Implant Technology Unit 6b Flashcards Preview

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Flashcards in Implant Technology Unit 6b Deck (147):
1

what are the 2 methods of fixing fractures

internal fixation
- bone screws and plates, IM nails

external fixation

2

what are the materials commonly used for fracture fixation and what must be considered

stainless steel
- commonly used
- strong, inexpensive and easy to manufacture
- however, stainless steel plates don't tolerate stress reversals very well

titanium
- strong, inexpensive, biologically more inert than stainless steel
- less likely to cause allergies

plates and screws must be of the SAME material, otherwise galvanic corrosion of the implants is likely to occur.

3

how does a screw work in fracture fixation

hold two broken fragments together

may be used in isolation or in combo with plates

4

what is a screw

mechanism that produces linear motion as it is rotated

screw in ortho consists of a helix shaped thread on a shaft.

turn the head and the screw will move through a stationary object.

The object may be made of a material, such as cancellous bone, which is softer than the screw, so that the screw can create its own thread as it passes through. This requires a suitably designed tip.


5

how does a screw work

fixes together two (or more) objects by compressing them against each other

6

what is required for a screw to be able to fix together two objects

1 - that the head of the screw is wider than the diameter of the shaft so that it pushes Block 1 against Block 2

2 - thread does not grip Block 1

7

how can it be assured that the thread does not grip block 1

1 - either the screw must have no thread on the section nearest to the head, where it touches Block 1

2 - or if a screw thread is present, Block 1 must have a pre-drilled hole in it which is larger than the screw thread

8

what are the 3 components of a screw

head
shaft
tip

9

what are the 3 factors that determine the strength of a screw fixation

- strength of the screw material
- strength of the object material [bone in ortho]
- design of the screw thread

10

what are the 2 functions of the head of a screw

1 - provides a buttress to stop the whole screw sinking into the bone. Buttress can be made bigger by placing a washer between the head and the bone, thus spreading the load over a larger area. This is used in soft bone.

2 - provides a connection with the screwdriver. It is the interface which transmits to the screw the twisting force applied by the person putting in the screw. The screw turns and advances forwards in response to a twisting force (or torque) so it is important that torque can be applied effectively.

11

what is the shape of the connection on the head of the screw in bone screws and why

hexagonal, because

1 - it gives an effective coupling unlikely to be damaged in the screwing process.

2 - the very positive interlock between screwdriver and screw makes it easy to use. No axial force is required to retain the driver in the head. This is an advantage in tight corners

12

what is another aspect of the screw head design

the shape of the undersurface
- most standard bone screws this is rounded
- this allows max area of contact between screw head and bone after countersinking, thus reducing the risk of a zone of excessive stress which may crack the bone

13

why is careful screw design important

try to avoid causes of high stress called stress raisers

14

in the shaft of a screw, what are the diameters to consider

1 - the core diameter, which is the smallest diameter of the threaded section of the shaft.

2 - the shaft diameter, which is the diameter of the shaft where there is no thread.

3 - the thread diameter, which is the diameter of the widest part of the threaded
section.

15

what is the strength of the screw determined by

determined by the smallest diameter

the greater the smallest diameter, the stronger the screw will be

[In some screws the thread diameter is bigger than the shaft diameter (in a cancellous bone screw for example). In others the thread diameter is the same as the shaft diameter (in some partially threaded cortical bone screws for example).]

16

what are the 3 aspects of the thread

shape
depth
pitch

17

what is the function of the thread

inclined plane which is rotated in the thread hole in the bone so that the screw moves forward in response to being twisted

18

most bone screws have symmetrical threads - true or false - and why are they designed this way

false
- asymmetrical threads

they are flat on the upper surface in contact with the bone and rounded underneath

provide a wide surface on the pulling side and little frictional resistance on the underside

thus, more of the torque is used in pulling two objects together and less is wasted on simply overcoming friction during insertion of the screw

19

what is the depth of the thread

half the difference between the thread diameter and the core diameter

20

what determines how well a screw resists being pulled out of bone

The amount of thread in contact with the bone

21

what depth of thread is preferred in weak cancellous (spongy) bone

deeper thread as this will capture more material between the threads

thus, increase the resistance of the screw to pulling out

22

what is the pitch of a screw

the linear distance travelled by the screw for a complete (360 degree) turn of the screw

[not so critical in influencing holding strength]

23

what is tapping

process of cutting a thread

24

what is a self tapping screw

A screw which has a cutting tip that enables it to cut its own “female” thread track, as it is being inserted

All cancellous screws are self-tapping

25

what is special about the malleolar screw

it's self-drilling which means that it can drill a hole in the cancellous bone without the need to use a separate drill bit.

