Plaster of Paris — Comprehensive Overview

1. Composition & Chemistry

CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O

• Setting temperature: ~128°C for preparation (water is driven off)
• Setting time once wet: 5–15 minutes (setting stage followed by hardening stage)
• Setting reaction is slightly exothermic — releases heat as it crystallizes
• Setting involves a slight expansion in volume (first stage), which helps conform to the limb, followed by the hardening stage
• Water temperature used during application must stay <24°C (75°F) — warmer water accelerates the reaction and risks thermal burns

• Accelerators (shorten setting): salicylic acid, zinc sulfate, aluminum sulfate
• Retarders (lengthen setting): gums, glue, citrate

2. History

3. Medical Uses & Indications

3a. Orthopedic Indications (Pfenninger & Fowler's Procedures for Primary Care, p. 2985)

• Stable, nondisplaced closed fractures of long bones (radius, ulna, phalanges, metacarpals, metatarsals, malleoli)
• Reduced dislocations requiring immobilization
• Grade III ligament sprains (e.g., ankle)
• Achilles tendon disruptions
• Refractory tendonitis

• Easier to mould to the limb contour
• More forgiving with acute swelling
• Can be used as a "backslab" (partial cast) to allow post-injury swelling without compartment syndrome risk
• Inexpensive and widely available — Bailey and Love's Short Practice of Surgery 28th ed., p. 447–448

3b. Emergency / Soft Tissue Indications (Roberts and Hedges, p. 1180)

• Tendon lacerations (reduces stress on the repair)
• Deep lacerations crossing joints (reduces wound tension)
• Inflammatory disorders: tenosynovitis, acute gout
• Deep space infections of hands/feet (limits edema)
• Large abrasions crossing joint surfaces
• Multiple trauma: splinting fractures while resuscitation and imaging proceed

3c. Surgical / Reconstructive Applications

• Post-surgical support after tendon, nerve, or vascular repair
• Correction of congenital/acquired deformities (e.g., clubfoot / talipes equinovarus)
• Custom molds for prosthetics and orthotics

3d. Non-medical Applications

• Sculpture and art: fast-setting nature allows direct modeling; used since medieval times
• Dentistry: dental molds and impressions
• Architecture: ornamental plasterwork, ceiling cornices, fire-resistant wall surfaces
• Casting medium: does not significantly shrink or crack on drying — ideal for precision molds (Encyclopaedia Britannica)
• Art gesso: mixed with glue applied to wood/canvas panels as a ground for tempera and oil painting (medieval/Renaissance technique)

4. Types of POP Casts

5. Application Method

Equipment Required (Pfenninger & Fowler's Procedures for Primary Care, p. 3000)

• Rubber gloves, gown, shoe covers
• Stockinette (2-, 3-, 4-inch widths)
• Soft cotton (Webril) or synthetic padding bandages
• Plaster of Paris rolls/sheets (≤10 ply — avoid thicker; can cause thermal injury)
• Bucket of cool water (<24°C)
• Elastic bandages

Step-by-Step Procedure (Roberts and Hedges, p. 1183)

1. Measure the length of plaster against the area to be splinted
2. Roll out layers: ~8 layers for upper extremity, 12–15 layers for lower extremity
3. Apply stockinette over the extremity, extending 10–15 cm beyond both ends
4. Wrap 2–3 layers of Webril (cotton padding) circumferentially over the stockinette, overlapping 25–50% each pass; avoid wrinkling (pressure sores)
5. Submerge plaster strips in cool water until bubbling stops — do NOT use hot water (thermal burn risk)
6. Smooth plaster between fingers, remove excess water; lay flat and smooth to remove wrinkles and ensure uniform lamination
7. Apply plaster over Webril; smooth with palms (NOT fingertips — fingertip indentations cause pressure sores)
8. Fold stockinette over edges of plaster for smooth, padded margins
9. Secure with elastic wrap distal → proximal; do not wrap too tightly
10. Mould in desired position using palms, holding until fully set

"Bent casts make straight bones" — a correctly moulded cast may look crooked but uses three-point moulding to maintain fracture reduction against the intact periosteal hinge — Bailey and Love's, p. 448

Setting Times

• Setting stage: 5–10 minutes (exothermic, slight expansion)
• Hardening stage: follows setting; cast should not bear weight until fully hardened (typically 24–48 hours for full strength)
• Cool water slows setting (more working time); warm water accelerates it (burn risk)

6. Advantages & Disadvantages

Advantages (Bailey and Love's Short Practice of Surgery, Table 32.4)

• No operative wound or interference with fracture site
• Cheap and widely available
• Easily mouldable — especially in acute phase
• Adjustable (can be bivalved, windowed, or modified)
• No implants requiring later removal
• Good for acute swelling management (as backslab)

Disadvantages

• "Plaster disease": joint stiffness and muscle wasting with prolonged immobilization
• Limited access to soft tissues (difficult wound inspection)
• Cumbersome, especially in elderly patients
• Interferes with function and daily activities
• Not waterproof — degrades when wet
• Poor mechanical stability compared to fiberglass or internal fixation
• Heavier than synthetic alternatives
• Thermal injury risk if applied too thick (>10 ply) or with warm water

7. Complications

• Compartment syndrome: circumferential cast in acute swelling — mitigated by backslab or splitting the cast
• Pressure sores at bony prominences (olecranon, ulnar styloid, heel, fibular head)
• Thermal burns: exothermic reaction amplified by thick layers or hot water
• Skin maceration / dermatitis: under a wet or prolonged cast
• DVT / thromboembolic events: with lower limb immobilization
• Malunion: from inadequate moulding or loss of reduction within cast
• Nerve compression: from tight application

8. Alternatives to Plaster of Paris

Currently, fiberglass is the most common material used in definitive casting globally. It is approximately 2–3× stronger for any given thickness and creates less heat during application. However, POP remains preferred for acute fractures where swelling risk is high and precise moulding is needed — Pfenninger & Fowler's, p. 2980; Rockwood & Green's Fractures in Adults 10th ed., p. 700.

9. Summary

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 1180–1184
• Pfenninger and Fowler's Procedures for Primary Care, p. 2978–3080
• Bailey and Love's Short Practice of Surgery 28th ed., p. 447–448
• Rockwood and Green's Fractures in Adults 10th ed., p. 617–700
• Open Orthopaedics Journal — "Plaster of Paris: Short History of Casting and Injured Limb Immobilization"