CPO Prosthetic & Orthotic Design & Fabrication 2 — Questions and Answers
Question 1: Which thermoforming technique is most appropriate for fabricating a total contact orthosis requiring high conformity to bony prominences?
- Vacuum forming over a positive model (Correct answer)
- Drape molding at room temperature
- Injection molding with ABS plastic
- Compression molding with fiberglass
Correct answer: Vacuum forming over a positive model
Vacuum forming over a positive model applies uniform negative pressure to draw the heated thermoplastic into intimate contact with all surface contours.
Question 2: A patient with a transfemoral amputation has a residual limb that has decreased in volume significantly post-surgery. Which socket modification is the FIRST step to address this?
- Add distal padding or liner inserts (Correct answer)
- Fabricate an entirely new definitive socket
- Switch to a suction suspension system
- Apply a rigid pelvic band
Correct answer: Add distal padding or liner inserts
Adding distal padding or liner inserts compensates for volume loss and restores fit without the expense and time of fabricating a new socket.
Question 3: In carbon fiber laminate fabrication, the direction of fiber orientation most critical for maximizing longitudinal stiffness in a pylon is:
- 0° (parallel to long axis) (Correct answer)
- 45° bias
- 90° (perpendicular to long axis)
- Woven multidirectional
Correct answer: 0° (parallel to long axis)
Fibers oriented at 0° (parallel to the long axis) bear axial loads most efficiently, maximizing longitudinal stiffness.
Question 4: When casting a patient for a below-knee orthosis, the subtalar joint should ideally be positioned in:
- Neutral subtalar position (Correct answer)
- 5° of inversion
- 5° of eversion
- Maximum pronation
Correct answer: Neutral subtalar position
Casting in subtalar neutral captures the foot in its most biomechanically efficient alignment, which is the goal for most AFO designs.
Question 5: Which material property makes high-density polyethylene (HDPE) preferable over polypropylene for a patient requiring frequent impact loading in a lower-limb orthosis?
- Higher impact resistance and toughness (Correct answer)
- Greater rigidity and stiffness
- Lower melting temperature for easier fabrication
- Better chemical resistance
Correct answer: Higher impact resistance and toughness
HDPE's higher impact resistance and toughness make it better able to absorb repeated impact loads without fracture compared to polypropylene.
Question 6: A transtibial prosthetic socket with a roll-on liner and locking pin achieves suspension primarily through:
- Mechanical engagement of pin into a locking mechanism (Correct answer)
- Negative pressure within the socket
- Anatomical contours of the residual limb
- Supracondylar wedge trimlines
Correct answer: Mechanical engagement of pin into a locking mechanism
The pin at the distal end of the roll-on liner clicks into a lock in the socket, providing suspension via mechanical engagement.
Question 7: Which of the following best describes the purpose of a relief in a prosthetic socket over a bony prominence?
- To reduce pressure concentration over sensitive areas (Correct answer)
- To increase total contact surface area
- To improve suspension of the prosthesis
- To add structural rigidity to the socket walls
Correct answer: To reduce pressure concentration over sensitive areas
Reliefs are recessed areas in the socket that offload pressure from sensitive or bony structures, preventing tissue breakdown.
CPO Prosthetic & Orthotic Design & Fabrication 2
Which thermoforming technique is most appropriate for fabricating a total contact orthosis requiring high conformity to bony prominences?