Screws

Question 1

size of screw applied to bone?

Answer 1

Thread diameter should be 25% of the diameter of the bone (up to 33%)

Question 2

What is the difference between cortical and cancellous screw?

Answer 2

Cortical: Smaller pitch, less depth to thread
Calcellous: Bigger pitch, more depth to thread

Cortical is stronger due to core diameter being bigger relative to size

We prefer cortical screws

Question 3

Self-tapping vs non-self tapping?

Answer 3

Self-tapping have a cutting flute which means we do not need to “tap” the whole prior to placing the screws

Selftapping screws must exceed bone >2mm to ensure full contact throughout

Question 4

What is the bending strength related to?

Answer 4

Core diameter of the screw

Question 5

What is the pullout strength related to?

Answer 5

Outer diameter, thread depth and in what material it is placed

Question 6

What is a locking screw?

Answer 6

Threaded head -> locks into the plate

Question 7

Lag screws - how is it placed in lag fashion?

Answer 7

Near cortex is over-drilled (glide hole), far cortex is the one the screw engage with and with tightening it compresses the fracture bits together

Question 8

What direction must a lag screw be placed in relation to the fracture line for it to function?

Answer 8

perpendicular

Question 9

What size diameter drill bit do you use to make a glide hole for a 3,5mm cortical screw to be used as lag screw?

Answer 9

3,5mm glide hole drill bit

Question 10

What size diameter drill bit do you use to make a transcortex hole for your 3,5 mm lag screw?

Answer 10

2,5mm drillbit

Question 11

What is important to do with the head of a lag screw?

Answer 11

counter sink - to spread the stress created when compressing the fragments

Question 12

What are the steps of inserting a screw into bone?

Answer 12

1) drill
2) measure (measure gauge)
3) tap (skip if using self-tapping)
4) screw insertion
5) screw tightening

Question 13

What is important when drilling?

Answer 13

Sharp drill
Always drill guide to prevent slipping
Avoid too much pressure

Question 14

Measurement?

Answer 14

Careful with sitable depth gauge, not based on XR
Always add 1-2mm to ensure full engagement of screw in transcortex

Question 15

Tapping?

Answer 15

suitable for the screw you are using
always with drill guide/tissue protector
rotate a quarter antiblockwise every 3-4 turns to prevent clogging
replace when blunt

Question 16

Countersinking?

Answer 16

Always when screw engages bone directly and not a plate.
Results in increased contact area -> reducing stress at this point

Question 17

Where do we not countersink? What do we do?

Answer 17

Metaphyseal and epiphyseal bone - cortex too thin
Use a washer

Question 18

How do we maximise interfragmentary compression with lag screws?

Answer 18

The lag screw should be inserted in the middle of the fragment equidistalt from the fracture edges and directed at a right angle to the fracture plane

If not perpendicular -> can create shearing force

Question 19

What about single lag screws?

Answer 19

Not strong enough to adequately stabilise fractures, should be used in combination with neutralization plates or other fixation devices

If alone -> risk of overload -> break

Question 20

Cortical screw - thread, core, drillbit and lag screw?

Answer 20

Question 21

Cancellous screw - thread, core, drillbit?

Answer 21

Question 22

Locking screws size, drill bits?

Answer 22

Question 23

Plate screws?

Answer 23

Attach plate to bone
Locking or non-locking
Non-locking can be placed in a lag fashion through a bone plate