Its trocar shaped tip provides a suitable cutting edge to allow it to act as a drill, although in practice this is seldom done.

26

what can a screw do in soft, cancellous bone

screw may be permitted to ‘force’ a thread track without the need for a tapping instrument

[not possible in hard cortical bone because too much torque would be required, risking jamming or breaking the screw]

27

What advantages do hexagonal and star head screws have over cross head screws?

Good coupling so no damage to screw heads. Positive interlock between screwdriver and screw

28

Why are cortical screws not self-tapping?

Self tapping would cause damage to the bone and make screw impossible to insert due to excessive torque required.

29

what to tapping instruments and self-tapping screws have

flutes
- i.e. channels which provide a route for cuttings to escape

30

why do cortical screws have no flutes

because bone can grow in to the flute and so make removal difficult

31

what is lagging

process of compressing two objects together

[screws area a means of compressing objects together]

32

what is the purpose of screws in bone fixation

stabilisation of fragments provides a basis for early mobilisation of the limb

prevent excessive movement of metal implants which could lead to failure through metal fatigue.

33

Screws can achieve a lag effect in two ways: one are lag screws, how does this work

- partially threaded only at the section nearest the tip.

- When a partially threaded cancellous screw is placed in a hole the size of the core that has been drilled in the first bone fragment A, the screw will firstly cut its own thread in fragment A and then in the second bone fragment B, using the corkscrew tip

- unthreaded shaft then slides through the hole in the first fragment until the head touches the surface of bone A

- As the screw advances the head pulls fragment A towards fragment B (which now contains the threaded part of the screw).

- In this way the two pieces of cancellous bone have been compressed (lagged) together.

34

Screws can achieve a lag effect in two ways: one is how the screws are inserted, how does this work

- technique allows any screw to act as a lag screw, even if it is threaded along all its shaft

- If a hole (the gliding hole) is slightly larger than the screw thread diameter is made in cortical bone fragment A, then the screw will slip through the hole without any need to twist it with a screwdriver.

- If a hole (the thread hole) equivalent to the screw core diameter is made in fragment B and then tapped to match the dimensions of the screw thread, the screw will now advance by gripping the bone of the second fragment.

- When the screw head makes contact with the surface of fragment A and the screw is further twisted, the two fragments of cortical bone are compressed (lagged) together.

35

what is essential to ensure when using lagging as a fixation technique

- to position screws accurately

- so forces generated are evenly distributed across fractures, otherwise distortion of bone is easily possible

36

what are the situations that screws may be used

- to prevent sideways displacement of fragments
i.e screws commonly used along around joints to hold cancellous bone fragments together

- to hold a plate against bone
i.e. When cortical bone is being fixed, screws alone are not very effective in controlling large bending forces. Screw fixation is therefore usually supplemented by a plate. The screws and plate share the load with the reconstructed bone.

- to increase the grip of an IM nail on the bone

- To permit displacement in an axial direction

- As part of an external fixator assembly

37

when are screws and plates used together

- screws are used to hold the plate firmly to the bone so that the load is shared between the reconstructed bone and the plate

- used in upper limb to fix forearm fractures

- used around joints where complete reconstruction of the cancellous bone is not possible, so that the fragments of bone cannot be rigidly held together with screws alone

38

when is complete reconstruction of cancellous bone not possible

- particularly violent fractures
- soft bone found after a delay of a few days between injury and surgery
- bone is unnaturally soft as it may be in old age

39

when are plates used

in situations where fracture stability can be achieved and enhanced by compression at the fracture site by forcing bone fragments together

screw fixation can be reinforced or protected by plate

40

what is osteosynthesis

reconstruction of a fractured bone by surgical and mechanical means

should be complete so that the shape of the bone is restored with few or no defects in overall bony architecture

41

how does plates help with osteosynthesis

helps to maintain the restored bony shape

When the bone is loaded, for example by muscle activity, the bone and plate share the load.

