Design, Measurement, and Casting/Scanning
What the exam expects
This domain covers capturing an accurate negative or digital model. Know patient positioning (joint angles, muscle relaxation, weight-bearing status) and why it must match the intended device. Understand casting materials and technique: plaster wraps, fiberglass, and the difference between non-weight-bearing, partial weight-bearing, and weight-bearing casts. For digital capture, know scanner types (structured light, laser), the need for consistent landmarking, and the fact that a scan captures surface geometry but not tissue compliance - the clinician must still mark bony prominences and note tissue turgor.
Traps
- Casting in a position that does not match the device's functional position (e.g., casting a foot in plantarflexion when the device must hold neutral).
- Over-tight wraps causing pressure ridges; wrinkles in plaster create focal pressure.
- Failing to mark landmarks and bony prominences on the model.
- Assuming a scan substitutes for hands-on tissue assessment.
- Ignoring limb volume change between capture and delivery.
- Forgetting to record the date, position, and side on the model.
Rule of thumb
Capture the limb in the position the device will hold it, mark what must be relieved, and label what you captured. If the position or labeling is uncertain, the model is not usable.
Worked example
For a transtibial limb with a prominent tibial crest and a fibular head, the cast is taken with the knee in slight flexion, the patient relaxed, and the crest and head marked. On the positive model, the technician relieves those marks before forming the shell. A common exam trap is answering that the marks are unnecessary because the scan 'already has the shape' - geometry does not encode pressure tolerance.