Surgical Fracture Fixation - Screws

Question 1

What can be used to achieve internal fixation?

Answer 1

bone screws
bone screws and plates
intramedullary nails

Question 2

What two materials are most commonly used in internal fixation?

Answer 2

stainless steel

titanium

Question 3

Why must plates and screw be made of the same material

Answer 3

if not the galvanic corrosion will occur

Question 4

What is a screw?

Answer 4

a mechanism that produces linear motion when it is rotated

Question 5

How can a screw fix objects together?

Answer 5

the head of the screw is wider than the shaft diameter so that it pushes one block against the next

either the screw must have no thread on the head section where it touches block one

or

a pre-drilled hole in block 1 which is larger than the screw thread

Question 6

What 3 factors determine screw fixation

Answer 6

strength of the screw
strength of the object
design of the thread

Question 7

What is the function of the head of the screw?

Answer 7

provides a buttress to stop the whole screw sinking into the bone - a washer can create a bigger buttress

provides a connection with the screwdriver

Question 8

Why are bone screw drive connections commonly hexagonal shaped?

Answer 8

effective coupling unlikely to be damaged by screwing

positive interlock makes it easy to use and no axial forces is required to maintain the driver in the head

Question 9

What is an area of high stress in screw fixation called?

Answer 9

stress raisers

Question 10

What 3 diameters are important in screws? What do they mean

Answer 10

core diameter - smallest diameter of the threaded section

shaft diameter - diameter of the shaft where there is no thread

thread diameter - widest part of the threaded section

Question 11

Which diameter determines the strength of the screw?

Answer 11

the core

Question 12

What 3 factors are important in screw thread?

Answer 12

shape
depth
pitch

Question 13

Why are threads flat on the upper surface and rounded underneath

Answer 13

wide surface on the pulling side but little frictional resistance on the underside - more torque is used pulling two objects together and less wasted on simply overcoming friction

Question 14

How is the thread depth calculated?

Answer 14

half the distance between the thread diameter and the core diameter

Question 15

In what type of bone is a deep thread desirable? Why?

Answer 15

cancellous

deeper thread has a greater resistance to being pulled out

Question 16

What is the pitch of a screw?

Answer 16

the linear distance travelled for a complete 360 degree turn

Question 17

What is tapping?

Answer 17

the process of cutting a thread

Question 18

What is a self-tapping screw?

Answer 18

a screw with a cutting tip that enables it to cut its own female thread track to match the male thread

Question 19

Why is it not possible to self-tap in cortical bone?

Answer 19

too much torque is required resulting in jamming or breaking of the screw

Question 20

What kind of tip do cancellous bone screws have?

Answer 20

corkscrew tip to engage the screw in the bone - because there is no pre-tapped hole the soft bone is compressed as the screw is driven in - enhancing the grip in otherwise weak bone

Question 21

How are standard bone screws inserted?

Answer 21

require a tapping instrument to cut a female thread

this means it is more easy to insert as more torque is converted into compression

Question 22

What is a ‘flute’?

Answer 22

a channel in tapping instruments or self-tapping screws that provides a route for cuttings to escape

Question 23

What is lagging

Answer 23

compression of two objects together

Question 24

how can a screw achieve lag?

Answer 24

partially threaded crew

hole drilled through one fragment

Question 25

List 5 uses of screws

Answer 25

to prevent sideways displacement of fragments

to hold a plate against a bone

to increase grip of an IM nail

permit axial displacement

as part of an external fixator