Term Test 2: Tissue Mechanics Flashcards
External forces
- Resisted by internal forces
- Can cause deformation of the internal structures of
the body - Cartilage, tendons, ligaments, bones and muscle
Amount of deformation produced
Is related to the stress caused by the forces and the material that is loaded
Mechanical stress
The internal force divided by the cross-sectional area of the surface on which the internal force acts
Three types of stresses
- Tensile
- Compressive
- Shear
Tensile stress
- Axial stress (acts perpendicular to analysis plane)
- Result of a force that pulls molecules apart
Axially loaded
The object tends to deform by stretching or elongating in the direction of the external loads
Compressive stress
- Axial stress (acts perpendicular to analysis plane)
- Result of a force that pushes molecules together
Shear stress
- Transverse stress (acts parallel to the analysis plane)
- Result of a force that pushes molecules past each
other, acting parallel to this plane
Simple (uniaxial loads)
Only produce one type of stress, that is uniform across the whole plane
Bending
An object with greater depth (and more cross-sectional area) is able to withstand greater bending loads
Stresses during bending
Tensile and compressive stresses = lower during bending because they have a larger moment arm
Weight bearing
When the foot is weight-bearing, the load is distributed among several structures
- Bones of the feet bear compressive stress
- Plantar fascia and dorsal muscles bear tensile
stress
Torsion
An object with a greater diameter (greater cross-sectional area) is able to withstand greater torsional loads
Stresses during torsion
Shear stresses are lower because they have a larger moment arm
Uniaxial Tension
- Muscles, tendons and ligaments behave like ropes
or cables - Only carry one type of load; uniaxial tension
Combined Loads
- Bones and cartilage can be loaded in many ways
- Uniaxial tension, compression or shear loads
- Produces uniform stress, bending and torsion loads,
leading to more complex stress
Three types of strain
- Mechanical strain
- Linear strain
- Shear strain
Mechanical strain
Quantification of the deformation of a material
Linear strain
- Change in length
- Result of tensile or compressive stress
Shear strain
- Change in the orientation of adjacent molecules
- Result of molecules slipping past each other due to
shear stress. Quantified as a change in angle
Stress-strain relationship behaviors
- Elastic behaviour
- Linear elastic behaviour
- Plastic behaviour
Elastic behaviour
- Stretches under a tensile load
- Returns to its original shape when the load is
removed (like a rubber band)
Linear elastic behaviour
As the stress increases, the strain increases by a proportional amount (the line on the graph is linear)
Plastic behaviour
When a permanent deformation of the object occurs under a load
Material strength
- Maximum stress (or strain)
- The material is able to withstand failure
- Several qualifications of strength depend on which
function one is interested in
Yield point
Point on the stress-strain curve where further stress will cause permanent deformation
Yield strength
Stress at the elastic limit of a materials stress-strain curve
Ultimate stress
Maximum stress the material is capable of withstanding
Failure strength
- Stress where failure actually occurs
- Stress corresponds to the endpoint of the stress-
strain curve - Failure means breakage or rupture
Four types of materials
- Ductile materials
- Brittle materials
- Hard materials
- Soft materials
Ductile materials
Have large failure strains
Brittle materials
Have small failure strains
Hard materials
Have large failure stresses
Soft materials
Have small failure stresses
Toughness
- Ability to absorb energy
- A material is tougher if more energy is required to
break it - An estimation of the toughness of a material is given
by the area under the stress-strain curve
Elastic Modulus (Young’s Modulus)
The ratio of stress to strain is shown graphically as the slope of the stress-strain curve
Viscoelastic materials
- Material that exhibits both viscous and elastic
characteristics - In the body this is bone, tendon, ligament, cartilage
and muscle
Four properties of viscoelastic materials
- Strain-rate dependency
- Stress-relaxation
- Creep
- Hysteresis
Strain-rate dependency
- The rate at which you deform/strain a tissue will
affect the stress (internal load) it feels - A faster loading rate will create more stress than a
slower loading rate
Stress-relaxation
- When tissue is stretched and maintained at a
constant length (strain), the stress within the tissue
will reduce over time - Eventually, the stress will reach an equilibrium
Creep
If a constant load (stress) is applied to a tissue, it will slowly continue to deform and eventually reach an equilibrium length
Hysteresis
The loading and unloading stress-strain curves of viscoelastic materials will differ. This is energy lost due to heat
Mechanical Properties of Musculoskeletal system
- Muscle tissue and connective tissue
- (bone, cartilage, ligament and tendon)
Muscle tissue vs Connetive tissue
Active elements vs Passive elements
Connective tissue
Composed of living cells and extracellular components
- Collagen
- Elastin
- Ground substance
- Minerals
- Water
Collagen
- Fibrous protein is the most abundant substance
- Muscles of collagen align together to form collagen
fibrils that bind together to form fibres
Collagen Features
Very stiff
- Brittle
- Failure strain of 8% to 10%
- High tensile strength (hard)
- Unable to resist compression because its long
fibres are not supported laterally
Elastin
- Fibrous
- Pliant (soft)
- Extensible
- Ductile
- Failure strain as high as 160%
Isotropic materials
Have the same mechanical properties in every direction
Anisotropic materials
Have different mechanical properties depending on the direction of the load
Connective tissue: activity
- Strength of CT increases with regular use
- Due to an increase in the size of the tissue cross-
section - Inactivity and immobilization result in decreased
strength of tissues and shortening of ligaments and
tendons
Connective tissue: age
CT shows an increase in ultimate strength with age until the third decade of life, after which strength decreases. Bones become more brittle and less tough with increasing age. Tendons and ligaments become less stiff
Connective tissue: bone
- Bones are strongest in compression and
weakest in shear - High tensile strength
- High compressive strength
- Strongest and stiffest material of the
musculoskeletal system
Two types of bone
- Cortical or compact
- Cancellous
Cortical/ compact bone
Found in the dense and hard outer layers of bone
Cancellous bone
Less dense, porous bone that is spongy in appearance and found deep to cortical bone near the ends of long bones
Bone features
Bone is an anisotropic material. Strongest in compression, then tension and is weakest in shear
Bone loading types
- Unloaded
- Tension
- Compression
- Bending
- Shear
- Torsion
- Combined loading
Connective tissue: cartilage
Is able to withstand compressive, tensile and shear loads
Three types of cartilage
- Hyaline
- Fibrous
- Elastic
Hyaline cartilage
Covers the ends of long bones at joints. Collagen fibres are arranged parallel to the articular surface. At the surface of joints, therefore need to handle large compressive loads
Fibrous cartilage
Found within some joint cavities, in intervertebral discs, at the edges of joint cavities and at the insertions of tendons and ligaments into bone
Elastic cartilage
Found in the external ear and in several other organs that are not part of the musculoskeletal system
