Merck Manual

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How To Apply a Cast


James Y. McCue

, MD, University of Washington

Reviewed/Revised Jun 2021 | Modified Sep 2022

A cast is a rigid, circumferential, layered composite dressing intended to immobilize a body part, typically an extremity.

Casts usually consist of a soft fabric sleeve on the skin, over which is placed a layer of soft padding, followed by multiple, thin layers of flexible strips of plaster or fiberglass that quickly harden through a chemical reaction. Similar material is used to make splints, which differ mainly in that the plaster or fiberglass is not circumferential.



  • Acute fractures or dislocations at risk of continued swelling that could cause ischemia after circumferential casting



  • Stockinette

  • Roll padding

  • Plaster or fiberglass casting material*

  • Strong scissors and/or shears

  • Lukewarm water and a bucket or other container

  • Nonsterile gloves

* Both materials are equally effective. Choice depends on availability and user preference. Length and width of materials depend on the body part being immobilized.


  • The patient should be positioned so that the operator has appropriate access to the affected extremity.

  • The extremity should be positioned in the appropriate anatomic alignment for the specific injury.

Step-by-Step Description of Procedure

  • Choose stockinette of the appropriate width; it should be form fitting but not so tight that it compromises circulation.

  • Apply stockinette to cover the area (eg, about 5 to 10 cm) proximal and distal to the anticipated extent of casting material.

  • Place several layers of padding (typically, 4).

  • Wrap the padding circumferentially, from distal to proximal, over the area to which the cast will be applied. Overlap the underlying layer by half the width of the padding.

  • Apply the padding firmly against the skin without gaps but not so tightly that it compromises circulation.

  • Extend the padding slightly (about 3 to 5 cm) past the anticipated extent of the plaster or fiberglass.

  • Smooth the padding as necessary to avoid protrusions and lumps. Tear away some of the padding in areas of wrinkling to smooth the padding.

  • Add separate, non-circumferential pieces of padding over and around bony prominences.

  • Immerse the casting material in lukewarm water.

  • Gently squeeze excess water from the casting material. Do not wring out plaster.

  • Apply the casting material circumferentially from distal to proximal, overlapping the underlying layer by half the width of the casting material.

  • Use 4 to 6 layers of plaster (typically) or 2 to 4 layers of fiberglass to ensure adequate strength of the cast.

  • Smooth out casting material to fill in the interstices in the plaster, bond the layers together, and conform to the contour of the extremity. Use your palms rather than your fingertips to prevent the development of indentations that will predispose the patient to pressure ulcers.

  • Fold back the stockinette before adding the last layer of casting material. Roll back the extra stockinette and cotton padding at the outer margins of the cast to cover the raw edges of the splinting material and create a smooth edge; secure the stockinette under the casting material.

  • Hold the body part in the desired position until the cast material hardens sufficiently, typically 10 to 15 minutes.

  • Check for distal neurovascular status (eg, capillary refill and distal sensation) and motor function.


  • Determine functional status (eg, weight bearing on lower extremity or use of upper extremity).

  • Arrange or recommend appropriate follow-up.

  • Provide verbal and written instructions.

  • Advise the patient to elevate the casted extremity above heart level whenever possible for the first 48 to 72 hours.

  • Advise the patient to keep the cast clean and dry.

  • Advise the patient not to insert any objects between the skin and the cast and not to cut the cast.

  • Instruct the patient to watch for complications such as worsening pain, paresthesias/numbness, and color change to the fingers.

  • Instruct the patient to seek further care if pain cannot be controlled with oral drugs at home.

Warnings and Common Errors

Tips and Tricks

  • Using cooler water increases the time required for the casting material to harden, which will give the operator more time to mold the cast.

  • Consider adding additional padding over bony prominences to minimize the risk of pressure sores.

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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