Tissue Loading Flashcards
Force
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a push or pull; a load that tends to produce an acceleration of a body in
the direction of its application.
Forces may:
Change a body’s state of motion or cause deformation (Strain) in the body Or Both
Pressure
force per unit area; P=F/A; distributing force over a larger area can help avoid injury; units Pa
Measures distribution of forces acting outside of a body
Center of Pressure (COP)
• Point the represents the
place of application of
the sum all forces acting
on a surface.
NOT NECESSARILY LOCATED AT POINT OF CONTACT
Stress
distribution of force within a body, quantified as force divided by the cross-sectional area over which the force acts. • “Internal” pressure • Stress=F/A • Normal: 10-20 N/cm2
Internal pressure; measures distribution of forces within a body
Strain
amount of deformation with respect to the structure
- Normal (ε):ratio of the change in length (technically doesn’t have units)
- ε = ΔL / L = (l-L)/L
- L – original length, l=final length
- Can be compressive or tensile
Center of Pressure is not the same as center of mass
True
Bending
asymmetric loading that
produces tension on one side of a
body’s longitudinal axis and
compression on the other side
Posterior Pelvic Tilt and Anterior Pelvic Tilt
Anterior Pelvic tilt places greater shear stress on the intervertebral discs, whereas Posterior Pelvic Tilt (neutral, lordosis of lumbar spine) places greater axial loading on the intervertebral discs. The spinal column withstands axial loading much better than shear stress.
Greenstick Fracture
• Incomplete fracture caused by the bending of the bone. • The convex side ruptures due to tensile stress. • Failure on Tension side
In lumbar flexion which
area of the intervertebral
disc will undergo the
greatest compression?
Anterior vertebral disc will undergo greatest compression (posterior will undergo tension)
Torsion
• Load producing twisting of a body
around its longitudinal axis.
• Causes shear stress in the material
With torsion shear stress develops in the material and increases towards the periphery of the cylinder or in this case the intervertebral disc- IMPORTANT
Spiral Fracture
Oblique break due
to torsional loading.
Spondylolisthesis
anterior slippage of a superior vertebra with respect to an inferior one due to excess shear force.
Spondylolysis
Unilateral or Bilateral “scotty dog” fractures of the pars interarticularis (isthmus)
Most common stress felt at ligaments
Tensile stress
Most common stress felt by muscles
Tension
Most common stress felt by bones
Compressive Stress
Most common stress felt at mensici
Compressive stress and shear stress
Mitigating the Risk for Injury
• Maintain upright posture • Maintain a neutral spine • Sustain intra-abdominal pressure during lifting • Reduce moment arm of the load during lifting • Avoid twisting while lifting
Body position in order of
decreasing internal
pressure
- Sitting Slouched
- Standing Learning Forward
- Sitting Erect
- Standing Erect
- Lying Flat
Elastic materials
- Elastic materials: deform instantaneously when they are subjected to externally applied loads and resume their shape when load is removed.
- Stress is a function of strain only. No time dependent behavior
- Materials may exhibit elastic behavior up to certain loads beyond which they will exhibit plastic behavior
Plastic materials
deform instantaneously when they are subjected to
externally applied loads and do not fully recover their shape when load
is removed.
Modulus of elasticity (E)
defines the slope of the linear region of the
stress/strain curve
E = Modulus of Elasticity = Young’s modulus
E = Stress/Strain
Cortical (Compact) Bone
• Compact mineralized connective
tissue
• Low porosity; Stronger
• Found in the shafts of long bones
• Undergoes high level of stress without high degree of deformation/strain
• Has higher modulus of elasticity and can undergo a higher value of stress and strain elastically before deforming plastically after yield point
• More Brittle
• Cortical bone: stiffer, withstand more stress
but less strain before fracture
Accepts large loads, but can not deform much without risking fracture
Trabecular (cancellous)/spongy bone
• Less compact mineralized connective tissue • High porosity • Found in vertebrae and ends of long bones • Strains regularly at low loads • Much plastic deformation before Fracture • More Ductile • Trabecular bone: high strain but low stress before fracture -deforms easily and lower stiffness Area under the curve = energy stored in the tissue as it is deformed.
Ductile
Able to yield at normal temperatures
• Large plastic deformation prior to failure
• Mechanical: steel
• Anatomical: Trabecular Bone
Brittle
Rupture occurs during elastic deformation • Failure without undergoing plastic deformation • Mechanical: Cast iron, glass, stone • Anatomical: Cortical bone
Ductility/Brittleness of Bone and Age
Young Age: More ductile
Old Age: More Brittle
Load Rate (how quickly it takes to get to maximal load):
Young Age: Low Rate of Loading
Old Age: High Rate of Loading
Bone Growth
Longitudinal Growth: • Growth at epiphyseal plate (cartilaginous disc) • Most fuse by age 18 ending longitudinal growth
Circumferential Growth: growth • occurs throughout the lifespan • Putting concentric layers of bone on the shaft • Circumferential growth • Increases bone diameter • Occurs throughout most of the lifespan • Periosteum builds concentric layers of bone. • Bone is simultaneously resorbed on the medullary side.
Epiphyseal Fracture
• Damage to the growth plate area of younger bone <18yrs. • If hyaline cartilage is disrupted bone growth may end. • Can be difficult injury to treat, often with long term effects.
Bone Remodeling
- Occurs continuously throughout life.
- Density and shape of bone change with load. (Wolff’s law)
- Fatigued damaged older bone is resorbed
- Formation of new bone in response
- About 25% of trabecular bone is remodeled yearly
Bone Remodel- Osteocytes
• Osteocytes direct bone
remodeling activity
• Osteoblast deposit new bone
• Osteoclast resorb bone
Bone Hypertrophy
increase in bone mass with stress
Increase in osteoblast activity
Bone Atrophy
decrease in bone mass with disuse
Increase in osteoclast activity
Buccal Exostosis
Extra bone buildup in the gum sockets
Osteoporosis
• Bones become weak and may break from a minor fall or,
in serious cases, even from sneezing or bumping into
furniture
• Lifestyle disease that is dependent on habits
• No clear onset
• Peak bone mass during childhood is important predictor
• Weight bearing exercise in pre-puberty years may help
• Dietary Calcium (dependent on absorption via Vitamin D)
Playing field sports or participating in load-bearing exercise at a young age can help prevent this
Anisotropy
• Phenomenon in which a material (i.e., bone) responds differently to different directions of loading
• Some materials act better under different
directions of load because of the microstructure
of the material
• Loading angle also matters
• More axial the load the greater ability to
withstand the load
Wolff’s Law
Body will adapt to loads under which it is placed
Composition of Bone
- Minerals: calcium carbonate, calcium phosphate (60-70% dry bone weight); primarily provides compressive strength
- Collagen: protein, cable-like (25-30% dry weight); primarily provides tensile strength
- Water: carries nutrients to and waste away (25% of total weight) and adds to compressive strength
- Bone is a composite material
Stress to fracture
Greatest: Shear stress
Middle: Tension
Least: Compression
