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Fracture Repair 1 > Function and Application of Bone Screws > Flashcards

Function and Application of Bone Screws Flashcards

1
Q

What are the 3 applications of bone screws?

A
  • Combination with plate/interlocking nail
  • # reconstruction combined with plate, EDF, IM pin
  • # of cancellous bone
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2
Q

Oblique diaphyseal fractures or fractures with large bone fragments can be reduced and stabilised with lag screws. Why are Lag screws are preferable to cerclage wires?

A

Placed to give more compression

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3
Q

True or false
Bone screws should never be used in isolation for diaphyseal #?

A

TRUE

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4
Q

What are the 3 basic screw types?

A
  • cortical
  • cancellous
  • locking screw
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5
Q

Generally speaking, which bone type are cortical screws used in? (3)

A
  • Diaphyseal
  • Cortical
  • Metaphyseal (dogs + cats)
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6
Q

Cortical screw:
A) Thread?
B) Pitch steepness?
C) threads per unit length compared to cancellous?

A

A) Fully threaded
B) Less steep
C) More

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7
Q

Is a cancellous or cortical screw stronger for the same size?

A

Cortical

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8
Q

True or false:
Cortical bone is only 2-3 mm thick in places

A

True

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9
Q

True or false:
With cortical screws the priority is to have a minimum number of threads engaging

A

FALSE

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10
Q

What are cancellous screws used for? (2)

A

To compress fragments of epiphyseal or metaphyseal bone

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11
Q

Cancellous screws:
A) Thread?
B) Thread depth?
C) Number of threads per unit length?
D) Pitch?

A

A) Partial or full
B) Deep
C) Relatively few
D) High (steep angle)

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12
Q

What surface area of cancellous screws thread in contact with the relatively low-density bone?

A

Large

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13
Q

Why are cancellous screws weaker?

A

Narrower ore diameter

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14
Q

Locking screws:
A) Head?
B) Pitch?
C) Core diameter?

A

A) Threaded
B) Narrow
C) Large

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15
Q

What are locking screws used in combination with?

A

Locking plates (in which the locking head engages with the threaded locking plate hole.)

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16
Q

Too much pressure needs to be avoided when using a drill - why? (2)

A
  • Prevent damage to transcortex
  • Avoid too larger hole
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17
Q

After using a depth gauge; what size screw is used?

A

Insert a screw 1-2mm longer than the measurement obtained with the depth gauge to ensure a full thread engages the transcortex.

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18
Q

If a screw strips accidentally; how can this be rectified?

A

screw one size larger can be used as ‘rescue screw’.

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19
Q

What equipment is needed to place screws? (7)

A
  • Screwdriver
  • Drill guide
  • Tap
  • Measure
  • Drill
  • Screws
  • Bone plate
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20
Q

Which screws can be used alone for # repair? (2)

A

Positional screw
Lag screw

21
Q

Can positional or lag screws achieve interfragmentary compression?

A

Lag

22
Q

To accomplish interfragmentary compression, lag screw threads:
A) should NOT engage where?
B) SHOULD engage where?

A

A) Near (cis) cortex
B) Far (trans) cortex

23
Q

Define lag screw

A

The term lag screw applies to the way a screw is functioning. A fully threaded cortical screw can be used as a lag screw if the thread is prevented from engaging with the near cortex but engages with the far cortex.

24
Q

what drill bits for a 3.5mm lag screw?

A

2.5 mm and 3.5mm

25
Q

What is the effect of failing to overdrill the near cortex when placing a lag screw?

A

The screw will act as a positional screw and not compress the fracture fragments

26
Q

How can a partially threaded cancellous screw be used as a lag screw?

A

The threads engage the far cortex and as the screw is tightened the near fragment glides on the smooth shank of the screw

27
Q

What is the negative of using a partially threaded cancellous screw as a lag screw?

A

A weak screw with a risk of screw failure at the junction of the smooth shank to threaded part of the screw

28
Q

How is a fully threaded lag screw placed?

A
  • Near fragment must be ‘overdrilled’ with a hole equal in size to the diameter of the screw threads – the glide hole - so the threads do not engage the near cortex.
  • A hole equal in diameter to the screw core is then drilled in the far cortex – the thread hole.
  • An insert guide is placed in the glide hole prior to drilling the thread hole to ensure that the hole is drilled at the correct angle.
29
Q

Why would a countersink instrument be used?

A

Countersinking is performed when the screw head is directly in contact with cortical bone.

30
Q

What does countersinking result in?

A

increased contact area between the screw head and the bone.

31
Q

Which bones is countersinking NOT used in (2) and why?

A

Metaphyseal
Epiphyseal

As cortex too thin

32
Q

What can be used in areas where countersinking is not possible to allow load distribution?

A

Flat washer

33
Q

What can lead to loss of reduction? (3)

A
  • Malposition of screw
  • Not counter sinking
  • # comminution
34
Q

To maximise interfragmentary compression, lag screws should be inserted:
A) Where on the bone?
B) What angle?

A

A) In the middle of the fragment equidistant from the fracture edges
B) At right angle to # plane

35
Q

If the screw is not inserted perpendicular to the fracture plane, then as it is tightened, what force is introduced?

A

Shearing

36
Q

What do lag screws need to be used with? (3)

A
  • Another lag screw
  • Neutralisation plate
  • Other fixation device
37
Q

Positional screws - what drill diamater is used?

A

Almost the same

38
Q

Ture or false
For positional screws, both cortices are drilled in one step?

A

True

39
Q

Why might a positional scre be recommended?

A

If the insertion of a lag screw would cause an unsupported fragment to collapse into the medullary cavity.

40
Q

Using a depth gauge for positional screws:
When using non self-tapping screw measurements are rounded to what mark?

A

up to nearest 2mm

41
Q

Using a depth gauge for positional screws:
When using self-tapping screw measurements are rounded to what mark?

A

up to nearest 4mm mark

42
Q

What is the core diameter of a 3.5mm cortical bone screw?

A

2.4mm

43
Q

The resistance to cyclic failure or bending of a screw is related to its area moment of inertia and is directly related to..?

A

Core diameter to the power of four.

44
Q

Small increases in screw diameter results in what change of stiffness?

A

Increase

45
Q

What size screw should be chosen?

A

The largest screw possible, that does not compromise the mechanical properties of the bone in which it is to be inserted, should be chosen.

46
Q

Experimental work in bone has shown that holes made within the bone will not significantly affect its mechanical properties providing that the hole diameter does not exceed ? % the diameter of the bone.

A

20

47
Q

What size drill bit for a 3.5mm screw?

A

2.7mm

48
Q

For conventional (non-self-tapping) screws, the hole needs to be tapped with a tap of what size?

A

Same size

Fracture Repair 1 (10 decks)

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