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Implant technology > Surgical fracture fixation > Flashcards

Surgical fracture fixation Flashcards

1
Q

why must plates and screws be made of the same material

A

to prevent galvanic corrosion

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2
Q

how does a screw move through cancellous bone

A

cancellous bone is softer so the screw can create its own thread as it passes through the bone, provided it has a suitably designed tip

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3
Q

if a screw is to hold together 2 blocks, what is important about the thread in relation to one of the blocks

A

it must NOT grip one of the blocks, either by being unthreaded in the portion that passes through this block or one block has a pre-drilled hole larger than the thread of the screw

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4
Q

How can a screw buttress be made bigger

A

using a washer to spread the load over a larger area

this is used in soft bone

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5
Q

what is the function of the head of a screw

A

provides a buttress to stop the whole screw sinking into the bone
provides a connection with the screwdriver

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6
Q

why do bone screws normally have a hexagonal shape on their head

A

effective coupling unlikely to be damaged in the screwing process
very positive interlock between screw and screwdriver&raquo_space; easy to use, no axial force needed to retain the driver in the head.

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7
Q

what is the shape of the undersurface in most standard bone screws and why

A

round

for max contact area between screw head and bone after countersinking, reducing risk of a zone of excessive stress

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8
Q

what are the 3 diameters to consider in a screw

A

core diameter - smallest diameter of threaded part
thread diameter - largest diameter of threaded part
shaft diameter - diameter of the widest part

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9
Q

which diameter determines the strength of a screw?

A

the smallest diameter - whether that be the shaft diameter or thread diameter. the greater the smallest diameter, the stronger the screw will be

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10
Q

name the 3 important aspects of the thread

A

shape
depth
pitch

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11
Q

what is beneficial about the screw thread being asymmetrical

A

wide surface on the pulling surface and little frictional resistance on the underside &raquo_space; more of the torque is used for pulling the two objects together rather than wasted on overcoming friction when pulling the 2 objects together

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12
Q

what is the thread depth

A

half the difference between the thread diameter and the core diameter

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13
Q

how does thread depth affect the effectiveness of the screw

A

the amount of thread in contact with the bone determines how well the screw resists being pulled out of bone

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14
Q

would you want a large or small thread depth when inserting a screw into cancellous bone?

A

a deeper thread because this will capture more material between the threads and further resist it being pulled out

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15
Q

what is the pitch of the screw

A

the linear distance travelled by the screw for a complete (360) turn. this isn’t as important for bones screws as depth and shape of the screw

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16
Q

what is tapping?

A

the process of cutting a thread

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17
Q

what is a self-tapping screw

A

a screw which has a cutting tip that allows it to cut its own ‘female’ thread track as its being inserted

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18
Q

what does a trocar shaped tip allow

A

it allows the cutting edge to act as a drill

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19
Q

what type of tip do cancellous bone screws have

A

corkscrew

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20
Q

whats the benefit of their being no pre-taped hole when inserting a screw into cancellous bone

A

the material of the soft bone is compressed as the screw is driven in, enhancing grip strength

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21
Q

how does pre-tapping help

A

it means the screw is easier to insert so most of the torque applied when tightening the screw is converted into compression
also means there will be no bone fragments jamming between the bone thread and the screw, which would increase frictional resistance and risk damage to the bone thread

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22
Q

what do tapping instruments and self tapping screws have

A

flutes - channels which provide a route for cuttings to escape

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23
Q

how can a fully threaded screw be made to act like a lag screw

A

if the gliding hole is made to be slightly larger than the screw thread diameter and then another hole is made equivalent to the screw core diameter

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24
Q

why is it important to position screws accurately

A

to prevent uneven forces being generated across the fracture which can lead to bone distortion

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25
Q

name 5 applications of orthopaedic screws

A

to prevent sideways displacement of fragments
to hold a plate against bone
to increase grip of IM nail on bone
to permit displacement in an axial direction (e.g. dynamic hip and condylar screws)
as part of an external fixator

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26
Q

what fractures are screw-plate combinations recommended for

A
  • forearm fractures
  • 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
    (e. g. following particularly violent fractures; soft bone found after a delay of a few days between injury and surgery; if the bone is unnaturally soft as it may be in old age)
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27
Q

What is osteosynthesis?

