What is the proper procedure for applying a short arm thumb spica cast to a patient?

Short Arm Thumb Spica Cast Application

Indications and Context

A short arm thumb spica cast is the standard immobilization technique for stable, undisplaced scaphoid fractures and other thumb/wrist injuries requiring rigid immobilization of the thumb and wrist while allowing elbow motion. 1

Patient Preparation and Positioning

• Position the patient seated comfortably with the injured arm resting on a stable surface at approximately waist height 2
• The forearm should be in neutral rotation (thumb pointing upward) with the wrist in slight extension (approximately 15-20 degrees) 1
• The thumb should be positioned in volar abduction (as if holding a soda can), with the interphalangeal joint free to move 3
• Ensure the patient's shoulders are relaxed and the arm hangs naturally to maintain reproducible positioning 2

Materials and Equipment Setup

• Gather stockinette (3-inch width), cast padding, fiberglass or plaster casting material (3-4 inch width), and gloves 2
• Have water at appropriate temperature ready (cool for fiberglass, lukewarm for plaster) 2
• Prepare a stable surface for the patient's arm and ensure adequate lighting 2

Step-by-Step Application Technique

Initial Padding Layer

• Apply stockinette from mid-forearm to beyond the fingertips, cutting a hole for the thumb to protrude 2
• Apply cast padding circumferentially around the forearm, starting 2-3 cm below the elbow and extending to the metacarpal heads 2
• Use 2-3 layers of padding over bony prominences (radial and ulnar styloids, first metacarpal base) to prevent pressure sores 2
• Wrap padding around the thumb from the metacarpophalangeal joint to just proximal to the interphalangeal joint, leaving the thumb IP joint free 1, 3

Cast Material Application

• Immerse the casting material in water according to manufacturer specifications (typically 2-3 seconds for fiberglass) 2
• Begin wrapping at the proximal forearm, working distally with 50% overlap of each turn 2
• Mold the cast around the thenar eminence and thumb, ensuring the thumb remains in the abducted position 3
• Extend the cast distally on the volar surface to the distal palmar crease, allowing full flexion of the metacarpophalangeal joints of the fingers 2, 1
• On the dorsal surface, extend to just proximal to the metacarpal heads 2
• The radial border should extend along the radial aspect of the thumb to just distal to the metacarpophalangeal joint, leaving the interphalangeal joint completely free 1, 3

Molding and Finishing

• While the cast material is still malleable, mold it to conform to the natural contours of the forearm, wrist, and thumb 2
• Create a gentle three-point mold: apply pressure dorsally over the distal radius while supporting volarly at the proximal and distal forearm 2
• Ensure the thumb web space is well-molded to prevent the cast from sliding distally 1
• Fold the stockinette edges back over the cast margins and secure with a final layer of casting material for smooth, padded edges 2
• Trim any excess material that restricts finger or thumb IP joint motion 1

Critical Technical Points

• The cast must extend from 2-3 cm below the elbow proximally to the distal palmar crease volarly and just proximal to the MCP joints dorsally 2, 1
• The thumb IP joint must remain completely free to allow opposition and pinch function 1, 3
• Avoid excessive tightness during application, as swelling may occur in the first 24-48 hours 2
• The wrist should be maintained in slight extension (15-20 degrees) throughout the hardening process 1

Immediate Post-Application Assessment

• Check capillary refill in all fingertips and the thumb tip (should be <2 seconds) 2
• Verify that the patient can fully flex and extend all finger MCP, PIP, and DIP joints 2
• Confirm the thumb IP joint has full range of motion 1, 3
• Assess for any areas of excessive pressure or discomfort that require immediate cast modification 2

Patient Instructions

• Elevate the hand above heart level for the first 48-72 hours to minimize swelling 2
• Begin active finger motion exercises of the MCP, PIP, and DIP joints immediately to prevent stiffness 4
• Perform thumb IP joint flexion and extension exercises regularly 1
• Keep the cast completely dry; use a waterproof cover for bathing 2
• Return immediately if unremitting pain, numbness, tingling, or color changes develop 4
• Understand that even brief removal of the cast can restart the healing timeline and compromise fracture stability 4

Common Pitfalls to Avoid

• Do not include the thumb IP joint in the cast, as this significantly reduces hand function without improving fracture stability 1, 3
• Avoid applying the cast too tightly, particularly around the thumb web space, which can cause pressure necrosis 2
• Do not extend the cast too far distally on the palm, as this restricts finger flexion and grip strength 5
• Ensure adequate padding over bony prominences to prevent skin breakdown during the immobilization period 2
• Do not position the thumb in excessive extension or adduction, as this creates an unstable and uncomfortable position 3