in a cast?
6-8 weeks.
What happened to the monkeys who had their knees immobilized for 8 weeks?
40% decrease of maximum load to failur of their ACL, and a 30% decrease of energy storage capacity, and a loss of stiffness.
What happened to the monkeys after 5 months of rehab?
They load to failure was 80% of normal and energy storage was 80% of normal.
What happened to the monkeys after 12 months of rehab?
The load to failure was 90% of normal. And energy storage capacity was 90% of normal.
What duration of strenuous exercise might be required to rehabilitate ligaments?
A year
What does aging do to ligaments and tendons?
Causes loss of strength, stiffness and energy storage capacity
What is a sprain?
A tear of a ligament.
What is the function of a tendon?
Attach muscle to bone and transmit tensile loads from muscle to bone for the purpose of joint motion and stabilization.
What are tendon fibers made of?
Nearly 100% collagen.
How are the collagen fibers traveling in tendons?
Parallel arrangement.
What is a paratendon?
Loose arelar connective tissue that surrounds the tendon
What is the function of the paratendon?
Aids in gliding
Where does the paratendon run on the tendon?
May run the entire length of the tendon or just where it rubs against the bone
What is an epitendon?
Points of high friction on the paratendon
Where are epitendons found and what is the function?
Surrounds the paratendon and produces synovial fluid
What are the 2 major factors that will determine the amount of stress a tendon can handle?
1) Relative size of muscle and tendon.
2) Whether the muscle is contracting.
Normal activity produces how much load on a tendon?
Only 1/4 of its capacity.
Can a muscle contraction alone rupture a tendon?
No.
How is a tendon usually injured?
The muscle is contracted and tensile force rapidly extends the muscle before it can relax.
What is stronger muscles or tendons?
Tendos may be 2 times stronger therefore muscle strains are more common than tendon tears
What is a torn muscle called?
strain.
What is more common muscle strains or torn tendons?
Muscle tear.
Is stress good for a torn tendon to remodel?
Yes.
What is the healing time like for tendons and ligaments?
Slow.
Why is the healing time slow for tendons and ligaments?
Poor blood supply.
What muscle in our shoulder has poor vascularization?
Supraspinatus.
What are some resulting dysfunction of muscle strains? (6)
1) Weakness
2) Palpatory pain
3) hypermobility and instability
4) aberrant motion
5) subluxation (medical)
6) increased susceptibility to joint injury
What will aging do to tendons?
Collagen fibers get smaller.
What will mobilization exercises do to tendons?
They increase the tensile strength of tendons and the tendon-bone interface.
What do corticosteroids do to tendons?
They inhibit collagen synthesis, and can prevent early strenous rehab.
What percentage of microfailure in ligaments occur with repetitive micro-trauma.
4%.
With what condition does, tissue returns to pre-strain status yet it is still weaker because of loss of energy.
Hysteresis.
What is the strongest and stiffest Connective tissue?
Bones.
Does a stiffer material need more or less stress to deform?
More Stress.
What is stiffness?
the amount of stress/the amount of strain.
After a bone fracture heals what % of the height may return?
95% so 5% is lost to plastic deformation.
What happens to bone with compression?
It shortens and widens.
What is a bone deformation without a fracture?
Osteromalacia.
What is a schmorl’s node?
Protrusions of the cartilage of the intervertebral disc through the vertebral body enplate and into the adjacent vertebra
What are the two different types of fractures due to compression forces?
Compression fracture and end plate fracture
Are compression bone fractures typically stable or unstable?
They are stable.
What are 3 times to suspect a traumatic compression fracture?
- Fall on buttock.
- Land on heels.
- Lift a heavy load.
What is spinal percussion how is it done and why?
Tap on SP’s with a reflex hammer as a screen for a fracture. If a fractures has occurred then a sharp pain occurs
What is an avulsion?
Pulling a piece of bone off.
What happens with game keepers thumb?
The thumb is forced into abduction and extension and sprains ulnar ligament and may cuase a small avulsion from proximal phalange
What is the failure mechanism in tension forces on bone?
Debonding and separation of osteons
What is the anisotropic property?
Response of cortical bone is dependent on direction of a tension strength
What are bone fractures like with tensile conditions?
Avulsion.
What occurs with structure of bone under tensile conditions?
It lengthens and narrows
Fractures under tensile stress tend to be stable or unstable?
Unstable.
What type of bone is tensile forces seen in?
Cancellous bone
What are the 3 types of load?
Compression, tensile and shear
In what type of bone is shear stress found in?
Cancellous bone
Are shear stress fractures typically stable or unstable?
Unstable.
What type of loads does shear stress occur with?
Compressive and twisting loads
Shear strain is measured by in what units?
The angle (in radians) created by deformation.
What would cause compression of the spinpous processes in the cervical region?
Hyperextension.
Cortical bone can handle what type of stress more than others?
Compression> Tension > shear.
With bending stress will the convex or concave side be under tensile or compressive stress?
Convex- Tensile. Concave- Compressive.
What is the theory behind 3 point bending?
Fracture occurs at the point of application of the middle force
A boot top fracture happens while skiing where will the fracture occur?
