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
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
Describe the process of secondary (natural) bone healing
- Weeks 0-2: The haematoma is invaded by macrophages which absorb dead and damaged tissue/cells
- Weeks 2-6: new capillaries grow into haematoma bringing with them fibroblasts, fibrin and bone forming osteoblasts. The periosteum begins to regenerate and grow.
- Weeks 6-12: new bone tissue is laid down in the endosteal space and eventually the 2 ends are reunited as a ball of provisional callus, which appears dense on x-ray
- Upto about 12 months - provisional callus forms woven bone which remodels to form a cortex
- Up to 2yrs: callus matures and dissapears, trabecullar pattern reformed and the bone remodels to accommodate the stresses that the bone experiences in that anatomical region.
[callus develops, around the fracture site, from mesenchymal (primitive) tissue then chondroid (cartilage) and then osseous (bone) tissue.]
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 - the 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) fractures 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 and what is this called
Bony union will not take place - may be described as an “atrophic” or fibrous union
what is primary bone healing and when does it occur
- This is where the fracture heals without external callus formation in which new Haversian systems grow directly across the fracture gap.
- This occurs if there is no relative movement (or micromovement) taking place between fracture fragments during the healing process
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 is wolffs law ?
Bone will remodel, by altering its size, shape, and structure, to meet the mechanical demands placed on it.
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]
State the equation for calculating rigidity of a structure for bending
R, of a structure depends both on the material stiffness and the geometrical stiffness of the structure.
R = EI for bending where E is the Young’s modulus of the material and I is its second moment of area.
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)
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
Movement encourages bony union, what type of loading encourages bone healing
loading bone along its long axis
Are shearing forces determinatal to the fracture healing process?
There is little evidence to suggest they are
How does no movement affect the fracture healing process?
No movement at all completely inhibits the healing cycle.
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.
At about how many weeks does the provisional callus form?
Between 6-12 weeks
What mechanical properties does a callus confer to a broken bone as it heals?
It confers to the bone a larger surface area ==> an increased second moment area ==> properties of strength and stiffness
what is the aims of fracture management
- save life
- treat pain
- restore function
broken bones bleed and if many bones are broken then large amounts of blood may be lost from the circulation
how much blood can a femoral, pelvis and wrist fracture have
- femoral - 1L blood loss
- pelvis - 3L blood loss
- wrist - only few millilitres blood loss
how can pain be relieved in fractures
- opiates - has added effect of reducing anxiety and fear
- splintage - reduces the muscle spasm which are very painful
What is the The medium and long term management of fractures ?
to restore function as much as is reasonably possible
what are the stages of fracture management
- Reduction
- Holding (bone fracture fixation via external or internal fixation)
Define what reduction and holding is in fracture management
- Reduction = the original anatomical shape of the bone is restored, at least sufficiently to permit normal functions in daily living
- Holding = when the fractured bone is held in the reduced position until healing has taken place
What is given to the patient to make the process of reduction possible ?
Anaesthesia - makes the process tolerable and relaxes the muscles in spasm which would make it difficult to do
what are the types of reduction
closed reduction - manipulation of the fracture fragments (without gaining direct access to fracture site)
open (surgical) reduction - fracture site opened by surgical op and fracture fragments restored to alignment directly
whilst the type of reduction doesn’t influence the fixation used, what method is usually done with fractures that require open reduction
internal fixation
what are the 2 methods of holding a fracture in place
- external fixation
- internal fixation
List the 3 main external fixation methods
- plaster of paris [and its derivatives]
- traction
- external fixator
List the 3 main internal fixation methods
- plates and screws
- pins and wires
- rods and nails
why is relative stability between fracture fragments needed
After fracture, bone has lost its continuity as a rigid link to support the body and to facilitate muscle action and body movement
Relative stability between fracture fragments is needed to restore this continuity, since excessive movement would result in non-union.
what is the primary objective of all kind of fixation devices
minimise deformation (or movement) between fracture fragments
how do splints work
- stabilises a fracture through the soft tissue
- help the injured limb to resist bending forces after reduction but are of little use in resisting torsional and compression forces
- ==> can only be used for relatively stable fractures
how are the vast majority of fractures treated
treated non-surgically
