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 ).