CFO Orthotic Device Design & Fabrication 3 — Questions and Answers
Question 1: A wrist-hand orthosis (WHO) intended to immobilize the wrist in neutral should position the wrist at approximately:
- 0–10° extension (Correct answer)
- 30° flexion
- 20° ulnar deviation
- 45° extension
Correct answer: 0–10° extension
A functional or resting WHO is typically fabricated with the wrist in neutral to slight extension (0–10°) to maintain a functional hand position and prevent contracture.
Question 2: When selecting polypropylene sheet thickness for a pediatric foot orthosis for a child weighing 20 kg, the fitter should choose:
- 3/16 inch (4.8 mm) (Correct answer)
- 1/4 inch (6.4 mm)
- 3/8 inch (9.5 mm)
- 1/2 inch (12.7 mm)
Correct answer: 3/16 inch (4.8 mm)
A 3/16-inch (4.8 mm) polypropylene sheet provides adequate rigidity for a child around 20 kg without being unnecessarily heavy or stiff.
Question 3: The 'University of California Biomechanics Laboratory' (UCBL) insert is designed primarily to control:
- Subtalar valgus and hindfoot pronation (Correct answer)
- Forefoot supination
- Hallux valgus angle
- Tibial varum
Correct answer: Subtalar valgus and hindfoot pronation
The UCBL insert features deep heel cup walls that capture the calcaneus and control excessive subtalar eversion (hindfoot valgus/pronation).
Question 4: A fitter is using a heat gun to spot-heat a thermoplastic orthosis for minor adjustments. At what distance should the heat gun be held to avoid burning the material?
- 6–8 inches (15–20 cm) (Correct answer)
- 1–2 inches (2.5–5 cm)
- 12–15 inches (30–38 cm)
- 24 inches (60 cm)
Correct answer: 6–8 inches (15–20 cm)
Holding the heat gun 6–8 inches away concentrates heat enough to soften a small area without scorching or distorting adjacent material.
Question 5: Which edge-finishing technique BEST reduces the risk of skin abrasion along the proximal trim line of a hard plastic AFO?
- Flaring and flame-polishing the edge (Correct answer)
- Applying Velcro straps over the edge
- Adding a medial upright
- Increasing shell thickness
Correct answer: Flaring and flame-polishing the edge
Flaring the proximal edge outward and then flame- or belt-polishing it removes sharp burrs and creates a smooth, rolled edge that is less likely to abrade the skin.
Question 6: In fabricating a custom thoracic-lumbar-sacral orthosis (TLSO), what is the purpose of adding a polyethylene foam liner inside the rigid shell?
- Distribute pressure and improve comfort at bony prominences (Correct answer)
- Increase overall device rigidity
- Prevent corrosion of the plastic
- Reduce the overall height of the orthosis
Correct answer: Distribute pressure and improve comfort at bony prominences
A foam liner distributes contact forces over a larger area, protecting bony prominences such as the iliac crests and ribs from pressure injuries.
Question 7: When carbon fiber is used instead of polypropylene for an AFO shell, the primary clinical benefit is:
- Higher stiffness-to-weight ratio allowing a thinner, lighter device (Correct answer)
- Lower material cost
- Greater ease of trimming with standard scissors
- Improved moisture absorption
Correct answer: Higher stiffness-to-weight ratio allowing a thinner, lighter device
Carbon fiber composites offer superior stiffness per unit weight compared to polypropylene, enabling thinner, lighter orthotic shells without sacrificing structural support.
CFO Orthotic Device Design & Fabrication 3
A wrist-hand orthosis (WHO) intended to immobilize the wrist in neutral should position the wrist at approximately: