COF Materials & Fabrication Basics 3 — Questions and Answers
Question 1: What is 'creep' in the context of orthotic materials?
- A sudden fracture under impact
- Gradual permanent deformation under sustained load (Correct answer)
- Rapid softening when heated
- Surface cracking from UV exposure
Correct answer: Gradual permanent deformation under sustained load
Creep refers to the slow, time-dependent permanent deformation that occurs when a material is subjected to a constant load over an extended period.
Question 2: When selecting leather for an orthotic interface, which property is most critical for patient comfort over long wear periods?
- Tensile breaking strength
- Moisture-wicking and breathability (Correct answer)
- Electrical insulation
- UV color stability
Correct answer: Moisture-wicking and breathability
Leather's natural moisture-wicking and breathability properties help manage perspiration and reduce skin maceration during extended orthosis wear.
Question 3: What is the purpose of a plaster of Paris negative cast in the custom orthosis fabrication process?
- To apply corrective forces directly to the limb
- To capture an exact impression of the patient's anatomy for model creation (Correct answer)
- To test material durability
- To trim the final plastic shell
Correct answer: To capture an exact impression of the patient's anatomy for model creation
A plaster of Paris negative cast captures the precise shape of the patient's limb, which is then filled to create a positive model over which the orthotic material is formed.
Question 4: Which type of rivet is preferred when joining orthotic components that may need to be disassembled for adjustment?
- Solid rivet
- Pop (blind) rivet (Correct answer)
- Tubular rivet
- Drive screw
Correct answer: Pop (blind) rivet
Pop rivets can be drilled out for disassembly and replacement, making them practical for orthotic joints that may require periodic adjustment or component exchange.
Question 5: What does 'Shore hardness' measure in orthotic materials?
- Tensile strength at break
- Resistance to permanent indentation or penetration (Correct answer)
- Thermal expansion coefficient
- Fatigue cycle life
Correct answer: Resistance to permanent indentation or penetration
Shore hardness quantifies how resistant a material is to surface indentation, helping clinicians select padding and liner materials with appropriate firmness for the clinical application.
Question 6: Why is polyethylene foam (Plastazote) commonly used as an interface liner in diabetic foot orthotics?
- It provides maximum rigidity
- It molds to foot contours with body heat and offloads pressure (Correct answer)
- It is the most durable foam available
- It has the highest Shore hardness rating
Correct answer: It molds to foot contours with body heat and offloads pressure
Plastazote softens with body heat to conform to foot contours and redistributes pressure, reducing peak plantar pressures critical in diabetic neuropathy management.
Question 7: What is the significance of a material's 'glass transition temperature' (Tg) in orthotics?
- The temperature at which it becomes transparent
- The temperature below which a polymer becomes brittle (Correct answer)
- The melting point of metal alloys
- The maximum sterilization temperature
Correct answer: The temperature below which a polymer becomes brittle
Below Tg, amorphous polymers shift from a rubbery, flexible state to a brittle, glassy state, which is important when considering orthosis performance in cold weather environments.
COF Materials & Fabrication Basics 3
What is 'creep' in the context of orthotic materials?