The bone screw

Question 1

What is a screw?

Answer 1

A mechanism that produces linear motion as it is rotated

Question 2

What does an orthopaedic screw consist of?

Answer 2

Head
Helix shaped thread on a shaft
Tip

Question 3

What are the 2 conditions needed for a screw to fix together 2 objects?

Answer 3

The head of the screw is wider than the diameter of the shaft so that it pushes block 1 against block 2

The thread does not grip block 1

Question 4

How can it be made sure that the thread does not grip block 1?

Answer 4

Screw must have no thread on the section nearest to the head where it touches Block 1
or
If a screws thread is present, block 1 must have a pre-drilled hole in it which is larger than the screw thread

Question 5

What would happen if the thread were to grip both blocks?

Answer 5

The screw would move through both block so that it could not compress them together no matter how close they were

Question 6

What factors determine the “holding together” strength of the two objects?

Answer 6

Strength of the screw material
Strength of the object material
Design of the screw thread

Question 7

What are the 2 functions of the head of a screw?

Answer 7

Provide a buttress to stop the whole screw sinking into the bone
Provides a connection with the screwdriver

Question 8

Why is a washer used between the head and bone in soft bone screws?

Answer 8

Makes the buttress bigger - spreading the load over a larger area

Question 9

What drive connections are most commonly used in bone screws?

Answer 9

Hexagonal

Star shaped

Question 10

Why do bone screws commonly used a hexagonal shaped drive connection?

Answer 10

Gives an effective coupling unlikely to be damaged in the screwing process
Positive interlock makes it easy to use - no axial force needed to retain the driver in the head

Question 11

What shape is the undersurface of the head in bone screws? Why?

Answer 11

Rounded - so there is max area of contact between bone and screw head after countersinking - reducing the risk of a zone of excessive stress which may crack the bone

Question 12

What are the 3 diameters to be considered in a screw?

Answer 12

Core diameter (smallest diameter of the threaded section of the shaft) 
Shaft diameter (diameter of the shaft where there is no thread) 
Thread diameter (diameter of the widest part of the threaded section)

Question 13

In what type of screw is the thread diameter bigger than the shaft diameter?

Answer 13

Cancellous bone screw

Question 14

What diameter affects the strength of a bone screw?

Answer 14

The smallest diameter (whether it is core or shaft) - the greater the smallest diameter the stronger the screw

Question 15

What are the 3 important aspects of the thread?

Answer 15

Shape
Depth
Pitch

Question 16

Describe an assymmetrical thread

Answer 16

Flat on the upper surface in contact with the bone and rounded underneath

Question 17

Why do most bone screws have assymmetrical threads?

Answer 17

Provide a wide surface on the pulling side and little fricitional resistance on the underside - more of the torque is used in pulling 2 objects together and less is wasted overcoming friciton during insertion

Question 18

How is the thread depth calculated?

Answer 18

Half the difference between the thread diameter and the core diameter

Question 19

What determines how well the screw resists being pulled out of bone?

Answer 19

The amount of thread in contact with the bone

Question 20

When is a deeper thread more desirable? Why?

Answer 20

In weak cancellous (spongy) bone - will capture more material between the threads and increase the resistance of the screw to pulling out

Question 21

What is the pitch of a screw?

Answer 21

The linear distance travelled by the screw for a complete (360) turn of the screw

Question 22

How important is the pitch in influencing the holding strength of a screw?

Answer 22

Not as important as depth and shape

Question 23

What is tapping?

Answer 23

The process of cutting a thread

Question 24

What is a self-tapping screw?

Answer 24

A screw which has a cutting tip that enables it to cut its own “female” thread track as it is being inserted

Question 25

Which type of screws are self-tapping?

Answer 25

Cancellous bone screws

Question 26

When might a tapping instrument not be needed?

Answer 26

If bone is soft (e.g. cancellous)

Question 27

How are cancellous bone screws inserted?

Answer 27

Have corkscrew tip - no pre-tapped hole - material of soft bone is compressed as the screw is driven in, enhancing grip strength

Question 28

What is the advantage of ‘pre-tapping’?

Answer 28

Screw is easier to insert - so most of the torque applied when tightening the screw is converted into compression
Also no bone fragments jamming between the screw and thread in the bone which would increase frictional resistance and risk damage to the bone thread

Question 29

What are flutes?

Answer 29

Channels which provide a route for cuttings to escape

Question 30

Why do cortical screws not have flutes?

Answer 30

Because bone can grow into the flutes making removal difficult

Question 31

What is lagging?

Answer 31

The process of compressing 2 objects together

Question 32

What are the 2 methods by which legging can be achieved?

Answer 32

Using lag screws

Inserting a screw in such a way that any screw can act as a lag screw

Question 33

What is the advantage of stabilisation of bone fragments in bone fixation?

Answer 33

Provides basis for early mobilisation of the limb

Prevent excessive movement of metal implants which could lead to metal fatigue failure

Question 34

How does a lag screw work?

Answer 34

Hole in fragment A
Screw cuts its own head in fragment A
Screw cuts own thread in fragment B
Unthreaded shaft slides through fragment A until head stops surface of bone
As screw advances pulls 2 fragments together

Question 35

How can any screw be made to function as a lag screw?

Answer 35

Make a hole (gliding hole) slightly larger than the thread diameter in cortical bone fragment A
Screw will then slip through without the need to twist it with a screw driver

Make a hole equivalent to the core diameter in fragment B
Screw will now advance gripping the bone in fragment B

As head comes into contact with fragment A will compress the 2 fragments together

Question 36

Name 5 uses of screws

Answer 36

To prevent sideways displacement of fragments
Hold a plate against bone
Increase the grip of an intramedullary nail on the bone
Permit displacement in an axial direction
As part of an external fixator assembly

Question 37

Give an example of how a screw is used to prevent sideways displacment of fragments

Answer 37

Commonly used in around joints to hold cancellous fragments together - e.g. medial malleolar fragment in an ankle fracture