Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

BMSc - Applied Ortho Semester 2 > Implant technology - unit 6B deck 1 > Flashcards

Implant technology - unit 6B deck 1 Flashcards

1
Q

what are the 2 methods of fixing fractures that require surgical intervention ?

A
  1. Internal fixation
  2. External fixation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are the different ways internal and external fixation can be achieved ?

A

Internal fixation:

  1. bone screws,
  2. bone screws and plates,
  3. intramedullary (IM) nails.

External fixation via external fixators

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what are the materials commonly used for fracture fixation

A

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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Why must the plates and screws used in fracture fixation be of the same material?

A

If not then galvanic corrosion of the implant likely

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is a screw and describe its structure

A

A screw is a mechanism that produces linear motion as it is rotated

  • It has a helix shaped thread on a shaft
  • By turning one end, called the head, the screw will move through a stationary object.
  • The object may have a thread in it to match that of the screw, or it 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 well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what are the 3 components of a screw

A

head
shaft
tip

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what is the function of a screw

A

fixes together two (or more) objects by compressing them against each other

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what is required for a screw to be able to fix together two objects

A
  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

[think of the blocks as the fracture fragments]

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

how can it be assured that the thread does not grip block 1 (fragment 1)

A
  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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what are the 3 factors that determine the strength of a screw fixation

A
  1. Strength of the screw material
  2. Strength of the object material [bone in ortho]
  3. Design of the screw thread
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what are the 2 functions of the head of a screw

A
  1. Provides a buttress (support) 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.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What shape is the connection on the head of the screw in bone screws usually and why

A

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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe the usually undersurface shape of the head of a screw and why?

A

It is rounded (fig.) so that there is the 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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What does careful screw design help avoid

A

try to avoid causes of high stress called stress raisers

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What are the 3 different diameters to consider in a screw?

A
  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.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what is the strength of the screw determined by

A

Determined by the smallest diameter - the greater the smallest diameter, the stronger the screw will be

[In some screws the thread diameter is > shaft diameter e.g. in a cancellous bone screw. In others the thread diameter = shaft diameter e.g. some partially threaded cortical bone screws]

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the thread of a screw and what is its function

A

An inclined plane which is rotated in the thread hole in the bone so that the screw moves forward in response to being twisted.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

what are the 3 important aspects of the thread

A
  1. shape
  2. depth
  3. pitch
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

most bone screws have symmetrical threads - true or false - and why are they designed this way

A

false - they have asymmetrical threads

  1. They are flat on the upper surface in contact with the bone and rounded underneath this provides a wide surface on the pulling side and little frictional resistance on the underside
  2. 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
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

what is the depth of the thread determined by

A

It is half the difference between the thread diameter and the core diameter

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

what determines how well a screw resists being pulled out of bone

A

The amount of thread in contact with the bone (thread depth)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Why is a deeper thread depth desirable in cancellous (spongy) bone ?

A

A deeper thread will capture more material between the threads and so increase the resistance of the screw to pulling out which is needed in weaker cancellous bone

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

what is the pitch of a screw

A

the linear distance travelled by the screw for a complete (360 degree) turn of the screw

[not so critical in influencing holding strength]

24
Q

what is tapping

A

The process of cutting a thread

25
Q

what is a self tapping screw

A

A screw which has a cutting tip that enables it to cut its own “female” thread track, as it is being inserted. The “female” cut is helical to match the “male” thread on the screw.

26
Q

What bone screws are all self-tapping?

A

Cancellous bone screws

27
Q

what is special about the malleolar screw

A
  • 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.
28
Q

What may a bone screw be able to do in cancellous bone but not in cortical bone ?

A

Screw may be permitted to ‘force’ a thread track without the need for a tapping instrument

[this is not possible in hard cortical bone because too much torque would be required, risking jamming or breaking the screw]

29
Q

What type of type do cancellous bone screws have and why ?

A

A corkscrew tip to engage the screw in the bone and, because there is no pre-tapped hole, the material of the soft bone is compressed as the screw is driven in, enhancing grip strength in otherwise quite weak bone.

30
Q

Are standard screws self-tapping ?

A

No - they are pre-tapped which is where a separate tapping instrument to cut a female thread in the bone to match the male thread on the screw.

This ‘pre-tapping’ means that the screw is easier to insert so that most of the torque applied when tightening the screw is converted into compression. Also means no jamming of bone fragments between screw and the thread which would increase frictional resistance and risk damage to the bone thread.

31
Q

What do tapping instruments and self-tapping screws have ?

A

Flutes which provide a route for cuttings to escape

32
Q

Why are cortical screws not self-tapping?

A

Self tapping would cause damage to the bone and make screw impossible to insert due to excessive torque required.

33
Q

Why cant cortical screws have a flute ?

A

Because bone can grow in to the flute and so make removal difficult.

34
Q

What advantages do hexagonal and star head screws have over cross head screws?