42

what can happen in complex fractures that can hinder healing

complete anatomical restoration, incorporating all the pieces in the fixation, may be achieved only at the risk of damaging the blood supply to the fragments

43

what happens in complex fractures in regards to plates and what is it called

two main bony shaft fragments may be linked by means of a plate to restore bone length and alignment

small fragments in between are left unfixed but their blood supply is left undisturbed and they retain the potential to heal as part of the overall healing of the bone

[called "bridging technique"]

44

what is the design and structure of plates

plates are flat and relatively thin

must be as compact as possible and sufficiently malleable to allow shaping

means that they have limited capabilities to resist an applied load when stressed in certain directions

45

how does the structure of bone differ from structure of a ortho plate

bone is hollow tube and generally resists bending equally well in any direction

however, its overall strength depends very much on its wall thickness

thin walled bones are weaker than thick walled bones

46

what is the aim of plate fixation

achieve load sharing between plate and bone until the healing bone is strong enough to take all the load efficient i.e. when the fracture is united

[If load sharing is satisfactorily achieved then the plate is relatively safe from failure, which is normally due to metal fatigue]

47

what happens if the plates takes most of the load

plate will be prone to bending

can happen if there is a defect or gap at the fracture site
- normally due to fractured bone not been accurately re-assembled

plate bends backwards and forwards as the incomplete bone-plate construct is loaded

backwards and forwards cyclical movement known as stress reversal
- likely to result in early fatigue failure of the plate

48

what factors should be considered to prevent bending of the plate

- system of fixation with plates and screws should, together with the bone, form as stable a construct as possible,

- ensure as little damage as possible should be done to the blood supply to the bone [so healing of fracture complex takes place as quickly as possible

- plate should be placed in a position relative to the broken bone that it is minimally stressed

- plate should be placed so there is min soft tissue damage

- plate should be made of materials which are strong as possible and which can tolerate the fatigue effects of stress reversals

49

what is the main cause of fatigue fracture

Stress reversals due to inadequate load sharing, i.e. the bone not taking enough of the applied load.

50

what are indications for use of plates

- when anatomical alignment must be restored accurately

- where the use of screws alone is inadequate (because large bending forces distort # fixed only by screws)

- when load sharing may be achieved with confidence

51

what can be done if loading sharing between bone and the plate cannot be guaranteed

bone graft may be added at the site of any deficit.

preferably taken from elsewhere on the patient (an autogenous graft)

will encourage the healing of defects

will accelerate the healing of the bone before the plate becomes at risk from fatigue failure.

52

what areas of the body commonly require the use of a plate

around joints
- failure to restore a joint surface may lead to OA

bones of forearm

pelvis
- esp around acetabulum

on the face and the jaw

53

what can be done with plate-screw combos to make them more efficient

can be places strategically to make them more effective so that they are less likely to fail

i.e. using a plate on the tension side of a fracture opposite to where muscles remain intact

54

what is eccentric loading

when bones are not loaded evenly along their axes

can occur in a fracture if the soft tissue are stripped off one side of the bone while remaining intact on the other side
- produce a tendency for a loaded bone to distort more on one side than the other.

55

how can a plate help with eccentric loading

If a plate is fixed on the side tending to open - the tension side - then this will counteract the eccentric load, compressing the fragments together at the side under the plate.

The eccentric load will continue to compress the bone fragments together on the side of the bone opposite to the plate

Fracture is compressed throughout the bone cross section and the plate suffers an equal and opposite force - tension.

56

in what areas of the body is bone compression achieved using wires as tensions bands

around the olecranon of the elbow

patella

57

wherever possible where should plates be placed

tension side of a fracture

[may not always be possible for anatomical reasons, such as the need to respect the blood supply of the bony fragments or the risk of tendons]

58

what else can be done to improve the use of plates

contouring a plate before use, so that it is bent slightly more concave than the bone [will encourage compression of the bone opposite the site of attachment of the plate]

aids in load sharing between plate and bone and, by forcing the rough fracture fragments together across the whole cross- section of the bone, it adds greatly to stability.

