Orthotic Device Design & Fabrication Flashcards

Which thermoplastic material is considered HIGH-temperature and requires oven heating above 160°C for molding?

Answer: Polypropylene

Polypropylene is a high-temperature thermoplastic that requires heating to approximately 160–180°C before it can be molded into orthotic shells.

A fitter is trimming a rigid AFO and notices the trim line runs too proximal over the fibular head. What is the primary risk?

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.

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:

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.

Which rivet material is preferred for pediatric orthoses because it minimizes weight while maintaining adequate shear strength?

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.

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?

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.

During vacuum forming of a polypropylene AFO shell, the material shows webbing (thin folds) at the ankle. The MOST likely cause is:

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.

Which padding material has the LOWEST Shore A hardness and is therefore best suited for direct-contact insole cushioning in a diabetic foot orthosis?

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.