Lecture 3 - Tissue Mechanics Flashcards
What is tissue mechanics?
Study of mechanical behaviour or properties of the tissues of the body
Divisions of tissue mechanics
- Hard tissues e.g. bone and cartilage
- Soft tissue e.g. muscle, tendon, ligament, skin and nervous tissue
What does tissue mechanics help us to do?
- Predict thresholds/mechanisms of injury
- Predict effects of disease
- Investigate mechanics of structural disorders
- Develop appropriate finite element models and tissue engineered constructs
- Develop realistic surgical simulations
Components of a mechanical test system
- Sample source, preparation and coupling
- Environmental conditions
- Sensor systems
- Test protocol including sample preconditioning
- Equipment
How to choose test protocol?
- Examine the orientation, direction and magnitude of the applied force e.g. tension, compression, bending, torsion, shear
- Rate of load application
- Preconditioning
- Boundary conditions
- Is cyclic loading or rest periods necessary?
Types of samples
- Live humans
- Human cadavers
- Animal models
- Computer models
Considerations of selected samples
- In vivo vs in vitro
- Degree of dissections
- Age
- Time post mortem
- Health or diseased
- Shape, preparation or mounting of sample
Environmental conditions
- Perfusion
- Hydration
- Temperature
What is hierarchical multiscale modelling?
Involves carrying out modelling at multiple stages in biological life e.g. atomic, molecular, cellular, tissue, organ of organ system level
What is viscoelasticity?
- Resist shear flow and strain linearly with time when a stress is applied
- Exhibit both viscous and elastic characteristics when undergoing deformation
- Materials exhibit a time delay in returning the material to original shape with some energy loss
Properties of elastic materials
Strain when stretched and quickly return to original state once stress is removed
Conditions affecting viscoelasticity
Strain rate, time and temperature
Overview of elastin
- Consists of long, flexible molecules
- Cross linked to form 3D networks
Properties of elastin
- Slight differences in the loading and unloading cycles
- For strains up to 60% remains fairly linear
Modulus of elastin
0.4MPa
Overview of collagen
- 3 stranded helix protein
- At least 20 different forms
- Main constituent of tendons, ligaments and most membranes
Properties of collagen
- Non linear and viscoelastic
- When held at a constant strain, the load relaxes over time
- Key influencer of collagen properties is the extent of cross linking
Overview of cortical bone
- Made with cylindrical osteons/haversian systems with a network of veins and arteries
- Strength in bending and torsion e.g. in the middle of long bones
- High stiffness
- Fracture point at strain >2%
- Withstands greater stress
Overview of bone materials
- Made of HA and collagen
- HA is a strong, stiff material which gives bone rigidity
- Collagen fibers are more elastic and give bone its toughness, also prevents brittle cracking
Effect of osteocytes on bone properties
- Bone contains 13,000 osteocytes per cubic mm
- Form an interconnected network through dendrites
- Communicate with each other and bone surface lining local cells
- Measure strains from fluid flow through the bone matrix caused by tissue deformation
What is Wolff’s law?
- Bone remodels depending on the loading environment
- Bone which has no loading over time decreases in density and strength
- Occurs through the action of osteoblasts and osteoclasts which deposit and remove bone respectively
- Static strains do not lead to adaptive remodelling
- High frequency impact loading induces a greater adaptive remodelling response than low frequency loading
Anisotropy of bone
- Bone is quite anisotropic due to its composite structure
properties vary with age, sex, location and strain rate - Strongest in compression, followed by tension and shear
Implications of bone design on stress
- Compression tends to bend the bones on one side and stretch on the other
- Stresses are greatest at external surfaces at the epiphyses
- Stronger and denser compact bone is
- Medullary cavity which experiences no stress therefore can store things and lighten the bone
Modulus of HA
Tension 165GPA
Modulus of collagen
Tension 1.2GPa
Modulus of bone
Tension 18GPA
- Bone usually experiences only small strains in normal physiology
- Fairly linear elastic within these small deformations
Hierarchy of bones
- At each size scale, the structure of bones influences its susceptibility to freature
- Smaller levels affect intrinsic toughness
- Higher levels impacting extrinsic toughness
Factors affecting material biomechanical properties e.g. ultimate stress, modulus and toughness
Mineralisation, microdamage and the organic matrix
Factors affecting structural biomechanical properties e.g. ultimate load, stiffness and energy to fracture
Bone mass, material biomechanical properties and geometry/architecture
Overview of cancellous bone
- Lattice like framework
- Withstands greater strain
- Strength in compression
- Young’s modulus much greater in longitudinal direction
- Lower stiffness
- Fracture point at strain >75%
Elastic modulus of cortical bone
Tension 11-19GPa
Compression 15-20 GPa
Elastic modulus of cancellous bone
Tension 0.2-5GPa
Compression 0.1-3GPa
Ultimate stress of cortical bone
Tension 107-146MPa
Compression 156-212MPa
Shear 73-82MPa
Ultimate stress of cancellous bone
Tension 3-20MPa
Compression 1.5-50MPa
Shear 6.6+/-1.6MPa
Why are bones curved?