A

Good coupling so no damage to screw heads. Positive interlock between screwdriver and screw

35
Q

what is lagging

A

This is the process of compressing two objects together

[screws area a means of compressing objects together]

36
Q

In bone fixation, screws provide stabilisation of fragments, which provides a basis for what? and what do they help prevent

A

Early mobilisation of the limb and will prevent excessive movement of metal implants which could lead to failure through metal fatigue

37
Q

Screws can achieve a lag effect in two ways:

Using screws specifically designed as lag screws is one way this effect can be achieved, how do they work?

A

Specifically designed lag screws are partially only at the section nearest the tip. These work by:

  1. 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
  2. Unthreaded shaft then slides through the hole in the first fragment until the head touches the surface of bone A
  3. As the screw advances the head pulls fragment A towards fragment B (which now contains the threaded part of the screw).
  4. In this way the two pieces of cancellous bone have been compressed (lagged) together.
38
Q

Screws can achieve a lag effect in two ways:

One way allows any screw to act as a lag screw even if it is threaded along all its shaft i.e. even if it is not designed as a lag screw, how does this work?

A
  1. A hole (the gliding hole) 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.
  2. 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.
  3. 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
39
Q

what is essential to ensure when using lagging as a fixation technique

A

To position screws accurately - so forces generated are evenly distributed across fractures, otherwise distortion of bone is easily possible

40
Q

List the different applications/situations screws may be used in

A
  1. To prevent sideways displacement of fragments - Screws are commonly used alone around joints to hold cancellous bone fragments together
  2. To hold a plate against bone - 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.
  3. To increase the grip of an IM nail on the bone
  4. To permit displacement in an axial direction
  5. As part of an external fixator assembly
41
Q

What is the purpose of using screws in association with plates ?

A

Screws are used to hold the plate firmly to the bone so that the load is shared between the reconstructed bone and the plate.

42
Q

when are screws and plates used together

A
  • 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
43
Q

when is complete reconstruction of cancellous bone not possible

A
  1. Particularly violent fractures
  2. Soft bone found after a delay of a few days between injury and surgery
  3. Bone is unnaturally soft as it may be in old age
44
Q

In what situations may plates be used ?

A

In situations where fracture stability can be achieved and enhanced by compression at the fracture site by forcing bone fragments together

This reconstruction should be held together initially by screws, using lagging techniques. The screw fixation can then be reinforced or protected by means of a plate.

45
Q

Define what osteosynthesis is

A
  • The reconstruction of a fractured bone by surgical and mechanical means
  • In simple fractures this should be complete so that the shape of the bone is restored with few or no defects in overall bony architecture
46
Q

how do plates help with osteosynthesis

A

They help maintain the restored bony shape as when the bone is loaded e.g. by muscle activity, the bone and plate share the load.

47
Q

In some complex fractures with many fragments, 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.

In these situations what technique involving plates may be used ?

A

The bridging technique:

  • The two main bony shaft fragments may be linked by means of a plate to restore bone length and alignment.
  • The intervening small fragments 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.

==> bridging plates used to restore bone length and alignment and keep blood supply intact

48
Q

Describe the design of plates and their requirements

A

They are flat and relatively thin.

They must be as compact as possible and sufficiently malleable to allow shaping.

This means that they have limited capabilities to resist an applied load when stressed in certain directions

49
Q

how does the structure and loads bone resists differ from that of an ortho plate

A
  • Plates are flat and relatively thin and has capabilities to resist an applied load when stressed in certain directions
  • Bone on the other hand is a hollow tube and generally resists bending equally well in any direction
50
Q

What does the overall strength of bone depend on ?

A

Its wall thickness. Thin-walled bones are weaker than thick- walled bones.

51
Q

What is the aim of plate fixation

A

To 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

52
Q

What is plate failure usually due to ?

A

Metal fatigue caused by stress reversald due to inadequate load sharing i.e. the bone not taking enough of the applied load

[If load sharing is satisfactorily achieved then the plate is relatively safe from failure]

53
Q

what happens if the plates takes most of the load and when does this usually happen

A
  • The plate will be prone to bending. This can happen if there is a defect or gap at the fracture site, normally due to fractured bone not been accurately re-assembled
  • The plate bends backwards and forwards as the incomplete bone-plate construct is loaded, this backwards and forwards cyclical movement known as stress reversal which is likely to result in early fatigue failure of the plate
54
Q

what factors should be considered to prevent bending of the plate

A
  1. System of fixation with plates and screws should, together with the bone, form as stable a construct as possible,
  2. Ensure as little damage as possible should be done to the blood supply to the bone ==> healing of fracture is as quick as possible
  3. Plate should be placed in a position relative to the broken bone that it is minimally stressed
  4. Plate should be placed so there is min surrounding soft tissue damage
  5. Plate should be made of materials which are strong as possible and which can tolerate the fatigue effects of stress reversals
55
Q

State the main cause of fatigue failure.

A

BMSc - Applied Ortho Semester 2 (25 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions