Implant technology - unit 6A deck 1 Flashcards
what is the function of bones and therefore its most important mechanical properties
Functions:
- protect and support internal organs
- carry load
- enable locomotion
==> strength and stiffness
bone is anisotropic, what does this mean
that it displays different mechanical behaviour under different types and directions of loading
Rank the following in terms of which bone has the greatest resistance to (from strongest to weakest):
- Shear
- Tension
- Compression
Compression > Tension > shear
What can the location and mode of fracture be determined by ?
- geometry and structure of the bone
- loading mode, such as compression, bending, torsion
- loading rate i.e. how rapidly the load is applied
in tension and compression, what is the stiffness and load required to cause failure proportional to
- cross sectional area of the bone
- the larger the area, the stronger and stiffer the bone
under a bending load, what affects the bones mechanical behaviour and what is the quantity that takes into account these two factors
the cross-sectional area and the distribution of bone tissue around a neutral axis
second moment of area (area moment of inertia)
what does a larger second moment of area mean
a stronger and stiffer bone
How does the shape of long bones help resist bending moments in all directions ?
These bones have a larger second moment of area than would be possible for the same amount of bone material in a solid section because much of the bone tissue is distributed at a distance from the neutral axis.
what will happen if there is excessive movement at the fracture site
- cartilage rather than bone cells is laid down
- if there is a lot of movement a false joint (or pseudoarthrosis) may form between rapidly proliferating cartilage cells at either end - gives what is called an “elephant foot” appearance
under a torsional load, what affects the bones strength and stiffness and what is the quantity that takes into account these two factors
- the cross-sectional area and the distribution of bone tissue around a neutral axis
- the polar moment of inertia
As the polar moment of inertia increases what happens to the strength and stiffness of the bone ?
It increases
When the tibia is placed under torsion, why do torsional fractures of the tibia occur distally rather than proxially
Although the proximal section has a slightly smaller cross sectional area than the distal section, it has a much higher polar moment of inertia as much of the bone tissue is distributed away from the neutral axis
The magnitude of the torsional shear stress in the distal section is therefore approximately double that of the proximal section.
where would the fracture be in the fibula if it occurred with a fracture of the distal tibia under torsional loads and why
the proximal 1/3rd of the bone i.e. much higher - as fibula does not have same geometry as the tibia, and the fracture will occur at the weakest point
The structure of bone is another important factor in how fractures occur, if a long bone is placed under axial compressive loading where will it fail and why?
The cancellous bone in the metaphyses will fail before the cortical bone comprising of the mid-diaphysis, this is because cancellous bone is weaker under axial compressive loading
This results in fractures such as supracondylar and tibial plateau fractures of the knee
what fracture pattern will be seen under pure bending loads
The convex side will fail first in adults since the bone is weaker in tension than in compression, in children the concave side would fail first.
This loading usually results in a transverse fracture pattern
what fracture pattern will be seen under pure compression loads
Oblique
what fracture pattern will be seen under bending loads superimposed on axial compression
This results in a combination of the 2 fracture processes occurring:
- Bending produces a transverse crack on the tension side of the bone, while compression results in an oblique fracture.
- Under the combined load, as the bone deforms, the protruding oblique surface impacts the other surface.
- The result is the characteristic “butterfly segment”, which occurs on the compressed side of the bone
what fracture pattern will be seen under torsional loads
- spiral fracture with fracture line at about 45 degrees to the axis about which the torque was applied
[fracture line results from failure of the bone in tension, perpendicular to the crack]
what loading most commonly causes a long bone fracture
combo of more than 1 type of loading rather than a pure single loading mode
What is bone strength partly determined by ?
The loading rate i.e. it is stronger at a higher loading rate than at a lower loading rate
When a bone is loaded to failure by impact its energy absorption capacity can be how many times higher than if loaded slowly ?
2 times as high
When a bone is loaded by high energy impact resulting in failure what type of fracture does this result in ?
Comminuted fracture - caused by high energy being released from the bone and results in serve accompanying soft-tissue damage
(recall bone has a higher energy absorption capacity when impacted)
List the steps of the fracture process
- Energy delivered to the limb
- Energy transferred via the soft tissue to the bone which absorbs the energy
- Bone breaks and energy is released back to the soft tissues
- Broken bone and damaged soft tissues bleed and cause a build up of blood around damaged area; called a haematoma
- Acute inflammatory response occurs around damaged area which causes pain to the victim and commences process which lead to healing
Think EEBBA
what are 4 rules about bone healing
- bone will heal naturally if broken
- movement does not inhibit fracture healing, it encourages it
- bone “appreciates” a gap at the fracture site = if gap is smaller than the critical gap size it heals, but if largeer it does not.
- a good blood supply is essential
in the early stages of bone remodelling during fracture healing, what does the large callus achieve
- It increases the cross-sectional area in that part of the bone which significantly increases its second moment of area.
- This gives structural support that compensates for the lower strength and rigidity of the material of the callus.
In the callus, rigidity is maintained by the low Young’s modulus being compensated for by a higher value of I (see pic)