ON the convex side (posterior side) because it is where tensile stress will be and tensile stress is weaker in bones than compressive stress.
What is torsion?
A combination of shear and tensile and compressive stress.
Where does twisting cause shear stress?
Around the axis
Where is there greater shear stress, closer or farther from the axis?
The further from the axis, the greater the shear stress
In what plane does maximum compression and tension stress act on in the neutral axis?
a plane diagonal to the neutral axis
*When a bone breaks under torsional stress what part breaks first and second?
first will be from shear and second will be from tensile.
In a picture of a torsional fracture what will shear and tensile fractures look like?
Shear parallel to the bone. Tensile will travel acrossed the bone.
What are the complications with end plate fractures?
schmorls nodes, may cause loss of disc height, if enough disk height is lost then the nerve root may be compromised in IVF.
What stress can muscle groups produce on bone?
Compression stress
What does the compression caused by muscles on bone do in regards to tensile stress?
Reduce or neutralize the tensile stress of bending
How much stronger is bone with brisk walking than slow walking?
30% d/t faster load with faster walking therefore it needs more protection
What happens to energy storage when speed and load is increased?
The energy storage capacity can double
With increased speed and load it is said that “load to failure doubles” what does this mean?
Doubling the load if the load is applied quickly it can be supported however the same load applied slowly might cause the bone to fail or break
What kind of fracture is caused in low speed fractures?
A stable, single crack with little soft tissue damage
What kind of fracture is caused in high speed fractures?
A comminuted fracture (explosion of bone) that may be unstable with ore tissue damage
What can occurs in regards to the disk in an end plate fracture?
May allow nucleus pulpsis to migrate through cracks in the end plate
With end plate fractures, what occurs with the disk height?
May cause a loss of disc height at that level
If there is loss of disc height due to end plate fractures what can occur?
Nerve roots may be compromised in the IVF
What does repetitive loading due to bone strength?
It causes a decreased strength
When do fatigue fractures occur?
with high repetitions and low load or low repetitions and high loads. AND when fatigue outpaces remodeling.
Microfractures of trabeculae can occur with repetitive loading _____% of maximum failure rate.
20-30%.
When bones fatigue and are under repetitive stress what happens to stiffness and shock absorption?
The bone loses it’s stiffness and shock absorption
What happens to bones with muscle fatigue?
When muscles are fatigued they don’t protect the bones from stress.
How will density of bones change?
Increases with use and decreased with disues.
What are some adaptive changes by bone under mechanical demand? (3)
1) Density changes: increases with use, decreases with disuse
2) Remodeling
3) spurring
What is wolff’s law?
Bone is laid down or resorbed where needed, and load on bone is caused by muscle action or gravity. The greater the body mass the greater the bone mass.
How much bone mass is lost per week with bed rest?
1%.
Over decades, how much reduction will the skeletal mass (trabecular and cortical mass) undergo?
Trabecular = 50% and Cortical = 25%.
How much bone mass will women lose from the 4th decade on?
1.5-2%.
How much bone mass will men lose from the 4th decade on?
About half of women.
What is brittleness?
The quality whereby a material exhibits little plastic deformation before FAIURE. Failure < 5% total elongation of material.
What are some common sites of fracture with age?
lumbar vertebra, femoral head, proximal tibia.
X-rays should be ordered in what 3 cases?
- Osteoporosis.
- Patients over 50.
- Spontaneous fracture or with slight load.
How many people that break their hip can regain their independence?
1/3 of them.
How many people with hip fractures die within the first year?
1/4 of them.
What % of people that survive hip fractures will be able to walk without aid?
50% of them.
What is the second leading cause of admissions to nursing homes?
Broken hips
What % of people that use corticosteroids get hip fractures?
25%of them.
what will the prevalence of vertebral fractures be for asthma patients that use corticosteroids for less than a year ?
11%.
What is a good thing to do to keep bones strong?
Exercise.
What type of cartilage make up the nose and ear?
Elastic cartilage.
Where is fibrocartilage found in the body? (3)
1) annular fibers of the disc
2) iliac surface of SI joints
3) menisci of the knee
Where is hyaline cartilage found?
In synovial joints
What is the purpose of cartilage in joints?
To decrease contact stress on articular surfaces by spreading the load over a larger area. AND to alow movement of articular surfaces with minimal wear and friction.
What is the composition of hyaline cartilage be like?
No nerve, No blood supply, No lymph supply and low density so not visible on x-ray
How fast can cartilage heal and why?
Not fast because there is nothing supporting it like blood, lymph and nerves.
What are the 3 parts of cartilage?
- Solid matrix (collagen and proteoglycan gel).
- Chondrocytes.
- Water.
Of the 3 parts of cartilage what is the largest part?
Water.
What makes up the solid matrix of cartilage?
Collagen and proteoglycan gel.
What is the function of collagen in cartilage?
Provides ultrastructure for cartilage. AND provides tensile stiffness and strength.
What directional pattern do the lines of force move in occurs when cartilage is compressed?
Lines of force, when compressed, move in horizontal pattern
How will collagen be arranged in the 3 layers of cartilage?
