Prosthetic Measurement, Casting, and Fitting Techniques
What is tested
Accurate measurement, negative and positive casting, rectification and fit assessment.
Measurement rules
- Measure the bare limb or the prescribed sock thickness, never over clothing.
- Mark landmarks first: patellar tendon, tibial tubercle, fibular head, popliteal crease, ischial tuberosity, greater trochanter, acromion, olecranon.
- Record circumferences at fixed intervals, and the AP and ML dimensions at the same level.
- Measure limb length and the contralateral side for comparison; record joint ROM and any contracture.
Casting and rectification
Wrap plaster bandages evenly with 50% overlap, maintaining the prescribed joint position - typically 5–10° knee flexion for a transtibial cast and near extension for a TSB. Do not pull the wrap tight at the distal end. Pour a positive model, then rectify: build up relief over the tibial crest and fibular head, and remove plaster over the patellar tendon bar and popliteal area.
Worked example
A transtibial cast measures ML 10 cm and AP 12 cm. With semi-axes a = 5 and b = 6, Ramanujan's approximation gives C ≈ π[3(a+b) − √((3a+b)(a+3b))] = π[33 − √(21×23)] = π[33 − 21.98] ≈ 34.6 cm. This circumference guides liner and socket sizing; do not simply average the two dimensions.
Traps
Measuring over trousers; losing landmarks when the cast is removed; casting with the knee at 30° flexion; over-tightening the distal wrap; skipping the check socket before laminating.