Screws Flashcards
size of screw applied to bone?
Thread diameter should be 25% of the diameter of the bone (up to 33%)
What is the difference between cortical and cancellous screw?
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
Self-tapping vs non-self tapping?
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
What is the bending strength related to?
Core diameter of the screw
What is the pullout strength related to?
Outer diameter, thread depth and in what material it is placed
What is a locking screw?
Threaded head -> locks into the plate
Lag screws - how is it placed in lag fashion?
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
What direction must a lag screw be placed in relation to the fracture line for it to function?
perpendicular
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?
3,5mm glide hole drill bit
What size diameter drill bit do you use to make a transcortex hole for your 3,5 mm lag screw?
2,5mm drillbit
What is important to do with the head of a lag screw?
counter sink - to spread the stress created when compressing the fragments
What are the steps of inserting a screw into bone?
1) drill
2) measure (measure gauge)
3) tap (skip if using self-tapping)
4) screw insertion
5) screw tightening
What is important when drilling?
Sharp drill
Always drill guide to prevent slipping
Avoid too much pressure
Measurement?
Careful with sitable depth gauge, not based on XR
Always add 1-2mm to ensure full engagement of screw in transcortex
Tapping?
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
Countersinking?
Always when screw engages bone directly and not a plate.
Results in increased contact area -> reducing stress at this point
Where do we not countersink? What do we do?
Metaphyseal and epiphyseal bone - cortex too thin
Use a washer
How do we maximise interfragmentary compression with lag screws?
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
What about single lag screws?
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
Cortical screw - thread, core, drillbit and lag screw?
Cancellous screw - thread, core, drillbit?
Locking screws size, drill bits?
Plate screws?
Attach plate to bone
Locking or non-locking
Non-locking can be placed in a lag fashion through a bone plate
