Design
What the exam expects
Design questions test whether you can translate biomechanical requirements into geometry, materials, and trimlines. Core concepts: pressure = force / area (distribution strategy), moment arms and lever arms (where force is applied relative to a joint), three-point force systems for deformity control, total contact principles for load distribution, and stiffness vs. flexibility trade-offs driven by patient weight, activity, and control needs. Know that trimlines determine stiffness and motion control: a higher posterior shell limits plantarflexion; a more distal trim frees the knee; a wider proximal brim reduces focal pressure.
Traps
- Confusing more material with better control - mass without proper trimline geometry can add weight and pressure without controlling motion.
- Ignoring ground reaction forces and the effect of heel height on sagittal alignment.
- Designing for the deformity as it appears non-weight-bearing when the limb changes under load.
- Forgetting donning/doffing geometry - an opening too small for the foot or limb is a design failure, not a patient failure.
- Overlooking volume fluctuation; design should anticipate adjustability.
Rule of thumb
State the design intent in one sentence before choosing materials: "This device must [control/offload/support] at [joint/region] during [activity] for a patient of [weight/activity level]." Then choose trimlines and materials that serve that sentence.
Worked example
A patient needs to limit ankle dorsiflexion during stance to reduce anterior tibial pressure. Design reasoning: a rigid anterior shell with a posterior strap creates a counterforce; the moment arm from the ankle joint determines how much force is needed. Extending the proximal trim well up the calf increases the lever arm and reduces required strap tension - and therefore reduces focal skin pressure.