SCREWS

1.  What is a screw and what is the function of a screw?

A  screw is a mechanical device which converts a rotational force (torque) into a linear movement. Its function is to connect two or more objects by compressing them together.

2.  What are the names of the different parts labelled above?

•      A – Head

•      B – Shank

•      C – Run out (this is the area where screws tend to break)

•      D – Thread

•      E – Tip

•      F – Shaft

•      G – Pitch

•      H – Core diameter/minor diameter

•      I – Thread diameter/major diameter

•      J – Thread depth

3.  Can you tell me the function of the following parts of a screw?

•     Head – This provides an area for attachment of a screwdriver and is designed to prevent slip and to improve directional control. It also acts to prevent forward motion of the screw when it is fully seated against bone. Hexagonal head recess design is the most common.

•     Flutes – These are present in self-tapping screws and they provide a route for the removal of swarf (bone debris).

•     Pitch – This is the distance between threads. It is the same as the distance advanced for every single (360 degrees) turn of the screw.

•     Core – The size of the core determines the strength of screw and its fatigue resistance. The size of the drill bit used is equal to the core diameter.

•     Threads – Thread depth is half of the difference between thread diameter and core diameter. The thread depth determines the amount of contact with the bones, which in turn determines the resistance to pull out.

4.  What is the difference between tensile strength and pullout strength in relation to screws?

 The tensile strength of a screw is its resistance to breaking. This is proportional to the core diameter squared.

  The pullout strength of a screw depends on the outer diameter of the threads and the area of the threads in contact with the bone. This is the effective thread depth ( J in the picture above) and this is proportional to the pullout strength.

5.  What type of screw is pictured here?

      This is a partially threaded, cancellous screw.

6.  How does the pictured screw differ from a locking bolt used with an intramedullary nail?

A  locking bolt has superior rotational stability to a screw owing to its wide core diameter. It is particularly useful in osteoporotic bone of the distal femur to prevent the ‘broomstick in a trashcan’ phenomenon.

7.  What drill sizes are required to insert a small fragment cortical screw and a large fragment cortical screw in order for them to act as lag screws?

A  lag screw requires two different drill sizes in order to create a gliding hole (near cortex) and a threaded hole (far cortex) as per AO techniques.

  A small fragment screw has a major diameter/thread diameter of 3.5 mm and a minor diameter/core diameter of 2.5 mm. This therefore requires:

•      3.5  mm drill bit (silver) for the gliding hole

•      2.5  mm drill bit (gold) for the threaded hole