- When loaded in the longitudinal direction, the bone will deform in a predictable direction
- Will bend more when the load is increased
- Curved bones have enhanced ability to differentiate between loading conditions and then alter the osteogenic stimulus according to the magnitude of strain
Why can children’s bones bend before they break?
- Children’s bones are more porous than adults
- Have lower osteoid density than adults
- Haversian canals occupy a larger space in bone of a child
Overview of cartilage
- Articular cartilages lines the surfaces of most joints
- Hydrated tissue which has complex structure
- Chrondrocytes are found in superficial, transitional and deep zones
What makes up a cartilage matrix?
- Made up of cells and collagenous fibres in a fluid matrix
- Mostly water with proteoglycans, whoe interaction with fibres gives rise to complex nonlinear mechanical behaviour
General mechanical properties of cartilage
- Under high rates of loading the cartilage is stiff and protects the bone from harmful high frequency forces
- Under low rates of loading it is not stiff and passes the load onto the bone tissue (causing strain to bone tissue)
Specific mechanical properties of cartilage
- Cartilage/synovial fluid lubrication very efficient
- Coefficient of 0.02
Factors affecting stiffness of cartilage
- Deformation of the collagen matrix and flow through it
- Concentrations of proteoglycans
- Health
- Loading rate
- Direction and location of sample
Overview of ligaments
- Bind joints together
- Provide strength and stability
- Carry only tensile loads
- Poor blood supply so do not heal well when damaged
- Blood vessels are at periphery
What are ligaments comprised of?
Collagen and elastin fibres with interspersed fibroblasts
Mechanical properties of ligaments
- Also hydrated tissues meaning behaviour is also controlled by this
- Nonlinear viscoelastic
- Exhibiting J-shaped stress strain response and creep response
Overview of tendons
- Carry tensile forces from muscle to bone
- Carry compressive forces when wrapped around bone
- Structurally similar to ligaments and muscle fibers
- Often insert over a region via muscle
Important note about tendon, ligament and muscle properties
Often reported in terms of force vs length rather than stress v strain
Mechanical properties of tendons
Typical J-shaped curve with almost linear relationship after toe region
Ultimate stress of tendons
80-120MPa
Failure of tendons
Strains of 8-10%
Overview of muscle
Skeletal (direct voluntary), smooth and cardiac (involuntary)
Active properties of skeletal muscle
Relationship between muscle length and force generating ability
Passive properties of skeletal muscle
Relaxed force length relationship
What is the relationship between muscle length and force?
- As length increases AND decreases, fewer binding sites available to produce forces
- If cell is stretched so myosin and actin no longer overlap, no longer generate force
- If cell is shortened so thin filaments overlap, Z discs contact thick filaments and no more shortening can occur
What is the relationship between binding sites and force?
The greater number of binding sites means more force
What is the resting length of a muscle?
Length the muscle returns to when unloaded
Overview of skin
- Constructed of layers of collagen fibre networks
- Anisotropic
Mechanical properties of skin
- Nonlinear stress strain curve
What occurs in skin during wound healing?
Strength of skin changes and scar tissue is much stiffer due to dense collagen fibres
What is optical elastography?
Using a piezoelectric transducer to evaluate mechanical properties, with the potential to improve differentiation of tissue pathologies
Overview of nervous tissue
- Peripheral nerves are bundles of parallel nerve fibres
- Soft hydrated tissues with collagen fibres and high water conten
- Once again, mechanical behaviour influenced by this flow
What is myelin?
Protein coating wrapped around the nerve fibre (electrical insulation)
Mechanical properties of nervous tissue (brain)
- Nonlinear and visoelastic
- Highly strain rate dependent
What is higher, physical or functional tolerance to deformation?
- Physical tolerance to deformation is higher than functional
- Relevant during mechanical testing because we are only testing physical tolerance
What is stiffer, brain or spinal cord tissue?
Spinal cord stiffer than brain tissue due to highly organised longitudinal fibres
What is laboratory motion analysis and what is it used for?
Used to study tissue mechanics by measuring the kinematics and dynamics of human or animal motion
What are common applications of LMA?
Gait analysis whether abnormal or normal and sports biomechanics
What are the reasons for using LMA?
Improving performance or efficiency, clinical diagnosis or suggestions for therapeutic or surgical interventions
What methods for data acquisition are there?
Video, 3D optical or analogue
What does video acquisition involve?
Points are digitised manually or by attaching reflective markers and digitised automatically
What does 3D acquisition involve?
- Marker only system that uses 2-12 cameras and IR lights to collect 3D coordinates
- Cameras may be optical, IR, x-ray, fluoroescent
- Records motion of skin mounted markers on a moving person OR rigidly mounted markers on a specimen
- Software helps to digitise markers positions and calculate joint angle, displacement, strain fields or limb velocities
What does analog acquisition involve?
Analogue sampling from force platforms, EMG or other devices, and can be integrated with the other two
Limitations of LMA
- Skin to bone motion artefact
- Alternative markers are percutaneous pins but this is invasive and painful if conscious
- Cannot determine individual muscle forces
Applications of body kinematic
- Non invasive joint kinematics
- Analysis of patella tracking
- Comparison of pain against normal biomechanics to track disease progression
- In vivo performance of orthopaedic implants
