CFO Orthotic Device Design & Fabrication 2 — Questions and Answers
Question 1: Which thermoplastic material is considered HIGH-temperature and requires oven heating above 160°C for molding?
- Polypropylene (Correct answer)
- Polyethylene foam
- Low-density polyethylene
- Plastazote
Correct answer: Polypropylene
Polypropylene is a high-temperature thermoplastic that requires heating to approximately 160–180°C before it can be molded into orthotic shells.
Question 2: A fitter is trimming a rigid AFO and notices the trim line runs too proximal over the fibular head. What is the primary risk?
- Peroneal nerve compression (Correct answer)
- Achilles tendon irritation
- Tibial torsion
- Plantar fascia strain
Correct answer: Peroneal nerve compression
A trim line that extends too proximally over the fibular head places direct pressure on the peroneal nerve, which can cause foot drop or numbness.
Question 3: When fabricating a custom foot orthosis using a positive plaster cast, the fitter adds a 3 mm heel raise to the cast before draping the plastic. This technique is called:
- Intrinsic posting (Correct answer)
- Extrinsic posting
- Cast modification
- Balancing
Correct answer: Intrinsic posting
Intrinsic posting involves building corrections directly into the cast (and thus the device shell), as opposed to extrinsic posting which adds wedges beneath the finished orthosis.
Question 4: Which rivet material is preferred for pediatric orthoses because it minimizes weight while maintaining adequate shear strength?
- Aluminum pop rivets (Correct answer)
- Steel hex bolts
- Brass machine screws
- Titanium studs
Correct answer: Aluminum pop rivets
Aluminum pop rivets are lightweight, easy to apply, and provide sufficient shear strength for the lower loads found in pediatric orthoses.
Question 5: A solid-ankle AFO is prescribed for a patient with spastic drop foot. Which trim-line modification would allow a small amount of dorsiflexion while still controlling plantarflexion?
- Posterior leaf spring trim line (Correct answer)
- Anterior shell extension
- Proximal trim line reduction
- Medial flare addition
Correct answer: Posterior leaf spring trim line
A posterior leaf spring trim line narrows the posterior strut, allowing controlled dorsiflexion motion while the shell still blocks uncontrolled plantarflexion.
Question 6: During vacuum forming of a polypropylene AFO shell, the material shows webbing (thin folds) at the ankle. The MOST likely cause is:
- Insufficient drape tension before vacuum (Correct answer)
- Plastic overheated beyond optimal temperature
- Vacuum pressure set too high
- Cast was too dry
Correct answer: Insufficient drape tension before vacuum
Webbing occurs when the plastic sheet is not adequately stretched or tensioned over the cast before the vacuum is applied, leaving excess material that folds.
Question 7: Which padding material has the LOWEST Shore A hardness and is therefore best suited for direct-contact insole cushioning in a diabetic foot orthosis?
- Plastazote LD (low-density) (Correct answer)
- Pelite medium
- Aliplast XPE
- Polypropylene 1/8"
Correct answer: Plastazote LD (low-density)
Low-density Plastazote (Plastazote LD) has the softest durometer rating among common orthotic foams, making it ideal for pressure-relieving contact layers in diabetic insoles.
CFO Orthotic Device Design & Fabrication 2
Which thermoplastic material is considered HIGH-temperature and requires oven heating above 160°C for molding?