Non-surgical techniques: Plaster of Paris

Question 1

How is Plaster of Paris made?

Answer 1

Clacium sulphate is extracted in crystal form and heated to remove water producing powder (calcium sulphate hemihydrate)

This is mixed with water to form crystals again and “sets” to a solid

Question 2

What risk is involved in using Plaster of Paris?

Answer 2

Heat generated can cause damage to patients if care is not taken

Question 3

Describe a hard coated bandage

Answer 3

Calcium sulphate hemihydrate is dissolved in an organics solvent (such as ether) which contains no water
Starch is added to this mixture and the whole paste is spread on a cotton bandage
Bandage is therefore coated with calcium sulphate “held on” by starch

Question 4

Describe a loose coated bandage

Answer 4

Dust calcium sulphate hemihydrate onto a bandage so there is a weak association between bandage and plaster

Question 5

What role does the bandage play in Plaster of Paris?

Answer 5

Adds little strength but provides a vehicle for getting wet plaster on to the part to be splinted

Question 6

What is the purpose of starch in Plaster of Paris?

Answer 6

Acts an an accelerator to speed up setting

Question 7

What are retarders? Give 2 examples

Answer 7

Materials that can be added to slow down setting

e.g. alum and borax

Question 8

Describe the crystal structure of Plaster of Paris

Answer 8

Long sharp crystals - occur naturally as alabaster - give a finished cast a hard quality

Smaller crystals - give cast a softer feel

Question 9

What influences the interlocking between the plaster crystals?

Answer 9

How wet the plaster material is at the time of application

Question 10

What are the 2 ways in which a plaster functions?

Answer 10

Hydraulic therapy

Periosteal hinge

Question 11

What is hydraulic therapy?

Answer 11

Encasing the limb in a rigid exoskeleton providing support to the soft tissues which in turn support the broken bone

Question 12

What is a periosteal hinge?

Answer 12

Moulding the cast against to fracture to obtain a gentle 3 point fixation system

Question 13

In which situation is a 3 point mould particularly important in fracture ehaling?

Answer 13

Childhood

Question 14

The function of a cast is to control….

Answer 14

Length (prevent shortening)
Position (prevent tilt and shift)
Rotation

Question 15

How do standard casts control rotation?

Answer 15

By incorporating the whole of the broken bone and limb segment in the cast including the joints

Question 16

What is the disdvantage of incorporating the full limb segment and joints in the cast?

Answer 16

If casting is prolonged joints become stiff and muscles waste through disuse - prolongs overall rehab
Also immobility might prolong depndency and hospital stay

Question 17

What is the advantage of functional (cast) bracing?

Answer 17

Allows mobilisation of the joints

Question 18

Describe a femoral functional brace

Answer 18

Upper third of femoral component is gently squared odd (so soft tissues are slightly distorted but not enough to raise high points of pressure)
Knee is freed by use of hinges

Question 19

How does Sarmiento’s tibial brace achieved rotatory control?

Answer 19

Moulds around upper third of the tibia and by extensions to the cast which can encapture the femoral condyles in knee flexion

Question 20

When are functional braces applied? Why?

Answer 20

After the first 2 or 4 weeks (so soft tissue injury has settled down and there is no swelling)

Question 21

What are the 2 classes of new materials that are used for cast bracing?

Answer 21

Isoprene rubbers or polycaprolactone sheets

Glass fibre or artifical fibre and polyurethane composites

Question 22

What are the advantages of polycaprolactone/isoprene sheets?

Answer 22

Become ductile at fairly low temperatures

At room temp become firm but remain flexible enough to be gently adjustable

Question 23

Disadvantages of polycaprolactone/isoprene sheets?

Answer 23

Expensive
Require purchase of an oven
Require development of considerable skills to use well

Question 24

How is the expensiveness of polycaprolactone/isoprene sheets partially overcome?

Answer 24

Providing basic shapes in various sizes which can be finely adjusted during fitting

Question 25

What do fibre/polyurethane composites consist of?

Answer 25

Woven bandages made or glass fibre or fabric which is impregnated with a urethane monomer and a catalyst

Question 26

What are the advantages of fibre/polyurethane composites?

Answer 26

When exposed to warmth and moisture form a composite which is very light and extremely strong, yet flexible

Ideal for applying to a reasonably stable, healing fracture as they can form very sophisticated shapes an interface well with hinge materials

Question 27

Why are fibre/polyurethane composites not very useful as a primary splintage material?

Answer 27

They are conforming rather than mouldable - difficult to use on unstable and swollen limbs

Question 28

Why is Plaster of Paris the akinstay of most low velocity fracture management?

Answer 28

Cheap
Easy to obtain
Reasonably easy to apply
Very versatile