Fabrication, Modification, and Assembly
What the exam expects
Fabrication questions test knowledge of positive model modification, shell forming, and assembly. Understand model modification: adding plaster to create relief over bony prominences and reducing plaster to create total contact and load-bearing areas. Know thermoplastics by category - low-temperature vs. high-temperature, and the trade-off between drape/formability and durability/stiffness. Know that heating temperature, dwell time, and cooling determine whether a shell retains shape or relaxes. Assembly concepts include rivets vs. straps, hinge alignment with the anatomic joint axis, and fastener security.
Traps
- Reversing relief and reduction: add plaster where you want relief, remove where you want contact/load.
- Hinge axis misalignment causing shear at the joint during motion.
- Overheating thermoplastic, causing loss of shape or embrittlement; underheating causing incomplete forming.
- Straps placed over bony prominences rather than over soft tissue or shell.
- Skipping a check that fasteners do not protrude into the skin side.
Rule of thumb
Relief = add; contact = remove. And align the mechanical hinge with the anatomic joint axis before fastening anything permanently.
Worked example
A positive model shows a marked fibular head. The technician adds plaster at the mark to create relief in the finished shell, and removes plaster over the calf to create total contact and load sharing. Then the hinge is positioned by locating the anatomic axis with the limb in the device, and only then are rivets set. Reversing the relief step is the classic exam trap.