Implant Technology Unit 6a Flashcards
what is the function of bones and its most important mechanical properties
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
when is bone strongest and weakest
strongest - under compression
weakest - under shear forces
what can determine location and mode of fracture
- 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 and how is bone designed to resist bending loads
a stronger and stiffer bone
tubular long bones that allow a larger second moment of area than would be possible for the same amount of bone material in a solid section
under a torsional 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
the polar moment of inertia
why do torsional fractures of the tibia occur distally
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
the proximal third 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
how does the structure of bone contribute to fractures
mid-diaphysis is made of cortical bone, the metaphyses are constructed from cancellous bone.
This is significantly weaker under axial compressive loading and will fail before cortical bone, causing # such as supracondylar and tibial plateau # of the knee.
what fracture pattern will be seen under pure bending loads
In pure bending, the convex side is loaded in tension and the concave side in compression.
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
combo of 2 fracture processes
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 # with # line at about 45 degrees to the axis about which the torque was applied
[# 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
what is bone strength determined by
loading rate
i.e. it is stronger at a higher loading rate than at a lower loading rate
what happens when energy is suddenly released when the bone fails
high energy fracture
normally a comminuted fracture with severe soft tissue damage
what is 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
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 # site = if gap is small it heals, but if large it does not.
- a good blood supply is essential
what are the 2 types of bone healing
primary healing
secondary (natural) healing
what is natural (secondary) bone healing characterised by
callus formation around the fracture site
what is the process of natural healing
formation of callus developing, around the fracture site, from mesenchymal (primitive) tissue then chondroid (cartilage) and then osseous (bone) tissue.
Later, remodelling takes place and the external callus gradually disappears as the bone regains its original strength, shape and internal architecture
what can effect the rate of healing in secondary natural bone healing
depends on the degree of damage and time it takes for a new blood supply to be re-established
higher the energy of the injury the longer healing takes
what is the time frame for most long bone fractures to heal via secondary healing
6 to 12 weeks
[metaphyseal (cancellous bone) # heal in a slightly shorter period]
what can delay fracture healing
if movement is inhibited early in the healing process
what will happen if no blood supply is established at the fracture site
bony union will not take place
may be described as an “atrophic” or fibrous union
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
what is primary bone healing
If there is no relative movement (or micromovement) taking place between fracture fragments during the healing process, the fracture heals without external callus formation
new Haversian systems grow directly across the fracture gap.
how does primary bone healing compare to secondary bone healing
primary is quicker
however, bone does not quickly recover its original strength
therefore, fracture fixation devices that promote secondary bone healing have been preferred in recent years
what impact does physical exercise have on bone
increases in bone density and thickness of cortical bone, therefore increasing its strength and stiffness
[Wolff’s law]
in the early stages of bone remodelling, what does the large callus achieve
the large callus cross-sectional area at the fracture site 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
what does the rigidity of a structure depend on
R = EI
E = young's modulus I = second moment of area
in a callus, how is rigidity maintained
by low Young’s Modulus being compensated for by a higher value of I
what happens to the callus, in later stages of healing
with the increase of strength and stiffness of the callus, its cross sectional area decreases until bone regains its original shape
what type of loading encourages bone healing
loading bone along its long axis
what are 3 possible factors which may explain why movement at fracture site influences bone healing
piezoelectric effects
- Electrical effects caused by moving crystals of hydroxyapatite, which are the basic mineral constituents of bone. Hydroxyapatite is known to be piezoelectric, that is it develops an electric charge when loaded.
hormonal factors
- hormone “substance P” found to be produced at fracture sites
electromagnetic effects
- Electro-magnetic effects produced through electron flow away from the fracture site
Define primary bone healing.
In primary bone healing, a fracture heals without a callus.
what is the aims of fracture management
save life
treat pain
restore function