Surgical fracture fixation Flashcards
why must plates and screws be made of the same material
to prevent galvanic corrosion
how does a screw move through cancellous bone
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
if a screw is to hold together 2 blocks, what is important about the thread in relation to one of the blocks
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
How can a screw buttress be made bigger
using a washer to spread the load over a larger area
this is used in soft bone
what is the function of the head of a screw
provides a buttress to stop the whole screw sinking into the bone
provides a connection with the screwdriver
why do bone screws normally have a hexagonal shape on their head
effective coupling unlikely to be damaged in the screwing process
very positive interlock between screw and screwdriver»_space; easy to use, no axial force needed to retain the driver in the head.
what is the shape of the undersurface in most standard bone screws and why
round
for max contact area between screw head and bone after countersinking, reducing risk of a zone of excessive stress
what are the 3 diameters to consider in a screw
core diameter - smallest diameter of threaded part
thread diameter - largest diameter of threaded part
shaft diameter - diameter of the widest part
which diameter determines the strength of a screw?
the smallest diameter - whether that be the shaft diameter or thread diameter. the greater the smallest diameter, the stronger the screw will be
name the 3 important aspects of the thread
shape
depth
pitch
what is beneficial about the screw thread being asymmetrical
wide surface on the pulling surface and little frictional resistance on the underside »_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
what is the thread depth
half the difference between the thread diameter and the core diameter
how does thread depth affect the effectiveness of the screw
the amount of thread in contact with the bone determines how well the screw resists being pulled out of bone
would you want a large or small thread depth when inserting a screw into cancellous bone?
a deeper thread because this will capture more material between the threads and further resist it being pulled out
what is the pitch of the screw
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
what is tapping?
the process of cutting a thread
what is a self-tapping screw
a screw which has a cutting tip that allows it to cut its own ‘female’ thread track as its being inserted
what does a trocar shaped tip allow
it allows the cutting edge to act as a drill
what type of tip do cancellous bone screws have
corkscrew
whats the benefit of their being no pre-taped hole when inserting a screw into cancellous bone
the material of the soft bone is compressed as the screw is driven in, enhancing grip strength
how does pre-tapping help
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
what do tapping instruments and self tapping screws have
flutes - channels which provide a route for cuttings to escape
how can a fully threaded screw be made to act like a lag screw
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
why is it important to position screws accurately
to prevent uneven forces being generated across the fracture which can lead to bone distortion
name 5 applications of orthopaedic screws
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
what fractures are screw-plate combinations recommended for
- 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)
What is osteosynthesis?
the reconstruction of fractured bone by surgical and mechanical means
when is the bridging technique used and what does it involve
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
what is the usual aim of plate fixation
to achieve load sharing until the bone is strong enough to take all the load efficiently
what would make the plate prone to bending
if the plate takes most of the load and there is a defect or gap at the fracture site
what is stress reversal and what can it cause
backwards and forwards cyclical movement of the plate
can lead to early fatigue failure
when might a bone graft be added at the fracture site
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
what areas do fractures most commonly need bone graft
bones of forearm because they rotate about each other
pelvis especially acetabulum
around joints
face and jaw
what side of the fracture should a plate be placed
on the tension side where possible
what is the disadvantage of all plating techniques
there must be a lot of soft tissue stripping which further damages the blood supply
what is the consequence of soft tissue stripping at fracture site
infection
delayed healing
why are pins generally used in pairs
so that the rotary element is minimised in the final bone/pin construct
why would pins be used in conjunction with flexible wires
to achieve compression between 2 small bone surfaces eg patella fracture
what does cerclage of wires mean
encircling/crossing the fragments to force them together
how can wires be used dynamically?
to act as a tension band by utilising the power of surrounding muscles to induce compression at a fracture site
it is important to work beneath the ? to avoid damage to blood supply
periosteum
in terms of access why can wires be more useful than
nails and screws
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
what is the principle reason that wire has fallen out of use
awkwardness in use and the need to twist it properly
over twisting causes breakages so its hard to get it right
how do IM nails withstand axial and bending stress
they are sturdy and approximately round in shape
what is the antegrade technique
inserting the nail into the bone from one end without disturbing the fracture site at all
give an advantage and a disadvantage of the antegrade technique
advantage - little damage to soft tissues / blood supply
disadvantage - an x-ray intensifier is essential in the operating room during this
what is the retrograde technique
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!!
is aggressive medullary fixation good or bad in elderly/multiply injured people
good! been shown to reduce injury mortality
why are most nails made of stainless steel
good strength and stiffness characteristics
easy to handle during manufacturing
well tolerated by body tissues
Titanium would hypothetically be good for nails so why is it not used?
