Implant Technology Unit 6b Flashcards
what are the 2 methods of fixing fractures
internal fixation
- bone screws and plates, IM nails
external fixation
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.
how does a screw work in fracture fixation
hold two broken fragments together
may be used in isolation or in combo with plates
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.

how does a screw work
fixes together two (or more) objects by compressing them against each other
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
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
what are the 3 components of a screw
head
shaft
tip
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
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.
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
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
why is careful screw design important
try to avoid causes of high stress called stress raisers
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.
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).]
what are the 3 aspects of the thread
shape
depth
pitch
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
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
what is the depth of the thread
half the difference between the thread diameter and the core diameter
what determines how well a screw resists being pulled out of bone
The amount of thread in contact with the bone
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
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]
what is tapping
process of cutting a thread
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
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.
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]
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
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.
what to tapping instruments and self-tapping screws have
flutes
- i.e. channels which provide a route for cuttings to escape
why do cortical screws have no flutes
because bone can grow in to the flute and so make removal difficult
what is lagging
process of compressing two objects together
[screws area a means of compressing objects together]
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.
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.
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.
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
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
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
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
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
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
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.
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
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”]
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
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
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]
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
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
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.
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
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.
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
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
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.
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.
in what areas of the body is bone compression achieved using wires as tensions bands
around the olecranon of the elbow
patella
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]
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.
