Properties of Bone Flashcards
Functions of the skeleton
- Protect vital soft tissue organs
- Support and maintain posture
- Movement (attachment for muscles, act as levers)
- Mineral storage (calcium and phosphorus)
- Hematopoiesis (red marrow)
Bones are composed of
- Calcium hydroxyapatite
- Water
- Collage
Calcium hydroxyapatite content of bone
- Calcium carbonate and calcium phosphate
- 60-70% of all minerals
Water content of bone
- 25-30%
Role of collagen in bone structure
- Provides flexibility
- Provides strength
- Loss of collagen with agin
Wolff’s Law
- Changes in the form and function of a bone are followed by changes in its internal structure
Application of Wolff’s Law
- Bone “adapts” to the load it is placed under
- Will become stronger with increased load
- Will become weaker with decreased load
Bone adapts to its macro and microarchitecture
- Prevents fragility
- Prevents fracture
Bone changes its shape
- Absorbs compression energy
Bone is light in weight
- Allows for rapid movement
Bone is a dynamic structure
- Porosity can change:
- Aging
- Osteoporosis
- Adaptive response
- Bone adapts to its mechanical environment
Criteria for “ideal” bone
- Resist mechanical loads
- Resist torsional loads
- Permit movement
- Provides a source of calcium
Bones meet criteria for ideal via
- Bone mass
- Geometry
- Tissue material composition
External forces applies perpendicular to bone
- Axial load (along the axis)
- Compression
- Tension
External forces applied parallel to bone
- Shearing or torsional
Axial load effects
- May be applied in compression or tension
- In walking body weight and ground reactive force provide an axial load
Bending of bone occurs when
- Compressive axial load occurs eccentrically
Two types of biomechanical forces on bone
- Stresses or loads
- Strains
Stresses or loads
- Force applied to the outside of a structure
- Ground reactive force
- Body weight borne by the foot
Strain
- Reaction of bone when a load is applied
- Deformation of tissue
- Bone can undergo 0.3% strain without deforming
- Beyond 2% fracture will occur
Mechanical forces on bone
- Compression
- Tension
- Shearing/Torsional
Compression stress
- A force in matter that resists being pushed together
- May be observed as Pressure
- Body weight on the foot bones
Pressure
- Pressure = F/area
- Measured in pascals
Tension stress
- The force in matter that resists being pulled apart or stretched
Tendo Achilles rupture
- “Watershed” area
- ~3-6 cm superior to insertion
- Reduction in both actual number and mean relative volume of vessels
Shearing forces
- Sliding forces
- With walking during contact, forces against the foot parallel to the walking surface
Torsional forces
- Rotational or twisting forces
- Ankle fracture from inversion ankle sprain
Bending forces
- A combination of compression and tension forces
- Greenstick fractures in pediatric patient
- Butterfly fracture
Compression strain (deformation)
- Shortening or “squashing”
Tension strain (deformation)
- Elongation or “stretching”
Shear strain (deformation)
- Displacement or delamination
Tissues react in accordance with Newton’s Third Law
- When a force is exerted it causes an equal and opposite force on the substance acted upon
Tissue reaction to stress is dependent upon
- Innate structural characteristics of bone that resist external loads
Regarding joint stability in general
- The more mobile, the less stable
- The more stabile, the less mobile
Predominant collagen of bone
- Type I collagen
Factors affecting the stiffness needed for lever systems to work
- Nonhomogeneous
- Anisotropic
- Viscoelastic
- Brittle
Nonhomoceneity macroscopically
- Dense outer cortex carries the load
- Loosely arranges network of cancellous bone
Loosely arranged network of cancellous bone
- Distributes the load evenly to the cortex
- Decreases tension and shear to the cortex
- Equalizes compression forces
Cortical bone accounts for
- ~ 80% of all skeletal bone
Role of cortical bone
- Provides strength
- Contributes primarily to the mechanical role of bone
Cortical bone stress endurance
- Can sustain greater stress but less strain before failure
- It is “stiffer”
- Will fail is strain >2%
Trabecular bone accounts for
- ~ 20% of skeletal bone
Trabecular bone characteristics
- Greater capacity to store energy