A

the reconstruction of fractured bone by surgical and mechanical means

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28
Q

when is the bridging technique used and what does it involve

A

complex fractures with many fragments

the 2 main bony shaft fragments are linked with a plate to restore bone length and alignment but the intervening small fragments are left unfixed so that their blood supply is undisturbed

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29
Q

what is the usual aim of plate fixation

A

to achieve load sharing until the bone is strong enough to take all the load efficiently

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30
Q

what would make the plate prone to bending

A

if the plate takes most of the load and there is a defect or gap at the fracture site

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31
Q

what is stress reversal and what can it cause

A

backwards and forwards cyclical movement of the plate

can lead to early fatigue failure

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32
Q

when might a bone graft be added at the fracture site

A

if anatomical alignment must be restored accurately but screws alone are not adequate, but load sharing cannot be achieved with confidence.
the bone graft should preferably be autogenous

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33
Q

what areas do fractures most commonly need bone graft

A

bones of forearm because they rotate about each other
pelvis especially acetabulum
around joints
face and jaw

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34
Q

what side of the fracture should a plate be placed

A

on the tension side where possible

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35
Q

what is the disadvantage of all plating techniques

A

there must be a lot of soft tissue stripping which further damages the blood supply

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36
Q

what is the consequence of soft tissue stripping at fracture site

A

infection

delayed healing

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37
Q

why are pins generally used in pairs

A

so that the rotary element is minimised in the final bone/pin construct

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38
Q

why would pins be used in conjunction with flexible wires

A

to achieve compression between 2 small bone surfaces eg patella fracture

39
Q

what does cerclage of wires mean

A

encircling/crossing the fragments to force them together

40
Q

how can wires be used dynamically?

A

to act as a tension band by utilising the power of surrounding muscles to induce compression at a fracture site

41
Q

it is important to work beneath the ? to avoid damage to blood supply

A

periosteum

42
Q

in terms of access why can wires be more useful than

nails and screws

A

useful in situations where there is limited access eg spinal surgery it can be threaded through restricted holes and its versatility can still produce powerful fixation

43
Q

what is the principle reason that wire has fallen out of use

A

awkwardness in use and the need to twist it properly

over twisting causes breakages so its hard to get it right

44
Q

how do IM nails withstand axial and bending stress

A

they are sturdy and approximately round in shape

45
Q

what is the antegrade technique

A

inserting the nail into the bone from one end without disturbing the fracture site at all

46
Q

give an advantage and a disadvantage of the antegrade technique

A

advantage - little damage to soft tissues / blood supply

disadvantage - an x-ray intensifier is essential in the operating room during this

47
Q

what is the retrograde technique

A

fracture site opened up by soft tissue dissection and fracture end delivered into the wound for reaming and nail insertion - this is not a good technique!!

48
Q

is aggressive medullary fixation good or bad in elderly/multiply injured people

A

good! been shown to reduce injury mortality

49
Q

why are most nails made of stainless steel

A

good strength and stiffness characteristics
easy to handle during manufacturing
well tolerated by body tissues

50
Q

Titanium would hypothetically be good for nails so why is it not used?

A

due to the phenomenon of notch sensitivity: its more susceptible to weakening if a hole is drilled across it or if its accidentally abraded during insertion or locking

51
Q

what is the working length of a nail

A

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

52
Q

what is inversely related to a nails working length

A

stiffness of the nail in rotation and bending

53
Q

is a hollow nail stronger and stiffer if the wall is thick or thin

A

stronger if thicker

54
Q

whats the advantage of having a somewhat flexible nail

A

easier to put in and allows the nail to deform slightly to conform to the natural shape of the bone

55
Q

do nails get more or less stiff as they get stronger

A

stiffer as they get stronger

56
Q

for what purpose is a longitudinal slot added to a nail

A

to reduce stiffness

57
Q

when are IM nails contraindicated

A

children and when there is a fracture involving an adjacent joint

58
Q

Which part of long bones are nails effective at

A

middle only

59
Q

why is the femur constantly bent when loaded

A

because of the 135 degrees offset of the femoral neck

60
Q

what angle exists between the axes of the femur and the tibia

A

about 7 degrees

61
Q

what is the natural tendency of an extra capsular femoral fracture

A

prox fragment to keel over medially&raquo_space; femur shortens

62
Q

restoration of medial fragmentation of the upper femur is impractical. what is done instead?

A

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 fem neck

63
Q

what has to be done to the medullary cavity to permit a sufficiently strong nail being inserted?

A

has to be widened usually

64
Q

what is the complication of widening of the medullary cavity

A

reamers used to do this can get stuck, or penetrate through to the outside of the bone

65
Q

apart from those associated wit widening of the medullary cavity, what are complications of nailing?