59

what is disadv of plating technique

Tissue stripping may damage blood supply

will delay healing and contribute to risk of infection

60

what are the function of K wires

maintain fragments in alignment by pushing them or holding them together, depending on the fragments fitting together like a jigsaw puzzle so that their shape and interlock contribute to the bone/pin or bone/wire construct.

61

when are IM nails used and what is used along side them by the surgeon

hold long bones of the leg [and sometimes humerus]

x-ray intensifier - permits the surgeon to take freq "snapshots" of a bone during an op so that they can check progress of a nail as it passes through the bone

62

what is the shape of pins

sharp "trochar points" or occasionally threaded, self tapping ends and are driven into bone fragments by hand driven T-handles or by hand

63

when are pins commonly used

small bones or occasionally in upper limb long bones in small children

foot and hand bones in adults commonly

64

is it important the no. of pins used ?

not that important

but generally pins used in pairs in order to minimise the rotatory element in final pin/bone construct

65

when may pins and flexible wires be used in conjunction

to achieve compression between two small bone surfaces, e.g. in patella fracture

66

what are flexible wires used for

to induce compression

67

what are the two ways to use flexible wires

- used statically by encircling or crossing the fragments (cerclage), pushing them together so that the cleavage lines produced by the fracture are pushed together

- used dynamically as a tension band, utilising the power of surrounding muscle to produce compression at a fracture site
[useful in patella fractures]

68

what are examples of flexible wires being used statically

used in repairing long bones which have been cracked

also cerclage of cracked fragments of bone around endoprostheses is useful
i.e. during revision hip surgery ometimes useful to deliberately crack open the femur to facilitate removal of the primary prosthesis and its surrounding cement; afterwards the femur can be reconstituted with cerclage wires, maintaining a medullary cavity so that the new prosthesis can still be inserted.

69

what have in recent years replaced cerclage in most situations and why

nails
- as nails do less damage to the blood supply is used properly and are much more robust

70

where are sites, other than the patella, that the flexible wires work as dynamic compression

olecranon

medial malleous of the ankle

71

how does dynamic compression by flexible wires work

Muscle contractions working against the fulcrum caused by the wire holding the fragments together immediately under the wire produce compression at the far side of the fracture fragments

the more the muscles work, the firmer the fracture is compressed and the more stable it will be.

[ideal way of rehab damaged joints as movement promotes stability and encourages joint nutrition and cartilage as well as bone recovery]

72

what is essential to ensure when working with wire

to work beneath the periosteum as otherwise the blood supply will be damaged

also to avoid over-twisting the wire which would cause a weak or potentially loose fixation

73

What is a cerclage wire?

A wire that crosses or encircles a fracture fragment

74

where is a intramedullary (IM) nail places and what does it work to do

in the medullary canal of a fractured bone and functions as a form of internal splint which stabilises long bone fractures with minimal damage to the surrounding soft tissues

75

IM nails are sturdy and round in shape, what does this mean

that they can withstand axial and bending stresses

76

what can be achieved by combining nails with screws and plates

rotatory control of the bone can be achieved and length maintained

Limbs with nailed fractures may be mobilised early after surgery, with weight bearing before bony union.

77

what is the name of modern nailing technique and what does it entail

antegrade technique
- the nail is inserted into the bone from one end whilst not disturbing the fracture site at all
- does little damage to surrounding soft tissue or blood supply
- x-ray image intensifier is essential

78

what is the name of the old nailing technique and what does it entail

retrograde technique
- the fracture site was opened by soft tissue dissection and the fracture end delivered into the wound for reaming and insertion of the nail

79

what is development of new IM nails designs centred around

whether it is necessary to widen the intramedullary canal through paring off the inner surface of the bone (reaming) or whether to use nails which are solid and thinner so that they may be inserted without damage to the inner blood supply of bones (unreamed technique).

80

why is IM nailing used so much

shown that early aggressive medullary fixation dramatically reduces injury mortality and post-operative morbidity

81

what are the 3 important considerations of the design of IM nails

- material of which it is made
- how much of the nail is in contact with the bone for any particular fracture
- dimensions and shape of the nail [and its wall thickness if it is hollow]

82

what are most nails made of

stainless steel
- has good strength and stiffness characteristics
- easy to handle during the manufacturing process
- well tolerated by the body tissues

83

why is titanium not used for nails

more susceptible to weakening either if a hole is drilled across it, or if it is accidentally abraded during insertion or locking - known as notch sensitivity.

