Page 196 - ASOP ROT Study Guide

Signs that these areas may be affected adversely include:

♦ Loss of function - such as development of a weak foot dorsiflexion from common peroneal
nerve damage.
♦ Change in sensation - this may be numbness or tingling below a pressure point.
♦ Pain and burning sensations - these must be taken seriously.
♦ Discoloration - if the skin is dusky or blue or if, when pressed until the skin blanches, it takes a
long time to go pink again.

If any of these features are present then the cast must be removed and the area inspected.

3.6 The Effects of Heat

As described, both plaster of Paris and polyurethane polymers give off heat during setting. The
amount of heat produced depends on two factors:

♦ The temperature of the water used in wetting the cast bandage.
♦ The total quantity of material used.

1. Take three rolls of old plaster of Paris.
2. Divide one roll into two halves. Wet one half with water as per manufacturer's
instructions. Time how long it takes to set. Feel the temperature difference between
the wet roll and the dry roll.
3. Take one of the remaining rolls and dip it in water as per manufacturer's
instructions. Time how long it takes to set.
4. Dip the last roll (using forceps) in water that is too hot for your hands. Time how
long it takes to set.

Up to a certain temperature, increasing the temperature of the water used will decrease the setting
time. Beyond this optimum temperature the setting reaction will not continue to be accelerated.
Using water at a higher temperature than the optimum only adds to the general heat hazard. Water
temperatures should be kept well within the limits set by a manufacturer for their particular
product.

The heat released per molecule involved in the chemical reaction is always the same. The more
molecules there are, the more heat is given off. This means that thick heavy casts are more likely
to generate higher temperatures than thin casts. In addition, a thick cast will tend to act as an
insulator and retain the heat produced, thus increasing the risk.

4. RISKS IN REMOVAL OF PLASTERS

4.1 Cutting

Only one commercially available material has a zipper in it and nearly all casts have to be split for
removal. There are two basic methods:

♦ Hand cast cutters
♦ Power cast cutters.