Orthopaedic Biomechanics 1 (10/11b) [Biomedical] Flashcards
Force
push or pull that produces, arrests or modifies movement
Torque
product of a force and its moment arm (EX: rotational force)
Forces and torques are both ___ or ___, and occur simultaneously
internal or external
Load
a force that acts on the body
Externally derived loads — gravity, impact, friction, wind
Internally derived loads — muscle activation, tissue deformation (EX: stretch, compression
Tissue Loading
Joint reaction forces load different tissues
Tissue deform as much as they need to to accommodate stretching or compressing, results in loads arising within tissues
Cumulative loading can lead to eventual injury
How do healthy tissues respond to load?
Can deform but resist change in structure and shape
Internal forces that arise within the structure under load can resist the external forces placing the tissue under the load
Load response is tissue dependent
Tissue Stress
Force or load generated within the tissue to resists deformation, divided by its cross sectional area
A measure of load or energy that stored within a tissue
Pressure = Force (N) /Area (m2)
EX: in a balloon → forces resisting being stretched out/expanded
Tissue Strain
The amount a tissue deforms under a force or load
Usually expressed as a percent change in length (%), distance (mm), although truly a unit less measure
EX: in a balloon → measure of how much the balloon stretches/expands
Stress Strain Diagram - Toe Region
represents taking up slack in the tissue
the nonlinear beginning of the diagram
Stress Strain Diagram - Slope of Linear Region
represents stiffness of the tissue
Young’s Modulus
the linear upward slope of the diagram
Stress Strain Diagram - Young’s Modulus (ε)
represents how much the tissue deforms in response to certain loads (aka stiffness)
High ε = high stiffness
Low ε = low stiffness
This behavior only exists in linear slope region of elastic region
Stress Strain Diagram - Elastic Region
represents elastic deformation energy
Tissue returns to original shape/length after loading
All stored energy is released once tissue is unloaded
dark blue region of the diagram
Stress Strain Diagram - Yield Point
represents the transition between elastic and plastic behavior
Point of no return (aka once you pass this point, you create permanent tissue change)
Additional load results in marginal increase in stress
point where it goes from dark blue to light blue on the diagram
Stress Strain Diagram - Plastic Region
represents plastic deformation caused by micro-failure of tissue under continued load
Overstrained tissue is permanently deformed
Plastic deformation energy cannot be recovered once load is released
Can be good (EX: stretching, weight lifting, serial casting, joint mobilization) or bad (EX: injury)
light blue region of the diagram
Stress Strain Diagram - Ultimate Failure Point
represents point where tissue fails and is unable to hold additional load
point in light blue region where sharp downward slope begins
Biologic tissues exhibit ___-like and ___-like behaviors
fluid-like and elastic-like
Biologic Tissues - Fluid Behaviors
Viscosity — fluid-like component to behavior of tissue, resistance to flow
EX: honey = high viscosity, water = low viscosity
Fluid-like behavior is a time dependent behavior
Slow loading can lead to stretching, fast loading can lead to tear (EX: silly putty)
- in general, not always the case
Biologic Tissues - Elastic Behaviors
Elasticity — ability of material to return to original shape after loading
Does not mean “stretchy”, but rather a measure of how stiff/unstiff it is
Bone, capsule and ligament, muscle, tendon, fibrocartilage, and articular cartilage all respond differently based on ___ ___
their makeup
Tissues in the body are ___
viscoelastic
Creep
continued deformation of a material over time as it’s subjected to a constant load
Prolonged, low load stretch
Time required depends on type of tissue
EX: serial casting → PT will use a cast to hold body in a certain position to force it to adapt to the stretch, and incrementally increase the stretch to improve motion
Ankle Sprain Data
Most common cause is inversion plantarflexion
Most common injury in ankle is ATFL
ACL Strain Data
It is usually a rapid load that causes an ACL tear
In vitro testing — in the lab testing, outside of the living organism
In vivo testing — in a living organism
In silica — completely computer modeled
Factors that Impact Musculoskeletal Loads
Loading Magnitude
Loading Rate
Loading Type
Loading Magnitude
how much the tissue is loaded
High vs Low Load
Cumulative load — you can keep increasing the yield capacity or reach yield capacity faster since you are starting at a higher load due to cumulatively loading or getting stronger
Loading Rate
the speed at which a tissue is loaded
Loading rate is a function of time
Biologic tissues are sensitive to the rate at which they are loaded
Tissues behave differently under different loading rate conditions
Fast or slow loading rates lead to different injuries
EX: loading in the ACL
- Fast/high velocity → leads to complete ACL rupture
- Slow/low velocity → leads to intact ACL with avulsion of tibial eminence (ligament pulls away from tibia)
Loading Type
the way in which the tissue is loaded
Tension, Compression, Bending, Shear, Torsion, Combined
Loading Type - Tension
two forces pull on an object in different directions
EX: lateral ankle ligaments are severely tensioned as the foot rotates inward
Loading Type - Compression
forces that push/pull surfaces of objects together, or brings the end of an object closer
EX: humerus is pulled against the glenoid by the deltoid muscle creating a compressive load between the bones
Loading Type - Bending
deformation tissue that occurs at right angles to its longitudinal axis
Concave side undergoes compression load
Convex side undergoes tension load
EX: coxa vara results in increased bending load on neck of the femur
Loading Type - Shear
unaligned parallel forces that move on part of a body in one direction and another part in the opposite direction
EX: cam type femoroacetabular impingement creates abnormal shear load between the femur and acetabulum, causing delamination (peeling off articular cartilage)
Loading Type - Torsion
twisting force applied to tissue around longitudinal axis
EX: noncontact ACL rupture
Loading Type - Combined
multiple types of loading interacting on the tissue
Factors That Influence Tissue Ability to Accept Loads
Age
Disease/trauma
Overuse
Underuse
Tissue Loading Ability - Age
cells in tissues change as we get older, which dictates tissue response
(EX: osteoporosis)
Tissue Loading Ability - Disease/Trauma
soft tissue disorders influence genes that code for the protein of collagen, if this is impacted it affects elasticity of tissues
EX: Ehlers Danlos syndrome where tissues are overly stretchy and lack ability to return to normal shape
Tissue Loading Ability - Overuse
when loading of the tissue exceeds the repair capacity of the tissue, leads to plastic deformation
Can cause microtearing which changes tissue, you may give time to rest but not enough time for tissue to go back to its baseline
EX: tendonitis
Tissue Loading Ability - Underuse
when you don’t load tissue, it starts to lose its ability to accept loads
Changes, like loss of bone or muscle mass, can occur over time
Bone grows in response to stress — if you don’t load them, they will get weak