84

what is the working length of a nail

the length of a nail that transmits load from one main fragment of a fractured bone to the other

85

why is the working length of a nail important

as the stiffness of a nail in both rotation and bending is related inversely to its working length

i.e. a longer working length and it is easier to bend and twist [the tube analogy]

86

what does it mean if a IM nail has a firm grip on the endosteal surface of the bone immediately above and below a transverse fracture

that it will have a short working length

i.e. the ability of the nail to resist bending and torsional forces will be high

87

in what scenarios does IM nail have a long working length

if a nail is inserted across a multifragmentary shaft fracture, the nail must be anchored to the bone via cross-locking screws.

If gripping of the bone is solely by virtue of the nail’s proximal and distal locking screws, then it will have a longer working length equal to the distance between top and bottom locking screws

88

what does a long working length of a IM nail mean

will be less able to resist bending and torsional forces

The longer the working length, the greater the relative movement between the main bone fragments.

89

nails can either be solid or hollow - what is the difference

Solid nails are stronger than hollow ones of the same diameter simply because they have more metal for their volume

Solid nails may be thinner than hollow ones but they require cross screws to get a grip on the hollow bone.

Hollow nails are supplied in a range of diameters so that a size can be chosen which will grip the inside of a reamed bone, whatever its size, along a large length of its internal diameter.

90

nails can be curved to conform roughly to the shape of the bone for which they are designed - how are femoral nails shaped compared to tibial nails

femoral nails are gently curved in an arc over their whole length whereas the tibial nail has a sharper angulation one third of the way down from the top.

91

Hollow nails are less stiff in bending than solid ones, although their stiffness may be altered by making the walls thicker or thinner - what makes them stronger?

The thicker the wall the stronger and stiffer the nail.

92

what is an adv of a slightly flexible nail

will have a little “give” in it on insertion which will make it easier to put in and will permit the nail to deform slightly to conform to the natural shape of the bone.

93

why are very stiff nails not ideal either

may damage the bone if there is any discrepancy between the shape of a nail and that of the bone - this situation may arise because nails are of a standard shape and people are not

94

what is one way of reducing stiffness of a nail

to put a longitudinal slot in the wall of a nail

makes it much more flexible but does so at the cost of it losing some overall bending strength and, especially, torsional strength

95

what is normally the design and dimensions of a nail

usually curved according to the shape of the bone involved and slotted along their length

wall thickness of 1.2 mm

longitudinal slot

gives an optimal balance between strength and flexibility and permit good contact between nail and femur

96

Why are some nails made less stiff by the use of a longitudinal slot?

so they are easier to insert

97

what bone are IM nails used in

weight bearing long bones
- femur and tibia

98

what are the indications for nailing

- transverse and short oblique fractures of the tibial and femoral shafts,

- comminuted fractures of tibia and femur, provided cross locking capabilities are
available,

- pathological shaft fractures, especially in osteoporotic bone,

- delayed or non-union of the shafts of the femur or tibia,

- selected open fractures, in the hands of surgeons experienced in the management of trauma.

99

why is IM nailing used so much

provides stable fixation with minimal damage to soft tissues

100

what are contraindications for nailing

in children as the nail may damage growth plates

also when there is a fracture involving the adjacent joint

101

what are the 3 configurations IM nails can be used in

- as a simple nail with no additions,

- in association with screws situated obliquely or at right angles to the axis of the nail and passing through holes in the nail,

- in association with plates, particularly to treat fractures of the proximal and distal femur.

102

what are nails within medullary cavities useful at doing

maintaining a fairly accurate anatomical alignment whilst permitting early weight bearing

103

what do nails need to work

to be in contact with the bone

and are only effective in the middle part of long bones

104

how can the effective working length of a nail be increased

by adding cross screws
- increases the length of long bones which can be nailed

105

what areas of bone present problems in fracture fixation

upper and lower ends of the femur
- femur is being constantly bent when under load because of the 135 degree offset of the femoral neck, which in turn creates an angle of about 7 degrees between the axis of the femur and the tibia when the tibial plateau is orientated horizontally, parallel with the ground.

