CTO Orthotic Materials & Fabrication Techniques 2 — Questions and Answers
Question 1: Which thermoplastic material is most appropriate for fabricating a rigid ankle-foot orthosis (AFO) requiring high durability and resistance to deformation under load?
- Polyethylene foam
- Polypropylene (Correct answer)
- Neoprene
- Plastazote
Correct answer: Polypropylene
Polypropylene is the standard rigid thermoplastic for AFOs due to its high durability, stiffness, and resistance to deformation under body weight.
Question 2: When using a heat gun during thermoplastic molding, what is the primary risk if the material is overheated?
- The material becomes too rigid to mold
- The material loses its thermoplastic properties permanently
- The material may bubble, thin unevenly, or burn (Correct answer)
- The material changes color but retains all properties
Correct answer: The material may bubble, thin unevenly, or burn
Overheating thermoplastic can cause bubbling, uneven thinning, and burning, compromising structural integrity.
Question 3: Which type of orthotic padding is best suited for accommodating a bony prominence to relieve focal pressure?
- Closed-cell polyethylene foam (Correct answer)
- Open-cell polyurethane foam
- Neoprene sheet
- Carbon fiber laminate
Correct answer: Closed-cell polyethylene foam
Closed-cell polyethylene foam (e.g., Plastazote) is preferred for accommodating bony prominences because it conforms and does not bottom out as quickly as open-cell foam.
Question 4: In lamination fabrication, what is the purpose of the PVA (polyvinyl alcohol) bag used during the process?
- To add color to the finished orthosis
- To create a vacuum-sealed barrier between resin and stockinette (Correct answer)
- To provide structural reinforcement
- To act as a bonding agent between layers
Correct answer: To create a vacuum-sealed barrier between resin and stockinette
The PVA bag creates an airtight barrier that prevents resin from contaminating the vacuum pump and ensures even resin distribution.
Question 5: Which rivet type is most commonly used when permanently joining two rigid thermoplastic components in orthotics?
- Wood rivet
- Solid aluminum rivet
- Pop (blind) rivet (Correct answer)
- Stainless steel machine screw
Correct answer: Pop (blind) rivet
Pop (blind) rivets are most commonly used in orthotics because they can be installed from one side without access to the back of the joint.
Question 6: What is the primary advantage of carbon fiber composite over polypropylene for orthotic fabrication?
- Lower material cost
- Greater weight and bulk
- Higher strength-to-weight ratio (Correct answer)
- Easier to modify with a heat gun
Correct answer: Higher strength-to-weight ratio
Carbon fiber composites offer superior strength-to-weight ratio, allowing for lighter, thinner orthoses with equivalent or greater rigidity.
Question 7: When vacuum-forming a thermoplastic over a positive model, which draping technique helps prevent bridging at concave areas?
- Stretching the material before heating
- Applying the material while it is still cold
- Manually pressing the softened material into concavities before applying vacuum (Correct answer)
- Using a thicker gauge material
Correct answer: Manually pressing the softened material into concavities before applying vacuum
Manually pressing softened thermoplastic into concave areas before vacuum is applied prevents bridging and ensures intimate contact with the model.
CTO Orthotic Materials & Fabrication Techniques 2
Which thermoplastic material is most appropriate for fabricating a rigid ankle-foot orthosis (AFO) requiring high durability and resistance to deformation under load?