With the exception of the specific chemical substance involved, references to plaster in the following description are equally applicable to fiberglass.
The chemical reaction that causes plaster of paris (calcium sulfate) to crystallize, or set, is initiated by contact with water. The higher the water temperature, the faster the hardening process. However, the setting of plaster is an exothermic chemical reaction, which liberates heat. The faster plaster sets, the more heat it generates. This means, the maximum temperature to which the patient's skin is exposed will be the additive result of the water temperature plus the heat released by the plaster. For this reason, severe burns can result when plaster has been immersed in hot water, even though the temperature of the water itself was not sufficient to cause such burns. Although there is no universally prescribed ideal water temperature, a safe practice is to make the water slightly warmer than room temperature. If steam is visible, the water is almost certainly too hot.
To avoid irritation and minimize the potential for pressure sores, plaster dressings need to include several layers of padding between the plaster and the skin. When longitudinal splints are used, the padding need not be circumferential. Longitudinal padding will effectively protect the skin as long as it slightly exceeds the width and length of the splint. The best way to ensure this is to fashion the dry splint first and then measure the padding over it.
The length of a splint should be sufficient to provide ample leverage to immobilize the injured joint. To immobilize the elbow, for example, a splint should begin distal to the wrist and extend high up the lateral arm, to the level of the humeral neck. To effectively immobilize the ankle, a splint should extend from beneath the metatarsal heads to high calf. If the fracture is located along the midshaft of the distal extremity rather than at a joint, the splint should be long enough to immobilize the joint above and the joint below the fracture.
Splints may be fashioned from the plaster rolls normally used for casting or from prepadded material supplied on a continuous roll, which can be cut to length. When using common plaster rolls, determine the necessary length of the splint by measuring out a single layer along the extremity. Then, on a flat surface, unroll the plaster back and forth over itself to make a multilayered splint. For an adult, the splint should be at least 12 layers thick. Even more layers should be used for children, who typically remain as active as possible and have little regard for protecting the dressing.
When the dry splint has been prepared, measure out several layers of padding over it, making the padding longer and wider than the plaster. After setting the padding aside, grip each end of the splint and immerse it in water, keeping it submerged until bubbling stops (indicating the water has been fully absorbed into the interstices of the material). Then withdraw the splint and strip out the excess water by sliding the thumb and index finger along the length of the plaster on each side. (Be sure to use a stripping motion, rather than crumpling the dressing to wring out the excess water, or much of the plaster will be wrung out as well.)
The next step, frequently overlooked, is to lay the splint on a flat surface and massage the layers into one another so that they fuse. This creates a strong dressing that is solid on cross section. A splint whose separate layers are still visible on cross section is much weaker.
The padding should now be laid over the plaster and the dressing applied to the extremity, with the padded surface against the skin. An assistant can hold the splint against the extremity while it is wrapped in place with gauze bandage. Make sure the assistant uses the palms, rather than fingertips, when holding the plaster. Hardened finger dents can cause irritation or even pressure sores. If a compressive effect is desired, an elastic bandage may be wrapped over the gauze. (If an elastic bandage is wrapped directly onto plaster without an intervening layer of gauze, it will set into the plaster and lose its compressive function.)
While the plaster is setting, the physician may need to maintain the affected joint in a particular position. Again, the palms, rather than the fingers, should be used.
Once the setting process is well underway, if the position of a joint is changed, the dressing will crack and become functionally useless. If the joint has gradually migrated from the desired position, the physician must decide either to accept the current position or remove the dressing and start over. There is no need to feel self-conscious about the latter course. Patients generally appreciate a desire for perfection by their physician.
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