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
how much blood can a femoral, pelvis and wrist fracture have
femoral - 1 litre blood loss
pelvis - 3 litres blood loss
wrist - only few millilitres blood loss
how can pain be relieved in fractures
opiates
splintage
- reduces the muscle spasm which are very painful
what are the stages of fracture management
- reduction
- holding
- bone fracture fixation
what are the types of reduction
closed reduction
- manipulation of the fracture fragments
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
what are examples of external fixation
plaster of paris [and its derivatives]
traction
external fixator
what are examples of internal fixation
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
To restore this continuity, it is essential to regain relative stability between fracture fragments, 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
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
how is a hard coated plaster of paris bandage made
- by dissolving calcium sulphate hemihydrate in an organic solvent, such as ether, which contains no water.
- Starch is added to this mixture and the whole paste is spread on a cotton bandage.
- wet bandage is then dried and the solvent collected for re-use.
- The bandage is therefore coated with calcium sulphate ‘held on’ by starch.
what is a plaster of paris cast made using
cotton bandages, hard coated with crystals of calcium sulphate hemihydrate
bandage adds little to the strength but provides a vehicle for getting the wet plaster on the part to be splinted
consists of cotton thread which may be double woven to prevent it fraying (known as leno cloth)
what influences the speed of setting of a plaster of paris cast
the starch
- called an accelerator
- speeds up the chemical reaction
also the temp of the water
what are retarders
materials that slow down the setting of POP cast
examples
- alum and borax
what needs to be considered why applying POP cast
considerable heat production when calcium sulphate hemihydrate mixed with water
need to make sure it isn’t gonna hurt the patient
what are the two types of crystals that make up the POP cast
long crystals
- sharp
- called alabaster
- give cast hard quality
smaller crystals
- give cast softer feel
properties of the material of the final splint are largely dependent on the physical interlocking of the 2 types of crystals
how does the POP plaster function and what method predominates
1 - By encasing the limb in a rigid exoskeleton it provides support to the soft tissues which in turn support the broken bone. This so called hydraulic theory has been well proven.
2 - By moulding the cast against the fracture it is possible to obtain a gentle three point fixation system, giving a so-called periosteal hinge.
[Most casts work by a combination of both methods and the 1st probably predominates in most situations except in childhood when the tough periosteum provides a gentle hinge about which fulcrum a three point mould can be achieved]
most cast have to control the position of a broken bone in three dimensions - these dimensions are
1 - length i.e. prevent shortening
2 - position i.e. prevent tilt and shift in anterior/posterior and medio/lateral planes
3 - rotation i.e. about the long axis of the bone
how do casts control rotation
by incorporating the whole of the broken bone and limb segment in the cast including the joint
i.e. above knee cast, below knee cast, cylinder
what is disadv of incorporating the joint into a cast
- if casting is prolonged, the encased joints become stiff and their muscle waste through disuse
- prolongs overall rehabilitation
- impairment caused by immobilisation of joints may lead to disability sufficient to induce dependency and prolong the stay in hospital - especially for elderly patients.
how can the problems that come from immobilisation with a cast be overcome
- careful moulding
- application of hinges incorporated into the cast i.e. known as functional or cast bracing
how does a functional cast work in a femoral brace
the upper third of the femoral component is gently squared off so that the soft tissues are slightly distorted but not sufficiently to raise high points of pressure
knee is freed by the use of hinges which permit the knee to move normally
the position of the broken fragments are held reduced whilst the joints move normally and the muscles can rehabilitate early.
how does Sarmiento’s classic brace for tibia fractures work
achieves rotatory control through moulding around the upper third of the tibia and by extensions to the cast which encaptures the femoral condyles in knee flexion.
why do braces need to be applied after the first two/three weeks after fracture
so the soft tissue injuries have settled down and there is no swelling
how so braces ideally be designed
adjustable
always need to be in contact with skin
what are the classes of new materials of adjustable braces
isoprene rubbers (or polycarprolactone sheets) and glass fibre
artificial fibre and polyurethane composites
what are the properties of Polycaprolactone/isoprene sheets
become ductile at fairly low temperatures, so that whilst warm they can be moulded directly onto the skin achieving a reciprocal shape to the limb
When at room temperature they become firm, but remain flexible enough to be gently adjustable, retaining a “memory” of their formed shape.
what is disadv of Polycaprolactone/isoprene sheets
expensive
- require purchase of an oven
require a lot of skill
what do Fibre/polyurethane composites consist of
woven bandages made of glass fibre or fabric which is impregnated with a urethane monomer and a catalyst
why are Fibre/polyurethane composites useful
When exposed to warmth and moisture, forms a true fibre/polyurethane composite which is very light and extremely strong, yet flexible
when are Fibre/polyurethane composites useful
1 - forming braces when they are applied to a reasonably stable, healing fracture as they can form very sophisticated shapes and interface well with hinge materials, ensuring a firm anchorage.
