Lecture 1: Introduction to Tissue Mechanics (copy)

Discuss Internal vs. External Forces.

Internal Force

• Produces from muscle (tendon), ligaments, skin, and fascia

External Force

• Produced from gravity, external resistance (weights, bands, etc.), friction, water (drag)

Force that are applied at a distance away from an axis will produce _______.

Torque

Forces are a vector, meaning that they must have…

• Direction

• Magnitude

What type of lever is the most common in the human body? By having this type of set up, what is this lever good for and what isn’t it good for?

Most Common in Human Body:

• 3rd Class Lever System

  • Axis —> Int. Torque —> Ext. Torque

Good For: Speed / Range of Motion

Bad For: Strength / Production of Force

What is the “pull?”

Tension: Muscle contracts and tendon lengthens

What is the “push"?”

Compression: Muscle contracts around the joint without moment arm (no movement)

What are the different types of force application?

• Parallel

• Perpendicular

• Oblique

What does it mean if the force application is parallel?

• Means the forces will be summed

• Direction matters

  • If forces in same direction —> add

  • If forces in opposite direction —> subtract

What does it mean if the force application is perpendicular?

• The forces will be summed to move into a different direction

What does it mean if the force application is oblique?

• Forces enter at an oblique angle need to have component parts:

  • Y-component: the rotational force or ‘normal’ force

  • X-component: the compressive or distractive force; the ‘tangential’ force

Which of the types of force application does the human body function at?

Perpendicular Force

Can oblique forces change during movement?

True / Yes

Biological tissues are…

Deformable bodies (not rigid)

What is static equilibrium?

Means all the forces are balanced in all directions

If an object is in static equilibrium, how/when will it change shape or deform?

It will deform when it is acted on by an unbalanced force

What factors determine the extent of deformation?

• External Force

  • Magnitude, direction, and duration

• Characteristics of tissue being deformed

  • Material properties (e.g. elastic vs. solid)

  • Size and shape of object

• Environmental Factors

  • Temperature / Humidity

What are the different types of forces?

• Compressive

• Tensile

• Shear

• Bending

• Torsion

What type of forces does the human body dislike?

• Shear

• Torsion

• Bending?

What forces are related to moments and torque?

• Torsion

• Bending

Which force(s) are colinear and known as Normal Forces?

• Tensile

• Compressive

What force(s) are parallel to the object and known as Tangential Forces?

Shear

With a bending force, what is being compressed and what is receiving tension?

• Concave side is being compressed

• Convex side is receiving tension

With a torsion force, what is being compressed and what is receiving tension?

• Inner segment is being compressed

• Outer segment is receiving tension

What is normal stress (σ)?

• Force per cross sectional area (load)

• Resultant force is perpendicular to surface of an object

• Normal Stress = force/area of the object

• Measured in Newtons/meters² or Pascal (Pa)

Does the size of a material affect the overall amount of force/stress it can resist?

True

What is strain?

The measure of the degree of deformation

What is normal strain (ε)?

• Normal strain = ratio of change in length from the resting length

• ε = change in length (Δx/x)

• Reported as percentage of change

‘Positive’ strain is…

Tension

‘Negative’ strain is…

Compression

What is shear stress (𝜏)?

• If a force is applied parallel to the surface of an object, then the internal forces are acting in tangent, meaning it is shear stress (𝜏)

• Reflects the change by components of forces parallel to cross-sectional area

  • 𝜏 = F/A

  • essentially the same as sigma, just force shear/tangential force)

How do you know if you should use tau (𝜏) or signma (σ)?

If you are applying a shear force —> tau (𝜏)

If you are applying a normal force —> sigma (σ)

What is shear strain (γ)?

• Occurs due to distortions caused by shear stresses

• Relative displacement of the object caused by shear stress is Δx

• γ = Δx / height (h)

What is the relationship between stress and strain?

Directly related; ↑ stress = ↑ strain

Stress-Strain Curve

What is stress (for stress-strain curve)?

The internal resistance to change, which is the amount of external force applied (N/m²)

What is strain (for stress-strain curve)?

The percentage (%) of stretch past the resting length

What is the elastic region of a stress-strain curve?

Where the tissue can resist change and will return to resting position once the external force has been removed

What is the plastic region of a stress-strain curve?

Where there is long-term changes to the tissue

What is the failure point of a stress-strain curve?

Where the tissue breaks / tears

Do all tissues have a failure point?

Yes, every tissue has a failure point

What is the yield point of a stress-strain curve?

• Marks the start of the plastic region (between the elastic and plastic region)

• This is where you feel resistance in the tissue

• Elongation can occur without an increase in load

What is the origin of the stress-strain curve?

Area with no load and no deformation

What is Young’s Modulus?

