Intro to Tissue Mechanics

Tensile and compressive forces are … and known as … forces

colinear, normal

Shear forces are applied … to the object and are known as … forces

parallel, tangential

Tensile and compressive forces are applied … to the object

Perpendicular

Stress is measured in…

Newtons/meters² or Pascal (Pa)

σ (sigma)

Normal stress

Normal strain

The ratio of change in length from the resting length

ε (epsilon)

Normal strain

Normal strain =

Δx/x

Shear stress

Results from a force applied parallel to the surface of an office, with the internal forces acting in tangent.

τ (tau)

Shear stress

Formula for shear stress when the intensity of the force is uniform across the surface of the object

τ=F/A

Φ (phi)

Relative displacement of an object

γ (gamma)

Shear strain

Average shear strain formula

γ=Δx/h

Strain

percentage of stretch past the resting length

Elastic region

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

Plastic region

Where there is long-term changes to the tissue

All tissues have a…, which is where the tissue will break/tear

Failure point

Yield point

separates the elastic and plastic regions

U

Ultimate strength, the highest point on the stress/strain curve

R

Rupture or failure point

Y

yield point, elongation can occur without an increase in load, beginning of the plastic region

P

Proportionality limit, between the origin and this point the relationship between stress and strain (slope) is linear

Young’s modulus

When the relationship between stress and strain is linear, between the origin and proportionality limit

O

Origin, area with no load and no deformation

Strength

Load at ultimate stress point, often at or just before the failure point

Stiffness

Load required to deform the structure a given amount, equal to load/elongation, a ratio specific to each sample specimen, represented by the slope of the stress/strain curve

What the slope of the stress strain curve represents

The compliance and stiffness of a tissue

Compliance

Measure of the ease of deforming of a specific structure, reciprocal of stiffness

Reciprocal of stiffness

Compliance

Reciprocal of compliance

stiffness

Toughness

How difficult it is to create a change in elongation, defined as work or energy, represented by the area under the curve

What is represented by the area under the curve on a stress/strain diagram

Toughness

E=

σ/ε (slope, stress over strain)

A large E value indicates that a material is…

less flexible and more stiff

A small E value indicates that a material is…

more flexible and less stiff

E is usually a large number because the … is small

Strain

Solids are everything but…

fluids

Flow is created by…

Water content

Elasticity

The ability of a material to return to starting size and shape when the load is removed, time independent, a predictable, linear relationship

All biomaterials are…

Viscoelastic and not ideal

Viscoelastic materials are dependent on

Time and rate

Viscoelasticity has characteristics of both a … and …

Fluid, solid

A continuously applied force on a fluid body will cause a…

continuous deformation, known as flow

Viscosity

The quantitative measure of resistance to flow

Three viscoelastic material mechanical properties

Hysteresis, creep, and stress relaxation

Hysteresis

Change in the stress/strain relationship with elongation and relaxation

Area within the loop of a loading/elongation curve

Energy dissipated as heat during the process of elongation and relaxation

Rebound resilience

Ratio of area under unloading curve to the area under the loading curve expressed as a percentage, a ratio of 1 indicates a purely linear elastic material

Rebound resilience is impacted by…

repetitions and temperature

When hysteresis is reduced…

the response of the tissue is quicker and becomes more ideal

A decrease in hysteresis implies…

A decrease in stiffness

Creep

Change in elongation or strain over time as a load is applied and maintained over time

Most changes in creep occur in the first…

10 minutes in tissues other than ligaments (in which most change occurs in the first 2 minutes)

Cyclic creep tests

Mimic athletic activities to see how viscoelastic materials fatigue, the hold is short but up to 100 cycles would be performed

Tendinitis is an example of…

fatigue failure

Stress relaxation

Maintain elongation over time and the load will gradually decrease over time (hours)

Uncrimping

When collagen fibers go from being wavy to straight with applied stress (toe region)

Stress reaction

When a tissue responds to repeated stresses to make itself strong (Wolff’s Law)

Poisson’s ratio

Lateral strain/axial strain present under uni-axial load either compression or applicable within elastic region

v (nu)

Poisson’s Ratio

v=

change in lateral strain/ change in longitudinal strain

Range of v values for biomaterials

0-0.5

As v (nu) approaches 0, structures are…

resistant to change in shape

As v (nu) approaches 0.5, it is more… and can …

viscous, adapt to external stresses more easily

Single insult

One large blow takes a tissue beyond the plastic range, the strength of the material has been breeched, also known as a macrotrauma

Fatigue failure

Repeated low-level insults, also known as microtrauma

Microtrauma can put a tissue at higher risk of developing a …

Macrotrauma

Shatter fracture

e.g. glass, comminuted bone fracture, poor prognosis, high impact load

Crack propagation fracture

results more from fatigue failure, e.g. shredding of tissue, stress fracture

Concerns for fracture mechanics

Number of defects present, size of defects, location of defects

Examples of natural stress concentrations

tissue interfaces (ligament to bone, tendon to bone, etc.)

In children and elderly, bone is often weaker than ligaments, which results more frequently in … than in middle aged adults

avulsion fractures

4 types of tissue in the body

Connective, muscle, neural, epithelial

Connective tissue types

Irregular connective tissue (loose and dense), regular connective tissue (dense), cartilage, adipose tissue, haemopoietic tissue (structures in blood, e.g. plasma), blood, bone

All connective tissue has these components

Extracellular matrix, collagen, elastin

ECM components

proteoglycan, glycoaminoglycans (GAGs), water

Ground substance components

Proteoglycans and glycosaminoglycans (GAGs)

Ground substance is a component of the…

ECM

Elastin

a protein that provides elastic recoil

Highest concentration of elastin is in the…

ligamentum flavum

Anisotropic

not symmetrical in how it resists things

Collagen fibers offer little to no resistance against…

compressive loads

Collagen fibers are only good at resisting tension along the…

length of the fiber

Type I Collagen

most common, found in all types of tissue, thick, stiff, strong, found in mature scars, most abundant in bone, tendons, and ligaments

Type II collagen

Thin support, hyaline cartilage, 90-95% of collagen in cartilage, good for compressive forces

Type III Collagen

Thin filaments that make tissue stong yet elastic, found in fresh scars, skin, blood vessels, uterus, GI tract, initially secreted by fibroblasts during wound healing and is the reabsorbed and converted to type I

As you age, your skin converts from type … to type … to collagen

III to I

The relationship between diameter and strength of collagen is…

direct

Flexibility of collagen increases with…

thread numbers and lower diameter size

Bonds in collagen (hydrogen and glycine) … over time, and are sensitive to an increase in heat

increase

Most collagen fibers run parallel, but ligament will have more … compared to tendons

Cross fibers

Creation of collagen requires

vitamin C

3 procollagen helices create

tropocollagen

Tropogcollagen composes

collagen fibrils

Fibrils compose

collagen fibers

Collagen is synthesized by

fibroblasts

Proteoglycan aggregate binds most of the …, which makes the structure gel-like.

extracellular water

Decorin contains

dermatan sulfate

Decorin is involved in…

collagen synthesis and development