- Can accept strains up to 75% before failure
Trabecular bone is comprised of
- Highly irregular (anisotropic) trabeculae
Trabecular bone in the foot
- The lesser tarsals
- Second cuneiform has the greatest mechanical advantage for resisting dorsal compression
Commonalities of cortical and cancellous bone
- Similar molecular composition
- Both have extracellular matrix with mineralized and non-mineralized parts
Both cortical and cancellous bone strength is determined by
- Calcium concentration (compressive strength)
- Collagenous proteins (tensile strength)
Long bone function
- Serve as levers
Short bone functions
- Small, cube shaped
- Large articular surface
- Good for shock absorption
Flat bone function
- Provides protection
Irregular bone function
- A variety of purposes
- Example: maxilla
Sesamoid bone function
- Embedded in tendon
- Provide improved mechanical advantage
Long bones are designed to
- Carry loads but remain light
Long bones load
- Loaded predominantly in bending
- Periosteal radius provides structural rigidity
Long bone structure
- Thin layer of cancellous bone on inner diaphyseal wall
- Femoral head is plate-like trabeculae
Long bone characteristics
- Emphasize rigidity over flexibility
- Longer than they are wide
- Possess growth plate on either end
- Hard, compact outer surface
- Spongy cancellous marrow containing interior
Long bone cartilage
- Hyaline cartilage
- Shock absorption and protection
Broad ends of trabecular bone (long bones)
- Distribute force better
Long bone joint surface
- Absorb stress load beneath joint surface
- Transmit that load into the cortex of long bone
Flat bone characteristics
- Anterior and posterior surfaces are compact
- Provides strength
Flat bone center
- Center consists of marrow-containing cancellous bone
- Red blood cell formation
Irregular bone characteristics
- Possess “spring” action
- Primarily cancellous bone
- Thin layer of compact cortical bone
- Vertebrae (rod-like trabeculae)
Sesamoid bone characteristics
- Possess special angulations and curvatures
- They resist compression, tension and torsion
Sesamoid bone shape is determined by
- Mechanical loading and modeling during growth
Nonhomogeneity microscopically
- Lacunae of osteocytes
- Haversian canal system
Haversian canal system
- Direction determines resistance to compression
- More resistant in direction parallel to system
- Less resistant in direction perpendicular to system
Benefits of nonhomogeneity in the calcaneous
- Most dense cancellous bone
- Neutral (silent) triangle
- Dense cortical bone
Cancellous bone of calcaneous is most dense
- Immediately inferior to the posterior facet
- Just proximal to the anterior calcaneal cuboid joint
The neutral (silent) triangle
- Below the lateral talar process
- Sparse trabeculae and osteons
Dense cortical bone of calcaneous
- Roof of the neutral triangle
- Provides strength during gait
Anisotropy
- Having properties that differ according to the
direction of measurement
Different resistance of stresses (anisotropy)
- Compressive stress: very resistant
- Tension stress: moderately resistant
- Shear stress: least resistant
Torsional forces on bone
- One end is twisted clockwise
- The other end is twisted counter-clockwise
- This results in a spiral fracture
- Fracture begins from the bone’s smallest diameter
Different stresses of bending bone
- Unequal stresses
- Tension forces: convex side
- Compressive forces: concave side
- Neutral Axis: point of no stress
When a bone bends on the concave side
- Compression
- Decreases end to end distance
When a bone bends on the conves side
- Tension
- Increases end to end distance
Neutral axis
- No strain when bending bone
- The compressive forces equal the tension forces
Bone is weaker when placed under tension
- It will fail on the tension side
As tension forces increase with a bending force
- On tension side, crack appears
- Neutral axis shifts
- More bone on tension side
- Crack propagates
As tension forces increase with high energy trauma
- Many cracks are formed as energy accumulates quickly
- This bone will shatter
As tension forces increase with low energy trauma
- Simple fractures without fragmentation
Pilon fracture
- Talus is driven into the tibial plafond
- Impaction with comminution
Pilon fracture mechanism
- Axial
- Lower Energy: skiing