106

who commonly gets fractures of the femoral neck

old age women suffering osteoporosis

107

due to the shaping and loading of the femur, what is the tendency to happen after a femoral neck fracture

for the proximal fragment to keel over medially and for the femur to shorten

degree of displacement depends on the degree of damage to the medial part of the upper femoral shaft

need to re-orientate the broken fragment

108

In practice, restoration of the medial fragmentation of the upper femur is impractical - what is done instead

an extra support is added to the lateral side of the femur in the form of a plate and from the plate is hung a nail, which is placed up the remaining proximal femoral neck.

femoral neck is effectively rejoined to the shaft by a nail acting as a cantilever against the buttressed lateral femoral cortex

109

what are complications of nailing

reamers used to widen the medullary cavity can get stuck or penetrate through to the outside of the bone

nail can be inserted in the wrong orientation and rotatory misalignment is common

infection - very difficult to treat

110

what are the main functions of cross screws

They hold apart bone fragments.
They provide rotatory control.

111

what does external fixators consist of

pins drilled into the bone to which a metal beam is attached in parallel to the long axis of the bone

beam and pins provide a means of support which stabilises the fracture and permits access to the soft tissues during wound healing

112

what were external fixators designed for

to help in the treatment of difficult and extensive wounds which involved fractures

Stabilise the bone, allowing soft tissue to be dealt with
- application of dressings by the nurses was easier, more complicated surgery to damaged blood vessels could be performed, skin grafting or even complex plastic surgery could be preformed

113

what are the two categories of external fixators use

orthopaedic use

post-trauma use

114

what are the orthopaedic use of external fixation

limb lengthening

limb shortening

joint fusion (arthrodesis)

correction of angulatory or rotatory deformity

bone segment transportation

115

what are the 2 categories of post traumatic use of external fixation

temporary

definitive

116

when is temporary external fixation used

in open fractures with extensive soft tissue damage
- bone healing is unlikely to occur until the blood supply to the soft tissues has recovered and the wounds have healed.

117

what does temporary external fixation allow

maintains stability of the bone whilst access is safely gained to the soft tissues for (a) dressing and (b) further surgery.

providing an easy way of achieving elevation of the limb

118

what will happen with temporary external fixation after the soft tissue has healed

external fixation technique may be changed for another treatment, such as nailing (surgical), or cast bracing (non-surgical)

119

what situations is temporary EF used

life threatening situations where speed is essential.

stop bleeding following unstable pelvic fractures

poly-trauma

120

what is meant by definitive EF

used for soft tissue healing and right through to fracture healing

121

what is dynamisation

Release of the fixator to allow axial (proximal-distal) movement.

need to ensure that pin placement does not interfere with soft tissue during mobilisation and to ensure that the external fixation construct can perform other functions such as the sliding of one fracture fragment relative to another to stimulate callus formation.

122

what are the two principles of configuration of modular EF frames

the bone/frame construct should be stable (i.e. not rigid)

pin placement must not tether soft tissues or restrict access to wounds

123

in the past, bilateral frames were used for EF, what does this method consist of and why is this method not used anymore

bone pins are positioned so that they crossed both cortices and passed through the skin and soft tissues on both sides of the limb

cause unacceptable soft tissue tethering and limit limb motion
- This is painful and limits rehabilitation so that joints become unnecessarily stiff.

124

currently unilateral frames are used for EF, what does this consist of

pass through the skin on one side of the limb, enter the proximal cortex and end by just passing through the opposite cortex

confer adequate stability to fractures whilst permitting mobilisation, good access to wounds and also keep soft tissue tethering to a min

have their pins carefully placed so that soft tissue damage and tethering are minimised.

125

occasionally, unilateral frames do not give adequate stability [bone might be too soft] - what can be done instead

A or V frames
- where pins are sited at right angles to each other through the same side of the limb

126

what are unilateral frames designed to do

hold bones in functional alignment whilst soft tissues heal and until the fracture is fixed or healed sufficiently to permit protected or unprotected weight bearing

127

what is the aim of EF frames

achieve fracture stability, NOT absolute rigidity

[total or near total rigidity may inhibit bone healing]

desired stability is somewhere between total instability and rigidity so that healing by callous formation is not inhibited and fracture alignment is maintained.

