Unit 6b: Surgical fracture fixation Flashcards
What is the most commonly used material in surgical fracture fixation?
stainless steel
Give a disadvantage of stainless steel plates
they do not tolerate stress reversals very well.
Why is titanium a good material?
strong
inexpensive
biologically more inert than stainless steel
less likely to cause allergies
Why must plates and screws be of the same material?
otherwise galvanic corrosion occurs
What is a screw?
a mechanism that produces linear motion as it is rotated
What type of screw is used in orthopaedics?
helix shaped thread on a shaft
What is the main use of a screw?
fix together two (or more) objects by compressing them against each other
Give 2 design criteria for a screw to work
head wider than shaft
thread doesnt grip first object: unthreaded section or pre-drilled hole
What does the strength of the holding together of two objects by a screw depend on?
the strength of the screw material
the strength of the bone
design of screw thread
What are the three main components of a screw?
head
shaft
tip
What is the function of the head of the screw?
- buttress to stop screw sinking
2. connects to screwdriver so torque can be applied
When is a washer used? why?
in soft bone
spreads load, which sinks head into shaft, over an increased area
Which shape of drive connection do bone screws use? Why?
hexagonal
effective coupling unlikely to be damaged in the screwing process
easy to use as no axial force is required to retain the driver in the head
What shape is the underface in most bone screws? why?
rounded
maximum area of contact between screw head and bone after countersinking, reducing the risk of a zone of excessive stress which may crack the bone
Name the three diameters of a screw
- core
- shaft
- thread
What is the core diameter of a screw?
smallest diameter of the threaded section of the shaft.
What is the shaft diameter of a screw?
the diameter of the shaft where there is no thread
What is the thread diameter of a screw?
diameter of widest threaded section
In which type of screw is the thread diameter is bigger than the shaft diameter?
cancellous bone screw
In which type of screw is the thread diameter is the same as the shaft diameter?
partially threaded cortical bone screw
Which characteristic determines the screw strength?
smallest diameter
the bigger the smallest diameter the stronger the screw
What shape are most bone screw threads?
asymmetrical threads
flat on the upper surface in contact with the bone and rounded underneath
Why do most bone screws have asymmetrical threads?
wide SA on pulling side
little frictional resistance on underside
so that, more of the torque is used in pulling two objects together and less is wasted on simply overcoming friction during insertion of the screw.
What is the relationship between thread depth and resistance to pulling screw out?
increase together
a deeper thread will capture more material between the threads
What is the thread depth?
half the difference between the thread diameter and the core diameter.
What is the pitch of a screw?
the linear distance travelled by the screw for a complete (360°) turn of the screw
What is tapping?
the process of cutting a thread
Which types of screws are self-tapping?
cancellous bone screws
What shape of tip do cancellous bone screws have?
corkscrew
Why are cortical screws not self-tapping?
would cause damage to the bone and make screw impossible to insert due to excessive torque required.
What is lagging?
compressing two objects together
How can screws achieve a lagging effect?
- be designed partially threaded
2. pre-drill a gliding hole: bigger than thread diameter in
Name three applications of orthopaedic screws
- hold a plate to bone
- stop sideways desplacement of a bone fragment
- To increase the grip of an intramedullary nail on the bone
Where are screws and plates used to fix long bone fractures?
around joints
bones of forearm
pelvis
face + jaw
What is osteosynthesis?
the reconstruction of a fractured bone
When can plates and screws carry out osteosynthesis?
simple fracture when bone can be restructed with few defects so that the bone and the plate share the load
Which technique is used in complex fractures where osteosynthesis cannot be done? why is it done
bridging technique = plate over unfixed section connects two fixed sections
To restore bone length and alignment, and keep blood supply intact.
What is the aim of plate fixation?
to create load sharing between bone and plate
State the main cause of fatigue failure of a plate
Stress reversals due to inadequate load sharing, i.e. the bone not taking enough of the applied load.
When are plates more easily bent?
when there is a gap at the fracture site
When is it appropriate to use a plate?