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
what is the working length of a nail
the length of a nail that transmits load from one main fragment of fractured bone to another
what is inversely related to a nails working length
stiffness of the nail in rotation and bending
is a hollow nail stronger and stiffer if the wall is thick or thin
stronger if thicker
whats the advantage of having a somewhat flexible nail
easier to put in and allows the nail to deform slightly to conform to the natural shape of the bone
do nails get more or less stiff as they get stronger
stiffer as they get stronger
for what purpose is a longitudinal slot added to a nail
to reduce stiffness
when are IM nails contraindicated
children and when there is a fracture involving an adjacent joint
Which part of long bones are nails effective at
middle only
why is the femur constantly bent when loaded
because of the 135 degrees offset of the femoral neck
what angle exists between the axes of the femur and the tibia
about 7 degrees
what is the natural tendency of an extra capsular femoral fracture
prox fragment to keel over medially»_space; femur shortens
restoration of 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 fem neck
what has to be done to the medullary cavity to permit a sufficiently strong nail being inserted?
has to be widened usually
what is the complication of widening of the medullary cavity
reamers used to do this can get stuck, or penetrate through to the outside of the bone
apart from those associated wit widening of the medullary cavity, what are complications of nailing?
nail can be inserted in the wrong direction and rotary misalignment is common
infection may occur
what do 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
why is external fixation good for open fractures
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
what is the use of external fixators in orthopaedic (non-trauma) cases
limb lengthening limb shortening arthrodesis correction of angulatory / rotary deformity bone segment transportation
what can post trauma external fixation be divided into
temporary and definitive
why is temporary external fixation useful in open fractures
access to soft tissues whilst also stabilising the bone and its also an easy way of achieving elevation of the limb
why is planning somewhat different in definitive external fixation compared to temporary
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
why is external fixation especially useful for tibial fractures
soft tissue damage is usually extensive and the bone has a relatively poor blood supply
why is a unilateral frame better than a bilateral frame in external fixation
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
why is bone-bone contact being achieved important in the load sharing mechanism of an external fixator
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
after fracture and application of the external fixator, what factors determine stability
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
what are the 2 principles in external fixator configuration
bone/frame construct should be stable
pin placement shouldn’t tether soft tissues or restrict access to wounds
strains in what direction are thought to be a good stimulus for bone healing
along the long axis of the bone
how much movement is needed to encourage healing and how much is needed to inhibit healing
to encourage healing only 0.5 - 1 mm is needed
excess of 2mm may inhibit
define dynamisation
the modification of the construct which permits the transmission of forces across a fracture without allowing distraction of the fragments
what are the advantages of an external fixator
- can be fitted fairly quickly»_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»_space; keeps storage stocks low
- excellent soft tissue access
what are the disadvantages of an external fixator
- 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
what does soft tissue tethering lead to
inhibited voluntary movement and discomfort, preventing joint motion and comfortable weight bearing
why are regular radiographs important when an external fixator is applied
to keep an eye on components loosening
painful pins are likely to be … ?q
loose pins
can loose pins generally just be tightened?
usually not and need re-sited
what can be done to keep pin tracts clean as possible
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
a loose infected pin should always be ..
re-sited
what is a sequestra?
area of dead bone
what should be done if a sequestra is found
it should be drilled out and the wound cleaned
how may tight infected pin sites be retrieved?
systemic antibiotics
aggressive wound cleaning
pins should pass through as little ? as possible
muscle
when applying pins consideration should be given to joint position. why should the ankle be kept in a plantigrade position?
so that muscle tethering doesn’t induce the patient to adopt an equinus position
why should fixators that cross joints be removed as soon as possible
they cause joint stiffness
what is reaming of the medullary canal
widening to allow insertion of larger IM nail
