Plaster of Paris takes its name from Paris, France, where it was first widely used chemically, surgically and in construction works.
Its first documented medical use dates back to 1852 when A. Mathyson, a Dutch Army Surgeon, rubbed powdered plaster into cotton bandages to form splints.
Plaster of Paris, in its raw state, is termed gypsum hydrated calcium sulphate with impurities. The surgical form is pure anhydrous calcium sulphate.
The essential chemical step is the heating of gypsum to 120 degree Celsius (250 degree F).
Adding water allows for a return to the original crystalline state of full hydration. Twenty percent of added water is incorporated into the hydrated crystal lattice but the other 80 percent of water eventually evaporates.
The absorption of water while setting gives out heat (an exothermic reaction).
Plaster of Paris extremely safe and does not produce allergies. It is infinitely adaptable to the part being splinted and can be applied speedily without gloves. It cheaper in comparison with more modern materials.
However, plaster of Paris is slow to dry, to gain full strength and is seriously weakened if it becomes wet again. It is very heavy when wet but becomes much lighter when dry.
It is partially radio-opaque, obscuring bone detail on radiographs.
Low temperatures and sugar solutions retard setting of plaster of Paris while high temperatures and salt or borax solutions accelerate it.
The setting time is three times longer at 5 degree Celsius (40 degree F) than at 50 degree Celsius (125 degree F).
Although setting takes only a few minutes, drying may take many hours – roughly 36 hours for an arm cast, 48-60 hours for a leg cast and up to 7 days for a hip spica, especially if the atmosphere is moist and cool.
Movement of the plaster while it is setting will cause gross weakening.
The optimum strength is achieved when it is completely dry .
Plaster of Paris casts are frequently used to put casts in fracture patients.Although Plaster of Paris only depends on one simple chemical reaction for its setting, it is possible to vary the features of the process according to different requirements.
If a bandage is immersed in cold water the initial set will be delayed and thus “working time” lengthened. However, if a very rapid set is required soaking the bandage in warm water will accelerate the rate of reaction.
The cast strength is depends on upon the thickness of plaster and the shape of the cast which follows the contours of the affected limb. However, excess plaster will also increase weight and bulk and heat product. Therefore these different factors must be weighed against each other.
Because plaster of Paris is infinitely mouldable in the wet state it can be set around cast brace hinges and walking heels with ease.
Alternatives to Plaster of Paris
Fiberglass is a commonly used material for casts and external splints. These are light, durable and waterproof but require protective packaging and are difficult to apply without wearing gloves.
They are more radiolucent than plaster.
A number of preformed plastic components are available as an alternative to plaster. They are made to fit different sizes of limbs and to allow movement at joints.
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