- when you can achieve load sharing
- when anatomical alignment is very important
- if bending forces would distort screw only
Why might a loaded bone distort more on one side than the other?
eccentric loading if soft tissues stripped only from one side of the bone
Should a plate go on the tension or compression side of the bone? Why?
the tension side to counteract the eccentric load
Why should a plate be bent slightly more concave than the bone?
encourage compression of the bone opposite the site of attachment of the plate which aids in load sharing
What is the main disadvantage of plating?
stripping of the soft tissues damages the blood supply which can delay healing and risk infection
Is a K-wire a pin or a wire?
a pin
Why are pins used in pairs?
minimises rotation
When are pins used in combination with wires? give an example
to achieve compression between two small bone surfaces
eg. patellar fracture
Give two uses of flexible wire?
- cerclage
2. tension band
What is cerclage?
encircling or crossing the fragments, pushing them together
Why have nails mostly replaced cerclage? when is it still used
because cerclage damages blood supply more
still used around endoprostheses
Give 2 sites where tension band wiring is used
patella
olecranon
medial malleolus of ankle
How does tension band wiring 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. In effect, the more the muscles work, the firmer the fracture is compressed and the more stable it will be
What are the main advantages of intramedullary nailing?
stabilises fracture without damaging blood supply
withstand axial and bending stresses
early mobilisation after surgery
What is the antegrade nailing technique?
nail goes in to other side of bone from fracture site so as not to disturb soft tissues
What is reaming?
widen intramedullary canal
What are most intramedullary nails made of?
stainless steel
Why isn’t titanium suitable for IM nails?
susceptible to notch senstivity where it weakens if a hole is drilled across it or it is abraded
What is the working length of an IM nail?
The length of a nail that transmits load from one main fragment of a fractured bone to the other
How is the resistance of the tubing to deformation by bending and twisting related to the working length of an IM nail?
inversely proportional
Is a solid or hollow nail of the same diameter stronger?
solid as more metal per volume
Is a solid or hollow nail stiffer in bending?
solid
How can a hollow nail be made stronger and stiffer?
increase thickness of the wall
In which bones are IM nails most commonly used?
femur and the tibia
What is a simple IM nail?
when used alone eg. in long bones
How can the effective working length of a nail be increased?
by adding cross-screws
Give an example of a plate being used together with an intramedullary nail.
Fractures of femoral neck
Due to the shape and loading of the femur, the natural tendency in an extracapsular fracture is for the proximal fragment to keel over medially and for the femur to shorten.
What are the main complications of IM nails?
reamer penetrates to outside bone
rotatory misalignment of the bone
infection
Give some non-trauma uses of external fixators
limb shortening
limb lengthening
arthrodesis
correction of angulatory or rotatory deformity
What is the difference between temporary and definitive external fixation?
Temporary usage is commonly associated with open fractures where the damage to the soft tissues is extensive.
Definitive external fixation may be used until soft tissue healing and right through to fracture healing.
What does planning of the fixation frame in definitive external fixation have to take in to account that temporarty use would not?
pin placement does not interfere with soft tissue during mobilisation, and fragments can slide so that a callus will form
What is dynamisation of an external fixator?
Release of the fixator to allow axial (proximal-distal) movement.
Which 2 principles must be kept in mind when configuring the frame of an external fixator ?
- stable not rigid bone/frame construct
2. pin placement must not tether soft tissues or restrict access to wounds.
Why are unilateral frames preferred over bilateral frames?
give adequate stability to fractures whilst permitting mobilisation, excellent access to wounds and also keeping soft tissue tethering to a minimum.
Post-fracture, how can external fixator stability be determined
frame configuration degree of contact between the bone ends. extent of the soft tissue injury. quality of the bone/pin interface. degree to which the clamps have been properly tightened. total number of pins used.
When is dynamisation used?
soft tissues well on way to healing
frame being used as definitive treatment
What are the main disadvantages of external fixation?
infection at pin sites
loosening of pins
soft tissue tethering is inevitable
