Orthotic Device Design & Fabrication Flashcards

A fitter notices a polypropylene AFO has developed a stress fracture along the posterior ankle strut after 8 months. The MOST likely cause is:

Answer: Material fatigue from repeated cyclical loading

Repeated dorsiflexion-plantarflexion cycles create cyclical tensile stress along the posterior strut, which eventually exceeds the fatigue limit of polypropylene, causing cracking.

An extrinsic forefoot post on a functional foot orthosis is placed:

Answer: Beneath the shell in the forefoot region

An extrinsic post is a wedge glued or ground onto the plantar surface of the completed orthosis shell, specifically under the forefoot, to alter forefoot loading.

Which casting method captures a non-weight-bearing, three-dimensional impression of the foot in a controlled subtalar neutral position using plaster bandages?

Answer: Slipper cast technique

The slipper cast technique involves wrapping the foot with wet plaster bandages while holding it in subtalar neutral, then removing the hardened shell as a negative mold.

When fabricating a custom thoracolumbar corset with stays, the stays should be positioned to:

Answer: Run parallel to the spine bilaterally without impinging on the spinous processes

Stays are placed paravertebrally (lateral to the midline) so they provide corset rigidity without pressing directly on the spinous processes, which would cause discomfort.

A fitter is fabricating a dynamic wrist extension orthosis using low-temperature thermoplastic. The outrigger wire used to mount the finger loops should be oriented at what angle to the metacarpals to achieve optimal finger extension force?

Answer: 90° (perpendicular) to the proximal phalanges

A 90° angle of pull on the proximal phalanges maximizes the rotational moment for extension and minimizes unwanted joint compression or distraction forces.

After grinding the edges of a polypropylene orthosis, the fitter should perform a final edge-smoothing step using:

Answer: Scotch-Brite pad or fine sandpaper followed by flame polishing

A fine abrasive pad removes micro-burrs left by grinding, and a quick flame pass melts the surface slightly to produce a smooth, rounded edge that will not irritate skin.

When applying a prefabricated patellar tendon-bearing (PTB) orthosis to offload a tibial stress fracture, the primary pressure relief area should be over the:

Answer: Patellar tendon and flares of the tibial plateau

A PTB orthosis transfers weight from the foot to the patellar tendon and the medial and lateral tibial flares, bypassing the fracture site in the tibial shaft.