- High Energy: MVA, fall from height
The neutral axis of bone
- An imaginary plane inside the bone
- Compressive forces = tension forces
- The further mass is from this axis, the more difficult it is to break
Stress (definition)
- Force applied to a material
- Usually measured in Newtons
- Application of stress (force) may result in deformation
Deformation (strain) in different materials
- An elastic material: full recovery
- Viscoelastic material: creep
Strain is measured as
- A % of the original dimension of the object
Stress-strain curve
- The relationship of the amount of stress applied to the % of deformation
When a force is applied,
- Initial strain is elastic
- When stress is removed, the bone will return to its normal shape
- This will be graphed as a straight line
Young’s modulus of elasticity
- The slope of this line in the elastic region
- It is always linear
Elastic region
- Area under the stress-strain curve
- A measure of potential energy
- Energy produced from deforming forces is not lost
- Returned back into the system as kinetic energy when force is removed (conservation of energy)
Plastic region
- Area under the stress-strain curve beyond the yield point
Yield point
- Stress-strain curve begins to flatten
- Decreased stress load is required to increase strain
- Tissue deformation becomes permanent
- Potential energy is dissipated (usually as heat)
Failure or fracture point
- Material comes physically apart
- Fracture will be easily visible
- Remaining stored potential energy is released suddenly
Consequences of failure or fracture
- Ruptured vessels
- Marked inflammation
- Pain
Viscoelasticity of bone
- Elastic properties
- Viscous properties
Viscous properties of bone
- Totally non-elastic stress-strain curve
- Flows according to the density of the substance
- Remains deformed after a force acts upon them
Young’s modulus changes with the speed the force is applied
- Stress applied very quickly = increased Young’s modulus
- Slowly applied stress = decreased Young’s modulus
Regardless of viscoelasticity
- Bone will still fracture at the same percent of strain
Viscoelasticity with high speed sports
- Allows bones to withstand higher forces
- Increases Young’s modulus
If the high speed force causes the bone to fracture
- More stored potential energy
- Greater dissipation of energy into surrounding tissue
- Greater soft tissue damage
Creep
- A constant force applied in the elastic region
- The longer this force is applied, the greater the strain retained by bone when the force is removed
- Bone will gradually return to its former shape
Creep is time dependent strain (deformation)
- Stress is constant
- Strain accumulates as a result of long term stress
- High levels of stress below the yield point
General characteristics of creep
- Constant load (stress)
- Gradual deformation over time
- Complete recovery over time
Stress relaxation
- Progressive decrease in load with time as the deformation of the structure remains constant
- Strain is constant
- The stress required to maintain this strain will decrease over time
Deformation is maintained with decreasing stress (load) over time. With release of this stress,
- There is an immediate elastic response followed by a gradual return to its original shape
Hysteresis
- Loss of energy in a loading/unloading cycle
- Reflects the dissipation of mechanical energy
Hysteresis occurs due to
- Time delay in returning to its original shape
- Purely elastic materials do not dissipate heat
- Bone is not purely elastic
Brittleness
- A measure of the length of the plastic portion of the stress-strain curve compared to the elastic portion
Brittle (short plastic region)
- Can endure a limited amount of energy loss and deformation
- Bone is brittle
Ductile (long plastic region)
- Capable of withstanding a greater amount of deformation
- Metals are ductile
Brittleness in bone is determined by
- Ratio of hydroxyapetite to collagen
Collagen is ductile
- High ratio of protein to mineral: less “brittle” bones (children)
- Low ratio of protein to mineral: “brittle” bones (elderly)
Highly mineralized bone
- Stiff and brittle
- Area under the plastic phase of the curve is much less
- Less energy required to cause fracture
Relationship between bone stiffness and ultimate strain
- Inverse relationship