A

nail can be inserted in the wrong direction and rotary misalignment is common
infection may occur

66
Q

what do external fixators consist of

A

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

67
Q

why is external fixation good for open fractures

A

permits good access to soft tissues
» application of dressings easier, more complicated surgery to damaged structures can be performed whilst stabilisation of the bone is maintained, skin grafting / transportation of muscle flaps can even be performed

68
Q

what is the use of external fixators in orthopaedic (non-trauma) cases

A 
limb lengthening 
limb shortening
arthrodesis
correction of angulatory / rotary deformity 
bone segment transportation
69
Q

what can post trauma external fixation be divided into

A

temporary and definitive

70
Q

why is temporary external fixation useful in open fractures

A

access to soft tissues whilst also stabilising the bone and its also an easy way of achieving elevation of the limb

71
Q

why is planning somewhat different in definitive external fixation compared to temporary

A

due to the need to ensure that the pin placement doesn’t interfere with the soft tissues during mobilisation and to ensure the construct can perform other functions such as the sliding of one fracture fragment relative to another to stimulate callus formation
this is called DYNAMISATION

72
Q

why is external fixation especially useful for tibial fractures

A

soft tissue damage is usually extensive and the bone has a relatively poor blood supply

73
Q

why is a unilateral frame better than a bilateral frame in external fixation

A

bilateral frames cause unacceptable soft tissue tethering which causes pain and limits rehabilitation
unilateral frames confer adequate stability to the fracture whilst allowing mobilisation, good access to wounds, and limits soft tissue tethering

74
Q

why is bone-bone contact being achieved important in the load sharing mechanism of an external fixator

A

if there is no contact between bone ends then the frame alone (with minor soft tissue contribution) will be responsible for the stability of the limb

75
Q

after fracture and application of the external fixator, what factors determine stability

A 
jframe configuration
degree of contact between the bone ends 
extent of soft tissue injury 
quality of the bone-pin interface 
degree to which clamps have been properly tightened 
total number of pins used
76
Q

what are the 2 principles in external fixator configuration

A

bone/frame construct should be stable

pin placement shouldn’t tether soft tissues or restrict access to wounds

77
Q

strains in what direction are thought to be a good stimulus for bone healing

A

along the long axis of the bone

78
Q

how much movement is needed to encourage healing and how much is needed to inhibit healing

A

to encourage healing only 0.5 - 1 mm is needed

excess of 2mm may inhibit

79
Q

define dynamisation

A

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

80
Q

what are the advantages of an external fixator

A
  • can be fitted fairly quickly&raquo_space; good in emergencies or in cases of multiple injury
  • can be adjusted later if bone positioning becomes anatomically unacceptable
  • beam of the fixator can be removed to allow clear X-rays to be taken and the stability of the fracture to be felt
  • most are versatile enough to be used in many sites without changing the basic model&raquo_space; keeps storage stocks low
  • excellent soft tissue access
81
Q

what are the disadvantages of an external fixator

A
  • bone/pin interfaces are a potential site of infection
  • pin loosening is a real possibility because great bending forces are exerted on pins leading to stresses and strains at the interface
  • a degree of soft tissue tethering by pins between skin and bone is inevitable
82
Q

what does soft tissue tethering lead to

A

inhibited voluntary movement and discomfort, preventing joint motion and comfortable weight bearing

83
Q

why are regular radiographs important when an external fixator is applied

A

to keep an eye on components loosening

84
Q

painful pins are likely to be … ?q

A

loose pins

85
Q

can loose pins generally just be tightened?

A

usually not and need re-sited

86
Q

what can be done to keep pin tracts clean as possible

A 
keep dry (if applying dressings they should be dry)
keep clear of dried exudate by using cotton buds
avoid antiseptic creams or lotions because they have been found to keep pin sites too moist and dont prevent infection significantly enough
87
Q

a loose infected pin should always be ..

A

re-sited

88
Q

what is a sequestra?

A

area of dead bone

89
Q

what should be done if a sequestra is found

A

it should be drilled out and the wound cleaned

90
Q

how may tight infected pin sites be retrieved?

A

systemic antibiotics

aggressive wound cleaning

91
Q

pins should pass through as little ? as possible

A

muscle

92
Q

when applying pins consideration should be given to joint position. why should the ankle be kept in a plantigrade position?

A

so that muscle tethering doesn’t induce the patient to adopt an equinus position

93
Q

why should fixators that cross joints be removed as soon as possible

A

they cause joint stiffness

94
Q

what is reaming of the medullary canal

A

widening to allow insertion of larger IM nail

Implant technology (10 decks)

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