Implant technology - unit 6B deck 2 Flashcards
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 fractures 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
- A bone graft may be added at the site of any deficit (preferably taken from elsewhere on the patient - an autogenous graft)
- This will encourage the healing of defects and accelerate the healing of the bone before the plate becomes at risk from fatigue failure.
What areas of the body where the indications for plate use commonly arise
- 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 placed 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
Define eccentric loading and state how may it occur in fractures
It is when bones are not loaded evenly along their axes
It 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 in fractures ?
- 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 and preventing fracture gap widening.
- 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 this situ the plate is working as a tension band/device as it is experiencing an equal and opposite reaction 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]
Other than placing the plate on the tension side of a fracture what else can be done to encourage compression of the bone opposite the site of attachment of the plate
- Contouring a plate before use, so that it is bent slightly more concave than the bone
- This 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.
what is disadv of plating technique
- A lot of soft tissue stripping occurs which further damages the blood supply
- This contributes to a delay in healing and increased risk of infection
What were the first devices available to orthopaedic and fracture surgeons?
Flexible wire and relatively stiffer, sharp ended pins called k-wires
What is the function of flexible wires and K wires/pins?
They 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.
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
Describe the shape of pins/K-wires and how they are inserted into bone
They have sharp “trochar points” or occasionally threaded, self tapping ends which are driven into bone fragments by hand driven T-handles or on hand or even air powered drills.
when are pins/k-wires commonly used
- In small bones
- Occasionally in upper limb long bones in small children
- Foot and hand bones in adults commonly
Why are pins/k-wires generally used in pairs ?
To minimise the rotatory element in the final pin/bone construct.
when may pins and flexible wires be used in conjunction
To achieve compression between two small bone surfaces, for example in a patella fracture
what are flexible wires used for
to induce compression
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
how does dynamic compression by flexible wires work
Muscle contractions working against the fulcrum caused by the wire which holds the fragments together immediately under the wire and produces compression at the far side of the fracture fragments (basically holds fragments in compression)
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]
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 and easier to use
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.
where are sites, other than the patella, that the flexible wires work as dynamic compression
- olecranon
- medial malleous of the ankle
what is essential to ensure when working with flexible wires
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
What is a cerclage wire?
A wire that crosses or encircles a fracture fragment
where is a intramedullary (IM) nail placed and how does it function
It is placed 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.
Because IM nails are study and round in shape what does this confer in terms of resisting stresses on them?
They can withstand axial and bending stresses
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 - True or false?
True
what is the name of modern IM 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
what is the name of the old IM 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
What is the current debate on what new IM nailing techniques should do ?
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).
Why has high quality IM nailing been one of the most powerful developments in modern trauma surgery?
It has shown that early aggressive IM fixation dramatically reduces injury mortality and post-operative morbidity
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
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
why is titanium not used for IM nails
More susceptible to weakening either if a hole is drilled across it, or if it is accidentally abraded during insertion or locking - a phenomenon known as notch sensitivity.
Define what the working length of an IM nail is
This is the length over which the IM nail transmits load from one main bone fragment of a fractured bone to another
why is the working length of an IM nail important
Because 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
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 in terms of working length
It will have a short working length ==> the ability of the nail to resist bending and torsional forces will be high
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
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.
IM 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.
How can IM nails be made to further conform to the internal diameter/shape of the bone ?
IM nails can be curved to conform roughly to the shape of the bone for which they are designed - e.g. 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
Hollow nails are less stiff in bending than solid ones, how may there stiffness and ==> strength be altered?
The thicker the wall the stronger and stiffer the nail
[stiffness in bending is proportional to the diameter4 and the strength proportional to diameter3. This means that as nails get a bit stronger they get considerably more stiff.]
what is an adv of a slightly flexible IM 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.
why are very stiff IM nails not ideal
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
What is one way of reducing stiffness of a IM nail but state its consequence
- To put a longitudinal slot in the wall of a nail which makes it much more flexible.
- But does so at the cost of it losing some overall bending strength and, especially, torsional strength
what is normally the design and dimensions of a nail
- usually curved according to the shape of the bone involved and have longitudinal slots along their length
- wall thickness of 1.2 mm
gives an optimal balance between strength and flexibility and permit good contact between nail and femur
