Surgical screw fixation

Question 1

Name different parts of screw

Answer 1

Core: main part
Thread: helical outside
Effective thread depth: (thread diameter-core diameter)/2

Question 2

Pitch of screw

Answer 2

Distance screw advances per 360 degrees

The finer the pitch the less the screw travels

The coarser the pitch, the more it travels

Question 3

How does a self threading self tapping cortical screw look like

Answer 3

2 or more flutes at the tip allowing cutting a thread and avoiding jamming

Question 4

How to measure self-cutting screws length (as opposed to a normal screw)

Answer 4

Measure screw length and add 2 mm

The flute shouldnt stay in the cortex, otherwise bone grows into it and makes removal very diffictult

Question 5

What is countersinking

Answer 5

Sinking the head of the screw into cortex

It applies the compression to a wider area, reducing the stress on bone

Question 6

What is wrong with cancellous screws (epiphyseal/metaphyseal bone)

Answer 6

Underlying cortex too thin to countersink
Head compresses cancellous bone beneath and may fail

Instead, use a washer to increase the surface area

Question 7

What are the measurements for a standard cortical screw

Answer 7

4.5 mm (outside diameter)
8 mm head
Core: 3.1mm
3.2 mm pilot hole

Question 8

What are measurements for a small cortical screw

Answer 8

3.5 cortical screw
2.4 mm core
2.5 mm pilot hole drilling

If used as lag:
3.5 mm gliding hole
Thread cut 3.5 cortical tap

Question 9

Conditions for a screw being used for lag technique

Answer 9

1. Gliding hole drilled in near cortex
2. thread of screws only purchase in far cortex
3. screw head to arrest screw progressing

Question 10

Lag technique steps

Answer 10

1. gliding hole drilled from outside
2. reduce fracture
3. pilot hole drilled in far cortex
4. near cortex countersunk
5. measure
6. tap far cortex using tap sleeve
7. insert screw

Question 11

How to correctly measure screw length in an oblique fracture

Answer 11

Hook on to obstuse angle instead of acute

If must hook on to acute angle, add 2-4 mm as otherwise too short

Question 12

What force does a lag screw resist if inserted perpendicular to long axis of the bone vs perpendicular to fracture line

Answer 12

Max resistance to shear forces

vs

Max interfragmentary compression

Question 13

How to achieve interfragmentary compression and resistance to shear forces in an oblique fracture

Answer 13

plate (in neutralisation mode) and screws (perpendicular to fracture)

Question 14

Cortical screws vs cancellous screws

Answer 14

Cancellous:
- Deeper thread
- different design tip
- spiral increases from tip to full thread diameter over 2 turns

Cancellous screw creates its own thread by pushing bone aside

Question 15

Standard large cancellous screw measurements

Answer 15

6.5 mm cancellous screw
Core: 3.1 mm
Pilot: 3.2 mm
unthreaded shaft: 4.5 mm
Outside thread: 6.5 mm

Question 16

Standard small cancellous screw

Answer 16

3.5 mm cancellous screw
core: 1.9mm
Pilot hole: 2mm
unthreaded shaft: 2.4 mm
Outside thread: 3.5 mm

Question 17

Steps for a cancellous screw insertion

Answer 17

1. reduce fracture
2. drill a pilot hole (3.2 for large)
3. tap the cortex, begining of cancellous bone
4. choose the length (16 or 32 mm)
5. consider using a washer
6. insert screw

Question 18

Types of locking head screws

Answer 18

Self tapping: used in metaphyseal area after predrilling

Self tapping self drilling: used in diaphysis, unicortically