2 - make excellent secondary casts once swelling has settled and the soft tissues confer a degree of stability to a fractured limb.
[They are not very useful as a primary splintage material as they are conforming rather than being very mouldable and are difficult to use on unstable and swollen limbs.]
why does POP remain the mainstay of treatment for low velocity fractures
cheap
easy to obtain
easy to apply
very versatile
what can traction be used for
Reducing a fracture
And
Holding a fracture
how does traction work to hold a fracture
altering muscle tone in order to maintain a position achieved at reduction.
what load is needed for traction in the lower limb
10N per 100N of BW
must be countered by an equal force otherwise patient would be pulled out of the bed
counter force achieved by tilting the bed backwards
what are the two ways the load may be applied to the limb
skin traction
skeletal traction
how does skin traction work
load is applied via a foam or sticky bandage applied to the skin.
The attachment to the limb is dependent on the adhesiveness of the bandage or the frictional resistance of the foam.
This method, although convenient, can only be used for loads up to 50 N as otherwise there is a very real danger of injuring the skin.
how does skeletal traction work
load is applied via a pin inserted through the bone.
can be used to apply large loads and has advantage that the load can be precisely relative to the long axis of the bone - this is important when traction is being used in dynamic situations.
Disadvantage of skeletal traction is the risk of bone infection at the pin bone interface [can be countered by careful pin insertion and good pin site care by nursing staff]
what are the 3 methods of traction
static (fixed) traction
dynamic traction
balanced traction
what is static (fixed) traction and an example of it
load is applied to the limb and attached to a splint so that the splint itself provides the counter force
Thomas splint
when is static traction used
mainly used for treating children’s fractures because they do not cope well with complicated traction and their fractures heal quickly.
what is disadv of static traction
acceptable for a week or two but the immobility prevents joint movement, does not induce axial movement at the fracture site and leads to muscle disuse
how does dynamic traction work
same principles apply as in static traction but the patient is encouraged to use their joints and the load is arranged so that, irrespective of limb position, the net pull is maintained along the axis of the bone
achieved through PULLEYS
what are the functions of the pulleys in dynamic traction
1 - they alter the direction of the force by being statically mounted on a surrounding bed frame
2 - they may alter the magnitude of the traction force by being mounted on the limb or “free floating” within the traction cord system.
when are free floating pulley systems useful in dynamic traction
useful when the physiotherapist wants to counter the weight of a limb segment so that very weak muscles can be exercised without the full weight of the limb early in treatment.
when is balanced traction used
used as a supplement to either static or dynamic traction
used to offset pressure effects caused by splints
i.e. used for Thomas splints where a counter force is applied to the groin
how can pressure effects be offset in the Thomas splint
by applying a small load to the splint as a whole which tends to draw the pressure off the groin area
what are complications of traction
[problems associated with having to lie in bed for long periods of time]
bed sores chest infection UTI atrophy of muscles and bones [need active physio program]
why is traction not used very often
requires long period of hospitalisation
what makes fractures heal faster
bones are in reasonable apposition
subjected to axially orientated [not shearing] loads
little bit of movement
what factors need to be considered when deciding on Tx of a fracture
the patient
the injury
the facilities available
the skill of the operator
what needs to be considered about the patient
age
- elderly have more co-morbities
- children, bones heal more rapidly and have more capacity to naturally remodel their shape after fracture
health
- co-morbidites
- functional requirements
what is the natural healing process:
- weeks 0-2
- weeks 0-2
haematoma is invaded by macrophages in surrounding tissue which are responsible for “mopping up” dead and damaged tissue. The haematoma and dead cells are absorbed into the macrophages.
what is the natural healing process:
- weeks 2-6
- weeks 2-6
New capillaries grow into the fracture haematoma bringing with them cells of healing and repair including fibroblasts, which form fibrin (scar tissue) and also other cells including bone forming osteoblasts. At the same time the surviving periosteum begins to regenerate and grow between the bone fragments
what is the natural healing process:
- weeks 6-12
- weeks 6-12
New bone tissue is laid down in the endosteal space from the residual living bone and eventually the two ends are reunited as a ball of “provisional callus” which appears as a dense area on an X-ray.
what is the natural healing process:
- up to 12 months
- up to about 12 months
provisional callus continues to form woven bone which gradually remodels to form a cortex.
what is the natural healing process:
- up to 2 years
- up to 2 years
callus matures so that the trabecular pattern is reformed and the bone remodels to accommodate the stresses that the bone experiences in that anatomical region.