Layer1- Random packed and random orientation in planes parallel to the surface.
Layer 2- Random orientation loosely packed.
Layer 3- Perpendicular orientation to anchor bone and fibrocartialge.
What is proteoglycan gel made of?
Protein core, aggrecans (condroitin sulfate, keratin sulfate).
What type of force will proteoglycan gel resists and how?
Compression and the anions on the proteoglycan gel will repeal the other parts of the proteoglycan gel. Cations create a cloud around the proteoglycan gel as they get pulled in by the anions and creates an osmotic pressure.
Are proteoglycan gels hydrophylic or hydrophobic?
Hydrophylic.
Cartilage swells and gets stiff due to what 2 things?
- Repulsive forces of the agrrecan.
2. Osmotic-pressure attracting water.
What happens to water under increasing stress of a joint?
It is pushed out, and produces increased swelling pressure and equilibrium is re-established when the swelling pressures balances the stress.
What part of the cartilage will the water be in?
Most of it is in the superficial zone.
What is creep?
Under a constant load, a viscoelastic material will deform quickly at first and then more slowly over time
What is most of the creep in cartilage due to?
Most of the creep in cartilage is due to the exudation of fluid (partly based on the permeability of the solid phase of cartilage
With creep, how does it work to resist deformation of the cartilage?
Copious exudation of fluid, then exudation slows and finally stops as swelling pressure balalnces the load and the swelling pressure and matrix strength resist futher deformation to a large degree
How does cartilage swell back up once the pressure/load is reduced?
The cations have stayed inside the cartilage and when the pressure is reduced it swells back up with water like a sponge
What are the properties that determine the effectiveness of the biomechanical properties? (2)
1) material properties of the matrix
2) Permeability of the matrix
What is permeability?
Measure of resistance to fluid flow
Does cartilage have low or high permeability?
Low permeability
Under which circmstances does permeability decrease? (2)
1) increased pressure gradient
2) compression deformation which doesn’t allow the last drops of fluid to be squeezed out of the cartilage
What is the function of permeability in cartilage?
Important for nutrition, lubrication and wear to prevent total loss of fluid
Under which circumstances does permeability increase?
In athological conditions due to collagen fiber defects and loss of proeoglycan
What happens to cartilage with high speed loading?
1) insufficient time to squeeze water out of the tissue causing the cartilage to be very stiff and little deformation
What is damping?
An increase in stiffness when a load is applied with increased speed
Is high speed loading due to damping a good or bad shock absorber ?
It is an excellent shock absorber and absorbs compressive shock that would otherwise overload the bone
What happens with the unloading following slow loading of cartilage?
1) Immediate restoration to 90% original thickness via elastic recoil
2) slower restoration via swelling pressure to original thickness
What is lubrication?
Protection from wear and tear
What acts as a lubricant between articular surfaces?
Synovial fluid
What are the two types of lubrication?
Fluid film and boundary
What is fluid film?
A thick film of lubricant that separates the surfaces
When does fluid film lubrication occur?
In physiologic loading and unlike man-made material, there is almost zero friction at the joint surfaces
What are the types of fluid film lubrication? (2)
Hydrodynamic and squeeze film
Where is hydrodynamic lubrication found?
Non-parallel surfaces
What kind of movement and action is hydrodynamic lubrication?
Tangential movement that creates a lifting action, keeping surfaces apart and reducing friction
What is squeeze lubrication?
Keeps surfaces apart and provides a perpendicular force
How is the lubricant held together and prevented from escaping from the surfaces?
The viscosity holds the lubricant together so that it doesn’t escae from between the surfaces allowing it to be sufficient for high loads for short durations
What is a boundary lubrication?
Last resort for cartilage as it is a monolayer of synovial fluid and important in severe loading
What is cartilaginous wear?
The unwanted removal of material from solid surfaces by mechanical action
What are the two types of cartilage wear?
Interfacial and fatigue wear
What is interfacial wear?
Due to interaction of articular surfaces caused by adhesion and abrasion
Is interfacial wear likely in healthy cartilage?
No, because of effective lubrication
What does interfacial wear cause?
Surface defect leading to softing and increased permeability -> fluid leakage -> loss of lubrication -> further wear and degeneration
What is fatigue wear?
Occurs with repetitive cyclic loading and reloading before the cartilage has time to fully reload
What does fatigue wear cause?
Collagen fiber disruption (perhaps due to tesile failure) and rapid high impact loads don’t allow the fluid to redistribute rapidly enough causing high internal fluid pressure at the PG-collagen matrix
What are some high reptitive load examples?
Football player’s knees, ballet dancer’s ankles
What are is an example of loss of muscle balance?
Loss of flexibility that prevents loads from being evenly distributed throughout joint surface and causes a defect that focuses load into one area
What are some disease that can cause matrix destruction? (3)
RA, joint space hemorrhage and collagen disorders
What does immobility do to synovial fluid circulation?
It decrease it
What does degeneration lead to?
Promotes further degeneration yielding deterioration of tensile properties which can lead to progressive breakdown and OA
Is the repair capcity of the body limited or unlimited?
Limited