128

what is stability in EF determined by

(a) the configuration of the frame,

(b) the degree of contact between the bone ends,
[depends on trauma]

(c) the extent of the soft tissue injury
[depends on trauma]

(d) the quality of the bone/pin interface
[depends on care in pin placement and quality of the bone]

(e) the degree to which the clamps have been properly tightened
[can be caused by oversight, should be checked regularly during Tx]

(f) the total number of pins used.

129

what should be achieved at reduction with the idea of using a EF for holding

bone to bone contact between the main fragments should be achieved, if at all possible, w/out affecting alignment as this maintains load sharing between the frame and the bone

If there is no contact between bone ends (which may be inevitable if there has been gross bone loss) then the frame alone, with a minor contribution from the soft tissues, will be responsible for the stability of the limb

130

what is thought to be a good stimulus for bone healing

strains along the long axis of the bone
- movement between fragments of 0.5 to 1mm
- [movements in excess of 2mm may inhibit healing]
- no movement at all inhibits callous formation

131

what can Dynamisation be defined as

the modification of the construct which permits the transition of forces across a fracture without allowing distraction of the fragments.

132

when is Dynamisation usually used

when the soft tissues are well on the way to healing,

when the simple unilateral frame is to be used for definitive fracture control.

133

what are adv of using EF

- can be assembled and fitted to the body fairly quickly, which is useful in emergencies

- can be adjusted later if position of the bones is not anatomically acceptable after first application

- beam of the fixator can be removed to take clear x-rays or to feel the stability of the #

- Most fixators are versatile enough to be used in many sites w/out changing the basic model so keeping down stocks in store.

- gives excellent access to the soft tissues

134

what are disadv of using EF

- bone/pin interface are potential site of infection

- pins loosening [also increases the risk of infection]

- soft tissue tethering by pins between skin and bone [is inevitable to a greater or lesser degree]

135

what increases the risk of the bone/pin interface becoming infected

if the patient walks on the fixator

If a pin has become infected then if the fixator does not work well and the method of treatment has to be changed then the options are limited

136

what increases the risk of pins loosening

if weight bearing is applied

Great bending forces are exerted on the pins about a fulcrum at the bone pin interface.

This leads to stresses and strains at the interface which usually result in damage to the bone rather than the pin, which of course loosens the pin.

Once pins loosen, if no further action is taken, further loosening is inevitable.

137

what are the categories of complications of external fixation

fixator complications
- loosening of the modular components
- all components should be regularly tightended

bone/pin interface complications

complications from the #

138

what needs to be considered after application of the fixator

stability in all planes and altering the configuration should be considered if testing demonstrates instability.

139

what does a painful pin in EF mean

the pin is loose

generally, loose pins cannot be tightened and should be re-sited

140

what is the care management plan for pins

Pin tracks should be cleaned regularly and kept dry.

Patients should be instructed in keeping pin sites clear of dried exudate with clean cotton buds.

In general antiseptic creams or lotions tend to keep pin sites too moist and do not prevent infection unless there is also regular cleaning.

If any dressing is applied it should be dry.

141

what is sequestra

area of dead bone
- may be ring-shaped
- if present should be drilled out and the wound cleaned

142

Tx for infected pin sites

aggressive wound cleaning (surgically if necessary) and systematic antibiotics

143

what needs to be considered when positioning the pins

Carelessly inserted pins can cause soft tissue tethering.

Pin site should pass through as little muscle as possible. Muscle tethering leads to pain, and limited movement can lead to joint stiffness because of limited rehabilitation.

Need to consider joint position when applying pins
- ankle should be kept in the plantigrade position so that muscle tethering does not induce the patient to adopt an equinus posture

144

what will fixators that cross joints cause

some joint stiffness

these fixators should be removed as soon as possible, even if only temporarily for rehabilitation.

145

what are the complications from the # seen in EF

malunion

non-union

infection
- if the EF is holding the # is in a stable configuration and the bone pins are not involved, the infection can be treated by local surgery and systemic antibiotics until union.
- if EF unstable, then need to change to more stable structure

146

When is dynamisation normally used?

The soft tissues are well on the way to healing.

The simple unilateral frame is to be used for definitive fracture control.

147

List three complications that can occur with external fixation.

Pins may loosen.

Pin tract infection.

Soft tissue tethering.