Plaster of Paris casts

Plaster of Paris (POP) casts are the most commonly used material for immobilizing injured limbs. 

History

Various materials have been used to immobilize limbs since antiquity. Splints have been found by Egyptian archaeologists dating from 2500 BCE. Various mixtures of waxes, resins, gums and flour were used up until the early 19^th century.

Plaster of Paris as a building material has been used since the Roman period, however its utilization for orthopedic immobilization has only been since the mid-19^th century. There has been little change in its method of preparation or clinical use since then.

Chemistry

Plaster of Paris (2CaSO₄.H₂O) is a hydrated form of calcium sulfate. Gypsum (a mined form of hydrated calcium sulfate) is heated to 120 ℃ driving off some of the water as steam, creating a hemihydrate form of calcium sulfate (CaSO₄.0.5H₂O). This desiccated form is then re-mixed with water when the plaster cast is applied by the plaster technician in the clinic. This reaction produces heat, as experienced by anyone who has had a plaster cast applied.

The first setting stage lasts up to 15 minutes when the wet plaster hardens. 

Modern alternatives

Fibreglass casts are increasingly used instead of plaster of Paris. Fibreglass is approximately one-third lighter, and three-times as strong. Fibreglass is a form of plastic containing thin strands of glass.

On the downside fibreglass is avoided in the acute setting as it is less forgiving to post-immobilization limb swelling than plaster and cannot be reshaped once set. Instead it is employed in the longer term management of fractures, once the acute phase has passed. 

Radiographic features

Plaster of Paris is radiopaque and when in place makes evaluation of fractures and bone positioning more difficult. It has very low signal on MRI. Ultrasound waves cannot pass through plaster of Paris and therefore it needs to be removed if ultrasound of the limb is needed.

Fibreglass, unlike plaster, is much more radiolucent, therefore it has much less impact on the ability of the reader to assess the bony injury.

Complications

Potential complications of using plaster immobilization include:

• deep vein thrombosis
  • risk of DVT with immobilization of lower limb > upper limb ¹
• compartment syndrome
• soft tissue edema
• pressure sores
• impaired venous return
• muscle atrophy
• disuse osteopenia
• ligamental atrophy