Plaster of Paris takes its name from Paris, France, where it was first widely used chemically, surgically and constructionally. However, one of the earliest surgical uses was recorded in 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) but not enough to cause discomfort or injury.
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 cheap in comparison with more modern materials.
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 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 (but as mentioned there is still a water content of 20 percent within the crystalline structure). Mechanical failure of a cast is due to the different elastic moduli in gauze fibres and hydrated calcium sulphate.
Alternatives to Plaster of Paris
Fibreglass and resinous materials can be safely applied as external splints. These are light, durable and waterproof but require protective packaging and are difficult to apply without wearing gloves.
They are considerably more expensive than plaster at present, but to balance this disadvantage, fewer bandages are required and they are much more durable and so are particularly suitable for active or elderly patients. 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.