Plaster of Paris (POP) for Leg Fractures

What is Plaster of Paris?

Indications

• Hold stable fractures in position during healing
• Supplement fixation of unstable fractures (e.g. after K-wire fixation)
• Maintain reduction after manipulation of displaced fractures
• Immobilize after operative procedures on the ankle/foot

Types of Leg Casts

1. Below-Knee (Short Leg) Plaster Cast

1. Patient sits on a high chair or lies with the lower leg hanging vertically, foot supported on the operator's knee; small pad/sandbag under the thigh
2. Place a rim of orthopaedic felt around the leg immediately below the flexed knee joint
3. Apply plaster over stockinet or a single layer of plaster wool
4. First slab: two 15 cm wide bandages from the medial condyle of the tibia, along the medial aspect of the leg, beneath the heel, up the lateral aspect to the lateral condyle
5. Second slab: extends from the back of the knee, behind the heel, to the base of the toes
6. Make transverse cuts on either side of the heel and imbricate the edges evenly
7. Fix slabs with circular moist plaster bandages rolled from above downwards
8. Extend to form a platform under the toes; trim so dorsal surfaces of toes remain free
9. Can be reinforced with a weight-bearing heel or sorbo rubber sole if required

2. Full-Leg (Long Leg) Plaster Cast

1. Apply stretch stockinet or plaster wool continuously from toes to groin
2. First complete the below-knee component (as above, but omit the felt rim around the upper tibia)
3. Assistant holds the wet-but-solid below-knee section, maintaining knee flexion at 20-30°
4. Extend with 15 cm wide bandage turns up to the groin
5. Ensure a good bond at the knee; finish the upper end by folding stockinet/wool outwards and incorporating into the final turns
6. The adage applies: "bent casts make straight bones" - a correctly moulded cast may look crooked but holds the fracture in proper alignment (three-point moulding technique)

3. Plaster Cylinder

1. Apply stockinet or plaster wool; pad each malleolus with orthopaedic felt
2. Two slabs of 15 cm wide bandage applied on each side from malleolus to groin; completed with circular turns
3. Must be carefully moulded above the malleoli to prevent the cast from slipping down
4. Knee held in a few degrees of flexion (not full extension) for comfort

4. Hip Spica

General Application Steps (Step-by-Step Summary)

Backslab vs Full Cast

• Backslab: Partial cast covering roughly half the circumference - used in acute injuries where significant swelling is expected (risk of compartment syndrome). Does not eliminate the risk of compartment syndrome - close clinical observation is still mandatory.
• Full cast split along its length is an alternative that allows for swelling.

Cast Wedging (Correcting Malalignment)

• Use radiographs/fluoroscopy to identify fracture level
• Cut the cast at fracture level, leaving 2-3 cm hinge on the concave side of deformity
• Open the gap to apply corrective force; insert wooden dowel to maintain position
• Complete with fresh POP
• Closing wedge (taking out a wedge on the convex side) is sometimes preferred for very unstable fractures where an opening wedge risks complete displacement

Advantages and Disadvantages

Complications

1. Compartment Syndrome (Most Serious)

• Caused by swelling inside a tight cast leading to neurovascular compromise
• Most common in the calf (leg) and forearm
• Warning signs: Pain beyond expected, pain on passive stretch, tightness, digital swelling
• Action: Immediately split (bivalve/univalve) the cast + all padding down to skin along the full length; elevate limb
• Univalving reduces pressure by ~30%; bivalving (both sides) + spreading reduces it further
• If relief does not occur - remove cast entirely, measure compartment pressures, perform fasciotomy if elevated
• Treatment of fracture is ALWAYS secondary to preserving blood supply

2. Pressure Sores

• Due to inadequate padding over bony prominences (malleoli, tibial crest, fibular head)
• Prevented by proper wool padding at these points before casting

3. Plaster Blisters and Skin Problems

• Plaster blisters and allergic dermatitis can occur under casts
• Patient should be advised not to insert objects under the cast

4. Peripheral Nerve Injury

• Elevated intracompartmental pressure can injure nerves

5. Delayed Union / Malunion / Nonunion

• Risk increased by: smoking (doubles healing time in ankle fractures), NSAIDs, diabetes, osteoporosis, poor reduction, non-compliance

6. Joint Stiffness and Muscle Wasting ("Plaster Disease")

• Caused by prolonged immobilization; worse in diabetics and smokers

7. Deep Vein Thrombosis

• DVT prophylaxis should be considered, especially with lower limb casts

Patient Instructions After POP

• Elevate the limb for the first week (reduces swelling and risk of compartment syndrome)
• Do not get the cast wet; do not insert objects down the cast
• Return immediately if: cast feels too tight, pain increases, digits become swollen or numb, or cast cracks/becomes soft
• Attend follow-up X-ray (usually at 1-2 weeks) to check fracture position has been maintained