L7 Introduction to mechanical behavior of biological materials- Biomechanics of Bones Flashcards
How many bones are present in the adult human body?
206
How many bones are present in a new born?
270
Smallest bone in the human body?
Stapes
Stress (σ)
- Define
- Units
- Force applied to deform a structure
◦ Force per unit area
◦ Measured in N/m2 or pascals
◦ σ = F/A
Strain (ε)
- Define
- Units
◦ Deformation caused by applied stress
◦ ε = ΔL/L
Elastic modulus (k)
- Define
- Formula
Stress-Strain Curve
- Stiffness of a material
- k = stress/strain (σ/ε )
Yield point
- Describe
- Up to yield point, structure is in its elastic region
- Past the yield point is the structure’s plastic region
Failure
- Describe
- If the applied force continues past the plastic region, the tissue will eventually fail.
Describe the Stress-Strain Curve
- Stress is on the Y-axis
- Strain is on the X-axis
- The elastic region is mainly linear and is followed by the yield pt, which indicates the initiation of the plastic region
- The end of the curve is failure.
Modulus of elasticity/Young’s modulus
- Define
- Modulus of elasticity is a property of the material,
not of the structure - Slope of the stress/strain plot = Modulus of
elasticity - Can also be called Young’s modulus
Residual strain
- Difference between original length and length resulting from stress into the plastic region
Safety factor
- Engineers usually design structures to be able to withstand 5–10× typical stress on structure.
- Similarly, the stresses placed on biological structures in everyday activities are much less than the
structure can handle.
How many bones are in the spine?
- 32 consisting of 7 cervical, 12 thoracic, 5 lumbar, 5 sacrum, 3 coccyx
If the weight of a person is 200lbs, the weight of bones is
~30lbs
Bones are 15% of body wt.
What are the degrees of freedom?
3 linear - Up/ Down - Right/ Left - Back/ Forward Rotation - Pitch - Yaw - Roll
Functions of the Skeleton
- Leverage*
- Support*
- Protection
- Storage
- Blood cell formation
Which functions of the Skeleton are critical for movement?
- Leverage*
- Support*
Describe protection (related to bone functions)
- Brain
- Internal organs
Describe storage (related to bone functions)
- Fat
- Minerals
Blood cell formation
- Where does it occur?
- (Hematopoiesis)
- Occurs inside cavities of bone
Describe the architecture of bone
- Bone: matrix of inorganic salts and collagen
- Osteocytes
- Osseous
Osteocytes
- Define
- What do they include?
- bone cells
- Osteoblasts: cells that create bone
- Osteoclasts: cells that resorb bone
Osseous
- Define
- What does it include?
- Describe each
- bone tissue
- Cortical: compact, very dense, outer layer, porosity = 5-30%
- Cancellous: spongy, very porous, inner layer, porosity= 30-90%
Types of Bones
Long
◦ Ulna, clavicle, femur
Short
◦ Tarsals, carpals
Flat
◦ Ribs, scapula, sternum
Irregular
◦ Skull, vertebrae
Sesamoid
◦ Patella
Wolff’s Law
Bone is deposited and resorbed in accordance to the stresses placed upon it
- resorption
- deposition
Describe Resorption
◦ Response to decreased stress
◦ Osteoclasts dominate
◦ Disuse, immobilization, microgravity
Describe Deposition
◦ Response to increased stress
◦ Osteoblasts dominate
◦ Weight-bearing exercise
Describe bones & Physical Activity
- Bones require mechanical stress to grow and strengthen. - Loading → Deposition → Increased Density
Which activities provide substantial mechanical stress to bones?
◦ Weight lifting
◦ Running
◦ Swimming
◦ Football
Describe Osteoporosis
- Resorption exceeds deposition
- Increases risk of fracture
- less density – reduction of 10 – 20% every decade after 55 years
- more mineralized
- more brittle
What is osteoporosis related to?
Hormonal factors (i.e., menopause) ◦ Nutritional imbalances (i.e., calcium intake) ◦ Lack of exercise
What are the locations for likely fractures?
Results in fractures during low or moderate impacts: wrist, hip, vertebrae
What is the female athlete triad ?
Disordered eating, amenorrhea, osteoporosis
Bone is both ___ & ___
Ansiotropic and viscoelastic
Anisotropic
- Response depends on direction of load application
Viscoelastic
- Response depends on rate and duration of loading
What are anisotropic properties of bone?
- Compression (best), tension (intermediate), & shear (worst)
- What are the 2 types of materials?
- Describe them.
Elastic
- Linear relationship between stress and strain
Viscoelastic
- Nonlinear relationship between stress and strain
- Hysteresis: energy lost in a viscoelastic material
Describe the types of materials curve
- stress is on the y-axis
- strain is on the x-axis
- energy lost is in between the 2 arrows and energy recovered is underneath the curve
Behavior of viscoelastic tissues is rate-dependent
- If you apply a different rate of loading, you will see different elastic properties.
- Viscoelastic tissues demonstrate creep and stress relaxation
Creep
- progressive strain under constant stress
Stress/load relaxation
- decreasing stress under constant strain
Stress-Strain Curve for Human Bone
- describe how it affects bone
- describe how it behaves past its loading pt
- Bone initially exhibits an elastic response
◦ Deformation in response to loading
◦ Load removed → Return to original shape/length - Continued loading past yield point → Plastic response
◦ Microtears and debonding
◦ Load removed → Permanently deformed
-Continued loading → Eventual fracture
What are 2 ways that bone can fracture?
- sudden increase in rate
- sudden increase in wt
Define strength
- Failure point or load sustained before failure
Failure is caused by:
- Single traumatic event
- Accumulation of microfractures
Assessed by:
- Energy storage
- Area under stress-strain curve
Stiffness
- Modulus of elasticity
- Slope of the load deformation curve
- Bone is flexible and weak
Desc materials in terms of flexible/ strong; stiff/ strong; flexible/ weak; stiff/ weak
Flexible/ strong: fiberglass, silk
Stiff/ strong: steel, iron, gold
Flexible/ weak: spiderweb, BONE, oak, lead
Stiff/ weak: copper, glass
Types of Loading
Compression
◦ Presses ends of bones together
Tension
◦ Pulls or stretches bone apart
Shear
◦ Parallel to the surface of object
Bending
◦ Applied to area having no direct support
Torsional
◦ Twisting force
Describe bone structure
- Made up of osteons/Haversian systems
- Concentric lamellae of mineralized ground substance
note rich blood supply - Note constant dynamic process of remodeling
Desc loading in relation to bone, mm, and injury risk.
- Bone adapts to changes in loading
- High rate of loading → Injury risk
- Muscles apply compression and tension
What do Stress Fractures result from?
- Resorption weakens bone
- Deposition occurs too slowly
- Results from:
◦ Repetitive muscle forces pulling on bone
◦ Muscle fatigue → Reduced shock absorption - 10% of injuries to athletes
Bending
- What does this result in?
- Desc 3 pt bending.
- Results in both tensile and compressive forces
- Greatest stress in the middle of the bone
- Boot-top fracture
Describe the injury threshold.
- load is on the y-axis and repetition is on the x-axis.
- high load & more repetitions = high injury risk
What is cartilage?
- Skeletal tissue–maintains certain shape and form
- Very resilient (bouncy or rubbery), mostly water
- Grows fast–forms embryonic skeleton
Kinds of cartilage
- Hyaline cartilage–most common, found in joints
- Elastic cartilage–epiglottis, ear
- Fibrocartilage–annular fibrosis of intervertebral disk, menisci of knee
