Non-surgical techniques: Plaster of Paris Flashcards
How is Plaster of Paris made?
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
What risk is involved in using Plaster of Paris?
Heat generated can cause damage to patients if care is not taken
Describe a hard coated bandage
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
Describe a loose coated bandage
Dust calcium sulphate hemihydrate onto a bandage so there is a weak association between bandage and plaster
What role does the bandage play in Plaster of Paris?
Adds little strength but provides a vehicle for getting wet plaster on to the part to be splinted
What is the purpose of starch in Plaster of Paris?
Acts an an accelerator to speed up setting
What are retarders? Give 2 examples
Materials that can be added to slow down setting
e.g. alum and borax
Describe the crystal structure of Plaster of Paris
Long sharp crystals - occur naturally as alabaster - give a finished cast a hard quality
Smaller crystals - give cast a softer feel
What influences the interlocking between the plaster crystals?
How wet the plaster material is at the time of application
What are the 2 ways in which a plaster functions?
Hydraulic therapy
Periosteal hinge
What is hydraulic therapy?
Encasing the limb in a rigid exoskeleton providing support to the soft tissues which in turn support the broken bone
What is a periosteal hinge?
Moulding the cast against to fracture to obtain a gentle 3 point fixation system
In which situation is a 3 point mould particularly important in fracture ehaling?
Childhood
The function of a cast is to control….
Length (prevent shortening)
Position (prevent tilt and shift)
Rotation
How do standard casts control rotation?
By incorporating the whole of the broken bone and limb segment in the cast including the joints
What is the disdvantage of incorporating the full limb segment and joints in the cast?
If casting is prolonged joints become stiff and muscles waste through disuse - prolongs overall rehab
Also immobility might prolong depndency and hospital stay
What is the advantage of functional (cast) bracing?
Allows mobilisation of the joints
Describe a femoral functional brace
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
How does Sarmiento’s tibial brace achieved rotatory control?
Moulds around upper third of the tibia and by extensions to the cast which can encapture the femoral condyles in knee flexion
When are functional braces applied? Why?
After the first 2 or 4 weeks (so soft tissue injury has settled down and there is no swelling)
What are the 2 classes of new materials that are used for cast bracing?
Isoprene rubbers or polycaprolactone sheets
Glass fibre or artifical fibre and polyurethane composites
What are the advantages of polycaprolactone/isoprene sheets?
Become ductile at fairly low temperatures
At room temp become firm but remain flexible enough to be gently adjustable
Disadvantages of polycaprolactone/isoprene sheets?
Expensive
Require purchase of an oven
Require development of considerable skills to use well
How is the expensiveness of polycaprolactone/isoprene sheets partially overcome?
Providing basic shapes in various sizes which can be finely adjusted during fitting
What do fibre/polyurethane composites consist of?
Woven bandages made or glass fibre or fabric which is impregnated with a urethane monomer and a catalyst
What are the advantages of fibre/polyurethane composites?
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
Why are fibre/polyurethane composites not very useful as a primary splintage material?
They are conforming rather than mouldable - difficult to use on unstable and swollen limbs
Why is Plaster of Paris the akinstay of most low velocity fracture management?
Cheap
Easy to obtain
Reasonably easy to apply
Very versatile
