Plaster of Paris is 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. However over 50 degree C the setting rte slows and at 100 degree C no set occurs.
The final strength of the cast depends on the crystal structure. If the cast is manipulated after the “initial set” or prevented from drying out, it will be weak. Drying out will be delayed in cold or moist conditions and accelerated in a warm and dry environment.
Obviously, the cast strength is dependent 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.
Padding is used under a plaster cast to mechanically protect skin, soft tissue and bony prominences from pressure and abrasion and cast removal. Also to protect the skin from thermal injury because of the exothermic generated during setting. Poorly applied padding may produce sores. Over padding will reduce the closeness of the fit of the cast, and possibly permit excess movement at the fracture site resulting in impaired healing. Thus skill in padding is as essential as plaster moulding in the production of a therapeutic cast.
Because plaster of paris is infinitely mouldable in the wet state it can be set around cast brace hinges and walking heels with ease. However for maximum strength, the plaster should dry out first and the walking attachment affixed with a fresh plaster of Paris bandage.
The property of Plaster of Paris to absorb moisture has meant that casts have been used as wound dressings. However, too much moisture from inside or out will reduce the strength of the cast and make it liable to breakdown.
The strength of Plaster Paris builds up over a period of time in 3 stages and so load may be gradually increased. Excessive premature loading will produce a breakdown of the cast and patients need to be advised how much load they can apply and at what time.