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Flashcards in Implant Technology Unit 6a Deck (98):

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



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


- 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]


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

- 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

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

- 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

easy to obtain
easy to apply
very versatile


what can traction be used for

Reducing a fracture
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
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

- elderly have more co-morbities
- children, bones heal more rapidly and have more capacity to naturally remodel their shape after fracture

- 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.