Tissue Mechanics Definitions Lecture 1

The extent of deformation of a tissue depends on:

applied external forces, characteristics of the tissue, and environmental factors

Tensile force

normal force

lengthening of the tissue

Compressive force

normal force

shrinking of tissue

Shear force

tangential force

parallel to the joint surface, tissues are not as strong

bending force

tensile force and compressive force acting together, there must be 3 points of contact in order for this force to be in play

torsion force

rotational force acting around a vertical axis

one side will be getting longer while the other is shortening

normal stress

the load or force applied to an object

the resultant force is perpendicular to the surface of the object

normal strain

change in the length of an object or the measure of degree of deformation

shear stress

load or force applied tangential or parallel to an object

when a parallel force is applied to the surface of the object the internal force will act in tangent

elastic region

where the tissue can resist change and will return to resting position once the external force is removed

plastic region

the region where permanent deformation of a tissue begins

elastic limit

point where elascity changes

yield point

elongation can occur without an increase in load and begins the start of the plastic region

strength

load at the ultimate stress point; often just before failure

stiffness

load required to deform the structure a given amount

compliance

measure of the ease of deforming of a specific structure

toughness

area under the curve, defined as work or energy

viscoelastic materal

all biomaterials

not ideal and is a combination of viscous and solid response

hysteresis

change in the stress/strain relationship with elongation and relaxation

lag time for the tissue to return back to 0

rebound resilience

ratio of area under unloading curve to area under loading curve expressed as a percentage

creep

maintain load over a period of time

if you apply load for period of time, the tissue will elongate over time

usually caused by stretching

stress relaxation

maintain elongation over a period of time and observe load changes

constant elongation will reduce stress over time

uncrimping

when collagen fibers go from being wavy to straight with applied stress

stress reaction

when the tissue responds to repeated stresses to make itself stronger

what would happen to the stress/strain curve if “normal” length tissue becomes tight

slope would be steeper and the elastic zone would shift to the left

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

a change in length, 0 would no longer be 0, and a shift to the right would occur

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

permanent deformation and damage to the fibers

how would you increase temperature effrect of the stress/strain curve

rebound resiliance would need to be closer to 1, would lower the hysteresis slope and would begin to behave more elastic