Refers to the area of the stress-strain curve where the relationship is linear (also known as the proportionality limit)

What is the highest point on the stress-strain curve referred to as?

• The highest stress point

• Refers to the ultimate strength of the material (σu)

What happens if stress is repetitively applied at the yield point of a stress-strain curve?

Injury (Stress Fx = Inflammation)

• Eventually, the tissue will fail (push over yield point) due to continuous application of stress over time

What happens if a “slow” stress is applied?

Increase in stretch

• This will move the yield point

• Increase in strain = therapeutic

What cues do we use for stretch intensity?

1. Pain (use as a guide)

2. Resistance (take them to yield point unless they have pain)

What are the different structural properties of tissues?

• Strength

• Stiffness

• Compliance

• Toughness

What is strength?

Load at ultimate stress point (often at or before the failure point)

What is stiffness?

• The load required to deform the structure a given amount (load/elongation)

• Stiffness is a ratio specific to each sample specimen

• SLOPE of the stress/strain curve

What does a steep slope in a stress-strain curve mean?

Steep Slope = Stiffer

What is compliance?

Measure of the ease of deforming of a specific structure (aka, the reciprocal of stiffness)

What does a shallow slope in a stress-strain curve mean?

Shallow Slope = Less stiff / More compliant

What is toughness?

• The area under the curve; defined as work or energy

• How much energy can the tissue withstand before it deforms

• How difficult is it to create a change in elongation?

What does a large E mean?

Small strain

What does a small E mean?

Large strain (meaning a gentler slope and the tissue is more flexible)

What is a solid?

Everything but fluids; has various stress/strain relationships (e.g., steel alloy is ductile where glass is brittle)

When consider ideal materials, what is the different between bone and collagen?

Bone: strong and brittle

Collagen: weak and ductile

What is elasticity?

A property of solids; perfectly linear stress/strain relationship

• The ability of a material to return to starting size and shape when load is removed

• Time independent

• Predictable relationship (F = kd)

What is viscoelasticity?

High liquid content (usually water) resulting in fluid flow when a material is loaded; also has some properties that are more “solid like”

• “Flow” is created by water content (which means you also have resistance of flow which is viscosity)

  • More viscosity = more stiff

What type of ideal material are biomaterials (biological tissues / tissues within the human body)?

• Viscoelastic material

  • Not “ideal” in the sense of material properties being predictive; BUT these materials serve our bodies well

  • Combination of viscous and solid (elastic) response

  • Time dependent and rate dependent

Viscoelastic materials are dependent on what?

Time-Dependent (how long? / length of time that stresses are applied and how it effects it)

Rate-Dependent (how often? / how often the stresses are applied)

A continuously applied force on a fluid body causes…

Continuous deformation (known as flow)

• Viscosity is the property that is the quantitative measure of resistance to flow

What is hysteresis?

Change in the stress/strain relationship with elongation and relaxation

• Essentially, it doesn’t go back to normal shape at the same rate; there is a “lag”

With hysteresis, is more or less energy absorbed during loading than is released during unloading?

More/greater energy is absorbed during loading

With hysteresis, the area within the loop represents..

The energy that has dissipated as heat during the process

• This “lag” is rebounding in hysteresis

What is rebound resilience?

• The area within the loop of hysteresis

• It is the ratio of area under unloading curve to area under loading curve expressed as a percentage

• When ratio = 1, it is purely linear, elastic material

• Greater the “lag” (or smaller the ratio), the more energy that is dissipated as heat

What factors have an effect rebound resilience?

• Number of repetitions

• Temperature

If hysteresis is reduced, then the response of the tissue is ______ and would become more _____.

Response of the tissue is quicker and would become more ideal

A decrease in hysteresis implies a decrease in ________.

Stiffness

What is creep?

Maintain load over time and observe reaction

• Change in elongation or strain over time —> tissue elongates

• (Constant stress)

• The position where you apply force changes but force does not change (i.e., stretching example in class)

Within how long does creep occur? How does this differ with ligaments?

• Creep / Change occurs mostly within first 10 minutes

• Ligaments, it occurs mostly within 2 minutes

What is a cyclic creep test?

Mimics athletic activities better (hold is short, but up to 100 cycles were be performed

• It examines fatigue characteristics of viscoelastic materials

What is stress relaxation?

Maintain elongation over time and observe the load changes

• “Opposite of creep”

• Constant strain and load will decrease over time

• Takes 6-8 hours for it to occur in most biological tissues

What is uncrimping?

When collagen fibers go from being “wavy” to straight with an applied stress (occurs in Toe Region)

What is stress reaction?

