Implant technology - unit 6B deck 2

Question 1

what are indications for use of plates

Answer 1

1. When anatomical alignment must be restored accurately
2. Where the use of screws alone is inadequate (because large bending forces distort fractures fixed only by screws)
3. When load sharing may be achieved with confidence

Question 2

what can be done if loading sharing between bone and the plate cannot be guaranteed

Answer 2

• 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.

Question 3

What areas of the body where the indications for plate use commonly arise

Answer 3

1. Around joints - failure to restore a joint surface may lead to OA
2. Bones of forearm
3. Pelvis - esp around acetabulum
4. On the face and the jaw

Question 4

what can be done with plate-screw combos to make them more efficient

Answer 4

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

Question 5

Define eccentric loading and state how may it occur in fractures

Answer 5

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.

Question 6

how can a plate help with eccentric loading in fractures ?

Answer 6

• 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]

Question 7

in what areas of the body is bone compression achieved using wires as tensions bands

Answer 7

1. around the olecranon of the elbow
2. patella

Question 8

wherever possible where should plates be placed

Answer 8

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]

Question 9

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

Answer 9

• 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.

Question 10

what is disadv of plating technique

Answer 10

• 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

Question 11

What were the first devices available to orthopaedic and fracture surgeons?

Answer 11

Flexible wire and relatively stiffer, sharp ended pins called k-wires

Question 12

What is the function of flexible wires and K wires/pins?

Answer 12

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.

Question 13

when are IM nails used and what is used along side them by the surgeon

Answer 13

• 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

Question 14

Describe the shape of pins/K-wires and how they are inserted into bone

Answer 14

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.

Question 15

when are pins/k-wires commonly used

Answer 15

• In small bones
• Occasionally in upper limb long bones in small children
• Foot and hand bones in adults commonly

Question 16

Why are pins/k-wires generally used in pairs ?

Answer 16

To minimise the rotatory element in the final pin/bone construct.

Question 17

when may pins and flexible wires be used in conjunction

Answer 17

To achieve compression between two small bone surfaces, for example in a patella fracture

Question 18

what are flexible wires used for

Answer 18

to induce compression

Question 19

what are the two ways to use flexible wires

Answer 19

1. Used statically by encircling or crossing the fragments (cerclage), pushing them together so that the cleavage lines produced by the fracture are pushed together
2. Used dynamically as a tension band, utilising the power of surrounding muscle to produce compression at a fracture site - useful in patella fractures

Question 20

how does dynamic compression by flexible wires work

Answer 20

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]

Question 21

what have in recent years replaced cerclage in most situations and why

Answer 21

nails - as nails do less damage to the blood supply is used properly and are much more robust and easier to use

Question 22

what are examples of flexible wires being used statically

Answer 22

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.

Question 23

where are sites, other than the patella, that the flexible wires work as dynamic compression

Answer 23

• olecranon
• medial malleous of the ankle

Question 24

what is essential to ensure when working with flexible wires

Answer 24

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

Question 25

What is a cerclage wire?

Answer 25

A wire that crosses or encircles a fracture fragment

Question 26

where is a intramedullary (IM) nail placed and how does it function

Answer 26

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.

Question 27

Because IM nails are study and round in shape what does this confer in terms of resisting stresses on them?

Answer 27

They can withstand axial and bending stresses

Question 28

What can be achieved by combining nails with screws and plates

Answer 28

Rotatory control of the bone can be achieved and length maintained

Question 29

Limbs with nailed fractures may be mobilised early after surgery, with weight bearing before bony union - True or false?

Answer 29

True

Question 30

what is the name of modern IM nailing technique and what does it entail

Answer 30

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

Question 31

what is the name of the old IM nailing technique and what does it entail

Answer 31

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

Question 32

What is the current debate on what new IM nailing techniques should do ?

Answer 32

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).

Question 33

Why has high quality IM nailing been one of the most powerful developments in modern trauma surgery?

Answer 33

It has shown that early aggressive IM fixation dramatically reduces injury mortality and post-operative morbidity

Question 34

what are the 3 important considerations of the design of IM nails

Answer 34

1. Material of which it is made
2. How much of the nail is in contact with the bone for any particular fracture
3. Dimensions and shape of the nail - and its wall thickness if it is hollow

Question 35

what are most nails made of

Answer 35

stainless steel:

• Has good strength and stiffness characteristics
• Easy to handle during the manufacturing process
• Well tolerated by the body tissues

Question 36

why is titanium not used for IM nails

Answer 36

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.

Question 37

Define what the working length of an IM nail is

Answer 37

This is the length over which the IM nail transmits load from one main bone fragment of a fractured bone to another

Question 38

why is the working length of an IM nail important

Answer 38

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

Question 39

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

Answer 39

It will have a short working length ==> the ability of the nail to resist bending and torsional forces will be high

Question 40

in what scenarios does IM nail have a long working length

Answer 40

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

Question 41

what does a long working length of a IM nail mean

Answer 41

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.

Question 42

IM nails can either be solid or hollow - what is the difference

Answer 42

• 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.

Question 43

How can IM nails be made to further conform to the internal diameter/shape of the bone ?

Answer 43

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

Question 44

Hollow nails are less stiff in bending than solid ones, how may there stiffness and ==> strength be altered?

Answer 44

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.]

Question 45

what is an adv of a slightly flexible IM nail

Answer 45

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.

Question 46

why are very stiff IM nails not ideal

Answer 46

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

Question 47

What is one way of reducing stiffness of a IM nail but state its consequence

Answer 47

• 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

Question 48

what is normally the design and dimensions of a nail

Answer 48

• 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