6- Tissue Mechanics

Elasticity

The ability of a tissue to return to normal length/shape after stress is removed

Stress

Force applied to any type of tissue

Plasticity

The ability to change based on physiological processes and cellular and histochemical characteristics

Viscosity is _________

Resistance to flow/deformation

Viscoelasticity is ________

ability to lengthen or shorten over time

What does deformation depend on?

Force (timing, duration, amount), environment (temp), Properties of object (shape, length, material)

Force pertains to

Timing, duration, amount

Environment pertains to

Temperature

Properties of object

Shape, length, material

Stress

-measure of an external force acting over the area of an object
-N/m2

Strain

-measure of deformation representing rather relative displacement of particles within a material

What do stress and strain depend on?

Applied force, thickness of material, prior loading/tension

Hooke's law

In elastic material, stress is directly proportional to strain within the elastic limit

Stress-strain curve

The relationship between the stress and strain that a particular material displays

Stress-Strain Curve: Non-linear region (toe region)

Collagen fibers within tissue are wavy or crimped and must be drawn taut before tension occurs

Linear region

measure of the tissue’s stiffness

Elastic region

-Tissue will return to its original length once the force is removed
-Important for ligaments and capsule that provide stabilization

Yield point

-the point at which increased strain will result in only marginalized stress (tension)
-overstretched or overcompressed tissue has experienced plastic deformation
-microscopic failure has occurred and the tissue will remain permanently deformed

Plastic region

Tissue remains in elongated position after force is removed

Ultimate failure point

-tissue partially or completely separates and loses its ability to hold tension

The rate of tissue loading matters for _______ tissues

Viscoelastic

Viscoelastic tissues

-tissues in which the physical properties associated with the stress-strain curve change as a function of time
-Both viscous and elastic properties during deformation

If we apply a stress _______, we don't have to use the same force to get same deformation of tissue

slowly

If we apply a stress ____, we have to provide more force to get the same deformation of tissue

quickly

Slope of the Stress-Strain curve increases when

under tension or compression throughout the elastic range as the rate of loading increases

Stiffness

-a material's resistance to deformation
-Material with a steeper curve
-compliance is inversely proportional to of this

compliance of tissue

tendency of tissue to resume its original position after an applied force has been removed

Resiliency

Material that can resume shape after being stretched or deformed (i.e. long elastic region and short plastic region)

Ductility

Large plastic deformation before failure

Brittleness

Material that exhibits very little plastic deformation before failure

Toughness

-the ability of a material to resist fracture
-large area under stress-strain curve increases

Elastic components

-stiffness vs. compliance
-hooke’s law/young’s modulus

Plastic components

yield point, brittleness, toughness, ductility

Area under curve represents ________

amount of energy that is dissipated during loading due to internal friction

Creep

-constant stress applied to a tissue causes progressive strain
-apply a force, get a change in tissue

Stress-relaxation:
If a Viscoelastic tissue is under constant strain,

the force (stress) will decrease over time

A bone loaded in transverse direction is __________

more brittle

What do bone mechanical properties depend on?

Rate of loading, direction of load, bone designed to resist torsion, bending, and compression, tubular design a benefit

Cancellous bone mechanical properties

-porous structure (decreased density)
-less stiff
-more ductile
-fractures easier with tensile loads

Factors affecting bone integrity

Density, size and geometry

How do bone mechanics change with age?

Mineral quality changes, increased stiffness, decreased ductility, decreased quantity of bone tissue, easier to deform, decreased stiffness of bone density, easier to fracture

Things that are stiffer are ____ compliant

less

Not stiff is _____ deformation and ___ energy

More; less

Mechanics of ligaments

permit movement at joints: elastic region, higher elastin content = less stiffness & strength and more extensibility, yield strain = 5x higher than tendons

Mechanics of cartilage

-higher water content, ECM → collagen stiffness when on tension, ground substance stiff in compression
-compressive stiffness depends on proteoglycan content