When the tissue responds to repeated stresses to make itself strong (i.e., Wolff’s Law)

• Very important with resistance training in tendons

• e.g., someone goes from NWB —> WB, this will cause bone growth (positive); could be negative if we get osteophytes

What protective responses to stress do tissues have?

• Uncrimping

• Viscoelasticity

  • Elastic Deformation

  • Creep

  • Stress Relaxation

• Stress Reaction

What would happen to the stress/strain curve if “normal” length tissue becomes “tight” (i.e., tight hamstrings)?

• The stress-strain curve would shift to the left (closer to y-axis / zero)

• Stiffness / Slope increases

What would happen to tissue if repeated stresses are performed up to but not beyond the yield point?

• Potentially nothing; tissue goes back to original length since still in elastic region

• Stress reaction; tissue gets stronger (or could be negative stress reaction)

• If viscoelastic —> hysteresis becomes more ideal due to heat being released

What would happen to the stress/strain curve if a prolonged stretch was applied beyond the yield point?

• Elongation of tissue that is either adaptive or maldaptive

• Plastic deformation (permanent deformity of resting length, etc.)

• Stress-strain curve would shift to the right

• Max strength shifted up and to the right (yield point)

How would an increase in temperature effect the stress strain curve?

• Hysteresis is minimized and the tissue behaves more ideal

• More compliant tissue (more shallow / less steep slope/stiffness)

• Ultimate strength does not change

What is Poisson’s Ratio?

Lateral strain (width) / Axial strain (height)

• Ranges from 0 - 0.5 for biomaterials

• Present under uni-axial load either compression or applicable within elastic region

If a material/structure has a Poisson’s Ratio close to 0, what does this mean?

• Structure has highly bonded atoms

• Resistant to change in shape

• Structure will break before deforming

• (e.g., bone = .13-.3, concrete = 0.1)

If a material/structure has a Poisson’s Ratio close to 0.5, what does this mean?

• More viscous material

• Has atoms that are moving freely

• Adapts to external stresses more easily

• Shape changes but volume does not

• (e.g. rubber)

Fracture ____ and _______ are important to the rehabilitation of tissue and prognosis

Fracture type and location (this tells us the force and direction of impact)

What are the methods of fracture?

Single Insult (Macrotrauma)

• One large blow takes tissue past plastic range

• Strength of material is breached

• To change fracture point you would need to increase the strength of the material

Repeated Low-Level Insult (Microtrauma)

• Cyclic loading, especially in creep leading to fatigue failure

• To change the fracture point, more endurance is needed (not strength)

What are the different types of fracture?

Shatter: comminuted bone fracture (e.g., glass)

• Prognosis is poor; high impact load

Crack Propagation: more from fatigue failure

• Shredding of the tissue; bone stress fracture

Both types of fractures are exacerbated by tissue defects — causes tissue to break with less force (single insult case) or with less cycles

What are the key concerns for fracture mechanics?

• Number of defects present

• Size of the defects

  • Narrow gaps or wide gaps in tissue

  • Longer and wider cracks lead to more damage

• Location of the defects

  • Location of defect relative to highly stressed portions of structure

  • e.g., defect in femoral head is more concerning than a defect near gluteal tuberosity

Tissue defects can lead to…

Stress concentration areas

• Uneven distribution of loads that is concentrated on the defect

• Higher load per ara (stress)

Where do natural areas of stress concentrations occur?

Tissue interfaces (e.g., ligament to bone; tendon to bone; etc.)

Is an avulsion injury or midsubstance tear more common in children? Why?

Avulsion injury more common in children

• Bone is still cartilaginous, meaning it is weaker than collagen of ligaments

Is an avulsion injury or midsubstance tear more common in adults? Why?

Midsubstance tear is more common in adults

• Collagen is weaker than the osseous bone

What are the four types of tissues in the body?

1. Connective Tissue

2. Neural Tissue

3. Muscle Tissue

4. Epithelial Tissue

What are different types of connective tissue?

• Irregular connective tissue

  • Loose and dense

• Regular Connective Tissue

  • Dense

• Cartilage

• Adipose tissue

• Haemopoietic Tissue

• Blood

• Bone

What is the make up of connective tissue?

Extracellular Matrix (ECM)

• Proteoglycan

• GAGs

• Water

Collagen

Elastin (provides elastic recoil of the tissue)

What is a proteoglycan?

Protein polysaccharide molecule with protein core and sulfate chains attached

What are proteoglycans made of?

Made of hylauronic acid (with link protein chains with GAGs attached)

Hylauronic acid has a high concentration of…

Viscoelastic materials

What is the function of proteoglycans?

Stabilize collagenous skeleton of tendons and ligaments and contributes to overall strength

What are the three different types of GAGs?

• Aggrecan

• Biglycan

• Decorin