Screw Parts
Screw parts
https://www.slideshare.net/ssusere6b07d/screws-in-orthopaedic-surgerypptx
1. Head - hexagonal - increased torque with maximum contact - provides attachment for screwdriver
2. Counter sink -
run out between head/shaft and thread.
3. Shaft
inner core diameter
4. Thread outer diameter is directly proportionate to pull out strength.
80 % grip - near cortex
20 % grip from far cortex.
5. Crest/root of thread
6. Pitch (lead )-distance advanced for one 360 degrees turn (cancellous >cortical > locking).
7. Flutes -removes debris
8. Tip - cortical -blunt
cancellous-cork screw.
What are mechanical properties of a screw ?
A device that converts a torsional force into a linear force.
Effective thread depth-combination of pitch and thread (outer diameter ) which is proportional to pull out strength .
Tensile strength is directly proportional to inner core radius cubed.
How does a small fragment screw differ from locking bolt for a IM nail ?
Locking bolt- For creating a rotationally stable construct.
Wide inner core diameter (Ultimate tensile stress)
Screws do have small thread (does not require a large amount pull out strength).
Lag screw for a small fragment screw
large lag hole (near cortex = 3.5 mm drill bit )
small hole (far cortex 2.5 mm drill bit ).
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